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

Wuqiushi Yao, Jianhua Lu, Yimin Liu

By defining a pseudo meridional overturning streamfunction (Ψpseudo) at any zonal sector and defining a new vector-type, dual-component index (Abrupt Seasonal Change Index, ASCI), we diagnose zonally asymmetric abrupt seasonal change (ASC) of tropical meridional circulation. Ψpseudo converges to traditional, meridional overturning streamfunction (Ψm) after being averaged over a zonal circle around any latitude. By applying the Helmholtz decomposition to horizontal velocity fields so as to decompose Ψpseudo 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.