Rotational Flow Dominates Abrupt Seasonal Change in Zonally Asymmetric Tropical Meridional Circulation

TL;DR

This study reveals that rotational flow, rather than divergent flow, predominantly drives abrupt seasonal changes in zonally asymmetric tropical meridional circulation, especially over landmasses with deep convection, highlighting the role of eddy interactions.

Contribution

The paper introduces a new diagnostic index (ASCI) and decomposes the circulation into rotational and divergent components to analyze zonally asymmetric seasonal changes.

Findings

Rotational flow dominates the abrupt seasonal change in tropical circulation.

Deep convection zones show the most pronounced seasonal shifts.

Landmasses with low heat inertia are key regions for these changes.

Abstract

The seasonality of the tropical meridional circulation evolves differently across different regions, governs the onset and retreat of monsoons and migration of tropical precipitation, thereby influencing agricultural productivity and disaster preparedness in the tropics and subtropics. By defining a pseudo meridional overturning streamfunction ({\Psi}pseudo) and defining a new vector-type, dual-component index (ASCI), we diagnose zonally asymmetric abrupt seasonal change (ASC) of tropical meridional circulation. {\Psi}pseudo converges to traditional, meridional overturning streamfunction ({\Psi}m) after being averaged over a zonal circle around any latitude. By applying the Helmholtz decomposition to horizontal velocity fields so as to decompose {\Psi}pseudo into rotational and divergent components, we quantitatively compare the contributions of horizontally rotational and divergent flows to the abrupt seasonal change. We find that the zonal sectors associated with strong deep convection exhibit the most pronounced ASC of tropical meridional circulation, and all of subregions exhibiting ASC contain landmass with low heat inertia. Particularly, in contrast to the case of zonally symmetric Hadley cell, rotational flow, rather than the thermal-direct divergent flow, dominates the zonally asymmetric ASC in the tropics, although the divergent flow also contributes to the ASC over the zonal sectors associated with deep convection. We suggest that the interplay between tropical Rossby-type eddies with extratropical eddies and tropical circulation is essential to the zonally asymmetric ASC of tropical Hadley circulation.