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.

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.

Abstract

We investigate the pattern of Amazon Basin rainfall forecasts of ten Global Weather Experiment (GWE) cases. Although the computations are based upon a rather crude wavenumber 15 resolution, the control forecasts exhibit a rather fine structure of the rainfall over tropical South America, including enhancements over the interior of the Amazon Basin and suppression on the northeast coast of Brazil. The forecasts appear to be in rather good agreement with climatology. The sensitivity of this model forecast to the presence of anomalous east Pacific heating is investigated through experiments in which a nonadiabatic term is added to the thermodynamic equation. These experiments suggest significant suppression of rainfall over the central Amazon Basin, and especially over the northeast portion of Brazil. This suppression is associated with the downward branch of a Walker circulation whose development is determined by a region of subsidence which propagates eastward from the eastern Pacific at a rate of about 30 m s⁻¹. This evolution, which is consistent with the Kelvin wave contribution to the Walker cell, affects Brazil within about two days of the heating onset.

The evolution of upper-level convergence, implied sinking motion, and suppression of rainfall over tropical South America in the forecasts does not depend sensitively upon the placement of the anomalous tropical Pacific heating. In particular, enhancements of the North Pacific are approximately as effective as those of the South Pacific.