Climate Prediction of precipitation for the Nordeste rainy season of MAM 2001

contributed by Iracema F.A.Cavalcanti; Jose A. Marengo; Marcos B.Sanches, Helio Camargo, David Mendes

Centro de Previso de Tempo e Estudos Climticos (CPTEC) Instituto de Pesquisas Espaciais (INPE) Brazil

Monthly to seasonal dynamical atmospheric prediction at CPTEC have 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 a sophisticated biosphere model, SiB (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 ensemble 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 the Amazon river, in MAM, as analyzed 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 and southern Nordeste is well represented in ten years simulations (Marengo et al, 2001; Cavalcanti et al, 2000).

Two sets of integrations were performed in the beginning of March 2001 for the seasonal prediction of the rainy season of Nordeste (MAM). One set (I) was a result of 25 integrations using initial conditions of December, with predicted SST in the Tropical Pacific Ocean (NCEP) and Atlantic Ocean (SIMOC), and February persisted SST elsewhere. Other set (II) was performed with the same initial conditions but with persisted SST anomalies of February in all oceans. The model climatology is taken from results of 23 integrations using climatological SST.

Fig. 1a shows the ensemble mean of set (I) for the seasonal (MAM) average. Similar pattern predicted for this season last year was forecasted over Nordeste, although shifted to west. Positive anomalies are predicted for the extreme northwest of Nordeste and negative anomalies in the extreme eastern side of the region. Ensemble mean of set (II) is shown in Fig. 1b. The model also predicts above normal precipitation over the northwest sector and below normal rainfall over the eastern sector. The main difference is in the intensity and extension of anomalies. In both cases, the ITCZ is located to the north of equator, influencing only the extreme northwest of Nordeste.

Dispersion maps for the northern Nordeste region in Fig.2 show the behavior of individual members and of the ensemble mean for each month. In March, the ensemble mean shows positive anomalies when using predicted or persisted SST, but in May there are negative anomalies in the ensemble mean. This is the behavior for the area 3oS-10oS; 45oW-35oW, which presented opposite anomalies in the west-east extremes.

Verification of MAM 2000 prediction is given in Fig.3a, Fig. 3b, Fig. 3c. The observed anomalies show that the northwest of Nordeste had precipitation above normal and the eastern sector had dry conditions, indicating that the ITCZ was situated to the north of equator. This pattern was predicted by the model when using predicted SST. However it is seen that the characteristics of the model resolution (type and size) does not allow a prediction for the perfect position of the anomalies sign.

References

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