El Nino is NOT controlling South America climate
Fig. 1: The 30-day 850 MB (5,000 feet) wind flow identifies the strong high pressure zone over Southeast Brazil sustaining a dry climate.
Fig. 2: The western South Atlantic subtropical ridge is stronger than normal causing waters to warm dramatically off the east coast of Brazil.
Fig. 3-4: The 7-day and 30-day percent of normal rainfall across South America identifies the feast or famine rainfall pattern.
Discussion: The DRAMATIC wet Argentina/dry Brazil regime is not related to El Nino southern oscillation (ENSO). Subtropical ridging off the South Atlantic nosing westward across Brazil encouraged dryness last spring. Meanwhile a stronger than normal southern hemisphere storm track pushed active cold fronts into the subtropical ridge over Brazil and stalling the frontal rains over Argentina.
Now that we’re into summer the soil moisture regime is acting as a feedback mechanism to sustain the late spring/early summer climate set-up. Over southeastern Brazil a strong subtropical high pressure center has formed and is preventing showers that can help suppress drought from forming. Well to the north a low pressure area over northeast Brazil is preventing a moist Atlantic fetch from moving around or into the subtropical high also encouraging Brazilian dryness.
Stronger than normal westerlies remain across Argentina encouraging occasional frontal zones to stall and entrain moisture from wet soils producing more rainfall.
The mid-summer climate across Brazil and Argentina is a classic soil moisture feedback to the atmosphere climate pattern very unlikely to change during mid-to-late summer!