The Impact of Tropospheric and Stratospheric Tropical Variability on the Location, Frequency, and Duration of Cool-Season Extratropical Weather Systems

The variability in the tropical atmosphere has the potential to shift the midlatitude flow into preferred states that can influence the life cycles of extratropical weather systems. Using the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications Version 2 data, this analysis evaluates the impact of tropospheric and stratospheric tropical variability on the location, frequency, and duration of cool-season (i.e., October – April) extratropical weather systems in the Northern Hemisphere. The synoptic events considered include cool season occurrences of blocks, extratropical cyclones that meet the bomb criteria, and tropical cyclones (TCs) that undergo extratropical transition (ET) from 1980-2015. The analysis considers tropical variability that is known to impact the atmosphere on subseasonal–to–seasonal (S2S) timescales. The synoptic events are investigated in the context of both tropospheric variability, qualified by the Madden-Julian Oscillation (MJO) and El Nino-Southern Oscillation (ENSO), and stratospheric variability qualified by the Quasi-Biennial Oscillation (QBO). It is hypothesized that stratospheric variability impacts extratropical weather systems at the same level of significance as tropospheric variability.

The results show that both tropospheric and stratospheric tropical modes of variability can have statistically significant associations with the frequency of the extratropical synoptic events. The analysis suggests that Northern Hemisphere cool-season block and ET events are associated with a statistically significant frequency minimum in phases 7 and 3 of the MJO, respectively. With respect to neutral QBO conditions, there is a statistically significant maximum during bomb events and a statistically significant minimum during ET events. An analysis of the latitude of bomb location reveals that the frequency of meeting the bomb criteria in the subtropical-to-midlatitude regions is significantly impacted by the MJO while the frequency of cyclones meeting the bomb in the Arctic region is significantly impacted by the phase of the QBO. The results suggest that tropical stratospheric variability may be a useful metric in assessing the potential for major Arctic bomb cyclones that produce large destructive oceanic waves at the sea ice margin.