The diurnal mixed layer and upper ocean heat budget in the western equatorial Pacific

This paper presents the results of an experiment in the western equatorial Pacific centered on the equator at 165°E which was designed to study the changes to the structure of the upper ocean on timescales of a few days and spatial scales of tens of kilometers. The results show that the response of the upper ocean to atmospheric forcing is very sensitive to the vertical structure of both the temperature and salinity. The diurnal response of the near-surface temperature to daytime heating and nighttime cooling was found to have an amplitude of a few tenths of a degree Celsius, This compares with a horizontal variation of temperature on scales of a few tens of kilometers of a similar magnitude. Even away from the very fresh surface layers typical of the area, salinity is found to play an important role in limiting the depth of nighttime mixing. In this case a subsurface salinity maximum restricts the depth to around 40 m. The nighttime convection is severely limited by either a small change in the surface forcing or the horizontal advection of slightly cooler waters from the east; we are unable to determine which is the dominant mechanism in the present case. The reduced mixing leads to an increase of the diurnal variation of sea surface temperature to over 1°C. The estimated net surface heat flux from the atmosphere to the ocean was found to be not significantly different from zero at 10 W m?2, in agreement with recent measurements. The net surface heat flux during the period of the heat budget experiment, which took place on the equator, was substantially higher at 65 W m?2. Changes of in situ temperature are found to be dominated by advection. The vertical velocity is estimated to be of order 10 m d?1 and to be caused by advection along east-west sloping density surfaces. Changes to the temperature structure of the upper ocean induced by motions with a timescale of a few days (possibly planetary waves) are found to be significantly greater than longer-term wind-induced upwelling or advectio