In situ observations of diurnal warming at the ocean surface

Observations of diurnal temperature variability at the ocean surface have been primarily available only from satellite SST retrievals themselves. Since most satellite observations revisit the same location only infrequently, determining how the ocean surface diurnal heating responds to variability in forcing (mainly insolation and wind speed) has been primarily addressed through theoretical modeling or extrapolation of results from in situ (buoy) observations measured 0.5 m to 1.5 m below the skin layer. Diurnal heating in the skin layer may be quite different than heating at 0.5 m as this layer responds very rapidly to changes in heat and momentum. The Explorer of the Seas, a cruise ship, makes weekly cruises on two alternating tracks through the Caribbean Sea. Measurements from the Marine Atmospheric Emitted Radiance Interferometer (M-AERI) carried on the Explorer of the Seas provide one of the few skin SST data sets, along with ancillary measurements necessary for diurnal investigations. Initial analyses show that the surface signature of diurnal warming in the skin layer is chiefly controlled by the wind speed. The daily peak in diurnal warming is directly related to the minimum wind speed during the day, causing the time of the peak to shift depending on when the minimum winds occur. Fluctuations in wind speed can result in multiple peaks in diurnal heating during a single afternoon. Wind speed is negatively lag-correlated with diurnal warming while insolation is positively lag-correlated. The maximum lag-correlation of wind speed (insolation) with diurnal warming is at a time lag of 30 (50) minutes. Several models of diurnal variability exist. A comparison of several models with each other reveals considerable differences in estimates of diurnal warming. Further validation of the models using M-AERI observed diurnal warming again reveals considerable differences in estimates of warming related to model forcing parameterizations.