The diurnal cycle of surface divergence over the global oceans

The diurnal cycle of surface divergence over the oceans is examined using 4 × daily data from the tandem SeaWinds satellite missions during April‐September 2003. A statistically significant diurnal cycle of surface divergence is observed over a large fraction of the tropical and subtropical oceans. The highest amplitudes are found adjacent to tropical landmasses and decrease exponentially with distance away from the coast, with a representative e ‐folding distance of approximately 200 km. In most of these near‐coastal regions the surface divergence peaks at ∼15‐20 h locally and there is evidence of offshore propagation of the signal. There are regions, however, where strong cycles persist for much greater distances offshore. Over the southeast Pacific Ocean, there is evidence of a diurnal subsidence wave that propagates for over 2000 km away from the South American coast at approximately 25 m s −1 . A cloud response to the subsidence wave moving westward from the Andes mountains is detected using passive microwave observations of liquid‐water path. Over the remote tropical Pacific, significant diurnal amplitude is found in and around regions of strong mean surface convergence, with a phase suggesting a surface response to the early morning diurnal maximum in precipitation and propagation away from these sources consistent with gravity waves forced by deep tropospheric heating. European Centre for Medium‐Range Weather Forecasts (ECMWF) model simulations of 850 hPa subsidence show a diurnal cycle that is largely consistent with the SeaWinds divergence observations. Copyright © 2009 Royal Meteorological Society