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  • Copernicus Publications (EGU)  (3)
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  • 1
    Publication Date: 2024-02-07
    Description: In this paper, we review observational and modelling results on the upwelling in the tropical Atlantic between 10∘ N and 20∘ S. We focus on the physical processes that drive the seasonal variability of surface cooling and the upward nutrient flux required to explain the seasonality of biological productivity. We separately consider the equatorial upwelling system, the coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. All three tropical Atlantic upwelling systems have in common a strong seasonal cycle, with peak biological productivity during boreal summer. However, the physical processes driving the upwelling vary between the three systems. For the equatorial regime, we discuss the wind forcing of upwelling velocity and turbulent mixing, as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is located along its northern boundary and is driven by both local and remote forcing. Particular emphasis is placed on the Guinea Current, its separation from the coast and the shape of the coastline. For the tropical Angolan upwelling, we show that this system is not driven by local winds but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 2
    Publication Date: 2024-02-07
    Description: Surface chlorophyll concentrations inferred from satellite images suggest a strong influence of the mesoscale activity on biogeochemical variability within the oligotrophic regions of the Gulf of Mexico (GoM). More specifically, long-living anticyclonic Loop Current Eddies (LCEs) are shed episodically from the Yucatan Chanel and propagate westward. This study addresses the biogeochemical response of the LCEs to seasonal forcing and show their role in driving phytoplankton biomass distribution in the GoM. Using an eddy resolving (1/12°) interannual regional simulation based on the coupled physical-biogeochemical model NEMO-PISCES that yields a realistic representation of the surface chlorophyll distribution, it is shown that the LCEs foster a large biomass increase in winter in the upper ocean. The primary production in the LCEs is larger than the average rate in the surrounding open waters of the GoM. This behavior cannot be directly identified from surface chlorophyll distribution alone since LCEs are associated with a negative surface chlorophyll anomaly all year long. This anomalous biomass increase in the LCEs is explained by the mixed-layer response to winter convective mixing that reaches deeper and nutrient-richer waters.
    Type: Article , PeerReviewed
    Format: text
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  • 3
    Publication Date: 2024-02-07
    Description: In this study, we use a joint observation-model approachto investigate the mixed-layer heat and salt annual mean as well as seasonalbudgets in the eastern tropical Atlantic. The regional PREFCLIM (PREFACE Climatology)observational climatology provides the budget terms with a relatively lowspatial and temporal resolution compared to the online NEMO (Nucleus for European Modelingof the Ocean; Madec, G., 2014) model, and thisis later resampled as in PREFCLIM climatology. In addition, advectionterms are recomputed offline from the model as PREFCLIM gridded advectioncomputation. In the Senegal, Angola, and Benguela regions, the seasonal cycle ofmixed-layer temperature is mainly governed by surface heat fluxes; however,it is essentially driven by vertical heat diffusion in the equatorial region.The seasonal cycle of mixed-layer salinity is largely controlled byfreshwater flux in the Senegal and Benguela regions; however, it follows thevariability of zonal and meridional salt advection in the equatorial and Angolaregions, respectively. Our results show that the time-averaged spatialdistribution of NEMO offline heat and salt advection terms compares much betterto PREFCLIM horizontal advection terms than the online heat and salt advectionterms. However, the seasonal cycle of horizontal advection in selectedregions shows that NEMO offline terms do not always compare well withPREFCLIM, sometimes less than online terms. Despite this difference, theseresults suggest the important role of small-scale variability in mixed-layerheat and salt budgets.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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