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Douglas G. Hahn
and
J. Shukla

Abstract

A short record of year-to-year variations of summer monsoon rainfall over India is compared with that of winter snow cover over Eurasia as derived from satellite data. An inverse relationship between these two quantities is indicated, i.e., winters with extensive (little) snow cover over Eurasia tend to be followed by summers with less (more) rainfall over India.

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Douglas G. Hahn
and
Syukuro Manabe

Abstract

An 11–level numerical model of the atmospheric circulation which has a prescribed seasonal variation of insulation and sea surface temperatures is integrated with respect to time for approximately three model years. The model is global in domain and incorporates a smoothed mountain topography. In order to investigate the role that mountains play in the south Asian monsoon circulation, a second numerical experiment, exactly the same as the first except that all mountains are removed, is integrated with respect to time from 25 March through July.

Analysis of the model with mountains reveals that the large–scale circulation associated with the south Asian monsoon is well simulated. However, the onset of the monsoon is approximately 10–15 days later than normal, and the atmosphere over the western Pacific seems to he dynamically too active, while the atmosphere over the northern reaches of the Bay of Bengal and northern India is relatively inactive.

Comparison of the simulation with mountains with the simulation without mountains reveals that the presence of mountains is instrumental in maintaining the south Asian low pressure system as the continental low forms far to the north and east in the simulation without mountain topography. In the model with mountains, much higher temperatures are maintained in the middle and upper troposphere over the Tibetan Plateau, a region where upward motion and latent heating dominate. Without mountains, downward motion and sensible heating by the earth's surface dominate in this region. In the simulation with mountains, high temperatures over Tibet produce a low pressure envelope over these mountains which extends southward over the plains of south Asia. The low pressure belt being located farther south than in the simulation without mountains produces a stronger north–south pressure gradient which enables moist southerly flow at the surface to penetrate farther northward into Asia. Many of the features of the monsoon break persist in the model without mountains as copious precipitation extends northward only to south India. Clearly, mountain effects help to extend a monsoon climate farther north onto the Asian continent.

The evolution of the south Asian monsoon is also influenced by the effects of mountains. Near the time of onset in the model with mountains, the subtropical jet abruptly jumps northward from a latitude just south of Tibet, 25°N, to a mean summertime position along 45°N. In the model without mountains, the subtropical jet gradually moves northward over a period of about two months, finally reaching a summertime position approximately 10° farther south than in the model with mountains. At the time of onset in the model with mountains, humid southerly flow near the earth's surface suddenly extends northward from equatorial latitudes to the south Asian low pressure belt centered at 30°N. In the model without mountains, humid southerly flow extends northward from equatorial regions, but it doesn't extend as far northward as northern and central India. These differences are attributed to mechanical and thermodynamical effects of the Tibetan Plateau.

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Syukuro Manabe
and
Douglas G. Hahn

Abstract

A spectral atmospheric circulation model is time-integrated for approximately 18 years. The model has a global computational domain and realistic geography and topography. The model undergoes an annual cycle as daily values of seasonally varying insolation and sea surface temperature are prescribed without any interannual variation. It has a relatively low computational resolution with 15 spectral components retained in both zonal and meridional directions. Analysis of the results from the last 15 years of the time integration indicates that, in middle and high latitudes, the model approximately reproduces the observed geographical distribution of the variability (i.e., standard deviation) of daily, monthly and yearly mean surface pressure and temperature.

In the tropics, the model tends to underestimate the variability of surface pressure, particularly at longer time scales. This result suggests the importance of processes with long time scales such as ocean–atmosphere interaction, in maintaining the variability of the atmosphere in low latitudes.

It is shown that global mean values of standard deviation of daily, 5-daily, 10-daily, monthly, seasonal and annual mean surface pressure of the model atmosphere may be approximately fitted by a corresponding set of standard deviations of a red noise time series with a decay time scale of slightly longer than four days. However, it appears that the temporal variation of surface pressure also includes minor contributions from disturbances with much loner decay time scales.

In general, the model tends to underestimate the persistence (or decay time scale) of atmospheric disturbances. However, it reproduces some of the features of the observed geographical distribution of decay time scale of the surface pressure fluctuations in middle and high latitudes.

The observed standard deviation of annual, hemispheric mean surface air temperature also is compared with model results. Although a clearcut evaluation of model performance is somewhat hampered by observational uncertainty, it appears that the model's value amounts to a substantial fraction of the corresponding standard deviation derived from observational studies.

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Syukuro Manabe
,
Douglas G. Hahn
, and
J. Leith Holloway Jr.

Abstract

A mathematical model of the atmosphere with a seasonal variation of insulation and sea surface temperatures is integrated numerically with respect to time over three model years. The model has a global computational domain and a realistic distribution of mountains. It contains a highly idealized parameterization of convection, i.e., dry and moist convective adjustment.

It is found that the model accurately simulates the seasonal variation of the location of the tropical rainbelt as well as that of the flow field associated with it. Over the continental regions of the model, the tropical rainbelt tends to form very close to the equator, whereas, in certain oceanic regions, it has a tendency to form away from the equator. Based upon a comparison of these results with those of another numerical experiment, it is concluded that this tendency is not due to an inherent characteristic of the rainbelt of the model to avoid the equator in oceanic regions, but rather it is due to the equatorial belt of low sea surface temperatures which is not favorable for the formation of a rainbelt. Over the sea, the surface temperature distribution seems to be the primary factor in determining the location of the rainbelt and accompanying tropical disturbances.

The primary source of kinetic energy of the disturbances in the model tropics is the conversion of eddy available potential energy which is generated by the effects of moist convection. A secondary source is the energy supplied from middle latitudes through pressure interaction. This effect has a significant magnitude in the subtropics of the model. The belt of maximum eddy conversion moves from one summer hemisphere to the other with respect to season in a manner similar to the tropical rainbelt. On the other hand, the contribution of pressure interaction to the production of eddy kinetic energy is significant in the winter hemisphere and thus supplements the contribution of eddy conversion. In general, the rate of eddy conversion due to transient eddies is particularly large in areas of relatively warm sea surface temperatures, where the tropical rainbelt and its accompanying disturbances predominate.

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