Variations in the stratospheric transport circulation 1991-1998 and effects on methane concentrations

Observations from the Upper Atmosphere Research Satellite (UARS) between 1991-1998 are used to infer the properties of the residual meridional circulation in the stratosphere. The stratospheric residual circulation transports trace constituents upward at tropical latitudes and downward and poleward at higher latitudes. Long-term variations during the UARS period are observed in this transport circulation that may be related to long-term variations observed in long-lived stratospheric trace gases.

We isolate predicted changes in stratospheric methane due solely to the changes in the stratospheric transport circulation during this period. The method utilizes a model of methane trend based on the flux form of the continuity equation in isobaric coordinates. The solutions are integrated over latitude to remove any dependence on horizontal mixing due to planetary wave breaking. The difference between two models isolates the effects of the long-term variations in the transport circulation: (1) The first model uses the average of the UARS period yearly cycles in methane mixing ratio and the residual circulation. (2) The second model includes long-term variations in the residual circulation only. The difference in the global average profile of the methane ``trend'' in these two models is compared to the global trend observed during the UARS record [Randel et al., 1999]. The results suggest that the observed trend may be largely explained by a combination of the increasing tropospheric methane trend plus the changes in the stratospheric transport circulation.