The Anomalous Regional Hadley Circulation in the ENSO Cycle

Based on our previous work on El Niño-Southern Oscillation (ENSO) theory, this article investigated the relationship between regional Hadley circulation and ENSO evolution. We found that an anomalous meridional circulation (AMC) steadily appeared on both sides of equator around the dateline from a mature ENSO (September) to ENSO decay (the next May), which played a role in enhancing or weakening in situ Hadley circulation. Meanwhile, a positive or negative sea level pressure anomaly (SLPa) in the western Pacific Ocean evolved into a shape in which the western equatorial part was recessed near the dateline and protruded near the subtropical dateline regions on both sides of equator. And the SLPa area with the opposite sign over the eastern Pacific was inserted into the recessed part at the equator around the dateline like a wedge. The mirror image of capital sigma shape that is the boundary line dividing opposite SLPa distribution is formed (mirror sigma pattern). The southern and northern subtropical SLP strengths around the dateline tended to increase or decrease accordingly with mirror sigma pattern, which responded to the AMC about two months later. The subtropical SLPa area around the dateline tended to expand eastward into the Nino 3 longitude sector until next August, which impacts reducing the initially warm or cold ENSO strength months later. A mechanism of a combined operation of regional Hadley circulation and Walker circulation existed in this situation, which implies that ENSO episodes play a role in maintaining the ENSO cycle by balancing heat and circulations between tropics and mid-latitudes.