Yongsheng Zhang
International Pacific Research Center, School of Ocean and Earth Science
and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
Tim
Li and Bin Wang
International Pacific Research Center and Department of Meteorology,
School of Ocean and Earth Science and Technology, University of Hawaii at Manoa,
Honolulu, Hawaii
Guoxiong Wu
LASG, Institute of Atmospheric Physics, The Chinese Academy of Sciences,
Beijing, China
ABSTRACT
The temporal and spatial structures of the atmospheric circulation
associated with the climatology and interannual variations of the summer
monsoon onset over the Indochina Peninsula were studied using the observed
daily rainfall at 30 stations and the NCEP每NCAR reanalysis from 1951 to 1996.
The climatological monsoon onset over Indochina is on 9 May, with a standard
deviation of 12 days. The monsoon onset is characterized by the pronounced
northeastward progression of the low-level southwesterlies over the Indian
Ocean and the intensification and northward extension of the tropical
convection from Sumatra. It coincides with the weakening of the midlatitude
westerly over south Asia, and the westward propagation of the intraseasonal
oscillation (ISO) originated in the South China Sea (SCS) and the western
Pacific with a dominant timescale of 12每25 days.
A
close relationship between the interannual variations of the monsoon onset and
El Niño/La Niña was identified. Years with warm (cold) sea surface temperature
(SST) anomalies in the western Pacific and cold (warm) SST anomalies in the
central-eastern Pacific in the preceding spring have an early (late) onset.
For an early onset year, strong convective activities appear over the southern
Indochina Peninsula and the southern SCS in the preceding winter and spring.
Associated with the changes of the Walker circulation and the local Hadley
circulation related to La Niña, strong convective activities were maintained
by the convergence between the anomalous southwesterlies in the Indian Ocean
and northeasterlies over the northern SCS. The anomalous southwesterlies in
the Indian Ocean were induced by both the anomalous Walker circulation
associated with La Niña and anomalous land每sea thermal contrast. The anomalous
northeasterlies over the northern SCS were originated in northern winter due
to the combined effects of the cold east China land and the warm Philippine
Sea, and further maintained by a positive thermodynamic air每sea feedback
mechanism related to La Niña. An opposite scenario is found for a late onset
year with warm SST anomalies in the central-eastern Pacific (El
Niño).
