The oceanic circulation in the upper layers of the north of the Gulf of Guinea is analyzed, as inferred from in situ observations and numerical simulations. This particular region, in spite of the presence of a coastal upwelling and its impact on resources and regional climate, is still poorly documented. Cruises carried out in the framework of different international programs (e.g., EGEE/AMMA, PIRATA) allowed to show the existence of an eastward flowing undercurrent, found under the Guinea Current, named the Guinea UnderCurrent (GUC). Numerical results from high resolution simulation allowed the description of the seasonal variability of this current. It appears that the GUC is stronger in spring and reverses westward in August–September. We also depict the fate and the sources of the GUC based on selected trajectories from numerical particle tracking. The simulated trajectories reveal: (i) a preferred route of the GUC along 4°N from Cape Palmas to Cape Three Points and following the coast east of Cape Three Points; (ii) strong recirculations in the most eastern part of the Gulf of Guinea and off Cape Palmas including warm and salty waters of the South Equatorial Current; (iii) a weak inflow from northern latitudes through a subsurface current flowing southward along the West African coast. In addition, Lagrangian experiments show that the GUC is not an extension of the North Equatorial UnderCurrent and confirm that this current does not penetrate into the Gulf of Guinea.

The upper circulation in the southern Bay of Biscay is analysed over winter and fall 2004 using a coastal altimetric dataset, moorings, sea surface temperature (SST) observations and a regional simulation from the SYMPHONIE model. The aim is to determine whether altimetric data can detect occurrences of a slope current (the Iberian Poleward Current, IPC). We first analyse in situ and SST data. The results show pulses of a warm surface poleward flow (IPC) limited to the westernmost part of the northern Spanish coast. Along-track TOPEX/Poseidon and Jason-1 data are analysed over three 'events' (Jan., Feb., and Oct. 2004). Altimetric data can depict the main pulses. They provide some information on their spatial variability but the present in situ dataset is not dense enough to assess this information. Local inconsistencies between neighbouring tracks may be related to uncertainties in altimetric corrections. The simulated fields are consistent with the in situ observations. They allow us to better understand the IPC subsurface properties and its signature in altimetric data. The ability of altimetry to monitor IPC pulses under any atmospheric conditions or when the SST signature is weak makes it an essential component for the IPC observing system in addition to satellite SST and moorings data.

The region located in the far northeast of the Gulf of Guinea (NEGG), eastern tropical Atlantic, remains poorly documented due to a lack of available in situ ocean data. Heavy rainfall and intense river discharges observed in this region induce a strong salinity stratification that may have a significant impact on the mixed layer depth and on sea surface temperatures, through the so-called barrier-layer effect. By using recent in situ data and climatological outputs from a numerical simulation, we reveal the existence of a barrier layer in the NEGG and describe its seasonal occurrence. In the NEGG, the barrier layer limits the mixed layer depth. From January to March, significant values for the barrier-layer thickness are observed mostly due to the horizontal advection of fresh water. From April, vertical mixing along with vertical advection increase the sea surface salinity; hence, the barrier-layer thickness decreases and reaches its minimum in July. During the rest of the year, values for the barrier-layer thickness are again high, mostly under the influence of the Niger River discharge and precipitation, with the highest values recorded in October, when the river discharge and precipitation are at a maximum.

Abstract. The impact of boreal spring intraseasonal wind bursts on sea surface temperature variability in the eastern tropical Atlantic Ocean in 2005 and 2006 is investigated using numerical simulation and observations. We especially focus on the coastal region east of 5° E and between the Equator and 7° S that has not been studied in detail so far. For both years, the southerly wind anomalies induced cooling episodes through (i) upwelling processes, (ii) vertical mixing due to the vertical shear of the current, and for some particular events (iii) a decrease in incoming surface shortwave radiation. The strength of the cooling episodes was modulated by subsurface conditions affected by the arrival of Kelvin waves from the west influencing the depth of the thermocline. Once impinging the eastern boundary, the Kelvin waves excited westward-propagating Rossby waves, which combined with the effect of enhanced westward surface currents contributed to the westward extension of the cold water. A particularly strong wind event occurred in mid-May 2005 and caused an anomalous strong cooling off Cape Lopez and in the whole eastern tropical Atlantic Ocean. From the analysis of oceanic and atmospheric conditions during this particular event, it appears that anomalously strong boreal spring wind strengthening associated with anomalously strong Hadley cell activity prematurely triggered the onset of coastal rainfall in the northern Gulf of Guinea, making it the earliest over the 1998–2008 period. No similar atmospheric conditions were observed in May over the 1998–2008 period. It is also found that the anomalous oceanic and atmospheric conditions associated with the event exerted a strong influence on rainfall off northeast Brazil. This study highlights the different processes through which the wind power from the South Atlantic is brought to the ocean in the Gulf of Guinea and emphasizes the need to further document and monitor the South Atlantic region.