Impacts of Extratropical Rossby Wave Breaking on Atlantic Tropical Cyclone Activity

Abstract:

With favorable tropical SST conditions, the 2013 Atlantic hurricane season turns out surprisingly quiet. The suppressed Atlantic tropical cyclone (TC) activity can be attributed to the unusually frequent breaking of midlatitude Rossby waves. We will show that the frequency of RWB over the North Atlantic and indices of Atlantic TC activity are negatively correlated in 1979-2013, at confidence levels comparable to the correlation between tropical Atlantic SST and TC activity.

Our case study suggests that RWB induces tropics-extratropics mixing and perturbs the tropical Atlantic atmosphere. This is supported by the composites of RWB-induced anomalies using RWB cases identified in July-October in 1979-2013. RWB frequency over the North Atlantic varies substantially from season to season, and its variations are closely associated with the variability of tropical Atlantic environment. More (less) frequent RWB contributes to higher (lower) vertical wind shear and lower (higher) moisture content. Consequently, TC activity is substantially suppressed when RWB is frequent. We also find that the correlation is mostly contributed by RWB over the West Atlantic, although RWB over the East Atlantic also have significant impacts on TC activity. Taken together, our study highlights the extratropical impacts on Atlantic TC activity.