Seasonal North Atlantic tropical cyclone (TC) activity is largely regulated by El Niño-Southern Oscillation (ENSO), the Atlantic Meridional Mode (AMM) and Multidecadal Variability (AMV), and the associated difference in sea surface temperature (SST) between the Atlantic main development region (10°-20°N, 60°-20°W) and the equatorial Pacific. However, the fluctuating amplitudes and phases of ENSO, AMM, and other climate modulators contribute to a spatial inhomogeneity of seasonal hurricane activity throughout the North Atlantic basin, over which TC impacts may vary significantly. For instance, ENSO is known to modulate Atlantic vertical wind shear more acutely in the western tropical North Atlantic, thus influencing TC development in close proximity to the US. Additionally, Atlantic Niño has recently been shown to enhance the development of powerful TCs in the eastern tropical North Atlantic. Therefore, analysis of sub-basin seasonal TC activity would enable the development of “user-oriented” outlooks (e.g., Gulf of Mexico energy sector) to better inform regional stakeholder interests. While seasonal outlooks of North Atlantic hurricane activity have been available for decades, skilled predictions of sub-basin TC activity have been elusive and are not yet incorporated in official seasonal forecasts. We refine the Pacific-Atlantic interbasin SST relationship to develop seasonal outlooks for five specific North Atlantic sub-basins (e.g., Caribbean, Gulf of Mexico, US East Coast, Subtropical North Atlantic and Tropical North Atlantic). We find useful skill in forecasting seasonal sub-basin accumulated cyclone energy for above- and below-normal seasons, particularly for the Caribbean, using SST predictors from the North American Multi-Model Ensemble.

Recent studies have demonstrated that the difference in sea surface temperature anomalies (SSTAs) between the tropical Atlantic main development region (MDR) and the tropical Pacific (Niño 3) modulates Atlantic tropical cyclone activity. This study further explores the seasonality of Pacific and Atlantic contributions to Atlantic hurricane activity. Our analysis shows that while MDR and Niño 3 SSTAs are equally important for late-season (September-November) activity, early-season (June-August) activity is largely modulated by MDR SSTAs. This reflects the increased (reduced) variance of MDR (Niño 3) SSTAs in the early-season due to their phase locking to the seasonal cycle. Further analysis yields skillful forecasts using an MDR-Niño 3 interbasin index derived from hindcasts of the North American Multi-Model Ensemble with May initial conditions. However, the prediction skill for MDR SSTAs is lower than that of Niño 3 SSTAs, suggesting that increasing the prediction skill for MDR SSTAs is key to improving seasonal outlooks.

Heat waves are among the deadliest natural hazards affecting the United States (US). Therefore, understanding the physical mechanisms modulating their occurrence is essential for improving their predictions and future projections. Using observational data and model simulations, this study finds that the interannual variability of the tropical Atlantic warm pool (AWP, measured as the area enclosed by the 28.5°C sea surface temperature isotherm) modulates heat wave occurrence over the US Great Plains during boreal summer. For example, a larger than normal AWP enhances atmospheric convection over the Caribbean Sea, driving an upper tropospheric anticyclonic anomaly over the Gulf of Mexico and Great Plains, which strengthens subsidence, reduces cloud cover, and increases surface warming. This circulation anomaly thus weakens the Great Plains low-level jet (GPLLJ) and associated moisture transport into the Great Plains, leading to drought conditions and increased heat wave occurrence for most of the US east of the Rockies.