IRI Home
Back to January 2002 digest

Northeast Brazil's Ceara State Tests High Resolution Forecast

The state of Ceara's weather and climate institute, FUNCEME, in collaboration with the IRI, have produced the first experimental seasonal forecast for the Northeast region using a dynamical downscaling prediction system.

The prediction system
This prediction system, developed for the northeast region of Brazil by scientists at FUNCEME and climate research specialists at the IRI, is comprised of the ECHAM4.5 AGCM (T42) and the NCEP Regional Spectral Model version 97 (RSM97) with horizontal resolution of 60 km. IRI global climate forecasts (i.e., ECHAM4.5 forecasts) are used to drive the NCEP RSM97 to produce high resolution climate forecasts for northeast Brazil. A number of statistical tools are developed to correct the systematic and conditional biases of the model forecasts.

The maps above show experimental dynamically downscaled forecast result, which is but one of a number of tools used in generating the final product of a consensus seasonal forecast. The probabilities (as defined by the vertical colorbar) are given for each of three categories ("Above Normal", "Near Normal", "Below Normal"), based the model's past performance with respect to the observations when predicting that category. The information given in the "Above", "Normal", and "Below" maps is then combined into a "Re-built Forecast" (see the lower left panel). This "Re-built Forecast" gives the most likely qualitative forecast. There are 5 categories for the "Re-built Forecast": Above (A), Not Below (NB), Normal (N), Not Above (NA), and Below (B). The maps above show or the upcoming rainy season (i.e., Jan-Feb-Mar 2002), "Below" category over Marajo Island, and the coast of Para, Maranhao, and Piaui, "Above" category over States of Goias and Minas Geais, and "Not Above" category along the coast of Ceara. This is an experimental system, and these results are used in conjunction with other tools to create the final forecast, which is discussed below.

Climate variability in the Northeast
Northeast Brazil has a history of crippling droughts and a significant population which relies on rain-fed agriculture. A single rainy season from February-May provides much needed moisture for this semi-arid region, which receives on average less than 1000 mm of rainfall annualy. Rainfall is largely dependent on the southward movement of a band of rainfall across the Atlantic Ocean which follows the warm surface ocean temperatures in late austral summer. During years when the warm ocean temperatures remain too far north, or alternatively when a strong El Nino shifts the tropical circulation, the rainfall band does not reach far enough south to provide sufficient rains for the Northeast.

This figure shows Northeast Brazil Feb-May rainfall anomalies for the period
1971-2001, based on monthly station rainfall from Fortaleza and Quixeramobim, with climatological base period 1971-2000.

The Recent Past
A severe drought resulted from the 1997/98 El Nino throughout the region. However in the three years since, near average rains were beneficial in replenishing aquifers and reserviors in 1999 and 2000, while last year was again somewhat dry. Current reservior levels in the Northeast are reported to be near 25% of capacity.

The Seasonal Forecast
For the upcoming February-May rainy season the FUNCEME forecast, which is based on a number of tools including the experimental downscaling system, calls for a greater likelihood for normal rains. This is consistent with the forecast from CPTEC and also from the IRI.

To top     Back to January 2002 digest