ORCID Profile
0000-0002-1619-7448
Current Organisations
Centre for Ecology & Hydrology
,
University of Oxford
,
Michigan State University
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Publisher: Springer Science and Business Media LLC
Date: 15-09-2020
DOI: 10.1038/S41467-020-18299-7
Abstract: Extreme polar vortex events known as sudden stratospheric warmings can influence surface winter weather conditions, but their timing is difficult to predict. Here, we examine factors that influence their occurrence, with a focus on their timing and vertical extent. We consider the roles of the troposphere and equatorial stratosphere separately, using a split vortex event in January 2009 as the primary case study. This event cannot be reproduced by constraining wind and temperatures in the troposphere alone, even when the equatorial lower stratosphere is in the correct phase of the quasi biennial oscillation. When the flow in the equatorial upper stratosphere is also constrained, the timing and spatial evolution of the vortex event is captured remarkably well. This highlights an influence from this region previously unrecognised by the seasonal forecast community. We suggest that better representation of the flow in this region is likely to improve predictability of extreme polar vortex events and hence their associated impacts at the surface.
Publisher: Elsevier BV
Date: 02-2021
Publisher: Wiley
Date: 13-03-2022
DOI: 10.1002/QJ.4256
Abstract: The influence of the Semi‐Annual Oscillation (SAO) on the timing and evolution of major sudden stratospheric warmings (SSWs) is examined using the 2008/2009 SSW as the primary case‐study. When the zonal winds in both the troposphere and the SAO region of the equatorial upper stratosphere/lower mesosphere are relaxed towards reanalysis fields in the UK Met Office Unified Model, a remarkably accurate representation of the January 2009 SSW is achieved. The accurate timing of the SSW is determined by the SAO zonal wind relaxation. The westerly‐to‐easterly phase transition of the SAO in the lower mesosphere (0.1–0.5 hPa) is found to be a key factor for this influence. It defines an initial conical‐shaped vortex that determines the upward propagation of wave activity and subsequent evolution of wave mean‐flow interaction. Internal transient wave reflection in the subtropics and associated wave‐induced acceleration of the mean‐flow is found to be an important component, strengthening the vortex and thus delaying the onset of the SSW. The sensitivity of SSW timing to the equatorial westerly winds in the lower mesosphere is further explored in the context of all major SSWs during the 1979–2018 period. The timing of SSWs is found to be significantly correlated with the timing of the equinoctial westerly‐to‐easterly phase transition at 0.3 hPa in early winter ( r = 0.79). This relationship is discussed in the context of the more widely recognised influence of the quasi‐biennial oscillation (QBO). These results suggest that accurate simulation of the timing of SAO phase transitions, as well as knowledge of the QBO phase, is likely to provide additional and extended Northern Hemisphere wintertime seasonal forecast skill.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Lijie Sun.