A Lagrangian investigation of vertical turbulent heat fluxes in the upper ocean during Tropical Ocean–Global Atmosphere/Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE)

Abstract

We investigated the diurnal cycle of the near‐surface ocean temperatures in the western Pacific Warm Pool, using temperature data from drifters with 20 m thermistor chains, during the Coupled Ocean‐Atmosphere Response Experiment (COARE) Intensive Observation Period (December 1992 to February 1993). We estimated turbulent heat fluxes at 20 m as a residual in the hourly heat budget along each drifter trajectory using hourly surface fluxes. Fortnightly averaged turbulent heat fluxes at 20 m were upward during periods of sustained winds, indicating a warming of the daily thermocline by the deeper layers. During calm periods the inferred turbulent heat fluxes were downward. We also calculated the day‐to‐night differences of the turbulent heat fluxes at 20 m as a residual in the budget of the daily cycle of heat content in the upper 20 m. This method largely avoids the uncertainties of surface turbulent heat fluxes. The residual averaged over the 2 month period was negative for the drifters that experienced strong westerly wind conditions (with heat fluxes positive downward). These surprising results for the warm pool region are at odds with what was found earlier from moored measurements in the central Pacific. Interestingly, however, periods characterized by a westerly wind burst during December 1992 to March 1993 contrasted with the periods with weaker winds: during westerly wind bursts the difference was negative, whereas it was positive during calm or warming periods. This is consistent with our analysis of the daily budget and also with other studies in the COARE domain. It confirms that the nighttime mixed layer was usually much deeper than 20 m during this period. However, our estimates are different for drifters deployed north of 1°N than for drifters near or south of the equator, indicating marked spatial variability in the heat budget of the warm pool.