2011 - Cooperative Institute for Research in Environmental Sciences ...

Milestone 2. Using diagnosed proxies for tropical inmixing
based on meteorological analyses and satellite
measured chemical species, examine changes in the width
and isolation of the tropical pipe on seasonal, interannual
and decadal time scales. Impact: Changes in the characteristics
of mixing between the tropics and mid-latitudes in
the lowermost stratosphere can affect residence time and
possibly species distribution throughout the stratosphere.
Globally, air cycles into the stratosphere from the
troposphere in the tropics, and out of the stratosphere at
higher latitudes. The amount of time air spends in the
stratosphere affects the distribution of trace gases that affect
the ozone layer and play a role in climate change. An
important quantity in this regard is the so-called mean age
of stratospheric air. This quantity is determined in part by
how fast air rises in the tropics, and how vigorously air
mixes between the tropics and higher latitudes. The goals
of this project are to evaluate long-term changes in mixing
between the tropics and extra tropics, and study the implications
of these changes for stratospheric age of air. To
accomplish this, we calculated effective diffusivity using
Figure 1: This figure shows decadal trends (% decade-1) in effective
diffusivity from four reanalyses: the European Centre for Medium-Range
Weather Forecasts reanalysis (ERA-40, upper left), the Japanese reanalysis
(JRA, upper right), the National Centers for Environmental Prediction/
National Center for Atmospheric Research reanalysis (NCEP/NCAR, bottom
left), and the NASA Modern Era Retrospective-Analysis for Research
and Applications reanalysis (MERRA, bottom right). Crosses show areas
where the trends are statistically significant, vertical dashed lines show
the latitudinal edge of the tropical upwelling region, and the horizontal
dashed line shows the tropopause. Most reanalyses show an increase in
effective diffusivity (i.e., increase in mixing) along the SH tropical edge, as
indicated by the broad regions of red, although there are notable differences
in the precise location and magnitude of the increase.
Figure 2: Decreasing mean ages caused by an increasing strength of the
stratospheric circulation are consistent robust results in nearly all CCMs
(green lines). Observed indicators of NH midlatitude mean age (blue and
red squares) show the opposite trend over the past several decades.
wind fields from multiple reanalyses. This data was fit to
a regression model to remove year-to-year variability and
evaluate long-term trends. We found that the reanalyses
exhibit a long-term trend toward greater mixing between
the tropics and extratropics in the lower stratosphere.
These trends were combined with trace gas observations
in a simple model to investigate long-term changes in the
stratospheric age-of-air. It was found that the long-term
changes in stratospheric mixing have a potentially strong
impact on age-of-air. These results may help explain why
global climate models, which predict age-of-air decreases,
and observations, which don’t show age-of-air decreases,
appear to be at odds regarding their long-term trends.
Product: Davis, SM, EA Ray, and KH Rosenlof
(2011),Variability and trends in effective diffusivity in
the stratosphere, and their implications for stratospheric
circulation changes, AMS Spring Meeting, Seattle, WA.
Ray, EA, et al. (2010), Evidence for changes in
stratospheric transport and mixing over the past
three decades based on multiple data sets and tropical
leaky pipe analysis, J. Geophys. Res., 115, D21304,
doi:10.1029/2010JD014206.
Milestone 3. Examine stratospheric mean meridional
circulation and mixing changes during recent decades
with observations, reanalysis data, chemistry-climate
model output and a simple stratospheric model. Impact:
Understanding how the stratospheric circulation has recently
changed will likely help us better understand how
changes in the troposphere affect the stratosphere.
Changes in the mean meridional circulation and mixing
in the stratosphere are important to understand
since these changes are a direct result of changes in the
tropospheric circulation and weather patterns. Since we
don’t have measurements of the stratospheric circulation
or mixing, we used a combination of mean age of air
estimates from in situ balloon observations, satellite ozone
observations, mixing trends from reanalysis data and a
simple model of the stratosphere to compare to a suite of
chemistry-climate model simulations. We found that the
simple stratospheric model could reproduce the observed
mean age and ozone changes over the past several decades
with a small strengthening of the mean circulation
in the lower stratosphere, a moderate weakening of the
mean circulation in the middle and upper stratosphere
and a moderate increase in the horizontal mixing into the
tropics. This circulation change profile does not compare
well to that produced by chemistry-climate models, which
tend to have a large strengthening of the circulation in
the lower stratosphere and weak strengthening in the
lower stratosphere. Mixing changes were not able to be
determined from the chemistry-climate model simulations
since the full resolution output was not saved in
the archive. The differences between circulation changes
inferred from observations and chemistry-climate model
results are substantial. One of the main implications of our
work is that mixing between the midlatitude and tropical
stratosphere plays an important role in understanding the
changes seen in observations of trace gases in the stratosphere.
Product: Ray, EA, et al. (2010), Evidence for changes
in stratospheric transport and mixing over the past
three decades based on multiple data sets and tropical
leaky pipe analysis, J. Geophys. Res., 115, D21304,
doi:10.1029/2010JD014206.
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