Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

Abstract

[1] Eighteen cross-equatorial shipboard current profiling sections along 10°W with conductivity-temperature-depth measurements taken between 1997 and 2007 are used to analyze the mean meridional structure and the seasonal variability of the Equatorial Undercurrent (EUC) at 10°W. Our analysis suggests a seasonal cycle for the EUC transport at 10°W, with a well-defined annual harmonic and some indication of a semiannual component, with a first maximum in January and a second stronger maximum from June to September. The mean EUC transport at 10°W is estimated to be 12.1 Sv and, compared to earlier estimates farther in the west, at 35°W (20.9 Sv) and 26°W (13.8 Sv). The eastward flow transport exhibits a strong variability at 10°W (with a total range of transports from 7.1 Sv to more than 31.7 Sv). The seasonal amplitude of the eastward flow variability is ±8.9 Sv, from a minimum of 8.2 Sv in November to 25.9 Sv in August. The eastward flows within the thermocline are divided in two parts: a permanent part within the s q = 24.5-26.5 isopycnal layer, with a semiannual cycle, known as the EUC, and a nonpermanent part in the deep thermocline (beneath s q = 26.5 isopycnal), associated with a strong eastward transport (up to 15 Sv) during the boreal summer, that is not observed during the rest of the year. The current at the equator in the deep thermocline is even westward during boreal fall. The disappearance of the salinity core of the EUC during the boreal summer, associated with the upwelling of the hydrological structure at 10°W, reveals that the saline subtropical waters carried by the EUC within the thermocline no longer flow into the Gulf of Guinea during the boreal summer. Our data also show the presence of the South Equatorial Undercurrent (SEUC) at 10°W in the Gulf of Guinea with a strong latitudinal and depth variability throughout the year. The mean SEUC at 10°W is centered near 5°S and is farther south than observed at 35°W and 26°W, suggesting its poleward shift from west to east.

On the Nutrient distribution and phytoplankton biomass in the Gulf of Guinea equatorial band as inferred from In-situ measurements

“…Consistent with the findings of Oudot and Morin (1987), uplift of the pycnocline, which supplies the EUC with nutrients, plays an important role in the enrichment process within the equatorial band. The seasonal differences in various parameters revealed in our study are in agreement with previous findings regarding seasonal variability in the Gulf of Guinea, where the seasonal equatorial upwelling and thermocline lifting are more enhanced in June than in September (Voituriez and Herbland, 1979;Oudot and Morin, 1987;Kolodziejczyk et al, 2009). Also, in agreement with Jouanno et al (2011), it is shown here that the intensification of the northern branch of the SEC around 1°N in June 2005 enhanced the vertical shear just above the core of the EUC, causing vertical mixing and equatorial cooling to be stronger in June than in September.…”

“…The general track of the six EGEE cruises conducted between 2005 and 2007 is presented in Figure 1 (Kolodziejczyk et al, 2009(Kolodziejczyk et al, , 2014Nubi et al, 2014 for additional details). Seawater samples were collected in 11 or 22 (depending on the cruise/research vessel used) hydrological bottles attached to a rosette, together with a CTD-O2 (bathysonde) and ship-mounted acoustic Doppler current profiler (SADCP).…”

“…The CTD sensors, which were calibrated before and after the cruises, were accurate within 0.003°C (temperature) and 0.003 (salinity). The horizontal resolution of the CTD profiles along 10°W was generally of ½° of latitude, but ⅓° between 1°S and 1°N (Kolodziejczyk et al, 2009). Nitrate analysis (accurate to 0.1 µmol.kg -1 ) was conducted according to Benschneider and Robinson (1952), and phosphate analysis (accurate to ~0.2 µmol.kg -1 ) according to Murphy and Riley (1962); also, Baurand (2008) provides further detail about nutrient analysis.…”

Observation of the marine atmospheric boundary layer in the Gulf of Guinea during the 2006 boreal spring

“…Thanks to this programme, the oceanic mixed layer and upwelling processes are better known Caniaux et al, 2010;Wade et al, 2010), as well as the oceanic circulation there (Kolodziejczyk et al, 2009). However, the marine atmospheric boundary layer (MABL) processes remain poorly documented.…”

On the physical and biogeochemical processes driving the high frequency variability of CO2 fugacity at 6°S, 10°W: Potential role of the internal waves

“…Given the small amount of data in the region, it is not possible to precisely validate these values, however their orders of magnitude are reasonably consistent with existing climatologies [Key et al, 2004;Kolodziejczyk et al, 2009;Garcia et al, 2014].…”