A model perspective on the dynamics of the shadow zone of the eastern tropical North Atlantic – Part 1: the poleward slope currents along West Africa

Abstract

Abstract. The West African seaboard is one of the upwelling sectors that has received the least attention, and in situ observations relevant to its dynamics are particularly scarce. The current system in this sector is not well known and understood, e.g., in terms of seasonal variability, across-shore structure, and forcing processes. This knowledge gap is addressed in two studies that analyze the mean seasonal cycle of an eddy-permitting numerical simulation of the tropical Atlantic. Part 1 is concerned with the circulation over the West African continental slope at the southernmost reach of the Canary Current system, between ∼8 and 20∘ N. The focus is on the depth range most directly implicated in the wind-driven circulation (offshore and coastal upwellings and Sverdrup transport) located above the potential density σt=26.7 kg m−3 in the model (approx. above 250 m of depth). In this sector and for this depth range, the flow is predominantly poleward as a direct consequence of positive wind stress curl forcing, but the degree to which the magnitude of the upper ocean poleward transport reflects Sverdrup theory varies with latitude. The model poleward flow also exhibits a marked semiannual cycle with transport maxima in spring and fall. Dynamical rationalizations of these characteristics are offered in terms of wind forcing of coastal trapped waves and Rossby wave dynamics. Remote forcing by seasonal fluctuations of coastal winds in the Gulf of Guinea plays an instrumental role in the fall intensification of the poleward flow. The spring intensification appears to be related to wind fluctuations taking place at shorter distances north of the Gulf of Guinea entrance and also locally. Rossby wave activity accompanying the semiannual fluctuations of the poleward flow in the coastal waveguide varies greatly with latitude, which in turn exerts a major influence on the vertical structure of the poleward flow. Although the realism of the model West African boundary currents is difficult to determine precisely, the present in-depth investigation provides a renewed framework for future observational programs in the region.

Seasonal Variability of the Mauritania Current and Hydrography at 18°N

“…The magnitude of their poleward transport from that season is thus somewhat larger than the poleward transports during upwelling and relaxation season of 0.7 and 1.8 Sv, respectively, reported here. ROMS by Kounta et al (2018) suggested elevated poleward transports at the eastern boundary during the boreal fall season due to CTWs excited in the Gulf of Guinea, which may explain this discrepancy.…”

“…Alongshore transport variability at the eastern boundary can be forced locally by variability of Ekman divergence through CTWs (McCreary, 1981;Suginohara, 1982;Wang, 1982) and offshore propagating Rossby waves (e.g., McCreary & Chao, 1985;Suginohara & Kitamura, 1984) or remotely by CTWs propagating into the region from the South (e.g., Kounta et al, 2018). While locally forced barotropic Rossby waves and barotropic and low-vertical mode baroclinic CTWs can establish an alongshore pressure gradient and related poleward flow at the boundary within a few days, geostrophic adjustment by baroclinic Rossby waves requires several months or more.…”

“…The dynamics of eastern boundary upwelling systems have been studied through a variety of theoretical and modeling approaches for various regional settings (e.g., Fennel, 1999;Fennel et al, 2012;Kounta et al, 2018;McCreary et al, 1987;McCreary & Chao, 1985;Philander & Yoon, 1982). McCreary and Chao (1985) using a linear general circulation model showed that a PUC, set up by coastally trapped waves (CTWs) due to the sudden onset of wind forcing (Philander & Yoon, 1982), only matched those typically observed in the California current system when a local wind stress curl was introduced.…”

First Evidence of Anoxia and Nitrogen Loss in the Southern Canary Upwelling System

“…A discussion is offered in section 4 on the conditions and processes having led to anoxia and on the broader local to regional implications. (Arístegui et al, 2009;Kounta et al, 2018)…”

“…(a) Thermohaline and flow features of the Northeast tropical Atlantic (Arístegui et al, ; Kounta et al, ) including the Cape Verde frontal zone (CVFZ), North Equatorial Current (NEC), North Equatorial Counter Current (NECC), North Equatorial Undercurrent (NEUC), and West African poleward Boundary Current (WABC). The typical upwelling circulation off southern Senegal is represented in the upper‐right panel: The WABC flows northward and feeds the coastal upwelling with nearly hypoxic subsurface waters.…”