Relationship between the North Pacific Gyre Oscillation and the onset of stratospheric final warming in the northern Hemisphere.

The seasonal timing or onset date of the stratospheric final warming (SFWOD) events has a considerable interannual variability. This paper reports a statistically significant relationship between the North Pacific Gyre Oscillation (NPGO) and SFWOD in the Northern Hemisphere in two sub-periods (1951-1978 and 1979-2015). Specifically, in the first (second) sub-period, the NPGO is negatively (positively) linked with SFWOD. Composite analyses associated with anomalous NPGO years are conducted to diagnose the dynamic processes of the NPGO-SFWOD link. During 1951-1978, positive NPGO years tend to strengthen the Pacific-North America (PNA) pattern in the mid-troposphere in boreal winter. The strengthened PNA pattern in February leads to strong planetary wave activity in the extratropical stratosphere from late February to March and causes the early onset of SFW in early April. By contrast, a strengthened Western Pacific pattern from January to early February in negative NPGO years causes a burst of planetary waves in both the troposphere and extratropical stratosphere from late January to mid-February and results in more winter stratospheric sudden warming events, which, in turn, leads to a dormant spring and a late onset of SFW in late April. During 1979-2015, positive (negative) NPGO years strongly strengthen (weaken) the mid-tropospheric Aleutian low and the Western Pacific pattern from January to mid-March, leading to increased (decreased) planetary wavenumber-1 activity in the stratosphere from mid- to late winter and thus more (less) winter stratospheric sudden warming events and late (early) onsets of SFW in early May (mid-April). [ABSTRACT FROM AUTHOR]