Journal of Climate: Vol. 16, No. 8, pp. 1195每1211.
Atmosphere每Warm Ocean Interaction and Its Impacts on Asian每Australian Monsoon Variation*
Bin Wang
International Pacific Research Center, and Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii
Renguang Wu
International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii
Tim Li
International Pacific Research Center, and Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii
(Manuscript received 5 April 2002, in final form 9 September 2002)
ABSTRACT
Asian每Australian monsoon (A每AM) anomalies depend strongly on phases of
El Niño (La Niña). Based on this distinctive feature, a method of extended
singular value decomposition analysis was developed to analyze the changing
characteristics of A每AM anomalies during El Niño (La Niña) from its development
to decay. Two off-equatorial surface anticyclones dominate the A每AM anomalies
during an El Niño〞one over the south Indian Ocean (SIO) and the other over the
western North Pacific (WNP). The SIO anticyclone, which affects climate
conditions over the Indian Ocean, eastern Africa, and India, originates during
the summer of a growing El Niño, rapidly reaches its peak intensity in fall,
and decays when El Niño matures. The WNP anticyclone, on the other hand, forms
in fall, attains maximum intensity after El Niño matures, and persists through
the subsequent spring and summer, providing a prolonged impact on the WNP and
east Asian climate. The monsoon anomalies associated with a La Niña resemble
those during an El Niño but with cyclonic anomalies. From the development
summer to the decay summer of an El Niño (La Niña), the anomalous sea level
pressure, low-level winds, and vertical motion tend to reverse their signs in
the equatorial Indian and western Pacific Oceans (10∼S每20∼N, 40∼每160∼E). This
suggests that the tropospheric biennial oscillation is intimately linked to the
turnabouts of El Niño and La Niña.
The remote El Niño forcing alone can
explain neither the unusual amplification of the SIO anticyclone during a
developing El Niño nor the maintenance of the WNP anticyclone during a decaying
El Niño. The atmosphere每ocean conditions in the two anticyclone regions are
similar, namely, a zonal sea surface temperature (SST) dipole with cold water
to the east and warm water to the west of the anticyclone center. These
conditions result from positive feedback between the anomalous anticyclone and
the SST dipole, which intensifies the coupled mode in the SIO during El Niño
growth and maintains the coupled mode in the WNP during El Niño decay. The
interactions in the two anticyclone regions share common wind
evaporation/entrainment and cloud每radiation feedback processes but they differ
with regard to the oceanic dynamics (vertical and horizontal advection and
thermocline adjustment by oceanic waves). The outcome of the interactions in
both regions, however, depends crucially on the climatological surface winds.
The SIO-coupled mode is triggered by El Niño-induced subsidence and alongshore
winds off the coast of Sumatra. However, other independent El Niño local and
remote forcing can also trigger this coupled mode.
The traditional view has regarded SST
anomalies in the Indian and western Pacific Oceans as causing the A每AM
variability. The present analysis suggests that the SST anomalies in these warm
ocean regions are, to a large extent, a result of anomalous monsoons. Thus, the
atmosphere每warm ocean interaction may significantly modify the impacts of
remote El Niño forcing and should be regarded as one of the physical factors
that determine the variability of the A每AM.
During the summer of El Niño development,
the remote El Niño forcing plays a major role in the A每AM anomalies that
exhibit obvious equatorial asymmetry. A tilted anticyclonic ridge originates in
the Maritime Continent and extends to southern India, weakening the Indian
monsoon while strengthening the WNP monsoon. Numerical modeling experiments
suggest that the mean monsoon circulation enhances the equatorial Rossby wave
response in the easterly vertical shear region of the Northern Hemisphere and
creates the equatorial asymmetry.