Polar vortex conditions during the 1995–96 Arctic Winter: Meteorology and MLS ozone

Abstract

The 1995–96 northern hemisphere (NH) winter stratosphere was colder than in any of the previous 17 winters, with lower stratospheric temperatures continuously below the type 1 (primarily HNO₃) polar stratospheric cloud (PSC) threshold for over 2 1/2 months. Upper tropospheric ridges in late Feb and early Mar 1996 led to the lowest observed NH lower stratospheric temperatures, and the latest observed NH temperatures below the type 2 (water ice) PSC threshold. Consistent with the unusual cold and chemical processing on PSCs, UARS MLS observed a greater decrease in lower stratospheric ozone (O₃) in 1995–96 than in any of the previous 4 NH winters. O₃ decreased throughout the vortex over an altitude range nearly as large as that typical of the southern hemisphere (SH). The decrease between late Dec 1995 and early Mar 1996 was ∼2/3 of that over the equivalent SH period. As in other NH winters, temperatures in 1996 rose above the PSC threshold before the spring equinox, ending chemical processing in the NH vortex much earlier than is usual in the SH. A downward trend in column O₃ above 100 hPa during Jan and Feb 1996 appears to be related to the lower stratospheric O₃ depletion.