In the U.S., peak summer (June-July) rainfall variability, especially east of the Rockies, is largely linked to North Atlantic sea surface temperature (SST) anomalies and associated variations in the Bermuda High. However, these well-established relationships almost completely break down in late summer to mid fall (August-October). Thus, operational seasonal forecast models have generally low skill in August-October season for U.S. regional rainfall, leaving a large portion of U.S. populations vulnerable to unpredictable extreme events in that season. In a new article published in the Geophysical Research Letters (Kim et al., 2020), a team of scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) identified key physical processes that modulate U.S. rainfall variability in August-October season. This study analyzed both historical observations and model simulations to show that in August-October season atmospheric convective activity in the Caribbean Sea, modulated by the Pacific‐Atlantic interbasin SST anomaly contrast, directly influences the atmospheric low‐level jet that carries warm and moist air from the Gulf of Mexico into the United States and thus affects U.S. rainfall variability. This newly identified mechanism suggests that the Pacific-Atlantic interbasin SST anomaly contrast and Caribbean Sea convective activity could serve as potential predictors for U.S. rainfall in August-October season, which may address the U.S. rainfall prediction gap in that season and thus improve the operational seasonal U.S.rainfall forecast systems.
Kim, D., S.-K. Lee and H. Lopez, G. Foltz, V. Misra, and A. Kumar, 2020: On the role of Pacific-Atlantic SST contrast and associated Caribbean Sea Convection in August-October U.S. regional rainfall variability. Geophys. Res. Lett., 47, e2020GL087736. https://doi.org/10.1029/2020GL087736.