Changes in upper stratospheric CH₄ and NO₂ as measured by HALOE and implications for changes in transport

Measurements from HALOE indicate that there has been a significant decrease in CH 4 in the middle to upper stratosphere from 1991–1997. Two dimensional model calculations suggest that a decrease in the tropical upwelling is the most plausible explanation for the CH 4 decrease. The changes in CH 4 resulting from solar cycle variations or changes in tropospheric CH 4 and CFC emission rates are shown to be much smaller than the measured variations from ∼35–65 km. The HALOE measurements of NO 2 in the lower stratosphere near the equator also show an increase which is consistent with a reduction in the upward transport rate. Above ∼40 km the NO 2 measurements appear to be consistent with the net effect of opposing solar and dynamical trends.