Diurnal and Semidiurnal Cycles of Global Tropical Ocean Surface Wind Speed Based on the NASA CYGNSS Mission

The annual and seasonal mean diurnal (24-hour) and semidiurnal (12-hour) amplitudes and phases of surface wind speed over global tropical oceans (38°S-38°N) are documented using roughly one-year of new surface wind speed data provided by the NASA Cyclone Global Navigation Satellite System (CYGNSS) missionwith high temporal (~3-hourly) and spatial (25-km) resolutions. The diurnal and semidiurnal amplitudes and phases are defined as half the daily maximum-minimum difference and the local solar time (LST)of the daily maximum of the diurnal and semidiurnal harmonics, respectively. It is shown that the diurnal amplitude is relatively small (<0.4 m s^-1and ~5% of the daily mean) over most of the equatorial oceans but relatively large (>0.5 m s^-1and ~10-15% of the daily mean) over the subtropical oceans. In contrast, the semidiurnal amplitude is relatively small (<0.4 m s^-1and <5% of the daily mean) over the subtropical oceans but relatively large (>0.5 m s^-1and ~10-15% of the daily mean) over the equatorial oceans. The diurnal cycle is more important over the subtropical oceans while the semidiurnal cycle is more vital over the equatorial oceans. The diurnal phase is around midnight (0000 LST) over most of tropical oceans but the semidiurnal phase has a greater spatial variability. The diurnal and semidiurnal cycles show a strong seasonal variation. Our results reveal important new information about the daily cycles of global ocean surface wind speed and provide important new constraints on climate models with regard to their ability to properly reproduce these cycles.