Age of air and the circulation of the stratosphere
Age of air and the circulation of the stratosphere
Date
2017-09
Authors
Linz, Marianna
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DOI
10.1575/1912/9228
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Keywords
Stratosphere
Ozone
Stratospheric circulation
Ozone
Stratospheric circulation
Abstract
The circulation of air in the stratosphere is important for the distribution of radiatively important
trace gases, such as ozone and water vapor, and other chemical species, including
ozone-depleting chlorofluorocarbons. Age of air in the stratosphere is an idealized tracer
with unique mathematical properties, which we exploit to derive a theory for the relationship
of tracer observations to the stratospheric circulation. We show that the meridional
age gradient is a measure of the global diabatic circulation, the total overturning strength
through an isentropic surface, and test this time-dependent theory in a simple atmospheric
general circulation model. We apply the theory to satellite data of sulfur hexafluoride (SF6)
and nitrous oxide to derive the first observationally-based estimates of the global meridional
overturning circulation strength at all levels in the stratosphere. These two independent
global satellite data products agree to within 5% on the strength of the diabatic circulation
in the lower stratosphere. We compare to reanalyses and find broad agreement in the lower
stratosphere and disagreement (∼ 100%) in the upper stratosphere. To understand the relationship
between the diabatic circulation and other metrics of the circulation, we calculate it
in a state-of-the-science atmospheric model and in three different reanalysis data products.
The variability of the global diabatic circulation is very similar to one typical circulation
metric, and it is correlated with total column ozone in the tropics and in Southern hemisphere
midlatitudes in both a model and in reanalysis–data comparisons. Furthermore, we
develop a metric for the mean adiabatic mixing, showing that it is related to the meridional
age difference and the vertical gradient of age. We calculate this metric for a range of simple
model runs to determine its utility as a measure of mixing. We find very little mixing of air
into the tropics in the mid-stratosphere, and the vertical structure of mixing in the lower
stratosphere and upper stratosphere varies among model runs and between hemispheres. A
picture of global average stratospheric circulation could thus be obtained using age of air
data, given reliable long-term records.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2017
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Citation
Linz, M. (2017). Age of air and the circulation of the stratosphere [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/9228