LAUSR

Cool-season occurrences of blocks, extratropical cyclones that undergo explosive cyclogenesis, and tropical cyclones (TCs) that undergo extratropical transition (ET) from 1980 to 2015 are analyzed using the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications, version 2, dataset. These synoptic events are first examined in a climatological analysis that includes identifying consecutive synoptic events, namely, blocks that follow bombs or ET events as well as extratropical cyclones that follow ET events. These synoptic events are then analyzed with respect to three tropical modes of variability: the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation, and the stratospheric quasi-biennial oscillation (QBO). The QBO was considered from both a momentum and thermal point of view, using the equatorial 30-hPa zonal-mean wind and the equatorial zonal wind shear between 30 and 50 hPa, respectively. The results show that in the seven days prior to cool-season blocks and ET events, there is a statistically significant frequency minimum in MJO phases 7 and 3, respectively. With respect to the QBO, there is a statistically significant frequency maximum in neutral QBO conditions during bomb onset and a frequency minimum during ET onset. When stratifying bombs by latitude, there is a significant reduction in Arctic (i.e., poleward of 55°N) bomb onset during easterly QBO conditions. The results show that both tropospheric and stratospheric tropical modes of variability can modulate the frequency of extratropical synoptic events to a similar degree.