Synoptic feedback to intraseasonal variability through the modulation of the surface heat flux

The relative contribution of intraseasonal (20-90day) and higher-frequency (2-20day) motion (including surface wind and air-sea humidity difference variability) on the intraseasonal surface latent heat fluxes in the global tropics is investigated using the observational data and a band-pass filtering scheme. It is noted that the higher-frequency (2-20day) wind and humidity variability accounts for about 50% of the total intraseasonal surface latent heat flux in the tropical Indian Ocean, western Pacific, eastern Pacific, and Gulf of Mexico. This up-scale heat flux contribution is attributed to the nonlinear dependence of the heat flux on the zonal and meridional wind components and air-sea humidity difference. The ratio of the heat flux contribution by the higher-frequency motion versus that by the intraseasonal motion exceeds a half in various ocean regions including the tropical eastern Pacific, Gulf of Mexico, northwestern Pacific, and western Indian Ocean. The high percentage of the intraseasonal latent heat flux contributed by the higher-frequency (2-20day) motion implies that the synoptic-scale variability may exert a significant impact on the atmospheric intraseasonal oscillation through the modulation of the wind-induced surface heat exchange (WISHE). A further analysis reveals that the nonlinear up-scale feedback exhibits a strong seasonal- and geographic- dependent characteristic.