The Seasonal Structure of Temperature Trends in the Tropical Lower Stratosphere

Radiosonde data show a large seasonal difference in trends since 1979 in the tropical lower stratosphere, with a maximum cooling of similar to 1 K decade(-1) in December and January and a minimum in March or April at 50 mb between 10 degrees N and 10 degrees S. The statistically significant difference of up to similar to 1 K decade(-1) between trends in December and those in March amounts to up to 20% of the climatological seasonal cycle. Although the size of annual mean cooling trends differs substantially among datasets, the seasonal pattern of trends is similar in all six radiosonde datasets used here and is consistent with MSU satellite data for the lower stratosphere. This greater cooling in boreal winter essentially disappears below 100 mb, and the troposphere has a different and smaller seasonal trend pattern. Trends in the tropical stratosphere show an inverse relationship with those in the Arctic for 1979-2009, which might be related to changes in stratospheric circulation. In most radiosonde data, however, the seasonal pattern of tropical trends at 50 mb since 1979 seems to come from a seasonal difference in the size of the stratospheric cooling in the mid-1990s, and trends for longer time periods or those for 1995-2009 do not show the same seasonal dependence. Whether the strengthening of the seasonal cycle in the stratosphere represents a long-term change related to greenhouse gas forcing, a shorter-lived shift related to ozone depletion or unforced interdecadal variability requires careful further study.