Simple <scp>GCM</scp> Simulations of Rainfall Over Northeast Brazil, Part 1: Systematic Effect of Canonical Sea Surface Temperature Patterns

ABSTRACT The response of February to April (FMA) Northeast Brazil precipitation to tropical sea surface temperature anomaly (SSTA) patterns is analysed by making ensemble seasonal forecasts with a simple GCM. The physical and dynamical link between the SSTAs and regional precipitation is examined for a set of tropical Pacific and Atlantic SSTAs, both separately and in combination. Five canonical SSTA patterns are considered: El Niño (strong eastern Pacific and moderate central Pacific); La Niña and the tropical North and South Atlantic. The model's anomaly responses to persisted SSTAs correspond quite well to observed precipitation regressions against SSTA timeseries: dry for El Niño and warm North Atlantic, wet for La Niña and warm South Atlantic. Experiments with reversed SSTAs show that model nonlinearity has a drying effect over northeastern South America. Combined tropical Atlantic SSTA patterns result in wet conditions over northern Northeast Brazil for a warm Atlantic or a negative dipole, and dry conditions for the contrary, so the South Atlantic has a greater impact than the North Atlantic. Combining SSTA patterns in the Pacific and Atlantic gives results that are similar to the sum of individual effects, with the stronger Pacific SSTAs exerting more influence despite the model's sensitivity to Atlantic conditions. Dry dynamical experiments with fixed basic states and heating perturbations confirm that interactive moist thermodynamics is crucial for accurately representing continental rainfall responses, even over short timescales. These results are generally promising for applications in operational seasonal forecasting, and this is reported in the companion paper, part 2.