Climate Influences on South America Summer Climate

Fig. 1: IMME global SSTA forecast for JAN/FEB/MAR 2023 identifying a historic warm band across the subtropics.

Discussion: Look at the warm SSTA forecast across the subtropical southern hemisphere valid for mid-to-late summer (Fig. 1). The projection by the IMME model is likely the warmest signature on record. Above and downwind warm SSTA zones, upper-level high-pressure ridging in the subtropics is generated. The Climate Impact Company constructed analog (CIC-CA) projection of the southern hemisphere subtropical ridge during the summer season is immense and stretches across South America increasing summer drought risk on that continent (Fig. 2).

Fig. 2: Climate Impact Company constructed analog meteorological summer 2022-23 projected 500 MB anomalies.

Fig. 3: Climate Impact Company constructed analog meteorological summer 2022-23 precipitation anomaly forecast.

The CIC-CA precipitation anomaly forecast for meteorological summer indicates dryness emerges across Northern Argentina to Southeast Brazil (Fig. 3). Southwest and southeast Brazil have wet soils now after the wet climate pattern observed during the past 3 months. Somewhat unusual is the development of a cry summer climate across regions of wet soils forming during preceding spring. However, the CIC-CA forecast is stressing the influence of unusually strong subtropical high-pressure ridging attached to warm SSTA either side of Argentina to drive the evolving dry climate pattern. The forecast is made with average confidence. Other forecasts such as the rainfall probability prediction by the International Research Institute for Climate and Society focus dryness on zones already experiencing drought while to the north probability is more uncertain (Fig. 4).

Fig. 4: International Research Institute for Climate and Society precipitation probability forecast for meteorological summer 2022-23.

Short-term forecasts are interesting. An amplified upper-level trough delivers wet weather to Brazil during the next week or so (Fig. 5). Some of the rain is excessive (>3 in.). Unusually cool weather accompanies the rainfall. In the 8-14-day period the cold front focusing the wet weather shifts northward with widespread dry weather to follow (Fig. 6) while remaining mostly cool. The issue is whether the persistent trough responsible for wet weather in Southern Brazil the past 3 months weakens. The CIC-CA forecast pushes the trough offshore for summertime.

Fig. 5-6: ECM ENS percent of normal rainfall across Brazil and Argentina for the next 2 weeks.