Multiscale Air–Sea Interactions during TOGA COARE

Abstract

Two distinct intraseasonal oscillations (ISO) are found in the tropical ocean atmosphere in the western Pacific region during Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE). The ISO is characterized by cycles of dry?wet phases in the atmosphere due to the passage of Madden?Julian oscillations, and corresponding warming/shoaling?cooling/deepening cycles in the ocean mixed layer (OML). During the wet phase, 2?3-day disturbances and diurnal variations in the atmosphere are pronounced. During the dry phase, diurnal cycles in sea surface temperature (SST) is much enhanced while the OML is shallow. These multiscale coupled air?sea variations are further investigated with an ocean mixed-layer model forced by the observed surface heat, water, and momentum fluxes. The variations of ocean mixed layer are shown to be crucially dependent on the vertical distribution of solar radiation, that is, diurnal SST variability primarily determined by the absorbed solar radiation in the surface layer (?1 m), and intraseasonal variations determined by penetrating solar radiation below the surface layer. Results further reveal that the accumulative effect of diurnal mixing cycles (solar heating/nocturnal deepening) is essential to maintain a stable temperature stratification and a realistic evolution of mixed-layer depth and temperature at the intraseasonal scale. The nonlinear response of the ocean mixed layer to the surface heat and momentum fluxes indicates the need to resolve the high-frequency response including diurnal atmospheric radiative?convective processes and ocean mixing processes in a coupled model to simulate the whole spectrum of multiscale variations within ISOs.