Coastal Upwelling Dynamics in the Gulf of Guinea, Quantified

IntroductionCoastal upwelling along the northern Gulf of Guinea sustains fisheries and regulates tropical Atlantic sea-surface temperature, yet the combined roles of atmospheric and oceanic forcing remain incompletely characterized. This study quantifies the joint interaction between wind-driven Ekman transport and geostrophic circulation along the Ghana-Togo shelf, focusing on the Keta sector (0.5° E–1.5° E, 5.5° N–6.0° N) using deseasonalized monthly anomalies (2010–2022).MethodsTwo indices are derived: the Ekman Coastal Upwelling Index (ECUI), from wind stress rotated to a 20° coastline angle, and the Geostrophic Coastal Upwelling Index (GCUI), diagnosed directly from sea-surface height (SSH) gradients via geostrophic balance. These are integrated with GLORYS12V1 sea-surface temperature (SST) and SSH, utilizing Bretherton-corrected lagged correlations and multivariate empirical orthogonal function (mEOF) analysis.ResultsLagged correlations, computed with variable-specific effective degrees of freedom, demonstrate that atmospheric forcing (ECUI) initiates immediate surface cooling (r = 0.47, lag 0, Neff = 126) and sustains a significant response through lags +1 and +2. GCUI shows no significant relationship with SST at lag 0 (r = -0.01), but exhibits a significant negative correlation at lags -5 and -6 (r = -0.23 and -0.20), indicating SST anomalies precede geostrophic anomalies by roughly half a year—distinct from the concurrent ECUI-SST response. The mEOF analysis identifies a dominant co-variability mode explaining 34.0% of shared interannual variance, well-separated by North's rule and stable under block-bootstrap resampling (congruence = 0.98). Temporal evolution indicates no discrete regime shift; instead, change-point analysis reveals a long-term trend (predominantly in SSH) superimposed on shorter-timescale interannual variability.DiscussionThe Keta sector combines the domain's strongest diagnosed atmospheric forcing with a spatially consistent geostrophic signal (absent in the western Gulf), identifying it as a particularly favorable dynamical sector for coastal upwelling. Sensitivity analyses confirm these findings are robust to coastline angle (10–30°), drag coefficient, and upwelling length scale variations. Ultimately, this study provides a transferable, robust diagnostic framework for examining joint atmospheric-oceanic upwelling dynamics in tropical coastal systems.