Stratosphere-troposphere coupling and
effects on high-latitude climate
Mark P. Baldwin
Northwest Research Associates
Bellevue, WA USA
ARCUS 28 October 2003
1000-hPa Northern Annular Mode (NAM) = Arctic Oscillation
Contours in meters
Annular Mode Terminology
ß
ß
ß
ß
Arctic Oscillation (AO): At or near Earth’s
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.
Composite surface maps for
high and low AO index.
(From Thompson and
Wallace, Science 2001)
>0.9°C
Composite surface maps for
high and low AO index.
(From Thompson and
Wallace, Science 2001)
>0.9°C
AO Winter T Anomalies
(from Todd Mitchell, UW)
Northern Annular Mode (NAM)
1000 hPa (Arctic Oscillation)
10 hPa
Annular mode patterns are similar from Earth’s surface to 50+km.
S/T coupling occurs during extended winter.
Composites of strong and
weak vortex events
ß
Select strong events based only on the
daily 10-hPa NAM index.
ß
Include only strong events.
ß
Examine average behavior for the
composites.
ß
Not optimized for forecasting
Surface pressure anomalies after stratospheric
events look like the Arctic Oscillation.
AO Forecasts
ß
NAM anomalies in the lowermost
stratosphere (~150 hPa) are long lived.
ß
Stratospheric NAM values can be used
as a predictor of the future AO.
ß
There is some AO predictability out to
~60 days.
250-day running mean AM time series.
Simulated and observed geopotential height and
temperature changes
Model results from Gillett & Thompson, 2003
Tropospheric changes
Model results
from Gillett &
Thompson, 2003
A 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]
Polar 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).
Autocorrelation of daily surface AO index
(W. Norton, GRL, 2003)
ControlModel
Run
Normal
Model with damped
stratosphere
Without stratospheric variability, the
timescale of the surface AO is shorter.
In the troposphere the longest timescale occurs
during winter.
Does this effect depend on the stratosphere, or
is it simply an annual cycle?
Summary
ßThe
stratospheric circulation affects Arctic
climate mainly during the extended winter
season. The surface pattern looks like the AO.
ßOn
climate change timescales stratospheric
changes are expected to be mimicked at the
surface. But, future stratospheric changes are
uncertain.
[email protected]
ßReprints
available.
www.nwra.com/baldwin