Deep Layer Soil Moisture Deficits in South America and Australia

Pacific Decadal Oscillation/Atlantic Multi-decadal Oscillation Outlook
12/17/2020, 11:00 am EST
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Mid-to-late summer regional soil moisture deficit in the deep layers (10-200 CM) are a key diagnostic for monitoring potential flash drought risk. If a period of 7-10 days (or more) of dry and anomalous hot weather occur over these regions a fast-emerging drought will develop.

Highlight: South America 15-Day Rainfall Trajectory and Concerning Deep Layer Soil Moisture Deficit Areas. A Wet Australia Forecast.

Fig. 1: The 15-day upper air pattern across South America according to ECM ENS with annotated sensible weather effects.

South America discussion: An unusual presence of a low latitude upper trough in the tropical North Atlantic enhances northwest South America rainfall for late December into early January (Fig. 1). Meanwhile a second upper trough drops anchor just east of Uruguay and extends to Southeast Brazil much of the next 15 days. The combination of the two upper trough(s) inspires a wet weather trajectory across southwest to southeast Brazil sometimes reaching the northern Brazil soybean areas over the next 15 days. The wet weather risk misses an evolving drought zone in northeast Brazil and another developing dry zone in Northeast Argentina.

Meanwhile the drought concern areas in Brazil remain zones where deep layer soil moisture is in deficit due to long periods of rainfall shortages (Fig. 2). The dry zone is focused on the northern soybean crop areas of Brazil. A second area is developing in northeast Argentina. These zones are at risk of flash drought if 7-10 days of anomalous heat and dry weather affect these zones mid-to-late summer.

Fig. 2: Deep layer (10-200 CM) soil moisture deficits in Brazil.

Australia discussion: Surprisingly, Northeast Australia has a deep layer soil moisture issue as marginal dryness has developed (Fig. 3). The unexpected drier than normal start to the warm season is the culprit. However, primarily due to the La Nina climate and a stationary convection phase of the Madden Julian oscillation (MJO) to the north of Australia, a wet 15-day forecast is indicated by all models (Fig. 4).

Fig. 3: Deep layer soil moisture analysis across Australia.

Fig. 4: The ECM ENS 15-day rainfall anomaly forecast across Australia.