Model simulations of the influences of ENSO on the East Asian Monsoon

The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment, in which the observed variations of the sea surface temperature (SST) field in 1950-1999 have been prescribed in the tropical eastern and central Pacific. At maritime gridpoints outside of this forcing region, the atmosphere has been coupled to a model of the oceanic mixed layer.

During the summer of El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific. This feature is a Rossby wave response to the enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to cold temperature tendencies along a zonal belt extending eastward from northern China to the western North Pacific, and negative vorticity tendencies over the subtropical northwestern Pacific. These pre-conditions set the stage for the abrupt establishment of a strong Philippine Sea Anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold air surges over East Asia.

The PSAC anomaly attains maximum amplitude in the boreal winter of El Niño events. The near-surface atmospheric circulation and cloud cover associated with the mature PSAC anomaly affect the local surface heat and radiative fluxes, thereby altering the underlying SST conditions. The ensuing air-sea interactions in turn contribute to maintenance and eastward displacement of the PSAC and SST anomalies in the winter and spring seasons.

The space-time characteristics of the intraseasonal variations (ISV) in the East Asian monsoon region have been examined using extended empirical orthogonal functions. The onset of the PSAC anomaly is coincident with a prominent episode of the leading ISV mode. The air-sea interactions accompanying the ISV in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks between the SST and wind fields associated with the ISV.

ENSO events influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air-sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. Weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold air surges associated with the leading ISV mode in that season.