Distinct Roles of Base-State and Wind-Burst-Induced Sea Surface Latent Heat Flux in MJO Maintenance and Propagation

The Madden-Julian Oscillation (MJO) is the dominant and most influential intraseasonal oscillation in the tropics, as well as a prominent source of subseasonal-to-seasonal predictability. Its maintenance and propagation, particularly across the Maritime Continent, remain a challenge for theory, simulation and forecasting. In modern MJO theories, moisture plays a central role. As the primary source of moisture, sea surface latent heat flux has received significant attention. The sea surface latent heat flux can be induced by both base-state winds and wind bursts, but the different effects of these two types of latent heat flux are not well understood.

This study addresses this issue by analyzing CESM hindcasts of a strong MJO event in April 2009 that crossed the Maritime Continent with little decay. The control simulation reproduces the observed propagation characteristics and intensity of this event. We then impose an upper limit on wind speed in the bulk formula used by the model to calculate sea surface latent heat flux. By lowering this limit in different simulations, we reduce the latent heat flux from strong winds first, followed by the flux from base-state winds. Based on frequency distributions over the Indian Ocean and the Maritime Continent, the peaks for base-state winds and wind bursts occur at 3 m/s and 10 m/s respectively.

Results indicate that wind-burst-induced latent heat flux is essential to maintaining MJO amplitude over the Maritime Continent, due to the region’s lower sea fraction. As convection over land tends to disrupt the coherent organization of the MJO convection envelope, lower sea fraction increases the sensitive of MJO amplitude to sea surface latent heat flux. On the other hand, the base-state surface latent heat flux modulates MJO propagation speed due to its effectiveness in moistening the atmosphere. As the base-state latent heat flux is reduced, the atmosphere dries, moisture advection decreases, and the MJO slows down. Additional simulations confirm these findings in other MJO cases. This study underscores the importance of accurately simulating strong winds for maintaining MJO amplitude over the Maritime Continent and overcoming the barrier effect.