Long-Term Variation of the Principal Mode of Boreal Spring Hadley Circulation Linked to SST over the Indo-Pacific Warm Pool

Abstract

he variability of the boreal spring [March?May (MAM)] Hadley circulation (HC) is investigated, focusing on the long-term variation of the first principal mode for 1951?2008, which is an equatorially asymmetric mode (AM) with the rising branch located around 10°S. This mode explains about 70% of the variance of the MAM HC and shows an obvious upward trend and thus contributes to the strengthening of the MAM HC. The robust warming trends of sea surface temperature (SST) over the Indo-Pacific warm pool (IPWP) play an essential role in the variations of the MAM HC. When SST over the IPWP is warm, anomalous meridional circulation is induced with descending branches located in regions 30°?20°S and 5°?15°N and rising motion located near 10°S. The anomalous rising south of the equator is due to the inhomogeneous warming of SST over the IPWP. SST within the IPWP in the Southern Hemisphere shows a larger warming trend than that in the Northern Hemisphere. The position of the anomalous convergence associated with SST variations over the IPWP is aligned with the maximum meridional gradient of zonal mean SST, resulting in an equatorially asymmetric meridional circulation. This point is further established in theoretical analyses. However, the meridional SST gradient within the IPWP shows a decreasing trend, suggesting the associated anomalous meridional circulation intensifies, which in turn explains the strengthening of the MAM HC. Under this scenario, the accompanied descent in the regions of 30°?20°S and 5°?15°N is enhanced, implying a frequent drought in these regions during MAM.