Decadal Changes in the Atlantic Water in the Eurasian Basin

Master’s Thesis

University of Bremen Postgraduate Programme Environmental Physics

Alfred Wegener Institute - Helmholtz Centre for Polar and Marine Research

Author: First Examiner: Wiebke Körtke Prof. Dr. Torsten Kanzow [email protected] October 10, 1994 Second Examiner: 3085553 Prof. Dr. Monika Rhein

June 20, 2019

Abstract

The reduction of Arctic sea ice is a strong indicator of climate change. Until now it is mostly attributed to atmospheric forcing. Atlantic Water is a main source of heat for the Arctic Ocean. This study aims to show that a shoaling of the Atlantic Water in the Eurasian Basin combined with a weakening in stratification increases the potential of upward heat flux. CTD data from 1980 to 2018 are analysed to determine the depth of the Atlantic Water and the stratification of the upper water column. The focus of this study is on the Laptev Sea providing a good data coverage. A shoaling of the Atlantic Water layer takes place in the Laptev Sea from 2002 to 2018 by 50 m in total. A decreasing depth of the upper boundary of the Atlantic Water is found between 1993 to 2002. Analyses of density differences between the Atlantic Water and (i) the Polar Water and (ii) the Lower Halocline Water indicate a weakening of the stratification in the upper water column for the recent years in the period observed. As well, an increase in temperature and salinity is present in the upper water column. This study concludes that the Laptev Sea is in a transition towards an Atlantic-dominated regime and that aspects of atlantification are present. Further, a water mass definition based on fixed temperature and salinity values is not longer appropriate in the current state of the eastern Eurasian Basin. Future investigations are needed to determine the origin and forcing of the observed salinification of the upper water column and the shoaling of the Atlantic Water layer.

CONTENTS

Contents

Abstract i

1 Introduction1 1.1 Arctic Ocean...... 1 1.2 Water Masses and Circulation...... 5 1.3 Motivation and Focus of this Study...... 7

2 Data and Methods9 2.1 Study Site...... 9 2.2 Data Sets...... 10 2.3 Methods...... 13 2.3.1 Interpolation...... 13 2.3.2 Determination of Water Mass Boundaries...... 15 2.3.3 Stratiﬁcation Analysis...... 16

3 Results 19 3.1 Laptev Sea...... 19 3.1.1 Temperature and Salinity...... 19 3.1.2 Stratiﬁcation Analysis...... 24 3.2 Origin for Changes and Propagation through the Basin...... 33 3.2.1 Franz Joseph Land...... 33 3.2.2 Svalbard...... 40 3.2.3 Fram Strait...... 42 3.2.4 Propagation...... 44

4 Discussion 47 4.1 General Variability in the Eurasian Basin...... 47 4.1.1 Distance to the Coast...... 47 4.1.2 Seasonality...... 48 4.1.3 Atlantic Water in the Arctic Ocean...... 49 4.2 Atlantic Water Depth and Stratification...... 51 4.2.1 Shoaling of Atlantic Water Layer...... 51 4.2.2 Halocline Layer...... 52 4.2.3 Stratification...... 54 4.2.4 Atlantification...... 56 4.3 General Importance...... 57

ii CONTENTS

5 Summary and Outlook 58

Acknowledgements iv

References v

Appendix xi

iii 1. INTRODUCTION

1 | Introduction

The Arctic Ocean has undergone pronounced changes during the last decades. Changes have been observed in all components of the Arctic climate system (e. g. Jeffries et al., 2013; Pnyushkov et al., 2015, and references therein). Especially, the sea ice loss is an important indicator of changing conditions. Sea ice loss takes place in the complete Arctic Ocean and during all seasons (Serreze et al., 2007; Cavalieri and Parkinson, 2012; Onarheim et al., 2018). The change in sea ice can potentially have both local and remote impacts on the climate system. Different parameters such as the surface energy budget, the atmospheric and oceanic circulation patterns, marine ecosystems and mammals can be influenced by these changes (see Årthun et al., 2019, and references therein for further details). In March 2019 the sea ice extent in the Arctic has reached a historic low with a mean extent of 14.32 million square kilometres. The extent was lower by approximately one million square kilometres compared to the long term mean (1981 to 2010). This is the fourth-lowest extent in the 40-year time series starting in 1979 with the beginning of regular satellite observations (Ionita-Scholz et al., 2019). The lowest mean in March was recorded in 2017 with 14.42 million square kilometres (Grosfeld et al., 2016). To gain a better understanding of the further development of the Arctic sea ice and its drivers it is essential to determine external and internal forcing. One possible driver of shifts in the sea ice situation is a change in Atlantic Water. The role of Atlantic Water might have been underestimated so far, but might gain a greater role especially if the stratification above the Atlantic Water is decreasing (Carmack et al., 2015; Polyakov et al., 2017). Atlantic Water is entering the Arctic Ocean through Fram Strait, being a major heat source to the basin’s interior. An increase in the heat budget may influence the sea ice situation in the ocean. Shallower depth of the Atlantic Water layer also increase the likelihood of upward heat fluxes due to vertical mixing penetrating into this layer. Hence, this study will focus on changes in the depth of the Atlantic Water layer.

In this section, the theoretical background of the study will be covered. First of all, a general introduction in the hydrography of the Arctic ocean is given. It is followed by the deﬁnition of the occurring water masses and the propagation through the basin. The last part of the chapter concentrates on the motivation and the focus of this study.

1.1 Arctic Ocean

The Arctic ocean is the smallest of all seven oceans. It is part of the Arctic Mediterranean Sea and has a size of 9.4 million square kilometres (Rudels, 2009). Shallow shelf regions are typical for the Arctic Ocean. More than half of its area consists of shallow shelve regions. The deep parts of the Arctic Ocean are the Eurasian and Canadian Basin. They are separated by the Lomonosov Ridge (1600 m deep). The Gakkel Ridge separates the Eurasian Basin into

1 1. INTRODUCTION the Nansen and Amundsen Basin. The Canadian Basin is further divided into Makarov and Canada Basin. With a depth of approximately 4500 m the Amundsen Basin is the deepest one (Rudels, 2009). Figure1 shows map of the Arctic ocean in which the basins and topography are indicated. The Arctic ocean receives water of Atlantic origin through Fram Strait and over the sills in the Barents Sea (Rudels, 2009). Low saline Paciﬁc Water enters through Bering Strait adding heat to the interior. The outﬂow of water occurs mainly through Fram Strait and also through channels in the Canadian Arctic Archipelago.

General importance

Even if the Arctic Ocean is small, it has a quite large impact on the overall climate and ocean circulations. The thermohaline circulation, a large-scale ocean circulation, is driven by global density gradients created by surface heat fluxes and freshwater fluxes. In low latitudes heating at the surface takes place, primarily by solar radiation. Warming at the surface is not sufficient enough to cause a circulation, because a further rising of the water masses is not possible. In higher latitudes heat from the ocean is lost to the colder atmosphere. The heat loss leads to a cooling of the water and thus the density is increased. The increase in density can be directly, due to cooling at the surface, or indirectly by ice formation. Salt is rejected during the formation of sea ice and increases the density. These dense water masses might be sufficient enough to start a circulation by sinking into deeper parts. Due to the continuity of volume, the sinking water displaces deeper water and a movement is started (Pickard and Emery, 1990). Changes in the formation of dense water in the Arctic Ocean thus might have an influence on this large-scale ocean circulation.

Sea ice plays an important role in terms of feedback processes. Ice has a high albedo, reﬂecting solar radiation back into the atmosphere. In contrast, an ice free ocean is absorbing large parts of the radiation resulting in an additional warming which will in turn cause more melting of the ice (Rahmstorf, 2006). A decreased sea ice extent will most likely change properties of the climate system.

Atlantiﬁcation

Density differences are responsible for stratification in the ocean, since high gradients hinder mixing of water masses. The density in the oceans mainly depends on salinity and temperature. In cold water, as in the Arctic Ocean, the density is mainly controlled by the salt content. In the Arctic Ocean the surface water is less saline than the underlying water masses. A large amount of freshwater is transported into the Arctic Ocean through river runoff (Garrison and Ellis, 2016) and additionally freshwater is added to the surface during the sea ice melt. The halocline, a layer with a strong vertical salinity gradient, is important for the stratification

2 1. INTRODUCTION

Figure 1: Map of the Arctic Ocean using the International Bathymetric Chart of the Arctic Ocean (IBCAO, Jakobsson et al.(2012)). The two main basins, Canadian and Eurasian Basin, are separated by the Lomonosov Ridge. The Gakkel Ridge divides the Eurasian Basin into the Amundsen and Nansen Basin. The main basins are labelled in yellow, the sub basins in white. The ridges are in green, the in and out ﬂows straits in white. Landmasses are labelled in black. FJL: Franz-Joseph-Land.

3 1. INTRODUCTION

Figure 2: Conceptual model of "atlantification" of the eastern Eurasian Basin continental margin in recent years taken from Polyakov et al.(2017). Different processes are shown in the model. An increased penetration of surface signature of Atlantic Water into the eastern Eurasian basin is visible. A reduction of sea ice cover takes places, leading to greater surface fluxes and an increased depth of winter penetrative convection. Due to a higher convection, more heat and nutrients are brought up from the Atlantic Water into the Arctic surface waters. The red vertical red arrows indicate upward heat fluxes, the horizontal red arrows show inflows. WC: Winter convection, SML: Surface mixed layer, CHL: cold halocline layer, UPP: Up- per permanent pycnocline, AW: Atlantic Water. More details on this figure can be found in (Polyakov et al., 2017) in the Arctic Ocean. It limits the heat exchange between the surface water and the warm Atlantic Water (Jones, 2001). Changes in stratification could influence the heat budget, as well as the interaction between warm water masses and the sea ice cover. Atlantic Water reaching the Nansen Basin through Fram Strait is weakly stratified. Due to the weak stratification, this water is subject to winter ventilation, which is driven by cooling and haline convection (Ivanov et al., 2016). The sea ice thickness is reduced by this ventilation in the western Nasen Basin, since warm water is brought to the surface (Ivanov et al., 2012; Onarheim et al., 2014). The winter ventilation of Atlantic Water was limited to the western Eurasian Basin in the past. In the eastern Eurasian Basin the ventilation is restricted by a stronger stratification. Recent studies observed conditions similar to the one of the western Nansen Basin in the eastern Eurasian Basin (Polyakov et al., 2017, 2018). Polyakov et al. (2017) names this progression of conditions from the western Eurasian Basin to the eastern Eurasian Basin "atlantification". In Figure2 a conceptual model of the atlantification is shown (Polyakov et al., 2017). The penetration depth of the winter convection (WC) increased in the western Eurasian Basin and potentially is still deep enough to reach the upper permanent pycnocline (UPP) in the eastern Eurasian Basin. A larger heat flux from the Atlantic layer to the surface layer and thus a reduction of the sea ice cover is present in the new regime. The permanent cold halocline layer is transformed to a seasonal halocline. The reduction in sea ice, the weakening of the halocline, and the shoaling of the depth of the Atlantic Water have increased the winter ventilation and making the eastern Eurasian Basin structurally similar to the western Eurasian Basin (Polyakov et al., 2017).

4 1. INTRODUCTION

1.2 Water Masses and Circulation

Most changes and variability of temperature and salinity occur in the upper most 1000 m. In the intermediate and deep layer the variability is modest and slow (Korhonen et al., 2013). The upper 1000 m can be separated into different water masses. Several different definitions and names of the water masses exist, especially for the halocline waters (e. g. Kikuchi et al., 2004; Steele et al., 2004). This study utilise the definition of Korhonen et al.(2013), who defines six different water masses in the upper 1000 m. The Polar Mixed Layer is located closest to the surface. Following Rudels et al.(1996), the thickness is defined by the temperature minimum, which is a relict of the last winter convection and homogenization (Korhonen et al., 2013). The layer between the temperature minimum and the 34-isohaline is defined as upper halocline and is quite heterogeneous. It consists of different water masses, transported mainly by the Bering Strait inflow branch. In the Canada Basin, water from the Siberian rivers also contributes to the formation of this layer (see Korhonen et al., 2013, and references therein for more details). The Lower Halocline has the 34-isohaline as upper boundary and the zero degree isotherm as lower limit. It generally is formed when the saline surface layer of the Nansen Basin advects further into the basin where it is overlain by low saline shelf water (Rudels et al., 1996). The Atlantic Water is defined by temperatures over zero degree (Korhonen et al., 2013) and is separated by the temperature maximum into upper and lower Atlantic Water. The upper zero degree isotherm is the upper boundary of the Atlantic Water, the deeper zero degree isotherm is taken as the lower boundary (Korhonen et al., 2013). In this study, the focus will be on the the Atlantic Water. To determine its depth, the upper zero degree isotherm is taken as reference for the upper boundary of the Atlantic Water.

Table 1: Definitions of the water masses using temperature (T) and salinity (S) values. The upper and lower limit for the different water layers is specified. Adapted from Korhonen et al. (2013).

Water mass Upper limit Lower limit Polar Mixed Layer p = 0 dbar (water surface) T = Tmin Upper Halocline T = Tmin S = 34 PSU Lower Halocline S = 34 PSU T = 0 ◦C ◦ Upper Atlantic Water T = 0 C T = Tmax ◦ Lower Atlantic Water T = Tmax T = 0 C

General Circulation

The circulation of Arctic Ocean water masses is largely controlled by the ocean’s bathymetry (Rudels, 2009). In the Canada Basin, the surface layers are mainly dominated by wind forcing.

5 1. INTRODUCTION

The high-pressure cell over the Arctic creates Beaufort Gyre, a large wind-driven ocean circulation containing ice and water (Rudels, 2009). The Transpolar Drift (see Figure3, light blue arrow) brings ice from the offshore side of the gyre towards Fram Strait. Ice is also transported from the Siberian shelves across the Eurasian Basin to Fram Strait, through which 90 % of the ice export takes place (Rudels, 2009). Pacific Water enters the Arctic Ocean through Bering Strait. The Pacific Water enters at intermediate depths below the Surface Mixed Layer and above the Atlantic Water layer. This creates a strong stratification and a preservation of the halocline (Rudels, 2009; Carmack et al., 2015). A heat exchange between the warm Atlantic Water and the surface layer is thus prevented in this region (McLaughlin et al., 2004; Steele et al., 2004). Warm, fresh, and buoyant water is entering from Siberian and North American rivers. The river runoff is a heat source, especially during summer (Carmack et al., 2015). However, the major heat source of the Arctic Ocean is the Atlantic Water entering through Fram Strait and over the Barents Sea (see Figure3). The Atlantic Water is transported northwards with the Norwegian Atlantic Current. This current splits into a part entering the Barents Sea together with the Norwegian Coastal Current, and another part that continues as the West Spitzbergen Current towards Fram Strait (Rudels, 2009). In the Fram Strait, parts of the West Spitzbergen Current recirculate and join the southward- flowing East Greenland Current (Rudels, 2009; Carmack et al., 2015). The remaining part is transporting heat into the interior of the Arctic Ocean. In comparison, the Barents Sea branch only brings little heat to the deep Arctic basin (Carmack et al., 2015). The water cools and freshens rapidly in the western Barents Sea, because it is located at the surface and stays in contact with the cold atmosphere (Lien and Trofimov, 2013). The Barents Sea branch flows into the Eurasian Basin through St. Anna Trough, east of Franz Joseph Land, and then follows the continental slope. The Fram Strait branch is deflected by the inflowing Barents Sea branch into larger depths (Rudels, 2009). North of the Laptev Sea, a mixing of the branches took already place. Temperatures above zero degree indicate the presence of the Fram Strait branch in the slope. The Atlantic Water temperature in the boundary current is reduced, partly due to mixing of the two branches and partly due to heat loss to the upper layers and sea ice (Rudels, 2009). The boundary current circulates around the Canada Basin. It splits at the different ridges and enters the smaller basins. The different circulations (see Figure3) and their interactions with the boundary current generate distinct water masses in the different basins (Rudels, 2009).

Propagation through the basin

With the help of warm water pulses, the propagation of water through the basin can be detected. It is possible to track the way of a warm water pulse from Fram Strait into the interior of the basin. A temperature anomaly originating in the Norwegian Sea took 1.5 years to the Fram Strait region. It was found in 2004 in the Eurasian Basin and needed another 4.5

6 1. INTRODUCTION

Figure 3: Circulation in the Arctic Ocean. The circulation of the surface water is indicated with blue arrows, the intermediate Pacific Water is pink-blue. The Atlantic Water is shown by red arrows. The two branches of the inflowing Atlantic Water are visible as well as the different recirculation patterns in the basin. Schematic is taken from Carmack et al.(2015) to 5 years to reach the Laptev Sea slope (Polyakov et al., 2005). In Polyakov et al.(2010), the authors show that another warm water pulse of Atlantic Water, which entered the Arctic Ocean in the early 1990s, has reached the Canadian Basin in the 2000s. A warm pulse entering in the mid-2000s has passed through the Eurasian Basin and is now on the way further downstream. In 1990, Quadfasel et al.(1991) found a warm water pulse in the Nansen Basin with an Atlantic Water layer warmer by 1 ◦C than the long-term mean, which also could be traced further into the Arctic interior. Changes or warming events occurring in Norwegian Sea thus can influence the water masses and temperature anomalies in the entire Arctic Ocean.

1.3 Motivation and Focus of this Study

This chapter summarises the large importance of the Atlantic Water not only on a global scale, but also within the Arctic Ocean. Atlantic Water entering the Arctic Ocean interacts with the sea ice and the atmosphere. The upward heat ﬂux from the warm Atlantic Water results in sea ice melt (IPCC, 2013) and might thus reduce the sea ice concentration in the Arctic Ocean. Carmack et al.(2015) states that the role of Atlantic Water is not as negligible in terms of sea ice reduction as assumed in the past. Polyakov et al.(2017) shows that a reduction in sea ice,

7 1. INTRODUCTION a weakening of the halocline, and a shoaling of the intermediate-depth Atlantic Water layer in the eastern Eurasian Basin have increased winter ventilation (see Figure2). Winter sea ice formation has reduced due to the associated enhanced release of oceanic heat. Polyakov et al. (2017) analysed data of ﬁve moorings deployed in the eastern Eurasian Basin between 2002 and 2015 to draw the above-mentioned conclusion. This thesis aims to detect decadal changes in the Atlantic Water in the Eurasian Basin. Data covering the time span from 1980 to 2015, and even up to 2018 in the eastern Eurasian Basin, are analysed. The focus will be on changes within the Atlantic Water, mainly concerning the depth of this layer. This depth analysis will be connected to the stability of the overlaying water masses. Diﬀerent regions within the Eurasian Basin are compared to draw conclusions about propagation through the basin and the location of potential changes.

This study therefore covers the following questions:

• Is the Atlantic Water layer shoaling?

• Is the stratiﬁcation becoming weaker in the layers overlaying the Atlantic Water?

• Is it possible to track signals through the basin?

This study is structured in the following way. First, an introduction into the topic with a general description of the Arctic ocean and its properties was given in this chapter (Chapter1). It is followed by Chapter2 "Data and Methods", which explains the used study site, data sets, and methods. The results of this study are presented in Chapter3. Chapter3 is divided into subsections covering the results of the Laptev Sea, as well as the propagation through the basin in search of the origin of changes in the Laptev Sea. The results are discussed in Chapter4. Last, a summary and outlook is given in Chapter5.

8 2. DATA AND METHODS

2 | Data and Methods

In this chapter the study site, the used data sets, and the methods are explained. First, the choice of areas within the Eurasian Basin is given. Afterwards, this chapter focuses on data sets forming the base of this study and on how they were obtained, quality checked and utilised in this analysis. In the end, the methods are explained covering the interpolation method, determination of water mass boundaries, and the stability analysis.

2.1 Study Site

In this study the impact of Atlantic Water on the Arctic Ocean is analysed. Since the Atlantic Water enters through Fram Strait, the Eurasian Basin is strongly influenced by this water mass. Thus, the Eurasian Basin is chosen as study site. Due to a large diversity in this basin, smaller regions are defined to perform the analysis. Four subregions are located along the boundary current. The first region is found in the entrance to the Eurasian Basin, within the Fram Strait. Following the current, the next region can be found north of Svalbard, before the Barents Sea branch enters through St. Anna Trough. The third region is located north of Franz Joseph Land. The largest region is north of the Laptev Sea. In the following, the four regions are named: Fram Strait, Svalbard, Franz Joseph Land (FJL), and Laptev Sea. In Figure4 the location of the individual regions together with the available profiles (see also Table3) is shown. In Table2 the exact boundaries of the regions are defined.

Table 2: Deﬁnitions of the regions by latitude and longitude. Lat- itude (Lat) and Longitude (Lon) boundaries are given in degree North and degree East, respectively for each region.

Region Lat Min Lat Max Lon Min Lon Max Fram Strait 78 80 4 7 Svalbard 81 84 10 25 Franz Joseph Land (FJL) 82 85 45 62 Laptev Sea 77 82 110 140

Fram Strait

Fram Strait as entrance to the Arctic Ocean is providing information about signals entering the Eurasian Basin. This region is located close to the shelf edge west of Spitzbergen. Here, the West Spitzbergen Current transports Atlantic Water into the Arctic. The region only has a small meridional width to exclude recirculating water. These water masses have diﬀerent characteristics due to cooling, freshening, and mixing processes taking place in the Arctic Ocean interior than the water masses entering the ocean.

9 2. DATA AND METHODS

Svalbard

This region is a small area north of Svalbard. It captures the area east of the Yermak Plateau as far as 25◦ East. Even without directly including the shelf area of the Yermak Plateau, water flowing over this shallow area is present in this region. It shows different characteristics than water entering through deeper sections. Water flowing over shallow shelf regions is exposed strongly to cooling and heat fluxes towards the atmosphere. In the region north of Svalbard water masses with different origins and different states of cooling are present, making analysis related to decadal changes difficult.

Franz Joseph Land

The Franz Joseph Land is located west of the St. Anna Trough and thus before the inﬂow of waters from the Barents Sea takes place. To exclude water masses modiﬁed by shelf processes in the Barents Sea this region is limited to 62◦ East.

Laptev Sea

The Laptev Sea region is the region most upstream in this study. It is the largest one of the four regions, covering deep parts north of the Laptev Sea up to 82◦ North. This region has the best data coverage of all sub regions, including data from 2018. Due to this fact, this region is in the focus of the analysis in this study. In this region, a mixing of the two branches, Barents Sea and Fram Strait branch, already has happened, so that the water mass is quite homogeneous.

2.2 Data Sets

This study is based on oceanographic data. The analysis is mainly based on conductivity- temperature-depth (CTD) data out of the Uniﬁed Database for Arctic and Subarctic Hydrog- raphy (UDASH) (Behrendt et al., 2017). UDASH combines all publicly available data at the time the data set was created (2017). The data set covers the entire Arctic Ocean, including Fram Strait from 1980 to 2015. For this study, a sub data set of the Eurasian Basin is created, afterwards data sets of the single regions (see Figure4 and Chapter 2.1) are built. To gain a better data coverage, data of the latest NABOS cruise are added (not published yet, but free to use). These measurements were taken in the Laptev Sea in 2018.

10 2. DATA AND METHODS

Figure 4: Map of subregions in the Eurasian Basin. The regions used for the analysis are framed by black boxes. Grey dots symbolise all available profiles in the different regions, the yearly means are indicated by coloured dots with different colour schemes for each decade. More details on the region are shown in Table3 and 2. FJL: Franz Joseph Land

UDASH

The Unified Database for Arctic and Subarctic Hydrography (UDASH) by Behrendt et al. (2017) combines all publicly available oceanographic data in the Arctic Mediterraniean north of 65◦ North in the period of 1980 to 2015. In total there are 288 532 oceanographic profiles with approximately 74 million individual measurements of temperature and salinity. UDASH inclueds a thoroughly quality-check e. g. through duplication checks, positions checks, and gradient checks. The uncertainties of the different instrument types are also taken into account. For further details on the qualitiy check see Behrendt et al.(2017) (e. g. chapter 3, Figure 9).

11 2. DATA AND METHODS

The oceanographic profiles were mainly measured with conductivity-temperature-depth (CTD) probes, bottels, and mechanical and expendable thermographs. More than half of the measurements were taken by CTD probes (56.1 %, Behrendt et al.(2017)). In recent years, ice-tethered profilers (ITPs) became more important, since they are able to take measurements in the central Arctic. To this date, ships are the most used platform for data aquisition, but the amount of ITPs is increasing. The data coverage has large differences between the seasons. During the summer months, more data are available since ship measurements are easier to perform during this time. The data coverage during seasons with sea ice concentration is increasing as the ITPs provide year- round measurements. For details on the seasonal data distribution see Figure 13 in Behrendt et al.(2017). More details on depth distribution and further information on the instruments and platforms can be found in Behrendt et al.(2017) (e. g. Figure 15: depth distribution, Table 1: data sources, Table 4 and 5: instruments and platforms).

Out of the complete UDASH data set a sub data set is created for the Eurasian Basin. All available platforms (see Table 5 in Behrendt et al.(2017)) were used, with the chosen instrument types CTD and Bottle casts. They cover 83.8 % of the full archive (see Table 5 in Behrendt et al.(2017)). As the depth boundary of the Eurasian Basin a bottom depth of 1000 m is chosen, to exclude measurements taken on the shelf regions. Different longitude and latitude boundaries are defined to cover the entire Eurasian Basin. In total 16 955 profiles are available in this basin. For the different study regions, the data set is further divided. In Table3 the number of available profiles for the four regions is shown. It is differentiated between the overall number of profiles, the available amount of years and months, and the number of winter and summer months in the respective region.

NABOS

In 2018 a cruise within the NABOS project (Nansen and Amundsen Basin Observational Sys- tem) took place with the Russian icebreaker R/V Akademik Tryoshnikov. In the region of the Laptev Sea measurements were carried out. Since this region is part of this study, the data of this cruise ﬁtting into the Laptev Sea region of this study are added to the UDASH data set.

Seasonality

Table3 shows the amount of profiles available in the regions. The distribution between summer and winter months is shown as well. For the summer months, all data of August, September, and October are taken. Winter months are defined from January to the end of March. The year-round data coverage is too thin to make statements on a seasonal bias. A comparison between winter and summer data to define a bias is not possible, because too less data are available during the winter months.

12 2. DATA AND METHODS

Table 3: Available profiles in the different regions. The total number of profiles is shown, as well as the number of years and months in each region. The number of summer months (August, September, October) and winter months (Jan- uary, February, March) is shown, respectively for each region.

Region No. of proﬁles No. of years No. of months Summer months Winter months Fram Strait 1 266 36 133 60 12 Svalbard 222 17 29 16 2 Franz Joseph Land (FJL) 230 7 14 9 2 Laptev Sea 1 482 21 49 33 5

To get results which are not inﬂuenced by a potential seasonal oﬀset, just the summer data are used in this study. Monthly or yearly means are calculated including only the measurements of August, September and October.

2.3 Methods

In this part of the chapter the methods are explained. It shows how the data are processed to get the format used in the analysis. This chapter describes the interpolation method used and explains the determination of water mass boundaries. At the end, the method of the stability analysis is explained.

2.3.1 Interpolation

The data used in this study are taken by different platforms and instruments, having different depth resolutions. Especially, the bottle data from early years have a low resolution. These data have a maximal depths of about 300 m. Within this 300 m, measurements were taken in approximately six different depths (10 m, 40 m, 70 m, 120 m, 200 m, 300 m). One part of this work is the identification of the upper boundary of the Atlantic Water. Using the original measurements of the bottle-sampled data, would give higher depths than the actual location of the Atlantic Water layer defined by the upper zero degree isotherm. Detecting the depth by determining the temperature being higher than zero degree for the first time, would give deeper depth than the actual location of the zero degree isotherm. To avoid this effect, especially when comparing to highly resolved profiles of recent years, the data are interpolated.

Different methods of interpolating are tested: linear interpolation, nearest neighbour interpolation, spline interpolation, and Reiniger-Ross interpolation (special interpolation with application to oceanography Reiniger and Ross(1968)). To draw conclusions of the best fitting method, all available profiles are reduced to six measurements in the upper 300 m, similar to the early bottle-profiles. The different interpolation methods are applied to these, here named, short profiles. In Figure5 the results of the different interpolation methods are shown. The left panel shows a profile from 1988, which just has six measurement points (red dots). The

13 2. DATA AND METHODS right panel shows a profile from 2007 with a high vertical resolution. The red dots are the points between which the interpolations are carried out, while the dotted black line shows the high resolved (original) profile. The different interpolation methods are drawn into the plot. The nearest neighbour interpolation is printed in light brown, the spline interpolation in blue, the Reiniger-Ross method in cyan, and the linear interpolation in green.

Figure 5: Different interpolation methods shown for two profiles. The left panel shows a profile from 1988, the right panel a profile from 2007. The red dots are the measurement points between which the different interpolations are applied. The dotted black line shows the (high resolved) original profile. The different interpolation methods are drawn into the plot: the nearest neighbour interpolation (NN) in light brown, the spline interpolation (Spline) in blue, the Reiniger-Ross method (RR) in cyan, and the linear interpolation (Linear) in green.

Since an important depth used in this study is the location upper boundary of the Atlantic Water, it is used as comparison parameter of the different interpolation methods. The depth of the zero degree isotherm is calculated for all interpolation methods and afterwards compared to the original depth of high resolved profiles. This study defines profiles as highly resolved when they have a maximal difference of 1.5 m between two measurement points. The mean of the difference between the interpolated depth and the original depth is taken over all highly resolved profiles. Subsequently, the standard deviation was calculated (see Figure6). The method of the nearest neighbour interpolation gives a poor fitting, while the result of the other three methods are quite similar. The Reiniger-Ross method has slightly higher values in the difference of the mean depths and in the standard deviation. The spline interpolation seems to give too much curvature to the profiles. This results in the conclusion that for this study the linear interpolation method is the most suitable. A difference between the linear

14 2. DATA AND METHODS interpolated depth and the original depth of the upper boundary of the Atlantic Water of about 4 m is found. This is passable since inter annual variations are in a similar range. Thus this diﬀerence will not inﬂuence the results in terms of decadal changes. Linearly interpolated data are therefore used for all following results. The step width for the interpolation is set to 1 m. Mostly, the monthly or yearly means of the summer month (August, September, and October) are calculated.

Figure 6: Results of comparison between interpolation methods. The right side presents the diﬀerence of the mean depth of the zero degree isotherm between the interpolated and highly resolved proﬁles (maximal 1.5 m between measurement points). The left side shows the corresponding standard deviation (Std). The colour code is the same as in Figure5: nearest neighbour interpolation (NN) in light brown, spline interpolation (Spline) in blue, Reiniger-Ross method (RR) in cyan, and linear interpolation (Linear) in green.

2.3.2 Determination of Water Mass Boundaries

To determine the upper boundary of the Atlantic Water layer, the upper zero degree isotherm is taken. In the temperature profiles, this is the point the temperature is equal (or higher) than zero degree for the first time. Since the interest is on the Atlantic Water, the zero degree isotherm is searched below 40 m to avoid the Near Surface Temperature Maximum (NSTM). The boundary is set to 40 m as suggested e. g. by Bourgain and Gascard(2012). The NSTM can be present in the upper part of the water column due to seasonal solar heating, having a typical depth of 20 to 30 m (Jackson et al., 2010). In Figure7 profiles from the Laptev Sea region are shown for the years 1995, 2002, 2011, and 2018. The NSTM is pronounced in

15 2. DATA AND METHODS most of the years and is above the 40 m boundary in all cases. The zero degree isotherm is indicated in the profiles, marking the upper boundary of the Atlantic Water layer. Later, additional water masses are needed in this study. To determine the lower limit of the Polar Mixed Layer (see Table1), the temperature minimum is necessary. Therefore, the depth of the minimal temperature for each profile is determined. Similar to the temperature profiles, the determination of water mass boundaries is done using the salinity profiles. To find the boundary between the Upper and Lower Halocline, the depth of the 34-isohaline is identified similar to the determination of the zero degree isotherm.

Figure 7: Determination of water mass boundaries. Yearly mean profiles of the years 1995 (green), 2002 (blue), 2011 (pink), 2018 (purple) in the region north of the Laptev Sea. The 40 m line is indicated as dotted black line, to mark the boundary of typical depths of the Near Surface Temperature Maximum (NSTM). The zero degree isotherm is shown by a dotted black line, the upper boundary of the Atlantic Water (AW) is indicated for the profiles by a circle. The temperature minima of the profiles are as well surrounded by a circle.

2.3.3 Stratiﬁcation Analysis

There are different ways to identify the stability and the stratification of a water column. One way is to look at the Brunt-Väisälä frequency. The Brunt-Väisälä frequency (N) is a measurement for layer stratification:

16 2. DATA AND METHODS

g ∂ρ N 2 = − · (1) ρ0 ∂z where g is the acceleration due to gravity, ρ the potential density of seawater, and ρ0 the reference density (1030 kg/m3,(Polyakov et al., 2018)). This frequency is used to create vertical profiles of the stability, giving an overview over the entire water column. To gain informations about the stratification above the Atlantic Water layer, two points in the vertical are defined for comparison. Doing this for the N 2 frequency mirrors changes in the density difference as well as in the layer thickness between the two depth points. In this study, the density difference between the two points was calculated, to obtain information about the stratification in the water column.

The density difference is taken between the upper boundary of the Atlantic Water and (i) the temperature minimum defining the lower limit of the Polar Mixed layer, as well as (ii) the 34-isohaline being the boundary between the Upper and Lower Halocline (see Korhonen et al. (2013) and Table1). Comparison between the two methods do not show strong deviations. The correlation between the density difference and the Brunt-Väisälä frequency is significant as shown in Figure8. The density difference is taken between the upper boundary of the Atlantic Water layer (zero degree isotherm) and the lower boundary of the Polar Water (temperature minimum), the mean of the Brunt-Väisälä frequency is calculated between these two depths. The calculation was done for yearly profiles in the the region north of the Laptev Sea. However, this study will use the density differences to determine stability in the water column above the Atlantic Water layer.

17 2. DATA AND METHODS

Figure 8: Comparison between density difference and Brunt-Väisälä frequency for analysis of the water column stability above the Atlantic Water layer. The density difference is taken between the upper boundary of the Atlantic Water layer (zero degree isotherm) and the lower boundary of the Polar Water (temperature minimum), the mean of the Brunt-Väisälä frequency is calculated between these two depths. The calculation are based on yearly mean profiles in the region north of the Laptev Sea.

18 3. RESULTS

3 | Results

There are different major purposes of this study. The analysis of the current state of the Eurasian Basin is evaluated. As well, the changes over time in the different regions and a potential propagation through the Eurasian Basin are analysed. In total four regions within the Eurasian Basin are defined (see Figure4). This thesis focuses on the region north of the Laptev Sea due to the best data coverage in this area. The smaller regions further downstream are investigated mainly to determine the origin of changed conditions in the Laptev Sea and to track signals along the boundary current. The results of this study are based on summer data (August, September, October) to avoid seasonal bias. As shown in Table3 not enough winter measurements are available to state year around variabilities or determine the seasonal bias. This chapter presents the current state in the Laptev Sea region, with aspects of temperature and salinity, as well the stratification in the upper water column together with the depth of the Atlantic Water layer. Further, the changes are tried to be identified in the regions downstream. For this purpose an analysis of the region north of Franz Joseph Land follows. Briefly, the the region north of Svalbard and the area located in Fram Strait are analysed.

3.1 Laptev Sea

The region located north of the Laptev Sea covers the time period from 1988 to 2018, giving a good overview over the years in the last decades. The water masses are quite homogeneous, since cooling and mixing processes take place while its transport through the basin (see Chap- ter1 for more details). This region is the most upstream region in this study.

3.1.1 Temperature and Salinity

Figure9 gives a good overview over the situation in this region. The colours are used to show the temperature development in respect to depth and through time. The red dotted line indicates the zero degree isotherm as upper boundary of the Atlantic Water. The isolines of the salinity values are shown in white. The density levels are indicated in grey. In the upper 250 m, the upper part of the Atlantic Water and the overlaying water layers are present. Within the Hovmöller diagram, the lower boundary of the Polar Water (temperature minimum), the upper boundary of the Lower Halocline (S = 34 PSU) and the upper boundary of the Atlantic Water (T = 0 ◦C) are indicated by dots (blue edging), squares (green edging), and diamonds (red edging), respectively. These parameters identify the boundaries between the different layers. Later, the depths of the respective boundaries are used to define the stratification of the upper water column (see Chapter 2.3.3 and Figure 12).

19 3. RESULTS imnswt e digmr h et fteuprbudr fteAlni ae T=0 = (T Water Atlantic used. the is of PSU), October) 34 boundary September, = (temperature (August, (S upper Layer values Halocline the Mixed summer Lower of Polar the the of of depth the limit mean 0 (in of the upper the the density mark boundary the year for is lower available edging of isolines line depth the red the red the indicate with PSU), indicate dotted edging diamonds edging (in the blue green salinity developments, with show with temperature squares to symbols the minimum), used show Round are to grey. lines) used density in (white are isohaline and colours The salinity, The isotherm. temperature, Sea. of Laptev the diagram in Hovmöller 9: Figure nteupr20mtruhtime through m 250 upper the in ◦ ) o each For C). kg / m 3 are ) ◦ C

20 3. RESULTS

Temperature

The temperature is varying between −1.8 ◦C and +1.8 ◦C. Variability within the individual years is present, but the temperatures tend to increase in more recent years. In the years from 1988 to 2003, the coldest water mass is present from the surface down to depths between 80 m and 90 m. After 2003, this water mass is not deeper than 80 m, sometimes not even deeper than 60 m. At the same time, warmer water is present close to the surface, reducing the overall thickness of the cold water layer to approximately 40 m. In diﬀerence from the 1980s where it is up to 80 m thick. The warm water masses (warmer as 1 ◦C) seem to rise from deeper parts in the last decade. A similar condition is present in 1993 with a warm water core in a shallower depth than in adjacent years. The zero degree isotherm (indicated by a dotted red line) is the upper boundary of warm Atlantic Water. A strong decrease in the depth of the zero degree isotherm is present in the time between 2002 and 2011. The shallowest depth of the upper Atlantic Water boundary is found in 2018, the most recent year. The temperature minima (dots with blue edging in Figure9) are mostly located in depths above 50 m. The temperature minimum in 2018 lies in a comparatively high depth compared to the 2000s and 2010s. In 1998 it is located below 50 m similar to 2018. A slight increase in depth of the temperature minimum is present from 2004 to 2008 and again from 2012 to 2018. The second time span correlates with higher temperatures at the sea surface.

Salinity

The higher salinities are found together with the higher temperatures in larger depths. Up to 2003 the isohalines are quite horizontal, except in 1993. In 1993, higher salinities are found in shallower depths, leading to peaks in the isohalines. Similar to the temperature values, the year 1993 shows characteristics more typical for recent years. Starting in 2004, the isohalines are rising to shallower depth. A strong upwards trend is visible especially after the year 2012 in salinities between 34 PSU and 34.4 PSU. In 2012, the 34-isohaline, marking the boundary between the Upper and Lower Halocline, is found in a depth of 71 m similar to the early 2000s. Whereas in 2018 it is located in a depth of 34 m, which is the shallowest depth of the isohalines in data evaluated. In recent years, the water shows a strong increase of salinity. Recently, salty water of 34 PSU lies in depths between 30 m to 40 m, while this salt concentration is found in depths between 80 m to 90 m before 2005 (with the exception of 1993). The surface water has low salinities due to freshwater input. Lower saline water is found close to surface in recent years. In 2005 the salinity at the surface has values of 30 PSU, after 2009 salinities of 31 PSU are commonly present. The decreasing depth of high salinity lines in the last decade increases the vertical salinity gradient. The isohalines are tighter in the upper 40 m compared to the ﬁrst decades. In 1988 the 33-isohaline and the 34-isohaline are 40 m apart from each other, whereas it is only 10 m in 2018. This stronger gradient is only present in the upper most part of the water column. The gradient is decreasing in deeper parts of

21 3. RESULTS the water column. The 34-isohaline and the 34.2-isohaline are separated by a water mass of 20 m in 1988 and only by less than 10 m in 2018. This represents a decrease in stratiﬁcation beneath the surface mixed layer and above the Atlantic Water layer.

Density

In cold water, the density mainly depends on salinity rather than on temperature. Due to this property the isopycnals show a development similar to the isohalines. They are quite stable in the earlier decades, and are rising into shallower depths in the last two decades. For example, this can be observed on the 28-isopycnal which is slightly increasing from 2003 on and having a stronger increase in the years after 2012. The isopycnals are getting tighter in depth above 30 m similar to the salinity lines and thus are increasing the density gradient. An increased gradient in density leads to a stronger stratification in the upper most part of the water column. In contrast, the density gradient is decreasing between 50 m and 100 m. In 1998, the density in a depth of 100 m is approximately 28 kg/m3 and in a depth of 50 m the water has a density of 27 kg/m3. This is giving a difference of 1 kg/m3. In 2018, the density in 100 m of 28.2 kg/m3 and the density in 50 m of 27.8 kg/m3 are giving a difference of only 0.4 kg/m3, decreasing the density stratification strongly. Further analysis on the stratification is executed in Chapter 3.1.2.

Figure9 is showing changes in the structural characteristics of the water masses north of the Laptev Sea. The zero degree isotherm is shoaling in recent years. This is accompanied by higher temperatures in shallow depth, as well as increased temperatures at the surface. At the same time, the salinity is increasing. Higher salinities are present in shallower water, especially between 2012 and 2018 a strong increase is present. Subsequently, the density changes as well, having stronger gradients close to the surface and lower gradients in the layers beneath. In Figure9, the boundaries of water layers are indicated as defined by Korhonen et al.(2013). The Polar Water (or Polar Mixed Layer) is the upper most water layer, from the surface to the depth of the temperature minimum. The temperature minima are pointed out by the grey dots with a blue edging in the figure. Below the Polar Mixed Layer the halocline layer is located, which can be divided into the Upper and Lower Halocline, separated by the 34-isohaline (green edged squares). The Atlantic Water is located beneath the halocline. The upper boundary is defined by the zero degree isotherm, shown with diamond shaped symbols and a red edge. The definition of water layers with fixed salinity and temperature is suitable in the most parts of the covered time. In 1993, the boundaries of the Polar Water and the Lower Halocline are located in the same depth, so by definition no Upper Halocline is present. The same applies for the years 2007 and 2008, in which the temperature minimum is slightly above the 34-isohaline. In 2015 and 2018, the 34-isohaline lies clearly above the temperature minimum, with 10 m in 2015 and 24 m in 2018. Therefore, a fixed water mass definition on the basis of salinity and

22 3. RESULTS temperature properties is difficult in the current state of the Eurasian Basin. It does not seem suitable, especially of the (Upper) Halocline water to have a fixed definition.

Figure 10: Temperature, salinity, and density profiles for new and old state. The left panel is showing temperature profiles in the upper 300 m in the Laptev Sea, the middle panel are salinity profiles, and the right panel shows the density profiles for this region. The profiles of the years 2011, 2012, and 2013 are shown in blue as representatives of the well-known situation (here old). The new state, in which the 34-isohaline lies above the temperature minimum and making current water mass definitions difficult, is shown in profiles in green. For this situation, the two most recent available years, 2015 and 2018, are chosen. The zero degree isotherm and the 34-isohaline are indicated by a dotted black line. .

To identify changes in the properties of the two different states, profiles of temperature, salinity, and density are plotted and shown in Figure 10. The blue profiles are from 2011 to 2013. These years are still considered as years, in which the water mass definition is applicable. The green profiles represent the new condition in which the 34-isohaline is located above the temperature minimum and making current water layer classification difficult. The zero degree isotherm and the 34-isohaline are indicated by a dotted black lines in the profiles. The temperatures are slightly higher during the last two years in the upper 30 m and below 100 m. The temperature minima of the profiles are having similar values, nevertheless in the new situation the warming of the water takes place in larger depth. The temperature gradient towards the temperature maximum is thus higher than in previous years. In the salinity profiles a clear difference between the new and the old state is visible. The two recent profiles show a higher salinity than the older ones. As already seen in Figure9, the location of the 34-isohaline is deeper in the well-known state. In the density profiles, a similar situation as

23 3. RESULTS in the salinity profiles is present, since the density strongly depends on salinity in cold water. The density is higher from the surface to depths of 150 m. Afterwards, the densities equalise. The gradient in the density profiles is stronger in the upper most part (until depth of around 30 m) in the recent two years, giving a hint on increased stratification in this part of the water layer. Reaching higher densities already in shallower depths, the vertical density gradient is lower in the following approximately 50 m, indicating a decreased stratification. The stratification of the water layers above the Atlantic Water will be analysed in the following paragraph.

3.1.2 Stratiﬁcation Analysis

As described in Chapter 2.3.3 the stratification analysis is done by calculating the density difference between the upper boundary of the Atlantic Water and (i) the lower limit of the Polar Mixed Layer (temperature minimum), and (ii) the 34-Isohaline, being the boundary between the Upper and Lower Halocline. To see whether a decrease in stratification may lead to an increased possibility in the upwelling of warm Atlantic Water, the density differences are correlated to the depth of the upper boundary of the Atlantic Water layer. High values in the density difference describe a relatively stratified water column. Lower values indicate less stratified water, since the density gradient is lower making mixing processes possible.

Density

To visualise the basis of the stratiﬁcation analysis, the densities in three depths of the Polar, Lower Halocline and Atlantic Water are plotted in Figure 11. In the upper panel (a) the density in the depth of the temperature minimum (lower boundary of Polar Mixed Layer) is shown. In the middle panel (b) the density in the depth the 34-isohaline, being the lower limit of the Lower Halocline layer is pictured. And in the lower panel (c) the density in the depth of the zero degree isotherm as upper boundary of the Atlantic Water layer is displayed. Variability occurs at all depth points through time. The blue, green, and red dots for Polar Mixed Layer, Lower Halocline layer, and Atlantic Water layer, respectively, are showing the individual measurements. The black symbols are the monthly means of the density during the summer months (August, September, October). The variability of the density measurements is strongest in the Polar Water and smallest in the depth of the 34-isohaline (Lower Halocline Water). The density at the lower boundary of the Polar Mixed Layer has a strong variability, both in time and in the monthly measurements. The density varies approximately between 26 kg/m3 and 28 kg/m3. An extremely low density is present in September 2013 with a monthly mean of approximately 26 kg/m3. The highest monthly means are found in the two most recent years (2015, 2018) with values of around 27.8 kg/m3. The variability within in most of the years is high, varying by amounts of more than 2 kg/m3. In 2013, the density varies within two

24 3. RESULTS

Figure 11: Time series of densities in the depth of (a) the Polar Mixed Layer lower boundary (blue), (b) the upper boundary of the Lower Halocline (green), and (c) the upper boundary of the Atlantic Water (red). Black symbols indicate the monthly mean of the summer month (August, September, October), coloured dots show the single measurements. Note the different limits of the density range on the y-axis. month from approximately 27.4 kg/m3 in August to 26 kg/m3 in September. The densities between 1990 and 2002 are more or less constant within a range of 27 kg/m3 to 27.5 kg/m3.A slight increase in density is visible between the years 2003 and 2007. Afterwards, the density stays relatively constant above 27.5 kg/m3 until 2011, with an exception in 2008 where lower densities are present. The density rises to its highest values in recent years, having monthly means close to 28 kg/m3. The boundary of the Polar Water is defined as the temperature minimum. This parameter is difficult to locate, if no pronounced temperature minimum is present. This is the case, if the temperature is cooled in the entire upper water column, so that no warmer surface layer is present. In the area north of the Laptev Sea, the temperature minimum is located in depths around 50 m, and distinct temperature minima are present in most of the profiles. However, since the density variability within this depth is large, a second parameter is chosen to analyse the stratification of the water column.

The monthly mean densities in the depth of the Lower Halocline Water are varying between approximately 27.5 kg/m3 and 27.8 kg/m3. This is a lower overall variation than in the densities in depths of the Polar Water. The lowest values of densities are found in the two most

25 3. RESULTS recent years (2015, 2018), and also in 2009 with a the density is below 27.6 kg/m3. This is contrary to the observations of the densities in the depth of the Polar Water, where the highest densities are present in 2015 and 2018. The densest water in the depth of the Lower Halocline layer is measured in 1990 (September), 1995 (August), and 2012 (August) with values of about 27.8 kg/m3. A period of slightly decreasing densities between 2002 and 2009 is present, again contrary to the findings in the Polar Water. Between 2009 and 2012 the density is increasing again before a decrease in density occurs from 2012 to 2018. The density in the depth of the Lower Halocline layer shows less variability in the density distribution than it is seen in the depth of the lower boundary of the Polar Water. Especially, the variations within the monthly measurements are lower. The 34-isohaline, which defines the boundary between the Upper and Lower Halocline, is normally located deeper in the water column than the temperature minimum (see Figure9). Although this changed during the last two years, the 34-isohaline normaly is less influenced by surface mixing processes.

In the depth of the Atlantic Water layer the variability of density is higher than in the Lower Halocline Water, and weaker than in the Polar Water. The monthly values lie between 28.2 kg/m3 and 28.6 kg/m3. A trend of decreasing density exists in this water layer between 2002 and 2009, as also seen in the Lower Halocline Water and is even more pronounced. The lowest values of salinity are present at the end of this density decline in 2009, the highest values at the beginning of the decline in 2002. The values before 2002 are varying within a range of 0.3 kg/m3 between approximately 28.2 kg/m3 and 28.5 kg/m3.

Polar Water: Stratiﬁcation and Atlantic Water Depth

In Figure 12 the density difference between the density in depths of the temperature minima (lower boundary of the Polar Water) and the density in depths of the zero degree isotherms (upper boundary of the Atlantic Water) is shown. The y-axis on the left is the corresponding axis (blue). The depth of the Atlantic Water layer is plotted against time in the same figure, with the corresponding axis on the right (red). Note the reversed direction of the y-axis with the largest depth at the top. The grey shading is indicating the standard errors of the mean of the depth and density difference, respectively. The density difference and the depth of the Atlantic Water layer show a similar development through time. The density difference in the first two years of the time series (1988, 1990) is relatively high (above 1.4 kg/m3), representing a relatively strong stratified water column. In 1993, the stratification is low with a density difference below 1 kg/m3. The density difference is increasing until it reaches the highest value in 2003 with 1.7 kg/m3. In the following years, the stratification is strongly increasing until it reaches its minimum so far in 2007 (0.8 kg/m3). The tendency of decreased stratification is present up to the last available measurement in 2018. There are peaks of a higher stratification in 2008, 2012, and 2013 within the overall

26 3. RESULTS

Figure 12: Time series of the density difference between Atlantic and Polar Water (blue) and the depth of the upper boundary of the Atlantic Water layer (red). Y-axis on the left is showing the density difference between Atlantic and Polar Water. Note the reverse orientation of the right y-axis (deepest depth at the top) displaying depth in meters. The grey shading indicates the standard error of the mean of the density difference and the depth, respectively. Yearly means of the summer months are taken to create this time series. A significant correlation between the two curves exists. The orange and pink circles are marking the measurements for which the profiles are shown in Figure 13. decreasing period. 2008 and 2012 even show density differences above 1.2 kg/m3 similar to the mid-1995s. The smallest difference in density between the Polar and Atlantic Water is present in the year 2018 with a value of 0.5 kg/m3. The time series of the density difference shows decreasing and increasing stratification periods over time in the water layer above the Atlantic Water: Strong stratification in 1988 and from 2003 to 2004, medium stratification from 1993 to 1995 and from 2006 to 2007. The comparatively weakest stratification is visible in the two most recent years, 2015 and 2018. The stratification based on the density difference between Polar and Atlantic Water shows the development towards a decreasing stratification in the recent decades already seen in Figure9. The analysis supports the idea of weaker stratification due to the seen change in salinity values.

The depth of the upper boundary of the Atlantic Water layer has a similar development as the density diﬀerence. The depth is varying between 122 m and 167 m. It is deepest in 1996 and 2002 and shallowest in 1993, from 2008 to 2011, and in 2018. In contrast to the low unprecedented density diﬀerences in 2015 and 2018, shallow Atlantic Water depths exist earlier (earliest in 1993). The depth in 2015 is close to the depth in 2012 and in 2013, whereas the

27 3. RESULTS stratification is already weaker in 2015 than it is in 2012 and in 2013. The peak of a large depth in 1996 occurs together with a higher stratification than in adjacent years, but is less pronounced than the peak in the Atlantic Water depth. Striking is the peak in the density difference in 2008, which is not present in the Atlantic Water layer depth.

The time series do not show a clearly decreasing stratification or shoaling of the Atlantic Wa- ter layer. They are more dominated by down- and upward movements. There is a significant correlation of 0.7 between the two curves. A shoaling of the Atlantic Water thus correlates to a decreased stratification taking the density difference between Polar and Atlantic Water. A combination of reduced stratification together with a warm Atlantic Water layer in shallower depth, can lead to an increased upward heat flux. As well, it is more likely for warmer water to be brought to the surface by winter convection (see Polyakov et al.(2017)) or other upwelling processes.

Figure 13: Profile of temperature, salinity, and density in stratified and less stratified situations. Stability is defined by the density difference between Polar and Atlantic Water. The left panel is showing temperature profiles, the middle panel salinity profiles, the right panel density profiles. As stronger stratified years (orange) 1988, 2003, and 2004 are chosen, as less stratified years (pink) 1993, 2009, and 2018 are picked. The difference between the two states is clearly visible in all three panels.

To obtain a better understanding of the ongoing changes, profiles of stratified and less stratified years are analysed. In Figure 12 the measurements of the shown profiles are indicated by circles. The orange circles are the stratified states, while the pink circles are the less stratified states. In Figure 13 profiles of the temperature (left), salinity (middle), and density (right)

28 3. RESULTS in both states are plotted. As stratified years 1988, 2003, and 2004 are picked, as examples for a less stratified years 1993, 2009, and 2018 are selected. The two states show distinct differences in temperature, salinity, and density. The temperatures are higher in depth greater than 100 m in the less stratified years. They show a much higher temperature maximum than the less stratified years. The gradient between the temperature minimum and the temperature maximum is higher in years of a stratified water column. Since the density is mainly dependent on salinity in cold water, this stronger gradient only has little impact on the density gradient creation. The zero degree isotherm is reached in a shallower depth in the state of a weaker stratification. The difference in depth is approximately 25 m. Differences occur in the salinity profiles as well. The profiles of stronger stratified years have lower salinity values in the upper 200 m. Deeper than 200 m the salinity values equalise. At the surface the salinities show similar values as well. In the stronger stratified years, the salinity is almost linearly increasing down to 50 m. Thereafter, it increases slower and crosses the 34-isohaline in depths of about 80 m. During the less stratified years, the strong increase in salinity happens in shallower depths. The 34-isohaline is partly reached in a depth of 30 m, and is latest reached in depth of approximately 50 m. The gradients in temperature and salinity between the minimum and maximum are stronger during the less stratified years. Changes are happening faster in shallower depths in these years. In the density profiles it is visible, that the stronger stratified profiles have a lower density at the depth of the temperature minimum (down to approximately 60 m) than the less stratified profiles. In the depth of the zero degree isotherm (120 m to 160 m) the densities are approximately the same. This results in the state seen in Figure 12 already, with higher density differences during the stronger stratified years. The density profiles show characteristics similar to the salinity profiles, since density mainly depends on salinity in cold water.

As seen, the density profiles show distinct differences in the depth regions of the temperature minima between the strongly and weakly stratified state. Thus the stratification of the water layer is mostly driven by changes in the Polar Water layer. The temperatures are quite similar in these depths, whereas the salinity differs explicitly. Accordingly chages are probably driven by changes in salinity near the surface.

The analysis of potential forces on the Polar Water is only covered very briefly since it is not the main purpose of this study. Anomalies of the densities against the overall density mean are calculated for the Polar Water and the Atlantic Water (see Figure 25 in Appendix). These anomalies were then compared to anomalies of potential forcing. Potential forcing are the anomalies of the Arctic Oscillation and of the sea ice concentration in this area. El Niño events are taken into account as well. At first glance, none of these parameters allow conclusions of a correlation to the density anomalies. Future and detailed investigations are needed to find potential forcing creating a shift in the density regime.

29 3. RESULTS

Lower Halocline Water: Stability and Atlantic Water Depth

The boundary between the Upper and Lower Halocline, the 34-isohaline, is taken as a second depth for the density difference calculation. It is chosen since the variability in density is smaller than it is in the Polar Water. In addition, it is located in a deeper water layer and is present in regions with no distinct temperature minimum. The overall density difference is smaller than the one between the Polar and Atlantic Water. It varies between 0.6 kg/m3 and 0.9 kg/m3 because Lower Halocline Water and the Atlantic Water are adjacent water masses. The development of the density difference and thus the stratification is characterized by rapid changes of high and low values between the individual years. This is strongly pronounced for example in the years from 1995 to 2002. The density difference in 1995 and 1998 is approximately 0.6 kg/m3, whereas it is 0.8 kg/m3 in 1996, and 0.85 kg/m3 in 2002. Between 1998 and 2002 no measurements are available in this region and thus no statement can be done on the development between these years. In 1988 the density difference has a value of about 0.7 kg/m3, slightly decreasing until 1993. It reaches 0.6 kg/m3 in 1995, which is the lowest value in the time series. Followed by the described high value of 0.8 kg/m3 in 1996, and the low value in 1998 at approximately the same level as in 1995. 2002 has a density difference of 0.85 kg/m3, one of the highest values of the time period. The stratification is decreasing to a value of 0.69 kg/m3 until 2005. This decrease is followed by a jump up to 0.85 kg/m3 in 2006, which is again followed by a decrease to 0.62 kg/m3 in 2008. A strong increase in stratification is visible from 2012 to 2013 by 0.15 kg/m3 within one year. The highest values are reached in 2014 with a density difference of 0.87 kg/m3. The last available measurement of the time series in 2018 has a slightly lower value of 0.84 kg/m3.

The depths of the Atlantic Water layer partly show similar developments as the density difference between the Atlantic Water and the Lower Halocline water. Especially in the years from 1995 to 2008 the two curves are showing similar characteristics. This is proven by a significant (p value = 0.011) correlation coefficient of 0.76 in this 14 years. Before 1995, the development of the two curves is roughly the same, but differs in the extent of peaks. After 2008, there no longer is a common development of both curves. No (significant) correlation between the density difference and the depth of the Atlantic Water is present, in parts, it is even anti-correlated. The last three years of the time series show a comparatively strong stratified water layer combined with relatively shallow depth of the Atlantic Water layer, what in this extent is unprecedented. Relating the observed changes in Figure 14 to the Hovmöller diagram in Figure9, similarities can be found. In years with a comparatively shallow 34-isohaline, the density difference is increasing. This is especially pronounced in 2015 and 2018. In these years, the 34-isohaline lies even above the temperature minimum and thus in quite shallow depths. Due to increased spatial difference of the two points taken for the calculation of the density difference, the density differences are also increasing. The increased stratification in Figure 14 in the last three

30 3. RESULTS

Figure 14: Time series of the density difference between Atlantic Water and Lower Halocline (green) and the depth of the upper boundary of the Atlantic Water layer (red). Y-axis on the left is showing the density difference between Atlantic and Lower Halocline Water. Note the reverse orientation of the right y-axis (deepest depth on top) showing depth in meters. The grey shading indicates the standard error of the mean of the density difference and the depth, respectively. Yearly means of the summer months are taken to create this time series. No significant correlation between the two curves exists for the entire time period. years is mainly due to the changed depth of the 34-isohaline and thus mirrors more a change in the thickness of the layer rather then a change in stratification. The strong variability in depth, especially during the last decade, may indicate, that the density difference between the Atlantic Water and the Lower Halocline water is not a suitable to define stratification without taking into account the layer thickness.

Temperature Proﬁles

The temperature profiles in the region north of the Laptev Sea show different characteristics in different years. Changes do not start at a certain time, rather certain regimes are present at different points in time. The profiles of the stratified and less stratified years in Figure 13 show, that similar conditions are present in the years 1988, 2003, and 2004 (orange profiles). It is true for the years 1993 and 2009 as well. Even if the year 2018 is showing a slightly different behaviour, the same characteristics are still present. The characteristic of recent years already occurred in previous years. To visualise the overall development of the temperature over the

31 3. RESULTS covered time period, Figure 15 is displaying the yearly mean temperature proﬁles. No clear trend towards warmer temperatures over time is visible.

Figure 15: Temperature profiles in the Laptev Sea for the available years. The temperatures are yearly means of the three summer months August, September and October. The colours are defining the different decades of the available time series. Yellowish colours mark the first decade from 1980 to 1989, greenish mark the decade from 1990 to 1999, blueish the decade from 2000 to 2009, and pinkish colours mark the last decade starting in 2010.

Depending on the depth, there are differences between the profiles. For example there is a stronger developed Near Surface Temperature Maximum (NSTM) present at recent years (pinkish colours), while the profiles of the 90s (green colours) almost do not show a NSTM at all. The temperature minimum has the highest temperature in the most recent year (2018), the years from 2002 to 2009 show slight colder temperatures in shallower depth, compared to profiles from 2011 to 2018. The temperature minimum is located deeper in the 90s, showing similar temperatures to those in the 2000s. The temperature maximum is varying strongly between the different profiles. The most recent profiles do not show the highest temperature maximum. They are located within the temperature range. Temperature maxima of the decade from 2000 to 2009 show a wide temperature range. The maximum temperatures in this decade vary between approximately 1 ◦C (which is the lowest temperature maximum) and approximately 1.8 ◦C (which is the second highest temperature maximum). The spread of the temperature maximum in the most recent decade, is much smaller. It includes values between approximately 1.3 ◦C to 1.5 ◦C. Although the most recent year (2018) shows the highest temperature minimum it does not show the highest temperature maximum as well.

32 3. RESULTS

3.2 Origin for Changes and Propagation through the Basin

To determine in which parts of the Eurasian Basin the changes occur and if they are already present further downstream, the results of the regions north of Franz Joseph Land, Svalbard, and within Fram Strait are presented in the following chapter. The focus is on the region north of Franz Joseph Land, located closest to the Laptev Sea region.

3.2.1 Franz Joseph Land

Franz Joseph Land is located further downstream along the boundary in the Eurasian Basin than the Laptev Sea region. It is located before the inﬂow of the Barents Sea branch through St. Anna Trough (see Chapter 1.2 for more details). The temporal data coverage is quite low in this region. Data only exist in 1989 and from 2006 to 2012. In this study only the summer months (August, September, October) are taken into account. The data from 1989 were measured by ice tethered platforms in winter and thus are not included in this study. This leads to a time series starting in 2006 with data only available from the two most recent decades. Thus, making qualitative statements about long term trends or decadal variations is diﬃcult. Nevertheless, it might give hints on whether changes observed in the Laptev Sea region have already been present in a further downstream region.

Figure 16: Temperature and salinity profiles north of Franz Joseph Land for the available years. The profiles are yearly means of the three summer months August, September and October. The colours are marking the different decades of the available time series. Blueish for 2000 to 2009, and pinkish for the last decade starting in 2010. The left panel is showing the temperature of the upper 250 m in degree Celsius. The right panel shows the salinity profiles of the upper 250 m in PSU. The zero degree isotherm and the 34-Isohaline are indicated, respectively.

33 3. RESULTS

Temperature and Salinity

The temperature profiles in the region north of Franz Joseph Land are shown in the left panel of Figure 16. Since the temporal data coverage is low, only the two decades from 2000 to 2009 (blue) and from 2010 to 2015 (purple) are available. The temperature is higher in available data of the first decade (2006, 2007, 2009). Especially the year 2006 is much warmer than in the remaining years. This warmer characteristic is present up to a depth of at least 400 m (end of data set). The Near Surface Temperature Maximum is not pronounced in this region during the covered time period, indicating a well-mixed surface layer. The surface temperatures thus are close to the freezing point. The temperature minima are located in different depth, deepest and coolest in the most recent year of 2012. The upper boundary of the Atlantic Water layer (zero degree isotherm included in Figure 16 as dotted line in the left panel) is deeper in the recent years than it is in the previous decade. The deepest Atlantic Water layer is found in 2011 with a depth of 97 m. In 2007, it is located in a depth of 64 m. The temperature maxima of the second decade are in deeper depths, consistent to the deeper boundary of the Atlantic Water. The temperatures equal in depth deeper than 250 m, with an exception in 2006. 2006 clearly shows higher temperatures in depths below 80 m. The temperatures reach values of about 2 ◦C in the depth of 400 m. Between the decades the salinity profiles do not show as much variations as the temperature profiles. The strongest variability occurs in depths between 50 m and 150 m. In these depths, there is a slight separation between the two decades. Especially the years 2006 and 2009 have higher salinity values than the later years. The lowest salinity is measured in 2011. The difference in salinity between 2009 and 2011 in a depth of 100 m is 0.23 PSU. The surface salinities are lower in the recent decade. In 2011 and 2012 the surface salinities reach values of approximately 32.6 PSU, while they are higher in 2006, 2007, and 2009 with values above 33 PSU, reaching up to 33.4 PSU in 2006. The salinity of all available years is approaching 35 PSU in depth deeper than 300 m and staying constant thereafter over the analysed depths.

A Hovmöller diagram is created to visualise the temperature and salinity situation shown in the profiles in Figure 16 in a different way. In Figure 17 the temperature is displayed through the background colours. The decreasing depth of the temperature maximum as well as minimum (dots, blue edging) are clearly visible. The cold temperatures (of about -2◦C) are present at the top of the water column. In 2006, this cold water mass reaches down to a depth of 15 m. In 2012, it is present in a depth of 70 m. The warmer water masses with temperatures above zero degree are following the development of the cold water masses at the surface. The zero degree isotherm has its highest location in 2007 with a depth of 67 m. The depth of the Atlantic Water boundary is increasing to its deepest depth of 99 m in 2011. The salinity is indicated by white isolines. As already seen in the profiles (Figure 16), the salinity distribution has no clear temporal development. At the top of the water column, less salty water is appearing in the year 2010, forming a layer with salinities of 33 PSU. The 34-

34 3. RESULTS isohaline is staying relatively constant at a depth of 30 m. The 34.2-isohaline is decreasing in depth in the last two years from 46 m to 32 m. In contrast, the 34.4-isohaline is deepening. In 2006 and 2007 it is located in a depth above 60 m and then slightly decreasing in the following years. Between 2010 and 2011 the deepening increases and the 34.4-isohaline is found in depths of 80 m in 2011 and 2012. Due to this contrary movement of adjacent salinity lines the stratiﬁcation decreases. The isopycnals (grey lines) indicate this as well, since the distance between the 27.8-isopycnal and 28-isopycnal is increasing, at least in depth between 30 m and 80 m. In Figure 17, the boundaries of the Polar Water (round dots), Lower Halocline (squared symbols), and Atlantic Water (diamonds) are indicated. The dots for the temperature minima are either near the surface (2006, 2007, 2010) or deeper than the 34-isohaline (2009, 2011, 2012). The deepest temperature minimum is found in a depth of 56 m in 2012. The upper boundary of the Lower Halocline water is staying quite stable in depth over time. The depth of the Atlantic Water layer is increasing with time.

Figure 17: Hovmöller diagram of temperature, salinity and density over time in the upper 150 m north of Franz Joseph Land. The colours are used to show the temperature developments, the dotted red line is the 0 ◦C isotherm. The isolines (white lines) are used to show salinity (in PSU), grey lines show the density (in kg/m3). Round symbols with blue edging indicate the lower boundary of the Polar Mixed Layer (temperature minimum), squares with green edging indicate the depth of the upper limit of the Lower Halocline (S = 34 PSU), and diamonds with red edging mark the depth of the upper boundary of the Atlantic Water (T = 0 ◦C). For each available year the mean of the summer months (August, September, October) is used.

35 3. RESULTS

Compared to the region in the Laptev Sea, the Franz Joseph Land region is mostly showing different characteristics in salinity and temperature. To draw clear conclusions is difficult, because of the low temporal coverage in this region. Especially the development of the upper boundary of the Atlantic Water layer is oppositional to the one observed in the Laptev Sea. While the Atlantic Water layer is shoaling in the Laptev Sea in recent years, it is deepening in Franz Joseph Land. Same occurs for the 34.4-isohaline. In the Laptev Sea region it is shoaling starting in 2004, while it is deepening in Franz Joseph Land, especially after 2010. The 34-isohaline is staying relatively constant in Franz Joseph Land, in the Laptev Sea it is showing a clearly decreasing depth. The 34.2-isohaline is strongly shoaling from 2012 to 2018 in the Laptev Sea. In Franz Joseph Land it is shoaling from 2011 to 2012, where the time series ends. The deepening of the temperature minimum is present in both regions. The increased thickness of the cold water mass at the surface is only seen in Franz Joseph Land. In Laptev Sea, warmer water is found at the surface from 2006 onwards. The increased distance between the isopycnals and thus a decrease in stratification occurs in both regions. Some of the strong changes in the Laptev Sea are also found in the region north of Franz Joseph Land. Other occurrences have opposing developments. Due to the short time period covered in Franz Joseph Land, it is hard to say which of the observed signals might be transported further east into the Laptev Sea and are seen there at a later time.

Polar Water: Stratiﬁcation and Atlantic Water Depth

Figure 18 is showing the time series of the region north of Franz Joseph Land. In blue the density difference between the Polar Water layer (temperature minimum) and the Atlantic Water layer (zero degree isotherm) is shown. The red line, with corresponding y-axis on the right, is the depth of the Atlantic Water layer. The development of the density difference between Polar and Atlantic Water shows some variability. It starts with a slight increase in stratification between the first two years (2006 to 2007). The density difference decreases towards the next measurement in 2009. In 2009 it is on the same level as in 2011 and 2012 (around 0.3 kg/m3). The highest value occurs in 2010 with a difference in density of 1.2 kg/m3, which is by far the highest. The overall tendency is towards a decreasing stratification over the six years in this region, with a strong outlier in 2010. The development of the Atlantic Water layer depth over time is not strictly uniform. The overall tendency is towards a deeper location of the Atlantic Water. The shallowest depth of 69 m is found in 2006 and the deepest in 2011 with a depth of 96 m. The standard error of the mean in the first two years (2006, 2007) is quite large, mirroring the strong variability of the data set. The Atlantic Water boundary of 2007 and 2009 is in approximately the same depth (about 77 m). An increase in depth is happening in the following year, reaching a depth

36 3. RESULTS

Figure 18: Time series of the density difference between Atlantic and Polar Water (blue) and the depth of the upper boundary of the Atlantic Water layer (red). Y-axis on the left is showing the density difference between Atlantic and Polar Water. Note the reverse orientation of the right y-axis (deepest depth at the top) showing depth in meters. The grey shading indicates the standard error of the mean of the density difference and the depth, respectively. Yearly means of the summer months measurements are taken to create this time series for the region North of Franz Joseph Land. No correlation between the two curves is found. of 93 m. The highest depth is found the year after, in 2011. The Atlantic Water layer slightly shoals again in 2012, the last available year, to approximately 90 m. There is no correlation between the Atlantic Water depth and the density difference looking at the entire time series. Especially in the last two years the Atlantic Water layer is deep while the stratification is only weakly pronounced. Excluding the last two years, the correlation coefficient is 0.74, but still not significant (p value of 0.25). The last two years of the time period show an opposing development, which is closer to a negative correlation. A too short time period is covered in this region, to state whether a change in the system took place over the last decades or whether it is a short-term variability.

The density difference and the depth of the Atlantic Water layer vary strongly within the three months of summer observations. Taking the yearly mean of theses values does not show this variability. The year 2010 is showing a high variability between the three summer months in the density difference and in the Atlantic Water layer depth. In August 2010, the density difference is low (0.37 kg/m3), whereas the Atlantic Water boundary is in a depth of 122 m. In contrast, the boundary of the Atlantic Water in September 2010 is in a shallow depth of

37 3. RESULTS

73 m, while the density difference is high with 1.85 kg/m3. The measurement in October do not show such a strong contrast between Atlantic Water layer depth and density difference. Taking the yearly mean out of these different months gives a relatively deep Atlantic Water boundary (93 m), combined with a high density difference (1.17 kg/m3, highest in this time series). This is not mirroring the conditions in all three months. This difficulty only occurs in this pronounced way in 2011. In 2012, there are measurements taken in August and September, showing approximately the same values. In all other years, only measurements in one month (mostly August) have been taken, so that the yearly and monthly mean is the same.

Lower Halocline Water: Stratiﬁcation and Atlantic Water Depth

In Figure 19 the depth of the Atlantic Water layer is plotted together with the density difference between the Lower Halocline water and the Atlantic Water against time. As described already in the previous paragraph, the uncertainty of the Atlantic Water depth is quite large in the first two years, especially in 2007. The increase in Atlantic Water layer depth is not constant. In 2009 the depth is slightly decreasing, in 2012 the depth is shallower than before. The density difference between the upper boundary of the Lower Halocline water (34-isohaline) and the upper Atlantic Water boundary is increasing with time. The strongest increase is found from 2006 to 2007 by 0.1 kg/m3 within one year. From 2009 to 2010 the density difference stagnates at a value of 0.6 kg/m3. In contrast, the Atlantic Water layer depth is increasing by 16 m in these two years. In the two most recent years (2011, 2012) the density difference is constantly high at 0.65 kg/m3, while the Atlantic Water layer depth is decreasing. There is a significant correlation (correlation coefficient = 0.84, corresponding p value = 0.04) between the curves. Although they show slight differences in the development. An increase in the depth of the Atlantic Water layer correlates with an increase in layer stratification above the Atlantic Water layer. In theory, a mixing of Atlantic Water will be less likely, because the higher stratification of the overlaying water mass will hinder this process. The Atlantic Water is deeper in the recent years, making it harder for processes as the winter convection to reach this water mass. The variability within the different summer months in the year 2010 is also high with respect to the Lower Halocline water. In this case there is a difference between the August value and the following two months, the density difference and the Atlantic Water depth are having the same characteristics. The density difference between Lower Halocline water and Atlantic Water is high in August (0.74 kg/m3) and medium in September and October (around 0.52 kg/m3). The depth of Atlantic Water is deep in August and medium (to low) in September and October. Thus taking the yearly mean does not make a large difference in this case. The correlation coefficient is significant in both cases and the correlation coefficient almost stays the same for the yearly and monthly mean (0.84 to 0.83).

38 3. RESULTS

Figure 19: Time series of the density difference between Atlantic and Polar Water (blue) and the depth of the upper boundary of the Atlantic Water layer (red). Y-axis on the left is showing the density difference between Atlantic and Polar Water. Note the reverse orientation of the right y-axis (deepest depth at the top) showing depth in meters. The grey shading indicates the standard error of the mean of the density difference and the depth, respectively. Yearly means of the summer months measurements are taken to create this time series for the region North of Franz Joseph Land. No correlation between the two curves is found.

In this region the temperature minimum is difficult to determine, since it is not as clearly pronounced as seen in the Laptev Sea. Therefore, a second parameter, the 34-isohaline is chosen. The time series of the single density values show a high variability within the Polar Water layer. The densities are varying between 26 kg/m3 and 28 kg/m3. The densities in the depth of the 34-isohaline as boundary of the Lower Halocline water are showing only little variations (by 0.2 kg/m3 in total). The densities in the depth of the Atlantic Water boundary are showing the highest variability in 2010, having the most measurements available. The values varying in a range of 1 kg/m3. Comparing Figure 19, Figure 18 and Figure 17 differences in the changing stratification occur. While Figure 18 and Figure 17 show tendencies towards a decreasing stratification, Figure 19 is showing an increase in stratification. In contrast to the Laptev Sea region, the 34-isohaline is stable in depth north of Franz Joseph Land. In this calculation, the density change is mainly influenced by the density change in the Atlantic Water layer. In 2006, the zero degree isotherm is at the 28 kg/m3 density level, whereas in 2011 it is at 28.2 kg/m3. The temperature minimum is varying strongly in depth,

39 3. RESULTS

Figure 20: T-S Diagram for the region north of Svalbard. Different colours indicate different summer months of the available years. The oldest measurement was taken in 1980 (red), the most recent in 2004 (blue). The zero degree isotherm and the 34-isohaline are indicated by dotted lines. The curved lines show the density. Different characteristics of water masses are visible in this region. giving a higher uncertainty in terms of the density differences, since the depth levels are varying from lower than 27 kg/m3 to higher than 27.8 kg/m3 for the temperature minimum. Due to these different factors influencing the stratification in this region, it is hard to determine the origin of changes.

3.2.2 Svalbard

The region north of Svalbard is located within the inflow branch of the West Spitsbergen Current. On the way into the Arctic Ocean the water masses experience strong cooling and freshening (Rudels et al., 2005). The processes of cooling and freshening are creating incon- sistent characteristics of water masses. Additionally, water entering from the Yermak Plateau shows characteristics typical for shelf water. Water flowing out of the Arctic Ocean might also present in this region since recirculation of water starts north of Franz Joseph Land. The data in this region are taken by many different platforms and instruments. In contrast to the previous regions, profiles with a poorer vertical resolution occur at different times within the time series (and not only in early years). The variability between the different months within a year is quite large, which is why monthly mean values are used instead of yearly means. Data are available between 1980 and 2004.

40 3. RESULTS

In Figure 20 the T-S diagram (Temperature - Salinity diagram) for the region north of Franz Joseph Land is shown. The different months are showing strong variability, both in temperature and in salinity. Some measurements show characteristics of water masses, which did not experience strong cooling yet. These measurements show warmer temperatures e. g. in September 1989 (yellow) and in August 2004 (blue). The warmer profiles also show quite high salinities in most of the cases. Water with temperatures higher than 2 ◦C and salinities above 34.8 PSU is identified as Atlantic Water in this region (Cokelet et al., 2008, and references therein, particularly Aagaard et al.(1985)). Temperatures below zero degree indicate Polar Water in this region. The different states of the cooling process are well recognisable in the temperature profiles shown in Figure 21. The yellowish profiles of the years 1988 and 1989 show warm water close to the surface. Cooling has not yet taken place. Profiles of the years 1980 (red) and 2004 (blue) show a transition state. The upper parts close to the surface have cooled down already, while deeper down cold water is still present. Other profiles (e. g. 1986, 1987 (orange)) have been cooled down already. In depths greater than 250 m, the profiles have similar temperatures and show more uniform characteristic. The temperature in the upper part is influenced by water flowing over shallow shelf regions. These water masses are strongly exposed to the atmosphere and thus cooled down fast. The salinity content of the water masses is mainly influenced by the inflow of salty water through Fram Strait.

Figure 21: Temperature profiles in the region north of Svalbard. Different colours indicate different summer month of the available years. The oldest measurement is from 1980 (red), the most recent of the year 2004 (blue). Different states of the cooling process are present.

41 3. RESULTS

There is no clear time depended development visible in these profiles. The profiles of the 1990s (green) show a high variability in temperature. Comparing the earliest measurements (1980, red) with the most recent ones (2001, 2004, blue), higher temperatures are present in recent years. Especially in depths below 250 m the difference is quite high. In a depth of 300 m the temperature in 2004 is 2.7 ◦C, which is approximately one degree higher than the temperature measured in 1980. In this region no analysis of the depth of the Atlantic Water layer was carried out. Differences in the depth of the zero degree isotherm are more likely to occur due to different water masses and states of mixing processes rather than changes in the overall pattern. It thus is hard to determine time dependent variations in the region north of Svalbard.

3.2.3 Fram Strait

The aim of the analysis of the region in Fram Strait is to ﬁnd signals in the inﬂowing water. The region is located in the West Spitsbergen Current which is transporting water masses like the Atlantic Water into the Arctic Ocean. The data coverage in the Fram Strait is high. Data are available in this region from 1980 to 2015.

Figure 22: Mean temperatures with standard deviation (black curve) of the upper 300 m in the Fram Strait region. The temperature values are in degree Celsius. They are averaged per year over the available summer months of August, September, and October. The error bars for the individual yearly months are indicated by the black vertical lines. Mean temperatures over the periods 1980- 1999, 2000-2010, and 2010-2015 are shown by solid lines in blue, orange and red, respectively. The shading in the equivalent color indicates their standard deviations.

42 3. RESULTS

The temperatures are higher in Fram Strait than they are in the interior Arctic Ocean, because cooling processes have not yet taken place. The time series are split into three time periods, 1980 to 1999 (blue), 2000 to 2010 (orange), and 2010 to 2015 (red) as similarily done by Lind et al.(2018). Figure 22 shows the time series of the mean temperature of the upper 300 m in the Fram Strait region. The three chosen time periods show an increase in the mean temperatures. With the lowest mean is present in the first period and similar means in the second and third period. The variability and thus the standard deviation is highest in the first period and lowest in the last. Within the single summer means of the years there is a strong variation (see error bars for single points in Figure 22). Only the measurement in 1986 has a temperature below 1 ◦C. Temperatures above 4 ◦C occur in 1994 and 1995 for the first time, regularly after 2004. The highest summer mean temperature is present in 2004 with a value of 4.85 ◦C. The mean of the first time period from 1980 to 2000 is 2.80 ± 1.09 ◦C. The mean of the two following time periods is 3.84 ◦C with standard deviations of ±0.51 ◦C and ±0.37 ◦C, respectively. The temperature increase between the years before and after 2000 is approximately 1 ◦C. An increase in the temperature of the water entering the Arctic ocean means a higher heat transport into the Arctic Ocean.

Figure 23: Mean salinities with standard deviation (black curve) of the upper 300 m in the Fram Strait region. The salinity values are in PSU averaged per year over the available summer months of August, September, and October. The error bars for the individual yearly months are indicated by the black vertical lines. Mean salinities over the periods 1980-1999, 2000-2010, and 2010-2015 are shown by solid lines in blue, orange and red, respectively. The shading in the equivalent color indicates their standard deviations.

43 3. RESULTS

The time series of the salinity in Fram Strait (Figure 23) is showing a similar behaviour as the temperature development. Comparing the three time periods, the period from 1980 to 1999 has the lowest salinity, whereas the most recent period has the highest still similar to the middle period. In parts, the individual yearly means show a high variability within the three summer months. Overall, the range of the yearly means is quite small. The lowest salinity was measured in 1989 with 34.85 PSU, the highest in 2013 with 35.09 PSU.

Since the density mainly depends on the salinity in cold water, the changes are similar to the salinity development and thus quite small. The density development shows only little variation in the yearly mean values over the entire time period. The single standard deviations of the yearly mean values are quite high showing the diﬀerent water masses present in this area. The means of the three time periods are almost exactly the same, so that no large change in density is visible. The largest outlier is present in 1986, probably due to the comparatively low temperature in that year. Else, almost all yearly means lie within the standard deviation of the periodical mean.

3.2.4 Propagation

In this section the propagation of temperature signals is analysed. The Fram Strait region is located at the entrance of the Arctic Ocean. Data are available for every year in the entire time span covered by the data set used. Hypothetically, events present in this region will travel through the basin. While propagating through the basin, different (mixing) processes occur and change the characteristics of the water masses. Thus some signals found at the entrance of the basin are lost, travelling through the basin. In this study, the covered time periods of the individual regions are quite different. Not having a constant overlap in time, makes it hard to see the propagation through the basin. In Fram Strait different water masses are present, that do not have the pronounced characteristics seen in the Arctic Ocean interior. Due to this, an average over temperature of the upper 300 m is taken to be compared to the maximal temperatures in the regions in the inner part of the basin.

Temperature is decreasing with propagation through the basin. Cooling processes take place within the Arctic Ocean basin due to heat loss to the atmosphere. The longer the water is exposed to the colder surrounding, the more the temperature is decreasing. This process takes place at the water surface. In Fram Strait the temperature maximum is located close to the surface, since warm water has a lower density than cold water. With cooling and changes in salinity, less dense water masses are formed and overlay the existing water masses. This process can most likely force the Atlantic Water layer into deeper depths. The Atlantic Water is located deeper in the Laptev Sea than in the region north of Svalbard. The zero degree isotherm varies between approximately 122 m and 167 m in the Laptev Sea and between 69 m

44 3. RESULTS

Figure 24: Time series of the temperature maxima in Svalbard (green), Franz Joseph Land (FJL, blue) and Laptev Sea (red). For the Fram Strait (magenta), a mean temperature of the upper 300 m is shown. The dots are the yearly means of the respective temperature measurements, the thick line indicates the decadal mean for each period. and 96 m north of Svalbard conﬁrming the theory. Water cooler than 4 ◦C is less dense, compared to warmer water due to its density anomaly. Thus it is possible that a cold surface layer does not sink to larger depths.

Figure 24 is showing the time series of temperature development in the different regions. In Fram Strait, the temperature mean over the upper 300 m is taken, for the reasons explained above. In the upstream regions, the temperature maximum in the Atlantic Water layer is detected. The single yearly measurements are shown as dots, the decadal means by the solid lines. In Fram Strait a temperature increase over the covered time span is visible. It is increasing in the first three decades and is staying constant in the last time span (as already seen in Figure 22). A cold water inflow happens in 1986, relatively warm water enters in 1990, 1994 and 1995. In the 2000s, a temperature clearly higher as the decadal mean is present in 2004. The region north of Svalbard is the first region which is reached by the water from Fram Strait. Looking at the decadal trends in Franz Joseph Land, it shows oppositional behaviour in the first two decades than the Fram Strait region. While in Fram Strait the temperature is increasing between the 1980s and the 1990s, the water temperature north of Svalbard is decreasing. The mean of the first decade is even higher than the one in Fram Strait, mainly driven by warm temperatures in 1988 and 1989. These extremely high temperatures are not observed in the Fram Strait. In the 2000s, the temperature is increasing in the region north

45 3. RESULTS of Svalbard, but not as strong as in Fram Strait. The last measurements are existing in 2004 showing a high temperature similar to the Fram Strait region. The cold water mass entering Fram Strait in 1986 can not be found in the region north of Svalbard. The data coverage in the 1990s in the Svalbard region is too low, to identify the warm Fram Strait water from 1990. A propagation of certain characteristics or events from Fram Strait to Svalbard can not be determined with this data set. The temporal data coverage in the region north of Franz Joseph land is low, without measurements from before 2006. No overlapping time period between the Svalbard and the Franz Joseph region is available. As expected, the temperatures in this region are lower than in the regions further downstream. During the available years, the temperature is decreasing. This does not match the behaviour seen in Fram Strait. The covered time span in the region north of Franz Joseph Land is too short to determine single events, which might have travelled through the basin. In the Laptev Sea the temperature is decreasing between the 1990s and the 2000s and is staying constant in the 2010s. Within the 2000s, there is a clear temperature increase of about 0.8 ◦C. As in the region north of Franz Joseph Land, the temperatures are stable in the last decade. The temperature increase of the 2000s is not visible in this clarity in any of the previous regions. In Fram Strait an increase starting in 1998 reaching its maximum temperature in 2004 is present. This increase does show some peaks in between. In the region north of Svalbard, there might also be a trend tendency between 1999 to 2004. There are only three measurements available over this time, so nothing can be said about peaks during this period. Only a vague statement can be made for the cooling tendency in the different regions. In Fram Strait, the temperature decreased from 1995 to 1998 by more than 1 ◦C. North of Svalbard, there is a relatively high temperature in 1994. The next measurement in 1999 shows a temperature lower by 1 ◦C than in 1994. There is no information before or after the high temperature in 1994. In the Laptev Sea, a high temperature is found in 1998 followed by a low temperature in 2002. In this region, also no measurements are available between these two years, making it hard to find clear trends or variations. Taking these small trend tendencies as indicators for the propagation through the basin, there is only a small offset between Fram Strait and the region north of Svalbard. The temperature minima and maxima are found within the same year or with one year offset. Comparing the start and end points between Fram Strait and the Laptev Sea, there is an offset by four to six years. These statements for the propagation time do have a large uncertainty, because of (large) gaps within the time series.

46 4. DISCUSSION

4 | Discussion

An overall shoaling trend of the upper boundary of the Atlantic Water layer in the Laptev Sea is detected in this study. Warmer water is present in the Laptev Sea and Fram Strait in the upper part of the water column in the last decades. An increase in salinity also has been observed in the upper parts in all regions throughout the Eurasian Basin. This chapter compares the results found in this study to results of previous studies by different authors. First, the general variations observed in the Arctic Ocean are discussed before the focus is set on the Atlantic Water depth and stratification of the water column with its conse- quences on (vertical) mixing processes. At the end, the general importance of the variability in the Arctic Ocean with regard to the impact on lower latitude climate is briefly covered.

4.1 General Variability in the Eurasian Basin

In the Eurasian Basin different variabilities are observed in this study. A factor not directly covered in this study is the influence of the distance of measurement points to the boundary current. This is discussed first, since it might have an impact on the results. Secondly the importance of the seasonality is covered, before the results of the Atlantic Water in the Arctic Ocean are discussed.

4.1.1 Distance to the Coast

The boundary current in the Eurasian Basin is the main transporter of the warm Atlantic Water into the basin’s interior. Since it follows the topography of the basin, it is located close to the shelf edge. In Figure 26 (Appendix) a section along 60 ◦ East is showing the presence of warm water, that is transported by the boundary current. Meyer et al.(2017) found an increased depth of Atlantic Water with increasing depth of the basin in the region north of Svalbard. This deepening of the basin goes along with an increased distance to the coast or rather the shelf edge. To identify the influence the positions of the measurements have on the Atlantic Water depth, the distance to the 300 m depth contour (coast) is connected to the Atlantic Water depth. In an earlier investigation of the Laptev Sea (preparatory project to this thesis) no dependence of the Atlantic Water depth on the distance to the coast was found. In Figure 27 (Appendix) the depth of the upper boundary of the Atlantic Water is plotted against the distance to the 300 m depth contour. An influence of the distance to the coast could be rebutted in the earlier study for the Laptev Sea. Differences in the Atlantic Water depth with an increasing distance to the coast are still present in the north of Franz Joseph Land. The boundary current still has a higher impact on the water characteristics in this region (see Figure 26, Appendix). The investigation is only done for the depth of the Atlantic Water layer. No research is per- formed on the connection between the distance to the coast and the depth of the temperature

47 4. DISCUSSION minimum. The depth of the temperature minimum (boundary of the Polar Mixed Layer) may vary with the distance to the coast, since stronger mixing processes can occur close to the shelf edge. For the depth of the 34-isohaline also no investigation is carried out. The impact of changes in the depth of these water layers with a varying distance to the coast is not covered in this (or the earlier) study. A change within the minimum temperature depth and / or the 34-isohaline depth might affect the stratification analysis carried out in this study, because the density is dependent on the depth of the water layers. In future studies, more attention should be placed on this possible impact especially regarding the temperature minimum and the 34-isohaline. However, in this study means are taken, so that data of different dis- tances to the coast are included in each year, also giving a mean over the distance to the coast.

4.1.2 Seasonality

In Figure 27 (Appendix) the Atlantic Water depths of all seasons are plotted against the distance to the 300 m depth contour. Besides the differences between the decades, differences with respect to the season occur. The measurements are devided into summer values (dots with black edge) and non-summer values (diamonds). In the data of 2010 to 2015 (yellow), the summer values are mostly located in shallower depth than the non-summer values. No year-round measurements are analysed in this study and thus it is hard to make a definite statement on seasonal variability. Atlantic Water entering the Arctic Ocean is known to have a strong seasonal signal (e. g. Muilwijk et al., 2018; Dmitrenko et al., 2009; Ivanov et al., 2009). Muilwijk et al.(2018) report from a strong seasonal varying Atlantic Water flow in Fram Strait. This signal brought into the Arctic Ocean is quite likely to propagate further to interior of the basin. Ivanov et al.(2009) found significant hydrographic seasonal signals in the Atlantic Water layer in the Arctic Ocean. The seasonal fluctuations are assumed to be large enough to have an impact on the entire system of this ocean. The authors thus recommend to be aware of the seasonality and to take it into account. Ivanov et al.(2009) analysed mooring data north of Svalbard to have year-round measurements. They state, that the seasonal signal propagates along the Eurasian continental slope and make up 50 % of the total variance. A temperature difference between the ’summer’ (warmer and fresher) and ’winter’ (colder and saltier) water of 1.2 ◦C is indicated. Polyakov et al.(2017) found that the seasonal temperature range has even increased recently. Data from mooring observations in the Laptev Sea show an increase in the range from 0.2 ◦C to 0.3 ◦C in 2004 to 2007 (Dmitrenko et al., 2009). The seasonal temperature range increased to over 1 ◦C in 2013 to 2015 in this mooring (Polyakov et al., 2017). Due to the known large variability of the different seasons, only summer data are used in this study. This reduces the amount of available data and thus the possibility to find more information with regards to year-round (long term) changes. It therefore shows the importance

48 4. DISCUSSION of autonomous instruments, like ITPs or Argo Floats, to gain year-round measurements in the entire basin.

4.1.3 Atlantic Water in the Arctic Ocean

The largest amount of heat in the Arctic Ocean is stored in the Atlantic Water layer, which is transported into the Arctic Ocean via Fram Strait and through the Bering Sea. Investigations in the Barents Sea are done e. g. by Barton et al.(2018) and Lind et al.(2018). Both studies found a sharp increase in temperature and salinity values starting in the mid-2000s accompanied by a decline in sea ice import into the Arctic Ocean. The results of the Fram Strait area are similar to the ones found by Lind et al.(2018) in the Barents Sea. The mean temperature increases between the years before 2000 (from 1970 to 1999 in Lind et al.(2018), and from 1980 to 1999 in this study) and after 2010 by approximately 1.5 ◦C (see Figure 22). Considering the heat content of the upper 100 m, Lind et al. (2018) found a mirroring of the temperature in the ocean heat content development. Thus there is an increased heat transport into the Arctic Ocean after 2010. Similar results are found for the salinity development (see Figure 23 and Figure 2 in Lind et al.(2018)).

An increased Atlantic Water core temperature was found in the first half of the 1990s by Grotefendt et al.(1998) and can also be seen in the dataset of this study. High temperatures of the upper 300 m are present in Fram Strait in 1994 and 1995. North of Svalbard a relatively high temperature is found as well in 1994. Further downstream in the Laptev Sea region, the warmest temperature maxima are found in 1993 and 1995. Since the water needs time to propagate through the basin, it is unlikely that a warm water pulse occurs in Fram Strait and in the Laptev Sea almost at the same time. The warm water observed in the Laptev Sea thus might be a relict of a former warm pulse. Or a general warm (atmospheric) situation was present during the first half of the 1990s over the entire basin. Beszczynska-Möller et al.(2012) found two warming anomalies of Atlantic Water in Fram Strait from 1999 to 2000 and from 2005 to 2007. This can only be confirmed partly in this study. In the analysis of the upper 300 m mean temperature in Fram Strait a warm temperature anomaly is found in 1999, but followed by a relatively low temperature in 2000. The temperatures from 2005 to 2007 only show a higher temperature than the decadal mean in 2005 and 2006. In contrast to Beszczynska-Möller et al.(2012) a warmer temperature is seen clearly in 2004 in Fram Strait. North of Franz Joseph Land the warmest maximum temperature was measured in 2006, matching the observed second warming event found by Beszczynska-Möller et al.(2012). A warming event found by Korhonen et al.(2013) in the Nansen Basin between 2001 and 2007 might be related to a warm period in the Laptev Sea. The highest temperature maxima (after a warm period in the mid-1990s) are found between 2007 and 2009 in this region. Korhonen et al.(2013) reported a temperature anomaly by 1.2 ◦C in 2007. Comparing the temperature

49 4. DISCUSSION in 2002 to the temperature in 2009 in the Laptev Sea region, a warming of almost 0.8 ◦C occurred matching the results by Korhonen et al.(2013). Zhurbas and Kuzmina(2019) found an absolute maximum of the Atlantic Water core temperature in 2006 to 2008 and a maximum in 2013 at 103 ◦ East. This transect is close to the region north of the Laptev Sea in this study. Here, a temperature maximum is found between 2007 and 2009 in the upper 300 m, having an offset of one year compared to Zhurbas and Kuzmina (2019). In 2013 only a slightly higher temperature than in 2012 is found, having the same temperatures as in 2011. The high temperatures in 2007 to 2009 are not absolute maxima as claimed by Zhurbas and Kuzmina(2019) since this study has measurements available in the 1990s where higher temperatures are present in this region. Further, Beszczynska-Möller et al.(2012) give an overall warming trend in the northward flowing Atlantic Water of 0.06 ◦C per year in the time period from 1997 to 2010. In Fram Strait a warming of 0.08 ◦C per year is found for the same time period. In the Laptev Sea, a gap in the data set exists between 1999 and 2002. Taking the trend between 2002 and 2009 (yearly data available) a warming of even 0.95 ◦C is occurring. This time period has the strongest warming signal in the Laptev Sea. A general warming trend in the time period of 1997 to 2008 was found by Polyakov et al.(2012, 2010, 2004), but not by Bourgain and Gascard(2012). Bourgain and Gascard(2012) attempted to explain the discrepancy with a shorter time series used for their analysis. Polyakov et al. (2012) argued with their use of ’raw’ data rather than using interpolated data as done by Bourgain and Gascard(2012). Bourgain and Gascard(2012) claim the increased temperatures are linked to warming pulses, rather than to an overall warming trend. In contrary, Polyakov et al.(2010) claim a steady increase of Atlantic Water temperature since the late 1970s. In Polyakov et al.(2004), a time-period of the last 100 years is analysed, giving that the variability of Atlantic Water is dominated by low-frequency oscillations on time scales of 50 to 80 years, in which periods with short-term trends are found. This study covers a maximal time period of 36 years in Fram Strait and shorter ones in the other regions. The statement of low-frequency oscillations cannot be confirmed nor rejected. In a sense the general idea of short-term variability within a long term trend is matching the results in this study, both in the observed temperature development and the development of the Atlantic Water depth. Especially, in the Laptev Sea region increasing and decreasing periods of the depth of the Atlantic Water layer are present.

In the comparison of the temperature maxima, and the upper 300 m mean temperature in Fram Strait, some hints of propagating warm water pulses are found (see Chapter 3.2.3). Looking at the warming trend found in Fram Strait with a minimum as start point in 1998 and its maximum in 2004, it is likely to say that this trend is visible in the Laptev Sea from 2002 to 2009. The oﬀset of the start and end point of this trend between Fram Strait and the Laptev Sea varies between four and six years. This is coherent with Polyakov et al.(2005),

50 4. DISCUSSION stating that warm pulses need 4.5 to 5 years to be transported from the entrance of Eurasian Basin to the Laptev Sea. Zhurbas and Kuzmina(2019) interpreted the maximum in the core temperature found between 2006 and 2008 as a result of heat impulses of the early 2000s found by Polyakov et al.(2011). This also can be partly conﬁrmed in this study.

4.2 Atlantic Water Depth and Stratiﬁcation

A shoaling of the Atlantic Water layer is found in this study and will be compared to results of earlier studies in the following paragraph. Changes in the halocline Layer are discussed in the second part before the analysis of the stratiﬁcation is compared to other authors. Lastly, the principle of the atlantiﬁcation is applied to the results of this thesis.

4.2.1 Shoaling of Atlantic Water Layer

This study finds a shoaling of the Atlantic Water layer in the Laptev Sea in the last two decades. With some exceptions an overall shoaling is present starting in 2003. This is coherent with the findings of Polyakov et al.(2017). Analysing profiles and mooring data in the Eurasian Basin, they found a shoaling of the intermediate-depth Atlantic Water layer in the eastern Eurasian Basin. Using the same definition of the upper boundary of the Atlantic Water layer, Polyakov et al.(2017) found a rising of this boundary from 140 m in 2003 - 2004 to 100 m in winter 2014 - 2015 and even to 85 m in winter 2013 - 2014. In this study with only analysed summer data a depth of around 153 m is found in 2003 and 2004. The rising in 2013 and 2015 is present, but less pronounced in summer. In 2013 and 2015 the Atlantic Water layer is found in a depth of 137 m and 135 m, respectively. With a depth of 119 m the data of 2019 are showing the shallowest depth in this data set. The year-round measurements available from the moorings show strong seasonal signals, explaining the divergent depth in these two studies. The study of Polyakov et al.(2017) covers a relatively short time period. The decrease in depth of the Atlantic Water layer in the years from 1993 to 2002, again with little exceptions, is thus not covered. Also some years are not covered in the mooring observations. In the CTD-data used in this study, an increasing depth in Atlantic Water layer is present from 2011 to 2012. These years are not covered in the study of Polyakov et al.(2017). This study thus somehow disagrees with a general shoaling trend of the Atlantic Water layer, since it is only present in the recent two decades. Even within this overall shoaling trend, periods of increasing depths are present. Polyakov et al.(2004) found a low-frequency oscillations on time scales of 50 to 80 years with short-term trends found within these oscillations. In this study a shallow depth of the zero degree isotherm is found in 1993 and next from 2008 to 2011. This can give a cycle of 15 to 18 years for the returning of shallow Atlantic Water depths. Since the available time

51 4. DISCUSSION span in the Laptev Sea only covers 32 years, no deﬁnite conclusion can be drawn about the constancy of this only once observed cycle. In contrast to the results found and explained above, the region north of Franz Joseph Land shows an opposed behaviour in terms of the Atlantic Water layer depth. In this region only a short time span of seven years is covered. Within these seven years, the upper boundary of the Atlantic Water layer is deepening. As stated by Beszczynska-Möller et al.(2012), trends on relatively short time-series should be considered carefully, since they may show variability in a shorter band rather than long term trends. In the Laptev Sea region periods of a deepening of the Atlantic Water layer depth are also present. However, the overall trend is still towards a shallower depth of the Atlantic Water layer in the Laptev Sea region.

Comparing the temperature maxima in the Atlantic Water layer (Figure 24) to the depth of the Atlantic Water layer (Figure 12, Figure 14), a correlation seems to exist. Higher temperatures are present in years of a shallow Atlantic Water depth (e. g. 1993, 2008). The other way around, there are low temperatures present in years of a deep Atlantic Water layer (e. g. 1996, 2002). A warming water mass is accompanied by an increase in volume (Korhonen et al., 2013), so that it is likely that a warming might be connected to shoaling of the Atlantic Water layer due to an increased volume. This observed connection is also found in the region north of Svalbard, where the highest temperature is measured in the earliest available year. In this year (2006) the Atlantic Water layer lies deeper than in the following years in which the temperature is decreasing.

4.2.2 Halocline Layer

This study defines the 34-isohaline as boundary between the Upper and the Lower Halocline (following Korhonen et al., 2013). Polyakov et al.(2017, 2018) defined the cold halocline layer as separation between the cold and fresh surface mixed layer and the warm and saline Atlantic Water. They utilised a density ratio as the boundary between the layers (see Torrence and Compo(1998); Bourgain and Gascard(2011)), rather than a fixed salinity value as done in this study. Unprecedented changes in the water layer above the Atlantic Water are found in Polyakov et al.(2017) and in this study. In the analysis of a year-long record of an Ice-Tethered Pro- filer (ITP) in 2013 to 2014, Polyakov et al.(2017) found a deepening of the surface mixed layer to approximately 130 m and a disappearance of the cold halocline layer in the central Nasen Basin. The authors connect the disappearance to the winter convection in March to April in 2014. The ITP passed north of Franz Joseph Land in that time. Unfortunately, no measurements are available in 2013 and 2014 for this region in this study. However, the Lower Halocline boundary is found above the temperature minimum, which defines the lower boundary of the Polar Mixed layer in this study. This is giving a hint of the Lower Halocline already disappearing in 2011. A similar situation is also present in 2009 in this region. This

52 4. DISCUSSION observation might be related to a seasonally ventilated halocline found on a Russian ice drift station in 2007 to 2008 (Carmack et al., 2015). Similar results were also found in earlier observations (e. g. Rudels et al., 1996; Dmitrenko et al., 2009).

From 2013 to 2015 Polyakov et al.(2017) found a disappearance of the cold halocline in the eastern Eurasian Basin (Latev Sea in this study). This is almost consistent to the results of this study. A deeper Polar Mixed Layer than Lower Halocline Water is found in 2015 and 2018 (no data available in 2014). In 2013 the known situation of Polar Water on top of the halocline waters is still present. This offset by one year can be explained by slightly different definitions of the region. Since a longer time period is covered in this study it is shown, that a similar situation is already present in 1993, through it is less pronounced. The temperature minimum and the 34-isohaline are in similar depths in 1993, this is equivalent to a disappearance of the halocline layer. In 2007 and 2008 the same situation occurs. This change in the water layer order is accompanied by a strong salinification of the upper 150 m in the Laptev Sea, observed by several studies (e. g. Zhurbas and Kuzmina, 2019; Aksenov P.V., 2018; Polyakov et al., 2018). A warming tendency is found in almost all recent studies (e. g. Årthun et al., 2019; Barton et al., 2018; Muilwijk et al., 2018). Lind et al.(2018) even stated in their work that a rapidly diminishing Arctic Water mass with subzero temperatures in the Barents Sea is present. This tendency is found in the region north of the Laptev Sea in this study as well. Warmer water masses are shrinking the cold water mass. At the surface warmer water arises and the warmer Atlantic Water layer is rising from below, reducing the thickness of the cold water layer. In contrast, the cold water mass in the region north of Franz Joseph Land even seems to increase in thickness. Cold water (-1.5 ◦C) is present in 2012 down to a depth of approximately 70 m, while in 2006 only a depth of 15 m is reached. This is an increase of 55 m in seven years. Korhonen et al.(2013) found a decrease in thickness of the Polar Mixed Layer by 19 m per decade in the Nansen Basin for 1991 to 2011. They were using the temperature minimum as boundary for the Polar Mixed Layer as done in this study. In the region north of Franz Joseph Land the depths of the temperature minima are increasing and thus also the thickness of the Polar Mixed Layer increases. The temperature minimum in 2012 is found below 50 m, while they are close to the surface in 2006 and 2007 and in a depth of 40 m in 2009 and 2011. A similar tendency as in Franz Joseph Land is found in the Laptev Sea with a depth of the temperature minimum in about 40 m in 1988 and in about 50 m in 2011 to 2013. The deepest minimum in 2018 is in a depth of 56 m. Only taking the thickness of the cold water body (-1.5 ◦C, dark blue in Figure9) a decreasing thickness is identifiable. However, the warmer surface layer being present from 2005 onwards, are not included into the thickness determinition here. While the cold layer in the first two years (1988, 1990) is between 75 m to 85 m thick, it decreases to 45 m in 2011 and to 55 m in 2018. This gives a decrease in thickness of the cold water body by approximately 20 m per decade as found by Korhonen et al.(2013) for Polar Mixed Layer.

53 4. DISCUSSION

Changes in the water masses above the Atlantic Water layer and especially in the halocline are influencing the stability and stratification of the water column. A decrease in stratification can lead to a higher probability that warm and salty Atlantic Water is brought into shallower depths and close to the surface where it most likely reduces the sea ice cover. Details on the stratification above the Atlantic Water layer are presented in the next paragraph.

4.2.3 Stratiﬁcation

This study finds a decrease in stratification in the regions north of the Laptev Sea and Franz Joseph Land analysed in detail in this study. The reduction is mainly due to a change in the salt content and partly connected to a warming. Additionally, a disappearance of the Lower Halocline is found in the Laptev Sea in 2015 and 2018. This is equivalent to a disappearance of the strongest limitation in terms of water exchange between the cold and fresh surface water and the warm and salty Atlantic Water. The analysis of vertical profiles by Polyakov et al.(2017) shows increasing temperature and salinity and decreasing stability by calculating the Brunt-Väisälä frequency. The decreasing stability is located in the cold halocline layer and the upper pycnocline (40 - 150 m) in the 2000s and 2010s. Polyakov et al.(2018) relates the depth of the halocline base to the stability of the water column: "the deeper the halocline’s base, the more stable the water column is". Aksenov P.V.(2018) reports a change in the vertical stratification of the water column in the western Nasen Basin. The same assumption is done by Lind et al.(2018) for the Barents Sea. A sharp increase in temperature and salinity beginning the mid-2000s is linked to a recent sea ice import decline and thus to a loss in freshwater. This leads to a weakened ocean stratification. They also declare that a weaker vertical salinity gradient is present. The observed salinification in the upper part of the water column is particularly present in the Laptev Sea after 2003 with a strong increase between 2012 and 2018. Higher salinities are present in shallower depths in recent years. This is decreasing the salinity gradient and thus the stratification of the water column. The analysed profiles (Figure 13) show a strong increase in salinity in the upper 80 m to 100 m in years with a weak stratification. This is matching the results found by Polyakov et al.(2017) for the upper parts of the ocean. It is also consistent to the assumptions made by Lind et al.(2018) of the weaker vertical salinity gradient. In the region north of Franz Joseph Land the salinification is only partly present. While the 34-isohaline shows a constant depth, the 34.2-isohaline is shoaling between 2011 an 2012. In contrast, the 34.4-isohaline is deepening from 2007 on. However, the increased distance between the 34.2 and 34.4-isohaline is resulting in a weaker vertical salinity gradient in depths between 40 m and 80 m.

The density differences used as indicator for the ocean stratification provide similar results. An overall decreasing stratification is present in the Laptev Sea starting in 2003. An increase

54 4. DISCUSSION in the density difference between the depth of the temperature minimum and the zero degree isotherm can be observed from 1993 to 2003. The time series (Figure 12) shows small peaks of higher stratification within the overall weakening trend. The density difference taken between the density in the depth of the 34-isohaline and the zero degree isotherm shows a stronger variability. A period of decreasing density differences is present between 2002 to 2008, maybe even until 2012. In contrast to what is claimed before, the density difference is increasing in the last three available years. This normally is related to an increased stratification of the water column. In this case, the increase in the difference is probably related mainly to the salinification and thus to the shallower depth of the 34-isohaline accompanied by lower densities. However, a decrease in the ocean stratification in the region north of the Laptev Sea is mainly driven by variations in salinity. A different situation is present in the region north of Franz Joseph Land. The density differences, both between Atlantic and Polar Water and Atlantic and Lower Halocline water, do not show an overall trend towards a decreased stratification. The density difference between the Atlantic and Polar Water mainly depends on the location of the temperature minimum. In years where it is located close to the surface, the density difference to the Atlantic Water is higher, than in years where the temperature minimum is located deeper in the water column. In the case of the Lower Halocline water, the density difference and thus the stratification shows an increasing trend. The 34-isohaline is relatively constant in depth and density level. The density difference is therefore a result of the increased depth (and thereby density) of the Atlantic Water layer. The increase in stratification indicated by this parameter contrasts the argumentation done before based on the observed salinification.

The choice of defining the boundaries between the water layers by fixed salinity and temperature values does not seem to work in all years. In the Laptev Sea, the recent changes show a reverse layering of the Polar and Lower Halocline water. This makes statements concerning the stratification more difficult as long as they are based on the density differences between these specific layers. An approach of solving the difficulties of fixed values is already done for the cold halocline layer in Polyakov et al.(2017, 2018). They are defining a density ratio following e. g. Torrence and Compo(1998) and Bourgain and Gascard(2011). Similar approaches are done for the definition of the Surface Mixed Layer (Ivanov et al., 2009). The usage of ratios is particularly suitable in situations where no clear temperature minima are present. In the region north of Franz Joseph Land, the temperature minimum is located near the surface in some years, while it is found deeper in the water column in other years. Thus, strong variabilities in the density are present, being mirrored in the stability analysis in this region. The location of the Lower Halocline above the temperature minimum in some years makes this parameter also not suitable for clear statements on the water column stratification. The temperature and salinity profiles (Figure 13, Figure 16) and the Hovmöller-diagrams (Fig- ure9, Figure 17) are giving evidence on a transition in temperature, salinity, and stratification for these regions located in the Eurasian Basin.

55 4. DISCUSSION

4.2.4 Atlantiﬁcation

A weakening of stratification in the water layers above the Atlantic Water increases the potential of the penetration of vertical thermal convection into the warm and saline Atlantic Water layer (Aksenov P.V., 2018). Lind et al.(2018) concluded that the vertical mixing is inversely related to the strength of stratification. In Barents Sea the downward freshwater flux is no longer sufficient to counterbalance the upward salt fluxes from the Atlantic Water layer. They state that a weaker vertical salinity gradient plus weaker stratification enhance the vertical mixing with the Atlantic layer below. Polyakov et al.(2017) applies the term "atlantification" to the northward movement of sea ice in the Barents Sea together with the present reduction of stratification, and the increased vertical mixing. This atlantification is found in several recent studies (e. g. Lind et al., 2018; Pnyushkov et al., 2018; Barton et al., 2018) often also described as an eastward displacement of characteristics typical for the western Eurasian Basin. Lind et al.(2018) describes the ongoing process for the Barents Sea as a transit from a cold and stratified Arctic to a warm and well- mixed Atlantic-dominated climate regime. The substantial reduction in sea ice thickness is according to Årthun et al.(2019) a result of the further increase in Atlantic heat transport reflected in a northward penetration of warm water in the Arctic Ocean. In Figure2 (taken from Polyakov et al., 2017) a conceptual model of the atlantification is shown. Two situations are presented, the one before the atlantification (early 2000s) and the situation of atlantification (mid-2010s) in the eastern Eurasian Basin. In the early 2000s the winter convection (WC) reaches the upper permanent pycnocline (UPP) in the western Eurasian Basin (Barents Sea). In the eastern Eurasian Basin (Laptev Sea) it is limited by the cold halocline layer (CHL). Only a small amount of heat is brought to the surface and the layers overlaying the Atlantic Water (AW). With the transition process in the mid-2010s a suite of processes associated with atlantification is brought further into the Arctic Ocean. The penetration of the surface signature of the Atlantic Water is stronger by an increased flow, by an increased heat content, or by both. A reduction in sea ice cover is observed, resulting in a greater surface heat and moisture flux due to more open ocean and thus a higher exchange with the atmosphere. The depth of winter penetrative convection increases due to a weaker stratification. Therefore additional heat and nutrients from the Atlantic Water are brought to the surface water. The permanent cold halocline layer is transformed to a seasonal halocline. The winter convection in the eastern Eurasian Basin nowadays still reaches the upper permanent pycnocline. More heat flux takes place at the surface reducing the ice cover. In the Laptev Sea higher heat fluxes are present from the cold halocline layer to the surface mixed layer than observed in the early 2000s. The Figure2 summarises the effect of atlantification schematically. This study did not investigate the strength or penetration depth of (winter) convection. However, a shoaling of the Atlantic Water is found as well as a weakening in stratification and a disappearance of the Lower Halocline in the Laptev Sea. Summing-up these results, a scenario as described

56 4. DISCUSSION by Polyakov et al.(2017) is likely to represent the current transition in the eastern Eurasian Basin from a strongly stratiﬁed to a well-mixed regime.

Polyakov et al.(2017) found in their study seasonal upward heat fluxes from the Atlantic Water never before observed in the eastern Eurasian Basin. They further argue, that Ekman pumping (which is a wind-driven upwelling) cannot explain the anomalous shoaling of the Atlantic Water present in recent years. As well, variations in the contribution of fresh shelf water only play a minor role. Zhurbas and Kuzmina(2019) could not explain the almost monotonously increase in salinity between 2003 and 2013. Following Polyakov et al.(2017) it is most likely to find the sources of changes associated with the Atlantic Water further upstream in regions as Fram Strait and the western Eurasian Basin. This study finds a clear warming and salinification tendency in Fram Strait in the recent two decades. How these findings are connected to the changes in the eastern Eurasian Basin has to be left for future investigations.

4.3 General Importance

The large influence especially on the sea ice thickness and formation shows the importance of this kind of investigations. It is important to separate climate trends and variability to be able to improve accuracy of climate projections (Polyakov et al., 2017). A transition to a changed regime as observed in the eastern Eurasian Basin brings the need to rethink some of the common (water mass) definitions; as shown in this study for fixed temperature and salinity values as boundaries between water layers. A change in the characteristics in the Arctic Ocean has the potential to influence the climate conditions and the ocean circulation in lower latitudes. Higher temperatures in the Arctic Ocean are reducing the equator-to-pole thermal gradient associated with a weakening of the mid-latitude circulation (Coumou et al., 2018). Variations in the Arctic freshwater export can affect lower latitudes via oceanic linkages since it alters the deep water formation in the North Atlantic. Freshwater variations thus have the potential to affect the variability of the Atlantic Meridional Overturning Circulation (Koenigk and Brodeau, 2017). A slightly decreasing density is found in Fram Strait in this study, proving that variations are present. Continues research on this topic is essential since changes in the Arctic Ocean will have con- sequences to lower latitude and thereby many associated systems.

57 5. SUMMARY AND OUTLOOK

5 | Summary and Outlook

This thesis explores the changes in the Atlantic Water in the Eurasian Basin. The focus is on the variability of the Atlantic Water layer depth and the stratification of overlaying water masses in the Laptev Sea. Oceanographic data, mainly CTD-observations, are used to fulfil the analysis. A time span over 31 years is covered in the region north of the Laptev Sea, for the remaining regions along the pathway of the Atlantic Water only shorter time spans are available. In this study the depth of the Atlantic Water layer is defined by the depth of the zero degree isotherm which is utilised to determine the shoaling process of this layer. Density differences between the Atlantic Water and (i) the Polar Water and (ii) the Lower Halocline Water are calculated to draw conclusions about the water column stratification above the Atlantic Water layer. Temperature maxima are used to find propagation of pulses through the Eurasian Basin.

These analyses allow to answer the following questions:

• Is the Atlantic Water layer shoaling? In the region north of the Laptev Sea a shoaling of the Atlantic Water layer takes place in recent years. The upper Atlantic Water boundary is approximately 50 m shallower in 2018 as in 2002. The decrease in depth is not monotonously over this time period, but is still showing a clear trend towards lower depths. Considering the entire time series, periods of increasing depths are present as well. In 1993, the Atlantic Water is located in similar depths as from 2008 to 2011. The shoaling of the Atlantic Water layer is thus dominated by short-term variability. A decreasing depth of Atlantic Water is present in the region north of Franz Joseph Land. Between 2006 to 2011 the depth increased almost monotonously by 25 m in total. Only a short time period (7 years) is covered in this region, thus it is not possible to diﬀerentiate between a short-term variability and an overall decreasing trend. Even through the data in the region north of Franz Joseph Land are not explicit, the observed shoaling of the Atlantic Water layer in the region north of the Laptev Sea clearly shows a changed situation in the eastern Eurasian Basin.

• Is the stratification becoming weaker in the layers overlaying the Atlantic Water? Density differences in the Laptev Sea between the Atlantic Water and the Polar Water show a similar behaviour to the one of the Atlantic Water layer depth. The Atlantic Water depth is decreasing in recent years, while an increase in stratification between 1993 and 2003 is present. The density difference between the Atlantic Water and the Lower Halocline water shows stronger variability and an increase in stratification in the last three available years (2013 to 2018). This can be explained by a strong salinification

58 5. SUMMARY AND OUTLOOK

during this time in the Laptev Sea shifting the 34-isohaline into shallower depths. The salinification of the upper 150 m is reducing the vertical salinity gradient and thus leading to a weaker stratification. In the region north of Franz Joseph Land, the density difference between the Atlantic and Polar Water shows too strong variations to draw an explicit conclusion about the stratification. The temperature minimum, taken as boundary of the Polar Water, is too variable in depth and thus in density. Whereas the density difference between At- lantic Water and Lower Halocline Water shows an increase in stratification. This is contrary to the observed salinification in this region in the last years. The salinification of the upper water column is reducing the salinity gradient as also seen in the Laptev Sea.

• Is it possible to track signals through the basin? The diﬀerent time series available in the regions of the Eurasian Basin show only a short temporal overlap in most of the cases. This makes it hard to track signals through the basin. However, pulses entering Fram Strait can be observed in the Laptev Sea after four to six years. This observation of the propagation time has a large uncertainty due to gaps in the time series.

Beyond these research questions the principle of atlantification is found in the Laptev Sea confirming the results of Polyakov et al.(2017). A weakened stratification and a decreased depth of the Atlantic Water layer are making a deeper penetration of (winter) convection possible. Furthermore, a strong salinification in recent years in the Laptev Sea is present and is even leading to a disappearance of the halocline layer. The results found in this study are mostly consistent to previous findings of other authors. This study is on top able to make statements on decadal variability rather than on short-term variations only. This is possible due to a relatively long and consistent time series in the Laptev Sea. To review, most of the observed changes in the Laptev Sea are driven by changes in the water salt content. How this can be explained is not clear and requires future investigations. Even though, different results are found in the region north of Franz Joseph Land, they support the hypothesis that a transition to a new regime is mainly happening in the eastern Eurasian Basin.

Additional to the shown results, this study also identiﬁes several objectives to be addressed in future research. One is to separate climate trends and variability. This is a large challenge, but necessary to improve the accuracy of climate projections. Long time series are required to allow explicit conclusions on oscillation patterns on a multi-decadal scale. ITPs and mooring observations are providing year-round measurements and thus are important in future investigations. Seasonal signals in the Atlantic Water and overlaying water masses can be analysed based on theses measurements. Additionally, mooring data can give information

59 5. SUMMARY AND OUTLOOK on changes in the currents and transports at a certain location. For this purpose, a long oper- ating time of moorings is necessary. ITPs can increase the understanding of current systems and transformation of e. g. Atlantic Water masses along its way through the basin. For this reason, future work should include the analysis of mooring and ITP data. Model simulation can help to ﬁll gaps in the time series and to provide longer time series.

It could be interesting to use the same approach of density differences for the stratification analysis in future studies and adjust the definition of the boundaries between the water layers. Instead of using fixed values, the definition could be based on ratios in salinity, density, and temperature or any new definition adapted to the recent conditions present in the eastern Eurasian Basin.

Accompanied by a weakened stratification of the upper water column, the upward heat flux and a higher potential of vertical mixing increase. So it is worth to investigate in the effect on the upward heat flux on the sea ice concentration and thickness. Questions like the following need to be answered: How large is the amount of heat which is brought to the ocean surface? What is the additional melting rate due to the additional heat input from the Atlantic layer? Related to these questions are the so far unknown depths of the (winter) convection and vertical mixing processes and their potential to reach into the Atlantic layer. Calcula- tions on heat fluxes and transports as well as on penetration depths are therefore indispensable.

One of the most interesting and at the same time most difficult question is on potential forcing leading to the observed transition in the eastern Eurasian Basin. To find the origins of the salinification it might be of interest to calculate the freshwater content in this region. Attention should be paid on the river runoff, shelf water production, and the freshwater input due to ice melt. This might change dramatically in the future, if a change in sea ice cover will establish. For the shoaling of the Atlantic Water layer it would be interesting to investigate whether the decreasing depth is due to a reduction of water added at the surface or due to changes in water masses below, forcing the Atlantic Water to rise.

60 ACKNOWLEDGEMENTS

Acknowledgements

First of all, I would like to thank Prof. Dr. Torsten Kanzow for the great supervision of this thesis. Thank you for giving me the opportunity to work on such an interesting topic and for supporting me throughout the time. I also would like to thank Prof. Dr. Ursula Schauer to join the discussions and for the enormous amount of knowledge shared with me. Further, I would like to express my gratitude to Prof. Dr. Monika Rhein for accepting to review my thesis. Many thanks go to the Alfred-Wegener-Institut for hosting me and giving me the chance to be part of a great research institute. Especially, I would like to thank the members of my office: Luisa, Cassandra, and Laura for several lunch breaks and interesting talks. I would like to thank my family for supporting me the entire time and that you did not stop believing in me. Many thanks also to my friends, who always gave me confidence and support, as well as days filled with lots of fun. Thank you Clara, for our amazing cycling tour to the Nordkapp, giving me enough motivation to start and finally finishing this thesis. Finally, I would like to give a big thank you to Finn. You always cheered me up, trusted in my abilities and helped me to finish this thesis. Thank you.

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x APPENDIX

Appendix

In this section additional ﬁgures to complement the investigations are presented. As well, a list of abbreviation is shown.

Density Anomaly

Figure 25: Anomalies of the density in (a) Polar Water and (b) Atlantic Water. Anomalies are taken to the overall mean of the relative time series. The upper panel (a) is showing the anomalies for the densities in Polar Water in the depth of the temperature minimum. The lower panel (b) is showing the density anomalies for the upper boundary of the Atlantic water. Red colour indicates a positive anomaly, blue a negative anomaly.

xi APPENDIX

Distance to 300 m contour line

Figure 26: Temperature section along 60 ◦East in the Eurasian Basin. The section starts north of Franz Joseph Land and continues to a latitude of almost 90 ◦North. The warm Fram Strait branch is visible within the boundary current. It has a core temperature higher than 2 ◦C north of Franz Joseph Land and is present up to a latitude of 87 ◦C. The ﬁgure was produced using OceanDataView (Schlitzer, 2018). It is taken from a previous study (preparatory project to this thesis, not published).

xii APPENDIX

Figure 27: Depth of the upper boundary of the Atlantic Water layer against the distance to the 300 m depth contour in Laptev Sea. The colour code indicated the time, the measurement is taken. Round symbols with black edging are showing summer data (August, September, October), diamond shaped symbols are the remaining month. A diﬀerence is visible between summer and non-summer values and between the decades, but no inﬂuence of the distance to coast is visible. Figure slightly changed compared to the original of the previous study (preparatory project to this thesis, not published).

xiii APPENDIX

Abbreviation List

Abbreviation Meaning AW Atlantic Water CHL Cold Halocline Layer CTD Conductivity-Temperature-Depth EB Eurasian Basin FJL Franz Joseph Land LHC Lower Halocline NABOS Nansen and Amundsen Basin Observational System NN Nearest Neighbour interpolation NSTM Near Surface Temperature Maximum ITP Ice-tethered proﬁler PSU Practical Salinity Unit PW Polar Water RR Reiniger-Ross interpolation S Salinity s.e.m. Standard Error of the Mean SML Surface Mixed Layer T Temperature UDASH Uniﬁed Database for Arctic and Subarctic Hydrography UPP Upper permanent pycnocline WC Winter Convection

xiv