Diurnal variability in sea surface temperature characterised from drifting buoy day

<p>Drifting-buoy sea surface temperature (SST) records have been used to characterise the diurnal variability of ocean temperature at a depth of order 20 cm. We use measurements covering the period 1990-2009 from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) version 2.5, which is a collection of marine surface observations that includes individual SST records from drifting buoys. Appropriately transformed, this dataset is well suited for estimation of the diurnal cycle, since many drifting buoys have high temporal coverage (many reports per day), and are globally distributed. For each drifter for each day, we compute the local-time daily SST variation relative to the local-time daily mean SST. Climatological estimates of sub-daily SST variability are found by averaging across various strata of the data: in 10-degree latitudinal bands as well as globally; and stratified with respect to season, wind speed and total cloud cover. A parameterisation of the diurnal variability is fitted as a function of the variables used to stratify the data, and the coefficients for this fit are also provided with the data. Results are consistent with expectations based on previous work: the diurnal temperature cycle peaks in early afternoon (circa 2 pm local time); there is an increase in amplitude and a decrease in seasonality towards the equator. Generally, the ocean at this depth cools on windy days and warms on calm days, so that a component of sub-daily variability is the SST tendency on slower timescales. By not “closing” the diurnal cycle when stratified by environmental conditions, this dataset differs from previously published diurnal-cycle parameterisations. This thorough characterisation of the SST diurnal cycle will assist in interpreting SST observations made at different local times of day for climatological purposes, and in testing and constraining models of the diurnal-cycle and air-sea interaction at high temporal resolution.