The role of Southern Ocean fronts in the global climate system

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The role of Southern Ocean fronts in the global climate system

Robert M. Graham

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Till min mamma & pappa Tack för allt

Cover picture by Dr. Jennifer A. Graham,

Printed in Sweden by US-AB Stockholm University, 2014 Distributor: Department of Geological Sciences

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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.

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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.

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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

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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.

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Abstract

List of papers

Acknowledgements

1. Introduction

2. The modern day frontal structure in the Southern Ocean 2.1. The importance of an accurate frontal climatology 2.2. Defining ocean fronts

2.2.1. Fronts as water mass boundaries 2.2.2. Fronts as strong currents

2.3. The Dynamic Subtropical Front

3. The relationship between ocean fronts and the wind field

3.1. Motivation: Southern Ocean fronts in a changing climate

3.2. The response of fronts to a southward shift of the westerly winds

3.3. The relationship between the Subtropical Front and zero wind stress curl

4. Biological activity at ocean fronts

4.1. Background: limits on primary production in the Southern Ocean 4.2. The role of western boundary currents for nutrient supply

5. Applications to the Last Glacial Maximum

5.1. Southern Ocean changes at the Last Glacial Maximum 5.2. Advances made in this thesis

5.2.1. Evaluating possible frontal shifts

5.2.2. Explaining enhanced export production in the Sub-‐Antarctic Zone

6. Unresolved questions and possible future directions

6.1. What sets the location of the Dynamic Subtropical Front?

6.2. Inter-‐model comparison of Southern Ocean fronts 6.3. Location of fronts at the Last Glacial Maximum 6.4. Shelf sediment iron source parameterisation

7. Key Results

7.1. Paper I 7.2. Paper II 7.3. Paper III 7.4. Paper IV

8. References

III

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VI

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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

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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.

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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?

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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.

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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].

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.

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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.

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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].

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]

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2.3. The Dynamic Subtropical Front

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 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.

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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).

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]

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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.

The role of Southern Ocean fronts in the global climate system