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Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

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  • 1 NOAA/Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, New Jersey
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Abstract

The influences of El Niño–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios.

The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding warm and cold ENSO events reveal that a majority of the warm episodes are accompanied by below-normal summer rainfall in India and northern Australia, and above-normal winter rainfall in southeast Asia. The polarity of these anomalies is reversed in the cold events. These relationships are particularly evident in the TOGA experiment.

Composite charts of the simulated flow patterns at 850 and 200 mb indicate that the above-mentioned precipitation changes are associated with well-defined circulation features over the affected monsoon regions. Dry conditions are typically coincident with low-level anticyclonic anomalies, and vice versa. These circulation centers are situated to the northwest and southwest of a prominent precipitation anomaly situated near 120°–150°E at the equator, which corresponds to the western half of a dipolar heating pattern resulting from east–west displacements of the ascending branch of the Walker circulation during ENSO. The large-scale anomalous circulation over the monsoon regions is similar to that of a Rossby wave pattern associated with a condensational heat source or sink in the western equatorial Pacific.

Diagnosis of the output from the TOGA-ML experiment reveals that variations in the circulation and cloud cover accompanying ENSO-induced monsoon anomalies could modulate the latent heat and shortwave radiative fluxes at the air–sea interface in the Indian Ocean, thereby changing the SST conditions in that basin. These simulated SST anomalies compare well with observational results. The local atmospheric response to these SST anomalies opposes the remote response of the south Asian monsoon flow to SST anomalies in the tropical Pacific, thus leading to a negative feedback loop in the air–sea coupled system.

Corresponding author address: Dr. Ngar-Cheung Lau, NOAA/Geophysical Fluid Dynamics Laboratory, Princeton University, P.O. Box 308, Princeton, NJ 08542.

Email: gl@gfdl.gov

Abstract

The influences of El Niño–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios.

The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding warm and cold ENSO events reveal that a majority of the warm episodes are accompanied by below-normal summer rainfall in India and northern Australia, and above-normal winter rainfall in southeast Asia. The polarity of these anomalies is reversed in the cold events. These relationships are particularly evident in the TOGA experiment.

Composite charts of the simulated flow patterns at 850 and 200 mb indicate that the above-mentioned precipitation changes are associated with well-defined circulation features over the affected monsoon regions. Dry conditions are typically coincident with low-level anticyclonic anomalies, and vice versa. These circulation centers are situated to the northwest and southwest of a prominent precipitation anomaly situated near 120°–150°E at the equator, which corresponds to the western half of a dipolar heating pattern resulting from east–west displacements of the ascending branch of the Walker circulation during ENSO. The large-scale anomalous circulation over the monsoon regions is similar to that of a Rossby wave pattern associated with a condensational heat source or sink in the western equatorial Pacific.

Diagnosis of the output from the TOGA-ML experiment reveals that variations in the circulation and cloud cover accompanying ENSO-induced monsoon anomalies could modulate the latent heat and shortwave radiative fluxes at the air–sea interface in the Indian Ocean, thereby changing the SST conditions in that basin. These simulated SST anomalies compare well with observational results. The local atmospheric response to these SST anomalies opposes the remote response of the south Asian monsoon flow to SST anomalies in the tropical Pacific, thus leading to a negative feedback loop in the air–sea coupled system.

Corresponding author address: Dr. Ngar-Cheung Lau, NOAA/Geophysical Fluid Dynamics Laboratory, Princeton University, P.O. Box 308, Princeton, NJ 08542.

Email: gl@gfdl.gov

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