Mechanisms of monsoon-Southern Oscillation coupling: insights from GCM experiments |
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Authors: | K-M Lau W Bua |
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Institution: | (1) Climate and Radiation Branch, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA E-mail: lau@climate.gsfc.nasa.gov, US;(2) Department of Meteorology, University of Maryland, College Park, MD 20742, USA, US |
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Abstract: | The relative roles of internal atmospheric dynamics, land surface evaporation and sea surface temperature (SST) forcings
on the coupling between the Asian monsoon (AM) and the Southern Oscillation (SO) are investigated in a series of GCM experiments.
Results confirm previous studies indicating that the characteristic large-scale pattern of the SO is due primarily to SST
anomaly (SSTA) forcing. The AM circulation anomalies are coupled to the SO via a characteristic upper level circulation couplet
over the equatorial central Pacific. This couplet acts as a radiating node for teleconnection signals originating from the
AM region to the extratropics. Generally, a weak AM is associated with warm SST over the eastern equatorial Pacific, concomitant
with the negative phase of the SO, i.e., low (high) surface pressure over Tahiti (Darwin). The reverse holds for strong AM.
Two wavetrains associated with the AM fluctuation have been identified: one arcing over northeastern Asia via the Aleutians
to North American, and another emanating from northwestern Europe, via Siberia to northern India. Internal dynamics appear
to underpin the origin of these wavetrains, which are strongly tempered by SSTA forcing and to a lesser degree by interactive
land processes. Regionally, land-atmosphere interaction seems to have the strongest impact over East Asia/Indochina and the
adjacent oceanic region of the South China Sea. Here, land-atmosphere interaction is responsible for the enhancement of a
subseasonal scale see-saw oscillation in precipitation between land and the adjacent oceans. A local land-atmosphere feedback
mechanism involving strong coupling between the hydrologic and energy cycles is identified. It is suggested that the interaction
among precipitation, moisture convergence and land surface turbulent heat fluxes and radiation processes play key roles in
determining the fast (subseasonal and shorter scales) response of the AM. On these time scales, the occurrences of cool/wet
and hot/dry states associated with the precipitation seesaw appear to be chaotic. However, the preferred occurrence of a given
state and the abrupt transition between states are dependent on the large-scale circulation and radiation forcings induced
by the SO. One of the more provocative findings here is that effects of land-atmosphere interaction do not seem to alter the
basic planetary scale features of the AM-SO system. As a result, the interannual variability of the coupled AM-SO is relatively
small in the absence of anomalous SST forcing. Yet, the local effect of land-atmosphere interaction on AM is quite pronounced
and dependent upon the large-scale forcings related to SO.
Received: 18 November 1997 / Accepted: 07 April 1998 |
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