Monitoring the 1979‐1985 equatorial Pacific current transports with expendable bathythermograph data

Abstract

Geostrophic transports of the major equatorial currents, including the Equatorial Undercurrent and the South Equatorial Current, have been estimated on a bimonthly basis in the central and western Pacific over the 1979‐1985 period. This was made possible by a collection of carefully checked expendable bathythermograph (XBT) data and a few hydrocast and conductivity‐temperature‐depth data. This collection considerably improved the number and quality of temperature profiles of the original Institut Français de Recherche Scientifique pour le Développement en Coopération (ORSTOM)‐Scripps Institute of Oceanography tropical Pacific XBT Ship‐of‐Opportunity Program. The temperature profiles were first grouped, independently of longitude, in regions around mean XBT tracks. Dynamic heights were then calculated relative to 400 dB through a T‐S relationship deduced from the climatological Levitus file. Geostrophic currents were calculated along the roughly south‐north oriented Fiji‐Hawaii and New Caledonia‐Japan mean XBT tracks, using a specific technique. This technique, which combines Fourier filtering in the meridional direction and in time, also ensures the continuity across the equator between geostrophic currents deduced from the first and second derivatives of the meridional pressure field. The filter coefficients in the meridional direction were determined through an adjustment of geostrophic currents to directly measured currents, using the mean of data collected along 165°E during six Tropical Ocean and Global Atmosphere (TOGA)‐ORSTOM cruises. Finally, transports of the main equatorial currents were computed in a subjective way between 20°N and 20°S, 0–400 m using monthly charts of geostrophic currents along the two mean XBT tracks. The transports in the central Pacific were compared to the transports derived from the semicontinuous direct current measurements taken during the Hawaii‐Tahiti Shuttle Experiment in 1979‐1980 and during the Line Island Profiling Project‐Pacific Equatorial Ocean Dynamics Experiment in 1982‐1983. Reasonable agreement demonstrates that it is possible to use XBT data for transport indexes of the major equatorial currents in the central and western Pacific.