Diurnal cycle of rainfall and surface salinity in the western Pacific warm pool

The diurnal cycles of rainfall and surface salinity in the western equatorial Pacific were computed for the period August 1991-December 1994 using hourly data from the Tropical Atmosphere Ocean (TAO) mooring array enhanced for the Coupled Ocean Atmosphere Response Experiment (COARE). The analysis shows preferential rainfall during predawn hours. Nighttime mixing typically caused the predawn rainfall to mix downward into higher salinity subsurface waters. In contrast, afternoon rainfall, although weaker, generally produced a stable shallow layer of very low salinity. Consequently, sea surface salinity (SSS) exhibited a weak diurnal cycle, with anomalously low salinity in the late afternoon and anomalously high salinity at night. At 1 m depth, the SSS diurnal cycle anomalies were similar to 0.005 psu. Although nearly two orders of magnitude less than the SSS standard deviation for this region, the anomalies were significant at the 95% confidence level.