Characterizing Extreme Diurnal Warming in Satellite-Derived Operational Sea Surface Temperature Products

Diurnal changes in the sea surface temperature (SST) can perturb the air-sea heat flux and have been demonstrated to have an impact on climate variability. Recent satellite observations have shown that diurnal warming can be more extreme than previously anticipated, particularly on finer spatial scales, with warming events in excess of 5 K not uncommon. While previous efforts have attempted to develop climatologies for the magnitude and extent of diurnal warming and determine its impact on air-sea interactions, none of these has fully considered the largest diurnal warming events at high spatial resolution on a global scale. If the frequency, geographic distribution, and spatial extent of the largest diurnal warming events are not well known, it is not possible to quantify the total impact of diurnal warming on climate variability or determine with what accuracy it must be considered.

This work assesses the veracity of satellite-derived estimates of diurnal SST warming and characterizes extreme diurnal variability of the SST at high spatial resolution on global scales. Estimates of the daily amplitude of diurnal warming from multiple existing operational satellite products are computed as the difference between daytime SST retrievals and derived “foundation” temperatures assumed representative of the temperature before dawn. While satellite SST retrievals are typically very accurate on average, small uncertainties can have an important impact on derived diurnal warming. Theoretical considerations, probability distributions, spatial coherence, and time series are employed to assess the accuracy of estimates of large diurnal warming from different sensors. The resulting validated diurnal warming estimates are used to construct a multi-year, climatology of extreme warming, which characterizes the scale and probability of occurrence of large diurnal warming events as a function of parameters including amplitude, region, season, and sensor resolution. The results demonstrate that, while satellite-derived warming estimates must be interpreted with care, reliable evidence exists for the occurrence of large diurnal warming with magnitudes, frequency, and spatial extents great enough to warrant explicit consideration of the impacts on applications dependent on accurate knowledge of the SST.