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International Journal of Marine Science 2013, Vol.3, No.34, 267-277
http://ijms.sophiapublisher.com
267
Research Article Open Access
Representation of Coastal Upwelling and Environmental Interactions in the
Southern Benguela in Satellite Era Reanalysis
Mark R. Jury
University of Zululand, KwaDlangezwa, 3886, South Africa, and Physics Dept, University of Puerto Rico, Mayagüez, PR 00681
Corresponding author email: mark.jury@upr.edu
International Journal of Marine Science, 2013, Vol.3, No.34 doi: 10.5376/ijms.2013.03.0034
Received: 07 May, 2013
Accepted: 03 Jun., 2013
Published: 12 Jul., 2013
Copyright
©
2013 Jury, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:
Jury, 2013, Representation of Coastal Upwelling and Environmental Interactions in the Southern Benguela in Satellite Era Reanalysis, International Journal of
Marine Science, Vol.3, No.34 267
-
277 (doi: 10.5376/ijms.2013.03.0034)
Abstract
The ability of ocean and atmosphere reanalysis products to represent coastal upwelling in the southern Benguela (28-36°S,
14-20°E) is studied in the period 1980-2008. Mean maps, depth sections and seasonal cycle are analyzed for ocean color, sea surface
temperature (SST), vertical velocity, winds and ocean currents in the zone. The paper first evaluates coastal winds and cross-shelf
gradients of temperature in a variety of products; then provides an analysis of inter-relationships among variables averaged in the St
Helena Bay sub-area. The 30km Coupled Forecast System reanalysis resolves the cool strip of SST with a 1-2
℃
warm bias, while
subsurface zonal overturning and a longshore current jet are featured in the 50 km Simple Ocean Data Assimilation reanalysis. Mean
coastal upwelling rates of 1-2 m/day are consistent with observations. The expected links are found between the annual cycle of
meridional wind stress and longshore currents, and between uplift, temperature and salinity. A new finding is a connection between
the zonal wind stress and chlorophyll anomalies (-35.86 mg m
-3
/N m
-2
). When the subtropical anticyclone is located southwest of
Arica, offshore winds prevail and phytoplankton blooms in the southern Benguela.
Keywords
Southern Benguela; Coastal upwelling; Reanalysis; Model representation
Introduction
The Benguela Current is a zone of equatorward flow
off southwestern Africa that is part of the South
Atlantic subtropical gyre (Garzoli and Gordon, 1996).
Wind driven upwelling over the shelf is seasonally
modulated in the latitudes 31-34°S, and strongest in
austral summer: October to March (Nelson and
Hutchings, 1983; Shannon, 1985; Shannon and Nelson,
1996). The Benguela upwelling zone is exposed to
coastal trapped low pressure cells (Jury and Brundrit,
1992; Yamagata and Iizuka, 1995) that amplify the
effects of frequent wind reversals. The upwelling
intensifies next to capes (Jury, 1988) where alongshore
winds and currents accelerate. The wind increases at
the shelf edge due to a shallow atmospheric boundary
layer and Bernoulli channeling, thus cyclonic wind
vorticity is enhanced (Bakun and Nelson, 1991;
Shannon and Nelson, 1996). Fennel (1999) considered
a model of the Benguela upwelling system to be
dominated by a band of alongshore wind with a
bell-shaped structure and seasonal cycle with frequent
perturbations. An ocean current jet develops between
the coastal upwelling and strongest wind. Hence any
model representation should capture the offshore
gradient in these features.
Monthly high (~0.5°) resolution ocean and surface
atmosphere reanalysis products that make use of
numerical assimilation of in-situ measurements and
satellite estimated winds, temperatures and sea surface
height (Carton et al., 2000; Carton and Giese, 2008)
are used here to characterize the hydrography of the
southern Benguela. The upwelling south of 31°S is
particularly challenging to represent in grid-model
assimilated fields due to its narrow extent (Hutchings
et al., 2009). The wind-forced upwelling (Nicholson,
2010) tends to focus at Hondeklip Bay 30°S, Cape
Columbine 33°S and the Cape Peninsula 34°S (Jury,
1988; Shannon and Nelson, 1996). In this narrow zone
of upwelling, chlorophyll levels are high (Cury and
Roy, 1989) and contribute to increased biotic
abundance (Kreiner et al., 2001). Variations in
upwelling allow larval retention and juvenile
recruitment in weak periods, and inhibit plankton
aggregation and export larvae in strong periods
(Parrish et al., 1983; Cury and Roy, 1989).