Stratospheric Circulation Features Deduced from SAMS Constituent Data

Abstract

Stratospheric circulation is investigated by further analyses of three years of Stratospheric And Mesospheric Sounder (SAMS) data. Eddy effects on constituent transport are investigated with two transport formulations the transformed Eulerian mean formulation and the effective transport formulation. The transformed Eulerian mean formulation, together with calculated residual mean winds (v?*, w?*), is used to delineate regions, or times, of significant eddy contributions to constituent transport. Significant regions are found in the stratosphere in all seasons, not only in the Northern Hemisphere (NH) winter high latitudes (where contributions from nonlinear and nonsteady perturbations in sudden and final warming events are expected), but also in the midlatitude, middle stratosphere in autumn and at winter-summer low latitudes near the stratopause. Questionably large calculated ?0?1? · M magnitudes near the stratopause during solstice suggest that the use of residual winds calculated here may be inadequate for describing mass transport at these heights. The effective transport formulation is used, involving individual calculations of the effective-transport velocity (v??, w??) for both CH4 and N2O. With CH4, the tracer continuity equation is diagnosed term-by-term for January, April, July, and October months. Both gases reveal a descent in the tropics during northern spring (associated with a ?double peak? in mixing ratio measured along latitude) and upwelling in the summer tropics of the stratosphere during solstice. The NH summer upwelling appears to cause larger CH4 increases than corresponding Southern Hemisphere (SH) summer tropical upwelling. The effective transport calculations involving CH4 show that in July most local change in mixing ratio in the upper stratosphere is attributable to mean advection. In the upper stratosphere and lower mesosphere during NH summer-autumn, pulses of enhanced mixing ratio (related to an apparent coupling of semiannual and annual circulation components) propagate from low northern latitudes into both hemispheres. The behavior is similar to that predicted by diabatic models, but under equinox conditions. The calculated (v??, w??) fields for June?July exhibit this cellular feature for both CH4 and N2O. In the extratropics at stratopause heights, particularly in the SH, large local amplitudes of the semiannual component may be attributable in part to this mean meridional advection from summer to winter hemisphere. Each year, both CH4 and N2O show features consistent with significant NH autumnal vertical and meridional transport at high latitudes in the lower mesosphere.