Influence of Large-Scale Antecedent Soil Moisture Anomalies on South American Hydroclimate
Southeastern South America (SESA) is renowned for its thriving rainfed agriculture. In SESA, an important moisture source is the South American low-level jet (SALLJ), which is related to intense rainfall occurrences. Hence, improving subseasonal predictability of the SALLJ, moisture transport, and precipitation in SESA could yield significant socioeconomic benefits. Additionally, SESA is a region with strong land-atmosphere (L-A) interactions. Through L-A interactions, soil moisture memory could potentially close gaps in subseasonal forecasting. Therefore, understanding the effect of soil moisture anomalies (SMAs) on regional hydroclimate is critical. First, we evaluate how dry SMAs influence SESA climate at monthly time scales and whether the response is sensitive to the location of SMAs. We employ the Community Earth System Model (CESM) to perform three experimental simulations with prescribed dry SMAs within (1) SESA, (2) western SESA, and (3) eastern SESA. The results show distinct moisture transport and precipitation response in different simulations. When the dry SMAs are located over SESA and eastern SESA, there are widespread negative precipitation anomalies. Conversely, when the dry SMAs are located over western SESA, the simulations show positive precipitation anomalies over northeastern Argentina, which are associated with the colocation of the enhanced southward moisture flux and the SALLJ exit region. Using the European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis v5 (ERA5) monthly data, we conduct a composite analysis of extremely dry cases over western SESA, and the results support the findings from our CESM simulations. Second, we investigate how SMAs affect SALLJ at daily time scales. We use ERA5 reanalysis to identify SALLJ events based on wind speed and vertical wind shear. These events have shown clear relationships between precipitation, antecedent northerly moisture flux, geopotential height at 850 hPa, and low-level temperature. Further studies will analyze the heat budget to characterize the sources of temperature change and link SMAs to the SALLJ events.
