Trends in lower stratospheric zonal winds, Rossby wave breaking behavior, and column ozone at northern midlatitudes

Statistical trend analyses of National Centers for Environmental Prediction (NCEP) gradient zonal winds for the months of February and March demonstrate that the zonal mean meridional wind sheer for these months in the midlatitude lower stratosphere has tended to become more anticyclonic with time over the period from 1979 to 1998, Such a tendency favors the increased occurrence at these latitudes of anticyclonic, poleward, Rossby wave breaking events that transport low potential vorticity (PV), ozone-poor air from the subtropical upper troposphere to the midlatitude lower stratosphere while favoring the decreased occurrence of equatorward breaking, cyclonic events. Composite mean differencing and statistical trend analyses of NCEP-derived PV on the 330 K isentropic surface show that zonal mean PV values at midlatitudes in February and March have decreased with time, consistent with the expected trends in Rossby wave breaking behavior. Similar analyses of Total Ozone Mapping Spectrometer (TOMS) total ozone for the same 2 months show that total ozone trends correlate geographically with PV trends. Regression relationships between 330 K PV and total ozone deviations derived from monthly mean measurements on the Northern Hemisphere are applied to estimate that as much as 40% of the zonal mean total ozone decline at midlatitudes in February during the analysis period may be attributed to long-term trends in Rossby wave breaking behavior. As much as 25% of the midlatitude ozone trend in March may be attributed to such trends in wave-breaking behavior. At specific longitudes the contribution to ozone trends from this source (as well as long-term changes in quasi-stationary wave amplitudes and phases) can be well over 50%.