Hadley Cell Instability as a Driver of Tropical Depression-Type Waves

Convectively coupled tropical depression-type (TD-type) waves are synoptic-scale disturbances that predominantly occur during the summer season. These systems play an important role in the hydrological cycle of many off-equatorial regions. However, the mechanisms driving the seasonal variability of TD-type waves remain unclear. A recent study proposed that the low-level vorticity in TD-type waves interacts with the horizontal gradient of mean-state column moisture, leading to wave growth through moisture–vortex instability (MVI). On the basis of this framework, we examine the seasonality of TD-type waves. The results show that strong TD-type waves are observed in the southern hemisphere from December to May and in the northern hemisphere from June to November. The waves are strongest when the mean vertical velocity gradient of the corresponding hemisphere is largest, supporting their growth through MVI. Furthermore, these TD-type waves induce strong poleward eddy moisture fluxes in the summer hemisphere, where the poleward energy transport by the Hadley cell weakens. The intensity of TD-type waves shows a significant negative correlation with Hadley cell strength, suggesting that the eddy moisture fluxes generated by the TD-type waves may partially compensate for the reduced energy transport by the Hadley cell. These findings indicate that the background meridional moisture gradient influences the seasonality of TD-type waves, which yield poleward eddy moisture fluxes that can further transport latent energy from tropical to extratropical regions.