Master’s Thesis Decadal Changes in the Atlantic Water in the Eurasian Basin 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. i 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 atmo- sphere. 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.