Dynamic Upwelling on the inshore edge of the Agulhas Current
Neil
*1
Malan ,
Mike
1Department
2
Roberts
and Isabelle
1
Ansorge
of Oceanography, University of Cape Town,
2Oceans and Coast, Department of Environmental Affairs, Cape Town, South Africa
*Corresponding author, e-mail [email protected]
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1.  Introduction:
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Examina'on of historical cruise data and satellite SST has established the area off Port Alfred as a centre of kinema'cally-­‐driven shelf-­‐upwelling ac'vity on the Eastern Agulhas Bank, with upwelling reported to have a surface expression 40% of the 'me (Lutjeharms, Cooper & Roberts 2000). It has also been suggested that this shelf-­‐edge upwelling may advec'vely supply boPom water to the Agulhas Bank region as a whole (Lutjeharms 2006, Largier & Swart 1978) – See Fig. 1. This upwelling ac'vity has an effect on nutrient supply, promo'ng large blooms. See Fig. 2. Cruise data show the presence of cold water on the shelf slope with varying degrees of outcropping (see Figs. 3 & 4). However a detailed examina'on of the subsurface processes that supply this upwelling cell and their interac'ons with the meso-­‐scale variability of the Agulhas Current has yet to be carried out. This study seeks to answer two basic ques'ons, using a combina'on of historical cruise data, remote sensing and three in-­‐situ current-­‐meter moorings. •  How o[en does shelf-­‐edge upwelling occur? •  What is the main driving mechanism of this upwelling? 2. Bottom temperature comparisons: 15
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4. Coastal upwelling – connec1ons between wind and surface expression 3. Connections to mesoscale activity in the Agulhas Current
Fig. 5(a) shows a 'me series of boPom temperatures taken at a mid-­‐shelf mooring site at a depth of 80m. Upwelling events with water below 16°C occur 80% of the 'me. These upwelling events exhibit a strong 60-­‐80 day cycle, overlaid with higher frequency variability in the 3-­‐10 day range. (see fig. 6). A corresponding paPern can be observed in the offshore mooring boPom temperatures (depth 175m). It should be noted that there is a similar range in temperatures between the two mooring sites, despite the ~100m depth difference. Also to be noted is the propaga'on of cold events up the shelf. cross−shelf
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Fig. 7 shows the interac'ons between the shelf and the Agulhas Current divided into 5 main categories. This is shown both schema'cally (right hand panel) and using satellite SST images (le[ hand panel). Fig. 8 shows an example of the interac'on between the boPom temperature and the currents in the boPom layer (175-­‐120m), taken from the offshore mooring. This shows the reversals in currents during the passage of an elongated plume (mode 3) and the associated changes in boPom temperature. Fig: 8 – Current Modes An examina'on of the boPom temperatures at the inshore mooring site (depth 31m, 4 km offshore) shows a higher variability compared to the mid-­‐shelf and offshore sites. The strong effect of wind driven upwelling on this inshore site is apparent in fig. 9. However, the prolonged cold events in November 2005 and January-­‐February 2006 show the effect of the current itself in supplying cold boPom water along the width of the shelf. Fig: 9 Alongshore wind (m.s−1)
Fig: 6 – Spectral analysis of midshelf boDom temperatures. 10
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6. Conclusions Mode 4 Mode 3 Mode 5 The presence of upwelled water on the shelf has been confirmed using both historical and recent cruise data. The variability in upwelling along the boPom of the shelf has been inves'gated and compared to mesoscale features of the Agulhas current visible from satellite SST imagery. The effect of these features can also be observed as reversals in the boPom currents on the shelf, with associated changes in temperature (see Case Study 1) The effect of the wind on coastal upwelling and the outcropping of water brought onto the slope via shelf-­‐edge upwelling can be observed by examina'on of wind and coastal temperatures. It could be suggested that various shelf-­‐edge mechanisms such as boPom Ekman veering, geostrophic re-­‐adjustment and changes in poten'al vor'city act in concert to transport cold water up the shelf slope and retain it on the shelf (refer to the boPom panel of fig 4). Thesse processes are then modulated by the passage of mesoscale features as iden'fuied by the 5 current modes. Thus the Port Alfred upwelling cell is maintained by an interac'on between shelf-­‐edge upwelling, the effect of Aghulhas current mesoscale features, and the effect of coastal upwelling. This results in large shelf-­‐upwelling events occuring on a roughly 2-­‐monthly 'mescale, However to unravel the various contribu'ons of these driving mechanism, a more detailed inves'ga'on into the dynamics of the region is necessary. Feb