Trend in stratospheric ozone over tropics

Abstract

A statistical analysis of zonal monthly average ozone from solar backscatter ultraviolet in 10° latitude bands over 20°S–20°N for the period November 1978 to December 2008 is performed to find heightwise trend in the tropical stratosphere. A turning point (TP) of January 1999 is found for tropics, whereas a TP of January 1997, attributed to the change in equivalent effective stratospheric chlorine (EESC), was found for midlatitudes in previous studies. The introduction of two more TPs corresponding to the beginning and ending of the effect of Mt. Pinatubo volcanic eruption improves the representation of observed behavior of ozone in the lower (20–30 km) and in the upper (>30 km) stratospheres. The trend in total stratospheric ozone column is small compared to those in lower and upper stratospheric ozone columns during the considered subperiods created by TPs. The values of piecewise trend in the lower and upper stratospheres during the periods 1994–1999 and 1999–2008 are significant. The directions of trend (downward/upward) in the upper and lower stratospheres are opposite, and more conspicuously after January 1999 when the trend in the lower stratosphere decreased in contrast to an increase in the upper stratosphere. The trend at Umkehr layer 5 is almost zero at all times during the period 1978–2008. The timewise variation in the trend is comparatively large between Umkehr layers 2–3 and 7–10. The Eliassen‐Palm‐flux gives negative contribution to ozone in the lower stratosphere and positive contribution in the upper stratosphere, indicating that ozone is affected by transport over tropics.