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The Role of Southern Ocean Fronts in the Global Climate System

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The Role of Southern Ocean Fronts in the Global Climate System The role of Southern Ocean fronts in the global climate system Robert M. Graham Till min mamma & pappa Tack för allt © Robert M. Graham, Stockholm University 2014 ISBN 978-91-7447-991-1 Cover picture by Dr. Jennifer A. Graham, Printed in Sweden by US-AB Stockholm University, 2014 Distributor: Department of Geological Sciences II Abstract The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts. A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary. A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl. Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. III Sammanfattning Fronters geografiska läge utövar en direkt påverkan på såväl fysiska som biologiska processer i Södra Ishavet. Hittills har fronter varit dåligt upplösta, både i oceanografiska observationsdata och i klimatmodeller. I föreliggande avhandling analyseras en kombination av högupplösta satellitdata, modelldata och ARGO-data i syfte att förbättra den grundläggande förståelsen av fronter. En metod har utarbetats varigenom fronter identifieras med lokala havsytenivågradientmaxima. Härigenom definieras fronter lokalt som jetströmmar snarare än som kontinuerliga cirkumpolära gränser mellan olika vattenmassor. En ny klimatologi för fronter i Södra Ishavet har utarbetats. Denna leder till en ny tolkning av den Subtropiska Fronten; strömmarna riktade östvart som förknippas med fronten motsvarar förlängningen av respektive bassängs västliga randström. Vi sammanfattar dessa strömmar genom beteckningen den Dynamiska Subtropiska Fronten. Tidigare studier har istället gjort gällande att den Subtropiska Fronten är ett kontinuerligt fenomen i Södra Ishavet, där den har sagts utgöra den nordliga gränsen för den cirkumpolära cirkulationen. En omfattande utredning har genomförts av förhållandet mellan dessa fronters läge och vindstressen. Först har fronternas respons undersökts vid en sydlig förskjutning av de västliga vindarna med hjälp av en hundraårig klimatsimulering från en högupplöst kopplad ocean/atmosfärmodell. Resultatet visar att en sydlig västvindsförskjutning på 1°33’ inte ger upphov till någon lägesförändring hos fronterna. Satellitdata visar även att den Subtropiska Frontens klimatologiska läge är 5-10° norr om den latitud där vindstressrotationen är noll, vilken många tidigare studier har antagit sammanfaller med den Subtropiska Frontens läge. Slutligen har visats att näringstillförseln vid havsfronter främst orsakas av horisontell advektion och inte av uppvällning. Näringsämnen från kustområden blandas in i västliga randströmmar och advekteras in i Södra Ishavet längs den Dynamiska Subtropiska fronten. IV List of Papers This thesis is comprised of an overview section that outlines the main aims of this PhD and summarises some of the key results. The following manuscripts are also included: i. Graham, R. M., A. M. de Boer, K. J. Heywood, M. R. Chapman, and D. P. Stevens (2012), Southern Ocean fronts: Controlled by wind or topography?, J. Geophys. Res. Oceans, 117, doi:10.1029/2012JC007887. ii. Graham, R. M., and A. M. De Boer (2013), The Dynamical Subtropical Front, J. Geophys. Res. Oceans, 118, doi:10.1002/ jgrc.20408. iii. De Boer, A. M., R. M. Graham, M. D. Thomas, and K. E. Kohfeld (2013), The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front, J. Geophys. Res. Oceans, 118, doi:10.1002/jgrc.20407. iv. Graham, R. M., A. M. De Boer, K. E. Kohfeld, C. Schlosser (Submitted, 16/10/2014), Identifying sources and transport pathways of iron in the Southern Ocean, Deep-Sea Research Part 1. R. Graham was the main contributor in terms of analyses and writing for manuscripts I, II and IV, together with the help of all co-authors. The main contributor for manuscript III was A. De Boer. R. Graham assisted by producing all figures and conducting the analyses on the satellite data and fronts. The analyses on the model output from HiGEM used in Figures 4 and 5 of manuscript III was completed by M. Thomas. The ideas for Manuscript I were developed primarily by A. De Boer and R. Graham. R. Graham proposed the ideas for Manuscripts II and IV. The idea behind Manuscript III was developed by A. De Boer. Reprints for all manuscripts are made with permissions from the publishers, Wiley & Sons. The following papers are not included as a part of this thesis: Kohfeld, K. E., Graham, R. M., de Boer, A. M., Sime, L. C., Wolff W. E., Le Quéré, C., Bopp, L. (2013), Southern Hemisphere Westerly Wind Changes during the Last Glacial Maximum: Paleo-data Synthesis. Quaternary Science Reviews. doi:10.1016/j.quascirev.2013.01.017 Sime, L. C., Kohfeld, K. E., Le Quéré, C., Wolff, W. E., de Boer, A. M., Graham, R. M., Bopp, L. (2013), Southern Hemisphere Westerly Wind Changes during the Last Glacial Maximum: Model-Data Comparison. Quaternary Science Reviews. doi:10.1016/j.quascirev.2012.12.008 V Acknowledgements First of all I would like to thank my family. If it were not for them I would not be where I am today. My parents have always been there for me – whether it be to help me with my English essays in high school; to help me with all of my important decisions in life such as whether to move to Stockholm; to provide me with a house to live in while at UEA; or simply to take me on a relaxing holiday! I cannot begin to thank you enough. My sister, Jenny, has also been a great help. While I like to pretend otherwise, there is little doubt that Jenny being a PhD student in physical oceanography was a major factor in my decision to undertake a PhD. Jenny also kindly taught me Matlab and introduced me to many of my friends in Norwich. More recently it has also been great fun to meet up with her at conferences and have a friend to go travelling with. I would also like to thank my supervisor, Agatha. Agatha has truly been the best supervisor I could possibly have wished for. She has always been there for me when I have needed her – both as a friend and a teacher. Agatha has provided me with great freedom to follow my own research interests and curiosities. However, perhaps most importantly, she always encourages me to give everything my best shot. I never would have dreamt when I began my PhD that I would be where I am today. I also do not think I ever would have considered moving to Sweden if it was not for Agatha, and for that alone I will always be grateful to you. I would also like to acknowledge all of my co-authors. Without you much of this thesis would not have been possible. Karen Kohfeld has been a major inspiration to me through the last few years of my PhD. She has taught me huge amounts about the paleo-world, and I am extremely grateful for the opportunity to become involved with her and Louise Sime’s westerly wind project. Karen Heywood was also a great help and very patient in improving my writing skills and English grammar. While not listed as co-authors, I would also like to thank Filippa, Malin, Sara and Peter for their superb job with writing my Swedish abstract! I would like to thank all of the staff and students here at Stockholm University, both in IGV and MISU, for providing such a fantastic working environment. In particular, I would like to thank all of those who have taught me over the last two years. Likewise, Arne, Dan, Eve, Margita and Monica have been a great help at keeping everything running smoothly behind the scenes. A special thank you must also go to Fabien and Sarah for organising lunch seminars, which I have enjoyed a great deal. I owe a huge amount to the Bolin Centre. They have provided me with countless opportunities to travel, present my work, take courses and purchase a new computer. Thank you! Along with work there is life! Never would I have got through the last four years if it was not for my friends here in Stockholm as well as further afield. My officemate Francesco has been a great source of motivation to work harder and accompanied me on an incredible trip to Norway, numerous after work drinking and sushi adventures, and has cooked me countless delicious meals! My other officemates Moo, Francis, Liselott and baby Franbert have also provided great support allowing me to practice presentations or accompany me to Fika! I am worried that if I attempt to list everyone here that I would like to thank I will miss someone important out.
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