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Aspects of the Atlantic Flow Through the Norwegian Sea Jan Even Øie Nilsen Dr. Scient. Thesis in Physical Oceanography

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Aspects of the Atlantic Flow Through the Norwegian Sea Jan Even Øie Nilsen Dr. Scient. Thesis in Physical Oceanography Aspects of the Atlantic Flow through the Norwegian Sea Jan Even Øie Nilsen Dr. Scient. Thesis in Physical Oceanography October 2003 Geophysical Institute G.C. Rieber Climate Institute University of Bergen Nansen Environmental and Remote Sensing Center °c Jan Even Øie Nilsen, 2003 ISSN 1502-5519 ISBN 82-90569-99-8 Reports in Meteorology and Oceanography University of Bergen Report No. 3-2003 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission. This work is typeset in LATEX 2", bookclass, b5paper, 10pt text. Preface An introductory part (this synthesis) and a collection of papers constitute my thesis presented in partial fulfillment of the requirements for the degree of Doctor Scientarium in physical oceanography at the Geophysical Institute, University of Bergen. In this work, different mechanisms related to the flow of Atlantic Water to the Arctic Mediterranean are studied. The focus areas are the openings in the Greenland–Scotland Ridge and the Norwegian Sea, through which all the northward traveling Atlantic Water passes. The period studied is 1948– 99. Some of the atmospheric impacts on the Norwegian Atlantic Current that modifies the water mass and alter its flow pattern are examined using a wide range of methods and approaches. The types of data used are hydrographic data, meteorological reanalysis as well as results from a global numerical model. An effort is made to emphasize the importance of both local processes and large scale atmospheric circulation for the behavior of the Atlantic Flow. The synthesis is organized as follows: The first chapter gives an introduction to the topic of the thesis, the second chapter presents the sources for the data used. The synthesis istelf is ended by a summary of results followed by a list of conclusions and an outlook. There is also an appendix with a discussion of some particular aspects of the methods used, where after the collection of four papers is attached. KEYWORDS: Atlantic Water, Nordic Seas, variability, topographic steering, atmo- spheric forcing, mixed layer, water mass modification, exchanges, Ocean Weather Ship Station M, global climate model. v vi Acknowledgments This work would not have been possible without the help, support and guidance from a number of colleagues and friends. I am first of all grateful to my supervisors, Knut Barthel, Tor Gammelsrød and Svein Østerhus for their support and invaluable feedback. Svein is thanked especially for providing easy access to the hydrographic data from OWSM and otherwise making the start of this work as painless as possible. All my coauthors are thanked for an inspiring and rewarding experience. Eva Falck especially for suddenly popping up in my office with such a fruitful idea that lead to the most extensive paper in this collection. I also wish to express tremendous gratitude to Helge Drange for including me in the BCM group and letting me include my work there in this thesis, and together with the other group members showing me the pleasures of true teamwork. Many thanks to Alexander Korablev for the very useful and comprehensive hydrographic database compiled at AARI and NERSC, and for discussions. Keep up the good work. A warm thanks goes to my friend, colleague and coauthor Frank Nilsen, for friendship, memorable times in the “Nilsen Lab” and for giving me the opportunity to spend the best month of the year on Svalbard. I am also indebted to Solfrid Hjøllo for collegial support in tackling the tougher obstacles on the path to the degree. Thanks to my good friends Inge B. Johannessen, Jan Erik Stiansen, Tore Furevik and all the other “reviewers” for help with the texts. I also want to thank all the staff and students at the Geophysical Institute and NERSC for the good working environment. This work was funded by the University of Bergen through a university scholarship, and partly by the G.C. Rieber Climate Institute, Nansen Environmental and Remote Sensing Center, Bergen through RegClim and KlimaProg’s “Spissforskningsmidler”. Eternal gratitude to my wife Randi and my daughters Amalie and Oline for bearing (with) me through this time. You are life. Bergen, October 21st 2003 vii viii Contents Preface v Acknowledgments vii Abbreviations xi List of Papers xiii 1 Introduction 1 1.1 The Region . 1 1.2 The Atlantic Flow . 2 1.3 Forcing on the Atlantic Flow . 5 1.4 Aims of the Study . 6 2 Data 9 2.1 Oceanic Time Series . 9 2.2 Ocean-wide Hydrography . 10 2.3 Atmospheric Data . 10 2.4 Numerical Model . 13 3 Summary of Results 15 4 Concluding Remarks 23 4.1 Conclusions . 23 4.2 Future Perspectives . 25 A Time Series from Ocean Weather Ship Station M 29 B Methodological Considerations 33 B.1 Wavelet Analysis . 33 B.2 Bin Mean Averaging . 36 B.3 Vertical Interpolation . 41 B.4 Temporal Interpolation . 43 ix x Abbreviations AARI Arctic and Antarctic Research Institute (St. Petersburg) AF Arctic Front AIW Arctic Intermediate Water AMOC Atlantic Meridional Overturning Circulation AW Atlantic Water AtI Atlantic Inflow CGFZ Charlie Gibbs Fracture Zone COADS Comprehensive Ocean Atmosphere Data Set CTD Conductivity Temperature Depth DS Denmark Strait EGC East Greenland Current FBC Faroe-Bank Channel FC Faroe Current FSC Faroe–Shetland Channel GSBW Greenland Sea Bottom Water GSR Greenland–Scotland ridge IC Irminger Current IFR Iceland–Faroe Ridge MICOM Miami Isopycnic Coordinate Ocean Model MLD Mixed layer depth ML Mixed layer NAC North Atlantic Current NAO North Atlantic Oscillation NCAR National Center for Atmospheric Research NCA Northern center of action (of NAO) NCC Norwegian Coastal Current NCEP National Centers for Environmental Prediction xi NWAC Norwegian Atlantic Current OWSM Ocean Weather Ship Station M OW Overflow Water POP Population (mean) QPV Quasigeostrophic potential vorticity SLP Sea-level pressure THC Thermohaline circulation WFT Windowed Fourier transform WPS Wavelet power spectrum WP Weighted parabolas WTR Wyville-Thomson Ridge xii List of Papers Paper I Nilsen, J.E.Ø. (2003). Variability at Ocean Weather Station M in the Norwegian Sea. ICES Marine Science Symposia, 219, 371–374. Paper II Nilsen, J.E.Ø. and Nilsen, F. Detecting Frontal Struc- tures in Oceanic Station Time Series. Manuscript. Paper III Nilsen, J.E.Ø. and Falck, E. Variations of Mixed Layer Depth and Water Properties in the Norwegian Sea for the period 1948–1999. Climate Variability in the Nordic Seas, Geophysical Monograph Series, AGU. Submitted. Paper IV Nilsen, J.E.Ø., Gao, Y., Drange, H., Furevik, T. and Bentsen, M. (2003). Simulated North Atlantic–Nordic Seas Water Mass Exchanges in an Isopycnic Coordinate OGCM. Geophysical Research Letters, 30(10), 1536, doi:10.1029/2002GL016597 The reprints were made with permission from the pub- lishers. xiii xiv Chapter 1 Introduction The North Atlantic thermohaline circulation (THC) is a dynamically active com- ponent of the global climate system, in particular on multi-annual to decadal time scales (e.g. Curry and McCartney, 2001; Bentsen et al., 2003). The amounts of heat and salt carried northward by Atlantic Water across the Greenland– Scotland Ridge (GSR) are substantial, and both quantities are of importance for the regional climate, water mass and ice distribution of the Nordic Seas and Arctic Ocean, and possibly for the deep mixing and water mass modifications and transformations taking place in the region (Mauritzen, 1996a,b; Furevik et al., 2002). Furthermore, the primary production and thus the fisheries and other aquaculture activities in the whole region benefit from the relatively high temperatures introduced to the area by the Atlantic flow. Changes in this heat supply will and have been seen to affect the fisheries (Svendsen et al., 1995; Iversen et al., 2002). Thus the changes in the characteristics of inflowing At- lantic Water to our region as well as the local modification of this water mass as it continues towards its participation in deep water formation processes and general presence in the Arctic, are of profound importance. 1.1 The Region The Nordic Seas lies between the North Atlantic Ocean to the south and the Arctic Ocean to the North, and thus act as a buffer zone between the warm, saline Atlantic Water (AW), and the cold, fresh Polar Water (Figure 1.1). These two water masses outline the main features of the surface waters in the Nordic Seas as the AW occupies the southern and eastern part of the basin and Polar Water the northern and western part. The main source for Polar Water to the area is the East Greenland Current (EGC) which flows southward from the Fram Strait and along the western margin of the basin, branching off Polar Water into 1 both the Greenland and Iceland Seas on its way. The mixing products between Polar Water and the other water masses in the area is called Arctic Water and resides in the central and southern areas of the Nordic Seas. The GSR provides three gateways for the exchange of water masses between the Atlantic Ocean and the Nordic Seas (Figure 1.1): The Denmark Strait (DS) between Greenland and Iceland, the Iceland–Faroe Ridge (IFR), and the Faroe– Shetland Channel (FSC) with the Faroe-Bank Channel (FBC) and Wyville- Thomson Ridge (WTR) at its entrance. 1.2 The Atlantic Flow The major features of the circulation in the North Atlantic–Nordic Seas re- gion were already described by Helland-Hansen and Nansen (1909). Recently the northward flow of Atlantic Water and the southward flow of dense Overflow Water (OW) through the three passages have been reviewed (Hansen and Øster- hus, 2000), and a thorough description of the sources for the Atlantic inflow can be found in McCartney and Mauritzen (2001). I will here only briefly describe the rather complex surface circulation features of the region (Figure 1.1).
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