Possible ozone-induced long-term changes in planetary wave activity in late winter

Abstract

Using NCEP-NCAR reanalysis data, decadal trends in planetary wave activity in Northern Hemisphere high latitudes (50°-90°N) in late winter and early spring (January-February-March) were studied. Results show that wave activity in both the stratosphere and the troposphere has been largely reduced and exhibits statistically significant downward trends since the 1980s. In the stratosphere, the wave activity is decreased by about 30%, which is mainly due to less Eliassen-Palm (E-P) flux from the troposphere into the stratosphere. In the troposphere, the vertical E-P flux is reduced by about 30%, while equatorward horizontal E-P flux is increased by 130%. This suggests a significant refraction of planetary waves away from the high latitudes. The significant negative trends in wave activity in late winter are in contrast to the authors' previous finding of no significant changes in planetary wave activity in early winter. The timing of the significant decline in wave activity, which starts from the early 1980s and exists only in late winter and springtime, suggests that such a decrease of wave activity is possibly a result of stratospheric ozone depletion in the Arctic. Therefore, a mechanism is proposed whereby Arctic ozone depletion leads to an enhanced meridional temperature gradient near the subpolar stratosphere, strengthening westerly winds. The strengthened winds refract planetary waves toward low latitudes and cause the reduction in wave activity in high latitudes. Decreasing vertical E-P fluxes are found to extend to near the surface. At 850 mb, vertical E-P fluxes have been reduced by about 10% since 1979. Such a reduction in wave activity might be responsible for the observed late-winter and springtime warming over Northern Hemisphere high-latitude continents during the last two decades.