Thermal Response of the Tropical Tropopause Region
To Short-Term Solar Ultraviolet Variations
Lon Hood
Lunar and Planetary Lab
University of Arizona
Related Paper: GRL, December, 2003
SORCE Meeting
Sonoma, CA
December 5, 2003
205 nm Solar Flux
(Nimbus 7 SBUV)
Previous work has documented the existence of a response of ozone in the
upper stratosphere to solar ultraviolet variations occurring on the time scale
of the solar rotation period (e.g., Hood, 1986; Keating et al., 1987). This
response is understood to be a consequence of direct photochemical changes
in ozone production rate (e.g., Brasseur, 1993).
205 nm Solar Flux
(Nimbus 7 SAMS)
Previous work has also yielded evidence for a thermal response of the upper
stratosphere to short-term solar ultraviolet variations (e.g., Hood, 1986;
Keating et al., 1987). This response is understood to be primarily a radiative
effect of the changes in UV flux and the associated changes in ozone (e.g.,
Brasseur, 1993).
I
Direct Effects of Solar UV Variations
On Ozone, Radiative Heating, and
Zonal Winds in the Upper Stratosphere
Indirect Effects on Wave Absorption at
Lower levels in the Extratropics
∇⋅F < 0
∂u/∂t < 0
∆w* > 0
dT/dt < 0
dO3/dt < 0
∆w* > 0
dT/dt > 0
dO3/dt > 0
Tropopause
Vertically Propagating, Planetary-Scale Waves
Equator
Pole
Winter Hemisphere
Time Intervals Selected for Analysis:
Consider 100 hPa NCEP Temperature at tropical latitudes
(20oS to 20oN). Shown below are deviations from 35-day
running means (Interval 1):
Cross correlation functions for the two separate analysis
intervals confirm that a tropical 100 hPa temperature
response exists at a phase lag of 0 to 4 days:
Regression analyses for the two
separate time intervals yield
response amplitudes of 0.28 +/- 0.06
K / 0.01 MgII units and 0.19 +/- 0.04
K / 0.01 MgII units, respectively.
These are consistent within the 2
standard deviation error limits.
Repetitions of the analysis at other levels in the lower stratosphere
show that the maximum response amplitude occurs near 100 hPa:
Phase lags increase with increasing altitude:
Extensions of the regression analysis to other latitudes
and pressure levels (preliminary):
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How can we test whether solar UV forcing (as opposed to
other solar-correlated forcings) is most probably
responsible for the observed thermal response? One
empirical approach is to carry out parallel analyses using a
series of other possible solar-correlated variables.
Repetition of the analysis using total solar irradiance
yields no clear evidence for a correlation.
The same is true for Galactic cosmic ray flux.
Sunspot numbers, which are a rough proxy for solar UV
variations, yield better evidence for a consistent
correlation.
The 10.7 cm radio flux, also a rough proxy for solar UV
variations, yields better evidence for a consistent
correlation.
However, the Mg II index, which is the most accurate proxy for
solar UV variations, yields the best evidence for a consistent
correlation.
CONCLUSIONS
•
Correlative and regression analyses of NCEP data confirm that
temperature in the tropical lower stratosphere and upper
troposphere responds significantly to 27-day solar forcing with
maximum amplitude near the 100 hPa level.
•
Correlative studies indicate that solar UV variations (as opposed
to total solar irradiance or other solar-correlated forcings) are
most probably responsible for the observed thermal responses in
the tropical lower stratosphere and upper troposphere.
•
A possible mechanism for explaining the observed thermal
response near the tropical tropopause is changes in upwelling
rates induced by the direct effects of solar UV forcing on
photochemistry, radiative heating, and dynamics in the upper
stratosphere.
Possible Mechanism:
The direct effects of solar UV spectral irradiance variations near
200 nm on temperatures near the tropical tropopause are small
or negligible. It is therefore necessary to consider whether the
observed thermal responses are an indirect consequence of solar
UV-induced changes in circulation. Specifically, …
Direct Effects of Solar UV Variations
On Ozone, Radiative Heating, and
Zonal Winds in the Upper Stratosphere
Indirect Effects on Wave Absorption at
Lower levels in the Extratropics
∇⋅F < 0
∂u/∂t < 0
∆w* > 0
dT/dt < 0
dO3/dt < 0
∆w* > 0
dT/dt > 0
dO3/dt > 0
Tropopause
Vertically Propagating, Planetary-Scale Waves
Equator
Pole
Winter Hemisphere