Title: Stratospheric variability and stratosphere-troposphere connections (MOD 3 and KEY 6)
1Stratospheric variability and stratosphere-troposp
here connections(MOD 3 and KEY 6)
- Mark P. Baldwin
- Northwest Research Associates, Bellevue, WA USA
- Cargese UTLS Summer School, 6 October 2005
2Overview
- EOFs
- Variability in the stratosphere-troposphere
- The Arctic Oscillation (AO) and Northern Annular
Mode (NAM) - Downward coupling to the troposphere
- The stratosphere and weather prediction
- Explanations for downward coupling
- The quasi-biennial oscillationand hurricanes
3EOFs
Empirical Orthogonal Functions Maximum Variance
Patterns 1) Pattern 2) time series
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6Composite surface maps for high and low AO
index. (From Thompson and Wallace, Science 2001)
gt0.9C
7Northern Annular Mode (NAM)
10 hPa
1000 hPa (Arctic Oscillation)
Annular mode patterns are the leading EOF of
low-frequency Z variability. Annular mode
patterns are similar from Earths surface to
50km.
8Annular Mode Terminology
- Arctic Oscillation (AO) At or near Earths
surface. - North Atlantic Oscillation (NAO) Very similar to
AO, but more oriented to the Atlantic sector. - Northern Annular Mode (NAM) Same as the AO, but
can describe higher levels in the atmosphere. - The surface NAM is the Arctic Oscillation.
9What are annular modes?
- Not fundamental dynamical quantities
- Not the solution to any equation
- Why do the NAM and SAM look so similar and in the
troposphere stratosphere? - Why does the atmosphere tend to vary in
ring-like structures? - Why not multiple rings?
- How are annular modes related to jets?
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11Composite surface maps for high and low AO
index. (From Thompson and Wallace, Science 2001)
gt0.9C
12Southern Hemisphere surface climate response to
ozone depletion
- Observations and model
- Springtime ozone loss appears to drive changes in
surface climate from late spring to summer.
13Simulated and observed geopotential height and
temperature changes
Model results from Gillett Thompson, Science
2003
14Tropospheric changes
Model results from Gillett Thompson, 2003
15NAM index for 19981999. 23 separate levels are
used. The lowest level is the AO index. From
Baldwin and Dunkerton, Science 2001
16Composites of strong and weak vortex events
- Select strong events based only on the daily
10-hPa NAM index. - Form positive and negative composites.
- Examine average behavior for the composites.
17Time Delay
Long timescale
Baldwin and Dunkerton, 2001
18Surface pressure anomalies after stratospheric
events look like the Arctic Oscillation.
Baldwin and Dunkerton, 2001
19Northern Annular Mode (NAM)
1000 hPa (Arctic Oscillation)
20Storm tracks during weak and strong regimes
21Baldwin and Dunkerton, 2001
22Thompson et el., JAS, 2005
23Predictability Beyond 10 Daysa role for the
stratosphere?
- Boundary Conditions (SSTs, Snow and Ice, Soil
Moisture, etc.) - Persistent Phenomena (MJO, QBO, ENSO, etc.)
- Persistent stratospheric anomaliescan they
affect the troposphere?
24Autocorrelation of daily surface AO index
(W. Norton, GRL, 2003)
Control Run
Normal Model
Model with damped stratosphere
Without stratospheric variability, the timescale
of the surface AO is shorter.
25e-folding timescale time for autocorrelation to
drop to 1/e (0.38). In the troposphere the
longest timescale occurs during winter. Does
this effect depend on the stratosphere, or is it
simply an annual cycle?
Baldwin et al., Science, 2003
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27Statistical AO Forecasts
- Exploit the enhanced timescale of the AO during
winter. - Predict the average AO during a future time
period, such as 1040 days. - Use one or more linear predictors
- 1) The present value of the AO
- 2) The present values of the annular modes at
all other levels, including the stratosphere.
28Baldwin et al., 2003
29Cross-validated AO Forecasts
- Remove one winter at a time forecast the
remaining winter. - 150-hPa NAM only during DJF skill is reduced
from 20 to 18. - AO predicts AO cross-validated skill is 12
- Adding the the AO as a second predictor does not
improve skill. -
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31Composite surface maps for high and low AO
index. (From Thompson and Wallace, Science 2001)
gt0.9C
32Possible Dynamical Mechanisms
- Direct effect of stratospheric wave driving and
stratospheric temperature anomalies - Indirect effects involving waves
- Effect on baroclinic waves/life cycles
- Effect on planetary-scale waves
- Wave reflection in the stratosphere
-
33From Thompson et al., JAS submitted
34From Thompson et al., JAS submitted Gstratospher
ic wave driving Qstratospheric radiative
cooling Ffriction
35Stratospheric influence on baroclinic lifecycles
and its connection to the Arctic Oscillation
- M. Wittman, L. Polvani, R. Scott, A. Charlton
- GRL, 2004
- Baroclinic lifecycle model (wave-6 sector model)
- Numerical experiments show that baroclinic
lifecycles are sensitive to changes in
stratospheric winds. - The surface response to stratospheric changes is
an AO-like pattern (the model is too simple to
have an AO). - The stratosphere has an effect on the surface by
influencing individual synoptic systems.
36From Alexander and Holton, 1997
37The Equatorial Quasi-Biennial Oscillation
Baldwin et al., Reviews of Geophysics, 2001
38500-hPa (mid-troposphere)
39QBO email from Barbara Naujokat during first talk
40500-hPa (mid-troposphere)
41 42Do Stratospheric Winds Affect Hurricanes?
43Tracks of intense hurricanes. Top 15 years when
the QBO winds were easterly. Bottom 15 years
when the QBO winds were westerly. 1950-1990
data. From Gray et al. 1992
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45Hurricane, as seen in column ozone
4620N
10N
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49E-W wind component averaged over all longitudes
50E-W wind component averaged over 20W to 100W
51Wind shear averaged over all longitudes
52Wind shear averaged over 20W to 100W
53A stronger, colder vortex?
- With increasing GHGs, the modeled stratospheric
vortex becomes stronger and colder, and there is
a positive AO trend. - Shindell et al., 1999
- With increasing GHGs, the model NAO index
decreases significantly from 1990 to 2015. The
polar stratosphere becomes warmer. - Schnadt and Dameris, 2003
54250-day running mean AM time series.
55Polar vortex response to 4x CO2
January Temperature Change
January U Change
- The polar vortex gets weaker and warmer due to
enhanced wave driving from the troposphere. - These plots show the response in a 4xCO2
integration - easier to distinguish from noise
(from Jamie Kettleborough).
56A simulation of the separate climate effects of
middle atmospheric and tropospheric CO2 doubling
- M. Sigmond, P.C. Siegmund, E. Manzini, H. Kelder
- J. Climate, 2004
- MA ECHAM4, CO2 doubling
- Increased residual circulation small warming in
the Arctic lower stratosphere weaker winds in
the Arctic middle atmosphere increase in
tropospheric westerlies. - The increase of the tropospheric NH midlatitude
westerlies can be attributed mainly to the
middle-atmospheric CO2 doubling, indicating a
crucial importance of the middle-atmospheric CO2
doubling for tropospheric climate change.
57IPCC Altitude of the Model Top
Stratopause
Tropopause
Number of levels
(IPCC slides from Eugene Cordero)
58Questions
- How important is the stratosphere to tropospheric
climate change and variability? - What impact does resolution have?
- How high and how many vertical levels
- How many levels near the tropopause?
- First step How well do these climate model
simulate stratospheric structure and variability?
59January Temperature (Global Avg 1900-2000)
601960-2000 Zonal Wind _at_ 60N and 50hPa
Zonal Wind (m/s)
61DJF Temperature (50S 80S) _at_ 10hPa
62IPCC Summary
- The IPCC AR4 simulations vary in their ability to
simulate stratospheric variability and change
during the 20th century. - The evolution of the polar vortex is a good
diagnostic for model variability and skill. - There is no model consensus regarding changes in
the 21st century stratospheric climate.
(From Eugene Cordero)
63The Future of the Stratosphere?
- Increasing GHGs cool the stratosphere.
- Stratospheric NAM index trend would depend on
relative cooling of the polar cap. - Ozone loss cools the lower stratosphere (late
winter/spring). Highly dependant on temperature. - Most models show an enhanced wave driving, a
stronger Brewer-Dobson circulation, and a warmer
NH vortex in winter.
64Summary
- Persistence and predictability of the AO depend
on the long timescale of large circulation
anomalies in the lowermost stratosphere. - This relationship yields statistical prediction
skill for the monthly-mean AO index. - Changes to the circulation of the lower
stratosphere affect surface climate. - The future evolution of the stratosphere will
depend on increasing GHGs, ozone changes, and
changes to the troposphere. - We do not know how the stratosphere will change
in the coming decades and centuries, and we do
not know how surface climate will be affected. - We do not fully understand the dynamics of
stratosphere-troposphere coupling.
mark_at_nwra.com www.nwra.com/baldwin