Low-frequency variability of the extratropical stratospheric circulation: Role of the low-latitude mean momentum

Results from a number of extended simulations with the GFDL SKYHI troposphere-stratosphere-mesosphere GCM are examined. All these experiments had a prescribed climatological seasonal cycle of sea surface temperatures. The experiments include a 25-year and three 34-year runs with standard versions of the model, a 50-year run with updated physics including a cloud prediction scheme, and 34 and 48-year runs with versions that had artificial constraints imposed on the zonal-mean wind in the low-latitude stratosphere. The control versions produce some apparent long-period (decadal or longer) variations in the large-scale extratropical stratospheric circulation. The degree of low-frequency internal variation of the circulation is remarkable and it has important practical implications for attribution of apparent trends in stratospheric circulation and chemistry. One possible source for the interannual "memory" in this system is the zonal-mean zonal momentum in the low latitude stratosphere, since the mean flow at low latitudes is only very weakly constrained by the effects of radiation in restoring the mean temperature distribution. The GCM experiments described here will be analyzed to see the effects of strongly constraining the tropical mean flow.