Observations and wind-forced model simulations of the mean seasonal cycle in tropical Pacific sea surface topography

We examine simulations of the mean seasonal cycle in the tropical Pacific using a multiple vertical mode linear numerical model forced with three different surface wind stress products average over the period 1979-1981. The model is run to equilibrium for each four vertical modes, and results are summed. Simulated mean seasonal cycles in dynamic height and sea level are then compared with observed variations based on expendable bathythermograph and island tide gauge data averaged over the same 1979-1981 period. All simulations show characteristic features of the mean meridional ridge-through structure in surface topography. However, north and south equatorial ridges at 20°N and 20°S are much higher than those observed, only weak equatorial ridges are generated near 4°N, and none of the simulations exhibits a significant equatorial trough. These discrepancies are due principally to limitations in model physics in the wind forcing. Observed and modeled mean seasonal variations in surface height are of the order of a few centimeters

Use of the geostrophic approximation to estimate time-varying zonal currents at the Equator

Moored thermistor chains at 2°N and 2°S and current-temperature moorings at 0° are used to examine the accuracy of geostrophically estimated zonal velocity on the equator in the eastern (110°W) and werstern (165°E) Pacific. The meridionally differentiated form of the geostrophic balance is used to eliminate large errors due to wind-balanced cross-equatorial pressure gradients. Statistical analyses indicate that for time scales longer then 30-50 days, the observed and geostrophically estimated zonal velocities are similar (correlation coefficients of O.6-0.9 and comparable amplitudes). Thus low-frequency equatorial current oscillations are reasonably well represented by the geostrophic approximation. However, the mean currents are poorly resolved with the available array. In the eastern Pacific the mean zonal speed difference over the 10 month comparison period is 25 cm s-1 at 25 m and increases to 60 cm s-1 at 125 m. At 165°E mean differences in the upper 250 m are tipically 50 cm s-1 over a 4-month record

On the sensitivity of Sverdrup transport estimates to the specification of wind stress forcing in the Tropical Pacific

We use Sverdrup dynamics to estimate geostrophic transports between 20°N and 20°S in the Tropical Pacific Ocean averaged over the period 1979-1981. Three wind stress products are used to force the model. Results are compared to geostrophic transports computed along expendable bathythermograph transects in the Western, Central, and Eastern Pacific for the same period. Depending on the choice of wind stress, modeled transports may differ from the observations by a factor of 2 and, in some cases, flow is opposite to that observed. Possible limitations of the Sverdrup theory are discussed; however, we conclude that detailed and accurate simulation of the general circulation in the Tropical Pacific is limited more by the uncertainties in presently available estimates of the surface wind stresses than by deviations from Sverdrup balance. (Résumé d'auteur

Atlas des températures et des courants géostrophiques de 1979 à 1985 déduits des mesures XBT le long des rails de navigation du Pacifique tropical

Dans le Pacifique tropical, la concentration de données du réseau XBT (expendable bathythermograph) ORSTOM-SIO (Scripps Institutio of Oceanography) autour de trois rails de navigation a permis de construire des champs de températures et de courants géostrophiques. Afin d'obtenir des champs de températures les plus précis possible, le fichier original ORSTOM-SIO a été tout d'abord triplé par la récupération d'un maximum de données XBT et de quelques données hydrologiques et de sondes CTD (conductivité, température, profondeur) auprès de différents services océanographiques. Les champs de températures à trois dimensions (latitude, profondeur, temps) ont été obtenu en regroupant, indépendamment de la longitude, les données de profils thermiques autour de trois rails moyens situés dans les zones ouest, centre et est du Pacifique tropical. Ces champs s'étendent en latitude de 20°S à 20°N pour les zones ouest et centre et de 20°S à 7°N pour la zone est, en profondeur de 0 à 4OO m et en temps de janvier 1979 à décembre 198

Evidence of remote forcing in the Equatorial Atlantic ocean

An analysis of sea-surface temperature (STT) and surface winds in selected areas of the Tropical Atlantic indicates that the nonseasonal variability of SST in the Eastern Equatorial Atlantic (Gulf of Guinea) is highly correlated with the nonseasonal variability of the zonal wind stress in the Western Equatorial Atlantic. A negative (positive) anomaly of the zonal wind stress near the North Brazilian coast is followed by a positive (negative) SST anomaly in the Gulf of Guinea about one month later. Furthermore, the correlation betweenthe local winds stress anomaly and STT anomaly in the Gulf of Guinea is considerably smaller. These preliminary results indicate that remote forcing in the Western Equatorial Atlantic ocean is an important factor affecting the Eastern Equatorial Atlantic sea-surface temperature. Recent equatorial theories are consistent with these observations. (Résumé d'auteur