Extratropical Impacts on Atlantic Tropical Cyclone Activity

Zhuo Wang,Gan Zhang,Melinda S Peng,Timothy J Dunkerton,Gudrun Magnusdottir

doi:10.1175/jas-d-15-0154.1

Abstract

Abstract With warm sea surface temperature (SST) anomalies in the tropical Atlantic and cold SST anomalies in the east Pacific, the unusually quiet hurricane season in 2013 was a surprise to the hurricane community. The authors’ analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August 2013 can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant mid- to upper-tropospheric dryness and strong vertical wind shear hindered TC development. Using the empirical orthogonal function analysis, the active Rossby wave breaking in August 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the main development region (MDR) relative SST and higher than that with the Niño-3.4 index. This study highlights the extratropical impacts on Atlantic TC activity, which may have important implications for the seasonal predictability of Atlantic TCs.

Highlights

Atlantic tropical cyclone (TC) activity exhibits substantial interannual variability
Our analyses suggest that the frequent occurrence of anticyclonic Rossby wave breaking (RWB) suppresses Atlantic TC activity, potential vorticity (PV) filaments associated with RWB may occasionally lead to tropical cyclogenesis (Galarneau et al 2015)
Despite the favorable tropical sea surface temperature (SST) anomalies and active easterly waves over West Africa and the east Atlantic, TC activity over the Atlantic was substantially suppressed in August and early September of 2013, which led to a seasonal prediction bust

Summary

Introduction

Atlantic tropical cyclone (TC) activity exhibits substantial interannual variability. Dynamical models show that AGCM ensembles, even when driven by the same SST forcing, exhibit a large spread in the predicted hurricane counts (e.g., Chen and Lin 2013) This suggests that the internal variability of the atmosphere or forcing other than the tropical SST, which are not included in statistical schemes or not well captured by dynamical models, may have significant impacts on the TC activity. The EKE anomalies (Fig. 2d) suggest that AEWs are anomalously active over West Africa and the east Atlantic but relatively inactive west of 408W This is consistent with the authors’ impression in the realtime wave tracking that tropical easterly waves tended to weaken over the central Atlantic Since our real-time wave tracking suggests that the west and central Atlantic is a crucial region for the suppression of hurricane activity, we will focus on this region and further examine the VWS and tropospheric humidity anomalies in the subsection. The characteristics of humidity and VWS features imply the anomalies in August 2013 may be related to the extratropical processes

A case study

Findings

Summary and discussion