Measurements of upper ocean heat and freshwater budgets near a drifting buoy in the equatorial Indian Ocean

R/V Franklin followed a drifter for 8 days in the equatorial Indian Ocean t o test the accuracy of mixed-layer heat and freshwater budget closure. Four -hourly triangles were repeated, towing a SeaSoar with a photometer and con ductivity-temperature-depth (CTD) profiler. Currents relative to the buoy a t one depth were obtained from (a) a current meter at 25 m below the buoy; (b) a Global Positioning System estimate; (c) a drag estimate. Acoustic Dop pler current profiler (ADCP) shears provided currents at other depths. (b) and (c) agreed closely but differed from (a), possibly due to severe condit ions at the current meter. Vertical advection was estimated from ADCP diver gence around each triangle. Measurements on the buoy and the ship, calibrat ed earlier in the Coupled Ocean-Atmosphere Response Experiment (COARE), pro vided bulk surface fluxes. Accuracy of net surface heat fluxes was estimate d at +/-10 W/m(2). Rainfall differed considerably between the buoy and ship , but other quantities including shortwave radiation were very similar, eve n on cloudy days. A formalism for the budgets above a given isopycnal is de veloped, to clearly distinguish horizontal and vertical advection. When (b) and (c) above were used with the best estimate of surface fluxes, the 8-da y heat budget misclose for water above the 21.5 isopycnal was (-4.6+/-5) W/ m(2); the diurnal heating cycle was well resolved. The freshwater budget mi sclose was (0.9+/-3.5) mm/d. A small amount of vertical diffusion, in the r atio determined by the slope of the T-S curve, will reduce both misclosures . Cruise design considerations for minimizing errors in the advection and s torage components of heat and freshwater budgets are discussed.