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R. N. Keshavamurty

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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R. N. Keshavamurty

Abstract

In order to improve our understanding of the role of equatorial Pacific sea surface temperature (SST) anomalies in the Southern Oscillation and the variability of the Walker Circulation, we have conducted sensitivity studies with warm SSTA's over different parts of the equatorial Pacific using a general circulation model of GFDL. The model response is compared with the variability of the 15-year control run and the statistical significance of the response is assessed.

The anomalies produce dominant changes in the vertical circulation in the equatorial longitude-height plane. Besides the large local increase of precipitation near the anomalies and a slight reduction to the west, there are also global-scale anomalies in the equatorial zonal winds (notably in the upper troposphere) and in tropical sea level pressure. The Pacific Walker Circulation is significantly altered. In the case of eastern and central Pacific anomalies these features resemble the observed circulation anomalies during the low index phase of the Southern Oscillation. Thus warm SST anomalies over equatorial eastern and central Pacific are shown to act in such a way as to reinforce the low index phase of the Southern Oscillation. The model response of the low-level zonal winds is such that the maximum westerly anomalies occur to the west of the SST anomaly. This would induce the anomalies to shift westwards. We also propose a self-generating mechanism for the Southern Oscillation.

SST anomalies over equatorial central and western Pacific are found to be more efficient in producing atmospheric circulation anomalies compared to equal ones over eastern Pacific. The details of the response are also longitude dependent.

The anomalies also produce well-marked subtropical and middle latitude responses, notably in the winter hemisphere. The subtropical anticyclones and the subtropical westerly jet streams in the upper troposphere are strengthened in the longitudes of the anomalies whereas the westerlies in the subpolar latitudes are weakened. In other longitudes there is a weakening of the subtropical anticyclones. The precipitation also increases southeastward and northeastward from the anomaly. There is also a stationary anomaly low (in the middle latitudes) to the southeast of the SST anomaly and an anomaly high further southeast.

The diabatic heating anomalies associated with the central and western Pacific anomalies induce circulation changes over south Asia similar to those observed during drought years.

The central Pacific anomaly also produces westerly anomalies in the tropical stratosphere, around the globe.

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R. N. Keshavamurty

Abstract

No abstract available.

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R. N. KESHAVAMURTY
and
S. T. AWADE

Abstract

The different terms in the equation for the balance of standing eddy kinetic energy are estimated in the region of the monsoon trough over north India. The main contribution to the maintenance of the trough against frictional dissipation is from the work done by the horizontal pressure forces. The contribution by the mean and transient advection terms is of a smaller order of magnitude.

There is loss of standing eddy kinetic energy by the rising of cold air and sinking of warm air in the x-p plane in the lower troposphere. The circulation is forced by the flux of potentia1 energy from outside, mainly from the bottom and southern boundaries.

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