New insights on the upper layer circulation north of the Gulf of Guinea

Abstract

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.

Impact of intraseasonal wind bursts on sea surface temperature variability in the far eastern tropical Atlantic Ocean during boreal spring 2005 and 2006: focus on the mid-May 2005 event

“…These results obtained for the years 2005 and 2006 years do not imply that same atmospheric conditions would be observed for winter upwelling Kelvin waves of other years. The year 2005 was very particular and also exhibited anomalously cold SSTs in the South Atlantic and anomalously warm SSTs in the North Atlantic initiated in fall 2004, the signature of a meridional mode (Virmani and Weisberg, 2006;Foltz and McPhaden, 2006;Hormann and Brandt, 2009).…”

“…The numerical model used in this paper is the Regional Oceanic Modeling System (ROMS) (Shchepetkin and McWilliams, 2005). The model configuration is the same as employed in Herbert et al (2016), and the following text is derived from there with minor modifications.…”

“…The large-scale domain extends from 60 • W to 15.3 • E and from 17 • S to 8 • N, and the nested high-resolution zoom focuses on the domain between 17 • S and 6.6 • N and between 10 • W and 14.1 • E. This configuration allows for equatorial Kelvin waves induced by trade wind variations in the western part of the basin to propagate into the Gulf of Guinea and influence the coastal upwelling (Servain et al, 1982;Picaut, 1983). The horizontal grid resolution is 1/5 • (i.e., 22 km) for the parent grid and 1/15 • (i.e., 7 km) for the child grid (see Herbert et al (2016), their Fig. 1).…”

“…Model analyses are based on the 2-day averaged model outputs from the years 1998 to 2008. The model has already been validated suc-cessfully with a large set of measurements and climatological data, and more detailed information about the model validations can be found in Herbert et al (2016).…”

PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting

“…Closer to the coast in the northern part of the Gulf of Guinea, the existence of the Guiana Undercurrent has been revealed. This eastward current, which is not directly supplied out of the North EUC, flows at subsurface north of the main path of the surface Guiana Current (Herbert et al, ). Further to the west at 23°W, the large number of shipboard velocity sections allowed the estimation of a robust mean flow characterized by an eastward surface flow between about 4°N and 9°N including the NECC and its northern branch (Brandt et al, ; Urbano et al, ).…”