Characterization of Agulhas Bank upwelling variability from satellite-derived sea surface temperature and ocean colour products

Introduction The broad continental shelf extension south of Africa is known as the Agulhas Bank. It is a complex oceanic region influenced by coastal upwelling processes typical of the Benguela Upwelling System, as well as shelf-edge dynamic upwelling resulting from the variable presence of the Agulhas Current. Hutchings et al (2002) describe the Agulhas Bank as roughly triangular-shaped ranging from Cape Point in the west to East London 800km further along at its eastward extreme (see topography <500m in Figure 1). At its apex it has a width of 250 km, spanning a total area of ~116,000 km2 with a mean depth of just over 100m. Figure 1: Bathymetry of the Agulhas Bank, showing the 100m, 200m and 500m contours. Study regions are

overlayed in red. Seasonal variations are evident in the water column with summer conditions predominantly indicative of a mixture of subtropical surface water, separated by strong thermoclines from cool waters. Bottom waters exhibit the characteristics of central Indian Ocean water in the east and central Atlantic Ocean water in the west (Chapman and Largier, 1989). On the eastern Agulhas Bank, dynamic processes linked to the divergence of the Agulhas Current from the coast as it follows the shelf break offshore, draws an onshore Ekman layer of cold, predominantly Indian Ocean central water from below the warm shelf-edge flow (Figure 2). A subsurface ridge of cool water, extending roughly along the 100 m isobath, appears to be a quasi-permanent feature of the large-scale thermal structure along the eastern and central Agulhas Bank during spring and summer (Probyn et al., 1994). Cyclonic eddies, associated with meanders in the Agulhas Current, provide a further source of shelf edge upwelling downstream of Port Elizabeth, predominantly in the region of the Agulhas Bank Bight (22-24oE) (Lutjeharms et al., 1989). Associated with the meandering current and cyclonic eddies are plumes of warm surface water that migrate shoreward along the whole eastern edge of the Agulhas Bank providing a source of Indian Ocean subtropical surface waters to the region (Lutjeharms et al., 2003). During summer intermittent easterly winds drive coastal upwelling at several promontories along the South African southern coast (Schumann, 1999).

Figure 2: Schematic of oceanographic features resulting from a range of physical atmospheric and oceanographic forcing around the coast of southern Africa (M. Roberts, 2005).

Coastal upwelling is also a prominent feature on the western Agulhas Bank between Cape Agulhas and Cape Point. However, the lower variability of the atmospheric forcing driving upwelling in this region, result in larger cold water plumes that may merge with each other to form a continuous upwelling regime off the southwest coast of South Africa (Lutjeharms and Stockton, 1991). It thus forms the southernmost extension of the Benguela Current Large Marine Ecosystem. Despite the seasonal occurrence of wind-driven upwelling east of Cape Agulhas, it is not necessarily coincident in time and spatial extent with that of the Western Agulhas Bank and not characteristic of an eastern boundary system (Largier et al., 1992). Warm Agulhas Current water is frequently observed (~60% of the time) along the Western Agulhas Bank shelf edge as surface plumes generated by meanders in the current that are advected northward as Agulhas filaments due to equatorward alongshore transport in a frontal jet system (Largier etal., 1992; Probyn et al., 1994; Lutjeharms and Cooper, 1996; Shillington et al., 2006). Average flow on the western Agulhas Bank is generally northwest ward in summer (Boyd et al., 1992; Shannon and Nelson, 1996), but current reversals have been observed due to the influence of cyclonic eddies.

Abstract

Satellite derived sea surface temperature and chlorophyll-a concentration data are analyzed to investigate the temporal and spatial variability of upwelling and primary productivity at various zones of the Agulhas Bank for

January 2003 to December 2011.

Methods Data Sources

MODIS (Moderate Resolution Imaging Spectrometer) Aqua level 2 chlorophyll-a concentration ([Chl-a]) and co-incident sea surface temperature (SST) data granules were downloaded from the NASA Ocean Biology Processing Group (OBPG) online database (http://oceancolor.gsfc.nasa.gov/) for January 2003 to December 2011. Data covering the Agulhas Bank region was extracted and mapped to an equirectangular projection with a pixel resolution of 96x96 pixels per degree of latitude and longitude within 32o S to 40o S and 16o E to 28o E. Global level 4 blended Multi-sensor Ultrahigh Resolution (MUR) sea surface temperature data produced by the Group for High Resolution Sea Surface Temperature (GHRSST) were downloaded from the Physical Oceanography Distributed Active Archive Centre (PODAAC: ftp://podaac-ftp.jpl.nasa.gov/allData/ghrsst/data/L4/ GLOB/JPL/MUR) to compliment the MODIS dataset. Level 4 SST products incorporate in situ SST data and satellite SST data from several microwave and infra-red sources resulting in a cloud-free product of 1~2km resolution. MUR SST data was extracted for the study region and mapped to the same projection as the MODIS data.

Analysis Monthly climatologies of SST and [chl-a] were produced to investigate the seasonal variability of these geophysical variables in the Southern Benguela and on the Agulhas Bank (Figure 3). Using [Chl-a] climatologies and peer reviewed literature as a guide, the Agulhas Bank region was divided into 7 zones (Western Agulhas Bank Coast (a), South Coast (b), Port Alfred (c), Eastern Agulhas Bank (d), Central Ridge (e), Central Agulhas Bank (f) and Western Agulhas Bank (g), as well as an eighth zone for the Southern Benguela (h)) for a comparative variability study (Figure 1). Daily data encapsulated within the boundaries of each selected zone were spatially averaged to produce a single daily mean. Monthly means were produced from these datasets to investigate the inter-annual variability of SST and [Chl-a] in each zone (Figure 4). A daily chlorophyll-a upwelling index was produced for each valid image pixel to identify productivity associated with different phases of upwelling in all study regions, using the following algorithm:

Chl-a index = ([Chl-a] day – [Chl-a] month climatology minimum) / [Chl-a] month climatology range Two case studies highlighting the complexity of physical phenomena conspiring to produce exceptionally high values of [Chl-a] on the Agulhas Bank are also presented.

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Results

Monthly climatologies shown in Figure 3 suggest strong seasonal cycles for both [Chl-a] and SST on some of the Agulhas Bank zones. Both sequences start with the climatology for January in the top left and finishes with the climatology for December at the bottom right. The warmest temperatures are found during February on the Central Agulhas Bank and it is also the location for the coolest temperatures in August. A ridge of cool water is evident from December to April, extending from the coast towards the tip of the Agulhas Bank. Higher [Chl-a] is also associated with this feature, but unlike the SST values [Chl-a] values remain much the same in this region throughout the year. Again the Central and Western Agulhas Bank exhibit the highest seasonal range with lowest [Cl-a] observed during December to February and highest [Chl-a] evident from August to October. A thin band of high [Chl] values closely hugs the South Coast and is most extensive during April to July.

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Figure 3: Monthly SST and [Chl-a] climatologies for 2003 to 2011 from GHRSST MUR SST and OBPG MODIS Aqua data resepctively.

Figure 4: Monthly mean SST and [Chl-a] regional averages for 2003 to 2011 from GHRSST MUR SST and OBPG MODIS Aqua data resepctively. The red line represents SST and the blue [Chl-a].

The dominant period of variability evident in both parameters for the coastal upwelling zones of Southern Benguela, Western Agulhas Bank Coastal and South Coast is seasonal. [Chl-a] and SST peaks coincide in summer for the West Coast regions, whereas the South Coast [Chl-a] peak lags that of the SST by 1-3 months. However, the strong seasonal SST signature is seen in all other zones as well. There is evidence of a decreasing trend in SST for all regions. [Chl-a] values are significantly higher and

more variable in the coastal zones. Although much lower [Chl-a] ranges are observed on the Central and Western Agulhas Bank, a dominant seasonal cycle with winter [Chl-a] peaks is in observed. The [Chl-a] signature for Port Alfred, the Eastern Agulhas Bank and the Upwelling Ridge seem to indicate strong modes of variability at a

much shorter period than the seasonal cycle.

Results

Only data with a mean upwelling index > 0.2 and regional cloud cover < 40% were considered when examining the spatial and temporal variability of upwelling associated with the respective zones (Figure 5). The year on year variability in number of upwelling days year on year were evaluated, and also the distribution of days representing different phases of increasing upwelling. Overall the Southern Benguela had the most upwelling events, with most events having values between 40-50% of the index and a relatively low ratio of extreme events. The Western Agulhas Bank coastline shared a similar profile, but had fewer upwelling days overall and was slightly more prone to extreme events. Less variability is seen on the South Coast, with most days in phase 2. A similar distribution is observed for the Ridge region, but it had almost the same amount of days for both phase 2 and 3. Both Port Alfred and the Eastern Agulhas Bank had more extreme days and more upwelling days than the rest of the Eastern Agulhas Bank. There is a significant decline in upwelling events during 2009 to 2011.

Figure 6: MODIS Aqua SST and [Chl-a] for 14 April 2005 and 17 March 2008

Figure 5: Regional [Chl-a] upwelling index analysis

1 : index < 0.3 2 : index < 0.4 3 : index < 0.5 4 : index < 0.6 5 : index < 0.7 6 : index > 0.7

Discussion and Conclusion

The Central and Western Agulhas Bank exhibit strong seasonality in both SST and [Chl-a], but the coastal zone of the Western Agulhas Bank is observed to respond as an extension to the Benguela Upwelling System, whereas the Central Agulhas Bank has peak [Chl-a] in winter when there is strong enough mixing to bring nutrients closer to the surface. On the South Coast upwelling is limited to a much smaller region closer inshore with peaks in the second quarter of the year. [Chl-a] variability shows no seasonal preference at Port Alfred and the Eastern Agulhas Bank indicating that the high upwelling variability is not dominated by the seasonal wind field, but by instabilities in the Agulhas Current as it follows the shelf edge. Figure 6 shows two examples of strong upwelling and primary production on both the West Coast and the Agulhas Bank in late summer for different years. In both instances the Western Agulhas Bank coastal zone forms an almost continuous super upwelling cell with the Cape Peninsula and Cape Columbine cells that developed as result of sustained south-easterly wind forcing. There is, however, a marked difference in the [chl-a] response to the upwelling related cool sea surface temperatures on the Eastern Agulhas Bank between the two dates. Although a cool ridge signature is clearly seen on both SST images, a much stronger [chl-a] response is noted for 14 April 2005. In this case the role of the Agulhas Current, it’s associated shear edge features, Agulhas Rings and also the Good Hope Jet should be investigated to determine the physical forcing behind the instigation and spread of high primary productivity on the Agulhas Bank. A steady Agulhas Current, following the shelf edge is apparent on 17 March 2008, whereas much meandering is observed in the earlier image. Future analysis will include the evaluation of the proximity of the Agulhas Current to the coast during extreme East Central (Ridge) Agulhas Bank upwelling events and also the role that shear edge eddies play in producing high productivity plumes along the eastern and western shelf edge .

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Christo Peter Whittle, Earth Observation, NRE, CSIR, South Africa cwhittle@csir.co.za