Climate Prediction of precipitation over South America for DJF 1998/99 and MAM 1999

Contributed by Iracema F.A.Cavalcanti, Luciano P.Pezzi, Jose Marengo , Gilvan Sampaio, Marcos Barbosa Sanches

Centro de Previsao de Tempo e Estudos Climaticos (CPTEC) Instituto de Pesquisas Espaciais (INPE) Brazil

Monthly to seasonal dynamical atmospheric predictions 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 (Xue et al., 1991). The horizontal resolution of the AGCM is T62 and there are 28 levels in the vertical (L28; Cavalcanti et al., 1995). 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 T42 L18) version of the model of the Nordeste rainy season of 1993 and 1994 using observed monthly Sea Surface Temperature (SST) as boundary conditions 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 an 11 year simulation (1986 to 1996), using the T42L18 model reproduces much of the interannual variability in this region. (Cavalcanti et al., 1998). The forecast of the February-May 1998 rainy season in Nordeste was quite good.

The climate prediction performed in December 1998 for the Southern Hemisphere summer 1998/99 and autumn 1999 were done with 25 initial conditions (The first 25 days of September 1998) and using predicted SST in the tropical Pacific (from the NCEP coupled ocean-atmosphere dynamical model) and predicted SST for the tropical Atlantic by with a statistical model based on canonical correlation analysis (Repelli and Nobre, 1998). Persisted SST anomalies were used in the other oceans. The model climatology was an average of 23 integrations using climatological SST. Parallel runs were made with persisted SST for both Pacific and Atlantic, but only for 4 integrations were made.

The predicted precipitation for South America using the 25 members ensemble is shown in Fig.1a(postscript) for the Southern Hemisphere summer 1998/99 and Fig.1b(postscript) for autumn 1999. In order to improve our confidence in the results, we subjectively subsampled the 25 member ensemble. The 12 members of this sub-ensemble were determined by how well they agreed (subjectively) with the observed precipitation anomaly during November 1998. The observed November 1998 precipitation anomaly is shown in Fig. 2a (postscript) and the simulated precipitation anomaly for the 12 member sub-ensemble is shown in Fib. 2b. The sub-ensemble forecasts for DJF 1998-99 and MAM1999 are shown in Figs. 3a and 3b, respectively.

The DJF 1998/99 precipitation anomaly forecast shows enhanced convective activity in the South Atlantic Convergence Zone (SACZ) affecting parts of Southeast, south of Nordeste and the related subsidence branch to the south. Below normal precipitation is forecasted for part of southeast Brazil and also in western Amazonia, while rainfall in extreme southern Brazil is forecasted to be near normal. The prediction for Nordeste indicates a tendency for average rainfall from December to January and a relatively weak beginning to the rainy season in February. During MAM the predicted rainfall is above normal in the extreme north of Nordeste and normal in other areas of the region. Rainfall above normal over the mouth of the La Plata River is consistent with large positive SST anomalies near that area off the coast of Argentina-Uruguay. The forecasts based on the 4 member with persisted SST shows an rainfall anomaly pattern similar to Fig. 1a and 3a.

The behavior of the 25 members for several areas over Brazil is presented in Fig.4. The solid dark lines refer to the chosen 12 members and the dashed light lines to the other 13 members. The best agreement among members during the whole period is in the North region of Brazil (Fig.4d,e,f). There is a contrasting pattern of anomalies between east (Fig.4f) and west (Fig.4e) Amazonia. A contrasting anomaly pattern of rainfall anomalies is also detected between north (Fig.4g) and south (Fig.4h) sections of the Northeast region. South (Fig.4a) and central (Fig.4 c) regions show ensemble means close to normal. The southeast region (Fig.4b) has the largest ensemble spread if all members are considered. Considering only the 12 member sub-ensemble there is agreement for above normal rainfall mainly in the austral summer season. Northern Northeast (Fig. 4g) shows a tendency for slightly above normal MAM rainy season.

Acknowledgments: To Dr. Pedro Silva Dias and Dr. Carlos A.Nobre for the discussions on the choice of the similar members.

References

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