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 Puerto Rico
Author    Correspondence author
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
© 2013 BioPublisher Publishing Platform
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 50km 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 south-west of Africa, offshore winds prevail and phytoplankton blooms in the southern Benguela.

Keywords
Southern Benguela; Coastal upwelling; Reanalysis; Model representation

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).
In this paper, the ability of reanalysis products to represent coastal upwelling in the southern Benguela is evaluated in the period 1980-2008 using mean maps and depth sections. The annual cycle and anomalies are studied in the St Helena Bay sub-area. The paper addresses the following research questions: How is the structure and variability of coastal winds and upwelling represented? Which model databases best capture the slope of isotherms over the shelf? What temporal variability is evident in coastal upwelling? What are the environmental influences on chlorophyll enrichment and fish catch? An overarching goal of the paper is to show that modern reanalysis products are capable of describing coastal upwelling and its variability and impacts in the southern Benguela.
2 Data and Methods
Three monthly reanalysis datasets are considered in the period 1980-2008, two for the lower atmosphere and one for the upper ocean. Winds, sea surface temperatures (SST) and atmospheric features just west of South Africa are analyzed from monthly Coupled Forecast System (CFS, Saha et al., 2010, 0.3º resolution) and European Community Medium-range Weather Forecast (ECMWF, Dee et al., 2011, 0.7º resolution) reanalysis fields drawn from the Climate Explorer website. These two products are generated using rather different numerical models and techniques of data assimilation. Results over the elevated plateau and deep ocean are masked to focus on the shelf and coastal environment (28-36°S, 14-20°E). Oceanographic fields are provided by monthly Simple Ocean Data Assimilation version 2.1.6 (SODA, Carton and Giese, 2008) from the IRI Climate Library, that include temperature, salinity, currents and vertical motion in the satellite era. SODA is forced by ECMWF wind stress and data are available at 0.5º resolution. Here, sub-surface data are compared with the National Center for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS, Behringer and Xue, 2004) and the World Ocean Atlas (WOA, Locarnini et al., 2006).
The monthly reanalysis products incorporate in-situ and remotely sensed data: coastal and ship observations, scatterometer and passive microwave winds, drifters and ARGO profilers, infrared and microwave surface temperature (Smith and Reynolds, 2004), microwave altimeter sea surface height, and satellite estimated air temperature, humidity and cloud-drift winds. Data from the South Africa Data Center for Oceanography and GODAS are incorporated to provide adequate coverage of the Benguela shelf in the satellite era. The reanalysis products assimilate observations via a numerical model, which for CFS and SODA is the Geophysical Fluid Dynamics Lab Modular Ocean Model version 4 (Griffies et al., 2004) and for ECMWF is the ORA-S3 model (Balmaseda et al., 2008). The representation of coastal upwelling and its forcing is evaluated from long-term means and standard deviations 1980-2008. Oceanographic and atmospheric structure is analyzed as annual hovmoller plots and depth or height sections west of St Helena Bay (32.5°S, 14°-19°E).
Surface upwelling and shelf productivity are analyzed using monthly ~0.1° resolution SST and chlorophyll estimates from MODIS and SeaWifs satellites in the period 1998-2010, obtained from the National Aeronautics and Space Administration (NASA). Past research has found that chlorophyll is better related to winds and currents than SST in west coast upwelling zones (Demarcq, 2009; Freon et al., 2009). Index-to-field correlations are calculated for the St Helena Bay sub-area: 32.2-32.7°S, 17.2-18.2°E (cf. Figure 7 a); a nursery for South Africa’s pelagic fishery (Agenbag et al., 2003) within the perennial Cape Columbine upwelling plume. Temporal cross-correlations are analyzed between SODA wind stress, 1-100 m depth-averaged currents, vertical motion, temperature and salinity (1990-2008), and chlorophyll
International Journal of Marine Science
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