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1 Modification of the Deep Salinity-Maximum in the Southern Ocean by Circulation in the Antarctic 1 2 2 Circumpolar Current and the Weddell Gyre 3 4 3 Matthew Donnelly1, Harry Leach2 and Volker Strass3 5 6 4 1British Oceanographic Data Centre, National Oceanography Centre, Joseph Proudman Building, 6 Brownlow 7 8 5 Street, Liverpool, L3 5DA, UK 9 10 6 2Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, 11 12 7 L69 3GP, UK 13 14 8 3Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Postfach 12 01 61, D-27515 15 16 9 Bremerhaven, Germany 17 18 10 Corresponding author: Matthew Donnelly 19 20 11 E-mail: [email protected] 21 22 12 Telephone: +44 151 795 4892 23 24 13 25 26 14 Abstract 27 28 29 15 The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is 30 31 16 assessed using a set of 37 hydrographic sections collected over a 20 year period in the Southern Ocean as part of 32 33 17 the WOCE/CLIVAR programme. A circumpolar decrease in the value of the salinity maximum is observed 34 35 18 eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through 36 37 19 the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, 38 39 20 and in the Atlantic sector this acts to limit the direct poleward propagation of the salinity signal. Limited 40 41 21 entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddy-dominated eastern 42 43 -4 2 -1 22 limb. A vertical mixing coefficient, κV of (2.86 ± 1.06) x 10 m s and an isopycnal mixing coefficient, κI of 44 45 2 2 -1 46 23 (8.97 ± 1.67) x 10 m s are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar 47 -5 2 -1 48 24 Current (ACC). A κV of (2.39 ± 2.83) x 10 m s , an order of magnitude smaller, and a κI of (2.47 ± 0.63) x 49 50 25 102 m2 s-1, three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more 51 52 26 turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, 53 54 27 we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector 55 56 28 evidenced by the eastward propagation of the deep salinity-maximum signal, insulating the Weddell Gyre from 57 58 29 short-term changes in NADW characteristics. 59 60 61 62 1 63 64 65 30 Keywords: Southern Ocean; Antarctic Circumpolar Current; Weddell Gyre; Warm Deep Water; Lower 1 2 31 Circumpolar Deep Water; North Atlantic Deep Water; deep salinity maximum; entrainment; 3 4 32 mixing. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 2 63 64 65 33 1 - Introduction 1 2 34 The Southern Ocean is dominated by two main circulation features: the Antarctic Circumpolar Current 3 4 35 (ACC) and the Meridional Overturning Circulation (MOC). The MOC acts on a global scale to transport North 5 6 36 Atlantic Deep Water (NADW) southwards into the Southern Ocean, where it is modified to form Circumpolar 7 8 37 Deep Water (CDW) by exchanges with the other ocean basins during its circulation around Antarctica with the 9 10 38 ACC. CDW is brought towards the surface along sloping isopycnals by wind-driven upwelling in the Antarctic 11 12 39 Divergence. The lighter fraction of the CDW returns back northwards with the upper branch of the MOC, and 13 14 40 after undergoing further modification makes a major contribution to the Antarctic Intermediate Water (AAIW) 15 16 41 which subducts in the Antarctic Convergence. A denser CDW fraction, including Lower Circumpolar Deep 17 18 42 Water (LCDW) identified by a deep salinity maximum derived from the NADW (Patterson and Whitworth 19 20 43 1990), is advected farther southwards along trajectories which are further complicated by the presence of 21 22 44 Antarctic gyres (Orsi et al. 1993; Fahrbach et al. 1994, 1995). Within these gyres modified local forms of 23 24 45 LCDW, such as the fresher Warm Deep Water (WDW) in the Weddell Gyre, undergo further changes driven by 25 26 46 sea-ice formation over the continental shelf and interactions with the ice shelves that lead to the formation of 27 28 47 Weddell Sea Deep and Bottom Waters (WSDW and WSBW). Mixing of WSDW and LCDW (Speer and Zenk 29 30 48 (1993), Zenk and Hogg (1996), Rhein et al. (1998), Stramma and England (1999), Vanicek and Siedler (2002)) 31 32 49 eventually forms the Antarctic Bottom Water (AABW) which spreads northwards to fill most of the deep basins 33 34 50 of the world ocean (Purkey and Johnson, 2010). Transformation of the LCDW in the Southern Ocean is 35 36 51 therefore intimately linked to closing the lower limb of the global overturning circulation between the high 37 38 52 northern and high southern latitudes (Marshall and Speer 2012). 39 40 41 53 Simultaneously and superimposed on the MOC, the eastward flowing ACC is a sustained circumpolar 42 43 54 flow within which Antarctic Surface Water (AASW), AAIW, CDW and AABW are transported. These water 44 45 55 masses are often treated as being relatively homogeneous due to their circumpolar distribution, however local 46 47 56 variations in properties have been demonstrated to exist owing to variable rates and characteristics of water 48 49 57 mass modification processes. 50 51 52 58 The transport of the ACC is associated with circumpolar fronts formed by strong meridional density 53 54 59 gradients. The common major fronts from north to south (see Figure 1) are: the Sub-Antarctic Front (SAF), 55 56 60 Polar Front (PF), the Southern ACC Front (SACCF) and the Southern Boundary (SB). North of the SAF lies 57 58 61 the Sub-Antarctic Zone (SAZ), which in terms of water masses is dominated at depth by the NADW in the 59 60 61 62 3 63 64 65 62 Atlantic sector of the Southern Ocean. The ACC includes the enhanced transport associated with the SAF, PF 1 2 63 and SACCF and incorporates the Polar Front Zone (PFZ) between the SAF and the PF and Antarctic Zone (AZ) 3 4 64 to the south of the PF, dominated at depth by the CDW. South of the SB of the ACC lie the coastal waters and 5 6 65 Antarctic gyres dominated by local deep water masses modified from CDW (Orsi et al. 1995). These represent 7 8 66 clear circulation regimes. 9 10 11 67 A more refined view, presented by Sokolov and Rintoul (2009a, 2009b), identifies the consistent 12 13 68 alignment of multiple ACC jets/frontal filaments with particular dynamic height contours along the circumpolar 14 15 69 path; they show that the positions of these fronts vary but are consistently present, which in turn supports a 16 17 70 quasi-constant transport for the ACC. This work suggests that the circulation of water masses several times 18 19 71 around the Antarctic within the ACC, which exhibits a stable volume transport in the long-term despite short- 20 21 72 term variability provided by mesoscale eddies, may provide the conditions for dampening temporal variability in 22 23 73 the LCDW. Mixing by the eddies causes a downstream change in the salinity-maximum of the LCDW as it is 24 25 74 advected around the Southern Ocean. 26 27 28 75 Many studies have sought to examine either one or the other circulation feature. However, it is their 29 30 76 combined spatial and temporal variability acting upon hydrographic properties which together give rise to the 31 32 77 distribution of conservative tracers. This is well demonstrated by the distribution of salinity along the Neutral 33 -3 34 78 Density surface, γn = 28.05 kg m shown in Figure 1. This Neutral Density surface corresponds to the density 35 36 79 of the spatially variable core of the deep salinity-maximum. The highest values are found in the South Atlantic, 37 38 80 with an eastward extending core evident through the Indian and Pacific sectors of the Southern Ocean. In Drake 39 40 81 Passage the LCDW salinity maximum has been observed as reaching a local practical salinity minimum of SP 41 42 82 ~34.73 by Whitworth and Nowlin (1987) and Naveira Garabato et al. (2002). Upon return to the South Atlantic 43 44 83 there is a strong meridional salinity gradient across the ACC spanning a depth range of approximately 2000 m, 45 46 84 with the most saline waters found at depths of ~2500m to the north and the freshest deep waters at depths of 47 48 85 ~500m found in the gyre to the south. Meanders in the surface fronts of the ACC are co-located with meanders 49 50 86 in the contours of salinity maxima. 51 52 53 87 Variability of the transport in the Southern Ocean and the characteristics of CDW over different 54 55 88 timescales have been noted by a variety of authors. Gille (2002) reports on a long term warming by comparing 56 57 89 Argo float data with earlier hydrographic data. Meredith and Hogg (2006) used satellite altimeter data and 58 59 90 numerical modelling to investigate how eddy kinetic energy was increased following an increase in the mean 60 61 62 4 63 64 65 91 zonal winds with a delay of a few years. Sokolov and Rintoul (2009a, 2009b) stress the long term stability of the 1 2 92 structure of the ACC despite short term and regional variability. Turner and Overland (2009) compare and 3 4 93 contrast recent climate trends in both polar regions: while the Arctic has been characterised by reducing sea ice 5 6 94 cover this has not been the case in the Antarctic. Here the Peninsula has warmed due to changing winds and ice 7 8 95 shelves have been subject to enhanced melting due to the intrusion of Deep Water onto the shelves. Indeed 9 10 96 many authors have investigated regional processes involving the LCDW in particular sectors of the Southern 11 12 97 Ocean (for example: Naveira Garabato et al. (2002) in the Scotia Sea, Gladyshev (2008) along 30oE, and 13 14 98 Bindoff et al. (2000, 2009) in the Ross Sea). Callahan (1972) examines the general distribution of water masses 15 16 99 across the deep Southern Ocean, identifying the broad trends in the LCDW: the LCDW originates from the 17 18 100 NADW and moves south from the South Atlantic and southwest Indian Ocean, with the distribution of LCDW 19 20 101 determined predominantly by zonal advection, whilst vertical advection and mixing are relatively weak. 21 22 102 Williams et al. (2006) provide a detailed study of the variability of the LCDW within Drake Passage and 23 24 103 observe no long-term trend in the salinity-maximum between 1926 and 2004. They also provide a brief 25 26 104 examination of the circumpolar characteristics of the LCDW, recognising a freshening and cooling of the 27 28 105 NADW signal eastwards from the South Atlantic through to Drake Passage, but do not examine the Atlantic 29 30 106 sector itself. However, a detailed recent examination of the circumpolar change of the salinity maximum along 31 32 107 the ACC seems absent since Callahan (1972). 33 34 35 108 A crucial role in the interaction between the ACC and the MOC, and in the balancing of the movement 36 37 109 of water masses, is taken by mixing. It is recognised that at different spatial and temporal scales different 38 39 110 mechanisms for mixing can be less or more important, with the size of vertical and horizontal mixing being 40 41 111 scale dependent (Okubo 1971, Ledwell et al. 1998). For instance, mesoscale stirring brings waters of different 42 43 112 properties into closer proximity, so that interleavings, overturnings, double diffusion and finally molecular 44 45 113 diffusion can contribute to the evolution of local water mass properties. In the spirit of Munk (1966), on a 46 47 114 global scale the bulk mixing between different water masses – irrespective of exactly how that mixing occurs – 48 49 115 forms an important and intrinsic part of the global thermohaline circulation. 50 51 52 116 There have been many studies of mixing conducted in a variety of ocean regimes which have produced 53 54 117 a variety of estimates for mixing in different ocean regimes. It is important to contrast this range of mixing 55 56 118 estimates to provide context for testing the hypothesis that the more quiescent Weddell Gyre and the more 57 58 119 turbulent ACC have significantly different rates of mixing due to the different topographic and hydrographic 59 60 61 62 5 63 64 65 120 conditions, despite both being elements of the Southern Ocean circulation. One of the earliest estimates of 1 3 2 121 mixing was made by Munk, who, considering the horizontal mixing coefficient, κH, required a value of 5 x 10 3 4 122 m2 s-1 in order to obtain a realistic scale for the western boundary current of a wind driven gyre in an analytical 5 -4 2 -1 6 123 calculation (Munk 1950). Munk (1966) estimates a bulk vertical mixing coefficient, κV of 1.3 x 10 m s for 7 8 124 the Pacific interior using a vertical advective-diffusive balance for a variety of parameters. In the Brazil Basin 9 10 125 Polzin et al. (1997), using both turbulence measurements and tracers, demonstrate the spatial variability of 11 -4 2 -1 -4 12 126 mixing in the abyssal ocean, with κD = 0.1 x 10 m s over smooth topography, and a higher value of 5 x 10 13 14 127 m2 s-1 within 150 m of the sea floor and over rougher topography. A more recent study by Ledwell et al. (2010), 15 16 128 again using turbulence measurements and tracers, diagnosed a diapycnal mixing coefficient, κD = (1.3 ± 0.2) x 17 18 129 10-5 m2 s-1 in the eastern Pacific sector of the ACC averaged over 1 year and thousands of kilometres. Naveira 19 20 130 Garabato et al. (2004a) using in situ measurements obtained a range of values for the vertical mixing coefficient, 21 -4 2 -1 -2 2 -1 22 131 κV, of 3 x 10 m s to 1 x 10 m s in weakly stratified Nordic Seas with the largest values relating to tidal 23 -4 2 -1 24 132 activity near rough topography. Cisewski et al. (2005, 2008) obtain a κV of 7 x 10 m s in the upper 25 26 133 pycnocline of the ACC at about 20oE based on in situ turbulence measurements. Hibbert et al. (2009) diagnosed 27 -4 2 -1 2 -1 28 134 a maximum κV of 3 x 10 m s and κH of 30-100 m s within a cold core eddy of the ACC by looking at the 29 30 135 heat budget of the Winter Water. Leach et al. (2011) looked at the modification of the WDW core as it flows 31 -6 2 -1 32 136 westwards and found the Weddell Gyre to be relatively quiescent with κV of only 3 x 10 m s , one of the 33 34 2 -1 137 lowest values reported anywhere, and κH of 70-140 m s . Lenn et al. (2009) found a similarly quiescent regime 35 36 138 in the Arctic, which is an interesting hydrographic regime to compare with the Weddell Gyre: both are areas of 37 38 139 seasonal sea-ice cover with a mid-depth intrusion of relatively warm water. Sheen et al. (2013) report values for 39 40 -5 2 -1 -3 2 -1 140 κV ranging from 10 m s at mid-depth to 10 m s near the bottom in the Scotia Sea based on in situ 41 42 141 measurements and Waterman et al. (2013) have similar results near Kerguelen. These provide a useful backdrop 43 44 142 for this study. As will be addressed in the methods section of this paper, there are different approaches and 45 46 143 definitions when determining mixing through the water column. For purposes of inter-study comparison, the 47 48 144 vertical mixing coefficient (κV) and diapycnic mixing coefficient (κD), along with the horizontal mixing 49 50 145 coefficient (κH) and isopycnic mixing coefficient (κI), can be considered to be roughly analogous, however they 51 52 are derived by different means within different frameworks and this must be remembered whilst comparing 53 146 54 55 147 values. In the centres of ocean gyres, where isopycnals do not significantly slope, the difference between the 56 57 148 two sets of quantities is less than in the ACC where the isopycnals associated with the ACC fronts are inclined 58 59 149 to the isobars. 60 61 62 6 63 64 65 150 The objectives of the work presented in this paper are to examine the temporal and spatial variability of 1 2 151 the deep salinity maximum associated with LCDW, assess how this salinity signal is propagated and to calculate 3 4 152 bulk estimates of the horizontal and vertical diffusion in the Southern Ocean. To this end the WOCE/CLIVAR 5 6 153 (http://cchdo.ucsd.edu/) CTD data set has been used. This represents a high quality homogeneous primary data 7 8 154 set, though it is not as uniform in space and time as might be desired, but is not subject to any assumptions about 9 10 155 the circulation or mixing in the Southern Ocean. To calculate the diffusion coefficients a relatively simple 11 12 156 advective-diffusive balance has been assumed. 13 14 15 157 16 17 18 158 2 - Data 19 20 21 159 This study has made use of the available ‘merged’-format Conductivity Temperature and Depth (CTD) 22 23 160 sonde data from the CLIVAR and Carbon Hydrographic Data Office (CCHDO) website (http://whpo.ucsd.edu/). 24 25 161 The CCHDO website provided a readily searchable range of international data which was openly available for 26 27 162 use and gave access to a circumpolar coverage of hydrographic data. An earlier study examining the LCDW by 28 29 163 Williams et al. (2006) primarily focused on Drake Passage, although used a similar but less extensive set of 30 31 164 sections to complete a cursory review of the circumpolar salinity changes. They likewise obtained their 32 33 165 circumpolar data from the CCHDO, however this study makes use of additional cruise data which has more 34 35 166 recently been released through the CCHDO. 36 37 38 167 In total 37 hydrographic sections were used in this study, as detailed in Table 1. The earliest cruise 39 40 168 undertaken was the SR02-A section along the Greenwich Meridian in 1989, and the latest was the I06S section 41 42 169 undertaken in 2008 along 30oE. Only those stations falling on the main section of each cruise were used, and 43 44 170 only those stations whose profile reached an observable salinity maximum peak associated with the core of 45 46 171 LCDW were considered for analysis. 47 48 49 172 A further section, P17A in the Pacific sector undertaken in 1992 by the RV Knorr was not included in 50 51 173 this study due to a large longitudinal shift in the middle of the cruise track. This shift in cruise track was 52 53 174 covered by only a single station spanning the vicinity of the Udintsev and Eltanin Fracture Zones (see Figure 2) 54 55 175 on an orientation nearly perpendicular to the known topographically steered ACC path in that area (Orsi et al. 56 57 176 1995). As such it was deemed unwise for inclusion in the analysis as any re-gridding of data would introduce 58 59 177 unacceptable levels of uncertainty. 60 61 62 7 63 64 65 178 Two further sections, P17E (1992) by the RV Knorr and P18 (1994) by the NOAAS Discoverer in the 1 2 179 Pacific sector, were excluded as the cruises did not extend sufficiently far south to reach the Southern Boundary 3 4 180 of the ACC, likely due to ice cover during the time of occupation. The Ross Gyre in general is poorly sampled 5 6 181 compared with the rest of the Southern Ocean, and as a result it has not been possible to include its impacts on 7 8 182 the LCDW within this study. 9 10 11 183 The approximate position of the Polar Front, as defined by Orsi et al. (1995), was used as a proxy for 12 13 184 the centre of the mean ACC path in order to obtain a circumpolar trajectory, as shown in Figure 3. 14 15 16 185 Finally, estimates of ocean velocity were obtained from the Estimating the Circulation & Climate of 17 18 186 the Ocean (ECCO) project. This data was obtained from the Live Access Server for ECCO-GODAE version 3, 19 20 187 iteration 73 using the adjoint method, with 1 degree horizontal resolution and 23 vertical levels. The model 21 22 188 solutions span 1992-2007 which includes the majority of the timeframe of the CCHDO data and was the most 23 24 189 suitable of the available solutions for this study. 25 26 27 190 3 - Temporal and Spatial Variability of the Salinity-Maximum 28 29 191 3.1 - Methods – Identifying the Salinity-Maximum and Delineating Sub-sections 30 31 192 The initial step in examining the variability of the NADW north of the ACC, the LCDW within the 32 33 193 ACC and the WDW in the Weddell Gyre is to evaluate the distribution of the salinity signal in space and time. 34 35 194 This includes examining individual cruise potential temperature-practical salinity (θ-SP) plots and comparing 36 37 195 conservative temperature and absolute salinity (Θ-SA) plots for the salinity-maximum across all cruises. 38 39 40 196 In order to examine the circumpolar spatial variability of the salinity-maximum associated with the 41 42 197 LCDW it is necessary to consider the different regimes in which it is present. Firstly, it is necessary to separate 43 44 198 NADW in the South Atlantic Gyre from the LCDW in the ACC, and the LCDW in the ACC from the WDW in 45 46 199 the Weddell Gyre. This was achieved through setting maximum (1.823 dyn m) and minimum (0.732 dyn m) 47 48 200 limits for the dynamic height at 20 m referenced to 2500 m as a proxy for the respective northern and southern 49 50 201 boundary of the ACC, as observed in the data for Drake Passage. A depth of 20 m provides a consistent level at 51 52 202 which all stations were sampled to avoid problems with missing data from the near-surface of profiles. 53 54 203 Williams et al. (2006) use a reference level of 2000 m for their investigation of LCDW variability, however 55 56 204 Sokolov and Rintoul (2009a) use a deeper reference level of 2500m which maximises the mass included in the 57 58 205 dynamic height calculation whilst minimising the area of ocean discounted due to shallower topography, and the 59 60 206 latter was deemed the preferable reference level. 61 62 8 63 64 65 207 The next step is to partition the Atlantic Sector of the ACC into the Polar Front Zone and the Antarctic 1 2 208 Zone by establishing a section specific position for the Polar Front. There are many different definitions based 3 4 209 on different parameters at different depths. In this study the 2.2oC isotherm at 800m (Orsi et al. 1995) was 5 6 210 chosen as the preferable option as it is a deep, as opposed to surface signature, which was clearly observable in 7 8 211 almost all sections. 9 10 11 212 The final step in section partitioning was to delineate between the northern and southern limbs of the 12 13 213 Weddell Gyre. After interrogation of the data, the criterion used to determine the central axis of the Weddell 14 15 214 Gyre for each individual occupation of a section was the latitude of maximum Neutral Density at 300 m, which 16 17 215 coincides approximately with the depth of the Salinity-maximum in the WDW. This doming of density surfaces 18 19 216 is thought to be related to variability in the transport of the Weddell Gyre (Meredith et al. 2011), with the 20 21 217 northward/southward sloping of the surfaces related to eastward/westward transport of the cyclonic circulation. 22 23 218 Thus the maximum in Neutral Density represents an estimate of the central axis where near zero-flow occurs, 24 25 219 and this was seen to vary in latitude from cruise to cruise. 26 27 28 220 Using this partitioning, an examination of the variability along the circumpolar path of the LCDW can 29 30 221 be conducted with respect to longitude and time, taking into consideration depth and the value of the salinity- 31 32 222 maximum. 33 34 35 223 3.2 - Methods - Appropriateness of Sampling 36 37 38 224 The majority of cruise sections used in this study (Figure 3) have been undertaken with a regular 39 40 225 sampling pattern where a standard distance exists between one station and the next, within the limits of a 41 42 226 research vessel being able to maintain station. Given that small scale fluctuations over these distances are at 43 44 227 least as large as the variability between stations; this negated the need to grid the CTD data onto an equidistant 45 46 228 spacing. Such a gridding process would introduce unnecessary error and false confidence in data coverage. 47 48 229 Furthermore, the partitioning of sections in the Atlantic Sector of the Southern Ocean into their various regimes 49 50 230 avoids the likelihood of cross-regime sampling bias. 51 52 53 231 The only region where station spacing is cause for concern is the varying northern coverage of the 54 55 232 South Atlantic. However, as this study only refers to this northerly data generically as a source of the LCDW 56 57 233 signal, and is not involved in subsequent mixing calculations there was no need to attempt to regularise the data 58 59 234 distribution. Given the generally sparse and irregular sampling of more northerly waters in this dataset, it would 60 61 62 9 63 64 65 235 be even less desirable to attempt to re-grid this northerly data than in the case of the more regularly spaced data 1 2 236 straddling the ACC and Weddell Gyre. 3 4 5 237 3.3 - Methods - ACC and Weddell Gyre Track Length 6 7 8 238 The along ACC path distance was calculated between each pair of sections using the position of the 9 10 239 Polar Front estimated by Orsi et al. (1995). The Polar Front roughly corresponds to the latitudinal centre of the

11 o 12 240 ACC, however frontal positions can vary in time by up to 5 latitude as described by Moore et al. (1999). In 13 14 241 addition, branches of the main fronts diverge and converge at various points in time and space around the 15 16 242 Southern Ocean, as described by Sokolov and Rintoul (2009a, 2009b). Therefore, any representation of the 17 18 243 fronts will at best be an approximation to the mean state and therefore the position of the Polar Front from Orsi 19 20 244 et al. (1995) is considered a sufficient approximation to the mean ACC path length. This was used for both 21 22 245 demonstrating spatial variability and for calculating a term of the mixing equation. 23 24 25 246 In order to determine an estimate of the pathway of the LCDW/WDW through the Weddell Gyre, the 26 27 247 partitioned sections were analysed to identify an approximate latitudinal centre for each section. In conjunction 28 29 248 with examining the circumpolar salinity trend, a corresponding distance track was calculated using Ocean Data 30 31 249 View’s ‘Graphic Objects’ tool which enabled the plotting of an estimated circulation pathway and calculation of 32 33 250 its length. 34 35 36 251 3.4 - Results - Regional Variability of the Salinity-Maximum 37 o 38 252 The salinity-maximum associated with the NADW, LCDW and WDW (SP~34.6-34.8, θ~0-3 C) is 39 40 253 evident at all latitudes as a ‘knee’ in the θ-SP distribution as shown in panel A of Figure 4 in the box labelled 41 42 254 ‘Deep Waters’. Intermediate and surface waters account for the fresher and/or warmer part of the distribution, 43 44 255 whilst the increasingly cooler and denser Weddell Sea Deep (WSDW) and Bottom Waters (WSBW) account for 45 46 256 the remainder of the distribution. The deep and bottom waters are enlarged and labelled in panel B of Figure 4. 47 48 49 257 The salinity-maximum ‘knee’ successively freshens and cools polewards from the NADW to the 50 51 258 CDW, and this pattern is present in each section used in this study and is therefore observable at all longitudes. 52 53 259 Of particular note, however, is the reversal of this trend, where the salinity-maximum decreases from the CDW 54 55 260 at ~55oS (turquoise) to reach its lowest value at ~60oS (blue), yet there are higher values further south towards 56 57 261 70oS (pink). While the higher salinities around 70oS are related to the southern limb of the Weddell Gyre which 58 59 60 61 62 10 63 64 65 262 entrains LCDW in the eastern Weddell Gyre, the lowest values at ~60°S point to the Weddell Gyre centre as the 1 2 263 final location to which the NADW can be traced by its salinity signature after circulation around Antarctica. 3 4 5 264 In order to examine the changes of the salinity-maximum along its circumpolar path it is useful to focus 6 7 265 on the salinity-maximum data only, as shown in Figure 5. Overall there is a pattern of warm, saline NADW 8 -3 9 266 with a potential density of ~27.85 kg m in the Sub-Tropical Zone of the South Atlantic (red) gradually 10 11 267 freshening and cooling as the salinity signature moves southward and is entrained into the ACC (black). The 12 13 268 cooling and freshening continues but the potential density remains relatively constant at ~27.80 kg m-3 until 14 15 269 entrainment into the Weddell Gyre, where a more pronounced cooling results in an increase in potential density 16 17 270 to 27.85 kg m-3. 18 19 20 271 However, though it has been necessary to plot Figure 5 with contours of potential density referenced to 21 22 272 the surface, this metric is inappropriate for the deep ocean. Whilst a deeper reference level such as 2000m 23 24 273 would be more useful, it still fails to properly quantify the changes in density, as will be demonstrated later 25 26 274 when examining the variability in the depth of the salinity maximum. The issue is further complicated when 27 28 275 examining the entrainment of LCDW into the Weddell Gyre. Panel A of Figure 6 shows practical salinity 29 30 276 plotted against potential density demonstrating the same distribution as in Figure 3. Note how the distribution 31 32 277 changes with latitude (top) reflecting Figure 4, how the distribution changes with longitude (bottom), with the 33 34 278 Greenwich Meridian (green) and western South Atlantic (turquoise) including the most saline NADW as well as 35 36 279 the freshest WDW. The intermediate SP values are present in the Indian and Pacific sectors of the Southern 37 38 280 Ocean (blue and red). The curve of the salinity-maximum data can be seen to be centred on the value of 27.80 39 40 281 kg m-3, with higher densities associated with NADW and WDW, and lower densities associated with the 41 42 282 LCDW. The equivalent Neutral Density distribution shows a greater range of densities with a mean of 43 -3 44 283 approximately the same as for the Neutral Density surface shown in Figure 1 of γn = 28.05 kg m associated 45 46 284 with the highest salinities in the NADW and LCDW (Figure 6, panel B). The prominent spread of Neutral 47 48 285 Density at lower salinities is associated with the WDW and this demonstrates the cross-isopycnal movement of 49 50 286 the salinity-maximum when moving from the ACC, across the Southern Boundary/Weddell Front into the 51 52 287 Weddell Gyre. The different perspectives offered by potential density and Neutral Density with respect to the 53 54 288 Southern Boundary of the ACC is worthy of note for future studies. 55 56 57 289 The mechanism for this change between regions of the Southern Ocean is described in the appendix 58 59 290 and can be summarised as being the result of cross-frontal mixing moving the salinity-maximum signature from 60 61 62 11 63 64 65 291 the ACC regime to the Weddell Gyre regime, whereby there is a change in the overlying and underlying water 1 2 292 masses and a vertical move upwards in the water column. This study is conducted under the assumption that 3 4 293 tracking the salinity maximum along the temperature-salinity maximum curve in Figure 5 is legitimate as the 5 6 294 changes in density are the result of real modification of the water mass as it moves from regime to regime. 7 8 9 295 A further examination of the spatial distribution and nature of the deep salinity maximum is supported 10 11 296 by Figure 7 which shows the variation of salinity with latitude, grouped by cruises on similar longitudes. A 12 13 297 number of key observations can be made using this figure, however care should be taken when comparing the 14 15 298 subplots in Figure 7 as they have different axes to allow focus on different features. Firstly, when compared to 16 17 299 other areas of the Southern Ocean, there is a relatively large variability in the salinity maximum in the Atlantic 18 19 300 sector of the Southern Ocean north of the Weddell Gyre. This could be explained by the effect of transient 20 21 301 eddies moving across a strong meridional gradient in the salinity maximum. In other sectors of the Southern 22 23 302 Ocean there is no such strong meridional gradient and so the effect of eddies is not as apparent. In the Weddell 24 25 303 Gyre, whilst it is generally considered more quiescent than the ACC, across the Greenwich Meridian eddy 26 27 304 activity associated with Maud Rise accounts for observed variability. Secondly, the salinity maximum of the 28 29 305 Indian sector (subplot A and dark blue - subplot B) intersects with the Atlantic sector sections at SP > 34.75, 30 o 31 306 latitude ~51 S and the profile becomes convex across a reduced range in SP. As the LCDW is drawn eastwards 32 33 307 through the Pacific sector (light blue - subplot B) and Drake Passage (red - subplot C) the salinity maxima 34 35 308 continues to decrease and the curve of the profile flattens. In subplot D (black and light and dark teal) the 36 o 37 309 southern ACC is visible north of the Weddell Front/Southern Boundary at ~55 S, along with the trough in SP 38 39 310 associated with the northern Weddell Gyre (~60oS) and the peak of the southern Weddell Gyre (~65oS). 40 41 42 311 It is also of interest that through the Indian and Pacific sectors the latitudinal peak in the salinity 43 44 312 maximum gradually shifts southwards, from being at the extreme north of the sections (~50oS) in the Indian 45 46 313 sector to being at the extreme south of the sections (~70oS) in the eastern Pacific. Whilst the transient eddy field 47 48 314 may have a role in this apparent southward shift, another possible cause is vertical mixing between the more 49 50 315 saline CDW and less saline AAIW north of the Polar Front. South of the polar front AAIW has not yet 51 52 316 subducted and is therefore not a factor in deep ocean mixing at higher latitudes. The result of such mixing 53 54 317 would be to enhance dilution of the LCDW in the north of the ACC compared to the south of the ACC, giving 55 56 318 the appearance of a southward movement when instead this may simply be a latitudinal variation in vertical 57 58 319 mixing. However, this hypothesis requires further investigation which is beyond the scope of this study. 59 60 61 62 12 63 64 65 320 The salinity-maximum is therefore used as a proxy for the overall impact of mixing upon the deep 1 2 321 salinity-maximum core of the NADW, LCDW and WDW. It can be used to examine spatial and temporal 3 4 322 variability, entrainment from one regime to another, and to estimate mixing rates along the circumpolar 5 6 323 pathway. 7 8 9 324 3.5 - Results - Along-Path and Temporal Change 10 11 12 325 The change in the salinity-maximum along the circumpolar pathway can be seen in Figure 8a. Starting 13 14 326 from the highest NADW salinities in the Sub-Antarctic Zone (SAZ, sections 1 & 2) of the Atlantic Sector, 15 16 327 NADW is then entrained into the Polar Front Zone (PFZ, 3/4) producing the highest salinity maximum values 17 18 328 along the ACC. The salinity-maximum decreases throughout the ACC in the Indian and Pacific sectors of the 19 20 329 Southern Ocean and back through Drake Passage and into the Antarctic Zone (A23-AZ) of the Atlantic sector 21 22 330 (sections 5-16). There is then a noticeable increase in salinity in the transition from the A23-AZ sections to the 23 24 331 SR02-AZ sections (section 17) with the only potential source of this increase being from cross-frontal mixing 25 26 332 with LCDW from further north across the PFZ. As the only source of higher salinity water masses in the deep - 27 28 333 but not bottom layers – LCDW is then entrained into the WDW in the eastern Weddell Gyre, with the decrease 29 30 334 continuing through the eastern and southern Weddell Gyre (sections 18-21). The salinity signature then 31 32 335 increases in the northern Weddell Gyre (sections 22-24) which must be due to cross-frontal mixing with the AZ 33 34 336 of the ACC and/or mixing with higher salinity WDW across the central axis of the Weddell Gyre. 35 36 37 337 The change in the depth of the salinity maximum is more variable than the actual salinity values, as 38 39 338 shown in Figure 8b. The NADW values place the salinity maximum at ~2750 m depth in the Sub-Antarctic 40 41 339 Zone, rising to ~2500 m in the Polar Front Zone of the Atlantic sector and stabilising at a depth of ~1500m 42 43 340 across the Indian sector. The depth then fluctuates between 1500 m and 2100 m across the Pacific sector 44 45 341 apparently due to the combined impact of the shift in the vertical distribution of water masses and topographic 46 47 342 effects (see Figure 2 for major topographic features). In detail, the ACC fronts remain on or south of the 48 49 343 Australian-Antarctic Ridge through the Indian sector resulting in the consistent vertical distribution of the 50 51 344 underlying AABW and overlying AAIW, corresponding to a steady salinity-maximum depth (see section group 52 53 345 5-8). However, to the east of the Macquarie Ridge the fronts move to the north of the Pacific Antarctic Ridge 54 55 346 (~3000m ridge depth) into the Southwest Pacific Basin where lower volumes of AABW are present due to 56 57 347 topographic barriers in the northwest Ross Sea and an absence of topographic barriers to the northeast in the 58 59 348 Bellingshausen Abyssal Plain which allows AABW to flow northwards at greater depth, resulting in the 60 61 62 13 63 64 65 349 downward displacement of the salinity-maximum in the water column (sections 9 & 10). The salinity- 1 2 350 maximum depth shallows as the ACC crosses the Pacific-Antarctic Ridge due to topographic uplift (section 11), 3 4 351 and returns to a deeper level across the Amundsen, Bellingshausen and Mornington Abyssal Plains (sections 12- 5 6 352 14): here it is due to the lack of topographic constraint on AABW flowing northeast out of the Ross Sea 7 8 353 resulting in a deeper and more saline bottom water signature. The salinity-maximum shallows towards 1000 m 9 10 354 as it moves through Drake Passage and into the Antarctic Zone of the Atlantic sector (sections 15-17). The 11 12 355 signature finally shallows to ~500 m within the WDW of the Weddell Gyre representing the end of the shoaling 13 14 356 of isopycnals associated with the LCDW (sections18-24). 15 16 17 357 The variability in the per-cruise mean salinity-maximum between repeat occupations of a section is 18 19 358 often only as large as the variability of the salinity-maximum (defined as standard error of the mean) between 20 21 359 the stations of a single cruise. This is demonstrated in Figure 9 where the section mean of the salinity-maximum 22 23 360 is plotted with bars of standard-error to demonstrate inter- and intra-section variability. 24 25 26 361 Considering the scarcity of data in time, the less than 20-year time span for even the best sampled 27 28 362 sections, and multi-decadal timescales of circumpolar circulation; there appears to be no significant multi- 29 30 363 annual trend in the section mean salinity maximum of the LCDW although small increases and decreases are 31 32 364 observed. In detail these are (numbers in brackets refer to sub-section number as per Table 1): 33 34 35 365  The SR02-SAZ (1) salinities are surprisingly stable considering that the data sampling was regionally 36 37 366 irregular, whilst there is actually greater variability within the SR02-PFZ (2) occupations which were 38 39 367 more consistently surveyed. This suggests a relatively homogenous and quiescent SAZ and more 40 41 368 variable and dynamic PFZ. 42 43 369  The occupations of sections I06S (5) through to SR03 (8) show low variability between occupations of 44 45 370 each section, and are collectively a consistently close grouping. 46 47 371  The Pacific sector and Drake Passage sections (11-16) have a greater degree of variability compared to 48 49 372 the previous group of sections, but still largely within the range of the associated error. 50 51 373  The SR02-AZ (17) sections show the greatest variability, with values as high as the lowest P16S 52 53 374 occupations and as low as the mid-range of the SR01 occupations. This suggests a variable cross- 54 55 375 frontal influence from the PFZ, possibly the result of short-term impacts from eddy-activity. As the 56 57 376 partition of section occupations is done on the basis of oceanographic features and not geographical 58 59 377 position, this cannot be due to meandering of the Polar Front. 60 61 62 14 63 64 65 378  The smaller error bars and lower inter-section variability within the northern Weddell Gyre (23), 1 2 379 compared to the more pronounced variability of the southern Weddell Gyre (20), suggests variability in 3 4 380 the salinity-maximum signature may be attenuated by local mixing processes in the western Weddell 5 6 381 Gyre. The greater variability in the southern Weddell Gyre may be linked to known variability in 7 8 382 volume transport into the region, as demonstrated by Cisewski et al. (2011), associated with jets 9 10 383 created by the topography of Maud Rise. 11 12 384  The most profound statement which can truly be made about those sections with only two occupations 13 14 385 (1, 3, 6, 7, 11, 13 and 16) is that the repeats are not unexpectedly different, as a meanginful trend 15 16 386 cannot be inferred from two data points. 17 18 19 387 There are three main conclusions to be drawn from these results. Firstly, there is no observable 20 21 388 temporal trend in the value of the salinity-maximum for the time period considered. Secondly, there are clear 22 23 389 regimes of spatial change in the value of the salinity-maximum to which a simple linear fit can be applied to 24 25 휕푆 390 determine the term in equation [7]. The following letters correspond to the linear fits shown in figure 8a: 26 휕푖 27 28 29 391 A. the Sub-Antarctic Zone/Polar Front in the Atlantic sector; 30 31 392 B. the Indian sector/Pacific sector/Drake Passage; 32 33 393 C. the Antarctic Zone of the Atlantic sector, and the eastern and southern Weddell Gyre; 34 35 394 D. the western and northern Weddell Gyre; 36 37 38 395 Thirdly, that the depth of the salinity maximum may vary during the circumpolar transit but it maintains a stable 39 40 396 range of density, which is only modified by significant shifts from one regime to another. 41 42 43 397 44 45 398 4 - Water Mass Analysis 46 47 399 4.1 – Water Mass Analysis Method 48 49 400 In an effort to quantify the relative downstream changes in the salinity maximum in terms of water 50 51 401 mass modification, a water mass analysis has been performed for the mean of the mean salinity maxima. This is 52 53 402 based on an inverse calculation of the θ-Sp properties of both the mean of the mean salinity maxima for each 54 55 403 section and the mixing end members of NADW, AASW and AABW. 56 57 404 The mixing end members have been set according the appropriate maxima/minima available from the 58 59 405 hydrographic datasets used in this study as follows: the highest observed salinity maximum for the NADW in 60 61 62 15 63 64 65 o 406 the Atlantic sector (Sp = 34.884, θ = 2.762 C); the lowest surface salinity for the AASW in the Weddell Gyre (Sp 1 o o 2 407 = 33.90, θ = 0 C), and the lowest observed salinity in AABW (Sp = 34.65, θ = -0.7 C). The output from this set 3 4 408 of calculations is an estimate of the relative percentage contribution of each mixing end-member. However, it 5 6 409 should be noted that as AASW and AABW have regional variations in their properties, that this is an indicative 7 8 410 contribution during the entire circumpolar transit of LCDW reflecting the erosion of the salinity maximum and 9 10 411 it is not an explicit contribution of AASW or AABW along any specific section. 11 12 412 4.2 – Water Mass Analysis Results 13 14 413 The results from the water mass analysis are contained in Table 2 which lists the percentage indicative 15 16 414 contribution in θ-Sp space for each mixing end member and the percentage change from the previous up-stream 17 18 415 section. The entrainment of NADW into the ACC between sections 1 and 5 shows an overall 32.7% decrease in 19 20 416 NADW, a 7.6% increase in AASW and 23.9% increase in AABW, whilst the NADW contribution in SAZ of 21 22 417 the eastern South Atlantic (section 3) is noticeably lower than in the eastern South Atlantic (section 1). These 23 24 418 are the only sections where the AASW contribution shows a significant increase, consistent with the down 25 26 419 welling on AASW to form AAIW at the Polar Front. 27 28 420 For sections 6-15 there are either small increases or small but overall significant decreases in NADW 29 30 421 contribution, the latter consistent with the gradual erosion of the salinity maximum through the Indian and 31 32 422 Pacific sectors. This is followed by a small increase or 2.9% in NADW contribution for sections 16 and 17 33 34 423 upon return to the Atlantic sector suggestive of entrainment southward across the Polar Front. When combined 35 36 424 with the additional comparison of section 17 with section 5 where there is a 7.8% increase eastwards, this result 37 38 425 is significant as it demonstrates the meridional barrier to transport posed by the ACC fronts acting to minimise 39 40 426 direct meridional input from the South Atlantic to the Weddell Gyre. 41 42 427 There is then a large decrease (-15.3%) in NADW contribution and a large increase (17%) in AABW 43 44 428 contribution marking the shift into the Weddell Gyre (section 18). This is followed by a minor increase 45 46 429 suggesting that section 19 is entirely dominated by water masses circulating from the eastern Weddell Gyre. 47 48 430 Sections 20-22 then show a renewed decrease in NADW and increase in AABW contributions consistent with 49 50 431 further erosion of the salinity maximum throughout the southern and western and north-western Weddell Gyre, 51 52 which is consistent with a net transport northward from the South Scotia Arc. 53 432 54 55 433 The final water mass modifications are in the northern Weddell Gyre where increasing NADW contributions 56 57 434 indicate that entrainment occurs into the Weddell Gyre from the ACC across the Weddell Front/Southern 58 59 435 Boundary east of the South Scotia Ridge, followed by a further 6.2% increase of NADW contribution during 60 61 62 16 63 64 65 436 recirculation to section 18 and 19. This suggests that the entrainment of LCDW into the Weddell Gyre mostly 1 2 437 occurs east of the Greenwich Meridian (5.1% + 6.2%) as opposed to the west (3.7%) by a ratio of approximately 3 4 438 3:1. 5 6 439 7 8 440 5 - Estimate of Mixing Coefficients 9 10 11 441 5.1 - Methods - The equation 12 13 442 In order to make an estimate of the rate of mixing along the path of the LCDW a diffusive-advective 14 15 443 mixing scheme is assumed to sufficiently approximate the large-scale effects of mixing across a range of smaller 16 17 444 scales: 18 19 20 휕푆´ 휕푆´ 휕푆´ 휕푆´ 휕2푆 휕2푆 휕2푆 445 [3] + 푢 + 푣 + 푤 = 휅 ( + ) + 휅 + 푅 21 휕푡 휕푥 휕푦 휕푧 퐻 휕푥2 휕푦2 푉 휕푧2 22 23 24 where 25 446 26 −휕 휕 휕 27 447 [4]푅 = (푢푆´ ) − (푢푆´ ) − (푢푆´ ) ; 휕푥 휕푦 휕푧 28 29 30 휕푆´ 31 448 is the change in salinity with time; 푢,푣,푤 are respectively the mean zonal, meridional and vertical 휕푡 32 33 휕푆´ 휕푆´ 휕푆´ 34 449 components of velocity; , , are respectively the mean change in salinity in the zonal, meridional and 휕푥 휕푦 휕푧 35 36 휕2푆 휕2푆 37 450 vertical directions; 휅퐻 is the horizontal mixing coefficient and 휅푉 is the vertical mixing coefficient; 2 , 2 , 38 휕푥 휕푦

39 2 40 휕 푆 451 2 are respectively the uniform rate of change in salinity gradient with respect to the zonal, meridional and 41 휕푥 42 43 452 vertical directions; and R represents the eddy fluxes of tracers terms as shown in [4]. Despite the turbulent 44 45 453 nature of the Southern Ocean where mesoscale and turbulent mixing are significant, these contribute to the bulk 46 47 454 mixing considered in this study and the terms of [3] therefore hold at scales of hundreds of kilometres. 48 49 50 455 5.2 - Methods - Assessment of terms 51 52 Using scale analysis we can determine which, if any of these terms can be neglected in calculating rates 53 456 54 55 457 of oceanic mixing. 56 57 58 59 60 61 62 17 63 64 65 휕푆´ 458 On the left hand side of the equation, firstly we can reasonably neglect the term as this study did not 1 휕푡 2 3 459 identify any significant temporal trend in the deep salinity maximum (see part 3.5 and Figure 9). Whilst the 4 5 460 absence of a trend is not conclusive, the variability between repeat occupations of sections is greater than the 6 7 461 size of any trend, operating on the timescale for circumnavigation of Antarctica of the order of decades. Next 8 휕푆´ 9 462 we can neglect the 푣 term as the meridional transport across the ACC is dominated by transient eddies and is 10 휕푦 11 12 463 relatively small compared to the zonal transport (Marshall & Speer, 2012; Lauderdale, 2013), whilst the 13 14 464 isopycnal salinity gradient is too small for sections outside the Atlantic sector of the ACC which are not the 15 16 휕푆´ 465 target for this calculation. Similarly, the 푤 term can also be neglected as vertical transport is an even smaller 17 휕푧 18 19 466 component than the meridional transport, and when considered within the context of shoaling isopycnals this 20 21 467 term becomes insignificant as it is reduced to local fluctuations related to turbulence. 22 23 휕2푆 24 468 On the right hand side of the equation we can neglect the term as it estimated to be ~10-16 m-2, 25 휕푥2 26 휕2푆 27 469 whereas the is estimated at ~10-14 m-2 and therefore dominates the overall term relating to 휅 ; both values 28 휕푦2 퐻 29 30 470 have been calculated by fitting quadratic curves to the data in the ACC along path and across path directions 31 32 471 respectively for the Indian and Pacific sectors (sections 5 to 16 in Figure 8a). In addition, despite being able to 33 34 472 calculate the along path term, it is difficult to distinguish an eastward moving salinity maximum from the term 35 36 473 for zonal mixing. Finally, there is insufficient information to accurately include the Reynold’s Stress terms and 37 38 474 so the effect of the terms must be implicitly included in the remaining terms. This leaves us with: 39 40 41 휕푆´ 휕2푆 휕2푆 42 475 [5]푢 = 휅 + 휅 휕푥 퐻 휕푦2 푉 휕푧2 43 44 45 However, owing to the nature of the ACC, where the isopycnal surfaces are strongly sloping, the horizontal 46 476 47 48 477 (휅퐻) mixing term is more accurately expressed as isopycnal (휅퐼) mixing: 49 50 51 휕푆´ 휕2푆 휕2푆 478 [6]푢 = 휅 + 휅 52 휕푥 퐼 휕푦2 푉 휕푧2 53 54 55 479 Furthermore, as the path of the ACC meanders across the Southern Ocean it is inappropriate to consider the 56 57 480 downstream flow to be strictly zonal or the rate of change in salinity across the ACC to be strictly meridional 58 59 481 and the equation therefore becomes: 60 61 62 18 63 64 65 휕푆´ 휕2푆 휕2푆 482 [7]푢 = 휅 + 휅 1 휕푖 퐼 휕푗2 푉 휕푧2 2 3 4 휕푆´ 5 483 Where 푢, the downstream velocity is obtained from the ECCO model; is the downstream change in salinity 휕푖 6 7 휕2푆 484 obtained from an appropriate linear fit of circumpolar changes in salinity; is the rate of change in salinity 8 휕푗2 9 10 485 gradient in the across-stream direction. By collecting information on each of these terms for every appropriate 11 12 486 section in the circumpolar transit of the LCDW an estimate of bulk mixing can be obtained for the deep ocean 13 14 487 using a least squares linear regression. A statistical estimate of error associated with these calculated mixing 15 16 terms is provided by the estimated standard error of the isopycnal and vertical mixing coefficients which is 17 488 18 calculated as the square root of the estimated diagonal covariance matrix. 19 489 20 21 490 5.3 - Methods - Velocity estimates 22 23 491 In order to provide a representative value for u, it was essential to obtain a realistic estimate for the 24 25 downstream velocity. At a basic level, if we assume an upper limit of 150 Sv transport for the ACC through 26 492 27 Drake Passage, with a mean depth of 4000 m and an approximate width of 5o of latitude, we arrive at a mean 28 493

29 -1 30 494 value for u of 7.1 cm s for the entire water column. This represents an absolute upper bound for velocities at 31 32 495 LCDW depth which is in the lower half of the water column. 33 34 Döös (1995) estimates a mean residence time for water particles moving between the different ocean 35 496 36 basins via the indirect ventilated route in the Southern Ocean to be 243 years. When scaled by the estimated 6 37 497 38 39 498 circumnavigations, it suggests a typical circumnavigation time of 40.5 years. Using an ACC circumpolar track 40 41 499 length of order 27 000 km derived from Orsi’s (1995) Polar Front; this would provide a mean current speed of

42 -1 43 500 2.1 cm s . This is an estimate for water masses at a range of mid water column depths and provides a 44 45 501 reasonable value well within the upper limit described previously. 46 47 48 502 Another useful point of reference is the estimate of the mean tangential velocity to the path of the ACC 49 50 503 estimated by Olbers et al. (2004). The mean estimate of Southern Ocean velocity suggests velocities ranging

51 -1 -1 52 504 from 8 cm s at depths of ~1000m, decreasing to 2 cm s at depths of ~3000m: this is consistent with both of 53 54 505 the above estimates. 55 56 57 506 To obtain a robust set of estimates for eastward velocities, the ECCO live access server was used to 58 59 507 obtain velocity data for each section spanning the full range of the model solutions, from 1992 to 2008. The 60 61 62 19 63 64 65 508 average (where repeat occupations occur) north and south limits defined by the section partitioning was used for 1 2 509 the meridional limits of the ECCO data at a constant depth relevant to the average depth of the salinity- 3 4 510 maximum for each section. A simple time-latitude average was calculated to obtain a mean velocity. This is a 5 6 511 preferable method to calculating individual section transport velocities, say based on hydrography, as such a 7 8 512 method would provide only snapshot estimates of velocities when the circumpolar distribution of tracers is 9 10 513 determined by the long-term flow. Within the ACC the minimum velocity determined by this method was 1.34 11 12 514 cm s-1 in the Polar Front Zone of the A23 sections, and the maximum was 4.82 cm s-1 across the SR01 Drake 13 14 515 Passage sections. As expected, overall lower velocities were determined for the Weddell Gyre, with a minimum 15 16 516 of 0.87 cm s-1 in the far eastern Weddell Gyre and a maximum of 1.83 cm s-1 in the northern limb of the 17 18 517 Weddell Gyre across the SR02 sections. These values are consistent with the earlier outline calculations. 19 20 21 518 5.4 - Methods - Second differentials 22 23 휕2푆 휕2푆 519 The terms and in equation [5] were estimated by calculating the second differential of a 24 휕푗2 휕푧2 25 26 휕2푆 520 quadratic fit to the salinity data. The term is obtained from a single fit to the profile of the salinity- 27 휕푗2 28 29 휕2푆 521 maximum along the section. The term is the mean for each section occupation of the second differential of 30 휕푧2 31 32 522 the quadratic fit to each station salinity profile, for appropriate ranges of salinity. Within the ACC the vertical 33 34 523 quadratic fit was obtained for SP > 34.68, and for the Weddell Gyre SP > 34.67. This discrimination was 35 36 524 necessary due to the lower salinity values in the upper water column of the Weddell Gyre. 37 38 39 525 5.5 – Results - Mixing Coefficients 40 41 휕2푆 휕2푆 526 With reference to Equation [7], whilst all stations provided a consistently negative term, the 42 휕푧2 휕푗2 43 44 527 term was not consistently negative across all sections. Due to the lack of distinct curvature in the cross-stream 45 46 528 gradient of the salinity-maxima across the Polar Front and Antarctic Zones of the Atlantic sector, the computed 47 48 휕2푆 529 values for term were near zero, being orders of magnitude smaller than for sections in the Indian and Pacific 49 휕푗2 50 51 530 sectors of the Southern Ocean, and varying between being positive or negative. I06S was similarly affected by 52 53 531 the continuation of the deep salinity-maximum beneath the Agulhas Retroflection. 54 55 56 532 Additionally, I08S was affected by the presence of the Kerguelen Plateau which results in a bimodal 57 58 533 peak in the salinity-maximum to the north and south of the plateau, with an intervening low salinity-maximum 59 60 534 associated with the deep Australian-Antarctic Gyre (McCartney and Donohue 2007). The shallow bathymetry 61 62 20 63 64 65 535 forces LCDW to be transported around the north and south of the plateau, as reflected in the diversion of the 1 2 536 ACC fronts in Figure 3. The effect of this deep gyre can also be seen in the profile of one of the two 3 4 537 occupations of I09S (Figure 7, Subplot B – seen as a dark blue low salinity spike), suggesting it has a variable 5 6 538 zonal impact. 7 8 9 539 Finally, in the northern Weddell Gyre the term varies between positive or negative values due to the 10 11 540 influence of the mixing with the Antarctic Zone to the north and with the southern Weddell Gyre. This also 12 휕푆 13 541 makes the downstream salinity gradient, , term positive. 14 휕푖 15 16 The impact of the above terms assessed for the above sections is that it renders them unsuitable for use 17 542 18 19 543 with linear regression as the mixing equation assumes a body diffusing as it advects and does not account for 20 21 544 additional sources of salinity. While it is therefore not possible to calculate mixing coefficients for the above 22 23 545 mentioned sections, the SR03 and Pacific sector sections as well as the sections covering the eastern and 24 25 546 southern Weddell Gyre are suitable for that purpose, as shown in Table 3. Within the ACC – from the SR03 26 -4 2 -1 27 547 sections to the A21 section – a vertical mixing coefficient is obtained of κV = 2.86 x 10 m s and an isopycnal 28 2 2 -1 29 548 mixing coefficient of κI = 8.97 x 10 m s . Each has estimated standard error of the coefficients of the same 30 31 549 order of magnitude, but numerically smaller as shown in Table 3. 32 33 34 550 In the eastern and southern Weddell Gyre the vertical mixing coefficient is an order of magnitude 35 -5 2 -1 36 551 smaller, at κV = 2.39 x 10 m s : likely due to the general absence of significant topographic features when 37 38 552 compared with mid-ocean ridges that lay below the ACC. The isopycnal coefficient is the same order of 39 2 2 -1 40 553 magnitude as for the ACC, at κI = 2.47 x 10 m s . Whilst the κI has an associated error which is an order of 41 42 554 magnitude smaller, the κV is not only the same order of magnitude but also numerically larger. This appears to 43 44 555 be due to the presence of the Maud Rise seamount on the Greenwich Meridian causing increased vertical mixing 45 46 556 compared to the rest of the Weddell Gyre, and the contrast between the rates of vertical mixing leads to a larger 47 48 557 error. 49 50 -5 2 -1 51 558 The combined calculation places the vertical mixing coefficient at κV = 2.86 x 10 m s and associated 52 53 559 error similar to that for the Weddell Gyre and for the same reasons. The isopycnal mixing coefficient remains at 54 2 2 -1 55 560 the same order of magnitude, at κI = 2.62 x 10 m s , with the associated error an order of magnitude smaller. 56 57 561 As the combined calculation produces coefficients which are similar to the eastern and southern Weddell Gyre 58 59 562 coefficients, and the SR03 and Pacific sections have a much larger associated error, this suggests that there is 60 61 62 21 63 64 65 563 greater variability in the vertical mixing rates in the Pacific sector than in the Weddell Gyre, with some regions 1 2 564 of the Pacific being more quiescent than others. This is consistent with known variations in underlying 3 4 565 bathymetry across the Pacific sector of the Southern Ocean. 5 6 7 566 Collectively, these values suggest it is best to consider the two regimes independently: in general the 8 9 567 Weddell-Enderby Basin is bathymetrically smooth, with the exception of major features such as Maud Rise, 10 11 568 whereas the SR03 and Pacific sector sections cross rough and complex topography, particularly to the south of 12 13 569 Australia and New Zealand and across the Pacific Antarctic Ridge. 14 15 16 570 6 - Discussion 17 18 571 6.1 - Variability and Stability in the Southern Ocean 19 20 572 6.1.1 - Circumpolar trends 21 22 573 The quantitative observations made in this study about the circumpolar freshening trend of the salinity- 23 24 574 maximum downstream from the NADW source is consistent with the brief circumpolar part of the study by 25 26 575 Williams et al. (2006) and reinforces the earlier observations of Callahan (1972) regarding the distribution of 27 28 576 deep water masses. Within Drake Passage, Williams et al. (2006) identify no clear trend in the core 29 30 577 thermohaline properties of the LCDW across the entire study period between 1926 and 2004, whilst our study 31 32 578 seems to confirm their initial assessment that there is no apparent temporal trend within the LCDW domain 33 34 579 across the rest of the Southern Ocean either. This does not preclude a long-term trend from existing in the 35 36 580 Southern Ocean wide deep salinity-maximum; however there is an insufficiently large hydrographic dataset at 37 38 581 this time to judge circumpolar inter-decadal changes. 39 40 41 582 Observations of warming in the Southern Ocean suggest thermohaline changes are or were taking 42 43 583 place. A prominent example is the 0.17 K warming between 1950 and 1980 at depths of 700-1100 m as 44 45 584 detected by Gille (2002) and thought to be linked to the annually ventilated mode waters forced by atmospheric 46 47 585 warming. As LCDW is predominantly deeper than this, it is entirely feasible that this temperature increase does 48 49 586 not apply to the deeper ocean, as supported by the results of Williams et al. (2006). 50 51 52 587 The ACC in general is known to exhibit variability in flow regimes, transport and frontal positions. 53 54 588 According to Turner and Overland (2009), certain parts of the Southern Ocean have changed rapidly in the last 55 56 589 century, but there is a complex pattern of change, with multiple mechanisms and feedbacks implicated in the 57 58 590 variability of ocean characteristics. They propose that the variability of LCDW water mass characteristics could 59 60 591 be accounted for by: an acceleration in the ACC affecting entrainment of source water masses; the southward 61 62 22 63 64 65 592 shift of the ACC and its fronts as a result of changes to the Southern Annular Mode (SAM); or greater eddy 1 2 593 activity resulting from increased westerly winds. Such variability could account for the small changes between 3 4 594 repeat occupations. 5 6 7 595 6.1.2 - Weddell Gyre variability 8 9 10 596 Decadal scale variations in the characteristics of the Weddell Gyre have been observed across the 11 12 597 Greenwich Meridian with the mean salinity of the WDW observed to vary between 34.677 and 34.681 by 13 14 598 Fahrbach et al. (2004), although the variation is within the magnitude of the systematic error due to calibration. 15 16 599 In agreement with the assessment of this study, they judge that minor variations are likely to be real. 17 18 600 Interestingly, Whitworth and Nowlin (1987) note persistent subtle signals, such as a mid-depth silicate 19 20 601 maximum which they identify as being sourced from the Indian Ocean. This suggests that despite mixing over 21 22 602 long distances in the circumpolar path and across dynamic regime boundaries, variations in deep water mass 23 24 603 properties can be conserved and have an observable presence in the Weddell Gyre, albeit as a weaker signal. 25 26 27 604 Such observed variability may be due to time-varying transports across the topographically restricted 28 29 605 southern Weddell Gyre. Cisewski et al. (2011) calculate a westward transport of 23.9 ± 19.9 Sv in austral 30 31 606 summer and 93.6 ± 20.1 Sv in austral winter from two separate cruises. This large variability in volume 32 33 607 transport was thought to be linked to gyre-scale forcing by changes in the wind stress curl and manifests itself 34 35 608 primarily in variations in the jet structures associated with the Maud Rise seamount and the Antarctic 36 37 609 continental slope. Thus changes in transport could lead to a pulsating propagation of salinity signals from east 38 39 610 to west. 40 41 42 611 6.2 - The exchange of water masses 43 44 612 Whilst at first consideration the water mass analysis appears to be a rather simple technique, it 45 46 613 nevertheless appears to offer a useful insight into deep water mass dynamics and supports the existing evidence 47 48 614 regarding NADW and LCDW entrainment. Underlying the transport of the ACC are the strong zonal fronts, 49 50 615 which according to Sokolov and Rintoul (2009a, 2009b) comprise numerous branches of the conventional ACC 51 52 616 fronts associated with sea surface height gradients. However, whilst their analysis accounts for ~90% of the 53 54 617 variability, the remaining variability speaks to the fact that these fronts are not always strong or continuous in 55 56 618 time and space. This owes to the propagation of eddies across these fronts which drives entrainment from one 57 58 619 regime to another. Paraphrasing Naveira Garabato et al. (2011), under these conditions the parallel jets lead to 59 60 620 homogenisation of water masses between fronts, however in regions of greatest eddy transport and mixing, these 61 62 23 63 64 65 621 fronts are ‘leaky’ and thus provide for cross-frontal exchange of water masses exceeding that which normally 1 2 622 occurs along the path of the ACC. We can see from the water mass analysis how this results in a more 3 4 623 homogenous LCDW in the Indian and Pacific sectors for each cruise when compared to the Atlantic sector 5 6 624 which influenced by the input from the north of NADW. 7 8 9 625 6.2.1 - Reasons for entrainment of NADW into the PFZ 10 11 626 The complex topography which results in the northward excursion of the ACC fronts into the 12 13 627 Argentine Basin appears to be a primary means by which NADW is entrained into the PFZ of the ACC. The 14 15 628 meandering of the fronts and the associated eddy activity allow a higher cross-frontal transfer of water mass 16 17 629 properties than along the rest of the SAF in the Atlantic sector, as can be seen in the southwest South Atlantic in 18 19 630 Figure 1 where the SAF shifts to higher salinities east of Drake Passage, and by the significantly higher 20 21 631 contributions to the PFZ in the Atlantic sector compared to the rest of the ACC as shown in Table 2. The 22 23 632 importance of the Argentine basin in mixing has been previously noted (e.g. Peterson and Whitworth 1989, 24 25 633 Arhan et al. 1999), and the topographically driven mechanisms for the movement of these fronts within the 26 27 634 Scotia Sea are well described by Naveira Garabato et al. (2002). 28 29 30 635 Of less importance seem to be the more easterly sections of the SAF and – surprisingly – the Agulhas 31 32 636 Retroflection where despite carrying a deep salinity-maximum beneath it appears to make a low contribution 33 34 637 based on significant decrease in NADW contribution from the water mass analysis method to I06S sections. 35 36 638 This is likely to be due to this feature contributing directly to the deep waters of Indian Ocean rather than being 37 38 639 carried east by the ACC. We therefore conclude that the southwest Atlantic is the most important region for the 39 40 640 entrainment of NADW. However, these same water mass estimates suggest that this region of the Southern 41 42 641 Ocean may play a greater role in transferring the more saline salinity-maximum signal across the PF, but not 43 44 642 directly into the Weddell Gyre. 45 46 47 643 6.2.2 - Reasons for entrainment into and insulation of the Weddell Gyre 48 49 644 A number of authors have identified the eastern Weddell Gyre as a region dominated by eddy activity 50 51 645 which is thought to contribute significantly to the entrainment of CDW into the Gyre (e.g. Deacon 1979, Orsi et 52 53 646 al. 1993, Orsi et al. 1995, Gouretski and Danilov 1993). Gouretski and Danilov (1994) describe the impact of 54 55 647 warm core eddies from the ACC moving south into the eastern Weddell Gyre and observed that they must lead 56 57 648 to the transport of heat and salt as the eddies decay. Whilst the ACC is in an eddy-saturated state, there are 58 59 649 regions of greater and lesser eddy activity. 60 61 62 24 63 64 65 650 Fahrbach et al. (1994) assert that the injection of LCDW can occur through the Weddell Front (the 1 2 651 local Southern Boundary of the ACC generated by the influence of the Antarctic Peninsula) and thus cross 3 4 652 frontal entrainment of LCDW can potentially occur west of the Greenwich Meridian. However, as the Weddell 5 6 653 Front is variable in intensity, injection of water masses into the northern Weddell Gyre must also be subject to 7 8 654 variability. This factor is considered less important than the eastern Weddell Gyre region, and they assert that 9 10 655 processes local to the Gyre, either open-ocean or coastal, are likely to play a major controlling factor on the 11 12 656 salinity signal of the WDW. This is confirmed by the water mass estimates which suggest the eastern Weddell 13 14 657 Gyre acts an intermediate step between the ACC and western Weddell Gyre, being an even mixture of LCDW 15 16 658 and WDW, and which is then mixed further with the WDW through the southern limb of the Weddell Gyre. 17 18 19 659 Naveira Garabato et al. (2004b) identify that turbulent mixing is enhanced over topography and this 20 21 660 plays a crucial role in closing the overturning circulation. A particular section of interest in this study is the 22 23 661 20E-NWG section in the north-eastern Weddell Gyre. The dynamic height contour we use to denote the 24 25 662 Southern Boundary of the ACC is present only in the extreme north of the section at ~53.5oS. The Southwest 26 27 663 Indian Ridge underlies the front at this latitude and deepens further east, through which the ACC deflects 28 29 664 southwards. Rough topography and a meandering ACC provide ideal conditions for turbulent mixing to act to 30 31 665 modify the LCDW and entrain it into the eastern Weddell Gyre, and the water mass estimates in this study 32 33 666 reinforce the concept of the eastern Weddell Gyre as a key gateway for LCDW entrainment. This view is 34 35 667 consistent with salinity distribution determined by the objective mapping of Argo float measurements in the 36 37 668 upper-ocean of the Weddell Gyre by Reeve et al. (2016). 38 39 40 669 6.2.3 - Water Mass Pathways 41 42 670 The frontal meridional barrier presented by the ACC outweighs the poleward transport of properties by 43 44 671 eddies ensuring the downstream propagation of the deep salinity-maximum signature in the ACC dominates the 45 46 672 deep pathway of the MOC. 47 48 49 673 This view of the Southern Ocean circulation is reflected by the particle exchange experiments of Döös 50 51 674 (1995) which contrasts various pathways of inter-basin exchange. Our study deals with the indirect ventilated 52 53 675 route of particles moving through the Southern Ocean, and subsequently upwelling south of the ACC. This 54 55 676 route has a mean of six repeat circulations of Antarctica before exposure to the surface ocean, and this number is 56 57 677 consistent with the water mass estimates for I06S - 6% larger than SR01 and 21% smaller than SR02-PFZ - 58 59 678 which suggest a 4:1 dominance of SR01 recirculation across the Atlantic sector of the ACC. This route is in 60 61 62 25 63 64 65 679 contrast to the direct, unventilated route whereby water moves from basin to basin without reaching the surface, 1 2 680 such as the movement of CDW directly into the Pacific and Indian Oceans. 3 4 5 681 The water mass estimates combined with these considerations suggest that the frontal system acts as a 6 7 682 meridional barrier to the propagation of NADW properties, insulating the Weddell Gyre from fluctuations in the 8 9 683 large-scale circulation, and thus negating a direct meridional pathway across the ACC for the MOC. 10 11 12 684 6.3 - Estimating Mixing Rates 13 14 685 The results of a number of investigations of the mixing rates from a variety of different regimes and 15 16 686 oceans have been published and it is pertinent to see whether the results of our calculations are in agreement 17 18 687 with them. 19 20 21 688 6.3.1 – Comparison of vertical and diapycnal mixing rates 22 23 689 The mixing rates calculated by this study are now compared and contrasted with other examples of mixing rates 24 25 690 determined by other authors to give context for examining variability of mixing rates within different Southern 26 27 691 Ocean regimes. Our estimates are (2.86 ± 1.06) x 10-4 m2s-1 for vertical mixing for the eastern Indian and 28 29 692 Pacific sectors of the ACC and (2.39 ± 2.83) x 10-5 m2s-1 in the Weddell Gyre. Munk's original (1966) estimate 30 -4 2 -1 31 693 of a bulk κV for the Pacific interior was 1.3 x 10 m s which is between the quiescent Weddell Gyre and the 32 33 694 turbulent ACC. In the Brazil Basin Polzin et al. (1997) demonstrate the spatial variability of mixing in the 34 -4 2 -1 35 695 abyssal ocean, with κD = 0.1 x 10 m s over smooth topography, similar to this study’s Weddell Gyre estimate 36 37 696 under similarly low energy conditions. They obtain a higher estimate of 5 x 10-4 m2 s-1 within 150 m of the sea 38 39 697 floor and over rougher topography, twice as large as our mid-depth estimate for the LCDW in the ACC. Naveira 40 41 698 Garabato et al. (2004a) obtained a range of estimates for weakly stratified Nordic Seas; at mid depths of 1500- 42 -4 -3 2 -1 43 699 2500m, κV ≈ 10 to 10 m s was dominant. Naveira Garabato et al. (2004b) likewise obtained a range of 44 45 700 mixing rates at the bottom of Drake Passage, with higher rates above rough topography of 10-4 m2 s-1 at 500m to 46 47 701 10-3 to 10-2 m2 s-1 near the sea floor. The lower estimate is consistent with our ACC estimate which tracks 48 -4 49 702 topography during its circumpolar path. Zika et al. (2009) obtain a diapycnal mixing rate of κD = (1 ± 0.5) x 10 50 51 703 m2 s-1 for the entire Southern Ocean. In the Indian Ocean Sloyan (2006) calculates an abyssal diapycnal mixing 52 -4 2 -1 53 704 rate, κD = 13-15 x 10 m s for the LCDW moving northward into the Perth Basin which is higher by a factor 54 55 705 of 3 compared to our highest estimate. Likewise, Heywood et al. (2002) provide evidence for enhanced mixing 56 -4 2 -1 57 706 over topography in the Scotia Sea with κD = (39 ± 10) x 10 m s therefore also an order of magnitude larger 58 59 -5 2 -1 707 than our estimate for the ACC. Sheen et al. (2013) report values for κV ranging from 10 m s at mid-depth to 60 61 62 26 63 64 65 708 10-3 m2 s-1 near the bottom in the Scotia Sea based on in situ measurements and Waterman et al. (2013) have 1 2 709 similar results near Kerguelen. Watson (2013) examined mixing in the UCDW using an open-ocean tracer 3 -5 2 -1 4 710 release of trifluoromethyl sulphur pentafluoride and found κD to vary from ~2 x 10 m s in the southeast 5 6 711 Pacific sector of the ACC, increasing to ~7 x 10-5 m2s-1 in Drake Passage. Lower values in the topographically 7 8 712 deep and smooth southeast Pacific are similar to our own Weddell Gyre estimates, whilst the Drake Passage 9 10 713 estimates is significantly lower than our SR03 and Pacific sector estimate where there is rough topography south 11 12 714 of Australia and New Zealand. From studies on vertical mixing in the upper pycnocline performed within 13 14 715 two transient mesoscale eddies in the Atlantic sector of the ACC, Cisewski et al. (2005, 2008) obtained KV = 7 x 15 -4 -3 2 -1 16 716 10 -10 m s with high variability of KV on horizontal and temporal scales of order 10 km and days, 17 18 717 respectively. For one of the eddies studied by Cisewski et al. (2008), but using a different method based on 19 -4 20 718 mean changes occurring over weeks in the eddy core, Hibbert et al. (2009) estimate a maximum KV = 3 x 10 21 22 719 m2 s-1 which is well within our own ACC estimate. A more recent study by Ledwell et al. (2011) diagnosed a 23 -5 2 -1 24 720 diapycnal diffusivity κD = (1.3 ± 0.2) x 10 m s in the eastern Pacific sector of the ACC averaged over 1 year 25 26 721 and thousands of kilometres: a value which is characteristic of the mid-latitude ocean interior. Given the less 27 28 722 rough topography of the region this value fits better with our estimate for the relatively quiescent Weddell Gyre, 29 30 723 rather than our ACC estimate. Finally, Leach et al. (2011) obtained the very low diapycnic diffusivity of 3 x 10-6 31 32 724 m2 s-1 in the westward flowing southern limb of the Weddell Gyre. 33 34 -4 2 -1 35 725 The consensus seems to be that in general a value for kv of 10 m s is typical for the ocean in general 36 37 726 with lower values of 10-5 m2 s-1 in quieter regions and 10-3 m2 s-1 or larger near topography or in the upper 38 39 727 ocean, and our values are consistent with this. 40 41 42 728 6.3.2 - Horizontal and isopycnal mixing 43 44 3 2 -1 45 729 An early estimate for horizontal mixing by Munk (1950) required κH = 5 x 10 m s in order to obtain 46 47 730 a realistic scale for the western boundary current of a wind driven gyre, but Okubo (1971) reviewed estimates of 48 49 731 horizontal eddy diffusivities and found that they were scale-dependent. Ledwell et al. (1998) based on a tracer- 50 51 732 release experiment in the eastern North Atlantic estimated values of 2 m2 s−1 for scales of 1–10 km increasing to 52 53 733 1000 m2 s−1 for scales of 30–300 km. Cunningham and Haine (1995) obtained a value of 950 m2 s−1 and a scale 54 55 734 of 166 km for the core of the Labrador Sea Water in the North Atlantic. Zika et al. (2009) obtained an isopycnal 56 2 -1 57 735 mixing rate of κI = 300 ± 150 m s for the entire Southern Ocean, while Hibbert et al. (2009) also estimated a 58 2 -1 59 736 KH = 30-100 m s within a cold core eddy. Leach et al. (2011) examined mixing of the WDW entering the 60 61 62 27 63 64 65 737 Weddell Sea with the south-eastern limb of the Weddell Gyre across the Greenwich Meridian and found an 1 2 -1 2 -1 2 738 isopycnal diffusivity of 70-140 m s . Our estimate of 897 ± 167 m s for isopycnal mixing in the ACC is 3 4 739 consistent with values for scales larger than that of individual eddies. Our smaller Weddell Gyre value of 247 ± 5 6 740 63 m2 s-1 is perhaps more consistent with Leach et al. (2011), who also had smaller values here, possibly due to 7 8 741 weaker eddy activity. 9 10 11 742 12 13 14 743 7 - Conclusion 15 16 744 Motivated by the need to better understand changes in the LCDW - the water mass that represents the 17 18 745 link between the NADW and the AABW in the Southern Ocean, and which is therefore intimately involved in 19 20 746 closing the lower limb of the global overturning circulation - this study quantifies the spatial trend of the deep 21 22 747 salinity-maximum that is associated with the LCDW. This is conducted within a framework of variable density 23 24 748 regimes to the north of, within and to the south of the ACC owing to water mass modification during the process 25 26 749 of entrainment from one regime to another. There is no observable temporal trend from the repeat occupations 27 28 750 of sections, however there is a clear spatial pattern in the salinity signature. 29 30 31 751 This signature originates from a water mass formed in the North Atlantic, the NADW, which is 32 33 752 transported southwards and is subsequently entrained into the ACC as LCDW. The salinity signature weakens 34 35 753 due to freshening as it is carried through the Indian and Pacific sectors of the Southern Ocean, subject to mixing 36 37 754 with the intermediate and bottom water masses above and below, respectively. The signature is transported 38 39 755 through Drake Passage and the Scotia Sea, after which it remixes to some extent with NADW in the Polar Front 40 41 756 Zone of the ACC causing a slight increase in salinity. The signature then continues to freshen after entrainment 42 43 757 into the eastern and southern limbs of the Weddell Gyre, before undergoing a weak increase in salinity in the 44 45 758 northern Weddell Gyre. This is due to lateral mixing with more saline waters in the ACC and the southern limb 46 47 759 of the Weddell Gyre, to the north and south respectively. This overall decreasing trend in salinity suggests that 48 49 760 the ACC insulates the Weddell Gyre from short-term fluctuations in the characteristics of NADW in the South 50 51 761 Atlantic. 52 53 54 762 This assertion is supported by the water mass analysis which indicates low-levels of cross-frontal 55 56 763 mixing, particularly evident within the western Atlantic sector. The water mass analysis estimates also suggest 57 58 764 that there are varying degrees of entrainment from the ACC into the Weddell Gyre at all of the observed 59 60 61 62 28 63 64 65 765 longitudes in the Atlantic sector, but that the greatest entrainment occurs within the eastern limb of the Weddell 1 2 766 Gyre. 3 4 5 767 Estimates of vertical and isopycnal mixing coefficients are in line with other estimates for the Southern 6 7 768 Ocean, although they are only valid for the Indian and Pacific sectors, and the eastern and southern Weddell 8 9 769 Gyre. Two separate estimates for these two regional sets produce understandable differences: the ACC has 10 11 770 higher mean transport velocities and flows over rougher topography than the slower moving Weddell Gyre, 12 13 771 corresponding to higher and lower rates of vertical mixing, respectively. 14 15 16 772 The overall view that emanates from this study, summarised in Figure 10, is that the ACC acts as a 17 18 773 long-term ‘oceanic blender’ with the blending action concentrated in the Atlantic sector. Cross-frontal mixing 19 20 774 of NADW into the Polar Front Zone is most prominent in the Argentine Basin, with mixing across the length of 21 22 775 the Sub-Antarctic Front elsewhere in the South Atlantic being of secondary importance. East of the Greenwich 23 24 776 Meridian mixing across the Polar Front and the Antarctic Front appear more prominent, however there is a clear 25 26 777 lack of a direct meridional overturning pathway in the Atlantic Sector of the Southern Ocean. Further study of 27 28 778 the mixing of NADW/LCDW/WDW across the fronts of the ACC is warranted to further clarify the distribution 29 30 779 of exchanges and to enhance understanding of the climatic implications for variability in the deep waters of the 31 32 780 Southern Ocean. 33 34 35 781 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 29 63 64 65 782 Appendix 1 2 783 The LCDW in the ACC exists at a fairly constant neutral density layer of 28.04-28.08 kg m-3 3 4 784 throughout its circumpolar journey as earlier detailed. However, when LCDW is entrained into the Weddell 5 6 785 Gyre as WDW the salinity maximum associated with the LCDW and WDW shallows dramatically and moves to 7 8 786 deeper density levels. A simple 1-dimensional model is used to illustrate how the transport of the 9 10 787 LCDW/WDW and its interaction with overlying and underlying water masses act to modify the density profile 11 12 788 of the water column and help explain why, outside of the ACC regime, the tracking of the salinity maximum 13 14 789 remains an acceptable technique for assessing water mass changes in the LCDW/WDW throughout its Southern 15 16 790 Ocean journey. 17 18 19 791 Imagine that a column of water is being carried along by the ACC and as it does so it is slowly being 20 21 792 drawn southwards towards Antarctica, whilst mixing with the overlying and underlying water masses. As a 22 23 793 result of the poleward movement, the characteristics of these boundary water masses will change. The 24 25 794 northward Ekman transport – out of the Weddell Gyre – results in near-surface waters occupying a shallower 26 27 795 depth range south of the ACC. This includes the absence of AAIW which only subducts north of the Polar 28 29 796 Front within the ACC (Sallée et al. 2010). 30 31 32 797 Dense water which forms over the Antarctic continental shelf mixes with deeper waters as it descends 33 34 798 the continental slope to the deep ocean. This leads to the formation of topographically constrained WSDW (- 35 36 799 0.7oC < θ < 0oC) and WSBW (θ < -0.7oC) (Fahrbach et al. 1995, Gordon et al. 2001), the former of which is 37 38 800 present at depths as shallow as 1100m compared to the salinity maximum of the LCDW in the ACC which can 39 40 801 lie at depths of up to 2500m. The presence of WSDW and WSBW acts to force the LCDW/WDW upwards 41 42 802 from beneath, resulting in the shoaling of isopycnals and the rise of LCDW/WDW in the water column. 43 44 45 803 The standard framework for considering the ACC includes transport being dominated by streamlines of 46 47 804 eastward flow aligned with the position of circumpolar fronts and a strong dependence on the sloping isopycnal 48 49 805 surfaces associated with these fronts for tracking conservative tracers. This is against the background of a 50 51 806 northward Ekman transport near the surface and an opposing southward flux below by the mesoscale eddy field. 52 53 807 However, whilst this approach is suitable for the ACC, the Weddell Gyre represents a different dynamical 54 55 808 regime characterised by the recirculation of gyre water masses, water mass transformation, and interaction with 56 57 809 the ACC along the northern and eastern boundaries of the Gyre. This appendix explores a simple example to 58 59 60 61 62 30 63 64 65 810 explain the observed changes in the properties of the LCDW as it circulates around Antarctica and is entrained 1 2 811 into the Weddell Gyre. 3 4 5 812 To provide realistic initial conditions, the hydrographic profile from station 25, cast 1 of the 2002 A12 6 7 813 cruise track undertaken during FS Polarstern’s expedition ANT-XX/2 was used as the starting hydrographic 8 o o 9 814 profile. The station was located at 49 S, 2.8 E placing it south of the Sub-Antarctic Front but north of the Polar 10 11 815 Front. 12 13 14 816 At the surface boundary, the sea surface temperature decreases by 0.005 K per time step to represent 15 16 817 the decreasing surface temperature moving towards Antarctica, whilst the sea surface salinity is constant to 17 18 818 represent the general stability and relative freshness of the Antarctic Surface Water. The potential temperature 19 20 819 at the bottom boundary of 4000m decreases by 0.0018 K per time step and the salinity decreases by 0.00005 per 21 22 820 time step, mimicking the transition into the WSBW/WSDW regime at depth. 23 24 25 821 These linear changes were derived from the total meridional temperature and salinity gradient over the 26 27 822 A12 section as a representative trend for surface and bottom waters. As both surface and bottom waters are 28 29 823 formed locally in the Southern Ocean, these boundary conditions provide a reasonable first order approximation 30 31 824 to the meridional gradient in oceanographic conditions, although clearly this is an idealized approach. 32 33 34 825 In the vertical, the grid was set at 101 depth levels, spaced every 40m from 0m to 4000m. The initial 35 36 826 A12 profile was used for the model down to a depth of 4000m (of a total cast depth of 4090m), whilst the 37 38 827 shallowest data (at 9 dbar) was used as the surface value. Except the surface values there were no missing data. 39 40 41 828 The 1-dimensional model is based around simple equations for the modification of potential 42 43 829 temperature and salinity: 44 45 푑휃 푑휃 (휃푖−1 − 2휃푖 + 휃푖+1) 휃푖 − 휃푖−1 46 830 [8]휃 = 휃 + 훥푡, 푤ℎ푒푟푒 = 휅 + 푤 47 푡 푡−1 푑푡 푑푡 푧 (훥푧)2 훥푧 48 49 50 푑푆 푑푆 (푆푖−1 − 2푆푖 + 푆푖+1) 푆푖 − 푆푖−1 51 831 [9]푆 = 푆 + 훥푡, 푤ℎ푒푟푒 = 휅 + 푤 푡 푡−1 푧 ( )2 52 푑푡 푑푡 훥푧 훥푧 53 54 55 832 where 휃 is potential temperature, 푆 is practical salinity, 푡 is time, 푖 is the vertical counter, positive downwards, 56 57 833 푧 is the vertical co-ordinate, positive downwards, 휅푧 is the diapycnal diffusivity coefficient, 푤 is the uplift rate, 58 59 834 훥푡 is the time step and 훥푧 the vertical grid spacing. 60 61 62 31 63 64 65 835 A 120 year transport duration was derived from an estimated 40000 km track from the station location 1 2 836 in the ACC eastward around Antarctica, subsequently returning to a more southerly position on the Greenwich 3 4 837 Meridian, and finally entrainment into the Weddell Gyre at an assumed 1 cm s-1, rounded down to the nearest 5 6 838 whole year. The number of time steps was set at 365 and the time step size set to 120 days for a simple scaling 7 8 839 of the duration. This is consistent with the distances and times discussed in the main body of this text, whereby 9 10 840 LCDW is shown to recirculate in the ACC before entering the polar gyres. 11 12 13 841 The vertical thermal diffusivity coefficient, κ, was set to 5 x 10-5 m2 s-1 which falls within the range of 14 15 842 many studies of vertical mixing in the ocean (e.g. Munk 1966, Naveira Garabato et al. 2004b, Zika et al. 2009). 16 17 843 In addition, to represent the rise of LCDW/WDW in the water column as a result of Ekman suction, a rate of 18 19 844 water column uplift of 20 m yr-1 was applied. 20 21 22 845 Both potential density and Neutral Density were calculated based upon the new potential temperature, 23 24 846 practical salinity and pressure at each time step. 25 26 27 847 The initial and final profiles of the 1-D model shows: an overall decrease in temperature at all depths; a 28 29 848 marked increase in salinity in the upper water column but a slight decrease in deeper waters resulting in the 30 31 849 upward shift of the salinity maximum; and an overall increase in the density of the profile. 32 33 34 850 The salinity and Neutral Density time-series section shown in Figure 11 illustrates the rapid upward 35 36 851 shift of the salinity maximum as the profile moves into the modelled regime of the Weddell Gyre. Salinity 37 38 852 stabilises at depth reflecting the shallow salinity gradient in the WSDW and WSBW whilst a fresh surface layer 39 40 853 is retained. The potential temperature (not shown) decreases steadily at all depths, with the temperature 41 42 854 maximum remaining at about 100m depth. 43 44 45 855 Neutral Density increases at depth resulting in a decreasingly stratified deep water column, whilst the 46 47 856 surface boundary constraint results in a strongly stratified upper water column. The key result is that density 48 49 857 increases at depth such that the salinity maximum initially uplifts at a similar rate to the neutral surfaces until 50 51 858 about year 70 when the density at the salinity maximum increases whilst the depth of the salinity maximum 52 53 859 stabilises at about 400m depth after 80 years. The intervening period represents the entrainment of the water 54 55 860 mass into the Weddell Gyre proper. 56 57 58 861 The framework for examining the ACC described earlier is well established, however, this framework 59 60 862 does not extend to the transition to the hydrographic regime of the Weddell Gyre (Schröder and Fahrbach 1999). 61 62 32 63 64 65 863 This 1-D model shows an initial decrease in the depth of LCDW from ~2500m to ~400m, paralleling the north- 1 2 864 south distribution of temperature, salinity and density evident on sections crossing the ACC into the Weddell 3 4 865 Gyre. 5 6 7 866 Whilst simple, this model – when viewed as a representation of the mean-state of more complex 8 9 867 Southern Ocean dynamics over long time scales – serves to illustrate the broad-scale effect of crossing from one 10 11 868 regime to another upon the core of the LCDW. The core of a traditional tracer can be mixed and advected over 12 13 869 multi-year time scales to produce a result which is consistent with observations. Whilst within the core of the 14 15 870 ACC the salinity maximum remains within a constant density layer, this is not the case as the regime shifts from 16 17 871 a circumpolar flow to the Weddell Gyre circulation. The normal framework for examining the ACC on 18 19 872 streamlines and isopycnal surfaces does not apply beyond the Southern Boundary of the ACC, where 20 21 873 complicated processes of advection, mesoscale eddies and frontal meandering together modify the water 22 23 874 column. Recognition must be given to the effects of the contrasting hydrographic regimes of the ACC and 24 25 875 Weddell Gyre, which demonstrate a situation in which the signal of the deep salinity maximum crosses 26 27 876 isopycnal surfaces. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 33 63 64 65 877 Acknowledgements 1 2 878 This work was funded by the National Environment Research Council through a PhD Studentship at the 3 4 879 University of Liverpool in partnership with the Alfred Wegener Institute. 5 6 7 880 Our thanks go to the scientists and crews of the various cruises whose data we have used, and to the CLIVAR 8 9 881 and Carbon Hydrographic Data Office for hosting the publicly available data. 10 11 12 882 The state estimates were provided by the ECCO Consortium for Estimating the Circulation and Climate of the 13 14 883 Ocean funded by the National Oceanographic Partnership Program (NOPP). 15 16 17 884 This work has been greatly improved by the comments of the initial two anonymous reviewers, and also 18 19 885 subsequent comments from Dr. Elizabeth Jones. We also thank the final two anonymous reviewers for their 20 21 886 contribution. 22 23 24 887 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 34 63 64 65 888 Table Captions 1 2 3 889 Table 1 – Details of cruises used from the CCHDO database. Partitioned sections denoted with the 4 5 890 accompanying symbols *, ^, # in column 4 are only occupied by the correspondingly marked sections in column 6 7 891 5. All other non-marked sections are covered by all occupations. 8 9 892 10 11 893 Table 2 - Results of the water mass analysis listing: section number and name in columns 1 and 2 match those 12 13 894 listed in Table 1 for reference; indicative percentage contributions from mixing end members of NADW, 14 15 895 AASW and AABW in columns 3, 5 and 7 with the darkest green indicating greatest water mass contribution and 16 17 896 white denoting lowest water mass contribution; and the change in indicative contribution since the previous 18 19 897 section in columns 4, 6 and 8, where blue denotes a negative change, red a positive change and white no change. 20 21 898 The change in indicative contribution for section numbers 2 and 4 are relative to the indicative contributions for 22 23 899 section numbers 1 and 3 respectively. Heavy black lines denote shifts in regime from large decreases in NADW 24 25 900 (sections 1-5), to overall small decreases in NADW (sections 6-15), to small increases in NADW (sections 16 26 27 901 and 17), back to mostly large decreases in NADW (sections 18-22), and finally large increases in NADW 28 29 902 (sections 23 and 24). 30 31 903 32 33 904 Table 3 – Results of 3 multiple linear regression calculations based on a given number of section realisations (#) 34 35 905 providing estimates of the vertical mixing coefficient (κV) and isopycnal mixing coefficient (κI). 36 37 906 38 39 40 907 Figure Captions 41 42 43 908 Figure 1 – Salinity on the 28.05 kg m-3 Neutral Density surface from the World Ocean Atlas 2009 with the 44 45 909 major fronts (black lines and grey highlighted labels) and zones (brown highlighted labels) as follows: Sub- 46 47 910 Tropical Front (STF), Sub-Antarctic Front (SAF), Polar Front (PF), Southern ACC Front (SACCF), Southern 48 49 911 Boundary (SB), Sub-Tropical Zone (STZ), Sub-Antarctic Zone (SAZ), Polar Front Zone (PFZ), Antarctic Zone 50 51 912 (AZ), Weddell Gyre (WG) and Ross Gyre (RG). The highest salinity values are found in the South Atlantic, 52 53 913 decreasing eastwards through the Indian Ocean, reaching minima to the north of the ACC in the Pacific Ocean 54 55 914 and to the south of the ACC in the high Southern Ocean. The edge of white areas indicates the intersection of 56 57 915 the Neutral Density surfaces with the seafloor. 58 59 60 61 62 35 63 64 65 916 Figure 2 - Major bathymetric features of the Southern Ocean. The following abbreviations have been used: 1 2 917 Basins and seas: Amundsen Abyssal Plain (AAP), Amundsen Sea (AS), Bellingshausen Abyssal Plain (BAP), 3 4 918 Mornington Abyssal Plain (MAP), Prydz Bay (PB). Major ridge systems: Mid-Atlantic Ridge (MAR), 5 6 919 Southwest Indian Ridge (SWIR), Kerguelen Plateau (KP), Southeast Indian Ridge (SEIR), Pacific-Antarctic 7 8 920 Ridge (PAR), North Scotia Ridge (NSR), South Scotia Ridge (SSR), American-Antarctic Ridge (AAR), Drake 9 10 921 Passage (DP), Falkland Escarpment (FE). Smaller bathymetric features: Maud Rise (MR), Astrid Ridge (AR), 11 12 922 Gunneras Ridge (GR), Conrad Rise (CR), Del Cano Rise (DCR), Macquarie Ridge (MacR), Balleny Islands 13 14 923 (BI), Balleny Islands Fracture Zone (BIFZ), Udintsev Fracture Zone (UFZ), Eltanin Fracture Zone (EFZ). 15 16 924 North to south, the Sub-Antarctic Front, Polar Front, Southern ACC Front and Southern Boundary from Orsi et 17 18 925 al. (1993) are displayed as yellow lines 19 20 21 926 Figure 3 – Distribution of all cruise stations used in this study (blue dots) with corresponding sections names. 22 23 927 Fronts from Orsi et al. (1995) are shown to illustrate the extent of the ACC: Southern Boundary (green), 24 25 928 Southern ACC Front (yellow), Polar Front (black) and Sub-Antarctic Front (red). The Polar Front is used to 26 27 929 calculate the circumpolar path length of the ACC. The estimated path for the Weddell Gyre is shown in grey. 28 29 30 930 Figure 4 – Potential temperature-practical salinity plot coloured by latitude for the A12-1992 cruise along the 31 32 931 Greenwich Meridian. Black boxes denote the following water masses: WSBW (θ < -0.7°C); WSDW (0°C < θ < 33 34 932 -0.7°C); Deep Waters including the NADW, UCDW, LCDW and WDW (θ > 0°C, SP > 34.6). AABW is the 35 36 933 deepest end-member of the circumpolar water masses. The general distributions of intermediate and surface 37 38 934 waters are shown, including AASW. 39 40 41 935 Figure 5 – Conservative Temperature-Absolute Salinity plot for all deep salinity-maximum values from all 42 43 936 cruises showing the relative distribution according to regimes of the Sub-Antarctic Zone and Sub-Tropical Zone 44 45 937 (red), ACC (black) and Weddell Gyre (blue). The potential density surfaces are marked by dotted lines. 46 47 48 938 Figure 6 – The variability of all the deep salinity-maximum values from all cruises versus: A) potential density 49 50 939 referenced to the surface and B) Neutral Density. Data points are coloured by latitude (positive northwards) in 51 52 940 the top panel and by longitude (positive eastward) in the bottom panel. 53 54 55 941 Figure 7 - Main plot: The deep salinity maximum (Smax) associated with the NADW, LCDW and WDW for 39 56 57 942 cruises, with subplots for specific regions. Subplot A (green dashed box): Smax of Indian Ocean sections 58 59 943 including SR03 repeat sections. Subplot B (blue dashed box): Smax of sections in Pacific sector of the Southern 60 61 62 36 63 64 65 944 Ocean (western Pacific: dark blue, eastern Pacific: light blue). Subplot C (red dashed box): Smax of 4 sections 1 2 945 in Drake Passage. Subplot D (teal dashed box): Smax of sections in the Atlantic sector of the Southern Ocean 3 4 946 focused on the Weddell Gyre (western Weddell Gyre: light teal, central Weddell Gyre: dark teal, eastern 5 6 947 Weddell Gyre: blue). 7 8 9 948 Figure 8 – Downstream spatial change of the salinity-maximum, with distances referenced to the Greenwich 10 11 949 Meridian. Red vertical lines and labels denote the different sectors of the Southern Ocean under consideration, 12 13 950 with a distinction between two different zones of the northern Atlantic Sector and two different regimes in the 14 15 951 Weddell Gyre. Numbers are in accordance with Table 1 referring to section groups as follows: 1) A23-SAZ, 2) 16 17 952 SR02-SAZ, 3) A23-PFZ, 4) SR02-PFZ, 5) I06S, 6) I08S, 7) I09S, 8) SR03, 9) P14S, 10) P15S, 11) P16S, 12) 18 19 953 P18, 13) P19S, 14) A21, 15) SR01, 16) A23-AZ, 17) SR02-AZ, 18) I06S-EWG, 19) 20E-SWG, 20) SR02- 20 21 954 SWG, 21) A23-SWG, 22) A23-NWG, 23) SR02-NWG and 24) 20E-NWG. 22 23 24 955 a) Variation in the value of the salinity-maximum. The letters refer to the lines of best fit for the (A) Polar Front 25 26 956 Zone; (B) Pacific/Indian/Atlantic western AZ Sectors; (C) Atlantic eastern AZ, Eastern & Southern Weddell 27 28 957 Gyre; and (D) Western & Northern Weddell Gyre, with the slope of each fit shown inset. 29 30 31 958 b) Variation in the pressure/depth of the salinity-maximum. 32 33 34 959 Figure 9 – Temporal distribution of the salinity maximum for each set of repeat occupations of a section group. 35 36 960 The numbering and naming of section groups in the key is in accordance with Table 1 and Figure 8, omitting 37 38 961 sections with only one available occupation. Error bars are the standard error of the mean of the maximum 39 40 962 salinity values for each section occupation. A mix of solid, dashed and dot-dash lines is used to distinguish 41 42 963 sections along with colour. Note the change of scale at SP = 34.77 to help examine detail in the ACC and 43 44 964 Weddell Gyre. 45 46 47 965 Figure 10 – Schematic of the propagation of the deep salinity-maximum through the Southern Ocean. NADW 48 49 966 flows southwards into the South Atlantic [1] and is entrained into the Polar Front Zone (PFZ) of the Antarctic 50 51 967 Circumpolar Front (ACC) in the Argentine Basin [2] as Lower Circumpolar Deep Water (LCDW). LCDW is 52 53 968 advected eastwards by the ACC, with limited cross-frontal mixing with the Antarctic Zone (AZ) in the Atlantic 54 55 969 sector. LCDW passes through the Pacific and Indian sectors [3], during which a gradual flux of salinity carried 56 57 970 pole ward by eddies crosses the zonal fronts. The recirculating LCDW meets the entraining NADW east of 58 59 971 Drake Passage in the north of the ACC, whilst the LCDW is slowly entrained into the northern limb of the 60 61 62 37 63 64 65 972 Weddell Gyre to the south of the ACC [4]. 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Journal of Physical Oceanography 39:2926-2940 doi:10.1175/2009jpo4052.1 61 62 44 63 64 65 Manuscript figures Click here to download Manuscript LCDW-sub-v4.0- figures.docx Click here to view linked References

Figure 1 – Produced using Ocean Data View 1 2 Practical Salinity on the 28.05 kg/m3 Neutral Density surface 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 2 – Produced using Ocean Data View 1 2 Major bathymetric features of the Southern Ocean 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 3 – Produced using Ocean Data View 1 2 Location of the hydrographic sections and indicative circumpolar fronts 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 4 – Produced in Matlab 1 2 Potential temperature vs. practical salinity plot for the entire A12-1992 cruise 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 5 – Produced in Matlab 1 2 Variations in the circumpolar salinity-maximum 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 6 – Produced in Matlab 1 2 Salinity maximum variability with potential density and Neutral Density 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 7 – Produced in Matlab 1 2 Salinity maximum values for all stations on all sections plotted against the latitude of the station 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 8a – Produced in Matlab 1 2 Spatial change in salinity along the downstream path of the salinity-maximum with regime-based 3 dS/dx linear fits 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Figure 8b – Produced in Matlab 27 28 Spatial change in pressure along the downstream path of the salinity-maximum 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 9 – Produced in Matlab, exported via PDF 1 2 Temporal change in the salinity maximum at different sections 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 10 – Ocean Data View combined with grouped shape art in Word 1 2 Schematic of propagation of the salinity maximum 3 4 5 6 7 8 9 1 10 11 12 PFZ 13 14 AZ 15 44 16 2 17 WG 18 19 20 21 22 23 3 24 ACC 25 26 27 28 29 30 31 ACC 32 3 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 11 – Produced in Matlab 1 2 1-d model results of the modification of practical salinity and Neutral Density 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Manuscript tables Click here to download Manuscript LCDW-sub-v4.0- tables.docx Click here to view linked References

Table 1 1 2 Eastward path distance from 3 Approx. Section Sub-section Greenwich Cruise ref. No. of 4 Longitude EXPO code (6) Cruise dates (7) code (1) no. (3) Meridian, Code (5) stations 5 (2) including sub- 6 section tags (4) 7 1 -2900 (SAZ) A23 103 A16S_74JC10_1 20th Mar - 6th May 1995 8 3 -2900 (PFZ) 35oW 9 A23 16 24400 (AZ) (for ACC) 10 21 32800 (SWG)* A16S-2005* 60 A23_33RO200501 11th Jan - 24th Feb, 2005 11 22 35400 (NWG)* 12 SR02 # ^ 51 SR02_06MT11_5 6th Sept - 8th Oct, 1989 13 SR02-A * 43 SR02_06AQANTVIII_2 6th Sept - 8th Oct, 1989

14 # 2 0 (SAZ) # A12-1992 ^* 52 A12_06AQANTX_4 21st May - 5th Aug, 1992 15 # 4 0 (PFZ) S04A * 48 S04_06AQANTXIII_4 17th Mar - 20th May, 1996 16 SR02 0o 17 27300 (AZ) ^ # 17 20 31800 (SWG) * SR04-E ^* 46 SR04_06AQANTXV_4 31st Mar - 21st May, 1998 18 23 37100 (NWG) * A12-1999 ^* 26 A12_06ANTXVI_2 9th Jan, 1999 - 16th Mar, 1999 19 # 20 A12-2000 ^* 28 A12_06ANTXVIII_3 11th Dec, 2000 - 11th Jan 2001 21 A12-2002 # ^* 11 A12_ANTXX_2 26th Nov, 2002 - 15th Jan, 2003 22 I06S-A 30 I06SA_35MFCIVA_1 23rd Jan - 9th Mar, 1993 23 5 2200 (ACC) I06S 30oE I06S-B 28 I06SB_35MF103_1 20th Feb - 22nd Mar, 1996 24 18 30300 (EWG) 25 I06S-2008 27 I06S_33RR20080204 4th Feb - 17th Mar, 2008 26 I08S 27 I08S_316N145_5 1st Dec, 1994 - 19th Jan, 1995 27 I08S 90oE 6 7800 I08S-2007 33 I8S_33RR20070204 4th Feb - 18th Mar, 2007 28 29 I09S 115oE 7 9600 I09S 35 I09S_316N145_5 1st Dec, 1994 - 19th Jan, 1995 30 SR03-A 20 I09S_09AR20041223 25th Sept - 27th Oct, 1991 31 SR03-B 30 SR03_09AR101_1 11th Mar - Apr 03, 1993 32 33 SR03-C 16 PR12_09AR309_1 1st Jan - 1st Mar, 1994 34 SR03 141oE 8 11500 S03 12 PR12_09AR407_1 13th Dec, 1994 - 2nd Feb, 1995 35 SR03-D 28 SR03_09AR9404_1 17th Jul - 2nd Sept, 1995 36 SR03-G 5 SR03_09AR9501_1 22nd Aug -22nd Sept, 1996 37 38 SR03-2001 22 SR03_09AR9601_1 29th Oct - 22nd Nov, 2001 39 P14S 172oE 9 13500 P14S 15 SR03_09AR200011029 5th Jan - 10th Mar, 1996 40 41 P15S 170oW 10 14500 P15S 23 P14S_31DSCG96_1 5th Jan - 10th Mar, 1996 42 P16A 23 P15S_31DSCG96_1 6th Oct - 25th Nov, 1992 43 P16S 150oW 11 15800 44 P16S 23 P16A_316N138_9 9th Jan - 19th Feb, 2005 45 P18 103oW 12 18800 P18-2008 43 P16S_33RR200501 15th Dec , 2007 - 23rd Feb, 2008 46 SR01-B 53 P17E_316N138_10 11th Nov - 17th Dec, 1992 47 P19S 88oW 13 19900 48 P19S 75 P18S_31DSCG94_2 4th Dec, 1992 - 22nd Jan 1993 49 A21 68oW 14 21200 A21 29 P18_33RO20080121 23rd Jan - 8th Mar, 1990 50 51 SR01-E 65 SR01_74DI198_1 20th Nov - 18th Dec, 1993 52 SR01-F 18 P19S_316N138_10 13th Nov - 12th Dec, 1994 SR01 57oW 15 22100 53 SR01-K 66 A21_06MT11_5 15th Nov - 20th Nov 1996 54 SR01-M 43 SR01_74JC27_1 27th Dec, 1997 - 7th Jan, 1998 55 19 30900 (SWG) 56 20E 20oE A12-20E 32 A12_ANTXX_2 26th Nov, 2002 - 15th Jan, 2003 24 38500 (NWG) 57 58 59 60 61 62 63 64 65 Table 2 1 2 Section no. Name NADW NADW AASW AASW AABW AABW 3 (refers to % change % change % change 4 5 table 1) 6 1 A23-SAZ 100.0% 0.0% 0.0% 7 2 8 A23-PFZ 89.8% -10.2% 6.1% 6.1% 4.1% 4.1% 9 3 SR02-SAZ 86.7% 2.1% 11.2% 10 4 SR02-PFZ 79.3% -7.4% 7.2% 5.1% 13.5% 2.3% 11 12 5 I06S 68.6% -10.7% 7.6% 0.4% 23.9% 10.3% 13 6 I08S 69.9% 1.3% 8.5% 0.9% 21.6% -2.2% 14 15 7 I09S 69.9% 0.0% 8.8% 0.4% 21.3% -0.3% 16 8 SR03 70.1% 0.3% 8.9% 0.0% 21.0% -0.3% 17 9 P14S 68.8% -1.3% 9.5% 0.6% 21.7% 0.7% 18 19 10 P15S 68.2% -0.6% 9.5% 0.0% 22.3% 0.6% 20 11 P16S 66.7% -1.5% 9.1% -0.3% 24.1% 1.8% 21 12 22 P18 65.2% -1.5% 9.4% 0.3% 25.4% 1.3% 23 13 P19S 64.4% -0.8% 9.5% 0.1% 26.1% 0.7% 24 14 A21 62.9% -1.5% 9.0% -0.4% 28.1% 2.0% 25 26 15 SR01 59.9% -2.9% 8.8% -0.2% 31.3% 3.2% 27 16 A23-AZ 60.3% 0.4% 9.1% 0.3% 30.6% -0.7% 28 29 17 SR02-AZ 62.8% 2.5% 9.2% 0.1% 28.0% -2.6% 30 18 I06S-EWG 47.5% -15.3% 7.5% -1.7% 44.9% 17.0% 31 19 32 20E-SWG 47.8% 0.3% 7.7% 0.1% 44.5% -0.4% 33 20 SR02-SWG 41.9% -5.9% 6.9% -0.8% 51.2% 6.6% 34 21 A23-SWG 39.2% -2.7% 6.6% -0.2% 54.1% 3.0% 35 36 22 A23-NWG 32.5% -6.7% 5.4% -1.2% 62.1% 7.9% 37 23 SR02-NWG 36.2% 3.7% 6.2% 0.8% 57.7% -4.4% 38 39 24 20E-NWG 41.3% 5.1% 7.1% 0.9% 51.7% -6.0% 40 41 42 Table 3 43 44 45 2 -1 2 -1 46 # κV (m s ) κI (m s ) 47 48 SR03 and Pacific sector 15 2.86 x 10-4 ± 1.06 x 10-4 8.97 x 102 ± 1.67 x 102 49 50 Eastern and southern Weddell 12 2.39 x 10-5 ± 2.83 x 10-5 2.47 x 102 ± 0.63 x 102 51 Gyre 52 53 -5 -5 2 2 54 All of above combined 27 2.86 x 10 ± 2.45 x 10 2.62 x 10 ± 0.46 x 10 55 56 57 58 59 60 61 62 63 64 65