The effect of large-scale interannual variations in the Gulf of Guinea

Abstract

The Gulf of Guinea is an important region of tropical Atlantic variability, and embedded within it are the Atlantic Ni~no and the seasonal tropical instability waves. Although, the dynamics of this region are accepted to modulate the monsoon system by the scientific community, it is still debated what controls temperatures and the exact coupling mechanism with the atmosphere. Here, I present the results of temperature advection, variability of the Equatorial Undercurrent (EUC) in relation to atmospheric forcing, influence of Gulf of Guinea sea surface temperature (SST) to rainfall variability over West Africa and the Sahel, spatio-temporal variability of nonseasonal ocean-atmosphere processes and the coupling of winds and SST. The central aim is to understand how anomalous currents advect temperatures, and how anomalous oceanic temperatures respond to the atmosphere to cause coupled variability.

In order to understand the enigmatic influence of temperature advection by tropical instability waves (TIW), I examined the influence of anomalous currents and temperatures associated with the waves. Results show that TIWs advect temperatures zonally and warm the Gulf of Guinea during the boreal summer, and at the central equatorial Atlantic, they advect temperatures to the north. The EUC, represented by salinity maximum is investigated from four regions to cover the northern, central and southern branches of the current. West of the Gulf of Guinea, anomalous northward currents associated with TIWs destabilise the vertical positioning of the EUC core during the boreal summer. In the Gulf of Guinea, where bathymetry, river discharge and winds vary seasonally, vertical fluctuations are apparent every three months and the shallowing of the EUC affects the upper layer temperatures.

The Gulf of Guinea SST correlates well with variability of rainfall over West Africa and the Sahel, where cold (warm) temperatures in 2005 (2007) explained the causes of drought (flooding) that occurred in those years respectively. Strongest intraseasonal SST variability is observed at the Guinea Dome and the Gulf of Guinea region and both occur due to seasonally varying northeasterlies and southeasterlies. However, north of 18N, zonal tropospheric winds associated with Madden-Julian Oscillation appear to alter the changes on surface winds. This connection between surface and atmospheric winds explains the inability of the winds to covary interannually with SST at the TIWs frontal zones. Investigations presented in this thesis increase our understanding of tropical Atlantic variability and results therein are useful as background conditions for coupled ocean-atmosphere models.

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