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Julio Buchmann, Lawrence E. Buja, Julia Nogués-Paegle, and Jan Paegle

Abstract

A series of real-data integrations of the National Center for Atmospheric Research Community Climate Model with tropical heat anomalies display regions of pronounced subsidence and drying surrounding the anomaly. The present emphasis is upon subsidence and drying centers located several thousand kilometers westward and poleward of the heating. These features are repeatedly found in several different series of medium to extended range forecast experiments, including cases of tropical Atlantic heating and tropical east Pacific heating. This highly predictable sinking response is established within the first five days of these integrations. The normal modes of a set of primitive equations linearized about a resting basic state are used to partition model response into gravity-inertia and Rossby modes. The results show that most of the vertical motion response can be explained by gravity-mode contributions. The sensitivity of the response is examined through a series of numerical experiments with a simple global forecast model. These integrations suggest that the subsiding response surrounding the heated region is somewhat sensitive to the ambient circulation. In particular, the extratropical response tends to be greatest in the winter hemisphere, and it is relatively less sensitive to the precise location of the tropical heating than to the nature of the zonally averaged background flow. Further experimentation suggests that the peak subsidence response is almost linear in the heating amplitude. These experiments also demonstrate that a significant portion of the early response occurs independently of any fluctuations of the vorticity field and therefore is not merely a secondary circulation associated with extratropical Rossby wave responses. The latter response is relatively more sensitive to the presence of longitudinal vorticity gradients, and the dynamical interpretation is then less clear.

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Julio Buchmann, Lawrence E. Buja, Jan Paegle, and Robert E. Dickinson

Abstract

A series of real-data experiments is performed with a general circulation model to study the sensitivity of extended range rain forecasts over the Americas to the structure and magnitude of tropical beating anomalies. The emphasis is upon heat inputs over the tropical Atlantic, which have shown significant drying influences over North America in the author's prior simulations. The heating imposed in the prior experiments, that is, shown to be excessive by a factor of 2, is compared with the condensation heating rates that naturally occur in the forecast model. Present experiments reduce the imposed anomaly by a factor of 3 and also impose sea surface temperature decreases over the eastern tropical Pacific Ocean. The new experimental results are in many ways consistent with the author's prior results. The dry North American response is statistically more significant than the South American response and occurs at least as frequently in the different members of the experimental ensembles as in our prior experiments. The drying effect is accentuated by the presence of east Pacific cooling, but this does not appear to be the dominant influence. Over tropical South America, the Pacific and Atlantic modifications produce compensating influences, with the former dominating, and allow increased rainfall over the Amazon Basin.

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Julio Buchmann, Jan Paegle, Lawrence E. Buja, and Robert E. Dickinson

Abstract

Severe droughts occurred over eastern sections of North America and central sections of South America in 1986 and 1988. We summarize data suggesting that both periods were characterized by above-normal tropical Atlantic sea surface temperatures and convection, and investigate the response of a general circulation model to positive heating anomalies in the tropical Atlantic sector. An eight-case control ensemble of 30 day global predictions is made starting from the atmospheric state observed on 1 January of each year from 1977 through 1984. The same eight cases are integrated in a second experimental ensemble that is identical to the first control ensemble, except that a heating term is added to the thermodynamic equation in a region centered at 30°W, 6.6°N. This is intended to simulate the latent heating of enhanced tropical Atlantic convection. The third ensemble is identical to the second, except the heating is centered at 6.6°S.

Both heated ensembles produce reductions of forecast precipitation over most of North and South America, but these appear to have greater statistical significance over North America. Here the greatest precipitation reductions are forecast over the southern and eastern United States, and this response does not change substantially between the two experiments. The South American response is more sensitive to the placement of the heating anomaly. When the anomaly is located north of the equator, drying occurs over northeast Brazil; meanwhile this region receives increased rainfall when the anomaly is located south of the equator. Both experiment ensembles display a region of reduced rainfall over the Andes Mountains, and over southern portions of Brazil. However, only the former region is statistically significant above the 95% confidence level. The present usage of real initial data and an ensemble of cases permits us to draw quantitatively meaningful estimates of the time scale of response and case-to-case variability. For presently tested cases, the South American response is evident by day 5, but exhibits substantial intersample variability, and the North American response is fully established by day 10, and exhibits less intersample variability. The model drying effects can be explained only partly by enhanced local subsidence; much of the rainfall reduction appears to be related to a reorientation of the synoptic scale wave pattern in which the lower tropospheric circulation is unfavorable for water vapor inflow from source regions over the tropical Atlantic and Amazon Basin.

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