Observation of large diurnal warming events in the near-surface layer of the western equatorial Pacific warm pool

Abstract

Because of the relatively calm winds which prevail over the western Pacific warm pool, the diurnal cycle of temperature in the near-surface layer of the ocean is often quite pronounced. During the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE), very high resolution measurements of near-surface thermohaline and turbulence structures were made using bowmounted probes and a free-rising profiler. Experimental data demonstrate a strong dependence of near-surface thermal structure on weather conditions, In calm weather, SST was observed to exceed 33.25°C; this was associated with a diurnal warming of more than 3°C in the top I m of the ocean. A 1-D model of transilient type reproduces the diurnal cycle at low wind speeds and the evening deepening of the diurnal thermocline. Precipitation influenced the diurnal cycle by trapping heat in the near-surface region. During daytime evaporation, surface salinity increased slightly, but deep convection was inhibited by the strong vertical temperature gradient. Contour plots calculated using observations from bow sensors “scanning” the upper meters of the ocean due to ship's pitching in some cases revealed strong horizontal variability of the shallow diurnal thermocline with amplitude ∼ 2°C on scales of 0.2–6 km.