Volume 88 (2010) Issue 2 Pages 183-202
Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February-March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January-February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface heat flux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat flux induced by the anomalous air-sea humidity difference. These results suggest that the anomalous downward motion associated with the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air-sea interaction associated with the local Hadley circulation is discussed.