Executive summary: Many climate forecasts for precipitation across Brazil during just-started meteorological spring (SEP/OCT/NOV) are wetter than normal. An important forecast given the ongoing Brazil drought and implication for crops that welcome rainfall would bring. The catalyst to the wetter climate pattern is a developing La Nina and warm SSTA pattern in the tropical Atlantic shifting toward northeast Brazil. However, development of La Nina is not agreed upon by all forecast models and the tropical Atlantic SSTA pattern has cooled somewhat during the past 30 days. Close monitoring of the ENSO and tropical Atlantic SSTA pattern in September is required as unwanted drier climate forecast adjustments could occur.
Fig. 1: Soil moisture anomalies across South America for August 21-30, 2021 identifying where the drought region is located.
Discussion: Brazil is immersed in a drought regime based on NCDC/PSD soil moisture anomaly analysis for the last third of August 2021 (Fig. 1). All Brazil crop areas are affected. Marginal dryness has also developed in northern Argentina. Compared to last year at this time, the 2021 drought is larger expanding from a (previous) long-term drought in north-central Brazil (Fig. 2).
Fig. 2: Soil moisture anomalies across South America for August 21-30, 2020 identifying where the drought region is located.
Fig. 3-4: Rainfall anomalies for the past 12 months and last summer temperature anomalies across South America.
The 12-month rainfall anomalies across South America identify why drought expansion occurred in Brazil (Fig. 3). Helping to accelerate the drought was the anomalous heat last summer across all of northeastern Brazil (Fig. 4). Interestingly, the hotter/drier climate regime in northeast Brazil during last summer into autumn was counter to typical La Nina climate (Fig. 5). La Nina is forecast to return by some (not all) forecast models heading into upcoming summer (Fig. 6).
Fig. 5: La Nina climatology for DEC/JAN/FEB according to NOAA. Note that north/northeast Brazil is typically wetter than normal.
Fig. 6: ENSO forecast models are somewhat split on whether Nino34 SSTA is cold enough for La Nina by November 2021.
According to probability forecasts issued by the International Research Institute (IRI) for Weather and Society the climate forecast bias for Brazil for meteorological spring (SEP/OCT/NOV) 2021 is wet (Fig. 7). The monthly precipitation forecasts from Climate Impact Company are also wet in Brazil (Fig. 8-10). The catalyst to the wet forecast is the historical relationship of La Nina for wet climate in Brazil and an anticipated enhancing effect on that wet forecast by warm SSTA in the tropical Atlantic Ocean forecast to shift toward northeast Brazil (Fig. 11).
Fig. 7: The meteorological spring 2021 climate probability forecast for South America by the International Research Institute for Climate and Society.
Fig. 8-10: Monthly precipitation anomaly forecasts for South America during meteorological spring by Climate Impact Company.
Fig. 11: Global SSTA forecast for November 2021 by the International Multi-model Ensemble (IMME).
The potential developing concern is that the Brazil drought area (Fig. 12) is not receiving the anticipated rainfall as spring climate forecasts are indicating. The 15-day percent of normal rainfall forecast across Brazil by GFS ENS is dry (Fig. 13).
Fig. 12-13: NOAA soil moisture anomaly analysis for South America and the 15-day percent of normal rainfall forecast for South America by GFS ENS.
Meteorological spring has just started and a wetter pattern can evolve later this month. However, there are some signs that the catalysts driving the wet forecast may underachieve. As previously indicated, not all models are in agreement on a developing La Nina. Additionally, the western shift of a warm SSTA pattern across the tropical South Atlantic toward the northeast coast of Brazil is not as strong as expected. Current SSTA analysis reveals the warm core is in the central tropical South Atlantic (not off the northeast coast of Brazil) and the 30-day change analysis indicates this warm water anomaly is weakening (Fig. 14-15).
Fig. 14-15: Current and 30-day change of the South Atlantic tropical SSTA pattern.
Conclusion: Most forecast models are maintaining a wet forecast for Brazil for meteorological spring. However, the catalysts for these wet forecasts require close monitoring over the next few weeks. Does La Nina develop? Does the warm tropical SSTA pattern in the Atlantic regain strength and shift toward Brazil? These are the September questions paramount to the spring rainfall forecast for Brazil. Right now, there is increasing concern that these wet (climate) generators will develop.