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um hexafluorophosphate solut~ons, using freshly dstilled tetrahydrofuran or methyene chloride. A soutons were prepared under an atmosphere of nitrogen and degassed completely before injecton into the SEC cell. Blank reference solut~ons contalnng 0.1 M tetra-n-butyl ammonlum hexafluorophosphate were used for the Fourer transform R solvent subtractons. A Princeton Appl~ed Research (PAR) model 175 Unversal Programmer w~th a PAR model 176 Current Follower were used to effect and monitor thn-ayer bulk eectroyses. The IR spectra were acquired wth a Mattson Research Seres Fourier transform R equipped w~th a MCT (mercurycadmium-telluride) detector. 23 R. E. W t t r g and C P. Kubak, J. Electroanal. Chem. 393. 75 (1995). 24 F -W. Grevels et a / . Angew. Chem int. Ed. Engi. 26.885 (1 987) 25. H. L. Strauss, J Am. Chem. Soc. 11 4, 905 (1 9921, 26 J J. Turner. C:'M. Gordon. S. M. Howde, J. Phys. Chem. 99. 17532 (1 995).

[1] In the present study, the physical processes that control the seasonal cycle of sea surface temperature in the tropical Atlantic Ocean are investigated. A high-resolution ocean general circulation model is used to diagnose the various contributions to the mixed layer heat budget. The simulation reproduces the main features of the circulation and thermal structure of the tropical Atlantic. A close examination of the mixed layer heat budget is then undertaken. At a first order, the mixed layer temperature balance in the equatorial band results from cooling by vertical processes and heating by atmospheric heat fluxes and eddies (mainly tropical instability waves). Cooling by subsurface processes is the strongest in June-August, when easterlies are strong, with a second maximum in December. Heating by the atmosphere is maximum in February-March and SeptemberOctober, whereas eddies are most active in boreal summer. Unlike previous observational studies, horizontal advection by low-frequency currents plays here only a minor role in the heat budget. Off equator, the sea surface temperature variability is mainly governed by atmospheric forcing all year long, except in the northeastern part of the basin where strong eddies generated at the location of the thermal front significantly contribute to the heat budget in boreal summer. Finally, comparisons with previously published heat budgets calculated from observations show good qualitative agreement, except that subsurface processes dominate the cooling over zonal advection in the present study.

[1] The variability of sea surface temperature (SST) in the equatorial Atlantic is characterized by strong cooling in May-June and a secondary cooling in NovemberDecember. 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.

International audienceWe investigate the causes of the seasonal cycle of the near-surface salinity using a mixed-layer salinity model and a combination of satellite products, atmospheric reanalyses, and in situ observations for the period 2000-2008, in the tropical Atlantic Ocean. We find that the balance differs from one region to another. In the western tropical Atlantic, it is controlled by horizontal advection from March to November and by freshwater flux and entrainment for the rest of the year. In the central tropical Atlantic, it is mainly due to the strong contribution of precipitation in agreement with previous results. In the northeastern tropical Atlantic, all terms contribute to the mixed layer salinity between December and March; during the rest of the year, precipitation and zonal advection mainly control the balance. In the Gulf of Guinea, it is driven by freshwater flux from October to February; from March to July, it is controlled by horizontal advection and entrainment; from August to September, mixed-layer salinity variability is weak. Finally, in the Congo region, it is driven by freshwater flux (precipitation and runoff from Congo River) from September to December, by horizontal advection during January to March, and by vertical entrainment during the rest of the year (April to August). There are some discrepancies between observed and modeled salinity tendencies. Some of them are due to our model formulation, which does not explicitly account for the effect of vertical diffusion. Uncertainties of observation products, which force the model, are also sources of errors

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We examine the variability of the sea surface temperatures in the eastern equatorial Atlantic during 2005–2007 by using Argo profiling floats, Prediction and Research Moored Array in the Atlantic buoys and satellite, in situ, and atmospheric data sets. The eastern equatorial Atlantic, characterized by shallow mixed layers all yearlong, is divided into nine boxes of nearly equal surface area, with respect to the dynamics and thermodynamics in this region. Monthly mixed layer heat budgets are computed in each box from 10 day Argo profiles. In all the boxes, the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years. The amount of short‐wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability, while entrainment has a weaker contribution. To balance the heat budget, a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms. This residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle, with minimum (negative) values in boreal summer and maximum values in winter. This residual compares well with available observations of vertical turbulent mixing collected during Etude de la Circulation Océanique et des Échanges Océan‐Atmosphère dans le Golfe de Guinee campaigns (2005–2007) in the eastern equatorial Atlantic. When assuming that the residual is mostly associated with vertical turbulent mixing, it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator.

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