Impacts of Extratropical Wave Breaking on Atlantic Tropical Cyclone Activity

With warm SST anomalies in the tropical Atlantic and cold SST anomalies in the East Pacific, the reduced Atlantic tropical cyclone activity from August to early September in 2013 was a surprise to the hurricane community. Our analyses suggest that the suppressed storm activity can be attributed to the frequent occurrence of dry air in the middle to upper troposphere along with strong vertical wind shear. Such unfavorable conditions are directly related to the equatorward propagation and breaking of midlatitude Rossby waves.

Further examination suggests the active anti-cyclonic Rossby wave breaking and frequent equatorward intrusions of extratropical air in August 2013 were associated with changes of the midlatitude jet stream (i.e., acceleration, eastward extension and greater strain rate). The EOF analysis of 200-hPa zonal wind identifies a recurrent mode of interannual variability over Atlantic. This mode is associated with the variations of the intensity and zonal extent of the mid-latitude jet and is found significantly correlated to Atlantic hurricane frequency in August. Our analyses thus emphasize the extratropical impacts on Atlantic tropical cyclones via the Rossby wave breaking. This physical link is missing in most statistical forecast schemes and may help explain the seasonal prediction bust in 2013.