Derived Trends of CH_4 in the Stratosphere From HALOE Measurements

摘要

We have used the analysed HALOE/UARS CH_4 data on a regular three-dimensional grid (10~0 longitude x 5~0 latitude x 19 pressure level) at 3-month temporal resolution in the period from Dec. 1991 to Nov. 2000. Trend is calculated by using a linear fit to the zonal average time series data at every latitude-pressure grid. The latitude-pressure cross-section shows an asymmetric distribution of the trends in 100-1 mb height region. The northern hemisphere (NH) exhibits negative growth rates (upto 2.5%/year) above 10 mb height. On the contrary, the trends in the southern hemisphere (SH) are commonly found to be positive. The estimated tropospheric increase rates from surface CH_4 observations by the CMDL-group (about 6-8 ppb/year) show close agreement with the lower stratospheric growth rates. The trend analysis over two time periods suggest that the tropospheric growth rates have reduced significantly in the second half of the 1990s.