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F. Vauclair, Y. du Penhoat, and G. Reverdin

Abstract

The mass and heat budgets of the warm upper-ocean layer are investigated in the equatorial Atlantic using in situ observations during the period 1979–99, which encompassed a series of warm events in the equatorial Atlantic. The warm water layer is defined as the layer having an in situ temperature higher than 20°C, which is within the core of the equatorial thermocline. The geostrophic transport is calculated by combining gridded temperatures with historical salinity data. The Ekman transport is estimated from observed wind data or model- based wind products. The change in warm water volume is then compared with the horizontal mass convergence, and the residuals are determined. The heat budget of the upper layer is investigated in the same way. Three regions are considered: the equatorial band between 8°N and 8°S to study the meridional redistribution of the warm water and two boxes (western and eastern boxes) to investigate the zonal redistribution of the warm water. Mass and heat budget variability in the equatorial band is discussed in relation to the zonal wind variability. The authors discovered that during the development of an equatorial warm event the meridional net divergence first decreases, reaching its minimum as the warm event matures. Meridional divergence increases again as conditions become normal in the equatorial band. The vertical velocity through the 20°C isotherm also reveals variations consistent with this scenario. Cross-isotherm mass transport decreases during warm events. The heat budget residual is more difficult to interpret. The average value is consistent with heat loss through turbulent mixing at the base (20° isotherm), but the fluctuations are most likely noise, resulting mainly from the limited accuracy of the model surface heat fluxes used.

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G. Reverdin, P. Rual, Y. Du Penhoat, and Y. Gouriou

Abstract

A set of temperature profiles from expendable bathythermographs collected from 1980 to April 1988 along two ship routes transecting the equatorial Atlantic from 11°N to 11°S is analyzed to inter the vertical structure of the annual variability of the temperature and the currents in the upper ocean.

During the average seasonal cycle, the vertical isotherm displacements occur earlier below 300 meters than near the surface at most locations within 4 degrees of the equator. At the equator the amplitude of the displacements does not decrease with depth in the upper 500 meters. This still holds down to 700 meters, but there are less data at these depths. The lead of the deeper isotherm displacements with respect to those in the upper thermocline implies that there is a contribution to the pressure forces from these layers that is not in phase with the contribution of the upper thermocline. This also suggests that the energy source of the seasonal variability is close to the surface. Dynamic height and geostrophic currents relative to 400 db are also estimated. A seasonal cycle is found on the subsurface currents, which vary by up to a factor two during the cycle.

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