Hindcasts of Equatorial sea surface dynamic height in the Atlantic in 1982-1984

Two different wind data sets of the years 1982-1984 are used to force in linear multimode model of the Tropical Atlantic ocean. We describe the variations of the Equatorial slope in both cases and investigate the differences betwenthe two runs. The main discrepancy occurs in 1983 for the amplitude of the seasonal signal all along the equator. Models results are also compared with observations (hydrocasts and inverted echo sounders) taken during the Programme Français Océan et Climat dans l'Atlantique Equatorial/Seasonal Response of the Equatorial Atlantic (FOCAL/SEQUAL) field experiment (july 1982 throught october 1984). The contrast between 1983 and 1984 is well captured by the two runs, especially the abnormal flat topography of the sea surface along the equator in the first 3 months of 1984 due to an unusual basin wide relaxation of the Equatorial wind stress. The major discrepancy between model results and observations is found during the upwelling periods east of 10°W, but at that time dynamic height measurements are too sparse to give a good description of these events. (Résumé d'auteur

Modeled surface dynamic height in 1964-1984 : an effort to assess how well the low frequencies in the Equatorial Atlantic were sampled in 1982-1984

A wind-forced linear model has been used to produce 21 years (1964--1984) of monthly time series of surface dynamic height in the Equatorial Atlantic. Theclimatological seasonal cycle is substracted, and the statistical characteristics of the residuals are analyzed. An empirical orthogonal function analysis reveals that the most significant pattern has deviations of one sign in the Western Equatorial Atlantic. The anomalies in the last 2 years, 1983 and 1984, are particularly large. The time component of the first empirical orthogonal function peaks in july 1983, changes sign at the end of 1983, and has an extremun of the opposite sign in April 1984. At that time, the zonal slope of dynamic height had reversed with respect to normal along the equator. (D'aprèsrésumé d'auteur

Sea surface dynamic height topography and the North Equatorial countercurrent as inferred from a linear model

Hellerman and Rosenstein's (1983) climatological winds are used to force a linear, continuously stratified model of the tropical Atlantic. The dynamic height topography relative to 500 meters is compared with available data. Special attention is paid to the computed North Equatorial countercurrent. We conclude that most of the observed seasonal variations can be explained using simple linear theory. (Résumé d'auteur

The effect of islands on low frequency equatorial motions

We develop a complete linear theory for the effects of islands on low frequency waves of the kind thought to be important in the seasonal and interannual variations of the Equatorial ocean circulation. For an island whose meridional extent is small compared to R, the Equatorial radius of deformation, the waves pass the island almost undisturbed : the current incident on the island flow around it to the north and south and continue downstream in the lee of the island. For islands comparable to or greater in extent than R the mass fluxincident on the island is largely reflected in a manner similar to the case of a meridionally infinite barrier. An incident Equatorial Kelvin wave is reflected as long Rossby waves; symetric long Rossby waves are reflected as Equatorial Kelvin waves while antisymetric ones stop at the island barrier. In allcases a boundary current composed a short Rossby waves forms at the Eastern side of the island and accomplishes the required meridional redistribution of the zonal mass flux. Calculations carried out for the major mid-ocean low latitude islands (The Gilberts and Galapagos in the Pacific; Sao Tome in the Atlantic; the Maldives in the Indian Ocean) show that the propagation of long waves will not be significantly affected by any of these islands and that smallperturbations occur only in their immediate vicinity. (Résumé d'auteur

Effect of low latitude Western boundary gaps on the reflection of Equatorial motions

The Western Tropical Pacific is thought to be an important zone for generating El Nino : reflections at the boundary make it a potential source region of Equatorial Kelvin waves. Calculations of the effect of a gappy Western boundary on the reflection process are carried out in the framework of the low frequency limit of the shallow water equations and are highly idealized. The method is also applied to a schematic version of the flow through the Indonesian seas from the Western Pacific to the Indian oceans. The results indicate somestrong sensitivities to the location of the gap and to the structure of the incoming flows. In addition, the results can be quite different, depending on whether the zonal extend of the gap is assumed to be infinite or finite. (More precisely, the latter means that the extend of the gap is short compared with the zonal wavelength of the relevant free waves at that frequency). In view of the complexity of the results for even such a simplified model, it will be very difficult to be confident of any modelling study of the Indonesian throughflow short of a highly resolved numerical calculation with a detailed representation of the geometry and bathymetr

Vertical structure of the seasonal cycle in the Central Equatorial Atlantic ocean : XBT sections from 1980 to 1988

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 infer 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 current 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. (Résumé d'auteur

Heat content displacement in the Pacific during the 1982-1983 El Nino event

Le contenu thermique intégré de la surface à 300 m de profondeur est calculé dans la partie tropicale de l'Océan Pacifique, en utilisant les données XBT recueillies de 1979 à 1985 le long des lignes de navigation qui coupent l'équateur à 160°E, 160°W et 100°W. L'analyse du contenu thermique montre que pendant El Nino 1982-1983, les parties nord et sud de l'océan tropical ne présentent pas la même évolution à 160°E et 160°W. Par contre, à 100°W, les variations du contenu thermique sont semblables de part et d'autre de l'équateur. Les variations de contenu thermique peuvent être interprétées comme résultant de l'influence d'ondes planétaires, ainsi que le suggèrent des corrélations croisées significatives. Une simulation à l'aide d'un modèle linéaire simple, forcé par des anomalies de vent provenant du Florida State University (FSU), est en accord avec l'évolution du contenu thermique dans l'hémisphère Sud et dans la zone équatoriale mais pas dans l'hémisphère Nord. (Résumé d'auteur

Objective analysis of simulated Equatorial Atlantic ocean data on seasonal time scales

In this study we objectively analyze simulated Equatorial Atlantic ocean data on seasonal time scales using a technique based on optimal interpolation. The purpose is twofold : (1) to estimate the accuracy of the FOCAL/SEQUAL (Programme Français Océan-Climat en Atlantique Equatorial/Seasonal Equatorial Atlantic Response Program) array for mapping large-scale seasonal variations in the depth of the 20° isotherm, and (2) to examine the potential of 20 FOCAL drifting buoys drogued with thermistor chains for enhancing that mapping accuracy. This latter point leads to the development of heuristic model for drifter motion in order to identify the most favorable time and location for buoy deployments. Results are discussed for a number of assumptions about oceanic variability required by both the optimal interpolation procedure and the drifting buoy model. (D'après résumé d'auteur