The changing structure of the upper ocean in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment

Abstract

As part of the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) the French vessel Le Noroit completed a series of 18 sections from 5°S to 5°N on 156.5°E, using a towed undulating CTD (SeaSoar) and a hull‐mounted acoustic doppler current profiler (ADCP). Here we present results from the cruise. We describe both the mean structures and the response of the upper ocean to changing meteorological conditions in terms of changes in the dynamic and thermodynamic structure over a 10° latitude band and over a period of approximately 70 days. The mean thermal and haline structures were consistent with the ongoing El Niño conditions of 1992–1993. Only between 1°N and 3°N did salinity play a greater role than that of temperature in the mean stratification of the upper 80 m. The zonal currents were weak in the mean but highly variable, correlating significantly in the 0–60 m depth range with the zonal winds. We show that a cyclonic, cold core structure, identified at 2°S around January 9, will heavily influence any heat budget performed there. Meteorological conditions varied dramatically during the experiment, with strong westerlies, strong easterlies, and periods of little wind observed. Throughout these conditions and at all latitudes a diurnal thermal cycle was seen. The maximum depth of the diurnal thermocline varied from 30 to 35 m, being shallower at more southerly latitudes. The near‐surface temperature showed a mean diurnal signal of approximately 0.4°C. The halocline depth also exhibited a diurnal signal of around 17 m, suggesting that salinity stratification plays a role in limiting vertical mixing at all latitudes, and more so in northern latitudes. Thicker barrier layers were found north of the equator; we show that barrier layers thicker than approximately 25 m correspond to significantly reduced entrainment cooling of the mixed layer (0(0.1°C)). The observed spatial and temporal variability is discussed in the context of the analyses made by others within the intensive flux array near 2°S.