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

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.

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

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. So I have decided instead, with the serious risk of offending everyone, to list some words that should mean something to all those who have stood by me over the last four years! Green Villa (pub and lunch), GEOPUB, Lunch!, Mosebacke, sushi, Hermans, Kellys, Lasagne, brownie-cookie-dough, Dominoes, Stirling, Norwich, Reading, The Boat, sea-ice, Svensk Lunch, Folkuniversitetet, bacon, bikes, running, swimming, Volley Ball, kayaking, grilling, Brunnsviken, The Party, Fell Club, Triathlon, water-skiing, Salt Lake City, Agulhas, Hawaii, Bergen, Nyksund, ACDC, Fell Club, Triathlon, Nacka Halvmarathon, MISU, Happy Hour, office golf, The meal for 1 challenge, Cologne, London, Tea!, Fika!. I am especially grateful to all of my friends who have stayed in touch with me during my PhD, despite me not always replying to emails. It has been great fun coming back home to visit you, and I have really enjoyed your trips out to Stockholm also. This also gives me confidence that I am will still be friends with all of you here in Stockholm for many years to come, even if life takes us to faraway lands in the future! Thank you.

Contents

Abstract III

List of papers V

Acknowledgements VI

1. Introduction 9

2. The modern day frontal structure in the Southern Ocean 11 2.1. The importance of an accurate frontal climatology 11 2.2. Defining ocean fronts 11 2.2.1. Fronts as water mass boundaries 12 2.2.2. Fronts as strong currents 13 2.3. The Dynamic Subtropical Front 14

3. The relationship between ocean fronts and the wind field 19 3.1. Motivation: Southern Ocean fronts in a changing climate 19 3.2. The response of fronts to a southward shift of the westerly winds 20 3.3. The relationship between the Subtropical Front and zero wind stress curl 21

4. Biological activity at ocean fronts 24 4.1. Background: limits on primary production in the Southern Ocean 24 4.2. The role of western boundary currents for nutrient supply 25

5. Applications to the Last Glacial Maximum 27 5.1. Southern Ocean changes at the Last Glacial Maximum 27 5.2. Advances made in this thesis 28 5.2.1. Evaluating possible frontal shifts 28 5.2.2. Explaining enhanced export production in the Sub-Antarctic Zone 28

6. Unresolved questions and possible future directions 30 6.1. What sets the location of the Dynamic Subtropical Front? 30 6.2. Inter-model comparison of Southern Ocean fronts 30 6.3. Location of fronts at the Last Glacial Maximum 30 6.4. Shelf sediment iron source parameterisation 31

7. Key Results 32 7.1. Paper I 32 7.2. Paper II 32 7.3. Paper III 32 7.4. Paper IV 32

8. References 33

VII

1. Introduction

The Southern Ocean is at the centre of the global climate system. It interconnects three other major ocean basins; the Atlantic, Pacific and Indian, and is therefore important for transmit climate signals from one region to another [Gille, 2002]. The Southern Ocean is also the only ocean on Earth today with no meridional boundary [Olbers et al., 2004]. Within the Southern Ocean is the Antarctic Circumpolar Current (Figure 1). This current is both the longest and strongest ocean current on our planet. It has a transport of approximately 130 Sv (1 Sv = 106 m3/s) [Whitworth, 1983]. Several deep reaching hydrographic boundaries, known as fronts, exist across the Antarctic Circumpolar Current [Deacon, 1982; Orsi et al., 1995; Belkin and Gordon, 1996]. Associated with these fronts are intense jets with high velocities [Sokolov and Rintoul, 2007a]. These jets contribute the majority of the Antarctic Circumpolar Current transport. Fronts in the Southern Ocean are believed to be an important component of the climate system for several reasons. High bottom velocities associated with ocean fronts generate lee waves as fronts cross over rough topography [Nikurashin and Ferrari, 2010; Sheen et al., 2014]. The breaking of these lee-waves acts to mix the ocean and transform dense bottom waters into lighter waters [Nikurashin and Ferrari, 2010; Sheen et al., 2014]. This mixing is thought to be an important process for closing the meridional overturning circulation [Melet et al., 2014], and this overturning circulation is responsible for substantial cross equatorial heat transport to the Northern Hemisphere in the Atlantic Ocean (Figure 1). Satellite images reveal higher chlorophyll concentrations along several ocean fronts compared to the low background concentrations ubiquitous of the Southern Ocean [Moore and Abbott, 2000, 2002; Sokolov and Rintoul, 2007b]. Chlorophyll is a green pigment found in plants and algae that is used in photosynthesis. These high chlorophyll concentrations indicate that biological activity is enhanced at frontal zones in the Southern Ocean [Read et al., 2000; Moore and Abbott, 2002; Saraceno et al., 2005; Sokolov and Rintoul, 2007b]. The high productivity associated with frontal zones is an important component of the global carbon cycle. Changes in productivity over the Southern Ocean have been invoked to explain a substantial portion of the 80 parts per million reduction in atmospheric carbon dioxide concentrations at the Last Glacial Maximum [Martin, 1990; Kohfeld et al., 2005]. Strong gradients in sea surface temperature exist over ocean fronts. These temperature gradients can result in substantial fluxes of heat energy between the atmosphere and ocean, as the larger scale atmospheric circulation adjusts to these small scale oceanic features [Small et al., 2008]. Furthermore, when strong winds blow parallel or across ocean fronts, regions of convergence and

Figure 1 Simplified sketch of global overturning circulation by Kuhlbrodt et al. [2007]. ACC = Antarctic Circumpolar Current. 9 divergence can be generated at the surface of the ocean and atmosphere, respectively. This is because the atmospheric boundary layer is more stable over cooler waters compared with warmer waters and therefore the surface wind stress is reduced on the cold side of fronts [O’Neill et al., 2003, 2010a; Chelton et al., 2004]. Thus, winds blowing parallel to an ocean front will generate a strong wind stress curl perturbation that will induce a region of convergence/divergence in the Ekman Layer of the ocean, while winds blowing across an ocean front will generate regions of convergence/ divergence in the atmospheric boundary layer [O’Neill et al., 2003, 2010b; Chelton et al., 2004; Small et al., 2008]. These areas of convergence and divergence can induce large vertical velocities in both the atmosphere and ocean. This is believed to influence local rainfall patterns [Small et al., 2008]. However, the net effects of these vertical velocities for the general ocean circulation remain unknown [Hogg et al., 2009]. The strong sea surface temperature gradients across ocean fronts are also thought to guide the path of the mid-latitude westerly winds under certain circumstances [Nakamura et al., 2008; Brayshaw et al., 2011]. The locations of certain fronts in the Southern Ocean are thought to influence inter-ocean exchange. For example, the latitude of the Subtropical Front is believed to regulate the volume of warm and saline Agulhas Leakage passing from the Indian Ocean to the Atlantic [Bard and Rickaby, 2009; Beal et al., 2011]. The salt flux from this leakage is suggested to be a crucial component of the Meridional Overturning Circulation. It has been hypothesized that northward shifts of the Subtropical Front during glacial intervals cut off the flow of Agulhas Leakage and led to a shutdown of the Meridional Overturning Circulation and its associated northward heat transport [Peeters et al., 2004; Bard and Rickaby, 2009; Beal et al., 2011; Marino et al., 2013]. While Southern Ocean fronts are thought to have an influence on many different aspects of the global climate system and carbon cycle, our understanding of these features remains relatively poor. Ocean fronts are small scale features compared with the vast Southern Ocean. Moreover, the Southern Ocean is remote and weather conditions harsh. It is therefore challenging to obtain sufficient temporal and spatial resolution of observations to monitor fronts well. Similarly, models are expensive to run at the high resolutions required to resolve frontal features. However, these challenges are gradually being overcome with improvements in satellite capabilities, computing power, and observational programs such as the ARGO network. In this thesis we utilise the wealth of new data from satellites and the ARGO network, as well as high resolution model output, to address three key questions regarding fronts in the Southern Ocean: i. What is the modern day frontal structure like? (Papers I & II) ii. What is the relationship between ocean fronts and the wind field? (Papers I & III) iii. Why is biological activity enhanced at ocean fronts? (Paper IV) In Sections 2 - 4 we will outline our motivation for asking each of these questions and describe the progress we have made towards answering them. A discussion is given in Section 5 detailing the applications of this work for our understanding oceanic changes at the Last Glacial Maximum. In Section 6 we highlight some of the important outstanding questions that remain following our analyses, and potential directions for future research to tackle these problems. As a reference, a brief summary of the key results from each of the four papers contained within this thesis is provided in Section 7.

2. The modern day frontal structure in the Southern Ocean 2.1. The importance of an accurate frontal climatology It is essential to have an accurate knowledge of where fronts are in the Southern Ocean in order to improve our understanding of the role fronts play in the global climate system and carbon cycle. Having an a priori knowledge of where ocean fronts are would greatly benefit sea-going observational studies. For example, studies wishing to investigate the mixing generated from lee-waves as fronts pass over rough topography could save considerable money and ship time if near-real time maps of front locations were available when planning their route. The same would be true for in-situ observational studies wishing to investigate the relationship between fronts and biological activity. Similarly, for those wanting to reconstruct front locations in past climates knowledge of the present day mean front locations is required. Without accurate knowledge regarding the location of fronts, incorrect conclusions may be drawn from the analyses of these studies. In order to create an accurate frontal climatology, or to have real time information about the location of fronts, a consistent and robust method of identifying fronts is needed. 2.2. Defining ocean fronts Defining ocean fronts is not trivial [Sokolov and Rintoul, 2007a; Chapman, 2014]. Two common definitions of fronts prevail in the literature [Graham and De Boer, 2013]. Traditionally fronts are defined as hydrographic features or water mass boundaries [Orsi et al., 1995; Belkin and Gordon, 1996]. However, as higher resolution data sets and ocean models have become available fronts are often defined as strong narrow currents known as jets [Thompson et al., 2010; Graham et al., 2012; Thompson and Sallée, 2012; De Boer et al., 2013; Graham and De Boer, 2013; Chapman, 2014]. Frequently these two definitions of fronts are used interchangeably [Sokolov and Rintoul, 2002, 2007a, 2009a]. There is strong physical reasoning to support the idea that a water mass boundary should coincide with a strong current [Chapman, 2014]. By definition, a water mass boundary is a region of strong gradients in water mass properties such as temperature and salinity [Orsi et al., 1995]. Gradients in temperature and salinity produce density gradients, and gradients in density drive geostrophic currents. When considering the problem from the other angle, it is known that strong currents act as barriers to mixing in the ocean [Dritschel and McIntyre, 2008; Ferrari and Nikurashin, 2010; Naveira- Garabato et al., 2011; Klocker et al., 2012]. Such a mixing barrier would isolate the water masses on either side of the current. Thus if there is a west to east orientated current, and the atmospheric conditions on its northern side are warm and dry and to the south is cold and wet, it follows that the water mass at the surface on the northern side of the current will become progressively warmer and more saline due to heating and evaporation, while the water to the south will become comparatively cooler and fresher due to heat loss and precipitation. Water mass boundaries and strong currents are often observed to coincide [Orsi et al., 1995; Belkin and Gordon, 1996; Sokolov and Rintoul, 2007a]. However, this is not always the case. One example of this is at subtropical latitudes, where strong gradients in temperature and salinity can be density compensating [James et al., 2002]. This means that the reduction in density due to the increase in temperature as one moves equatorward is equally offset by an increase in density due to the increase in salinity. Without a density gradient there will be no geostrophic current at the water mass boundary. Hence, the presence of a water mass boundary does not command the existence of a jet. This raises some important questions. For instance, when considering the role of fronts in generating lee-waves, or the relationship between front and enhanced biological activity, is it more relevant to think of fronts as water mass boundaries or strong currents?

2.2.1. Fronts as water mass boundaries When treating fronts as water mass boundaries, five classical fronts have been identified in the Southern Ocean [Orsi et al., 1995]. From north to south these are the Subtropical Front; the Sub- Antarctic Front; the Antarctic Polar Front; the Southern ACC Front; and the Southern Boundary Front (Figure 2). Each of these fronts are said to be continuous and circumpolar in extent [Orsi et al., 1995]. The changes in water masses across Southern Ocean fronts are related to changes in the stratification of the water column [Pollard et al., 2002]. In northern regions surface waters are warm due to strong surface heating, while the deep ocean is isolated from this heating and is therefore cold. Here temperature dominates the stratification of the water column. Surface heating is weaker closer to the pole and therefore the difference in temperature between ocean surface and deep-ocean is less. In Polar Regions salinity dominates the stratification of the water column. Surface waters are fresh while deep waters are more saline [Pollard et al., 2002]. The Sub-Antarctic Front delineates the southern limit of regions where temperature dominates the stratification of the water column [Pollard et al., 2002]. North of this boundary the strong temperature stratification permits a subsurface salinity minimum to exist (Figure 3). The water mass associated with this salinity minimum is known as Antarctic Intermediate Water. The Sub-Antarctic Front can thus be identified as the southern boundary of the Antarctic Intermediate Water [Orsi et al., 1995]. Similarly, the Antarctic Polar Front delineates the northern limit of the region where salinity dominates the stratification of the water column [Pollard et al., 2002]. South of this boundary the strong salinity stratification allows temperature to increase with depth (Figure 3). The water mass associated with this subsurface temperature maximum is known as Upper Circumpolar Deep Water [Orsi et al., 1995]. Frontal definitions associated with changes in the stratification of the water column are inherently continuous and circumpolar [Pollard et al., 2002]. One can identify the transition from where temperature dominates the stratification of the water column to salinity along any latitudinal transect in the Southern Ocean. However, due to the limited availability of subsurface data, it is not common to identify fronts based on these stratification criteria. Instead, the location of fronts are often approximated using water mass properties i.e. specific isotherms or isohalines [Orsi et al., 1995; Belkin and Gordon, 1996]. For example, the location of the Polar Front is commonly defined as the 2°C isotherm at 200 m depth [Orsi et al., 1995]. Fronts are also inherently continuous and circumpolar when defined in this way.

Figure 2 Climatology of Southern Ocean fronts defined by Orsi et al. [1995]. From north to south these fronts are the Subtropical Front (red), Sub-Antarctic Front (green), Antarctic Polar Front (blue), Southern ACC Front (magenta) and the Southern Boundary Front (cyan). The grey contours are the 500 m and 3500 m isobaths.

Figure 3 a) Salinity transect at 100°E and b) Temperature transect at 100°E, from a gridded ARGO data set [Hosoda et al., 2008] c) Criteria for identifying Southern Ocean fronts using water mass boundary definitions from Pollard [2002]. APF=Antarctic Polar Front, SAF=Sub-Antarctic Front, AAIW=Antarctic Intermediate Water, UCDW=Upper Circumpolar Deep Water.

2.2.2. Fronts as strong currents Unlike fronts defined as water mass boundaries, strong currents in the Southern Ocean are neither continuous nor circumpolar in extent [Sokolov and Rintoul, 2007a; Thompson et al., 2010; Graham et al., 2012]. The number of strong currents present in the Southern Ocean varies with longitude [Thompson et al., 2010]. The discord between continuous-circumpolar water mass boundaries and discontinuous frontal jets has been noted for several years [Hughes and Ash, 2001; Sokolov and Rintoul, 2002]. Sokolov and Rintoul [2002] investigated this discord using sea surface height data. They suggest that while frontal jets are discontinuous, the jets are consistently found along distinct sea surface height contours. They therefore conclude that the location of each of the circumpolar water mass boundary fronts can be represented by a single sea surface height contours along which jets occur [Sokolov and Rintoul, 2007a, 2009a, 2009b]. The method derived by Sokolov and Rintoul [2007] has proven to be a powerful tool. It has allowed the position of fronts to be tracked with high spatial and temporal resolution satellite data for the first time. Using this method statistics can easily be calculated to show the circumpolar average variability and trends in the latitude of ocean fronts [Sokolov and Rintoul, 2009b; Billany et al., 2010].

Furthermore, correlations can be calculated to investigate whether these trends and variability are related to atmospheric patterns such as the Southern Annular Mode and El Nino Southern Oscillation [Sallée et al., 2008; Kim and Orsi, 2014]. Despite the advantages of Sokolov and Rintoul’s [2007] method, it continues to treat fronts as circumpolar features. As a result the climatological positions of their fronts’ provide no information on zonal variations in frontal characteristics. Nor does the method inform us whether a jet is actually present at any given longitude [Graham et al., 2012]. For certain studies, such as those wishing to investigate the generation of lee-waves at ocean fronts, information about where jets are present would be useful. In this thesis we derive a new method of identifying ocean fronts. We identify fronts as local maxima in the mean annual sea surface height or temperature gradients above a given threshold (Figure 4). We show using output from a high resolution climate model (HiGEM) that maxima in sea surface height gradients correspond closely to strong currents (maxima in zonal transport). In contrast, maxima in temperature gradients may not correspond to strong currents if these fronts are shallow or density compensated (Figure 5). We present a new climatology of fronts in the Southern Ocean where fronts are defined specifically as strong currents [Graham and De Boer, 2013]. Thus, the locations of fronts in our climatology correspond directly to locations where strong currents are present in the annual mean field. By defining fronts this way our fronts are discontinuous. We do not classify fronts using their traditional names – e.g. the Sub-Antarctic Front or Polar Front. Arguably this definition of fronts is more relevant for studies investigating mixing in the ocean compared with the traditional method of defining fronts as water mass boundaries [Chapman, 2014]. The gradient threshold method we use has since been extended to study the time-varying location of fronts in the Southern Ocean [Chapman, 2014]. We present a further climatology of fronts where we define fronts as local maxima in sea surface temperature gradients [Graham and De Boer, 2013]. This definition of fronts is more relevant for studying air sea fluxes, because the largest air-sea fluxes will occur where sea surface temperature gradients are strong [Small et al., 2008]. This climatology may also be more relevant for paleo-climate studies, as paleo-proxies are able to record large changes in sea surface temperature which may be the result of a sea surface temperature front shifting [Kohfeld et al., 2013].

Figure 4 Identifying fronts using sea surface temperature and height gradients with HiGEM output (30 year mean). Figure adapted from Graham et al. [2012]

a) Transect of sea surface temperature (green) and height (black) gradients, and zonal transport (magenta) at 100°E. b) cross section of zonal velocities at 100°E. Black vertical lines show the location of fronts identified as local maxima in sea surface height gradients.

Figure 5 Mean location of fronts in HiGEM. a) Fronts located as local maxima in zonal transport (magenta) and sea surface height gradients (black), b) Fronts located as local maxima in zonal transport (magenta) and sea surface temperature gradients (green, b). Grey lines are the 1000 m and 3000 m isobaths. The figure is adapted from Graham and De Boer [2013]

The skill of our frontal identification method is underlined by its consistency. Outside of the Subtropics, very similar results are found regardless of whether fronts are identified as local maxima in zonal transport, sea surface temperature or sea surface height gradients when using HiGEM model output (Figure 5). Furthermore, the pattern of fronts identified using satellite data and HiGEM model output are remarkably similar [De Boer et al., 2013]. Similar front locations were also found when comparing two one hundred year simulations on HiGEM, one of which was a control run and the other a climate change run where CO2 concentrations increased by 400% [Graham et al., 2012]. The consistency when using our method on independent data sets, different time intervals, and when comparing model output with observations, provides strong confidence in the robustness of this method. It also reveals new insights into the behaviour of fronts. The consistency between the frontal patterns in each of these scenarios, despite differing wind fields, shows that the mean position of fronts is more stable than previously thought and that topography has a strong control on the mean position of fronts [Graham et al., 2012]. The method also reveals how the number of jets present in the Southern Ocean changes dramatically with longitude, and that the number of jets is controlled primarily by the bottom topography [Graham et al., 2012]. When using the Sokolov and Rintoul [2007] method, large seasonal fluctuations in the locations of fronts have been reported. However, we show here that when defining fronts specifically as strong currents there is minimal seasonal cycle in the location of fronts [Graham and De Boer, 2013]. This result raises some concerns about the accuracy and applicability of the Sokolov and Rintoul [2007] method. Graham et al. [2012] further show that large spurious frontal movements can be recorded when using the Sokolov and Rintoul [2007] method, at locations and times where sea surface height gradients are very weak and no jets are present. This result has important implications for certain studies using the Sokolov and Rintoul [2007] method, such as those examining cross frontal mixing [Thompson and Sallée, 2012].

2.3. The Dynamic Subtropical Front

Figure 6 Schematic of the greater Agulhas System by Beal et al. [2011]. Background colours show the

mean subtropical gyre circulation, depicted by climatological dynamic height integrated between the

surface and 2,000 dbar. Black arrows illustrate significant features of the flow and the Southern

Hemisphere supergyre is given by the grey dashed line. Southward expansion of the Southern

Hemisphere westerlies over a 30 year period is shown on the right. Red arrows show the expected

corresponding southward shift of the Subtropical Front, and how it could affect Agulhas Leakage and

the pathway between the leakage and the Atlantic Meridional Overturning Circulation.

Reconstructing the location of the Subtropical Front during past climate intervals is a key goal of paleoclimate research [Bard and Rickaby, 2009; Franzese et al., 2009; De Deckker et al., 2012; Kohfeld et al., 2013]. This is because the latitude of the Subtropical Front is believed to be related to the volume of warm and saline Agulhas Leakage passing from the Indian Ocean to the Atlantic [Bard and Rickaby, 2009; Beal et al., 2011]. It is hypothesised that a northward shift of the Subtropical Front during glacial intervals pushed the Subtropical Front up against the African Continent, cutting off the flow of Agulhas Leakage and the associated salt flux. The salt flux from Agulhas Leakage is an important component of the Atlantic Meridional Overturning Circulation (Figure 6), and it is thought that the cessation of this salt flux may have caused the circulation and its northward heat transport to shut down [Peeters et al., 2004; Bard and Rickaby, 2009; Beal et al., 2011]. While there is a major research effort among the paleo-climate community to study the Subtropical Front, our understanding of this feature during the present day remains confused [Graham and De Boer, 2013]. Traditional climatologies of the Subtropical Front water mass boundary depict a continuous and near zonal feature extending from the Western Atlantic to the Eastern Pacific (Figure 7). However, there are known zonal variations in the characteristics of the Subtropical Front along its path [Burls and Reason, 2006; Dencausse et al., 2011]. For example, depending on where a study is conducted, descriptions of the Subtropical Front range from a deep and narrow jet with large transport to a broad and shallow frontal zone with little-to-no transport, and there is even uncertainty over whether the front exhibits a small or large seasonal cycle [Lutjeharms and Valentine, 1984; Stramma and Peterson, 1990; Stramma, 1992; Orsi et al., 1995; Stramma et al., 1995; Belkin and Gordon, 1996; James et al., 2002; Kostianoy et al., 2004; Burls and Reason, 2006]. Moreover, climatologies disagree on the location and number of fronts within this frontal zone [Orsi et al., 1995; Belkin and Gordon, 1996].

Figure 7 Fronts identified as local maxima in satellite sea surface temperature (green) and height (black) gradients. Orange lines are climatological positions of the Sub-Antarctic and Subtropical Front from Orsi et al. [1995]. Pink lines are the Dynamical Subtropical Front. Grey contours are the 500 m and 3500 m isobaths. The figure is adapted from Graham and De Boer [2013]

Many studies neglect to consider how these variations in the characteristics of the Subtropical Front may affect the interpretation of their data [Nürnberg and Groeneveld, 2006; De Deckker et al., 2012]. This is because continuous frontal climatologies obscure known zonal differences along the Subtropical Front’s path. These differences only become evident when reading deep into the literature. Our new frontal identification method reveals a clearer picture of the physical features present in the Subtropics [Graham and De Boer, 2013]. We see that the only strong currents (identified as local maxima in sea surface height gradients) associated with the Subtropical Front water mass boundary are located on the western sides of basins (Figure 7). These strong currents, or ‘dynamic fronts’ as we name them, are the western boundary current extensions from each basin in the Southern Ocean i.e. the South Atlantic Current, the Agulhas Return Current, and South Pacific Current. Collectively, we call these features the Dynamic Subtropical Front. The Dynamic Subtropical Front tracks south-eastwards in each basin and merges with the Sub-Antarctic Front (Figure 7). This is a departure from traditional climatologies of Subtropical Front water mass boundary that depict a zonal route extending across the entire Southern Ocean [Orsi et al., 1995; Belkin and Gordon, 1996]. There are no dynamic fronts at the Subtropical Front water mass boundary on the eastern side of basins (Figure 7). Instead, there is a broad area of sea surface temperature fronts that are visible as local maxima in sea surface temperature gradients but not height gradients. We observe with ARGO data, as well as in model output, that these frontal features in the east are shallow and there are no jets associated with them [Graham et al., 2012; Graham and De Boer, 2013]. We call this area of sea surface temperature fronts the Subtropical Frontal Zone. Interestingly, we see from the satellite data that there is a large seasonal cycle in the latitude of the Subtropical Frontal Zone, on the order of 5-7° [Graham and De Boer, 2013]. In contrast, the Dynamic Subtropical Front, and fronts associated with the Antarctic Circumpolar Current have little-to-no seasonal cycle. We can also see from the ARGO data that the Dynamic Subtropical Front is a deep feature (~2 km), while the Subtropical Frontal Zone is shallow [Graham and De Boer, 2013]. Separating the Dynamic Subtropical Front from the Subtropical Frontal Zone is a region of weak sea surface temperature and height gradients (Figure 7).

We conclude that the Dynamic Subtropical Front and Subtropical Frontal Zone are distinct and unrelated features (Figure 8). Thus, they should be studied separately. No continuous Subtropical Front exists, since the features on the eastern and western sides of basins that are usually associated with the Subtropical Front are in fact unrelated. This has important implications for studies reconstructing the location of the Subtropical Front in the Indian Ocean during past climates, and any inferences that may be made about Agulhas Leakage. In particular, we suggest that this structure of the Subtropical Front can help explain the asymmetric sea surface temperature changes during glacial-interglacial cycles on the east and west of basins [Nürnberg and Groeneveld, 2006].

Figure 8 Schematic of frontal features at Subtropical latitudes by De Boer et al. [2013]. Blue lines show the path of the western boundary currents and their extensions that we call the Dynamical Subtropical Front. Red shaded areas indicate the location of a region of enhanced temperature gradients (no currents) that we refer to as the Subtropical Frontal Zone. Beige and purple lines show the location of the Subtropical and Sub-Antarctic Front water mass boundaries, respectively.

3. The relationship between ocean fronts and the wind field 3.1. Motivation: Southern Ocean fronts in a changing climate Climate models indicate that the reduction in stratospheric ozone concentrations and increase in greenhouse gases over recent decades is causing the Southern Hemisphere westerly winds to intensify and shift southwards [Kushner et al., 2001; Fyfe and Saenko, 2006; Cai and Cowan, 2007; Thompson et al., 2011]. We therefore need to understand how the location of fronts in the Southern Ocean may respond to a change in the wind field, in order to assess the likelihood of any potential feedbacks on our climate system in response to a southward shift of the wind field in future decades. It is often assumed that the latitude of the Subtropical Front is set by the zero wind stress curl, which corresponds to the maximum eastward wind stress [Ruijter et al., 1999; Peeters et al., 2004; Zharkov and Nof, 2008; Dencausse et al., 2011; De Boer et al., 2013]. This would imply that the Subtropical Front will shift southwards in concert with a southward shift of the Southern Hemisphere westerlies, and may therefore lead to an enhanced supply of Agulhas Leakage into the Atlantic Ocean [Biastoch et al., 2009; Beal et al., 2011]. It has been suggested that the increased salt flux from this leakage could stabilise the Atlantic Meridional Overturning Circulation with respect to the freshening of surface waters in the North Atlantic from increased precipitation and ice melt [Biastoch et al., 2009; Beal et al., 2011]. Reconstructed front positions predominately show fronts to be displaced northwards at glacial intervals (Figure 9). This has been interpreted as evidence that the westerly winds shifted northwards during glacial periods [De Deckker et al., 2012; Kohfeld et al., 2013]. However, there is little dynamical evidence to support the idea that Southern Ocean fronts should follow meridional shifts in the Westerly Winds. In fact, many studies argue that the location of Southern Ocean fronts is tightly constrained by the bottom topography [Sinha and Richards, 1999; Moore et al., 2000; Tansley and Marshall, 2001; Thompson, 2010]. Between these two arguments is the idea that where the ocean floor is flat, such as in the Pacific, fronts will follow changes in the wind field, while in regions of rough bottom topography the locations of fronts are fixed [Howard and Prell, 1992; Hayward et al., 2008; Sallée et al., 2008; Sokolov and Rintoul, 2009b; Kemp et al., 2010]. Until recently, the relationship between fronts, topography and wind has not been widely tested due to the lack of computing capabilities and the high resolution required in models to resolve frontal features. In this thesis we conduct two studies to explore the relationship between fronts in the Southern Ocean and the overlying wind field. In the first study we investigate the response of ocean fronts to a southward shift of the wind field during a 100 year climate change simulation on a high resolution coupled model [Graham et al., 2012]. In the second study we use satellite data to test whether the Subtropical Front coincides with the zero wind stress curl [De Boer et al., 2013].

Figure 9 Inferred frontal shifts at the Last Glacial Maximum, from Kohfeld et al. [2013]. Black lines show the Orsi et al. [1995] Subtropical, Sub-Antarctic and Polar Front climatologies. Blue arrows correspond to studies approximating the position of fronts using present day isotherms, red arrows are studies analysing only a single core and green arrows studies analysing a transect of cores. 19

3.2. The response of fronts to a southward shift of the westerly winds We investigate the response of fronts to a southward shift of winds in a high resolution coupled climate model. The model output we use is from HiGEM: the UK Met Office High Resolution Global Environment Model [Roberts et al., 2009; Shaffrey et al., 2009]. The ocean model has a resolution of 1/3° latitude x 1/3° longitude, and 40 vertical levels. We use output from two 100 year simulations. One is a control simulation. The second is a climate change simulation where CO2 concentrations increase by 2% per year for the first 70 years and are then held constant. This increase in greenhouse gas concentration causes the maximum in the Southern Hemisphere westerly wind stress to strengthen by 15% and shift southwards by 1.3° [Graham et al., 2012]. We investigate changes to the mean location of fronts as a result of this wind shift by analysing output averaged over the final 30 years of both simulations. We identify fronts as local maxima in the full depth integrated zonal transport, where the transport exceeds 10 Sv / 100 km [Graham et al., 2012]. In this way we treat fronts only as strong currents rather than water mass boundaries. Our results show that the mean location of dynamic fronts remained almost unchanged following the intensification and southward shift of the wind field (Figure 10). There was no uniform 1.3° southward shift of fronts in response to the change in winds. Even in the Pacific Sector where the bottom topography is relatively flat there was very little movement of fronts. These results lead us to conclude that the location of fronts is tightly controlled bathymetry. This includes not just local bathymetry but upstream and downstream topographic constraints such as Drake Passage and the Kerguelen Plateau [Sinha and Richards, 1999; Tansley and Marshall, 2001; Graham et al., 2012]. The only major frontal changes we observe in the climate change simulation are in the subtropical regions on the western sides of basins, along the Dynamic Subtropical Front (Figure 10). These fronts shift southwards. However, the magnitude of the shift is not directly proportional to the change in wind and the shift was not uniform along the fronts. The detachment points of the boundary currents change little, but further east the fronts shifted southwards and meanders were flattened out. The timing of the shift also varied between basins. Some fronts appear to ‘jump’ with a very rapid change in path over just a few years, while others shift southwards gradually over the 100 year simulation (Figure 11). Large frontal movements were even observed over steep topographic ridges (Figure 11).

Figure 10 30 year mean (years 70-100) front locations from the HiGEM control (black) and

climate change (red) simulations. Fronts are identified as local maxima in full depth integrated zonal transport. Grey lines are the 1000 m and 3000 m isobaths. The figure is adapted from Graham et al. [2012]

Figure 11 Time series of sea surface height gradients during the 100 year climate change simulation on HiGEM at a) 40°E and b) 80°E. Thirty year mean (years 70-100, control simulation) zonal velocities at c) 40°E and d) 80°E. The figure is adapted from

Graham et al. [2102]

The fronts that shifted or changed in strength during the climate change simulation were shallower and more baroclinic, compared with those that did not (Figure 11). Thus the fronts that shift tend to be surface intensified, with fast velocities at the surface that decrease rapidly with depth. In contrast, velocity profiles of the fronts which do not shift are more constant with depth. We conclude that shallow or baroclinic fronts are decoupled from the bathymetry and therefore more sensitive to changes in the wind field than deep barotropic fronts. Hence, the depth of a front is a more important indicator of whether a front is likely to shift in response to a change in the wind field compared with the local topography of the region [Graham et al., 2012].

3.3. The relationship between the Subtropical Front and zero wind stress curl. We wish to test whether the Subtropical Front coincides with the latitude of the zero wind stress curl in the present day climate. The reason why the Subtropical Front is assumed to coincide with the zero wind stress curl stems from the idea that the Subtropical Front is the southern boundary of the Subtropical Super Gyre [Smythe-Wright et al., 1998; Burls and Reason, 2006; Bard and Rickaby, 2009; Beal et al., 2011; Dencausse et al., 2011; Smith et al., 2013]. Sverdrup Theory states that the latitude of the gyre boundaries should be located where the winds stress curl is zero [De Ruijter, 1982; De Boer et al., 2013]. At the zero wind stress curl there is no wind driven meridional transport. South of the zero curl line in the Southern Ocean, the wind stress curl is negative. This drives Ekman Suction and a poleward Sverdrup transport. North of this line there is positive wind stress curl driving Ekman Pumping and an equator-ward Sverdrup transport into the Subtropical Gyre. Thus, if the flow were purely wind driven, the gyre boundary should coincide with the zero wind stress curl where the wind driven flow must be zonal [De Boer et al., 2013].

Figure 12 Mean wind stress curl field calculated from QuickSCAT data (1999-2008). The units are N m-2 104 km-1. Blue line is the Orsi et al. [1995] Subtropical Front. Pink line is

the Graham and De Boer [2013] Dynamical Subtropical Front. Black line is the zero wind stress curl contour. The figure is adapted from De Boer et al. [2013]

In Paper III we test the hypothesis that the Subtropical Front should coincide with the zero wind stress curl, using existing climatologies of the Subtropical Front water mass boundary and our new climatology of the Dynamic Subtropical Front [De Boer et al., 2013]. We compare these climatologies to the mean wind stress curl field calculated from Quick Scat satellite wind stress data. We find that the Subtropical Front, regardless of whether it is defined as the surface water mass boundary or a dynamic front, is situated on average 5 - 10° north of the zero wind stress curl line (Figure 12). To investigate why the Subtropical Front does not coincide with the zero wind stress curl line, we evaluate each term of the barotropic vorticity equation using model output from HiGEM [De Boer et al., 2013]. This allows us to decompose the relative importance of wind stress, bottom topography and non-linear processes in determining the meridional transport. We find that at large spatial scales (averaging over 6° latitude x 10° longitude) the contribution from the bottom pressure torque to the meridional transport dominates over that from the wind stress curl in the region between the Subtropical Front and the zero wind stress curl line (Figure 13). The strong contribution from the bottom pressure torque in the Southern Ocean is due to the high bottom velocities of deep fronts associated with the Antarctic Circumpolar Current. The bottom pressure torque generated by these deep fronts pushes the ocean out of Sverdrup Balance, and therefore the Subtropical Front does not coincide with the zero wind stress curl [De Boer et al., 2013]. At smaller spatial scales lateral friction and nonlinear advection become important [Hughes and Cuevas, 2001; De Boer et al., 2013]. As the bottom pressure torque overwhelms the contribution of the wind stress curl in determining the meridional transport at the latitude of the Subtropical Front, it is not obvious that a shift in the zero wind stress curl line should lead to a corresponding shift in the latitude of the Subtropical Front, as is often suggested [De Boer et al., 2013]. Nonetheless, we note that in certain locations the Dynamic Subtropical Front shifted southwards during the HiGEM climate change simulation in response to the southward shift of the wind field [Graham et al., 2012]. Whether this indicates a causal relation between the winds and the Dynamic Subtropical Front or whether the change in the front is due to other co-incidental processes requires further investigation. The results from these analyses demonstrate that the assumption that the Subtropical Front will shift southwards and Agulhas Leakage increase in response to a southward shift of the westerly winds under future climate change is overly simplified.

Figure 13 Terms in the vorticity balance in equation (1) calculated from HiGEM

model output and divided by ρ0β to transform to units of depth integrated transport. The change in planetary vorticity due to (a) the meridional transport, is balanced by the source of vorticity from (b) the wind and (c) the bottom pressure torque. (d) The residual indicates the deviation from linear vorticity balance. The black line is the zero wind stress curl contour and the yellow line shows the position of the zero barotropic streamfunction. Units are m2 s-1. Figure is by De Boer et al. [2013].

Vorticity Balance equation: ρ!βV = � ⋅ ∇×τ − ∇p!×∇H + δ. (1)

4. Biological activity at ocean fronts 4.1. Background: Limits on primary production in the Southern Ocean The biology in the Southern Ocean is a crucial component of the global carbon cycle [Le Quéré et al., 2007]. A substantial portion of the 80 parts per million reduction in atmospheric carbon dioxide concentrations during glacial intervals is thought to be the result of increased export production in the Southern Ocean [Martin, 1990; Kohfeld and Ridgwell, 2009; Ziegler et al., 2013; Martinez-Garcia et al., 2014]. Understanding the controls on ocean productivity is therefore essential for our ability to predict how ocean-atmosphere carbon fluxes may respond to future climate change. The Southern Ocean is the largest High Nutrient Low Chlorophyll region of the global ocean [Boyd et al., 2004; Tyrrell et al., 2005]. This means that chlorophyll concentrations and productivity are lower than would be expected from the high macronutrient concentrations (nitrate and phosphate). Productivity in the Southern Ocean is limited by the availability of the micronutrient iron [Martin, 1990]. Numerous in-situ mesoscale artificial iron fertilisation experiments in the Southern Ocean have demonstrated that the addition of iron to the ocean surface increases primary production as well as chlorophyll concentrations that can be measured by satellites [Coale et al., 1996; Cooper et al., 1996; Boyd et al., 2000; Bakker et al., 2005; Aumont and Bopp, 2006; Law et al., 2006; Smetacek et al., 2012; Assmy et al., 2013]. While chlorophyll concentrations are relatively low over the Southern Ocean as a whole, distinct chlorophyll blooms are known to develop in certain regions. These include; continental shelves, downstream of islands, and ocean fronts [Comiso et al., 1993; Sullivan et al., 1993; Moore and Abbott, 2002]. These regions are thought to be sites of natural iron fertilisation [Blain et al., 2007; Pollard et al., 2009; Chever et al., 2010b; de Jong et al., 2012]. Shelf sediments provide one of the largest fluxes of iron to the global ocean [Johnson et al., 1999; Elrod et al., 2004; Moore et al., 2004]. In-situ measurements from several shelf regions reveal a large diffusive flux of dissolved iron from the shelf sediments to the overlying water [Johnson et al., 1999; Elrod et al., 2004; Planquette et al., 2007; Chever et al., 2010a]. This is due to pore waters becoming enriched in dissolved iron through the anoxic reduction of iron oxide contained within the sediment layers [Homoky et al., 2012; Raiswell and Canfield, 2012]. This iron flux can explain the high chlorophyll concentrations observed on continental shelves. The iron flux from sediments on the shelves surrounding islands may contribute to the high chlorophyll concentrations observed downstream of islands in the Southern Ocean [Blain et al., 2007; Planquette et al., 2007; Chever et al., 2010a; Sanial et al., 2014]. However, it is also thought that the strong upwelling induced downstream of these obstacles, known as the island-wake effect, supplies iron to the ocean surface from deep sources such as hydrothermal vents [Saraceno et al., 2005; Romero et al., 2006; Klunder et al., 2012; Rosso et al., 2014]. Intense upwelling is induced throughout the water column where deep fronts pass over topographic ridges in the ocean. This upwelling at fronts is believed to deliver iron from deep sources to the ocean surface [Sokolov and Rintoul, 2007b], and this mechanism is often favoured to explain enhanced productivity and chlorophyll concentrations at ocean fronts [Moore and Abbott, 2000]. However, other studies argue that ocean fronts advect iron horizontally from sources near the ocean surface, such as shelf sediments, rather than upwelling iron from depths [de Jong et al., 2012; Whitehouse et al., 2012; Frants et al., 2013; Sanial et al., 2014]. In this thesis we aim to explore further the mechanisms that lead to enhanced productivity at frontal zones.

4.2. The role of western boundary currents for nutrient supply

Figure 14 a) Mean annual chlorophyll concentrations (mg m-3) and the location of

ocean fronts (black, [Graham and De Boer, 2013]). Grey contour is the 1000 m isobaths. b) 30 year mean upwelling velocities from HiGEM, averaged over the upper 650 m of the ocean. Grey lines are the 1000 m isobaths. Figure adapted from Manuscript IV.

We compare our new frontal climatology to mean annual chlorophyll concentrations calculated from satellite data. These analyses reveal that chlorophyll concentrations are only enhanced along fronts in the Southern Ocean downstream of islands or continental land masses (Figure 14a). We further compare chlorophyll concentrations to vertical velocities from the HiGEM model output (Figure 14b). We average these velocities over the upper 650 m of the water column. The strong agreement between the position of fronts in the HiGEM model output and satellite data provides confidence in the models position of the fronts and thus also the spatial pattern of topographically induced upwelling at ocean fronts [De Boer et al., 2013]. The largest vertical velocities found in HiGEM are located around southern edge of the Campbell Plateau (170°E) and in Drake Passage (300°E) (Figure 14). If this topographically induced upwelling were delivering iron to the ocean surface one would expect chlorophyll blooms to develop here. However, the satellite data indicate that chlorophyll concentrations are extremely low in these areas. Chlorophyll blooms in the Southern Ocean are tightly related to the sea surface height field (Figure 15). Sea surface height contours are roughly parallel to streamlines in the Southern Ocean, and thus give an indication of the horizontal flow field [Sokolov and Rintoul, 2007a]. The strong relationship between chlorophyll concentrations and the sea surface height field implies that horizontal advection plays an important role in shaping chlorophyll blooms in the Southern Ocean. The highest chlorophyll concentrations in the Southern Ocean are observed on the western sides of each basin, where the western boundary currents detach from the continental shelf and turn eastwards into the Southern Ocean (Figures 14 - 15). The chlorophyll concentrations decrease eastwards along the Dynamic Subtropical Front.

From our analyses we conclude that primary reason for enhanced productivity at fronts in the Southern Ocean is the horizontal advection of nutrients and not the upwelling of nutrients. Western boundary currents entrain iron from shelf sediment sources as they run alongside the continental shelves. At the same time these currents act as barriers and prevent iron from mixing off the continental shelf, across the current, and into the Subtropical Gyres [Ferrari and Nikurashin, 2010; Naveira- Garabato et al., 2011]. Where these boundary currents detach from the continental shelves, iron is rapidly transported off the shelf and into the Southern Ocean along the Dynamical Subtropical Front. This supply of shelf sediment iron sustains the large chlorophyll blooms in the Sub-Antarctic Zone of the Southern Ocean (Figure 14). Similarly, the chlorophyll blooms around islands in the Southern Ocean can be explained by iron sourced from the shelf sediments surrounding these islands becoming entrained into nearby fronts and rapidly advected downstream. As chlorophyll blooms are not observed over shallow seamounts and submerged plateaus, we argue that the shelf sediments on these features do not act as an active source of iron. However, at present, biogeochemical models prescribe these features as an equal iron source to the sediments on continental shelves and shelves surrounding islands. We suggest that the shelves only act as a source of iron when they are close to continental or island boundaries. This difference between continental/island shelves and isolated bathymetric features is because processes such as coastal erosion and fluvial transport build up thick sediment layers in coastal regions that are rich in iron oxide, whereas remote plateaus and seamounts are likely to have minimal sediment covering.

Figure 15 Mean annual chlorophyll concentrations in the western South Atlantic (colour scale, mg m-3) and sea surface height field (grey contours). White lines are the 400 m isobaths. The figure is adapted from Manuscript IV.

5. Applications to the Last Glacial Maximum 5.1. Southern Ocean changes at the Last Glacial Maximum A key aim of this research was to improve our understanding of changes recorded in the Southern Ocean at the Last Glacial Maximum. Perhaps the most robust signal of change in the Southern Ocean at the Last Glacial Maximum is from the Sub-Antarctic Zone, between the Subtropical Front and Sub-Antarctic Front (Figure 16). Proxy records from this region indicate that there was enhanced productivity and a pronounced cooling at the Last Glacial Maximum [Kohfeld et al., 2013]. One of the most common interpretations of these changes in the Southern Ocean is a northward shift of the Southern Ocean frontal system [Howard and Prell, 1992; Gersonde et al., 2005; Kohfeld et al., 2013]. This in turn is used as evidence to support a northward shift of the westerly winds [Kohfeld et al., 2013; Sime et al., 2013]. A northward shift of the westerly wind belt in the Southern Hemisphere is an influential hypothesis for explaining the drawdown of atmospheric carbon dioxide during glacial intervals [Toggweiler et al., 2006; Toggweiler and Russell, 2008]. It is believed that northward shifted winds would lead to reduced upwelling in the Southern Ocean and a more poorly ventilated deep ocean. This would allow more carbon dioxide to be stored in the deep ocean. However, modelling studies fail to demonstrate a northward shift of the westerly winds when Last Glacial Maximum boundary conditions are prescribed [Sime et al., 2013]. Moreover, simulations that suggest southward shifted winds can still accurately capture moisture changes recorded at the Last Glacial Maximum, in particular a wetting on the west coasts of continents [Sime et al., 2013]. These moisture changes are often cited as the most robust evidence of northward shifted winds [Toggweiler et al., 2006]. Thus, the fact models can simulate these moisture changes without northward shifted winds calls into question how confident we are that winds were shifted north at the Last Glacial Maximum [Sime et al., 2013].

Figure 16 Changes in a) sea surface temperature b) export production (red higher, blue lower) and c) inferred front locations at the Last Glacial Maximum. The figure is adapted from Kohfeld et al. [2013].

5.2. Advances made in this thesis 5.2.1. Evaluating possible frontal shifts In Paper I we show that a southward shift of the westerly winds in the Southern Hemisphere, during a climate change simulation on HiGEM, did not lead to a corresponding shift of the Southern Ocean frontal system. Furthermore, we show in Paper II that the modern day location of the Subtropical Front does not coincide with the zero wind stress curl, as is often assumed. This demonstrates that there is not a direct correspondence between the position of fronts and the westerly winds in the Southern Ocean. Therefore we cannot use reconstructed positions of ocean fronts from the Last Glacial Maximum to make inferences about changes to the westerly winds. Similarly, these results indicate that even if the Southern Hemisphere Westerly Winds were shifted northwards at the Last Glacial maximum, it is not obvious that the fronts within the Southern Ocean would have shifted northwards. The location of frontal jets in the Southern Ocean appears to be more stable than previously thought and primarily controlled by the bottom topography. The possibility should therefore be considered that fronts were not displaced at the Last Glacial Maximum. Indeed, we cast doubt on the most common methods used for reconstructing past frontal positions whereby location of fronts is tied to present day isotherms by showing that the sea surface temperatures change by several degrees along present day fronts in the Southern Ocean [Graham et al., 2012; Graham and De Boer, 2013; Kohfeld et al., 2013]. If fronts were not displaced northwards at the Last Glacial Maximum an alternative explanation is required for the cooling and increased export production recorded in the Sub-Antarctic Zone. One possibility is that the mean location of frontal jets in the Southern Ocean remained constant, but the relative transport along each of these jets changed. For example, the transport along the Sub-Antarctic Frontal jet may have decreased, while the transport along the Dynamic Subtropical Front increased. This would likely lead to a pronounced cooling of the Sub-Antarctic Zone. Our results from Papers II and III also cast doubt on the hypothesis that a northward shift of the westerly winds pushed the Subtropical Front northwards at glacial intervals and cut off the supply of Agulhas Leakage to the Atlantic Ocean, leading to a shutdown of the Atlantic Meridional Overturning Circulation [Bard and Rickaby, 2009; Beal et al., 2011; De Deckker et al., 2012]. 5.2.2. Explaining enhanced export production in the Sub-Antarctic Zone

One of the most prominent signals of change in the Southern Ocean is of enhanced export production in the Sub-Antarctic Zone at the Last Glacial Maximum (Figure 16b). This enhanced productivity is believed to be partly responsible for an 80 ppm reduction of atmospheric CO2 [Kohfeld et al., 2005]. Two influential hypotheses have been put forward to explain the increased export production in this region. The first hypothesis suggests that increased iron supply from dust during the cold and arid glacial climate increased productivity [Ziegler et al., 2013; Martinez-Garcia et al., 2014]. The second hypothesis is that the change in productivity is related to a frontal shift, as we see reduced productivity to the south and increased productivity to the north [Kemp et al., 2006, 2010]. In Paper IV we show that the most intense chlorophyll blooms observed in the Southern Ocean today are located within the Sub-Antarctic Zone. This is the region where large increases in export production were recorded at the Last Glacial Maximum (Figure 17). We suggest that these blooms are stimulated today by the off-shelf transport of shelf sediment iron by the Dynamic Subtropical Front,

28 rather than iron supplied by dust or open ocean upwelling. Our analyses further indicate that only shelf sediments in coastal regions act as a major source of iron, whereas previous studies assume that shelf sediments over the entire continental shelf are an active source of iron. During glacial periods sea levels were roughly 120 m lower than the present day. This meant that continental shelves were smaller than today and therefore coastlines were closer to the shelf break and western boundary currents. We hypothesise that this shifted the coastal shelf sediment iron source nearer to the western boundary currents, and thus increased the efficiency at which shelf iron was entrained into the western boundary currents and exported to the Southern Ocean. This iron would be delivered directly to the Sub-Antarctic Zone and would likely have increased productivity and export production there. We suggest that this mechanism should be considered as a possible cause for changes in export production, along with increased dust deposition and/or frontal shifts.

-3 Figure 17 Present day, mean annual chlorophyll concentrations (colour scale, mg m ), and export production anomalies at the Last Glacial Maximum (red higher, blue lower). Black lines are the Orsi et al. [1995] Subtropical, Sub-Antarctic and Polar Fronts. Export production data were compiled by Kohfeld et al. [2013]. The figure was produced for this thesis.

6. Unresolved questions and possible future directions 6.1. What sets the location of the Dynamic Subtropical Front? A central focus of this thesis has been the western boundary current extensions in the Southern Ocean. In Paper II we define these features as the Dynamic Subtropical Front, and describe how they are related to the traditional Subtropical Front water mass boundary [Graham and De Boer, 2013]. We found in Paper I that these features responded more sensitively to a change in the wind field, during a 100 year model simulation, compared with frontal features within the Antarctic Circumpolar Current [Graham et al., 2012]. We also showed in Paper III that these features do not coincide with the zero wind stress curl, as is often suggested. This is because the ocean in this region is pushed out of Sverdrup Balance by a strong bottom pressure torque generated by the deep currents crossing over the bottom topography [De Boer et al., 2013]. Finally, in Paper IV we show that the western boundary current extensions are a crucial mechanism for advecting iron from shelf sediment sources into the Sub- Antarctic Zone and stimulating primary productivity in the Southern Ocean. One major outstanding question from this thesis is what controls the location of the Dynamic Subtropical Front. Considerable research has been conducted on the causes of western boundary currents separating from continental slopes. Most of these studies consider individual boundary currents. Often changes in shape of the coastline, bottom topography, collisions with boundary currents, or various relations to the wind stress curl are imposed as reasons for detachment [Matano, 1993; Özgökmen et al., 1997; Munday and Marshall, 2005]. However, the theories are not well- developed enough to enable predictions in other wind-regimes and there are no overarching theories for all boundary currents, despite the fact that each of the boundary currents in the Southern Ocean detach at roughly the same latitude [Graham and De Boer, 2013]. 6.2. Inter-model comparison of Southern Ocean fronts In Paper III we identified a strong agreement between the location of dynamic fronts in the HiGEM control simulation and those observed with satellite data [De Boer et al., 2013]. A recent study using output from the high resolution model, OFES, shows a qualitatively similar frontal structure in the Eastern Hemisphere of the Southern Ocean [Chapman, 2014]. We wish to conduct an inter-model comparison study of ocean front positions using our new frontal detection mechanism. We plan use all available model output where the horizontal resolution in the ocean is > 1/3°, including both ocean-only models and coupled models. We would like to test whether the strong agreement between front positions in HiGEM and satellite data is unique to HiGEM, and possibly OFES, or consistent between all models. We also hope to establish whether there are any systematic biases between ocean-only models and coupled models. 6.3. Location of fronts at the Last Glacial Maximum We demonstrated in Paper I that the location of most dynamic fronts in the Southern Ocean remained unchanged in response to a southward shift of winds during a 100 year model simulation [Graham et al., 2012]. However, paleo-climatic studies overwhelmingly support the idea that fronts were displaced northwards at the Last Glacial Maximum as a result of northward shifted winds [Howard and Prell, 1992; Gersonde et al., 2003; Kohfeld et al., 2013]. We would like to repeat our analyses from Paper I using output from a high resolution simulation of the Last Glacial Maximum where the westerly winds are shifted northwards by several degrees.

Many paleo studies reconstruct the location of fronts using present day isotherms [Howard and Prell, 1992; Gersonde et al., 2005; Bard and Rickaby, 2009; Kohfeld et al., 2013]. We argue that this is not an appropriate method to locate dynamic fronts or strong currents because the sea surface temperature changes considerably with distance along present day fronts. One should not assume that the temperature would remain constant at a current in a difference climatic state. For example, a uniform cooling over the entire Southern Ocean, without any change to the meridional sea surface temperature gradients, would be interpreted as a northward shift of the frontal system if approximating the position of fronts using isotherms. Instead, one should ideally reconstruct the location of fronts by studying a transect of cores and trying to identify past changes in the meridional sea surface temperature gradient [Kohfeld et al., 2013]. A further study could be to re-analyse the MARGO database of sea surface temperature reconstructions for the Last Glacial Maximum [Gersonde et al., 2005]. These sea surface temperature reconstructions could be compared with our new frontal climatology [Graham and De Boer, 2013] to investigate how temperature gradients changed across modern day frontal zones between the present day and Last Glacial Maximum. We believe this would be a more robust method of testing whether frontal zones were displaced during glacial intervals, as opposed to tying fronts to specific isotherms. 6.4. Shelf sediment iron source parameterisation Our analyses of chlorophyll concentrations in the Southern Ocean, from Paper IV, revealed that chlorophyll blooms do not develop over submerged plateaus and seamounts. We argue that this is because there is no iron flux from the sediments on these features. However, biogeochemical models currently prescribe a large iron flux from these bathymetric rises [Moore et al., 2004; Moore and Braucher, 2008; Tagliabue et al., 2014]. We recommend that a more realistic parameterisation of the shelf sediment iron flux would be to prescribe this iron source only around coastal regions, rather than being dependent on ocean depth. We also suggest that the enhanced export production recorded in the Sub-Antarctic Zone of the Southern Ocean during glacial intervals may have been due to coastal regions migrating closer to western boundary currents as a result of lower sea levels. This could have increased the efficiency at which shelf sediment iron is entrained into boundary currents and exported into the Southern Ocean. As a future study one could implement this new iron parameterisation into a biogeochemical model and test if it can produce realistic conditions. It would also be interesting to run some idealised simulations with lowered sea levels and see if this can enhance primary productivity and export production in the Sub-Antarctic Zone, as was recorded at the Last Glacial Maximum.

7. Key Results 7.1. Paper I • We develop a new method of identifying ocean fronts. • The location of most Southern Ocean fronts remained unchanged in a 100 year high resolution

coupled climate model simulation where CO2 concentrations increased by 400% and the Southern Hemisphere westerly winds intensified by 15% and shifted southwards by 1.3° latitude. • Surface intensified / baroclinic fronts responded more sensitively to the southward shift of winds compared with barotropic fronts.

7.2. Paper II • We show that two distinct physical features exist at the traditional Subtropical Front water mass boundary. • On the western side of basins a strong jet is located along the Subtropical Front water mass boundary. We name this the Dynamical Subtropical Front. This corresponds to the western boundary current extension from each basin. This jet merges with the Sub-Antarctic Front at the centre of each basin. • On the eastern side of basins is a zone of shallow sea surface temperature fronts. We name this the Subtropical Frontal Zone. There are no jets associated with these fronts and unlike the Dynamical Subtropical Front they have a large seasonal cycle. • As the Dynamical Subtropical Front and Subtropical Frontal Zone have different seasonal cycles, transports, vertical extents, and occupy different areas of each basin, we conclude that it is misleading to refer to these features with the same name and that they should be studied independently.

7.3. Paper III • We show that, regardless of what definition or climatology is used, the Subtropical Front sits on average 5-10 degrees latitude north of the zero wind stress curl line in the Southern Ocean. • Using output from a high resolution coupled climate model we evaluate each term in the barotropic vorticity balance and show that the transport in the region of the Subtropical Front is determined mainly by the bottom pressure torque and not surface winds.

7.4. Paper IV • We use satellite chlorophyll and sea surface height data to show that western boundary currents entrain nutrients from the continental shelf. These nutrients are then exported to the Southern Ocean along the Dynamical Subtropical Front and stimulate primary production in the Sub-Antarctic Zone. • We compare chlorophyll data to upwelling velocities from a high resolution coupled climate model and show that chlorophyll blooms do not develop in regions of intense upwelling at ocean fronts. This suggests that frontal upwelling does not supply nutrients to the ocean surface. Instead, blooms only develop along ocean fronts downstream of islands and continents. This indicates that fronts simply act as a mechanism to advect nutrients horizontally into the open ocean from coastal sources. • We recommend a new method of parameterising iron sources in biogeochemical models.

8. References

Assmy, P. et al. (2013), Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current., Proc. Natl. Acad. Sci. U. S. A., 110(51), 20633–8, doi:10.1073/pnas.1309345110.

Aumont, O., and L. Bopp (2006), Globalizing results from ocean in situ iron fertilization studies, Global Biogeochem. Cycles, 20(2), doi:10.1029/2005GB002591.

Bakker, D. C. E., Y. Bozec, P. D. Nightingale, L. Goldson, M.-J. Messias, H. J. W. de Baar, M. Liddicoat, I. Skjelvan, V. Strass, and A. J. Watson (2005), Iron and mixing affect biological carbon uptake in SOIREE and EisenEx, two Southern Ocean iron fertilisation experiments, Deep Sea Res. Part I Oceanogr. Res. Pap., 52(6), 1001–1019, doi:10.1016/j.dsr.2004.11.015.

Bard, E., and R. E. M. Rickaby (2009), Migration of the subtropical front as a modulator of glacial climate., Nature, 460(7253), 380–3, doi:10.1038/nature08189.

Beal, L. M. et al. (2011), On the role of the Agulhas system in ocean circulation and climate, Nature, 472(7344), 429–436, doi:10.1038/nature09983.

Belkin, I. M., and A. L. Gordon (1996), Southern Ocean fronts from the Greenwich meridian, J. Geophys. Res., 101(NO. C2), 3675–3696.

Biastoch, a, C. W. Böning, F. U. Schwarzkopf, and J. R. E. Lutjeharms (2009), Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies., Nature, 462(7272), 495–8, doi:10.1038/nature08519.

Billany, W., S. Swart, J. Hermes, and C. J. C. Reason (2010), Variability of the Southern Ocean fronts at the Greenwich Meridian, J. Mar. Syst., 82(4), 304–310, doi:10.1016/j.jmarsys.2010.06.005.

Blain, S. et al. (2007), Effect of natural iron fertilization on carbon sequestration in the Southern Ocean., Nature, 446(7139), 1070–4, doi:10.1038/nature05700.

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. Ocean., 118(10), 5669–5675, doi:10.1002/jgrc.20407.

Boyd, P. W. et al. (2000), A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization., Nature, 407(6805), 695–702, doi:10.1038/35037500.

Boyd, P. W., G. McTainsh, V. Sherlock, K. Richardson, S. Nichol, M. Ellwood, and R. Frew (2004), Episodic enhancement of phytoplankton stocks in New Zealand subantarctic waters: Contribution of atmospheric and oceanic iron supply, Global Biogeochem. Cycles, 18(1), doi:10.1029/2002GB002020.

Brayshaw, D. J., B. Hoskins, and M. Blackburn (2011), The Basic Ingredients of the North Atlantic Storm Track. Part II: Sea Surface Temperatures, J. Atmos. Sci., 68(8), 1784–1805, doi:10.1175/2011JAS3674.1.

Burls, N. J., and C. J. C. Reason (2006), Sea surface temperature fronts in the midlatitude South Atlantic revealed by using microwave satellite data, J. Geophys. Res., 111(C8), doi:10.1029/2005JC003133.

Cai, W., and T. Cowan (2007), Trends in Southern Hemisphere Circulation in IPCC AR4 Models over 1950–99: Ozone Depletion versus Greenhouse Forcing, J. Clim., 20(4), 681–693, doi:10.1175/JCLI4028.1.

Chapman, C. (2014), Southern Ocean jets and how to find them: Improving and comparing common jet detection methods, J. Geophys. Res. Ocean., 119, 1–22, doi:10.1002/2014JC009810.

Chelton, D. B., M. G. Schlax, M. H. Freilich, and R. F. Milliff (2004), Satellite measurements reveal persistent small-scale features in ocean winds., Science, 303(5660), 978–83, doi:10.1126/science.1091901.

Chever, F., G. Sarthou, E. Bucciarelli, S. Blain, and a. R. Bowie (2010a), An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean), Biogeosciences, 7(2), 455–468, doi:10.5194/bg-7-455-2010.

Chever, F., E. Bucciarelli, G. Sarthou, S. Speich, M. Arhan, P. Penven, and a. Tagliabue (2010b), Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre, J. Geophys. Res., 115(C10), C10059, doi:10.1029/2009JC005880.

Coale, K. H. et al. (1996), A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean, Nature, 383(6600), 495 – 501, doi:10.1038/383495a0.

Comiso, J., C. McClain, C. Sullivan, J. Ryan, and C. Leonard (1993), Coastal Zone Color Scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features, J. Geophys. Res., 98(92), 2419–2451, doi:10.1029/92JC02505.

Cooper, D., A. Watson, and P. Nightingale (1996), Large decrease in ocean-surface CO2 fugacity in response to in situ iron fertilization, Nature, 383, 511 – 513, doi:10.1038/383511a0.

Deacon, G. E. R. (1982), Physical and biological zonation in the Southern Ocean, Deep Sea Res., 29(1978), 1–15.

De Deckker, P., M. Moros, K. Perner, and E. Jansen (2012), Influence of the tropics and southern westerlies on glacial interhemispheric asymmetry, Nat. Geosci., 5(4), 266–269, doi:10.1038/ngeo1431.

Dencausse, G., M. Arhan, and S. Speich (2011), Is there a continuous Subtropical Front south of Africa?, J. Geophys. Res., 116(C2), 1–14, doi:10.1029/2010JC006587.

Dritschel, D. G., and M. E. McIntyre (2008), Multiple Jets as PV Staircases: The Phillips Effect and the Resilience of Eddy-Transport Barriers, J. Atmos. Sci., 65(3), 855–874, doi:10.1175/2007JAS2227.1.

Elrod, V. A., W. M. Berelson, K. H. Coale, and K. S. Johnson (2004), The flux of iron from continental shelf sediments: A missing source for global budgets, Geophys. Res. Lett., 31(12), L12307, doi:10.1029/2004GL020216.

Ferrari, R., and M. Nikurashin (2010), Suppression of Eddy Diffusivity across Jets in the Southern Ocean, J. Phys. Oceanogr., 40(7), 1501–1519, doi:10.1175/2010JPO4278.1.

Frants, M., S. T. Gille, M. Hatta, W. T. Hiscock, M. Kahru, C. I. Measures, B. Greg Mitchell, and M. Zhou (2013), Analysis of horizontal and vertical processes contributing to natural iron supply in the mixed layer in southern Drake Passage, Deep Sea Res. Part II Top. Stud. Oceanogr., 90, 68–76, doi:10.1016/j.dsr2.2012.06.001.

Franzese, A. M., S. R. Hemming, and S. L. Goldstein (2009), Use of strontium isotopes in detrital sediments to constrain the glacial position of the Agulhas Retroflection, Paleoceanography, 24(2), n/a–n/a, doi:10.1029/2008PA001706.

Fyfe, J. C., and O. a. Saenko (2006), Simulated changes in the extratropical Southern Hemisphere winds and currents, Geophys. Res. Lett., 33(6), 1–4, doi:10.1029/2005GL025332.

Gersonde, R. et al. (2003), Last glacial sea surface temperatures and sea-ice extent in the Southern Ocean (Atlantic-Indian sector): A multiproxy approach, Paleoceanography, 18(3), doi:10.1029/2002PA000809.

Gersonde, R., X. Crosta, A. Abelmann, and L. Armand (2005), Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum—a circum-Antarctic view based on siliceous microfossil records, Quat. Sci. Rev., 24(7-9), 869–896, doi:10.1016/j.quascirev.2004.07.015.

Gille, S. T. (2002), Warming of the Southern Ocean since the 1950s., Science, 295(5558), 1275–7, doi:10.1126/science.1065863.

Graham, R. M., and A. M. De Boer (2013), The Dynamical Subtropical Front, J. Geophys. Res. Ocean., 118, doi:10.1002/jgrc.20408.

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., 117(C8), 1–14, doi:10.1029/2012JC007887.

Hayward, B. W. et al. (2008), The effect of submerged plateaux on Pleistocene gyral circulation and sea- surface temperatures in the Southwest Pacific, Glob. Planet. Change, 63(4), 309–316, doi:10.1016/j.gloplacha.2008.07.003.

Hogg, A. M. C., W. K. Dewar, P. Berloff, S. Kravtsov, and D. K. Hutchinson (2009), The Effects of Mesoscale Ocean–Atmosphere Coupling on the Large-Scale Ocean Circulation, J. Clim., 22(15), 4066–4082, doi:10.1175/2009JCLI2629.1.

Homoky, W. B., S. Severmann, J. McManus, W. M. Berelson, T. E. Riedel, P. J. Statham, and R. a. Mills (2012), Dissolved oxygen and suspended particles regulate the benthic flux of iron from continental margins, Mar. Chem., 134-135, 59–70, doi:10.1016/j.marchem.2012.03.003.

Hosoda, S., T. Ohira, and N. Tomoaki (2008), A monthly mean dataset of global oceanic temperature and salinity derived from Argo float observations, JAMSTEC Rep. Res. Dev., 47–59.

Howard, W. R., and W. L. Prell (1992), Late Quaternary Surface Circulation of the Southern Indian Ocean and its Relationship to Orbital Variations, Paleoceanography, 7(1), 79–117, doi:10.1029/91PA02994.

Hughes, C., and B. de Cuevas (2001), Why western boundary currents in realistic oceans are inviscid: A link between form stress and bottom pressure torques, J. Phys. Ocean., 2871–2885. doi: http://dx.doi.org/10.1175/1520-0485(2001)031<2871:WWBCIR>2.0.CO;2

Hughes, C. W., and E. R. Ash (2001), Eddy forcing of the mean flow in the Southern Ocean, J. Geophys. Res., 106(C2), 2713–2722, doi:10.1029/2000JC900332.

James, C., M. Tomczak, I. Helmond, and L. Pender (2002), Summer and winter surveys of the Subtropical Front of the southeastern Indian Ocean 1997–1998, J. Mar. Syst., 37(1-3), 129–149, doi:10.1016/S0924-7963(02)00199-9.

Johnson, K. S., F. P. Chavez, and G. E. Friederich (1999), Continental-shelf sediment as a primary source of iron for coastal phytoplankton, (398), 697–700. doi:10.1038/19511

De Jong, J., V. Schoemann, D. Lannuzel, P. Croot, H. de Baar, and J.-L. Tison (2012), Natural iron fertilization of the Atlantic sector of the Southern Ocean by continental shelf sources of the Antarctic Peninsula, J. Geophys. Res., 117(G1), G01029, doi:10.1029/2011JG001679.

Kemp, a. E. S., R. B. Pearce, I. Grigorov, J. Rance, C. B. Lange, P. Quilty, and I. Salter (2006), Production of giant marine diatoms and their export at oceanic frontal zones: Implications for Si and C flux from stratified oceans, Global Biogeochem. Cycles, 20(4), 1–13, doi:10.1029/2006GB002698.

Kemp, a. E. S., I. Grigorov, R. B. Pearce, and a. C. Naveira Garabato (2010), Migration of the Antarctic Polar Front through the mid-Pleistocene transition: evidence and climatic implications, Quat. Sci. Rev., 29(17-18), 1993–2009, doi:10.1016/j.quascirev.2010.04.027.

Kim, Y. S., and A. H. Orsi (2014), On the variability of Antarctic Circumpolar Current fronts inferred from 1992–2011 altimetry. J. Phys. Ocean. doi: http://dx.doi.org/10.1175/JPO-D-13-0217.1

Klocker, A., R. Ferrari, and J. H. LaCasce (2012), Estimating Suppression of Eddy Mixing by Mean Flows, J. Phys. Oceanogr., 42(9), 1566–1576, doi:10.1175/JPO-D-11-0205.1.

Klunder, M. B., P. Laan, R. Middag, H. J. W. de Baar, and K. Bakker (2012), Dissolved iron in the Arctic Ocean: Important role of hydrothermal sources, shelf input and scavenging removal, J. Geophys. Res., 117(C4), C04014, doi:10.1029/2011JC007135.

Kohfeld, K. E., and A. Ridgwell (2009), Glacial-Interglacial Variability in Atmospheric CO 2, Surf. Ocean - Low. Atmos. Process. Geophys. Res. Ser., 187, 251–286, doi:10.1029/2008GM000845.

Kohfeld, K. E., C. Le Quéré, S. P. Harrison, and R. F. Anderson (2005), Role of marine biology in glacial- interglacial CO2 cycles., Science, 308(5718), 74–8, doi:10.1126/science.1105375.

Kohfeld, K. E., R. M. Graham, A. M. de Boer, L. C. Sime, E. W. Wolff, C. Le Quéré, and L. Bopp (2013), Southern Hemisphere westerly wind changes during the Last Glacial Maximum: paleo-data synthesis, Quat. Sci. Rev., 68, 76–95, doi:10.1016/j.quascirev.2013.01.017.

Kostianoy, A., A. I. Ginzburg, M. Frankignoulle, and B. Delille (2004), Fronts in the Southern Indian Ocean as inferred from satellite sea surface temperature data, J. Mar. Syst., 45(1-2), 55–73, doi:10.1016/j.jmarsys.2003.09.004.

Kushner, P. J., I. M. Held, and T. L. Delworth (2001), Southern Hemisphere Atmospheric Circulation Response to Global Warming, J. Clim., 14(10), 2238–2249, doi:10.1175/1520- 0442(2001)014<0001:SHACRT>2.0.CO;2.

Law, C. S. et al. (2006), Patch evolution and the biogeochemical impact of entrainment during an iron fertilisation experiment in the sub-Arctic Pacific, Deep Sea Res. Part II Top. Stud. Oceanogr., 53(20- 22), 2012–2033, doi:10.1016/j.dsr2.2006.05.028.

Lutjeharms, J., and H. Valentine (1984), Southern ocean thermal fronts south of Africa, Deep Sea Res. Part A. Oceanogr. Res. Pap., 31(12), 1461–1475, doi:10.1016/0198-0149(84)90082-7.

Marino, G., R. Zahn, M. Ziegler, C. Purcell, G. Knorr, I. R. Hall, P. Ziveri, and H. Elderfield (2013), Agulhas salt-leakage oscillations during abrupt climate changes of the Late Pleistocene, Paleoceanography, 28, doi:10.1002/palo.20038.

Martin, J. (1990), Glacial-interglacial CO2 change: The iron hypothesis, Paleoceanography, 5(1), 1–13.

Martinez-Garcia, a., D. M. Sigman, H. Ren, R. F. Anderson, M. Straub, D. a. Hodell, S. L. Jaccard, T. I. Eglinton, and G. H. Haug (2014), Iron Fertilization of the Subantarctic Ocean During the Last Ice Age, Science, 343(6177), 1347–1350, doi:10.1126/science.1246848.

Matano, R. (1993), On the separation of the Brazil Current from the coast, J. Phys. Oceanogr.

Melet, A., R. Hallberg, S. Legg, and M. Nikurashin (2014), Sensitivity of the Ocean State to Lee Wave– Driven Mixing, J. Phys. Oceanogr., 44(3), 900–921, doi:10.1175/JPO-D-13-072.1.

Moore, J., and M. Abbott (2000), Phytoplankton chlorophyll distributions and primary production in the Southern Ocean, J. Geophys. Res., 105(1999).

Moore, J. K., and M. R. Abbott (2002), Surface chlorophyll concentrations in relation to the Antarctic Polar Front: seasonal and spatial patterns from satellite observations, J. Mar. Syst., 37(1-3), 69–86, doi:10.1016/S0924-7963(02)00196-3. doi:10.5194/bg-5-631-2008

Moore, J. K., and O. Braucher (2008), Sedimentary and mineral dust sources of dissolved iron to the world ocean, Biogeosciences, 5(1994), 631–656.

Moore, J. K., M. R. Abbott, G. Richman, and D. M. Nelson (2000), The Southern Ocean at the last glacial maximum- A strong sink for atmospheric carbon dioxide of this expanded SIZ throughout the Southern, Global Biogeochem. Cycles, 14(1), 455–475.

Moore, J. K., S. C. Doney, and K. Lindsay (2004), Upper ocean ecosystem dynamics and iron cycling in a global three-dimensional model, Global Biogeochem. Cycles, 18(4), doi:10.1029/2004GB002220.

Munday, D., and D. Marshall (2005), On the separation of a barotropic western boundary current from a cape, J. Phys. Oceanogr., 35, 1726–1743. doi: http://dx.doi.org/10.1175/JPO2783.1

Nakamura, H., T. Sampe, A. Goto, W. Ohfuchi, and S.-P. Xie (2008), On the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation, Geophys. Res. Lett., 35(15), L15709, doi:10.1029/2008GL034010.

Naveira-Garabato, A., R. Ferrari, and K. Polzin (2011), Eddy stirring in the Southern Ocean, J. Geophys. Res., 116(C9), 1–29, doi:10.1029/2010JC006818.

Nikurashin, M., and R. Ferrari (2010), Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Application to the Southern Ocean, J. Phys. Oceanogr., 40(9), 2025–2042, doi:10.1175/2010JPO4315.1.

Nürnberg, D., and J. Groeneveld (2006), Pleistocene variability of the Subtropical Convergence at East Tasman Plateau: Evidence from planktonic foraminiferal Mg/Ca (ODP Site 1172A), Geochemistry Geophys. Geosystems, 7(4), doi:10.1029/2005GC000984.

O’Neill, L. W., D. B. Chelton, and S. K. Esbensen (2003), Observations of SST-Induced Perturbations of the Wind Stress Field over the Southern Ocean on Seasonal Timescales, J. Clim., 16, 2340–2354.

O’Neill, L. W., S. K. Esbensen, N. Thum, R. M. Samelson, and D. B. Chelton (2010a), Dynamical Analysis of the Boundary Layer and Surface Wind Responses to Mesoscale SST Perturbations, J. Clim., 23(3), 559–581, doi:10.1175/2009JCLI2662.1.

O’Neill, L. W., D. B. Chelton, and S. K. Esbensen (2010b), The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence, J. Clim., 23(2), 255–281, doi:10.1175/2009JCLI2613.1.

Olbers, D., D. Borowski, C. Völker, and J.-O. Wölff (2004), The dynamical balance, transport and circulation of the Antarctic Circumpolar Current, Antarct. Sci., 16(4), 439–470, doi:10.1017/S0954102004002251.

Orsi, A., T. Whitworth, and W. D. N. Jr (1995), On the meridional extent and fronts of the Antarctic Circumpolar Current, Deep Sea Res. Part I Oceanogr., 42(5), 641–673, doi:10.1016/0967- 0637(95)00021-W.

Peeters, F. J. C., R. Acheson, G.-J. a Brummer, W. P. M. De Ruijter, R. R. Schneider, G. M. Ganssen, E. Ufkes, and D. Kroon (2004), Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods., Nature, 430(7000), 661–5, doi:10.1038/nature02785.

Planquette, H. et al. (2007), Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean, Deep Sea Res. Part II Top. Stud. Oceanogr., 54(18-20), 1999–2019, doi:10.1016/j.dsr2.2007.06.019.

Pollard, R., M. Lucas, and J. Read (2002), Physical controls on biogeochemical zonation in the Southern Ocean, Deep Sea Res. Part II Top. Stud., 49(2002), 3289–3305.

Pollard, R. T. et al. (2009), Southern Ocean deep-water carbon export enhanced by natural iron fertilization., Nature, 457(7229), 577–80, doi:10.1038/nature07716.

Le Quéré, C. et al. (2007), Saturation of the southern ocean CO2 sink due to recent climate change., Science, 316(5832), 1735–8, doi:10.1126/science.1136188.

Raiswell, R., and D. Canfield (2012), The iron biogeochemical cycle past and present, Geochem. Perspect, 1(1).

Read, J. ., M. . Lucas, S. . Holley, and R. . Pollard (2000), Phytoplankton, nutrients and hydrography in the frontal zone between the Southwest Indian Subtropical gyre and the Southern Ocean, Deep Sea Res. Part I Oceanogr. Res. Pap., 47(12), 2341–2367, doi:10.1016/S0967-0637(00)00021-2.

Roberts, M. J. et al. (2009), Impact of Resolution on the Tropical Pacific Circulation in a Matrix of Coupled Models, J. Clim., 22(10), 2541–2556, doi:10.1175/2008JCLI2537.1.

Romero, S. I., A. R. Piola, M. Charo, and C. a. E. Garcia (2006), Chlorophyll- a variability off Patagonia based on SeaWiFS data, J. Geophys. Res., 111(C5), C05021, doi:10.1029/2005JC003244.

Rosso, I., A. M. Hogg, P. G. Strutton, A. E. Kiss, R. Matear, A. Klocker, and E. van Sebille (2014), Vertical transport in the ocean due to sub-mesoscale structures: Impacts in the Kerguelen region, Ocean Model., 80, 10–23, doi:10.1016/j.ocemod.2014.05.001.

De Ruijter, W. (1982), Asymptopic Analysis of the Agulhas and Brazilian Current Systems, J. Phys. Oceanogr., 12(4), 361–373.

Ruijter, W. P. M. De, A. Biastoch, S. S. Drijfhout, J. R. E. Lutjeharms, R. P. Matano, T. Pichevin, V. Leeu, and W. Weijer (1999), Indian-Atlantic interocean Dynamics , estimation and impact ring shedding, , 104.

Sallée, J. B., K. Speer, and R. Morrow (2008), Response of the Antarctic Circumpolar Current to Atmospheric Variability, J. Clim., 21(12), 3020–3039, doi:10.1175/2007JCLI1702.1.

Sanial, V., P. van Beek, B. Lansard, F. D’Ovidio, E. Kestenare, M. Souhaut, M. Zhou, and S. Blain (2014), Study of the phytoplankton plume dynamics off the Crozet Islands (Southern Ocean): A geochemical-physical coupled approach, J. Geophys. Res. Ocean., 119, 1–11, doi:10.1002/2013JC009305.

Saraceno, M., C. Provost, and A. R. Piola (2005), On the relationship between satellite-retrieved surface temperature fronts and chlorophyll a in the western South Atlantic, J. Geophys. Res., 110(C11), C11016, doi:10.1029/2004JC002736.

Shaffrey, L. C. et al. (2009), U.K. HiGEM: The New U.K. High-Resolution Global Environment Model— Model Description and Basic Evaluation, J. Clim., 22(8), 1861–1896, doi:10.1175/2008JCLI2508.1.

Sheen, K. L. et al. (2014), Eddy-induced variability in Southern Ocean abyssal mixing on climatic timescales, , (July), 6–11, doi:10.1038/NGEO2200.

Sime, L. C., K. E. Kohfeld, C. Le, E. W. Wolff, A. M. De Boer, R. M. Graham, and L. Bopp (2013), Southern Hemisphere westerly wind changes during the Last Glacial Maximum : model-data comparison, Quarternary Sci. Rev., 64, 104–120. doi: 10.1016/quascirev.2012.12.008

Sinha, B., and K. J. Richards (1999), Jet Structure and Scaling in Southern Ocean Models, J. Phys. Oceanogr., 29(6), 1143–1155, doi:10.1175/1520-0485(1999)029<1143:JSASIS>2.0.CO;2.

Small, R. J., S. P. deSzoeke, S. P. Xie, L. O’Neill, H. Seo, Q. Song, P. Cornillon, M. Spall, and S. Minobe (2008), Air–sea interaction over ocean fronts and eddies, Dyn. Atmos. Ocean., 45(3-4), 274–319, doi:10.1016/j.dynatmoce.2008.01.001.

Smetacek, V. et al. (2012), Deep carbon export from a Southern Ocean iron-fertilized diatom bloom., Nature, 487(7407), 313–9, doi:10.1038/nature11229.

Smith, R. O., R. Vennell, H. C. Bostock, and M. J. M. Williams (2013), Interaction of the subtropical front with topography around southern new zealand, Deep Sea Res. Part I Oceanogr. Res. Pap., doi:10.1016/j.dsr.2013.02.007.

Smythe-Wright, D., P. Chapman, C. D. Rae, L. V. Shannon, and S. M. Boswell (1998), Characteristics of the South Atlantic subtropical frontal zone between 15°W and 5°E, Deep Sea Res. Part I Oceanogr. Res. Pap., 45(1), 167–192, doi:10.1016/S0967-0637(97)00068-X.

Sokolov, S., and S. R. Rintoul (2002), Structure of Southern Ocean fronts at 140°E, J. Mar. Syst., 37(1-3), 151–184, doi:10.1016/S0924-7963(02)00200-2.

Sokolov, S., and S. R. Rintoul (2007a), Multiple Jets of the Antarctic Circumpolar Current South of Australia*, J. Phys. Oceanogr., 37(5), 1394–1412, doi:10.1175/JPO3111.1.

Sokolov, S., and S. R. Rintoul (2007b), On the relationship between fronts of the Antarctic Circumpolar Current and surface chlorophyll concentrations in the Southern Ocean, J. Geophys. Res., 112(C7), 1–17, doi:10.1029/2006JC004072.

Sokolov, S., and S. R. Rintoul (2009a), Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths, J. Geophys. Res., 114(C11), 1–19, doi:10.1029/2008JC005108.

Sokolov, S., and S. R. Rintoul (2009b), Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 2. Variability and relationship to sea surface height, in Journal of Geophysical Research, vol. 114, pp. 1–15.

Stramma, L. (1992), The South Indian Ocean Current, J. Phys. Oceanogr., 22, 421–430.

Stramma, L., and R. Peterson (1990), The South Atlantic Current, J. Phys. Ocean., 20, 846–859.

Stramma, L., R. G. Peterson, and M. Tomczak (1995), The south Pacific current, J. Phys. Oceanogr., 25, 77–91.

Sullivan, C. W., K. R. Arrigo, C. R. McClain, J. C. Comiso, and J. Firestone (1993), Distributions of phytoplankton blooms in the southern ocean., Science (80-. )., 262(5141), 1832–7, doi:10.1126/science.262.5141.1832.

Tagliabue, A., O. Aumont, and L. Bopp (2014), The impact of different external sources of iron on the global carbon cycle, Geophys. Res. Lett., 41, 920–926, doi:10.1002/2013GL059059.

Tansley, C. E., and D. P. Marshall (2001), On the Dynamics of Wind-Driven Circumpolar Currents, J. Phys. Oceanogr., 31(11), 3258–3273, doi:10.1175/1520-0485(2001)031<3258:OTDOWD>2.0.CO;2.

Thompson, A. F. (2010), Jet Formation and Evolution in Baroclinic Turbulence with Simple Topography, J. Phys. Oceanogr., 40(2), 257–278, doi:10.1175/2009JPO4218.1.

Thompson, A. F., and J.-B. Sallée (2012), Jets and Topography: Jet Transitions and the Impact on Transport in the Antarctic Circumpolar Current, J. Phys. Oceanogr., 42(6), 956–972, doi:10.1175/JPO-D-11-0135.1.

Thompson, A. F., P. H. Haynes, C. Wilson, and K. J. Richards (2010), Rapid Southern Ocean front transitions in an eddy-resolving ocean GCM, Geophys. Res. Lett., 37(23), 1–5, doi:10.1029/2010GL045386.

Thompson, D. W. J., S. Solomon, P. J. Kushner, M. H. England, K. M. Grise, and D. J. Karoly (2011), Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change, Nat. Geosci., 4(11), 741–749, doi:10.1038/ngeo1296.

Toggweiler, J. R., and J. Russell (2008), Ocean circulation in a warming climate., Nature, 451(7176), 286– 8, doi:10.1038/nature06590.

Toggweiler, J. R., J. L. Russell, and S. R. Carson (2006), Midlatitude westerlies, atmospheric CO 2 , and climate change during the ice ages, Paleoceanography, 21(2), 1–15, doi:10.1029/2005PA001154.

Tyrrell, T., a. Merico, J. J. Waniek, C. S. Wong, N. Metzl, and F. Whitney (2005), Effect of seafloor depth on phytoplankton blooms in high-nitrate, low-chlorophyll (HNLC) regions, J. Geophys. Res., 110(G2), G02007, doi:10.1029/2005JG000041.

Whitehouse, M. J., a. Atkinson, R. E. Korb, H. J. Venables, D. W. Pond, and M. Gordon (2012), Substantial primary production in the land-remote region of the central and northern Scotia Sea, Deep Sea Res. Part II Top. Stud. Oceanogr., 59-60, 47–56, doi:10.1016/j.dsr2.2011.05.010.

Whitworth, T. (1983), Monitoring the transport of the Antarctic circumpolar current at Drake Passage, J. Phys. Oceanogr.

Zharkov, V., and D. Nof (2008), Agulhas ring injection into the South Atlantic during glacials and interglacials, Ocean Sci. Discuss., 5(1), 39–75, doi:10.5194/osd-5-39-2008.

Ziegler, M., P. Diz, I. R. Hall, and R. Zahn (2013), Millennial-scale changes in atmospheric CO2 levels linked to the Southern Ocean carbon isotope gradient and dust flux, Nat. Geosci., 6(6), 457–461, doi:10.1038/ngeo1782.

Özgökmen, T., E. P. Chassignet, and A. M. Paiva (1997), Impact of wind forcing, bottom topography, and inertia on midlatitude jet separation in a quasigeostrophic model, J. Phys. Oceanogr., 27, 2460– 2476.