Aircraft observations of sea surface temperature variability in the tropical Pacific

Abstract

During the southern summer of 1992–1993, a series of aircraft observations of sea surface temperature (SST) were obtained over the tropical western Pacific ocean as part of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Down‐looking thermal infrared (10–11 μm) observations of the sea surface, coincident with uplooking infrared observations (9–11 μm) and meteorological measurements from the same aircraft platform are described. The data are compared with buoy and ship‐based measurements of SST, and show good agreement. The radiometric measurements, corrected for surface emissivity effects, show diurnal warming which precedes warming measured by sub‐surface buoy sensors. Mesoscale surface features are also seen which have spatial dimensions similar to those of tropical cloud clusters in the wavelength range of 1–100 km. Significant horizontal temperature gradients of 1°C occur in less than 10 km. On some days, the gradients can be directly related to precipitation. Clouds appear to create ‘imprints’ of their structure on the ocean surface by shielding incoming solar radiation, consistent with calculations of such a process using a one‐dimensional ocean mixed layer model. Measurements of the height of the well‐mixed region of the boundary layer and air potential temperature show a positive relation with increasing SST. An increase in buoyancy flux with increasing SST is identified with mesoscale sub‐cloud processes in fair weather conditions, and suggests that SST maxima on scales of 10–50 km enhance the transfer of moisture in the boundary layer by natural convection processes.