Climate Prediction of Precipitation over South America for DJF 1999/2000 and MAM 2000 using the CPTEC/COLA AGCM

contributed by Iracema F.A.Cavalcanti, José A. Marengo, Christopher Castro, Gilvan Sampaio and Marcos Barbosa Sanches

Centro de Previsão de Tempo e Estudos Climáticos (CPTEC) Instituto de Pesquisas Espaciais (INPE), Brazil

Monthly to seasonal dynamical atmospheric prediction have been performed at CPTEC since January 1995. The model used for these predictions is a 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 (Cavalcanti et al. 1995). Predictions for the rainy season of Nordeste (Northeast of Brazil) in 1995 were discussed in Nobre et al. (1995). Seasonal predictions for DJF and MAM were shown in Cavalcanti et al. (1998a,b,1999). Results from simulations of the Nordeste rainy season of 1993and 1994 using observed monthly Sea Surface Temperature (SST) as boundary conditions and T42 L18 resolution compared well with observed values of precipitation, and with NCEP reanalyses data (Cavalcanti et al. 1996). Precipitation anomalies in Nordeste (averaged area of 20S-120S; 450W-350W) from a simulation of 11 years (1986 to 1996), using T42L18 resolution, reproduces much of the interannual variability in this region (Cavalcanti et al., 1998).

Two ensemble forecasts of 15 members were made in the beginning of December 1999 for the seasonal prediction of DJF 1999/2000 and MAM 2000. The initial conditions for the two ensembles were 15 consecutive days from 14th to 28th September, 1999. Persisted November SST anomaly was applied as forcing boundary condition for one set of the ensembles, and predicted SST anomalies for the other set. The predicted SST anomalies for the Pacific Ocean were taken from the dynamical coupled ocean-atmosphere model of NCEP, and predicted SST anomalies for the Atlantic Ocean were obtained from the statistical model based on canonical correlation analysis, SIMOC/CPTEC (Pezzi et al. 1998). The model climatology is taken from a 10 year run nine member ensemble with observed SST.

DJF 1999-2000 predicted precipitation anomaly is shown in Figs. 1a-b using the persisted SST anomaly and the predicted SST anomaly. The model predicts above normal precipitation over east Amazonia region and northwest area of Nordeste in both ensemble predictions. Above normal rainfall is also predicted over almost all areas of Nordeste, including the semi-arid inland areas ("sertão"), when using the predicted SST anomalies, (Fig. 1b). Negative precipitation anomalies predicted in the oceanic area of the SACZ reflect the negative SST anomalies observed in November. However, the continental area of the SACZ shows enhanced or normal precipitation.

Figs. 2a-b show MAM 2000 predicted precipitation anomaly using the persisted and predicted SST anomalies. Above normal precipitation is also seen over Nordeste during the rainy season of MAM, more intense when using the predicted SST. The main difference is related to the Atlantic conditions that show favorable SST dipole in the predicted SST case, indicated by SIMOC prediction. The negative anomalous precipitation over South Atlantic are still related to the negative SST anomaly in that area and to the resulting subsidence associated with the strong ascent motion in the ITCZ region.

The behaviour of the 15 members from October to May for the Nordeste Region is shown in Fig. 3.

References

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