Monsoonlike Circulations in a Zonally Averaged Numerical Model with Topography

V. Brahmananda Rao Instituto Nacional de Pesquisas Espaciais, INPE, Sao Jose dos Campos, Brazil

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J. Pablo Reyes Fernandez Instituto Nacional de Pesquisas Espaciais, INPE, Sao Jose dos Campos, Brazil

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Sergio H. Franchito Instituto Nacional de Pesquisas Espaciais, INPE, Sao Jose dos Campos, Brazil

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Abstract

A primitive equation global zonally averaged general circulation model is used to study the effects of the topography on the atmospheric annual cycle. A smoothed zonally averaged topography that has a form similar to that observed was used. The control experiment showed that the model was capable of capturing the zonally averaged behavior of the annual cycle. The model is able to capture some characteristics of the monsoonlike circulation such as the seasonal wind reversal and the easterly jet in the boreal summer. Even in the absence of topography the model was able to reproduce the monsoonlike features. However, the circulation was weak and the position of its components was altered. This suggests that the topography has an important role in modifying the intensity and position of the monsoon circulation. Sensitivity tests were made in order to investigate the effects of high elevation and its steep southern slope. Two experiments were performed: 1) increasing the elevation of orography without changing the steepness of the slope, and 2) increasing both the elevation and the steepness of the slope. The results indicated that the steepness of the southern slope seems to control the monsoonlike flow in the model. The model was also capable of reproducing a monsoonlike response to changed external conditions. When the values of the earth’s orbital parameters (precession, obliquity, and eccentricity) were changed to those of 9000 yr BP, the precipitation and circulation intensified, which seems to agree with paleoclimatic evidence.

Corresponding author address: Dr. V. Brahmananda Rao, Instituto Nacional de Pesquisas Espasiais, INPE, CP515, 12201-970, Sao Jose dos Campos, SP, Brazil.

Email: vbrao@met.inpe.br

Abstract

A primitive equation global zonally averaged general circulation model is used to study the effects of the topography on the atmospheric annual cycle. A smoothed zonally averaged topography that has a form similar to that observed was used. The control experiment showed that the model was capable of capturing the zonally averaged behavior of the annual cycle. The model is able to capture some characteristics of the monsoonlike circulation such as the seasonal wind reversal and the easterly jet in the boreal summer. Even in the absence of topography the model was able to reproduce the monsoonlike features. However, the circulation was weak and the position of its components was altered. This suggests that the topography has an important role in modifying the intensity and position of the monsoon circulation. Sensitivity tests were made in order to investigate the effects of high elevation and its steep southern slope. Two experiments were performed: 1) increasing the elevation of orography without changing the steepness of the slope, and 2) increasing both the elevation and the steepness of the slope. The results indicated that the steepness of the southern slope seems to control the monsoonlike flow in the model. The model was also capable of reproducing a monsoonlike response to changed external conditions. When the values of the earth’s orbital parameters (precession, obliquity, and eccentricity) were changed to those of 9000 yr BP, the precipitation and circulation intensified, which seems to agree with paleoclimatic evidence.

Corresponding author address: Dr. V. Brahmananda Rao, Instituto Nacional de Pesquisas Espasiais, INPE, CP515, 12201-970, Sao Jose dos Campos, SP, Brazil.

Email: vbrao@met.inpe.br

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