Sea surface temperature (SST) is one of the most crucial climate variables as it couples the ocean and atmosphere. As we try to understand and predict the climate system, higher levels of accuracy are called for. The desirable accuracy for knowing SST is <0.3K. Under these strict requirements, the influence of diurnal warming, which occurs under low wind and high insolation conditions, becomes important as SST measurements can vary by over 1°C depending on what time of day the measurement is taken.

Ten years of global infrared satellite data from NOAA’s Advanced Very High Resolution Radiometer (AVHRR) have been analysed to identify the global temporal and spatial characteristics of diurnal warming. Daily night-time SST's are subtracted from adjacent day-time SST to give an estimate of diurnal warming (DT_day-night). The results reveal large regions in the tropics and mid-latitudes that are frequently susceptible to diurnal warming each year, dictated by seasonal wind and insolation patterns. The analysis also reveals how the spatial distribution and magnitude of DT varies with the drift of the satellite orbit as it shifts from a local afternoon overpass time of 14h00 to 16h00. The results are compared with highly accurate in situ SST measurements to assess how closely the satellite-derived DT is representative of diurnal warming events. The results highlight the sensitivity of SST to its sampling time and reveal the importance of the diurnal cycle for SST measurements. These results feed into the new GODAE High Resolution Sea Surface Temperature Pilot Project which aims to produce high resolution accurate SST products, resolving biases in the measurements including the influence of diurnal stratification. The results also suggest important implications for air-sea interaction and upper ocean processes.