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.
Highlights
The meridional extent of the Canary Current system (CCS) is one of its remarkable features
Poleward boundary currents are ubiquitous along eastern boundary continental slopes (Brink et al, 1983; Huyer, 1983; Connolly et al, 2014), those subjected to upwelling-favorable winds for which subsurface undercurrents are essential flow features (McCreary, 1981; McCreary et al, 1987; Philander and Yoon, 1982)
An eddy-permitting numerical simulation with realistic forcings has been analyzed to investigate the dynamics of the boundary current along the West African seaboard
Summary
The meridional extent of the Canary Current system (CCS) is one of its remarkable features. Older studies tend to insist on an origin in the Gulf of Guinea for the summer pulse (Kirichek, 1971; Mittelstaedt et al, 1975) but this process has not been revisited for a long time In his 1989 review study Barton qualifies the knowledge of the poleward undercurrent along the eastern boundary of the North Atlantic as sketchy: “the arguments for its existence as a continuous entity are based upon relatively few direct current observations, some interpretations of temperature and salinity data, and a degree of speculation”. We instead will prefer to use the unique and neutral terminology “WA poleward boundary current” (or WABC in short) to refer to the northward flow present over or in the vicinity of the WA continental slope This overview strongly suggests that clarifications are needed on ETNA circulation and dynamics. The source pathways for WA coastal upwelling waters and the broader regional circulation (which turns out to be of key relevance to understanding these pathways) will be examined in Part 2
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