, published 15 January 2005 , doi: 10.1098/rsta.2004.1484 363 2005 Phil. Trans. R. Soc. A A. L. New, K. Stansfield, D. Smythe-Wright, D. A. Smeed, A. J. Evans and S. G. Alderson Mascarene Plateau in the Indian Ocean Physical and biochemical aspects of the flow across the Email alerting service here right-hand corner of the article or click Receive free email alerts when new articles cite this article - sign up in the box at the top http://rsta.royalsocietypublishing.org/subscriptions go to: Phil. Trans. R. Soc. A To subscribe to on June 4, 2013 rsta.royalsocietypublishing.org Downloaded from
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, published 15 January 2005, doi: 10.1098/rsta.2004.1484363 2005 Phil. Trans. R. Soc. A A. L. New, K. Stansfield, D. Smythe-Wright, D. A. Smeed, A. J. Evans and S. G. Alderson Mascarene Plateau in the Indian OceanPhysical and biochemical aspects of the flow across the
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Phil. Trans. R. Soc. A (2005) 363, 151–168doi:10.1098/rsta.2004.1484
Published online 9 November 2004
Physical and biochemical aspects ofthe flow across the Mascarene Plateau
in the Indian Ocean
By A. L. New, K. Stansfield, D. Smythe-Wright,
D. A. Smeed, A. J. Evans and S. G. Alderson
Southampton Oceanography Centre, European Way,Southampton SO14 3ZH, UK ([email protected])
This paper presents results from a detailed hydrographic survey of the MascarenePlateau and surrounding area undertaken by the RRS Charles Darwin in June–July 2002. We examine how the westward-flowing South Equatorial Current (SEC)crosses the plateau, and how the structure of the flow determines the supply ofnutrients to the surface waters. We find that the flow of the SEC across the plateau ishighly dependent on the complex structure of the banks which make up the plateau,and that a large part of the flow is channelled between the Saya de Malha andNazareth Banks. Furthermore, the SEC forms a sharp boundary between subtropicalwater masses from further south, which are low in nutrients, and waters from furthernorth, which are relatively nutrient rich. Overall, the SEC delivers relatively highlevels of nutrients to the near-surface waters of the central and northern regions ofthe plateau, compared with the southern regions of the plateau. This is partly dueto uplifting of density surfaces through Ekman suction on the northern side of theSEC, and partly due to the higher levels of nutrients on those density surfaces onthe northern side of the SEC. This may drive increased production of phytoplanktonin these areas, which would in turn be expected to fuel increased abundances ofzooplankton and higher levels of the food chain.
Keywords: South Equatorial Current; Mascarene Plateau;Western Indian Ocean; nutrient supply
1. Introduction
The Mascarene Plateau is situated in the western portion of the South Indian Ocean,and can be considered to extend between the Seychelles in the north (4◦ S, 56◦ E)and Mauritius in the south (20◦ S, 57◦ E). It is comprised of a series of shallowbanks or ‘shoals’ separated by deeper ridges and channels (figure 1). The principalbanks are the Seychelles Plateau, the Saya de Malha Bank, the Nazareth Bank andthe Cargados–Carajos Bank. These are typically 20–100 m deep, coral topped, andsometimes break the surface to form small islands. They are generally surrounded
One contribution of 24 to a Discussion Meeting ‘Atmosphere–ocean–ecology dynamics in the WesternIndian Ocean’.
Figure 1. The seafloor topography (m) around the Mascarene Plateau, and the track of the RRSCharles Darwin, cruise 141 (1 June–11 July 2002). Grey shades delimit depths of 1000, 2000,3000 and 4000 m, and principal observation stations are shown numbered. (Stations 02 and 03were tests at the position of 04, and ‘M’ was the site of a mooring not discussed further here.)
by steeply descending slopes so that water depths rapidly increase to 3000–4000 mon either side of the plateau.
The physical oceanography of the interior of the South Indian Ocean is somewhatless well understood than other comparable ocean basins, such as the South Atlantic,due to a relative lack of data. For instance, although the International Indian OceanExpedition (IIOE, comprising cruises mostly during the late 1950s and early 1960s)attempted the first comprehensive description of the large-scale circulation patternsand thermohaline structure of the Indian Ocean (Wyrtki 1971), the larger part ofthe datasets collected were from the North Indian Ocean. Since then, apart froma small number of trans-ocean sections which have been undertaken in the SouthIndian Ocean, such as those at 18◦ S (Warren 1981) and 32◦ S (Toole & Warren1993), interest has been mainly focused on specific regions near the boundaries.
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These include, for instance, the seasonally reversing Somali Current system (Schott& McCreary 2001) and the southwestern Indian Ocean between South Africa andMadagascar (Grundlingh et al . 1991). In addition, the exchanges between the Indianand Atlantic Oceans around South Africa have recently been the subject of substan-tial international interest (Gordon 2003), since the transfer of Indian waters to theAtlantic may play a role in the heat transport and overturning of the latter.
In spite of the relative lack of data in the interior of the South Indian Ocean, ithas nonetheless been possible to develop an understanding of the larger-scale flowpatterns and water masses which are present, and which may, in particular, affect theMascarene Plateau. The flow patterns in the upper layers (extending to 1000 m or so)of the South Indian Ocean have been described by Stramma & Lutjeharms (1997) toconsist of an anticyclonic (anticlockwise) subtropical gyre between approximately 40◦
and 15◦ S. At 40◦ S, the flow is strongly eastwards in the South Indian Ocean Current(SIOC), which is associated with a subtropical front and which can be consideredto form the northern portion of the Antarctic Circumpolar Current. The strength ofthe SIOC decreases with distance to the east (from 60 Sv† near South Africa to 10 Svnear Australia) as waters are drawn northwards in the subtropical gyre recirculation.The northern boundary of the subtropical gyre is formed by the South EquatorialCurrent (SEC), which is a broad return flow westwards between 10◦ and 20◦ S (witha core somewhere near 15◦ S). The waters flowing into the SEC derive partly fromthe subtropical gyre to the south, partly from the Indonesian Seas further to the eastand partly from further to the north through the southward Java Current (Schott &McCreary 2001). On reaching Madagascar, the SEC splits and flows both southwardsand northwestwards around the eastern coast of Madagascar (Swallow et al . 1988).The southward branch then flows around the southern tip of Madagascar, westwardstowards the African coast, and finally retroflects in the Agulhas retroflection zone tocomplete the subtropical gyre. The northward component flows around the northerntip of Madagascar, westwards to the African coast, and then northwards in the EastAfrica Coastal Current (Schott & McCreary 2001). This itself may either link upwith the Somali Current system, or be returned eastwards as the South EquatorialCounter Current (SECC) just south of the Equator, depending on the time of year(Schott & McCreary 2001). In the North Indian Ocean, the currents are complexand reverse seasonally in the monsoonal wind systems.
Schott & McCreary (2001) also show that the SEC is primarily driven by the strongSE trade winds, which exist as a reasonably steady and strong wind band between10◦ and 30◦ S. They indicated that these winds would drive a transport of ca. 50 Svin the SEC. They also remarked that Ekman suction (caused by divergence of thesurface currents due to spatial wind stress variations) would cause uplifting or domingof the water masses on the northern side of the SEC, lifting up density surfacesbetween 5◦ and 12◦ S in a band between 80◦ E and the African coast. Since nutrientlevels generally increase with depth (Wyrtki 1971), this would also bring higherlevels of nutrients nearer to the surface, and, possibly, local increases in biologicalproductivity. While this effect has not hitherto been studied in detail, we remark thatLonghurst (2001) reported a major phytoplankton bloom in the Madagascar Basinto the south of the SEC (between 20◦ and 30◦ S), caused by the seasonal deepeningof the mixed layer (between January 1999 and April 1999) which would inject higher
† 1 Sv = 106 m3 s−1.
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nutrients into the surface layers. We also remark that another bloom is just evidenton the northern side of his figures (north of 15◦ S) at the latitudes of the SEC. This iswell developed by April, and could perhaps be at least partly related to the upliftingmechanism.
Turning now to the water masses which are typically found in the South IndianOcean (between 0◦ and 40◦ S), these consist principally of the following. The sea sur-face comprises (Duncan 1970; Wyrtki 1971; Grundlingh et al . 1991; Prasanna Kumar& Prasad 1999) the fresh Tropical Surface Water (TSW) in a broad band betweenapproximately 5◦ and 20◦ S, which is flanked on its northern side by the high salinityArabian Sea High Salinity Water (ASHSW), and on its southern side by the highsalinity Subtropical Surface Water (STSW). The TSW exists as a thin surface layerin the upper 50–100 m and flows westwards from the Indonesian Seas to the Africancoast, carried by the SEC. The ASHSW forms in the Arabian Sea and from there iscarried southeastwards towards the Equator by the prevailing currents. The STSWis thought to form east of 55◦ E, and between 15◦ and 32◦ S, and to circulate north-wards and then westwards in the subtropical gyre, but it is drawn down to forma subsurface salinity maximum (usually between 100 and 400 m) when it meets thefresher, overlying TSW in the SEC. Below these waters are varieties of Sub-AntarcticMode Waters (SAMWs) which are formed near 40–45◦ S as a result of deep wintermixing, and subducted northwards (McCartney 1982). They are associated withan oxygen maximum usually centred between 300 and 500 m, and circulate in thesubtropical gyre northwards at least as far as 20◦ S. Deeper down, Antarctic Inter-mediate Water (AAIW) is revealed by a salinity minimum between 600 and 1400 mwhich may spread northwards to about 15◦ S (Wyrtki 1971). This water mass origi-nates in the southeastern Pacific (McCartney 1982) and in the South Atlantic (Piola& Gordon 1989), and appears to enter the South Indian Ocean in its southeasternregion (Fine 1993). However, another water mass, the high salinity Red Sea Water(RSW), also occupies this approximate depth range. Beal et al . (2000) show that theprimary spreading route for the RSW is, from its origins in the Red Sea, southwardsalong the length of the western boundary of the Indian Ocean, eventually passingthrough the Mozambique Channel. The RSW also spreads more slowly towards thesoutheast, and its signature is seen near the Mascarene Plateau, where it may meetwith the AAIW. Below these waters is a further range of identifiable deeper waters,derived either from the North Atlantic (the Circumpolar Deep Water) or from theAntarctic. These generally flow northwards into the deepest basins of the IndianOcean (Mantyla & Reid 1995), but will not be considered further in this paper.
From the above, it is apparent that the SEC flows directly towards the MascarenePlateau, and must somehow pass across it. A laudable attempt, using direct obser-vations, to investigate this process, and the water masses involved, was made byRagoonaden et al . (1987) for the portion of the Mascarene Plateau south of 10◦ S,using data collected during the IIOE survey. However, while the authors were ableto infer geostrophic flow patterns, the details of the passage of the flow across theplateau could not be clarified, perhaps due to the lack of measurements near theplateau itself (the closest with reasonable coverage of the north–south extent beingsome 200–300 km away), and perhaps due to the usage of data widely spaced in time(between 1960 and 1965). Indeed, the main flow of the SEC appeared to impingedirectly from the east on the centre of the Nazareth Bank (at 15◦ S), and to re-
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emerge on its western side, without any indication being given as to how the flowwould actually cross the Bank.
There have also been a number of modelling studies which shed light on the passageof the SEC across the plateau. Woodberry et al . (1989), for instance, used a 1.5 layerreduced-gravity model with a high horizontal resolution (0.2◦). This showed the SECsplitting almost evenly into two cores, one of which flowed between the Saya de Malhaand Nazareth Banks (11–13◦ S), and one of which flowed between the Cargados–Carajos Bank and Mauritius (17–19◦ S), with no seasonal variation. Lee & Marotzke(1998), on the other hand, implemented an ocean general circulation model (OGCM)with more complete physics, 24 levels in the vertical, but with a coarser horizontalresolution of 1.5◦. This model, which because of its resolution could not represent anyof the islands in the Mascarene Plateau, revealed the SEC as a broad westward flowbetween 5◦ and 20◦ S. The surface flow passed across the plateau in a broad, smoothsweep without any perturbation or constriction from the underlying topography.However, Garternicht & Schott (1997) used a similar OGCM (with 20 levels in thevertical) but with a higher horizontal resolution (effectively 0.4◦ for the region inquestion) which gave better definition of the islands and topography of the plateau.The SEC in this model had a greater degree of horizontal structure, and passed overthe plateau, at all times of year, as a relatively narrow current (2–3◦ in north–southextent) almost entirely between the Saya de Malha and Nazareth Banks. In summary,the structure of the SEC as it passes across the Mascarene Plateau is rather differentin these various models, and such differences appear to be at least partly due tohow well the models resolve the complex bathymetry of the region. This is governedboth by the horizontal resolution of the model and by the realism of the topographicdataset used to construct its bathymetry. (Such realism has until now been ratherpoor in certain key areas, as described below, though it is beyond the scope of thepresent paper to investigate the effect of an improved topographical description onmodel simulations.)
Overall, then, little is known about how the SEC actually crosses the plateau,or indeed how the flow governs the biochemistry of the region. The present paperinvestigates these aspects using data from a recent cruise (the RRS Charles Darwin,cruise 141) which completed a near-synoptic survey (within 40 days) of the region,including observations very close to the plateau, and using modern instrumentationsuch as the acoustic Doppler current profiler.
2. Circulation patterns and water masses
The track of the RRS Charles Darwin, cruise 141, is shown in relation to thebathymetry in figure 1, and the principal measurement stations are numbered.The cruise (1 June–11 July 2002), comprised three quasi-meridional sections, oneca. 300 km to the east of the plateau (stations 12–22), one placed centrally (in termsof the overall survey) at the eastern edge of the Plateau (stations 25–49, together with1–7), and a further section ca. 300 km west of the plateau (stations 52–63). Theselines were joined by two further zonal sections at 8◦ and 20◦ S. Of relevance to thepresent investigation, at each station, CTD/LADCP/water sampler measurementswere taken covering the full depth of the water column to provide observations oftemperature, salinity, currents and water chemistry. In addition, while the ship wasunderway, a further acoustic Doppler current profiler (ADCP) continuously recorded
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Figure 2. Perspective view from the southeast of EM12 swath bathymetry, showinga deep channel between the Saya de Malha and Nazareth Banks.
currents to depths of 200–300 m below the ship, and an EM12 ‘swath bathymetry’system mapped out the seafloor topography with a swath width of 3.5 times thewater depth.
An example of the swath bathymetry is shown in figure 2. This reveals a previouslyuncharted channel in the ridge system between the Saya de Malha and NazarethBanks, near 12◦30′ S, which is important for the passage of the waters between thesebanks (as explained below). The General Bathymetric Chart of the Oceans (GEBCO)chart 5.09 shows this area to comprise the northeastern corner of the SomervilleBank, 100–200 m deep, which should appear in the southwestern corner of figure 2,and a ridge of depths between 200 and 500 m to the northeast of this (covering theremainder of the figure). On the other hand, the Smith & Sandwell (1997) bathymetry(derived from satellite observations and ship soundings) does show a deep channelhere, in the correct location, but it is 1650 m deep. The EM12 (figure 2) reveals thetrue nature of the channel to be 1100 m deep and 8–10 km wide. In addition, for50 km (being the distance surveyed) or more on the southwestern side of the channel(crossing the position of the Somerville Bank on the GEBCO chart), there appearsto be a ridge (not shown in figure 2, but extending to the southsouthwest) alongwhich the shallowest depths (on the ridge crest) are 400–450 m.
Figure 3 reveals the upper layer circulation pattern (at a depth of 51 m) as recordedby the shipboard ADCP. On the eastern section, near 64◦ E, there is a strong andpronounced westward flow between 9◦ and 16◦ S, in which the currents are typically25 cm s−1. This broad current, extending for ca. 750 km in the north–south direction,is the South Equatorial Current (SEC) as it approaches the Mascarene Plateau. Onthe central section, just eastwards of the shoals, it is apparent that a large portion ofthe SEC (probably about half of the 50 Sv total: the subject of a separate examina-tion) is being diverted to flow between the Saya de Malha and Nazareth Banks, andis passing through the region of the channel at 12◦30′ S and its southsouthwestward
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Figure 3. Currents at 51 m from the ship’s (150 kHz) acoustic Doppler current profiler. Thevelocity vectors represent averages over 8 m in the vertical and 30 km in horizontal, and the50 cm s−1 scale arrow is shown.
ridge. Here, the flow speeds are typically 50 cm s−1. Other portions of the SEC arediverted to flow around the northern tip of the Saya de Malha Bank (although thismay be intermittent), and, further south, through the gap between the Cargados–Carajos Bank and Mauritius. In fact, it seems as though the easternmost tips of theSaya de Malha and Nazareth Banks (at 10◦ and 15◦ S, respectively) act as break-points to the current, deflecting the SEC both northwestwards and southwestwardsto flow largely around the bathymetric contours of the banks. On the western section(near 57◦ E), the SEC consists of two cores. The larger part of the flow has reformed,after passage through the gap between the Saya de Malha and Nazareth Banks, as abroad current between approximately 10◦ and 15◦ S, with typical speeds of 30 cm s−1.This is similar to the SEC structure on the upstream (eastern) side of the plateau,though slightly less extensive in the north–south direction, and with more spatialvariability. In addition to this ‘northern core’, there is also an appreciable westwardflow between 17◦ and 20◦ S. This latter band appears to form a ‘southern core’ to the
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Figure 4. (a) Salinity (psu) and (b) oxygen (% saturation) patterns from the CTD (stations 22to 12), along the eastern section of the survey, in the top 1000 m of the water column.
SEC, which is connected to the flow through the gap between the Cargados–CarajosBank and Mauritius. In addition, there is also an indication of an eddy or meandernear 16◦ S, 57◦ E, evidenced by the retroflection of the current back towards the eastjust north of this position, and then the turning of the current towards the south andwest, further to the south (between 16◦ and 18◦ S). The presence of this feature issupported by satellite passive microwave sea-surface-temperature imagery (e.g. fromthe Tropical Rainfall Monitoring Sensor on 29 June and 4 July 2002), which showsan eddy in the process of pinching off from the SEC. Overall, the effect of the Mas-carene Plateau is to split the single large core of the SEC on the upstream (eastern)side into two cores on the downstream (western) side.
We next examine the various water masses which are present in the upper layers(the top 1000 m) of the flow system, since these are the layers important for thebiochemistry. Figure 4 shows the salinity and oxygen on the eastern section, theother sections being similar for this purpose. Firstly, there is a thin, fresh layer
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Figure 5. Data from the central section of the survey showing (a) nitrate (µmol l−1) againstdepth for the full depth of the water column (contour interval 2 µmol l−1) and (b) potentialdensity (kg m−3) in the upper 1000 m (contour interval 0.2 kg m−3). The densities are referencedto the surface (values of σ0) and, in the usual manner, 1000 kg m−3 has been subtracted forconvenience.
(figure 4a) in the upper 50 m of the water column with a sharp salinity gradient atits base. The freshest water (salinities lower than 34.2) lies between 12◦ and 14◦ S,the central portion of the SEC, although there are low salinity extensions between9◦ and 18◦ S, covering the remainder of the SEC. This is the TSW referred to above,which is being transported by the SEC from the Malaysian/Indonesian region (see,for example, Wyrtki 1971). There are then two further striking water masses (infigure 4a) which exist on the south side and up to the centre of the SEC at about13–14◦ S. These are, firstly, a pronounced salinity maximum (salinities in excessof 35.4), between 150 and 350 m depth, and a salinity minimum (salinities below
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34.7), between 600 and 1000 m. Between these water masses, a further water massexists, evidenced by the high oxygen content (greater than 70% saturation) between400 and 600 m (figure 4b). These water masses are identified as STSW (the salinitymaximum), a variety of SAMW (the oxygen maximum) and AAIW (the salinityminimum). They are the typical water masses which exist in the subtropical gyre ofthe South Indian Ocean, as discussed earlier. By comparison with surface properties(again, the subject of a separate examination), it is likely that the STSW and SAMWform in the southeastern corner of the gyre (between 30◦ and 45◦ S) and are drawndown into the ocean interior through subduction from the sea surface. The AAIWis formed in the Pacific and South Atlantic, but its exact route to the MascarenePlateau is unclear at this stage. In addition, we identify the high-salinity lens (withsalinity above 35.2) in the upper 100 m between 8◦ and 11◦ S with the ASHSW, andthe elevated salinities (above 34.8) between 8◦ and 10◦ S and 500–1000 m with RSW.In summary, the water masses on the southern side of the SEC (up to 13–14◦ S) areof a more southerly origin, while the waters on the northern side of the SEC areof a more northerly origin. The SEC therefore acts as a sharp boundary betweensubtropical waters from further south and waters from further north.
3. Nutrients and biochemistry
We now examine the distribution of nitrate within the water column. This is usually agood indicator of the likely growth of phytoplankton in the surface layers of the ocean,and so is studied here. We also note that the other nutrients measured (phosphateand silicate) show qualitatively similar trends.
Figure 5a reveals the pattern of nitrate (against depth) on the central section of thesurvey between the Seychelles and Mauritius. Nitrate is generally higher in the deeperwater, below 1000 m, than above, with a maximum (exceeding 36 µmol l−1) between1000 and 2000 m and north of 15◦ S (indicating a northern source for the high-nitratewaters). In the upper 1000 m, nitrate levels are much lower on the southern side ofthe SEC (south of 13◦ S) than on its northern side. This is particularly pronouncedin the top 500 m, where levels are only 2–14 µmol l−1 on the southern side, but14–28 µmol l−1 on the northern side. This reinforces our conclusions that the SECacts as a sharp boundary between water masses, those of subtropical origin beingnutrient poor, and those from further north being nutrient rich. Indeed, examinationof the Wyrtki (1971) atlas shows that the sources of such high levels of nutrients arealmost certainly the Arabian Sea or the Bay of Bengal. (How such water masses crossthe Equator is not clear at present. However, this could occur in principle through,firstly, eastward transport in the Southwest Monsoon Current south of India (Schott& McCreary 2001), southeastward transport in the boundary current system nearSumatra and Java, and finally westward transport in the SEC. In addition, cross-equatorial exchange by eddies could play a role.)
The nutricline (e.g. the position of the 20 µmol l−1 contour) in figure 5a risesfrom 700–800 m deep in the south (at 20◦ S) to close to the surface between 6◦
and 12◦ S. The nutricline is closest to the surface (in the upper 100 m) between 7◦
and 9◦ S, and shows a slight descent again further north, between 7◦ and 5◦ S. Asalready discussed, Schott & McCreary (2001) have remarked that Ekman suction,resulting from divergence of the wind-induced Ekman transports near the surface,results in uplifting or doming of the underlying water masses on the northern side of
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Figure 6. Nitrate (µmol l−1) against density (σ0) for the (a) eastern, (b) central and (c) westernsections. Note that the central section extends further north (to 5◦ S) than the other sections(which extend to 8◦ S).
the SEC, between approximately 5◦ and 10◦ S (and 40–80◦ E). Since water massesare considered to largely conserve their density (once removed from diabatic surfaceeffects), such uplifting can be illustrated by examining the density structure of thewater column. Figure 5b therefore shows the density structure along the centralsection, for comparison with figure 5a. It is clear that density surfaces do indeed risetowards the north across the SEC, and are closest to the surface in the northern region
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Figure 7. (a) Depth (m) of density surface 26.0 kg m−3 and (b) nitrate (µmol l−1)on the 26.0 kg m−3 density surface. Arrows indicate inferred flow directions.
(between 6◦ and 12◦ S), paralleling the situation for the nitrate surfaces. However,the pycnocline (e.g. the position of the 26.0 kg m−3 density surface) rises from onlyca. 300 m at 20◦ S to 100 m in the northern regions. This clearly does not match themore pronounced rise of the nitrate surfaces. Hence, assuming that nitrate levels areconstant on density surfaces, the uplifting of those density surfaces due to Ekmansuction is insufficient to explain the observed increases in nitrate in the north.
It is apparent therefore that the nitrate levels must vary on density surfaces. Weinvestigate this in figure 6 by showing nitrate plotted against density for each of thesections. For the eastern section (figure 6a), there is a small increase between densities27.0 and 26.5 kg m−3 (affecting depths of approximately 400–700 m; figure 5b) at 15–16◦ S, the southern edge of the SEC. The major feature, however, is the pronouncedrise in nitrates on the northern side of the SEC between 8◦ and 11◦ S, affecting densitysurfaces as low as 25.0 kg m−3 (reaching well into the pycnocline at the base of thesurface mixed layer at 50–100 m in depth; figure 5b). In addition, nitrate levels arehigher on all surfaces denser than 25.0 kg m−3 everywhere north of 13◦ S, the centre ofthe SEC, than south of this position. These are unambiguous indications of a differentsource region for the waters on the northern side of the SEC, with high nitrate levels,probably the Arabian Sea or Bay of Bengal, as already discussed. The central section(figure 6b) indicates that the high-nitrate core (at 8–11◦ S on the eastern section)has now split into two cores, near 11–12◦ S and 8◦ S, which are being diverted to flowsouthwestwards and northwestwards, respectively, around the Saya de Malha Bank
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(see also figure 7). Furthermore, the change between low and high nitrate levels ondensity surfaces heavier than 25.0 kg m−3 now occurs very sharply at 12◦30′ S asthe flow is channelled through the gap between the Saya de Malha and NazarethBanks. The SEC is again seen to act as an abrupt divide between waters of low andhigh nitrate content. Finally, figure 6c shows the western section. Nitrate levels arestill generally high on the northern side of the SEC (north of 13◦ S). In addition,the region of high-nitrate waters (on densities 25.0–26.5 kg m−3) at 16◦ S reflects thepresence of the eddy previously referred to, which is being detached from the SECfront and is bringing nitrate-rich waters from the northern side of the SEC furthersouth.
There are therefore two effects which bring higher nutrients to the surface waterson the northern side of the SEC, bathing the central and northern regions of theMascarene Plateau. Firstly, there is uplifting of the density surfaces due to the effectof the wind, as noted by Schott & McCreary (2001). Secondly, nitrate levels are muchhigher on density surfaces on the northern side of the SEC due to these waters beingderived from a different (northerly) origin. These two effects are shown in figure 7.Figure 7a shows the depth of the 26.0 kg m−3 density surface. This rises from 275–300 m deep in the southern regions to 125–100 m on the northern side of the SEC.The density surface rises most rapidly through the central position of the SEC near12–13◦ S. Indeed, the lines of constant depth are closely related to the flow patternsseen in figure 3, and even reflect the presence of the eddy at 16◦ S. Figure 7b thenshows the variation of nitrate on this density surface. The highest values (initiallyabove 26 µmol l−1) sweep in on the northern side of the SEC, and are split andspread around the Saya de Malha Bank. The SEC clearly delimits the high-nitrateregion in the north from the lower nitrate levels in the south, and again, lines ofconstant nitrate levels reflect the flow patterns seen earlier. There is a sharp changein the nitrate levels through the central position of the SEC, particularly as it passesbetween the Saya de Malha and Nazareth Banks, and the clear presence of the eddyat 16◦ S (57◦ E). Overall, we also note the general decrease (from 26 to 20 µmol l−1)in the highest nitrate levels as the flow progresses from east to west, possibly asa result of biological consumption. We also note that the Mascarene Plateau actsto spread the region of high nitrates (e.g. the region in excess of 20 µmol l−1) from8–12◦ S on the eastern side, to 6–16◦ S on the western side. This results from bothtopographic steering of the currents (forcing them northwards of Saya de Malha andup towards the Seychelles) and the shedding of eddies after the SEC has crossed theplateau.
The high nutrient levels being delivered to the surface waters on the northern sideof the SEC would be expected to result in increased biological production in theseareas, and this should be made evident by higher levels of phytoplankton and zoo-plankton. That this may be so for the phytoplankton is shown in figure 8, which is acomposite SeaWiFS image of data between 13 and 30 June 2002 (covering the middlepart of the cruise) revealing surface chlorophyll concentrations. (Surface chlorophyllconcentrations depend on the levels of phytoplankton, though the relationship isnot necessarily straightforward (Zubkov & Quartly 2003).) On the eastern side ofthe plateau, we observe a band of relatively high (compared with waters furthernorth and south) concentrations (0.2–0.5 mg m−3) on the northern side of the SEC,between approximately 6◦ and 12◦ S. This has a sharp southern boundary associ-ated with the centre of the SEC, and which passes over the plateau at 12–13◦ S.
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Figure 8. SeaWiFS surface chlorophyll concentration (mg m−3) composite between 13 and30 June 2002. Black areas represent land or persistent cloud. (Reproduced courtesy of theRemote Sensing Data Analysis Service (RSDAS), Plymouth Marine Laboratory.)
We also note the apparent spreading of the region of high concentrations as the SECmeets the plateau, both moving northwards towards the Seychelles, and being spreadsouthwards (to 14–16◦ S) on the western side of the plateau. All these findings arein accord with our expectations from the nitrate analysis above. In addition, there isclearly enhanced local production above the banks themselves (see the green areasin the vicinity of the Seychelles, and above the Saya de Malha and Nazareth Banks),presumably by ecosystems trapped (since the currents tend to divert around, ratherthan flow over, the banks) in the shallow waters, which we were not able to investi-gate. Furthermore, these regions of enhanced local production give rise to streamersof high concentrations which can be stretched out on the western side of the banksby the flow (the example from the Seychelles being particularly marked). Finally, weremark that higher levels of zooplankton were indeed seen on the northern side ofthe SEC during the present investigation, and these are examined by Gallienne &Smythe-Wright (2005).
4. Summary and conclusions
This investigation has examined how the South Equatorial Current (SEC) crossesthe Mascarene Plateau, and how the structure of the flow determines the supply ofnutrients to the surface waters surrounding the plateau.
It is apparent, firstly, that the flow of the SEC across the plateau is highly depen-dent on the complex structure of the banks and shoals which make up the plateau.
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On the eastern (upstream) side of the plateau, the SEC appears as a broad flowbetween 9◦ and 16◦ S. When this meets the plateau, the flow is deflected both north-westwards and southwestwards by the Saya de Malha and Nazareth Banks. Theresult is that a large part of the SEC is forced through the gap between these twoBanks at 12–13◦ S. Here there is a deep, narrow channel (1100 m deep and 8–10 kmwide), and a shallower (400–450 m) ridge to the southsouthwest, which was mappedout by the bathymetric survey. The remainder of the SEC flows partly around thenorthern tip of the Saya de Malha Bank (although this appears to be intermittent),and partly through the gap between the Cargados–Carajos Bank and Mauritius. Onthe western (downstream) side of the plateau, the flow consists of two cores, between10–15◦ S and 17–20◦ S.
We have also seen that the SEC acts as a sharp barrier or front, dividing watermasses of southerly or subtropical origin on its southern side, from waters which arelikely to come from more northerly sources, on its northern side. Furthermore, theSEC delivers high levels of nutrients to the surface waters of the central and northernregions of the Mascarene Plateau. This is partly because the density surfaces arelifted upwards on the northern side of the SEC by the effect of the wind (as noted bySchott & McCreary (2001)), and partly because the water masses on the northernside of the front have much higher levels of nutrients (on density surfaces) than thoseon the southern side. Indeed, this latter effect clearly indicates that the waters onthe northern side of the front have a different source from the waters on its southernside, and almost certainly derive from the Arabian Sea or Bay of Bengal (which haveappropriately high levels of nutrients on the same density surfaces (Wyrtki 1971)).Finally, we remark that the higher levels of near-surface nutrients in the centraland northern regions of the plateau seem to be related to higher surface levels ofphytoplankton, which would in turn be expected to fuel increased abundances ofzooplankton (and higher levels of the food chain) in these areas.
We gratefully acknowledge the Master and crew of the RRS Charles Darwin, without whosetireless cooperation and assistance the collection of the datasets presented here would have beenimpossible. The work was also partly supported by the Royal Geographical Society (with IBG)–Royal Society Shoals of Capricorn Programme, ‘Western Indian Ocean, 1998–2001’, to whichwe are indebted. Consequently, this paper also forms Shoals Contribution no. P047. Finally,we thank RSDAS for providing remote sensing support during the cruise, and in particular theSeaWiFS data, which are also courtesy of the NASA SeaWiFS project and Orbital SciencesCorporation.
References
Beal, L. M., Ffield, A. & Gordon, A. L. 2000 Spreading of Red Sea overflow waters in the IndianOcean. J. Geophys. Res. 105, 8549–8564.
Duncan, C. P. 1970 The Agulhas Current. PhD thesis, University of Hawaii.Fine, R. A. 1993 Circulation of Antarctic Intermediate Water in the South Indian Ocean. Deep-
Sea Res. I 40, 2021–2042.Gallienne, C. P. & Smythe-Wright, D. 2005 Epipelagic mesozooplankton dynamics around the
Mascarene Plateau and Basin, Southwestern Indian Ocean. Phil. Trans. R. Soc. A 363, 191–202.
Garternicht, U. & Schott, F. 1997 Heat fluxes of the Indian Ocean from a global eddy-resolvingmodel. J. Geophys. Res. 102, 21 147–21 159.
Gordon, A. L. 2003 Oceanography: the brawniest retroflection. Nature 421, 904–905.
Phil. Trans. R. Soc. A (2005)
on June 4, 2013rsta.royalsocietypublishing.orgDownloaded from
Grundlingh, M. L., Carter, R. A. & Stanton, R. C. 1991 Circulation and water properties of thesouthwest Indian Ocean, spring 1987. Prog. Oceanogr. 28, 305–342.
Lee, T. & Marotzke, J. 1998 Seasonal cycles of meridional overturning and heat transport of theIndian Ocean. J. Phys. Oceanogr. 28, 923–943.
Longhurst, A. 2001 A major seasonal phytoplankton bloom in the Madagascar Basin. Deep-SeaRes. I 48, 2413–2422.
McCartney, M. S. 1982 The subtropical recirculation of mode waters. J. Mar. Res. 40 (suppl.),427–464.
Mantyla, A. W. & Reid, J. L. 1995 On the origins of deep and bottom waters of the IndianOcean. J. Geophys. Res. 100, 2417–2439.
Piola, A. R. & Gordon, A. L. 1989 Intermediate water in the south western South Atlantic.Deep-Sea Res. A36, 1–16.
Prasanna Kumar, S. & Prasad, T. G. 1999 Formation and spreading of Arabian Sea high-salinitywater mass. J. Geophys. Res. 104, 1455–1464.
Ragoonaden, S., Ramesh Babu, V. & Sastry, J. S. 1987 Physico-chemical characteristics &circulation of waters in the Mauritius–Seychelles ridge zone, Southwest Indian Ocean. IndianJ. Mar. Sci. 16, 184–191.
Schott, F. A. & McCreary Jr, J. P. 2001 The monsoon circulation of the Indian Ocean. Prog.Oceanogr. 51, 1–123.
Smith, W. H. F. & Sandwell, D. T. 1997 Global sea floor topography from satellite altimetryand ship depth soundings. Science 277, 1956–1962.
Stramma, L. & Lutjeharms, J. R. E. 1997 The flow field of the subtropical gyre of the SouthIndian Ocean. J. Geophys. Res. 102, 5513–5530.
Swallow, J., Fieux, M. & Schott, F. 1988 The boundary currents east and north of Madagascar.1. Geostrophic currents and transports. J. Geophys. Res. 93, 4951–4962.
Toole, J. M. & Warren, B. A. 1993 A hydrographic section across the subtropical South IndianOcean. Deep-Sea Res. I 40, 1973–2019.
Warren, B. A. 1981 Transindian hydrographic section at lat. 18◦ S: property distributions andcirculation in the South Indian Ocean. Deep-Sea Res. A28, 759–788.
Woodberry, K. E., Luther, M. E. & O’Brien, J. J. 1989 The wind-driven seasonal circulation inthe southern tropical Indian Ocean. J. Geophys. Res. 94, 17 985–18 002.
Wyrtki, K. 1971 Oceanographic atlas of the International Indian Ocean Expedition. Washington,DC: National Science Foundation.
Zubkov, M. V. & Quartly, G. D. 2003 Ultraplankton distribution in surface waters of the Mozam-bique Channel: flow cytometry and satellite imagery. Aquat. Microbiol. Ecol. 33, 155–161.
Discussion
F. A. Schott (IFM-GEOMAR Leibniz Institut fur Meereswissenschaften, Univer-sitat Kiel, Germany). Is there evidence for water mass transformation from the east-ern to the western section through topographic interaction, particularly tidal mixingin the narrow passages, similar to what is observed in the Indonesian passages?
A. L. New. There is evidence of mixing in the upper 200 m in the TSW and under-lying more saline waters, as these pass between the Saya de Malha and NazarethBanks, since the interface between these waters deepens from east to west, and theTSW becomes more saline. However, such changes could result from mixing inducedby the topography as suggested, or from mixing induced by increasingly strong sur-face fluxes (of buoyancy and wind stress) as winter conditions (in the southwestmonsoon) become better developed (i.e. the sections were run east to west duringJune and July at the onset of the monsoon).
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F. A. Schott. How do you explain the physics of subsurface water from the Bay ofBengal crossing the Equator and supplying the high nutrients in the northern partof the sections?
A. L. New. Exactly how these water masses cross the Equator is not known,although this could be possible in the frictional boundary current systems off Suma-tra and Java.
I. N. McCave (Department of Earth Sciences, University of Cambridge, UK ). Inote that you did not specifically mention Indonesian Throughflow Water (ITF) inyour cross-sections (other than the surface rain-affected fresh plume), but do youthink it is recognizable there? Surely it should be present?
A. L. New. It is indeed possible to recognize a weak presence of the ITF as the moresaline waters (salinity between 35.0 and 35.2) which underlie the fresh rain-affectedplume (the TSW) between (for example) about 11◦ and 14◦ S on the eastern section,at depths between 100 and 250 m. (Refer to Song et al . (2004) for more discussionof this water mass.)
A. S. Laughton (Southampton Oceanography Centre, University of Southampton,UK ). Is there any evidence of erosion by enhanced current flow in the newly discov-ered 12.5◦ S channel? What are the currents there?
A. L. New. Preliminary analysis is indicating that the currents at the bottom ofthis channel could be between 5 and 20 cm s−1. While this is often large enough tolead to erosion of bottom sediments, we cannot say that there is definite evidence oferosion without either a direct sample of the bottom material, or, possibly, a moredetailed analysis of the swath bathymetry. However, the sides and bottom of thechannel do appear to be quite smooth and regular, indicating possible current scour.
N. C. Flemming (Southampton Oceanography Centre, University of Southampton,UK ). The ADCP measurements and current calculations show the transport of theSEC on both sides of the Mascarene Plateau and the flow through the channels. Dothe flows through the channels and round the north end of the Plateau account forthe whole SEC, or is there a significant flow over the tops of the Banks?
A. L. New. This cannot be answered precisely without a detailed survey over thetops of the banks. However, the banks are typically very shallow (ca. 20–50 m). Fur-thermore, conservation of potential vorticity would tend to drive the flows aroundthe banks (following contours of constant depth to first order), rather than over theirtops. Overall, it is suspected that only a small fraction of the SEC transport wouldtherefore flow over the tops of the banks. For example, a mean flow of 10 cm s−1
in a water depth of 35 m, and for a bank which was 300 km in extent (such as theSaya de Malha or Nazareth Banks) would amount to ca. 1 Sv (or 106 m3 s−1), a smallfraction of the total SEC transport of some 50 Sv.
I. Lloyd (Science Policy Support Group, Petersfield, UK ). Will the science of oceandynamics, etc., be advanced further by (i) new measurements, (ii) more data or(iii) more processing capacity?
A. L. New. Our feeling is that ocean science could be most rapidly advanced bya greater quantity of direct observations at sea. This is particularly true in the
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South Indian Ocean (and other oceans) which are notably data sparse. Such obser-vations could be achieved through new technologies such as Argo floats and gliders,or through the use of a greater quantity of research ships. The collection and analysisof such data are not typically limited by the processing capacity on board present-dayresearch ships.
Additional reference
Song, Q., Gordon, A. L. & Visbeck, M. 2004 Spreading of the Indonesian Throughflow in theIndian Ocean. J. Phys. Oceanogr. 34, 772–792.
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