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Ocean Climate of the South East Atlantic Observed from Satellite Data and Wind Models N.J

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Ocean Climate of the South East Atlantic Observed from Satellite Data and Wind Models N.J Progress in Oceanography 59 (2003) 181–221 www.elsevier.com/locate/pocean Ocean climate of the South East Atlantic observed from satellite data and wind models N.J. Hardman-Mountford a,∗, A.J. Richardson b, 1, J.J. Agenbag c, E. Hagen d, L. Nykjaer e, F.A. Shillington b, C. Villacastin e a Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, Devon PL1 2PB, UK b Oceanography Department, University of Cape Town, Rondebosch 7701, Cape Town, South Africa c Marine and Coastal Management, Private Bag X2, Rogge Bay, 8012 Cape Town, South Africa d Insitute for Baltic Sea Research Warnemuende, Seestrasse 15, 19119 Warnemuende, Germany e Institute for Environment and Sustainability, Joint Research Centre, I-21020 Ispra, Va, Italy Revised 8 September 2003; accepted 14 October 2003 Abstract The near-coastal South East Atlantic Ocean off Africa is a unique and highly dynamic environment, comprising the cool Benguela Current, warm Angola Current and warm Agulhas Current. Strong coastal upwelling and the Congo River strongly influence primary production. Much of the present knowledge of the South East Atlantic has been derived from ship-borne measurements and in situ sensors, which cannot generally provide extensive spatial and tem- poral coverage. Similarly, previous satellite studies of the region have often focused on small spatial areas and limited time periods. This paper provides an improved understanding of seasonal and interannual variability in ocean dynamics along the South East Atlantic coast of Africa using time series of satellite and model derived data products. Eighteen years of satellite sea surface temperature data are complimented by 7 years of sea level data. Three years of chlorophyll a data illustrate the seasonal biological response, but the time series is not of sufficient length for investigating interan- nual variability in chlorophyll biomass. Modelled wind fields are used to describe atmospheric forcing of the surface ocean. This is the first synoptic-scale description of the South East Atlantic from a suite of large spatial coverage, long time series products. Previous studies of seasonal and interannual variability in the region are reviewed and used to interpret key oceanographic features and processes identified in the satellite data. Key findings of this study are: 1. Descriptions of seasonal and interannual variability from these data show climate forcing of the South East Atlantic coast of Africa from both the northern and southern boundaries. Bimodal seasonal signals of equatorial origin propagate poleward along the Angolan coast, while the trade winds and events in the Agulhas region dominate the Agulhas Bank and Southern Benguela. The Northern Benguela is a mixed regime, under the influence of forcing from both directions. 2. The Benguela Nin˜o years of 1984 and 1995 are clearly observed in sea surface temperature and sea level anomalies and correspond to anomalously weak southerly winds at the equator. These conditions were also observed in 1999, suggesting this too may have been a Benguela Nin˜o year. 3. Consideration of putative Nin˜o-type events in the equatorial Atlantic from this and other studies suggests that the ∗ Corresponding author. Tel.: +44-1752-633100; fax: +44-1752-633101. E-mail address: [email protected] (N.J. Hardman-Mountford). 1 Present address: Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK. 0079-6611/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2003.10.001 182 N.J. Hardman-Mountford et al. / Progress in Oceanography 59 (2003) 181–221 frequency of these events is much higher than previously estimated and may be similar to the frequency of El Nin˜o- Southern Oscillation (ENSO) events in the Pacific Ocean. Furthermore, years of anomalously strong southerly winds at the equator occur during Pacific ENSO years. 2003 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 183 1.1. Oceanographic and meteorological overview . 183 1.1.1. Meteorology . 183 1.1.2. Major currents and boundaries . 185 1.1.2.1. Angola Current . 185 1.1.2.2. Benguela Current . 185 1.1.2.3. Angola–Benguela front . 186 1.1.2.4. Agulhas Current . 186 1.1.3. Upwelling . 187 1.1.3.1. Gabon/Angola coastal system . 187 1.1.3.2. Benguela . 187 1.1.3.3. Agulhas Bank . 189 1.1.4. Interannual variability . 189 1.1.4.1. Benguela Nin˜os and tropical-origin anomalous events . 189 1.1.4.2. Agulhas Current intrusions . 190 1.2. Satellite remote sensing of the South East Atlantic Ocean . 190 2. Data and methods . 192 2.1. Data . 192 2.1.1. Sea surface temperature from satellite . 192 2.1.2. Sea level from satellite . 192 2.1.3. Chlorophyll a from satellite . 192 2.1.4. Surface winds from models . 192 2.2. Methods . 193 2.2.1. Monthly maps . 193 2.2.2. Latitude–time plots . 193 2.2.3. Annual sea surface temperature anomaly maps . 194 2.2.4. Time series of anomalies . 194 2.2.5. Spectral analysis . 194 3. Results and discussion . 195 3.1. General overview . 195 3.2. Seasonality . 198 3.2.1. Angola Current and Angola coastal upwelling . 198 3.2.2. Congo River plumes . 201 3.2.3. Benguela upwelling . 201 3.2.4. Agulhas Current and Western Agulhas Bank . 202 3.3. Interannual variability . 203 3.3.1. Equatorial forcing and Atlantic Nin˜o-type events . 203 3.3.2. Agulhas warming and warm water intrusions to the Southern Benguela . 214 4. Conclusions . 215 N.J. Hardman-Mountford et al. / Progress in Oceanography 59 (2003) 181–221 183 1. Introduction The near-coastal South East Atlantic Ocean off Africa is a highly dynamic environment (Shannon, 1985a; Shillington, 1998), unique in many respects. The cool Benguela Current is the only eastern boundary current to be bounded at both ends by warm water of tropical origin: the Angola Current in the north and the Agulhas Current to the south (Shannon & Nelson, 1996). Additionally, the Benguela Current system may possess the strongest sustained, locally wind-driven, coastal upwelling of any region of the world ocean, the Lu¨deritz upwelling cell of southern Namibia (Parrish, Bakun, Husby, & Nelson, 1983; Bakun, 1993). Furthermore, into this area flows the Congo River, the largest freshwater input to any eastern ocean bound- ary, and this river has a marked effect on the productivity of the region (Thomas, Carr, & Strub, 2001). Much of the present knowledge of the South East Atlantic has been derived from ship-borne measure- ments and in situ sensors, which cannot generally provide both extensive spatial and temporal coverage. Previous satellite studies of the region have often focused on small spatial areas and limited time periods. We are now in the position to view both physical (sea surface temperature and sea surface height) and biological (chlorophyll a) variables in the South East Atlantic from satellite over broad temporal and spatial scales. The aim of this paper is to provide an improved understanding of the ocean dynamics along the South East Atlantic coast of Africa by describing seasonal and interannual variability using satellite-derived data products. These products include 18 years of sea surface temperature (SST) data, 7 years of sea surface height data, and 3 years of ocean colour data. Additional information for interpretation is provided by 17 years of modelled wind data. This is the first synoptic-scale description of the South East Atlantic from a suite of long time series satellite products. It is hoped that the insights provided by this study will aid current scientific research programmes in the region, such as the Benguela Current Large Marine Ecosystem (BCLME) programme. 1.1. Oceanographic and meteorological overview The major oceanographic features of the South East Atlantic are shown in Fig. 1. 1.1.1. Meteorology Oceanographic conditions in the South East Atlantic are largely controlled by basin-scale, ocean–atmos- phere interactions over the South Atlantic. In the East Atlantic Ocean, the ‘meteorological equator’ or Intertropical Convergence Zone (ITCZ) is to be found several degrees north of the geographical equator. The climatic position of the ITCZ clearly shows an inclination from southwest to northeast (Citeau, Berge´s, Demarcq, & Mahe´, 1988). Low atmospheric pressure over tropical regions of the African continent causes a divergence of the southeast trade winds, creating the southwesterly monsoon winds along the Angolan coast. In the eastern equatorial Atlantic, seasonal and interannual changes in the position of the ITCZ reflect changes in the southwest monsoon and the southeast trade winds directly. Associated fluctuations are correlated with changes in sea level air pressure over the central South Atlantic, in both the position and intensity of the atmospheric high pressure cell that forms the South Atlantic Anticyclone. This anticyclone interacts with the equatorial low-pressure belt of the ITCZ in the north and continental low-pressure cells over southern Africa in the east to control the southeasterly trade winds along the west coast of southern Africa. Accordingly, it provides the driving force for the Benguela Current regime, which feeds into the South Equatorial Current as part of the basin-scale, wind-driven circulation (Peterson & Stramma, 1991). Intense solar radiation warms near-surface waters causing an accumulation of warm surface water in western equatorial zones (Wyrtki, 1982). Here, the current convergence results in positive sea level anomalies and downwelling accompanied by a deep pycnocline (Carton, 1994). Across the whole equatorial ocean, press- ure gradients are established and maintained within superficial layers along and parallel to the equator and 184 N.J. Hardman-Mountford et al. / Progress in Oceanography 59 (2003) 181–221 Fig. 1. Map of the South East Atlantic showing surface and near-surface currents, frontal zones, upwelling cells, major areas of freshwater input and bathymetry.
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