How does the air-sea coupling frequency affect convection during the MJO
passage?
Abstract
The importance of air-sea coupling in the simulation and prediction of
the Madden-Julian Oscillation (MJO) has been well established. However,
it remains unclear how air-sea coupling modulates the convection and
related oceanic features on the subdaily scale. Based on a regional
cloud-resolving coupled model, we evaluated the impact of the air-sea
coupling on the convection during the active phase of the MJO by varying
the coupling frequency. The model successfully reproduced the
atmospheric and oceanic variations observed by satellite and
measurements but with some quantitative biases. According to the
sensitivity experiments, we found that stronger convection was mainly
caused by the higher sea surface temperatures (SSTs) generated in highly
coupled experiments, especially when the coupling frequency was 1 hour
or shorter. A lower coupling frequency would generate the phase lags in
the diurnal cycle of SST and related turbulent heat fluxes. Our analyses
further demonstrated that the phase-lagged diurnal cycle of SST
suppressed deep convection through a decrease in daytime moistening in
the lower troposphere. Meanwhile, in the upper ocean, the high-frequency
air-sea coupling helped maintain the shallower mixed and isothermal
layers by diurnal heating and cooling at the sea surface, which led to a
higher mean SST. In contrast, the barely coupled experiments
underestimated SST and therefore convective activities. Overall, our
results demonstrated that high-frequency air-sea coupling (1 hour or
shorter) could improve the reproducibility of the intensity and temporal
variation in both diurnal convection and upper ocean processes.