Regional stratospheric warmings in the Pacific-Western Canada (PWC) sector during winter 2004/2005: implications for temperatures, winds, chemical constituents and the characterization of the Polar vortex

摘要

The vortex during winter 2004/2005 was interesting for several reasons. It hasbeen described as "cold" stratospherically, with relatively strongwesterly winds. Losses of ozone until the final warming in March wereconsiderable, and comparable to the cold 1999–2000 winter. There were alsomodest warming events, indicated by peaks in 10 hPa zonal mean temperaturesat high latitudes, near 1 January and 1 February. Events associated with asignificant regional stratospheric warming in the Pacific-Western Canada(PWC) sector then began and peaked toward the end of February, providingstrong longitudinal variations in dynamical characteristics (Chshyolkova etal., 2007; hereafter C07). The associated disturbed vortex of 25 Februarywas displaced from the pole and either elongated (upper) or split into twocyclonic centres (lower).Observations from Microwave Limb Sounder (MLS) on Aura are used here tostudy the thermal characteristics of the stratosphere in the Canadian-US(253° E) and Scandinavian-Europe (16° E) sectors. Undisturbed highlatitude stratopause (55 km) zonal mean temperatures during the mid-winter(December–February) reached 270 K, warmer than empirical-models such asCIRA-86, suggesting that seasonal polar warming due to dynamical influencesaffects the high altitude stratosphere as well as the mesosphere. There werealso significant stratopause differences between Scandinavia and Canadaduring the warming events of 1 January and 1 February, with highertemperatures near 275 K at 16° E. During the 25 February "PWC" event awarming occurred at low and middle stratospheric heights (10–30 km: 220 K at253° E) and the stratopause cooled; while over Scandinavia-Europe thestratosphere below ~30 km was relatively cold at 195 K and thestratopause became even warmer (>295 K) and lower (~45 km). The zonalwinds followed the associated temperature gradients so that the vertical andlatitudinal gradients of the winds differed strongly betweenScandinavia-Europe and Canada-US.The data-archive of Aura-MLS was also used to produce height versus latitudecontours of ozone and related constituents, using mixing ratios (r) for ClO,N₂O and HCl, for the 16° E and 253° E sectors. The Q-diagnosticwas used to display the positions of the cyclonic (polar) vortex, using datafrom the UK Meteorological Office (MetO) analyses. ClO/HCL maxima/minimaoccurred on 1 February in both sectors, consistent with loss of ozone byheterogeneous chemistry. Low N₂O values at high latitudes indicatedthat both sectors were inside the polar vortex, Time-difference plots showgreater reductions in O₃ in the Canadian sector. For the 25 FebruaryPWC warming event, O₃-rich air from lower latitudes continued to beexcluded from Europe, while O₃ penetrated to at least 82° N over theCanadian sector. The contours for ClO, N₂O and HCl at 16° E areconsistent with continued ozone loss within the vortex during the event.Finally the thermal and chemical changes at these 16° E and 253° Esectors are placed into a hemispheric context using polar-cylindrical plots,with the following results. Firstly, the mixing ratios of O₃, ClO,HNO₃, HCL and the temperatures from Aura-MLS were consistent withconsensus views of heterogeneous chemistry. Secondly, and consistent withthe polar plots of C07, the vortices and their edges were strongly distortedduring the 1 January, 1 and 25 February warming events, with sinusoidalshapes consistent with stationary planetary waves of wave-numbers 1 and 2.Thirdly, the distributions of the chemicals followed the curvatures(cyclonic and anticyclonic) of the vortex edges with O₃ lossesoccurring at the cold cyclonic locations. During February these were overScandinavia-Western Europe and Central-Eastern Canada. Trajectory analysiswas applied to the two February warming events. For the 1 February event,the rotation tim