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

摘要

Diurnal temperature variability in the top 50m 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 5m temperatures in low-wind conditions, as expected due to diurnal surface warming effects. SSTs also tend to be warmer than Argo 5m temperatures when columnar water vapor is less than ～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 5m 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 5m depth, the amplitude of the summer diurnal cycle ranges from about 0.1C at the equator to 0.05C near 60 latitude. At latitudes where the diurnal amplitude exceeds about 0.04C, 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.