Freshwater Processes and Water Mass Transformation in the Arctic Ocean

Freshwater processes and water mass transformation in the Arctic Ocean

Per Pemberton Cover image: Satellite image showing the East Greenland Current, Greenland Sea and Fram Strait. This is the major gateway exporting Arctic Ocean freshwater, in the form of low saline water and sea ice. Photo by: NASA, freely available at http://earthobservatory.nasa.gov/.

c Per Pemberton, Stockholm 2014

ISBN 978-91-7447-998-0

Printed in Sweden by US-AB, Stockholm 2014 Distributor: Department of Meteorology, Stockholm University Abstract

This thesis explores freshwater-related processes and water mass transformation in the Arctic Ocean. Knowledge of these processes is important from both a local and a global perspective. Globally, because the export of cold and low saline water and sea ice might influence the North Atlantic and global meridional overturning circulation. Locally, because freshwater processes affect the vertical stratification and permit favorable conditions for the ice cover. Models of different complexity are the main tools of the present work. A part of the material considers how these models can be used to examine the key processes governing freshwater balance. Additionally, the freshwater budgets amongst 10 different ocean general circulation models (OGCMs) are compared and robust features and weaknesses identified. A large part considers the freshwater processes governing the stratification with an emphasis on the low saline upper parts. The interactions between freshwater sources and sinks are studied in an OGCM using passive tracers. It is found that the composition, pathways and shelf-basin exchange of low saline water primarily involve processes linked to Siberian runoff, Pacific water and sea-ice melting and formation. Motivated by observed changes and paleorecords the sensitivity of the stratification is further explored in freshwater perturbation experiments with an OGCM. The response yields a deeper halocline for decreasing freshwater input, in line with a theoretical model. The final part focuses on a new framework for analyzing water mass transformations. In the framework volume, heat and salt budgets are computed in salinity-temperature space. Using different OGCMs it is shown how surface and interior processes transform inflowing waters towards colder and fresher waters and how the halocline renewal rate can be estimated. Limiting cases for the water mass transformation balance are identified by separating contributions from surface, internal and boundary fluxes.

List of Papers

The following papers, referred to in the text by their Roman numerals, are included in this thesis.

PAPER I: Arctic Ocean freshwater: How robust are model simulations? A. Jahn, Y. Aksenov, B. A. de Cuevas, L. de Steur, S. Häkkinen, E. Hansen, C. Herbaut, M.-N. Houssais, M. Karcher, F. Kauker, C. Lique, A. Nguyen, P. Pemberton, D. Worthen, and J. Zhang, Journal of geophysical research, 117, C00D16 (2012).

PAPER II: Arctic Ocean freshwater composition, pathways and transformations from a passive tracer simulation Pemberton P. , J. Nilsson and H.E.M. Meier, Tellus A, 66, 23988 (2014).

PAPER III: Arctic Ocean water mass transformation in S-T coordinates Pemberton P. , J. Nilsson, M. Hieronymus and H.E.M. Meier, submitted to Journal of physical oceanography, (2014).

PAPER IV: The response of the central Arctic Ocean stratiﬁcation to freshwater perturbations Pemberton P. and J. Nilsson, manuscript, (2014).

Reprints were made with permission from the publishers. Papers by the author not included in this thesis

• Ridged sea ice characteristics in the Arctic from a coupled multi category sea ice model Mårtensson S., H. E. M. Meier, P. Pemberton, and J. Haapala, Journal of geophysical research, 117, C00D15 (2012). Author’s contribution

The idea to paper I grew from discussions between the co-authors during an AOMIP workshop in Woods Hole. Alexandra Jahn did the analysis and wrote the main text with input from me and the other co-authors. I performed all the model experiments with the RCO model. The idea to paper II was mine based on earlier work by Alexandra Jahn. It was Johan Nilsson’s idea to use the salinity framework in the analysis. I performed the model experiment, analysis and writing with input from Johan Nilsson and Markus Meier. The idea to paper III was mine based on earlier work by Magnus Hierony- mus. I performed the model experiment, analysis and most of the writing. Johan Nilsson help with small parts of the writing and provided valuable input together with Magnus Hieronymus and Markus Meier. The idea to paper IV was Johan Nilsson’s. I performed the model experiments, analysis and main writing. Johan Nilsson wrote the conceptual model section and provide input to the rest of the paper.

Contents

Abstract iii

List of Papers v

Author’s contribution vii

List of Figures xi

1 Introduction 13

2 Modeling the Arctic Ocean 19 2.1 Conceptual models ...... 19 2.2 Ocean general circulation models ...... 22 2.3 Model challenges ...... 23

3 The role of freshwater in the Arctic Ocean 27 3.1 What is fresh, and what is not? ...... 27 3.2 The Arctic Ocean freshwater distribution and governing processes ...... 29 3.3 Freshwater sensitivity ...... 33

4 Water mass transformation in the Arctic Ocean 37 4.1 Walin formalism ...... 37 4.2 Transformations in the Arctic Ocean ...... 39

5 Outlook 43

Sammanfattning xlv

Acknowledgements xlvii

References xlix

List of Figures

1.1 The connection between the Arctic Ocean and the global ocean circulation ...... 14 1.2 Map of the Arctic region and vertical proﬁles for different sub- basins ...... 17

2.1 Schematic of a conceptual model of the Arctic Ocean . . . . . 21 2.2 Freshwater model intercomparison ...... 26

3.1 Arctic Ocean freshwater height ...... 29 3.2 Arctic Ocean freshwater budget ...... 30 3.3 Composition of Arctic Ocean freshwater content ...... 32 3.4 Pathways of freshwater sources and sinks ...... 34 3.5 Arctic Ocean freshwater sensitivity ...... 35

4.1 The Walin S coordinate framework ...... 38 4.2 Arctic Ocean water mass transformation in S coordinate . . . . 41 4.3 Arctic Ocean water mass transformation in S-T coordinates . . 42

1. Introduction

This thesis is the result of an investigation in how freshwater processes and water mass transformation affect the Arctic Ocean. Before starting to discuss the results that are presented in this thesis a short introduction is given. Here I will focus on the physical settings introducing the conditions that make the Arctic region an unique and interesting area to study.

The Arctic region is a vast area with an extreme environment where people and wildlife have adapted to the severe climate conditions that prevail. The region consists mostly of an almost landlocked ocean – the Arctic Ocean – which is largely ice covered all the year round. The Arctic Ocean is the smallest of the world’s five oceans and covers an area of 9.5 million km2, half of which is deep (∼ 4500 m) ocean and half of which is shallow shelf seas (Jakobsson, 2002), see Figure 1.2 for a map of the Arctic Ocean. The Arctic is connected to both the Pacific Ocean, via the Bering Strait, and the North Atlantic Ocean, via the Fram Strait, the Barents Sea Opening and the Canadian Arctic Archipelago (CAA). From a global perspective it has primarily two roles: (i) it provides a direct link between the Pacific and Atlantic oceans, and (ii) it receives Atlantic water, modifies it and returns it back into the North Atlantic Ocean. The Arctic Ocean can thus be seen as the northern most limb of the global ocean circulation, see Figure 1.1.

Because the Earth receives more energy at the equator than at the poles a net northward energy transport is implied. This leads to a poleward heat transport carried by both the ocean and the atmosphere. On its poleward transit the ocean loses heat to the atmosphere and a large part of the atmospheric energy is radiated out to space leading to the cold temperatures in the polar region. A part of the large-scale energy transport is in the form of latent heat leading to a net moisture convergence over the Arctic region (e.g. Nakamura and Oort, 1988). This results in a net input of freshwater over the Arctic Ocean, both from the net precipitation as well as the large amounts of river runoff discharg- ing into the shelf seas. In fact, some of the worlds largest rivers1 drain into

1Based on mean annual discharge the rivers Yenisei, Lena, Ob, and Mackenzie are the 5th, 7th, 11th, and 13th largest rivers in the world, respectively (Wohl, 2007).

13 Figure 1.1: The connection between the Arctic Ocean and the global ocean circulation - The schematic shows how the warm Pacific and Atlantic waters (red) are transformed within the Arctic Ocean towards colder and fresher water classes (blue) before exiting back to the North Atlantic Ocean. The figure is from Holloway and Proshutinsky (2007). the Arctic Ocean. The large-scale atmospheric and oceanic circulation also leads to a fresher Pacific Ocean. This results in a pressure gradient between the North Pacific Ocean and the Arctic Ocean forcing a net inflow of low saline water through the Bering Strait (Coachman and Aagaard, 1966; Stigebrandt, 1984). The redistribution of freshwater from the atmosphere to the ocean occurring in the Arctic region is part of what is called the Global Hydrological Cycle which transports freshwater between the atmosphere, land and oceans. Because the Arctic Ocean receives a large input of freshwater this means that the Arctic Ocean also exports freshwater to lower latitudes in the form of sea ice and low saline seawater.

Most of the water that enters the Arctic Ocean is of Atlantic origin and enters either via the deep Fram Strait or the shallow Barents Sea. Once the At- lantic water reaches the central deep basins the flow gets confined to a boundary current aligned with the isobaths circulating cyclonically around the basins before eventually exiting at the Fram Strait (e.g. Nøst and Isachsen, 2003; Ak- senov et al., 2011; Rudels et al., 2013). The water from the Pacific side, that constitutes an important source for the upper low saline waters, enters via the shallow and narrow Bering Strait and mainly occupies the Canada Basin before it primarily exits through the CAA. The cold and fresh surface waters strongly respond to the prevailing large-scale wind pattern which tends to be dominated by a high sea level pressure center over the Beaufort Sea (Overland,

14 2009). The exerted wind stress is transferred both directly to the ocean and the sea-ice, leading to an anti-cyclonic ocean circulation over the Amerasian side of the Arctic Ocean, see Figure 1.2. On the ﬂank towards the Eurasian Basin, roughly aligned with the Lomonosow Ridge, there is an across-basin transport called the Transpolar Drift Stream, transporting water from the eastern Eurasian shelves towards the Fram Strait and north of Greenland.

The inflowing Atlantic and Pacific waters, large-scale atmospheric circulation, river runoff and sea-ice melt and formation processes are all important components shaping the stratification of the Arctic Ocean. There is a charac- teristic stratification over large parts of the central Arctic Ocean where fresh and cold water lies on top of warm and more saline waters, see Figure 1.2. The vertical structure also owes its existence to the thermodynamic properties of seawater as the density is mainly determined by salinity rather then temperature at low temperatures. In fact, one can consider the Arctic Ocean as a part of an even larger region in the high latitude oceans (including parts of the North Pacific and North Atlantic oceans) that is salinity stratified (Carmack, 2007). A sharp halocline exists between the fresh and cold surface layer and the warm and saline Atlantic layer. Over the central Arctic Ocean the halocline mostly coincides with near-freezing temperatures and is hence called a "cold" halocline. The cold halocline is considered an important feature of the stratification because it permits heat from the Atlantic layer to penetrate upwards in the water column enabling the annual ice cover to exist (e.g. Steele and Boyd, 1998).

The water mass structure of the Arctic Ocean was considered stationary for a long time, however, during the recent decades changes in both the upper fresh layer (Polyakov et al., 2008; Rabe et al., 2014) and the warm Atlantic layer (Polyakov et al., 2012) have been observed. Modeling studies have also revealed a decadal variability of the freshwater content (Häkkinen and Proshutinsky, 2004; Karcher et al., 2005; Köberle and Gerdes, 2007; Karcher et al., 2011; Rabe et al., 2014) and the Atlantic water temperaures (Karcher et al., 2003, 2011). These studies showed that the Arctic freshwater content was anomalously low during 1970s and 1990s and anomalously high during the 1980s and the 2000s; and that since the 1980s two pulse of anomalously warm water, during the early 1990s and from 2000 and onwards, has been present in the inﬂowing Atlantic water.

One motivation for studying the Arctic Ocean freshwater balance has been its potential impact on the North Atlantic and even on the global ocean circulation. Changes in the export of low saline water from the Arctic have received

15 considerable attention due to its possible inﬂuence on the deep water formation in the Greenland Sea (e.g. Rennermalm et al., 2007). So called Great Salin- ity Anomalies of low saline water have also been observed to circulate around the North Atlantic Ocean in the recent decades (Dickson et al., 1988; Belkin et al., 1998). Arctic surface air temperatures has also been observed to increase (Polyakov et al., 2013) and in a warming climate the Global Hydrological Cy- cle is expected to intensify (Held and Soden, 2006). One of the most dramatic changes in the Arctic is seen in the decreasing end-of-summer sea-ice extent, which has been reduced by 12% per decade since 1979 (Stroeve et al., 2011). Since the export and formation of sea ice is one of the major components in the Arctic freshwater budget this might impact on the freshwater balance of the Arctic Ocean.

The common ground for the four papers that make up this thesis is that they all consider how the supply and extraction of freshwater can affect the Arctic Ocean. Particular interest is paid to how the warm and saline Atlantic water is transformed by interactions with different sources and sinks of freshwater, how this water spreads across the Arctic basins to maintain the present stratification, and how perturbations in the freshwater input might change the stratification. A substantial part of the analysis focuses on water mass transformation using a method where the advective and diffusive fluxes are described in terms of salinity and temperature. The four papers explore the Arctic Ocean using several different ocean general circulation models (OGCM) as well as a more simple conceptual model. These models are also discussed and compared in this thesis.

16 Bering Strait

Chukchi Sea Mackenzie East Sib. Sea

Lena Canada Basin Laptev Sea

Mendeleyev Ridge

CAA Makarov Basin

Alpha Ridge Lomonosow Ridge St Anna Trough Yenisei Amundsen Basin Kara Sea Nansen Basin Ob

Fram Strait Barents Sea

BSO

Greenland Sea

Norwegian Sea

Figure 1.2: Map of the Arctic region and vertical profiles for different sub-basins. - The upper panel shows a map over the Arctic region with the bathymetry from Jakobsson et al. (2008). The black lines show the 500 and 2000 m isobaths. The yellow paths show the Beaufort Gyre and the Trans- polar Drift Stream. The magenta and orange paths show the Fram Strait and Barents Sea branches, respectively, redrawn after Rudels et al. (2014). The bottom panel shows vertical profiles of potential temperature and salinity from the Nansen, Amundsen, Makarov and Canada basins. This figure is taken from Rudels (2009).

17 18 2. Modeling the Arctic Ocean

Despite a steady increase in observations over the Arctic Ocean as a result of advances in satellite remote sensing, increasing number of ship-based expedi- tions, and the development of autonomous observations platforms, the region is still very sparsely sampled. As a complement to long-term observations, various models have been used to provide an understanding of the key Arctic processes. Models also serve a purpose by providing versatile tools to test and possibly verify hypotheses in a way that observations can not provide.

There are different models with varying types complexity in their representation of the underlying physics that they include. The underlying physics are here the fundamental equations of motion, the conservation of mass, the conservation of internal energy in the fluid (usually through the conservative tracers potential temperature and salinity), and an equation of state relating the thermodynamic variables to the fluid density (Vallis, 2006). Due to the nonlinear nature of these equations all models need to resort to some kind of simplification. The papers presented here employ two types of models: greatly simplified models, which we call conceptual models; as well as more complex OGCMs. In what follows I will briefly discuss these two types of models trying to convey their weaknesses and strengths. The presentation is not meant to be exhaustive, rather just give an overview of the models used in the different papers.

2.1 Conceptual models

Here a conceptual model refers to a model where only the assumed key processes of a system are included. For the Arctic Ocean such a model was introduced by Stigebrandt (1981) and has also been explored in e.g. Nilsson and Walin (2010), Jakobsson et al. (2010) and Rudels (2010). In these models the Arctic Ocean is formulated as a horizontally homogenous two-layer system that is in a steady-state. The stratiﬁcation is represented by a fresh and cold upper layer and a warm and saline lower layer that respectively represents the Atlantic water and the deeper water masses, see Fig. 2.1. In paper IV we for- mulate a model based on these assumptions and below I will present some key

19 results.

First we let the steady-state conservation of volume and salt be given by

M = MA + MB + F0, (2.1) and S1M = SAMA + SBMB, (2.2) where M is the net outflow from the upper layer, MA and MB inflow of Atlantic and Pacific waters, respectively; and F0 the net freshwater input minus sea-ice production; S1, SA and SB the salinity of the upper layer, inflowing Atlantic water, and inflowing Pacific water, respectively.

Following Stigebrandt (1981) we assume that the outﬂow occurs in a number of straits which are wide enough to allow for a rotational control of the ﬂow to hold yielding a baroclinic current in geostrophic balance

M = µgβ∆SH2/(2 f ). (2.3)

Here β is the haline expansion coefﬁcient, f the Coriolis parameter, g the gravitational acceleration, ∆S = SA − S1 the salinity difference between the upper and lower layer, and µ an adjustable parameter to account for the number of straits exporting with full capacity. It is also assumed that the reduced grav- ity can be expressed as g0 ≈ β∆Sg, ignoring the buoyancy difference due to the temperature contrast between the upper and lower layer. Using this and assuming that the freshwater height is HF = H ∆S/SA, we can obtain

2 f F 1/2 HF = , (2.4) µgβSA where we have deﬁned the effective freshwater forcing F = F0 +MB(1−SB/SA).

The residence time of the freshwater layer is τF = HF A/F, where A is the surface area of the Arctic Ocean. Using Eq. (2.4) we get

/ 2 f A2 1 2 τF = . (2.5) µgβSAF Using realistic values for the Arctic Ocean and assuming that the outﬂow occurs in three straits Rudels (2010) showed that HF = 8.25 m, or 10.1 m if there is no ice export. Using the same values for the residence time one gets τF = 8 yr, or 7 yr if there is no ice export. In addition, Equations (2.4) and (2.5)

20 1/2 −1/2 show that HF ∝ F and τF ∝ F and that they in this case are completely independent of the details related to the mixing between the upper layer and the more saline Atlantic water.

However, to further explore how both ∆S and H vary with changing F we need to consider the properties that control the upwelling through the stratiﬁ- cation from the lower to the upper layer. In paper IV we qualitatively analyze and discuss this for a number of different cases and conclude that the most likely situation is that an increasing F leads to a decreasing upper layer thickness H.

The aim of the conceptual model is to retain the most important processes while keeping the model simple. The model assumes that the outflow is purely baroclinic which is not the case in the real Arctic Ocean. Barotropic currents can also exist in the outflow gateways. Barotropic currents can for example be induced by local wind patterns changing the pressure distribution upstream of the gateways. In addition, the boundary current of Atlantic water also has a barotropic component that presumably affects the outflow (Nøst and Isach- sen, 2003). However, the low saline upper layer is relatively thin and if the barotropic currents are relatively weak the assumption of a baroclinically con- trolled outflow should hold (Rudels, 2010). The conceptual model also assumes a horizontally homogenous Arctic Ocean, however, there are salinity gradients upstreams of the different gateways that drain the Arctic Ocean; gradients that might change in a complex way that is obviously not captured by the conceptual model. In paper IV we explore how the results from the conceptual model compare with a more complex OGCM. This will be discussed further in chapter 3.3.

2.2 Ocean general circulation models

With the development of computational resources new opportunities arose to solve the fundamental ocean equations. In the late 1960s the ﬁrst OGCM was introduced by Bryan (1969) and ocean modeling was born.

Usually an OGCM is a three-dimensional model described by the ocean primitive equations1, e.g.:

1 The primitive equations (e.g. Müller, 2006) are derived from the Navier-Stokes equations using the Boussinesq, the shallow water, the traditional and the hydrostatic approximations.

21 F0

MB SB M H S1

M SA A

Figure 2.1: Schematic of a conceptual model of the Arctic Ocean - The ﬁgure is taken from paper IV.

Du 1 ∂ p = f v − + DM + FM, (2.6) Dt ρ0 ∂x Dv 1 ∂ p = − f u − + DM + FM, (2.7) Dt ρ0 ∂y ∂ p = −ρ g, (2.8) ∂z

∇ · u = 0, (2.9)

DT = DT + FT , (2.10) Dt DS = DS + FS, (2.11) Dt

ρ = ρ(Ti,S, p), (2.12) D ∂ = + u · ∇, (2.13) Dt ∂t which are the horizontal momentum balances, the hydrostatic balance, the in- compressible continuity equation, the heat and salt conservation equations, equation of state and total derivative, respectively. Here u = (u,v,w) is the ﬂuid velocity, p the pressure, T the potential temperature, Ti the in situ temperature, S the salinity, g the gravitational acceleration and ρ the seawater

22 density. DM, DT and DS are the parameterizations of sub-grid scale processes for momentum, temperature and salinity, and FM, FT and FS are surface forcing terms. Equations (2.6)–(2.11) subject to suitable boundary conditions are solved numerically and integrated forward in time to yield a model solution. Usually the OGCM is coupled to an sea-ice model that solves the dynamics and thermodynamics of the ice cover.

The numerical discretization of Equations (2.6)–(2.11) involves numerous choices of how to represent the continuous operators and particularly the vertical discretization leads to different model formulations. The physical processes in the ocean occur on length scales ranging from a few mm (molecu- lar diffusion) to thousands of kms (basin-scale circulation), and on time scales ranging from seconds to thousands of years. With limited computational power all these scales can obviously not be resolved, even with todays high-performance computing facilities. The processes occurring on scales below the computational grid size are thus considered sub-grid processes and parametrized. From a dynamical point of view an important length scale is the ﬁrst baroclinic Rossby radius of deformation. This is a natural scale in the ocean often associated with boundary currents, fronts and eddies (Gill, 1982). An ocean model can be considered eddy-resolving if it has at least 4 − 5 grid points within a Rossby radius (Osinski et al., 2010). In the Arctic Ocean the ﬁrst baroclinic Rossby radius is ∼ 10 km and most present models, which usually have a horizontal resolution coarser than 10 km, are thus not eddy-resolving.

In this thesis several different OGCMs have been used to study the Arctic Ocean. Papers II–IV each use a different OGCM, namely the Rossby Cen- tre Ocean (RCO) model (Meier et al., 2003), the Nucleus for European Mod- elling of the Ocean (NEMO) (Madec, 2008), and the Massachussetts Institute of Technology general circulation model (MITgcm) (Marshall et al., 1997), in respective order. RCO and MITgcm were both run in a regional conﬁguration with a horizontal resolution of 28 and 18 km, respectively; while NEMO was run in the ORCA1 global conﬁguration with a local horizontal resolution of 30 − 60 km over the Arctic region. Furthermore, in paper I the Arctic Ocean freshwater budget in 10 different OGCMs are compared model-to-model as well as with observations.

2.3 Model challenges

An OGCM adds complexity to the representation of ocean circulation with the aim of ﬁnding a more realistic model solution. However, many of the parametrized components of the OGCM have free parameters, parameters that

23 cannot be constrained by e.g. observations, and some level of tuning is performed to yield the best solution. Another issue when coupling many complex processes together is that an error in one model component can be compensated by an error in another component. These shortcomings need to be considered when evaluating model results. This makes the results model dependent. Usu- ally the accuracy of the model solution is veriﬁed by comparing with observations and methods to combine models and observation to an optimized solution have been developed with data assimilation.

To improve our models and to understand their weaknesses model intercomparison can be instructive. Physical processes that are robustly represented across the models warrant more credibility in the simulated results. This approach has been undertaken in the Arctic Ocean Model Intercompari- son Project (AOMIP) where different groups from the Arctic Ocean modeling community participate in a number of coordinated experiments (Proshutinsky et al., 2011).

In paper I one such coordinated effort is presented where we compared the freshwater budget in 10 different state-of-the-art OGCMs. The participating models differed in both resolution, computational domain and forcing. Some models were fully global conﬁgurations while others were only regional con- ﬁgurations simulating only the Arctic Ocean and the Nordic Seas.

In paper I we found that although the models generally agree on the main sources and sinks in the Arctic Ocean freshwater budget there are considerable differences, particularly in the time-mean export of low saline water through the Fram Strait. The time-mean sea-ice export through the Fram Strait and the export of low saline water through the CAA, on the other hand, showed better agreement among the models. By decomposing the freshwater ﬂux into mean components h i and anomalies 0 as (e.g Jahn et al., 2009),

0 0 0 0 FFW = hCihvi + hCiv +C hvi +C v , (2.14) we studied the contributions from the mean flux hCihvi as well as volume 0 0 anomalies (hCiv ) and salinity anomalies (C hvi) to the total export FFW . Here C = (Sre f − S)/Sre f is the normalized salinity deviation from the reference salinity Sre f (the reference salinity approach is explained in chapter 3.1) and v the cross-sectional transport. Besides the differing mean fluxes the anomalies also show model-to-model differences. For the CAA the flux variability was dominated by the volume anomalies while the Fram Strait flux variability generally was determined by the salinity anomalies. In the study we con- cluded that it is mainly the differing upstream salinity fields that lead to the

24 model-to-model scatter in the Fram Strait freshwater export. The large variations in the upper ocean salinity field are also reflected in the freshwater height (see Eq. (3.1) how it is defined) shown in Figure 2.2. Note that the version of the RCO model used in paper II, which was somewhat of an outlier in paper I, had undergone several improvements (including an updated vertical mixing scheme, changed surface salinity boundary conditions and tuning of background mixing values).

In previous studies as well as papers II and III a number of potential issues that might affect the evolution of the salinity field has been identified. They are all related to the surface boundary condition of freshwater and salt in the models. One issue is that many models still use a virtual salt flux to represent the distillation/dilution effect of the atmosphere-ocean freshwater flux. However, the physically correct way, which yields both volume and mass conservation, is to represent this by a freshwater flux (Huang, 1993; Griffies et al., 2001; Prange and Gerdes, 2006). The virtual salt flux approach is a legacy from the rigid lid models which did not include a free surface. For the Arctic Ocean this can result in an excessive freshening on the shallow shelves where freshwater input is high (Yin et al., 2010). A related issue is that the coupling between the ocean model and ice model does not allow the sea ice model to exert a pressure on the water column (Huang and Jin, 2007). Consequently, the phase change between liquid and solid seawater is approximated by a salt flux rather than a salt and volume flux (Tartinville et al., 2001). Another issue is that many models use restoring of the sea surface salinity field to a background value (usually taken to be a climatological value) to account for uncertainties in the forcing, missing model physics and to limit model drift. This can introduce spurious salinity anomalies and lead to a shift in the seasonal cycle (Newton et al., 2008).

25 Figure 2.2: Freshwater model intercomparison - The ﬁgure shows the freshwater height over the Arctic Ocean from 10 different models participating in the AOMIP project as well as observations (top row). The color scale is in m. The ﬁgure is taken from paper I.

26 3. The role of freshwater in the Arctic Ocean

3.1 What is fresh, and what is not?

Seawater is a mixture of pure freshwater and dissolved salts. As the mass fraction between the salts is considered constant throughout the world oceans one can simply use their total fraction (multiplied by 1000) in a water parcel and denote it salinity1 in gkg−1. With this absolute deﬁnition the volumetric amount of freshwater in a parcel of seawater with salinity S is V(1 − S/1000) where V is the volume of the water parcel. However, it is much more common to consider the amount of freshwater relative to a reference salinity Sre f . With this relative deﬁnition the fraction of freshwater in a water parcel is (Sre f − S)/Sre f . The relative freshwater approach is used in this thesis.

Using the relative deﬁnition of freshwater we can calculate a freshwater inventory as, Z HFW = (Sre f − S)/Sre f dz, (3.1) z0 which yields the height of freshwater in the water column over the upper z0 me- ters. Typically one integrates over the upper 200–300 m where most freshwater resides or down to a given isohaline. Figure 3.1 shows the Arctic Ocean freshwater height calculated from a climatological data set. As seen most freshwater is stored in the Canada Basin and a sharp gradient towards the Eurasian basins is also seen. The accumulation of freshwater in the Canada Basin is largely a result of the large-scale atmospheric circulation and resulting Ekman transport (Proshutinsky et al., 2009). The accumulation over the Canada Basin due to changes in the wind forcing does not necessarily lead to an increase in the total amount of freshwater in the Arctic Ocean, it can also be a redistribution of freshwater. To assess the total storage of freshwater (which we do in paper II) we thus calculate a total

1For more practical purposes when salinity has to be measured in seawater more care has to be taken. To this end the Intergovernmental Oceanographic Commission has deﬁned the quantity absolute salinity (IOC, 2010).

27 volume VFW as, Z Z VFW = (Sre f − S)/Sre f dzdA, (3.2) A z0 where A is the total area of the Arctic Ocean. Then we can form a budget framework as (e.g. Serreze et al., 2006),

∂V Z Z Z FW = (P − E + R) dA − M dl dz, (3.3) ∂t A l z0 where P−E is the net precipitation, R is the river runoff, and M the freshwater transport across the lateral boundaries l. Usually we also include the freshwater stored as sea ice in the term on the left hand side as well as the export of sea ice in the M term on the right hand side. If the Arctic is in steady-state then the time-changes in storage ∂VFW /∂t ≈ 0 and changes in the net sources and sinks will be in balance.

Several studies have been undertaken to calculate such freshwater budgets for the Arctic Ocean (Aagaard and Carmack, 1989; Serreze et al., 2006; Dick- son et al., 2007). In Figure 3.2 the latest estimate, using a Sre f = 34.8 is shown. As seen the major sources are river runoff, net precipitation and the inflow of Pacific water; and the major sinks are the export of through the CAA and the Fram Strait. At the Fram Strait the export is almost equally divided between liquid freshwater and sea-ice export while at the CAA the export is dominated by the liquid freshwater outflow.

It should be noted that the use of a reference salinity, which is an arbi- trary parameter, can be problematic when calculating and comparing different freshwater budgets, especially when different reference values are used. A commonly used value is Sre f = 34.8, which dates back to Aagaard and Car- mack (1989), and is considered the volume-mean salinity of the Arctic Ocean. Dickson et al. (e.g. 2007), however, used both Sre f = 34.8 and Sre f = 35.2 in their budget, where the latter represents the salinity of the Atlantic water entering the Nordic Sea and the Arctic Ocean. The latter reference value has the advantage that the freshwater ﬂuxes and volume ﬂuxes are in the same direc- tion, which is not necessarily the case when using Sre f = 34.8. In addition, Tsubouchi et al. (2012) showed that in order to retain mass conservation in the freshwater budget calculation one should use the boundary-average salinity rather than the volume-averaged salinity. However, they also point out that using e.g. Sre f = 34.8 only leads to a 1 − 2% error.

28 Freshwater height [m]

Figure 3.1: Arctic Ocean freshwater height - The ﬁgure shows the freshwater height integrated over the upper 250 m using data from the Polar Science Center Hydrographic Climatology 3.0 (PHC) (Steele et al., 2001). The ﬁgure is taken from paper II.

3.2 The Arctic Ocean freshwater distribution and governing processes

Clearly the processes governing the freshwater distribution and the stratiﬁca- tion in the Arctic Ocean are important in a global perspective as well as for the Arctic region itself. To further investigate these processes numerous studies have been undertaken which I will brieﬂy review below before discussing some results from paper II.

A lot of research has been carried out on how the shifts in the large-scale atmospheric circulation, typically alternating between cyclonic and anticyclonic modes, affect the redistribution of freshwater (Hunkins and Whitehead, 1992; Proshutinsky and Johnson, 1997; Proshutinsky et al., 2009). These studies have pointed out that in the anti-cyclonic mode low saline surface waters tend to be accumulated (via Ekman transport) in the Beaufort Gyre from the pe- riphery. While in the cyclonic mode the low saline mode is released from the Beaufort Gyre and transported towards the CAA and the Fram Strait. This suggests that the freshwater content might not be in balance on timescales that theses changes occur (they are roughly decadal). Other studies (Newton et al., 2006; Dmitrenko et al., 2008) have shown how shifts in atmospheric circu-

29 Figure 3.2: Arctic Ocean freshwater budget - The ﬁgure shows the latest assessment of Arctic Ocean freshwater budget based on available observations compiled by Serreze et al. (2006).

30 lation control the shelf-basin exchange of low saline waters from the Siberian shelf seas. Where the shelf water tends to stay on the shelf and travel eastwards (towards the East Siberian Sea) when the atmospheric vorticity is more cyclonical over the shelf, and migrate northwards off the Laptev Sea shelf break when the atmospheric vorticity is more anti-cyclonical. In addition, Morison et al. (2012) showed that the cyclonic mode can lead to an accumulation of low saline Siberian shelf water in the Canada Basin.

The research has also been focused towards trying to explain how the cold halocline is formed, what the dominating sources of freshwater are, and at what rate it is renewed. Coachman and Barnes (1962) suggested that upwelling of warm and saline Atlantic water on the shallow shelves could be cooled and freshened before reentering the central basins. Aagaard et al. (1981) proposed that the formation of dense water from the low saline water on the shallow shelves could be a source of halocline water. Thereby, important processes might be ice formation, brine release and possibly entrainment with ambient water. Based on a halocline thickness of 100 m, covering an area of 8×106 km2 and being exchanged within 10 years they suggested a total renewal rate of 2.5 Sv (1 Sv = 106 m3 s−1). Many subsequent studies have tried to estimate the rate at which halocline water can form on the shelf using both models and satellite observations (Björk, 1989; Martin and Cavalieri, 1989; Cavalieri and Martin, 1994; Winsor and Björk, 2000; Nguyen et al., 2012). These estimates are in a wide range 0.2–1.5 Sv albeit not as much as the estimated total rate by Aagaard et al. (1981), suggesting it is either an overestimation or that another process than modified shelf water may form halocline water. Based on observations in the Barents Sea Steele et al. (1995) showed that the halocline waters could also form from a combination of sea-ice melting, heat loss and mixing when the warm Atlantic water meets the ice edge. They found that this process could produce an additional 0.4–0.6 Sv. Rudels et al. (1996, 2004) pointed out that when the warm Atlantic water enters the Nansen Basin or the Barents Sea the subsequent cycle of summertime melting and wintertime convection (due to heat loss, ice formation and brine release) transforms the upper layer to near- freezing temperatures and salinities in the range of the cold halocline. As this water mass travels cyclonically around the central basins it would be capped by low saline shelf water further downstream and produce the lower halocline in the Arctic Ocean. In the Canada Basin a large part of the upper halocline consists of modified Pacific water (e.g. Steele et al., 2004).

To better understand what source waters form the upper fresh and cold layer observational efforts measuring a number of approximately conservative geochemical tracers have been undertaken (e.g Östlund and Hut, 1984;

31 Schlosser et al., 1994; Bauch et al., 1995; Jones et al., 2008). Using observations of e.g. salinity, δ 18O, alkalinity and nutrient concentrations the relative fractions of Paciﬁc, Atlantic, net sea-ice melt, and river runoff waters can be deduced in a water sample (e.g. Jones et al., 2008).

As the distribution of Arctic-wide observations have been sparse both in time and space these studies have been complemented by modeling studies using passive tracers to mark one or several sources of freshwater. These studies have investigated the role of both river runoff and/or Paciﬁc water in the Arctic Ocean, primarily focusing on how the pathways are affected by the large-scale atmospheric circulation (Harms et al., 2000; Maslowski et al., 2000; Karcher and Oberhuber, 2002; Prange, 2003; Newton et al., 2008; Condron et al., 2009; Jahn et al., 2010).

To complement previous studies, we use, in paper II passive tracers to mark the major sources and sinks of (relative) freshwater in the RCO model. We not only mark river runoff and Pacific water but also sea-ice melt and formation, precipitation, evaporation and inflows of Atlantic water to get a more complete description of the relative role of the sources and sinks. Using the tracers we show how the composition of the upper low saline water depends on the different freshwater sources and sinks in different sub-regions, see Fig- ure 3.3. Over the whole Arctic region we find that the freshwater from the Eurasian rivers dominates the inventory. The most dominant negative contribution is from tracers representing net sea-ice formation. The composition varies, as expected, with Pacific water becoming increasingly important towards the Canada Basin and the Beaufort Gyre region. However, the tracer composition also suggests that the Canada Basin contains a significant amount of Eurasian river runoff. This is also in line with the estimate based on geochemical tracers by Yamamoto-Kawai et al. (2008).

In paper II we also use the passive tracers to discuss how the low saline waters spread from source regions to the central Arctic Ocean and where the tracers are exported out of the Arctic Ocean. Similar to previous studies we find that the large-scale atmospheric circulation is a major factor controlling the pathways. We also find a coherent pattern of Eurasian runoff and net sea- ice formation tracers stretching across the Arctic from the eastern Siberian shelves towards the Fram Strait, see Figure 3.4. Their exports through the Fram Strait are also strongly anti-correlated, in agreement with observations by Dodd et al. (2012). Based on the tracers we discuss how the inflowing Pacific and Atlantic water are transformed by different processes before exiting to the deep central basins. Using the Walin (1977) salinity coordinate framework

32 Tracer/FW volume [103 km3]

Figure 3.3: Composition of Arctic Ocean freshwater content - The ﬁg- ure shows the time-mean composition of freshwater storage over different sub- regions of the Arctic Ocean. The composition is based on model results from the RCO model and the ﬁgure is taken from paper II.

(which is further discussed in chapter 4.2) we also estimate a halocline renewal rate of 1 Sv in the RCO model. The simulated halocline renewal is equally divided between the processes leading to a shelf-basin exchange and processes mixing low saline and Atlantic water in the central Arctic Ocean. Based on the passive tracers we ﬁnd that the model’s halocline shelf-basin exchange is dominated by runoff and sea-ice processes at the western shelves (the Barents and Kara seas) and by Paciﬁc water at the eastern shelves (the Laptev, East Siberian and Chukchi seas).

3.3 Freshwater sensitivity

Long-term observational records show variability in the Arctic Ocean freshwater storage (Polyakov et al., 2008; Rabe et al., 2014) and increasing Atlantic water temperature has been observed during the last decade (Polyakov et al., 2012). In addition, climate projections suggest an intensiﬁcation in the Arctic freshwater cycle with increasing precipitation and river runoff (Holland et al., 2007; Kattsov et al., 2007) and on much longer time scales paleo-records in- dicate shifts in the structure of the Arctic Ocean stratiﬁcation (Cronin et al., 2012; Jakobsson et al., 2014).

Motivated by these observationally inferred changes we explore the sensitivity of the central Arctic stratiﬁcation to variations in the freshwater supply,

33 Western Eurasian runoff [m] Eastern Eurasian runoff [m]

a) b)

Net sea-ice melt [m] Pacific inflow [m]

c) d)

Figure 3.4: Pathways of freshwater sources and sinks - The ﬁgure shows how different sources and sinks of freshwater spread across the Arctic Ocean. The results are based on passive tracer simulation with the RCO model and the ﬁgure is taken from paper II.

34 in paper IV. A key results from the conceptual model in chapter 2.1 (and earlier work by Jakobsson et al. (2010)) is that a reduced freshwater input yields a deeper halocline and a depressed Atlantic water layer and vice versa for increased freshwater input. To explore whether the hypotheses is robust in a more realistic situation we perform a number of idealistic sensitivity experiments with the MITgcm. In the experiments we both decrease and increase the freshwater input from rivers and precipitation over the Arctic region.

In paper IV we ﬁnd that upper ocean response of the MITgcm model, see Figure 3.5, largely agrees with the conceptual model. With decreasing (increasing) freshwater input the upper layer depth scale (H in the ﬁgure) increase (decrease) and the freshwater height HF decrease (increase). However, a strong response is also found in the potential temperature of the Atlantic layer, particularly when the precipitation is changed. We link this to changes in upper ocean stability in the convectively active region of the Barents Sea where the precipitation perturbations lead to changes in the ice cover, heat loss and eventually the downstream temperature of the water entering the central Arctic Ocean.

35 a

(Rudels, 2010)

Figure 3.5: Arctic Ocean freshwater sensitivity - The figure shows the response of the Arctic Ocean stratification to freshwater perturbations. The upper left panel shows the basin-averaged vertical salinity profile, the lower left panel the potential temperature profile, and lower right panel the basin-averaged freshwater height HF , salinity difference ∆S between the surface and upper Atlantic layer, and the depth scale H for experiments with varying freshwater input (B2 has the lowest and C2 the highest input). The dashed line is the freshwater height predicted by the conceptual model. This figure is taken from paper IV.

36 4. Water mass transformation in the Arctic Ocean

In oceanography water mass transformation is considered to be a process where some properties of a water parcel are changed. Typically we consider how the conservative tracers potential temperature and/or salinity change due to some physical process acting on the water parcel. Depending on the nature of the transformation the density of the water parcel might change, which is of particular interest as density changes ultimately affect the ocean dynamics. For a water mass transformation to occur there must be a transport across a prop- erty surface (isohaline, isothermal or isopycnal). This can be accomplished by surface and internal diffusive ﬂuxes. A useful way to study water mass transformations is to apply the Walin (1977, 1982) frameworks where ﬂows across isothermal or isohaline surfaces are considered. In the following I will shortly explain the key concepts of these frameworks and summarize some key results where the Walin formalism has been used to study water mass transformation in the Arctic Ocean.

4.1 Walin formalism

In two seminal studies Walin (1977, 1982) showed how water mass transformation could be described from the surface and internal ﬂuxes in either potential temperature or salinity coordinates. Walin’s ideas have since been used in a number of studies e.g. to study water mass transformation between density classes (Tziperman, 1986; Speer and Tziperman, 1992; Iudicone et al., 2008), to study transformations due to wind-induced near-surface mixing (Nilsson, 1996), to study how the seasonality affect water mass transformations (Mar- shall et al., 1997, 1999), and to study the global ocean circulation in S-T space (Hieronymus et al., 2014).

In the following the Walin (1977) salinity framework, which is used in paper II (and to some extent in paper III), is brieﬂy reviewed.

37 Following Walin (1977) the conservation of a volume1 V(S) of water, spanning a region in the ocean where salinity is less than S, could be expressed in terms of: the advective flux GS(S) across an isohaline surface, the advective flux M(S) across an internal control surface, and the freshwater flux E(S) across the sea surface, see Figure 4.1 for how the different variables are defined. From this the volume conservation can be expressed as follows ∂V(S,t) = −G (S,t) − M(S,t) − E(S,t). (4.1) ∂t S Using Equation (4.1) and the conservation of salt the flux across an isohaline can be expressed as, ∂E ∂F G = S − S , (4.2) S ∂S ∂S where FS is the diffusive flux across an isohaline surface. Equation (4.2) shows that the advective flux GS(S) across an isohaline surface, which is a water mass transformation from one salinity class to another, occurs as a result of freshening or salinification by surface and interior diffusive fluxes. If we are in a steady-state and there are no internal boundaries (no M(S) flux) then the surface and interior fluxes should balance and coun- teract each other. If on the other hand, there is an internal boundary (which is the case in this application for the Arctic Ocean) we instead have the balance M(S) = −GS(S). In both these situations we have assumed that the freshwater exchange E(S) is relatively small.

E(S,t) A(S,T,t)

M(S,t) V(S,t) B(S,t)

FS IS(S,t)

Figure 4.1: The Walin S coordinate framework - The figure shows how the volume, boundary surfaces and associated fluxes are defined based on the Walin (1977) framework, which is used in paper II.

In a recent study Hieronymus et al. (2014) extended the frameworks by Walin (1977, 1982) and considered both potential temperature and salinity as 1Note that in Walin (1977) a volume deﬁned by salinity greater than S was considered and no surface freshwater ﬂux E(S) was included in the derivation.

38 coordinates and introduced a S-T framework. In this framework fluxes across both isohaline and isothermal surfaces are included. Based on earlier work by Speer (1993) they also defined a water mass transformation vector ∂G ∂G J = S , T , (4.3) ∂T ∂S that describes the transformation of water from one potential temperature and salinity class to another in terms of the advective fluxes GS/GT , see Figure 4.3 for an example. Here 1 ∂Q 1 ∂F G = − − T , (4.4) T c ∂T c ∂T Q the surface heat flux, FT the diffusive flux across an isothermal surface, and c the heat capacity of seawater. GT is, similar to GS, the advective flux across an isothermal surface occurring as a result of heating or cooling by surface and interior diffusive fluxes.

Hieronymus et al. (2014) also formulated a continuity equation for a water volume v(S,T)dSdT spanning a region where salinity is in the range S to S + dS and potential temperature in the range T to T + dT, ∂v ∂ 2M(S,T) ∂ 2E(S,T) = −∇ · J − − , (4.5) ∂t ∂S∂T ∂S∂T where ∂ 2V(S,T) ∂ ∂ v(s,T) ≡ ; ∇ ≡ , . (4.6) ∂S∂T ∂S ∂T Assuming that the freshwater exchange is negligible in Equation (4.5) and we are in a steady-state, then we have that ∇ · J = 0 for the global ocean as there are no internal boundaries and ∂ 2M(S,T)/∂S∂T = 0. For the Arctic Ocean, on the other hand, we have internal boundaries and need to consider the ∂ 2M(S,T)/∂S∂T. In fact, the boundary term by itself reveals a lot of information about the transformations occurring in the Arctic Ocean.

4.2 Transformations in the Arctic Ocean

For the Arctic Ocean it is of particular interest to study how the warm and saline Atlantic water and the warm and fresh Paciﬁc water is transformed in the Arctic region. Several such studies have been undertaken using salinity, temperature and geochemical tracer observation across the Arctic Ocean as well as models to distinguish between different water masses (e.g. identifying Polar mixed layer water, Upper and Lower halocline water, Atlantic water, In- termediate water, deep and bottom water; Rudels, 2009; Aksenov et al., 2010).

39 In this thesis, however, the Walin frameworks are explored to study water mass transformation in the Arctic Ocean. Below a short summary is given, high- lighting the main ﬁndings where we used the Walin frameworks to study the Arctic Ocean water mass transformation.

In paper II we calculate the volume exchange M(S) between the central Arctic Ocean and the surrounding seas to estimate how much water that is transformed and intruded into the Arctic Ocean halocline based on a simulation with the RCO model. Assuming that the model is in a pseudo steady-state we use that M(S) = −GS(S) and calculate the exchange between different salinity classes. From this we find a total halocline water production of 1 Sv where 0.5 Sv is produced on the large Siberian shelf seas and 0.5 Sv by internal processes mixing low saline and Atlantic water into the halocline in the central Arctic Ocean, see Fig. 4.2. In addition, we also find that the M(S) reveals the estuarine character of the Arctic Ocean showing how the saline Atlantic water is transformed by the low saline shelf water and Pacific water into water classes of intermediate salinities.

In paper III we further investigate the balance between the GS and M(S) in the salinity coordinate framework for the whole Arctic region. We ﬁnd that there are two regimes in the Arctic Ocean: (i) for low salinities (typically on the shelves) there is no M(S) exchange term and the main balance is between the surface and interior diffusive ﬂuxes in Equation (4.2); (ii) for high salinities the main balance is between the exchange M(S) and interior diffusive ∂F/∂S terms; see Fig. 4.2.

In paper III the approach is somewhat broader when considering the water mass transformation occurring in the Arctic Ocean. Here we apply the S − T framework of Hieronymus et al. (2014) to study how different parametrized surface and interior processes (in the OGCM NEMO ORCA1) contribute to transform the inflowing Pacific and Atlantic waters. The analysis by Hierony- mus et al. (2014) is adapted to the Arctic Ocean by including the ∂ 2M/∂T∂S term in Equation (4.5). The analysis of the ∂ 2M/∂T∂S term together with the J clearly reveals the role of the Arctic Ocean where the inflowing water is cooled and freshened before exiting back to the North Atlantic, see Fig. 4.3.

To further investigate how different processes contribute to the net cooling and freshening we make a decomposition of Equation (4.3) into the most important parametrized processes in the model. From this we ﬁnd that, in regions away from the freezing line, where most of the water volume resides, it is primarily the surface heat ﬂuxes that transform towards colder tempera-

40 Water mass transformation [Sv]

0.9 S < 31 0.1 1.0 Shelf 0.5 Water 1.0 31 < S < 34.3

Atlantic 3.7 S > 34.3 0.4 Water 4.1

E E

∂E S ∂S M

∂F - S ∂S ∂F - S ∂S

Figure 4.2: Arctic Ocean water mass transformation in S coordinate - The upper panel illustrates how the inflowing waters are transformed towards the out- flowing (surface and halocline) waters as in the RCO model. This figure is taken from paper II. The lower panel illustrates the main balance of Eq. (4.1) for the two limiting cases in the NEMO model, from paper III.

ture while surface freshwater fluxes play a minor role in transforming towards lower salinity. Instead, it is the down-gradient mixing of salt in the ocean interior that contributes most to freshen the inflowing Atlantic water. Near the freezing line, where most surface water resides, it is the seasonal melt and freeze cycle that influences the transformation pattern.

41 Figure 4.3: Arctic Ocean water mass transformation in S-T coordinates - The figure shows how the inflowing waters are transformed towards the outflow- ing water classes in a global general circulation model. The figure is taken from paper III.

42 5. Outlook

The Arctic Ocean is still an area of very active research and many interesting questions still remain to be explored. Below I have identiﬁed some interesting topics related to freshwater processes, the Arctic stratiﬁcation and water mass transformation that I think deserve further research.

Recent research on the Arctic Ocean freshwater balance has emphasized the role of the large-scale wind forcing and its variability for the distribution and ﬂow of the low salinity water (Proshutinsky et al., 2009; Giles et al., 2012; Rabe et al., 2013). In addition, observations and models largely agree on the pathways by which the upper low saline water spreads across the Arctic Ocean. A large part of the Arctic Ocean research has also been on the mean pathways and temporal changes of the inﬂowing warm Atlantic water, with particular focus on the Atlantic core boundary current. An interesting question is, if and how the low salinity near-surface water and Atlantic water circulations are coupled? In fact, analysis of long-term observations show a concurrent increase in both the freshwater content and the Atlantic water temperature, particularly for the last decade (Polyakov et al., 2013). The results of paper IV suggest that changes in precipitation lead to changes of, not only the features of the near-surface low salinity layer but also those of the subsurface Atlantic Water layer.

Furthermore, based on theoretical ideas by Yang (2005), suggesting that the cyclonic circulation of the Atlantic water in the Arctic Ocean is driven by lateral potential vorticity ﬂuxes, some modeling-based studies (Karcher et al., 2007; Aksenov et al., 2011) have proposed that the Atlantic branch entering through Barents Sea branch serves as an important source of potential vorticity. Here, the seasonal surface buoyancy ﬂuxes create Atlantic water with high potential vorticity which enters the central Arctic Ocean through the St. Anna Trough. Local changes in freshwater might thus be coupled to changes in the Atlantic water circulation. Another theory, however, based on an integral mass (or vorticity) balance over volume enclosed by an f /H contour1, suggests that the integrated wind forcing along the f /H contours that close within the Arc-

1Here f is the Coriolis parameter and H the depth.

43 tic Ocean is the chief driver of the cyclonic Atlantic water circulation (Nøst and Isachsen, 2003; Nost et al., 2008). It should be noted that there is no fully developed theory of the Arctic Atlantic water circulation that allows the driv- ing roles of local lateral eddy vorticity fluxes and global wind forcing to be quantified in an easy way. A further challenge is that many OGCMs still have difficulties in producing a cyclonic Arctic Atlantic water circulation (Holloway et al., 2007)

With increasing computational resources the model development is going towards higher horizontal grid resolution and some models are close to, or beyond the limit of being eddy-resolving (Maslowski et al., 2008; Nguyen et al., 2012; Spall, 2013). Compared to, for example, the Southern Ocean (e.g. Nikurashin and Vallis, 2012) the role of eddies is still poorly understood in the Arctic. What is e.g. the role of eddy transport in shaping the freshwater distribution? Interesting recent work by Spall (2013), using an eddy-resolving idealistic conﬁguration of the MITgcm and an analytical model, has developed ideas of how the upper fresh layer, halocline layer and Atlantic layer are coupled via lateral eddy advection and vertical diffusion of heat and salt. This work directly relates to the previous topic of how the freshwater and Atlantic water circulation changes are coupled.

The S − T framework is an interesting new way to analyze model results that deserves to be further explored. Using, for example, a high-resolution OGCM and focusing on a smaller sub-region of the Arctic Ocean such a study could reveal further details on the governing processes. The S − T framework might also serve as a tool for model intercomparison.

44 Sammanfattning

Denna avhandling behandlar färskvattensprocesser och transformation av vat- tenmassor i det Arktiska havet. Kunskap om dessa processer är av stor bety- delse både ur ett lokalt och globalt perspektiv. Globalt därför att exporten av lågsalint vatten och havsis potentiellt kan påverka cirkulationen i Nordatlanten och den globala havscirkulationen. Lokalt därför att färskvattensprocesser på- verkar vertikala skiktningen vilket ger gynnsamma förhållanden för havsisen. De huvudsakliga verktygen för studierna som utgör den här avhandlingen är modeller av olika komplexitet. En del av avhandling betraktar hur modeller används för att studera viktiga processer som kontrollerar färskvattensbalan- sen. Vidare så jämförs färskvattensbudgeten i 10 olika havscirkulationsmodeller där robusta egenskaper och svagheter modeller emellan identifieras. En stor del tillägnas att studera hur färskvattensprocesser påverkar skiktningen med speciellt fokus på det övre lågsalina lagret. Samverkan mellan käl- lor och sänkor av färskvatten undersöks i en havscirkulationsmodell med hjälp av passiva spårämnen. Resultaten visar hur sammansättningen, spridningsvä- garna samt utbyte mellan grund- och djupområden av lågsalint vatten beror av främst tillförseln från Sibiriska floder, Stilla havsvatten samt smält- och frys- processer. Motiverat av observerade förändringar och paleoceanografiska data undersöks skiktningens känslighet i experiment med varierande färskvattentill- försel. I enlighet med en teoretisk modell ökar det övre lågsalina lagrets djup med minskad tillförsel. Den avslutande delen riktar sig till att utforska en ny metod för att ana- lysera vattenmasstransformationer. Metoden bygger på att volym, värme och saltbudgetar beräknas i salthalt-temperatur rummet. Havscirkulationsmodeller nyttjas för att studera hur ytnära och interna processer transformerar inflödande vatten mot lägre temperaturer och salthalter samt hur utbytet av haloklinvatten kan uppskattas. Extremfall för balansen av vattenmasstransformationen identifieras genom att särskilja bidrag från ytnära, interna och randflöden.

Acknowledgements

Life as a PhD student can, at times, be a bumpy ride. Luckily however, many people have contributed in various ways making this journey a particularly smooth, stimulating and possible one – and of course deserve to be thanked. First and foremost, I would like to thank my supervisor Markus Meier for giving me the opportunity to pursue this PhD. You have always been generous with your advice, helpful and ﬂexible with arrangements. You have also provided valuable feedback, yet given me the freedom to pursue whatever path I decide to take. I am also grateful for all the help and advice my co-supervisor Johan Nilsson has given me. Especially for sharing great ideas and in-depth knowledge in physical oceanography, and for trying to make me less of an ocean modeler and more of a "theoretical" oceanographer. (Although that was sometimes a hopeless case!) The ﬁnancial support from SMHI and the FORMAS project ADSIMNOR is greatly acknowledged. Thank you also to Ralf Döcher who coordinated the project. My research is the result of many fruitful cooperations. The cooperation with Alexandra Jahn and the other AOMIP co-authors was greatly appreci- ated. Andrey Proshutinsky and Mike Steele also deserve to be acknowledged for organizing the stimulating AOMIP and FAMOS meetings, and for kindly supporting my participation in these meetings. They truly sparked my interest in the Arctic Ocean! A special thank you to Magnus Hieronymus for sharing your interesting work on water mass transformations and to Laurent Brodeau for technical assistance with the NEMO model. Thanks also to fellow and former PhD students, and everyone else at MISU that I have gotten to know during the years. I really enjoyed the warm and stimulating atmosphere and always appreciate coming back. My colleagues at SMHI in Göteborg and Norrköping also deserve to be thanked for cheering me on during my PhD and for generally making SMHI a really nice place to work at. A big and warm thank you to my friends and family for support and encouragement and especially dad for spending many hours brushing up the lan- guage of my manuscripts. Last but not least, my two lovely girls Mona and Ester deserve my deepest gratitude for their endless love and encouragement - without your support this would never have been possible!

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