Effects of high-frequency surface wind on the intraseasonal SST associated with the Madden-Julian oscillation
The effect of high-frequency (< 20 days) wind on the intraseasonal sea surface temperature (SST) anomaly associated with the Madden-Julian oscillation (MJO) is examined by diagnosing reanalysis and outputs from a set of oceanic general circulation model (OGCM) experiments. Warm SST anomaly (SSTA) ahead of MJO convective center induces anomalous boundary-layer convergence, favoring the eastward propagation of the MJO. To understand the key physical processes contributing to the warm SSTA, the mixed-layer heat budget equation is diagnosed. The time change of SSTA (∂ «T »/∂ t ) mostly comes from shortwave radiative heating, while latent heat flux (LHF) plays the secondary role. Due to the strong nonlinearity of LHF, the high-frequency (< 20 days) wind may affect the intraseasonal LHF variability via interacting with the background state, resulting in changes in intraseasonal SSTA. Our diagnosis shows that the upscale feedback associated with high-frequency wind variability accounts for around 23% of the intraseasonal LHF in the intraseasonal SST warming region, supporting the growth of ∂ «T »/∂ t . Sensitivity experiments are then designed using an OGCM that simulates the upper-ocean temperature well, to verify the high-frequency wind effect on the intraseasonal SST variability. Once the high-frequency component of surface winds is removed in the model integration, the amplitudes of intraseasonal LHF and ∂ «T »/∂ t are decreased, leading to reduced SSTA. The modeling results confirm the positive role of high-frequency wind in supporting the tropical intraseasonal SST variation. The findings of this study suggest that an accurate representation of high-frequency disturbances and their interaction with other components are crucial for MJO simulation and prediction.