Diurnal variability of upper ocean temperatures from microwave satellite measurements and Argo profiles

Diurnal temperature variability in the top 50 m of the ocean is assessed by pairing Argo temperature profiles with geographically colocated Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperatures (SSTs) collected within +/- 24 h of each other. Data pairs with time separations of up to +/- 3 h are used to evaluate systematic differences between the two data sets. Daytime SSTs are warmer than Argo 5 m temperatures in low-wind conditions, as expected due to diurnal surface warming effects. SSTs also tend to be warmer than Argo 5 m temperatures when columnar water vapor is less than similar to 7 mm. These effects are removed empirically. For Argo data collected within +/- 24 h of satellite overpass times, temperature differences between Argo and AMSR-E show evidence of a diurnal cycle detectable at 5 m depth and below. The diurnal amplitude decreases with increasing latitude and increasing depth to the base of the mixed layer and is stronger in summer than in winter. At 5 m depth, the amplitude of the summer diurnal cycle ranges from about 0.1 degrees C at the equator to 0.05 degrees C near 60 degrees latitude. At latitudes where the diurnal amplitude exceeds about 0.04 degrees C, maximum temperatures occur at about 16:50 +/- 0: 40 local time, and minimum temperatures occur at about 07:50 +/- 0: 40 local time. Above the base of the mixed layer, the time of the diurnal maximum increases with depth, consistent with downward propagation of the diurnal signal, while the time of the minimum implies an upward propagation.