85th AMS Annual Meeting

Thursday, 13 January 2005
Interannual variability in the troposphere-stratosphere climate system and the solar cycle
Robert X. Black, Georgia Institute of Technology, Atlanta, GA; and B. W. Whitesides
There is a surprisingly large amount of tropospheric climate variability linked to the solar cycle. This signal is manifested by annular temperature and wind anomaly structures in both the troposphere and stratosphere. The relatively strong variability in UV radiation observed during the solar cycle has focused attention on the impact of the solar cycle upon the stratosphere. This idea has been confirmed in modeling studies which also indicate a climate response extending into the troposphere. Better knowledge of the statistical and dynamic linkages between the middle and lower atmosphere is essential in pursuing a mechanistic understanding of solar cycle-climate linkages.

An overview is provided of the key mechanisms that dynamically couple the stratosphere and troposphere. The direct and indirect means by which the stratosphere can influence tropospheric climate are emphasized. Direct downward influences include tropospheric circulation changes induced by latitudinal redistributions of mass and potential vorticity within the stratosphere. Indirect influences involve alterations in the vertical and meridional propagation of tropospheric Rossby waves, which in turn provide local wave forcings to the longitudinally averaged wind field. Potential feedback and tropospheric amplification mechanisms are discussed

A statistical and diagnostic framework is introduced for studying the dynamic coupling between the stratosphere and troposphere in association with the solar cycle. We hypothesize that the solar cycle provides dynamical influences to the tropospheric circulation via variations in the strength of the wintertime stratospheric polar vortex. Synoptic and dynamic atmospheric circulation structures linked to the solar cycle are first isolated using multivariate statistical analyses. Diagnostic analyses are then performed to answer the following questions: What are the relative importance of stratospheric and tropospheric dynamic structures in comprising tropospheric circulation anomalies? How do the propagation characteristics of tropospheric Rossby waves vary in association with the solar cycle? What are the resulting local wave forcings acting upon the longitudinal winds? Indirect forcing mechanisms are studied using Eliassen-Palm (EP) fluxes, EP flux divergences, and resulting forcings of the Transformed Eulerian-mean circulation. EP fluxes provide a direct measure of Rossby-wave propagation in the meridional plane while the EP flux divergence is proportional to the net local wave forcing of the longitudinally-averaged wind field. Direct forcing mechanisms are studied by applying potential vorticity (PV) inversion methods to assess the relative contributions of stratospheric and tropospheric dynamical structures. Our preliminary results indicate roles for both direct and indirect mechanisms in stratospheric modulation of tropospheric climate during the solar cycle.

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