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Southern Oscillation: Further Studies with a GFDL General Circulation Model

R. N. KeshavamurtyGeophysical Fluid Dynamics Program, Princeton University, Princeton, NJ 08540

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Abstract

In this paper we report the results of the atmospheric response to realistic warm sea surface temperature anomalies over the equatorial Pacific during summer and winter (northern). We have used the same GFDL general circulation model as in 1982.

In the case of the realistic summer anomaly, the spatial structure of the atmospheric response is similar to that in the case of idealized warm anomalies over the eastern and central Pacific, but the magnitudes of the response are smaller and are of smaller latitudinal extent. The Walker circulation is significantly weakened, There is also a weaker anomaly circulation of the opposite sense over the equatorial Atlantic and Africa.

The planetary scale response of tropical sea-level pressure and equatorial zonal winds again is quite similar to the observed anomalies during the low index phase and thus, the realistic sea surface temperature anomaly acts in such a way as to reinforce the low index phase. As the induced low-level zonal wind anomalies over the equatorial central and western Pacific are far to the west of the maximum of sea surface temperature anomalies off the South American coast, there is a mechanism for the westward propagation of the latter.

The subtropical anticyclones and jet streams are strengthened mainly in the Southern Hemisphere and the subpolar westerlies are weakened. There is also a series of quasi-barotropic alternating cyclones and anticyclones over the South Pacific.

In the case of the realistic winter anomaly, on the other hand, the most significant and marked response is the subtropical and middle latitudes of the Northern Hemisphere. The subtropical jet significantly mainly strengthened mainly over the Northern Pacific, but also in almost all other longitudes. The subpolar westerlies are weakened. Quasi-barotropic cyclonic anomalies exist 1) over the northeast Pacific, 2) over the North Atlantic and adjoining Europe and 3) over north India. An anticyclonic anomaly exists over Alaska and the surrounding region. The Aleutian low is thus shifted southward. These anomalies extend even to the stratosphere. Here also the Pacific Walker circulation is weakened, and the tropical sea-level pressure anomaly has a global longitudinal extent.

We have identified the equatorial and tropical response as a forced Kelvin wave and baroclinic Rossby wave response, and the middle-latitude response as a forced barotropic Rossby wave response.

Abstract

In this paper we report the results of the atmospheric response to realistic warm sea surface temperature anomalies over the equatorial Pacific during summer and winter (northern). We have used the same GFDL general circulation model as in 1982.

In the case of the realistic summer anomaly, the spatial structure of the atmospheric response is similar to that in the case of idealized warm anomalies over the eastern and central Pacific, but the magnitudes of the response are smaller and are of smaller latitudinal extent. The Walker circulation is significantly weakened, There is also a weaker anomaly circulation of the opposite sense over the equatorial Atlantic and Africa.

The planetary scale response of tropical sea-level pressure and equatorial zonal winds again is quite similar to the observed anomalies during the low index phase and thus, the realistic sea surface temperature anomaly acts in such a way as to reinforce the low index phase. As the induced low-level zonal wind anomalies over the equatorial central and western Pacific are far to the west of the maximum of sea surface temperature anomalies off the South American coast, there is a mechanism for the westward propagation of the latter.

The subtropical anticyclones and jet streams are strengthened mainly in the Southern Hemisphere and the subpolar westerlies are weakened. There is also a series of quasi-barotropic alternating cyclones and anticyclones over the South Pacific.

In the case of the realistic winter anomaly, on the other hand, the most significant and marked response is the subtropical and middle latitudes of the Northern Hemisphere. The subtropical jet significantly mainly strengthened mainly over the Northern Pacific, but also in almost all other longitudes. The subpolar westerlies are weakened. Quasi-barotropic cyclonic anomalies exist 1) over the northeast Pacific, 2) over the North Atlantic and adjoining Europe and 3) over north India. An anticyclonic anomaly exists over Alaska and the surrounding region. The Aleutian low is thus shifted southward. These anomalies extend even to the stratosphere. Here also the Pacific Walker circulation is weakened, and the tropical sea-level pressure anomaly has a global longitudinal extent.

We have identified the equatorial and tropical response as a forced Kelvin wave and baroclinic Rossby wave response, and the middle-latitude response as a forced barotropic Rossby wave response.

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