Climate Prediction of precipitation over South America
for DJF 2001/2002 and MAM 2002
contributed by Iracema F.A.Cavalcanti; José A. Marengo; Helio Camargo
Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)
Instituto de Pesquisas Espaciais (INPE)
Monthly to seasonal dynamical atmospheric prediction at CPTEC has been performed since January 1995. The model used for these predictions is the CPTEC version of the COLA AGCM which was derived from the NCEP model ( Kinter et al. 1988) and includes the SSIB module (Xue et al. 1991). The horizontal resolution of the AGCM CPTEC/COLA is T62 and there are 28 levels in the vertical (Cavalcanti et al. 1995). The seasonal predictions at CPTEC, during the first 3 years, were results of an ensemble of 4 members, using 4 consecutive days as initial conditions. In 1998 there was an increase in the number of members, with the prediction based on 25 integrations. The application of boundary conditions also changed, from the use of persisted SST anomalies, to predicted SST. In the tropical Pacific the SST is given by NCEP coupled model forecast, and in the tropical Atlantic the SST is predicted using a statistical model (Pezzi et al. 1998). Outside these regions persisted SST anomalies are used.
Predictions for the rainy season of Nordeste (Northeast of Brazil) in 1995 were discussed in Nobre et al. (1995), and those for 1996, in Nobre and Cavalcanti (1996). Results from simulations (with a T42L18 version of the model) of the Nordeste rainy season of 1993 and 1994 using observed monthly Sea Surface Temperature (SST) as boundary conditions and T42L18 compared well with observed values of precipitation, and with NCEP reanalyses data (Cavalcanti et al. 1996). Precipitation anomalies in Nordeste (area averaged 20S-120S; 450W-350W) from a simulation of 11 years (1986 to 1996), using T42L18 reproduces much of the interannual variability in this region, (Cavalcanti et al. 1998). The forecast of the March-May 1998, 1999 and 2000 rainy season in Nordeste verified well.
Analysis of systematic errors over Northeast South America show excessive precipitation on the eastern Nordeste and deficit over Amazon river mouth, in MAM, as analysed in a climatological simulation (Cavalcanti et al. 2001; Cavalcanti et al. 2000). These errors are removed when the anomalies are calculated, and the interannual variability over northern Nordeste is well represented in ten years simulation (Marengo et al. 2001; Cavalcanti et al. 2000).
As usual, the seasonal prediction at CPTEC is performed in two ways, one using persisted SST anomalies in all oceans and another using predicted SST in the Tropical Pacific and Atlantic Oceans and persisted SST anomalies in the other areas. As the results were very similar, only the results using predicted SST are shown. The model integrations at this month were performed using 18 initial conditions. The predictions for both DJF 2001/2002 and MAM 2002 are shown in Fig.1a,b. Above normal precipitation is forecast for North and North-east South America, which comprises eastern Amazonia and Nordeste. The higher values of positive anomalies observed over eastern Amazonia in DJF are extended to areas of Nordeste in MAM. To the south of this positive broad band there is a NW/SE band of below normal precipitation, in DJF, located in the position of the SACZ. In MAM the prediction still shows negative anomalies over southeast Brazil, but with reduced values. This region has low predictability, showing large dispersion among members, opposite to the north and northeast behaviour, where all members show the same sign of anomalies (Fig. 2 a,b,c,d). The above normal precipitation to the south of 300 S over South Atlantic is associated with the positive november SST anomalies which are taken as boundary condition.
The prediction taking into account the probability of dry, not dry, wet and not wet, following IRI method ( Mason et al. 1999) is shown in Fig.3 a,b. It is seen high probability of wet conditions for the north and northeast South America, except for the eastern coast, and high probability of dry conditions for parts of the southern and south regions. Normal conditions are predicted only for the extreme southwestern Brazil.
Monthly climate meetings at CPTEC are conduced to analyse and discusse the monthly observed large scale atmospheric and ocean conditions and the model results. Considering the predictability of several regions of South America, the intensity of the SST forcing, and verifying the consistence with other GCMs, a seasonal prediction consensus is performed based on the models results. In this month, the consensus prediction was performed to JFM 2002 (Fig.4). In the next issue of March, the MAM prediction for the Nordeste rainy season will be updated.
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