The Location and Variability of Southern ocean Fronts

(1)

1

Licenciate Thesis Marine Geology

The Location and Variability of Southern Ocean Fronts

Robert M. Graham

Stockholm 2013

(2)

2

Abstract

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Moreover, the Subtropical Front (STF) is believed play a key role in the global climate system. Model simulations have shown that a wind induced poleward shift of the STF may strengthen the Atlantic Meridional Overturning Circulation by allowing a stronger salt flux from the Indian to the Atlantic Ocean. This hypothesis has important implications for our future climate, as global warming scenarios predict an intensification and southward shift of the Southern Hemisphere Westerlies. Nonetheless, confirmation of the theory has been limited by a lack of data and also our poor dynamical understanding of fronts. In this thesis we produce a new working dynamical definition of the STF and study the relation of this and other Southern Ocean fronts to the winds and topography.

We first explore the relative importance of bottom topography and winds for determining the location and structure of Southern Ocean fronts, using 100 years of a control and climate change simulation on the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3° in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current’s (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the STF shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography.

We then use satellite sea surface temperature (SST) data to show that the traditional STF, as defined by water mass properties, is comprised of two distinct dynamical regimes. On the western side of each basin the traditional STF coincides with a deep current that has strong SST gradients and no seasonal cycle. We define this as the Dynamical STF (DSTF). Further east, the DSTF diverges from the traditional STF and tracks south-eastwards into the centre of each basin to merge with the Sub- Antarctic Front. The traditional STF continues to the eastern side of the basins where it coincides with the so-called Subtropical Frontal Zone, a zone of shallow SST fronts that have little transport and large seasonal cycles.

Finally, we compare the position of our DSTF and previous STF climatologies to the mean wind stress curl field, from satellite scatterometry winds. We find that contrary to previous suggestions, the position of the STF does not coincide with the zero or maximum wind stress curl. Using output from the HiGEM model we show that instead of being controlled purely by the wind field, transport south of the subtropical gyre, including the latitude of the zero wind stress curl, is forced strongly by the bottom pressure torque that is a product of the interaction of the ACC with the ocean floor topography.

Here in these studies we have provided a new simple and reproducible method for identifying fronts. We have also given new insights into the seasonal and decadal variability of fronts, as well as how fronts may respond to future climate change. This has highlighted previous misconceptions regarding the relationship between the position of fronts and winds. Finally we have provided a new framework to study the behaviour of the STF and interpret observations, paving the way for better predictions on the likelihood and impact of future STF changes.

(3)

3

List of Papers

This thesis is comprised of a short introduction to the background of my PhD work, accompanied by the following manuscripts:

I. Graham, R. M., de Boer, A. M., Heywood, K. J., Chapman, M. R., & Stevens, D. P. Southern Ocean fronts: Controlled by wind or topography? Journal of Geophysical Research. 117, C08018, doi:10.1029/2012JC007887, 2012.

II. Graham, R. M., de Boer, A. M. The Two Dynamical Regimes of the Subtropical Front.

III. de Boer, A. M., Graham, R. M., Thomas, M. D., Kohfeld, K. E., The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front.

For Manuscripts I and II, R. M. Graham was the main contributor in terms of analysis and writing. All authors contributed with ideas and helped edit the manuscript. The idea for Manuscript II was initiated by R. M. Graham. A. M. de Boer was the main contributor in terms of analysis and writing for Manuscript III. R. M. Graham helped with the analysis of the satellite data and produced the final figures. M. D. Thomas helped with the analysis of the model output. All authors helped edit the manuscript and contributed ideas.

Reprints of Manuscript I are made with permission of John Wiley & Sons, Inc.

The following papers are not included as a part of this thesis:

Kohfelf, K. E., Graham, R. M., de Boer, A. M., Sime, L. C., Wolff W. E., Le Quere, C., Bopp, L.

Southern Hemisphere Westerly Wind Changes during the Last Glacial Maximum: Paleo-data Synthesis. Quarternary Science Reviews. (in press).

Sime, L. C., Kohfelf, K. E., Le Quere, C., Wolff, W. E., de Boer, A. M., Graham, R. M., Bopp, L.

Southern Hemisphere Westerly Wind Changes during the Last Glacial Maximum: Model-Data Comparison. Quarternary Science Reviews. 64, 104-120, doi: 10.1016/j.quascirev.2012.12.008, 2013.

(4)

4

The Southern Ocean encompasses all ocean south of 60°S. This ocean is unique for many reasons. For example, the Southern Ocean is the only ocean on Earth today with no meridional (north-south) boundary [Olbers et al., 2004] (Figure 1, [Kuhlbrodt et al., 2007]). This enables the Antarctic Circumpolar Current (ACC) to exist, which is both the longest and also strongest (~ 130 Sv = 130 x 10⁶ m³/s) ocean current [Whitworth, 1983]. The ACC interlinks the three other major ocean basins; the Atlantic, Pacific and Indian Oceans. This makes it an important component of the climate system as it can transmit signals from one region to another [Gille, 2002]. Interestingly the ACC is not a single current. Instead it is made up of a number of small and intense currents or jets. These act to separate water masses with different properties such as temperature and salinity. The boundary between these water masses are referred to as fronts. The location of these fronts is important for understanding the distribution of species within the Southern Ocean [Pollard et al., 2002].

Fronts of the Southern Ocean are an integral part of the ACC. Therefore, in order to monitor variability in the ACC, it is necessary to accurately monitor variability in both the intensity and position of individual fronts. Likewise, to improve our understanding of the driving mechanisms of

Figure 1 Strongly simplified sketch of the global overturning circulation system. In the Atlantic, warm and saline waters flow northward all the way from the Southern Ocean into the Labrador and Nordic Seas. By contrast, there is no deepwater formation in the North Pacific, and its surface waters are fresher. Deep waters formed in the Southern Ocean become denser and thus spread in deeper levels than those from the North Atlantic. Note the small, localized deepwater formation areas in comparison with the widespread zones of mixing-driven upwelling. Wind-driven upwelling occurs along the Antarctic Circumpolar Current (ACC). (From Kuhlbrodt et al. [2007]) After Rahmstorf [2002].

(6)

6 the ACC, and how it may change under future warming scenarios, we must improve our understanding of fronts.

The location of fronts, and in particular the Subtropical Front (STF), is also thought to be of great importance for climate [Bard and Rickaby, 2009; Biastoch et al., 2009; Beal et al., 2011]. The STF extends eastwards from the Western Atlantic, at ~40°S, below the southern tip of Africa and into the Indian Ocean [Orsi et al., 1995; Belkin and Gordon, 1996; Sokolov and Rintoul, 2009a]. On the east coast of Africa is the strong southward flowing Agulhas Current, which carries warm and saline water [Lutjeharms and Valentine, 1984]. This current continues south until it meets the STF. At the STF the Agulhas Current is blocked and forced to retroflect eastward, back into the Indian Ocean (Figure 2, [Beal et al., 2011]). However, there is a small gap between the STF and southern tip of Africa. This allows some of the warm and saline Indian Ocean water, transported by the Agulhas Current, to leak into the Atlantic Ocean [Beal et al., 2011]. This flux of water from the Indian to the Atlantic Ocean provides an important source of salinity that is needed to sustain the Atlantic Meridional Overturning Circulation (AMOC) [Beal et al., 2011].

Figure 2 Agulhas leakage affected by westerly winds and position of subtropical front. Schematic of the greater Agulhas system embedded in the Southern Hemisphere supergyre. Background colours show the mean subtropical gyre circulation, depicted by climatological dynamic height integrated between the surface and 2,000 dbar, from the CARS database [Ridgway and Dunn, 2007]. Black arrows and labels illustrate significant features of the flow. An outline of the Southern Hemisphere supergyre is given by the grey dashed line. The plot on the right shows the southward expansion of the Southern Hemisphere westerlies over a 30-yr period, from the CORE2 wind stress [Large and Yeager, 2004] averaged between longitudes 20uE and 110uE (Indian Ocean sector). The expected corresponding southward shift of the subtropical front is illustrated by red dashed arrows and would affect Agulhas leakage (shown as eddies) and the pathway between leakage and the AMOC, which is highlighted with a red box. (from Beal et al. [2011])

(7)

7 The AMOC transports a large amount of heat energy northwards, and is responsible for northern Europe’s mild climate [Barker et al., 2009; Denton et al., 2010]. It has been hypothesised that changes to the position of the STF, and therefore volume of Agulhas Leakage, may have helped initiate transitions between glacial and interglacial climate states, through their effect on the AMOC.

For example, during glacial intervals the STF may have been positioned further north, restricting the flow of salt into the Atlantic and shutting down the AMOC [Bard and Rickaby, 2009; Beal et al., 2011;

De Deckker et al., 2012].

(8)

8 2. Background

2.1. Identification and definitions of Southern Ocean fronts

Traditionally fronts have been defined as the boundary between different water masses. Often these boundaries are identified using scalar water mass properties such as isotherms or isohalines.

Early studies determined these boundaries from in situ hydrographic measurements. Due to the vast size of the Southern Ocean, these data have historically been sparse. However, some studies have successfully synthesised the available data to produce mean climatologies of these fronts. Five primary fronts were identified in these assessments of the Southern Ocean [Orsi et al., 1995].

Starting from the South these are the Southern Boundary Front, the Southern ACC Front, the Polar Front, the Sub-Antarctic Front and the Sub-Tropical Front (Figure 3) [Orsi et al., 1995]. By defining fronts as the boundary between different water masses, or using scalar quantities such as temperature and salinity, we inherently assume that fronts are continuous. Also, if we use an isotherm or isohaline to define a front we assume that these properties are conserved along the front’s path.

Recently satellite data have become widely available, in particular SST and sea surface height (SSH) products. These satellite data are ideal for observing fronts; they have high spatial resolution, wide spatial coverage and also high temporal resolution [Sokolov and Rintoul, 2002; Kostianoy et al., 2004]. High resolution data is favourable for studying fronts, due to the small spatial scale and highly variable nature of these features. The availability of satellite data has led to huge advances in our understanding of fronts. These high resolution data sets have allowed us, for the first time, to see in detail the intricate pattern of fronts over the Southern Ocean.

Figure 3 – Location of fronts defined by Orsi et al. [1995]. From the top these fronts are the Sub-Tropical Front, Sub-Antarctic Front and Antarctic Polar Front.

(9)

9 Fronts are often identified as enhanced horizontal gradients, in SST or SSH, when using satellite data [Moore et al., 1999; Sokolov and Rintoul, 2002; Kostianoy et al., 2004; Burls and Reason, 2006;

Dong et al., 2006; Dencausse et al., 2011]. If using this method for identifying fronts, fronts are considered to be dynamical features as opposed to water mass boundaries.

A dynamical front is essentially a strong current (Figure 4c). It is difficult for a parcel of water to move across a fast current. Instead of moving across the current, the parcel of water will be carried along with the flow. This means fronts act as barriers in the ocean, and this prevents water on either side of the front from mixing [Thompson, 2010]. As a result dynamical fronts often coincide with water mass boundaries and are coincident with enhanced horizontal gradients in water mass properties. Thus, it possible to identify a dynamical front simply as a peak in SST gradient (Figure 4a) [Hughes and Ash, 2001; Kostianoy et al., 2004].

When we consider fronts as dynamical features we lose the simple concept of five continuous circumpolar fronts, which was found from using water mass boundary definitions. Instead, a far more complex pattern of multiple fronts merging and splitting is seen. Sokolov and Rintoul [2007] tried to resolve this apparent discrepancy. They identified dynamical fronts as enhanced gradients in SSH (like the SST gradient, the SSH gradient is also increased over strong currents). In order to determine how certain fronts related to features up/downstream they derived a method of selecting SSH contours that have paths which coincide best with the regions of enhanced gradients. They define

Figure 4 – Ideal front. 30 year mean output from HiGEM control simulation. Meridional transect at 25°E. (a) Sea surface temperature gradient. (b) Sea Surface temperature. (c) Zonal velocity (red – eastwards, blue – westwards).

(10)

10 these SSH contours as fronts, in a similar way to which past studies have used isotherms and isohalines to define fronts. This new method has proven to be a simple and powerful tool. It makes fronts very easy to identify and track, enabling us to learn about their temporal variability. For example, it was shown that fronts migrate north and south seasonally [Sallée et al., 2008]. Also new and updated climatologies of fronts in the Southern Ocean were produced. In this climatology 10 fronts were defined, relating to separate branches of previously defined fronts [Sokolov and Rintoul, 2009a].

The work of Sokolov and Rintoul has been extremely influential in the field of ocean fronts. Many studies over the last decade have used their method of defining fronts as a SSH contour [Sokolov and Rintoul, 2002, 2007, 2009a, 2009b; Sallée et al., 2008; Billany et al., 2010; Volkov and Zlotnicki, 2012].

However, a number of important caveats of the method are often overlooked. The first is the key assumption that the SSH value of a front is constant in both space and time. It is suggested that this assumption is valid in the study Sokolov and Rintoul [2007]. However, previous studies have highlighted the fact that water mass properties such as temperature can change significantly at fronts in both space and time [Kostianoy et al., 2004]. Therefore tying the position of fronts to scalar quantities such as these can be misleading [Pollard et al., 2002]. Thus, we must treat these assumptions with caution. Secondly, Sokolov and Rintoul [2007] acknowledge that any SSH contour associated with a front will not coincide with a dynamical front at all points along the contour.

Therefore, it is possible to have a large shift of a SSH contour in time that resembles a frontal shift and yet have no current present at this particular point. This factor introduces considerable uncertainty to the conclusions of studies interpreting frontal variability using the SSH contour method of Sokolov and Rintoul [2002]. Sokolov and Rintoul [2009] also highlighted that their method performs poorly at tracking the STF, which is an important front for climate.

Other studies have identified fronts using satellite SST data. These studies have often used a threshold value of SST gradient to identify a front. Dong et al. [2006] and Moore et al. [1999] defined the polar front as being the southernmost point at any longitude where the SST gradient exceeded this threshold and created a new climatology of this front . By simply using a threshold value of SST gradients, this method potentially neglects regions where the SST gradient may be higher and therefore does not necessarily locate the core of a front. Moreover, these studies indicate that the Polar Front is a continuous dynamic feature. Kostianoy et al. [2004] also studied the variability of fronts using SST gradients and SST maps, but include the positions from past climatologies in their method to identify fronts.

(11)

11 The number of different methods used to identify fronts introduces considerable uncertainty when comparing the results of different studies. Moreover, each method of identifying fronts has its own uncertainties. There is also considerable confusion regarding whether fronts should be defined as water mass boundaries (temperature and salinity values) or dynamical features (strong currents).

This confusion is hindering our ability to accurately locate fronts and monitor variability. There is therefore a clear need for a simple and robust method of identifying fronts.

Reconstructing the position of fronts in past climates has become a key goal of paleo- oceanography. This is both to improve our understanding of the relationship between the latitude of the STF and volume of Agulhas Leakage [Bard and Rickaby, 2009], and to infer details about the position of the Southern hemisphere Westerly Winds [De Deckker et al., 2012; Ho et al., 2012;

Kohfeld et al., 2013]. When trying to establish the position of fronts in the paleo-record, fronts are identified using water mass properties. The distribution of deep sea cores is relatively sparse. It is therefore not possible to reconstruct paleo SST gradients on the scales required to resolve frontal features. As a result it has become common practice to locate fronts at past time intervals using mean SSTs at modern day frontal zones [Prell et al., 1980; Howard and Prell, 1992]. This assumes not only that SSTs are constant along a front (spatially), but also that the SST at a front remains constant in time (temporally). Notably, these are very long time periods considered here (~100 ka or more), and typically during intervals of major climatic change; glacial-interglacial cycles, for example.

Unfortunately this assumption is difficult to justify, not least because in the present day ocean, satellite studies have shown that the SST varies considerably both spatially and seasonally along a front [Kostianoy et al., 2004]. Thus it is difficult to see why the SST at a front should remain constant over glacial-interglacial time periods.

The reconstruction of paleo frontal positions is challenging. Most studies interpret frontal positions to be displaced northwards during glacial intervals [Kohfeld et al., 2013]. Fronts mark a boundary between warmer and cooler water. Thus if a front is displaced northwards (equatorward), the region of colder water would expand and extend further north. Therefore, we would expect to see a cooling signal in our record. The challenge in interpreting past frontal positions arises in trying to determine what portion of the cooling signal is the result of frontal shifts and which is due to a general global cooling, in response to the glacial climatic state [Kohfeld et al., 2013]. As of now this issue remains unresolved and creates large uncertainty in these interpretations.

(12)

12 2.2. The Relationship between fronts and wind stress

Many studies assume that the position of fronts is related to the pattern of overlying wind stress.

Winds insert energy into the ocean which helps drive the circulation. Thus, it seems logical that if you change the overlying wind stress you will alter the ocean circulation. However, studies disagree on the exact relationship between winds and fronts, and in particular the STF. Some studies suggest that the STF coincides with the latitude of the zero wind stress curl [Peeters et al., 2004; Zharkov and Nof, 2008; Dencausse et al., 2011], which corresponds to the maximum wind stress. In contrast, others suggest that the STF coincides with the maximum winds stress curl [Deacon, 1982; Burls and Reason, 2006; Bard and Rickaby, 2009], which relates to the maximum north-south gradient of zonal wind stress. Nevertheless, these studies all assume some near linear relationship between the STF and the band of Westerly winds over the Southern Ocean. Simulations of future warming scenarios on modern high resolution climate models suggest that the Southern Hemisphere Westerly Winds will shift southwards [Fyfe and Saenko, 2006; Wang et al., 2011]. Thus, if we are to assume such a relationship exists between the STF and Westerly Winds, we should expect a southward shift in the position of the STF in response to future warming, and the affect that this shift may have on climate [Biastoch et al., 2009; Beal et al., 2011].

Despite the assumed relationship between the position of fronts and the Westerly Winds, such a relationship has never been shown. Moreover, the fact there is confusion in the literature about whether the position of the STF corresponds to the maximum wind stress curl or zero winds stress curl, highlights that we do not yet understand how modern day frontal positions relate to the wind field, let alone past or future scenarios.

The idea that the STF should coincide with the zero wind stress curl stems from assuming that Sverdrup Balance holds in the Southern Ocean, and that the STF corresponds to the southern boundary of the Subtropical Gyre. If an ocean is in Sverdrup Balance then the circulation is driven purely by the wind. Key to the location of the STF, flow will be completely zonal (eastwards) at the latitude of the maximum wind stress, and this marks the southern boundary of the subtropical gyre.

This theory holds for the STF in flat bottom closed basins [De Ruijter, 1982]. However, the Southern Ocean is not a closed basin as it has no meridional boundary and furthermore, it does not have a flat bottom.

The concept that the STF should coincide with the maximum wind stress curl, stems from a different reasoning. Here people assume that the STF is formed as a result of wind driven convergence at the ocean surface [Burls and Reason, 2006; Bard and Rickaby, 2009]. Therefore the STF should sit where there is maximum convergence, which is at the maximum wind stress curl.

(13)

13 The confusion within the literature over what the STF is highlights our poor dynamical understanding of this oceanic feature, despite its potential importance in our climate system. Clearly there is a need for a detailed study of how the position of the present day STF is related to the overlying wind field.

While the exact relationship between the wind field and location of fronts remains unclear, a number of studies have shown that seasonal and inter-annual variability in frontal positions maybe driven by variability in the wind field. For example, a positive anomaly of the Southern Annular Mode is thought to push the Sub-Antarctic and Polar Fronts southwards in the Atlantic and Indian Sectors, and Northward in the Pacific Sector of the Southern Ocean [Sallée et al., 2008]. These correlations have been found using satellite data. These data only extend back ~20 years, and it is therefore unclear whether these relationships will hold over longer timescales or major climatic shifts. To answer these questions it is necessary to perform modelling studies.

(14)

14 2.3. The relationship between fronts and topography

The topography of the sea floor is believed to be important for determining the location of ocean fronts [Moore et al., 1999; Thompson, 2010]. It is clear from climatologies of fronts that their synoptic scale distribution follows a path around the major topographic features; namely the mid ocean ridges, continental shelves and plateaus [Orsi et al., 1995; Belkin and Gordon, 1996].

The flow of fronts around topography is primarily due to the conservation of potential vorticity in the ocean [Moore et al., 1999]. The potential vorticity (PV) is defined as:

PV = (ζ – f) / H

Where ζ is the relative vorticity, which is the curl of the velocity field (∇ x V = dv / dx – du / dy), H is the depth of the ocean and f is the planetary vorticity, which is a measure of the local effect of the rotation of the earth. The rotation of the earth becomes more important closer to the poles as we approach the axis of rotation (i.e. the magnitude of f increases) In reality the planetary vorticity is much larger than the relative vorticity and so the potential vorticity can be approximated as simply the planetary potential vorticity [Moore et al., 1999].

PV ≈ f/H

This means for example, if we move a column of water over a mid ocean ridge, the depth will decrease and we reduce the magnitude of H. Therefore, to conserve (keep constant) the potential vorticity we must also reduce f. This means reducing the effect of the earth’s rotation and so moving the water column equatorward, away from the axis of rotation. Thus, fronts are displaced equatorwards as they pass over a mid ocean ridge, and will return polewards again as they move off the ridge [Moore et al., 1999; Sinha and Richards, 1999; Sokolov and Rintoul, 2002; Thompson, 2010].

We call this affect topographic steering.

It is still unclear how well this approximation holds, and therefore how strong the topographic steering of fronts is in relation to the importance of winds. For example, if the band of westerly winds over the Southern Ocean were to shift north by 5 degrees latitude, and there was a front sitting on the southern side of a plateau spanning 10 degrees latitude, what would the effect be on the location of the front?

One of the main reasons we cannot yet answer this question is a lack of computer power. Most climate models are run at a resolution too coarse to be capable of resolving small scale frontal features. High resolution climate models that are able to accurately model fronts have been available for a number of years now [Sinha and Richards, 1999]. However, these models are extremely

(15)

15 computer intensive and so only short simulations (e.g. 10 years) are feasible. High-resolution models are still too computer intensive for the type of sensitivity experiments required, to investigate in detail the relationship between fronts, topography and winds, to be justifiable. As a result, the relative importance of wind and topography for determining the location of fronts is still unknown.

Nonetheless it is often stated in literature that in the vicinity of steep bottom slopes the location of fronts is constrained by the topography, while in regions where the ocean floor is flat fronts will follow changes in the wind field [Hayward et al., 2008; Ho et al., 2012]. However, there is little physical evidence to support this hypothesis.

(16)

16 3. Summary of Manuscripts

3.1. Manuscript I

In this study we take advantage of newly available high resolution (1/3 degree in the ocean) output from the eddy permitting model HiGEM; the UK Met Office’s High Resolution Global Environment Model. We analyse output from two 100 year simulations that were performed as part of a much larger modelling initiative to study the effects of global warming. There is a control and a climate change simulation in which CO2 concentrations are increased to four times present day values. This model output gives us, for the first time, high resolution and full depth fields of ocean properties with a longer time series than is currently available from satellites to study ocean fronts.

We derive a new, simple and robust method of identifying fronts; as local maxima in horizontal gradients of ocean properties (e.g SST, SSH) or zonal transport. Using this method we explore how the locations of fronts change in the climate change simulation. We also investigate how the mean location and number of fronts are related to the underlying topography.

We show that the number of fronts decreases in the vicinity of large topographic features and the intensity of fronts here increases. We also show that the location of fronts changes very little in response to a southward shift of the wind field in the climate change simulation, even where the sea floor is flat. The fronts which respond most to the changes in wind forcing are shallow (baroclinic) fronts such as the STF, and the presence of steep ridges does not hinder movement of these fronts.

3.2. Manuscript II

Here we use satellite SST data and our new method of identifying fronts to create the first climatology of the Dynamical STF. We find that there are two distinct dynamical regimes operating within the STF water mass boundary, as defined by previous studies. The STF is only a dynamical front on the western side of basins, and this current tracks south eastwards to merge with the Sub- Antarctic Front. On the eastern side of basins the STF water mass boundary coincides with an unrelated feature, the Subtropical Frontal Zone. This zone is comprised of numerous shallow SST fronts with little transport. In contrast to the Dynamical STF, which has no seasonal cycle, the Subtropical Frontal Zone experiences a large seasonal cycle of several degrees latitude. The identification of these two dynamical regimes helps resolve historic confusion within the literature about the characteristics of the STF, and paves the way for better observations to improve our understanding of the dynamics and variability of this feature.

(17)

17 3.3. Manuscript III

In this study we compare the position of our new climatology for the dynamical STF, and past climatologies of the STF water mass boundary, to the wind stress curl field. We show that, contrary to what is often stated in literature, the position of the present day STF does not correspond to the zero or maximum wind stress curl. The zero wind stress curl is located far south of the STF.

We next calculate the relative contribution of individual terms to the vorticity budget from HiGEM model output. This analysis shows that the Sverdrup Balance breaks down north of the STF.

South of the STF the importance of wind diminishes and the meridional flow is controlled primarily by the bottom pressure torque, due to the presence of the ACC. The position of the STF is therefore controlled also by the northward extent of the ACC compared with simply the latitude of zero wind stress curl.

(18)

18 4. Future Work

Our next goal is to use a combination of satellite data (sea surface height and temperature) and in-situ observations from ARGO floats and elephant seals to produce a new climatology of all dynamic fronts in the Southern Ocean and relate these to water mass boundaries.

We will also perform our own modelling study to investigate the effect of removing key topographic features from the Southern Ocean on the general circulation. In particular, we would like to test the hypothesis that removing the Scotia Ridge, east of Drake Passage, would reduce the northward penetration of the ACC in the Atlantic and shift the Subtropical gyre southward to the position of zero WSC.

(19)

19 5. Acknowledgements

First of all I would like to thank my parents and family for their continuous support for me all through school and university. I certainly would not be where I am today if it were not for them.

I must also thank my supervisor Agatha de Boer. The journey of my PhD to date has exceeded all of my expectations and has helped me achieve things and taken me to places I never would have dreamt (like South Africa, USA and moving to Stockholm!), and I owe much of this to her. The last two years have without doubt been some of the best in my life. She always makes time for interesting discussions whether they are scientific or social, and has done an excellent job of involving me in new projects and building collaborations. I must also thank my co-supervisors Karen Heywood, Mark Chapman and David Stevens from the University of East Anglia for all of their help on my first manuscript. I would also like to thank Karen Kohfeld, with whom I have been working a great deal over the last year. She has been a pleasure to work with and given me a great insight into the

‘paleo world’! A special thanks must go to my sister Jennifer Graham and officemates from ‘The Beach’ in Norwich, who had great patience in teaching me Matlab and also provided me with endless banter. I would also like to thank all of the staff from GEO and MISU for organising interesting seminars, courses, group discussions and such like.

A huge thanks must go to all of my friends, whichever country they may be in, who have supported me through the last few years. Especially my officemates; Moo, Francesco, Liselott and Francis, for putting up with my singing and also shouting at my computer! Also to our lunch and Fika group in GEO; Barbara, Adam, Oscar, Cliff, ‘Jing, Nut, Daniele, Catherine, Martina, Alexander and Roo etc., who never fail to brighten up my day. I would also like to thank all of my friends over in MISU, who have helped me so much with some of my courses. I must also give a massive thanks to Sarah, Laura and Paul who helped me out so much during my first few months in Stockholm, and have always been there for me… particularly when I have felt like a trip to the pub or eating some Lasagne! I would also like to thank my Oklahoma friends from my undergraduate; Becky, Kate, Rob, Rob, Laura and Pete, without whom I may never have chosen to do a PhD! I also wish to apologise for anyone whose name I have forgotten – I still love you!

(20)

20 6. References

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.

Barker, S., P. Diz, M. J. Vautravers, J. Pike, G. Knorr, I. R. Hall, and W. S. Broecker (2009),

Interhemispheric Atlantic seesaw response during the last deglaciation., Nature, 457(7233), 1097–102, doi:10.1038/nature07770.

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, Journal of Geophysical Research, 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, Journal of Marine Systems, 82(4), 304–310,

doi:10.1016/j.jmarsys.2010.06.005.

Broecker, W., and S. Barker (2007), A 190‰ drop in atmosphere’s Δ14C during the “Mystery Interval” (17.5 to 14.5 kyr), Earth and Planetary Science Letters, 256(1-2), 90–99, doi:10.1016/j.epsl.2007.01.015.

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

Deacon, G. E. R. (1982), Physical and biological zonation in the Southern Ocean, Deep Sea Research, 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, Nature Geoscience, 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?, Journal of Geophysical Research, 116(C2), 1–14, doi:10.1029/2010JC006587.

Denton, G. H., R. F. Anderson, J. R. Toggweiler, R. L. Edwards, J. M. Schaefer, and a. E. Putnam (2010), The Last Glacial Termination, Science, 328(5986), 1652–1656, doi:10.1126/science.1184119.

Dong, S., J. Sprintall, and S. T. Gille (2006), Location of the Antarctic Polar Front from AMSR-E Satellite Sea Surface Temperature Measurements, Journal of Physical Oceanography, 36(11), 2075–

2089, doi:10.1175/JPO2973.1.

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

(21)

21 Gille, S. T. (2002), Warming of the Southern Ocean since the 1950s., Science (New York, N.Y.),

295(5558), 1275–7, doi:10.1126/science.1065863.

Graham, R. M., A. M. D. Boer, K. J. Heywood, M. R. Chapman, and D. P. Stevens (2012), Southern Ocean fronts : Controlled by wind or topography ?, Journal of Geophysical Research, 46(0), 1–

35.

Hayward, B. W. et al. (2008), The effect of submerged plateaux on Pleistocene gyral circulation and sea-surface temperatures in the Southwest Pacific, Global and Planetary Change, 63(4), 309–

316, doi:10.1016/j.gloplacha.2008.07.003.

Ho, S. L., G. Mollenhauer, F. Lamy, A. Martínez-Garcia, M. Mohtadi, R. Gersonde, D. Hebbeln, S.

Nunez-Ricardo, A. Rosell-Melé, and R. Tiedemann (2012), Sea surface temperature variability in the Pacific sector of the Southern Ocean over the past 700 kyr, Paleoceanography, 27(4), 1–15, doi:10.1029/2012PA002317.

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. W., and E. R. Ash (2001), Eddy forcing of the mean flow in the Southern Ocean, Journal of Geophysical Research, 106(C2), 2713–2722, doi:10.1029/2000JC900332.

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, Journal of Marine Systems, 45(1- 2), 55–73, doi:10.1016/j.jmarsys.2003.09.004.

Kuhlbrodt, T., A. Griesel, and M. Montoya (2007), On the driving processes of the Atlantic meridional overturning circulation, Reviews of …, (2004), doi:10.1029/2004RG000166.1.INTRODUCTION.

Large, W. G., and S. G. Yeager (2004), Diurnal to Decadal Global Forcing For Ocean and Sea-Ice Models : The Data Sets and Flux Climatologies Diurnal to Decadal Global Forcing For Ocean and Sea-Ice Models : The Data Sets and Flux Climatologies, NCAR Technical Note, (May).

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

Moore, J. K., M. R. Abbott, and J. G. Richman (1999), Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data, Journal of Geophysical Research, 104, 3059–

3073.

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

Orsi, a. H., G. C. Johnson, and J. L. Bullister (1999), Circulation, mixing, and production of Antarctic Bottom Water, Progress in Oceanography, 43(1), 55–109, doi:10.1016/S0079-6611(99)00004-X.

(22)

22 Orsi, A., T. Whitworth, and W. D. N. Jr (1995), On the meridional extent and fronts of the Antarctic

Circumpolar Current, Deep Sea Research Part I: Oceanographic, 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.

Pollard, R., M. Lucas, and J. Read (2002), Physical controls on biogeochemical zonation in the Southern Ocean, Deep Sea Research Part II: Topical Studies …, 49(2002), 3289–3305.

Prell, W. L., W. H. Hutson, D. F. Williams, A. W. H. Bé, K. Geitzenauer, and B. Molfino (1980), Surface circulation of the Indian Ocean during the last glacial maximum, approximately 18,000 yr BP, Quaternary Research, 14(3), 309–336.

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

Rahmstorf, S. (2002), Ocean circulation and climate during the past 120,000 years., Nature, 419(6903), 207–14, doi:10.1038/nature01090.

Ridgway, K. R., and J. R. Dunn (2007), Observational evidence for a Southern Hemisphere oceanic supergyre, Geophysical Research Letters, 34(13), L13612, doi:10.1029/2007GL030392.

De Ruijter, W. (1982), Asymptopic Analysis of the Agulhas and Brazilian Current Systems, Journal of Physical Oceanography, 12(4), 361–373.

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

Sinha, B., and K. Richards (1999), Jet structure and scaling in Southern Ocean models, Journal of physical oceanography, 29(6), 1143–1155, doi:10.1175/1520-

0485(1999)029<1143:JSASIS>2.0.CO;2.

Sloyan, B. M., and S. R. Rintoul (2001), The Southern Ocean Limb of the Global Deep Overturning Circulation*, Journal of Physical Oceanography, 31(1), 143–173, doi:10.1175/1520-

0485(2001)031<0143:TSOLOT>2.0.CO;2.

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

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

Sokolov, S., and S. R. Rintoul (2009a), Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths, Journal of Geophysical Research, 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.

(23)

23 Thompson, A. F. (2010), Jet Formation and Evolution in Baroclinic Turbulence with Simple

Topography, Journal of Physical Oceanography, 40(2), 257–278, doi:10.1175/2009JPO4218.1.

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.

Volkov, D. L., and V. Zlotnicki (2012), Performance of GOCE and GRACE-derived mean dynamic topographies in resolving Antarctic Circumpolar Current fronts, Ocean Dynamics, 62(6), 893–

905, doi:10.1007/s10236-012-0541-9.

Wang, Z., T. Kuhlbrodt, and M. P. Meridith (2011), On the response of the Antarctic Circumpolar Current transport to climate change in coupled climate models, Journal of Geophysical Research, 1–49.

Whitworth, T. (1983), Monitoring the transport of the Antarctic circumpolar current at Drake Passage, Journal of Physical Oceanography.

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

The Location and Variability of Southern ocean Fronts

Licenciate Thesis Marine Geology