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On the balanced response of the troposphere to variability in the extratropical stratosphere

Date

2005

Authors

Furtado, Jason C., author
Department of Atmospheric Science, Colorado State University, publisher

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Abstract

Variability in the extratropical stratosphere arises primarily through wave-mean flow interactions, with the source of these waves lying in the troposphere. However, recent evidence suggests that the troposphere also responds to variability in the extratropical stratosphere, with persistent anomalies in the stratospheric circulation accompanying same-signed anomalies at tropospheric levels. The balanced response of the atmosphere to anomalous stratospheric wave breaking and heating qualitatively describes how the troposphere should respond to stratospheric variability. However, the balanced response has been viewed as incapable of explaining the observed tropospheric anomalies because of the relative small mass of the stratosphere compared to the troposphere. Hence, studies have looked at tropospheric eddy feedbacks and internal tropospheric wave dynamics as ways to explain the amplitude of the observed tropospheric response. This thesis demonstrates that the balanced response is, in fact, capable of explaining the amplitude of the observed tropospheric response to anomalous forcing in the stratosphere. First, we examine observations of stratosphere troposphere dynamical coupling in two frameworks: the Eulerian mean (EM) and transformed Eulerian mean (TEM) frameworks. Each framework offers its own interpretation of the dynamics of the coupled system, but only the TEM framework allows us to investigate stratosphere/troposphere dynamical coupling directly in terms of wave-mean flow interactions. The observational analyses suggest relationships between the observed changes in the stratosphere and troposphere and also which terms in the governing equations in each framework dominate the observed zonal-mean zonal wind and temperature fields. Then, we use a simple numerical model to quantify the balanced response of the atmosphere to prescribed anomalous stratospheric wave drag, anomalous stratospheric radiative cooling, and friction. The results of the model demonstrate that the balanced response sufficiently accounts for the amplitude of the observed tropospheric response to stratospheric anomalies. Hence, tropospheric eddy feedbacks are not required to explain the observed anomalies, contrary to previous studies. Moreover, the results show that persistent positive temperature anomalies in the lower stratosphere contribute to the persistence of the attendant tropospheric zonal-mean zonal wind anomalies by countering the restoring effects of friction. The results suggest parameterizations of radiative cooling used in modeling experiments are important in capturing these persistent anomalies.

Description

Summer 2005.
Includes bibliographical references (pages 99-102).

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Subject

Tropospheric circulation
Stratospheric circulation

Citation

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