Subseasonal Variability of Rossby Wave Breaking and Its Impacts on Tropical Cyclones during the North Atlantic Warm Season

This study investigates the subseasonal variability of the anticyclonic Rossby wave breaking (AWB) and its impacts on atmospheric circulations and tropical cyclones (TCs) over the North Atlantic in the warm season. It was found that that AWB affects the tropospheric wind and humidity fields over the tropical-subtropical Atlantic on the subseasonal timescale. Significant atmospheric anomalies mainly occur within 8 days of a peak of AWB activity and attenuate quickly. Such anomalies can lead to suppressed TC activity on the subseasonal timescale, but a significant negative correlation between AWB and TCs does not exist every year due to the modulation of TCs by other factors. Further analysis shows that the subseasonal AWB-TC correlation tends to be stronger during the years of enhanced MJO activity and during El Nino years.

The impacts of AWB on the predictability of Atlantic TCs were examined using the Global Ensemble Forecasting System (GEFS) reforecasts with the lead time up to 2 weeks. The GEFS has a lower hit rate of tropical cyclogenesis than climatology during the active AWB episodes and is less skillful in capturing the subseasonal variation of TC activity during the years of enhanced AWB activity. The lower predictability of TCs can be attributed to the lower predictability of environmental variables, such as vertical wind shear, moisture, and low-level vorticity.