J. Jouanno

Seasonal heat balance in the upper 100 m of the equatorial Atlantic Ocean

The variability of sea surface temperature (SST) in the equatorial Atlantic is characterized by strong cooling in May-June and a secondary cooling in November-December. A numerical simulation of the tropical Atlantic is used to diagnose the different contributions to the temperature tendencies in the upper ocean. Right at the equator, the coolest temperatures are observed between 20°W and 10°W due to enhanced turbulent heat flux in the center of the basin. This results from a strong vertical shear at the upper bound of the Equatorial Undercurrent (EUC). Cooling through vertical mixing exhibits a semiannual cycle with two peaks of comparable intensity. During the first peak, in May-June, vertical mixing drives the SST while during the second peak, in November-December, the strong heating due to air-sea fluxes leads to much weaker effective cooling than during boreal summer. Seasonal cooling events are closely linked to the enhancement of the vertical shear just above the core of the EUC, which appears to be not driven directly by the strength of the EUC but by the strength and the direction of the surface current. The vertical shear is maximum when the northern branch of the South Equatorial Current is intense. The surface cooling in the eastern equatorial Atlantic is not as marked as in the center of the basin. Mean thermocline and EUC rise eastward, but a strong stratification, caused by the presence of warm and low-saline surface waters, limits the vertical mixing to the upper 20 m and disconnects the surface from subsurface dynamics.

Importance of the Equatorial Undercurrent on the sea surface salinity in the eastern equatorial Atlantic in boreal spring

The physical processes implied in the sea surface salinity (SSS) increase in the equatorial Atlantic Cold Tongue (ACT) region during boreal spring and the lag observed between boreal spring SSS maximum and sea surface temperature (SST) summer minimum are examined using mixed-layer salinity budgets computed from observations and model during the period 2010–2012. The boreal spring SSS maximum is mainly explained by an upward flux of high salinity originating from the core of the Equatorial Undercurrent (EUC) through vertical mixing and advection. The vertical mixing contribution to the mixed-layer salt budget peaks in April–May. It is controlled primarily by (i) an increased zonal shear between the surface South Equatorial Current and the subsurface EUC and (ii) the presence of a strong salinity stratification at the mixed-layer base from December to May. This haline stratification that is due to both high precipitations below the Inter Tropical Convergence Zone and zonal advection of low-salinity water from the Gulf of Guinea explains largely the seasonal cycle of the vertical advection contribution to the mixed-layer salt budget. In the ACT region, the SST reaches its maximum in March/April and minimum in July/August. This SST minimum appears 1 month after the maximum of SSS. The 1 month lag observed between the maximum of SSS in June and the minimum of SST in July is explained by the shallowing of the EUC salinity core in June, then the weakening/erosion of the EUC in June–July which dramatically reduces the lateral subsurface supply of high-saline waters.

Potential of video cameras in assessing event and seasonal coastline behaviour: Grand Popo, Benin (Gulf of Guinea)

ABSTRACT Abessolo Ondoa, G.; Almar, R.; Kestenare, E.; Bahini, A.; Houngue, G-H.; Jouanno, J.; Du Penhoat, Y.; Castelle, B.; Melet, A.; Meyssignac, B.; Anthony E.; Laibi, R.; Alory, G., and Ranasinghe R., 2016. Potential of video cameras in assessing event and seasonal coastline behaviour: a case study at Grand Popo, Benin (Gulf of Guinea). In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No.75, pp. 442–446. Coconut Creek (Florida), ISSN 0749-0208. In this study, we explore the potential of a nearshore video system to obtain a long-term estimation of coastal variables (shoreline, beach slope, sea level elevation and wave forcing) at Grand Popo beach, Benin, West Africa, from March 2013 to February 2015. We first present a validation of the video system with field data over a 10-day experiment conducted on Grand Popo beach in 2014. Secondly, 2-years daily and monthly timeseries are extracted and their variability is described as a function of regional forcing and climatic modes. All variables show large monthly variability. The longshore sediment transport estimated locally from video is in agreement with that derived from Era-Interim wave data re-analyses. Results show that the shoreline responds predominantly to tides at the event scale and to waves. Overall, this study suggests that video stations are efficient tools to monitor coastal processes over the long term, in complement with other conventional approaches. Although no clear conclusions can be drawn on inter-annual variability, the results show that it is important to build up extended coastal observation networks to address coastline changes over a wide range of scales.

Modification of sea surface temperature by chlorophyll concentration in the Atlantic upwelling systems

The influence of the chlorophyll on the upper Tropical Atlantic ocean is investigated with long term (1979-2012) regional oceanic simulations with 1/4° horizontal resolution based on the NEMO3.6 model. The model solar radiation penetration scheme depends on the chlorophyll concentration. Simulations with time and spatially varying concentrations obtained from satellite ocean color observations are compared with a simulation forced with constant chlorophyll concentration of 0.05 mg m−3, representative of chlorophyll depleted waters. Results indicate that regions of the Tropical Atlantic with chlorophyll concentrations larger than in the reference simulation (i.e. [chl] > 0.05 mg m−3) get warmer at the surface, with the exception of the main upwelling regions where high chlorophyll concentrations are associated with a significant cooling of the sea surface (∼1°C in the Benguela upwelling). The analysis of the model heat balance shows that the biological differential heating causes negative temperature anomalies in subsurface source waters prior to their upwelling at the coast. The shallow mixed-layer in the eastern equatorial and tropical Atlantic favors the persistence of these subsurface anomalies and may explain why the Benguela is particularly sensitive to the biological differential heating. In spite of the presence of high chlorophyll concentrations in the upwelling regions, both the larger amount of shortwave radiation captured in the surface layers and the modifications of the horizontal and vertical advection at the coast are found to play a secondary role in the SST change in the upwelling region.

Sensitivity of Loop Current metrics and eddy detachments to different model configurations: The impact of topography and Caribbean perturbations

The dynamics of the Loop Current (LC) and the release of its anticyclonic eddy (Loop Current eddy, LCE) are some of the most important features of the circulation in the Gulf of Mexico (GoM) and key aspects to gauge the validity of numerical simulations. Using a numerical model, we investigate the sensitivity of the LC and LCE detachments to three different mechanisms deemed to be relevant to their behavior: a ) suppression of Caribbean vorticity perturbations entering the GoM; b ) smoothness of the topography, and c ) suppression of a deep canyon on the eastern Campeche Bank. The main results of these experiments in comparison to a reference run considered to be the more realistic one are: a. Suppression of Caribbean eddies reduces the number of LCE separations, but they are not the principal mechanism that triggers the separations. Locally generated instabilities over the northeastern Campeche Bank and the LC northward extension, appear to be the controlling factors. b. Smoothing the topography generates a wider and less intense LC and reduces the energy exchange terms related to flow instabilities. Nevertheless, the number of LCE separations is similar to the reference experiment. Extension of the LC controls the shedding that, in this case, tends to occur in the summer-fall season, when the LC is more extended, and the Yucatan transport abruptly weakens after its seasonal maximum. c. Removing the deep canyon in the eastern Campeche Bank, makes the LC extension more stable and reduces the number of LCE separations. The canyon appears to play an important role in spinning up cyclones generated over the LC eastern front that finally leads to an LCE release. The seasonal distribution of LCE separations in the experiments does not appear to be controlled by the strength of the barotropic and baroclinic instability source terms. Instead, a necessary but not sufficient condition for LCE separations is that the LC extends beyond 24o N. Our results indicate that caution should be exercised when interpreting LC statistics from a single numerical configuration.

Partitioning of the Open Waters of the Gulf of Mexico Based on the Seasonal and Interannual Variability of Chlorophyll Concentration

The seasonal and interannual variability of chlorophyll in the Gulf of Mexico open waters is studied using a three‐dimensional coupled physical‐biogeochemical model. A 5 years hindcast driven by realistic open‐boundary conditions, atmospheric forcings, and freshwater discharges from rivers is performed. The use of recent in situ observations allowed an in‐depth evaluation of the model nutrient and chlorophyll seasonal distributions, including the chlorophyll vertical structure. We find that different chlorophyll patterns of temporal variability coexist in the deep basin which thereby cannot be considered as a homogeneous region with respect to chlorophyll dynamics. A partitioning of the Gulf of Mexico open waters based on the winter chlorophyll concentration increase is then proposed. This partition is basically explained by the amount of nutrients injected into the euphotic layer which is highly constrained by the dynamic of the winter mixed layer. The seasonal and interannual variability appears to be affected by the variability of atmospheric fluxes and mesoscale dynamics (Loop Current eddies in particular). Finally, estimates of primary production in the deep basin are provided.