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Freshwater Processes and Water Mass Transformation in the Arctic Ocean

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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 transforma- tion 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 merid- ional 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 stratifica- tion 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 wa- ter and sea-ice melting and formation. Motivated by observed changes and paleorecords the sensitivity of the stratification is further explored in fresh- water 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 trans- formations. 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 contri- butions 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 simula- tions? 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 trans- formations 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 experi- ments, 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 pro- cesses . 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 re- gion 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 circula- tion, 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 trans- port 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 oc- curring 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.
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