Precipitation over Indochina during the monsoon transition: modulation by Indian Ocean and ENSO regimes

The interannual variability of precipitation during the summer monsoon transition over the Indochina Peninsula (ICP) is substantially influenced by the sea surface temperature anomalies (SSTAs) of the tropical ocean, showing a robust relationship between April and May (AM) precipitation and the El Niño/Southern Oscillation (ENSO) phenomenon. Dynamic composites and statistical analyses supported by model experiments indicate that the observed anomalous AM precipitation is associated with circulation anomalies over the Pacific and, in addition, affected by the response to the tropical SSTAs forcing from the Indian Ocean (IO): (i) Less (greater) than normal AM precipitation over the ICP occurs during the El Niño (La Niña) years, which is consistent with late (early) Bay of Bengal (BoB) summer monsoon onset. (ii) The dry (wet) AM precipitation years are associated with the anomalous western North Pacific (WNP) anti-cyclone (cyclone) induced by El Niño (La Niña) concurrent with the anti-cyclone (cyclone) over the BoB, suppressing (favoring) the meridional flow of warm and moist air from the Pacific and Indian ocean and thus cutting (providing) moisture supply for the ICP. (iii) The reduced tropical convective activity over Maritime Continent (MC) is related to the weakened local Hadley circulation concurrent with the weakened overturning Walker circulation, and favors a drier than normal AM precipitation over the ICP, to which the wetter years are opposite. These symmetric atmospheric circulation patterns characterizing dry and wet AM precipitation over the ICP are also reproduced by numerical experiments with an atmospheric general circulation model.