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

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

Variability in Equatorial Pacific sea surface topography during the verification phase of the Topex/Poseidon mission

As part of the verification phase of the TOPEX/POSEIDON mission, 10-day gridded fields of altimeter data derived from TOPEX geophysical data records are compared with 10-day gridded fields of dynamic height derived from more than 60 moorings of the Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) array in the Equatorial Pacific ocean. Acess to TAO data in real time permits the first 500 days of the TOPEX/POSEIDON mission to be placed in the context of complementary, in situ measurements of surface winds, sea surface temperatures, and upper ocean thermal structure, as well as the time history of these variables prior to lauch. Analysis of the space-time structure in the TOPEX and TAO surface topography data indicates sea level variability primarily due to equatorial Kelvin wave activity generated by intense windbursts west of the date line in association with the 1991-1993 El Nino. Cross correlations between the two data sets are generally >0.7, with RMS differences 5 cm north of the equator in the Central and Eastern Pacific. (Résumé d'auteur

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

A model study of potential sampling errors due to data scatter around expendable bathythermograph transects in the Tropical Pacific

We describe a series of sampling sensitivy experiments to examine potential errors due to data scatter around expendable bathythermograph (XBT) transects in the tropical Pacific. We use a linear, multiple vertical mode model forced with three differrent monthly mean wind stress sets for the period 1979-1983. The model is sampled along approximately straight lines of grid points corresponding to the mean positions of XBT tracks in the eastern, central, and western Pacific and then sampled again at the dates and location of actual XBT casts for 1979-1983. Model dynamic heights are calculated with a resolution of 1° of latitude and 1 month, then processed to a monthly mean seasonal cycle and anomalies associated with the 1982-1983 El Nino. When results are compared for the two methods of sampling, the model indicates that data scattered zonally around XBT transects in general can lead to about 2 dyn cm error in dynamic height (equivalent to a 10-m error in model pycnocline displacement) in composite sections of XBT data

Effects of westerly wind bursts upon the Western Equatorial Pacific ocean, february-april 1991

In February-April 1991, episodes of 2 to 8 m s-1 westerly winds of 3 to 11 days' duration occurred in the Western Pacific warm pool. Resulting modifications of the upper ocean in current and hydrology are quantified using data from an equatorial mooring at 165°E and from three cruises within 30 days of one another along 165°E. During westerly wind bursts (WWB) stronger tha m s-1, the upper 50 m becomes isothermal to within O.1°C and sea surface temperature (SST) drops by 0.3.-0.4°C between 5°S and 2.5°N. Conversely, STT starts warming and the upper 50 m restratifies in 4-5 days after the end of WWB. In contrasts to previous observations, salinity between 0 and 50 m appears almost unaffected by WWB; it freshens by 0.4 practical salinity unit in March within an area of 1°-2° of latitude aroud the equator but not necessarily in direct response to WWB. As for zonal circulation, surface equatorial flow accelerates eastward jets both develop from 2°N to 2°S in the upper and lower halves of the temperature mixed layer, respectively. Changes in zonal mass transport in this layer were as much as 30 Sv between 2.5°S and 2.5°N from one cruise to the next. (Résumé d'auteur

Mechanism of the zonal displacements of the Pacific warm pool : implications for ENSO

The western equatorial Pacific warm pool is subject to strong east-west migrations on interannual time scales in phase with the Southern Oscillation Index. The dominance of surface zonal advection in this migration is demonstrated with four different current data sets and three ocean models. The eastward advection of warm and less saline water from the western Pacific together with the westward advection of cold and more saline water from the central-eastern Pacific induces a convergence of water masses at the eastern edge of the warm pool and a well-defined salinity front. The location of this convergence is zonally displaced in association with El Nino-La Nina wind-driven surface current variations. These advective processes and water-mass convergences have significant implications for understanding and simulating coupled ocean-atmosphere interactions associated with El Nino-Southern Oscillation (ENSO). (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

The response of the Western Equatorial Pacific ocean to Westerly wind bursts during november 1989 to january 1990

Several 5 to 10 m s-1 westerly wind bursts of 10-15 days' duration occurred in the Western Equatorial Pacific during november 1989 to January 1990. The response to these wind bursts was characterized by a 400- to 600 km wide eastward jet in the upper 100-150 m along the equator between 135°E and the date line. Flow in this jet accelarated to speeds of over 100 cm s-1 within 1 week after the onset of westerly winds in november 1989 in association with super thyphoon Irma. In addition, a 20 to 40 cm s-1 westward counterflow developed between 2°N and 2°S below the surface jet separating it from the eastward flow of the Equatorial undercurrent in the thermocline. Changes in surface layer zonal volume transport in the Western Pacific due to westerly wind bursts were 25-56 Sv based on comparison of three shipboard velocity transects in november and december 1989. Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer, the Equatorial undercurrent decelerated to less than 20 cm s-1 (i.e., less than half its speed before the onset of westerlies) by early december 1989. (D'après résumé d'auteur