Transcript Long-Term Changes in Annual Mode Variability Part I: Observations

Long-Term Changes in
Northern and Southern
Annular Modes
Part I: Observations
Christopher L. Castro
AT 750
Trends in Annular Modes: Outline
• Part I: Observations
How have annular modes changed in the last decades
and how is this related to observed climate change?
Castro
• Part II: Mechanisms
Ozone and/or greenhouse gas forcing?
Tropical Pacific SSTs?
Ahlgrimm
Introduction: Highlights of Reviewed Papers
NAO Perspective (Hurrell 1995)
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NAO to its high phase over recent decades and corresponding climate changes in
North America and Europe.
NAM (or AO) Perspective (Thompson et al. 2000)
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Similar high index polarity of the AO, most apparent during active (winter) season.
AO explains most of long term trends in atmospheric observations.
Relationship between AO variability and ozone trends.
Consider also stratospheric variability.
SAM Perspective (Thompson and Solomon 2002)
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High index of SAM related to photochemical ozone loss and corresponding climate
change southern hemisphere.
NAO Perspective: Index Trends
NAO index shows downward
trend from 1940s to early
1970s, but sharp increase
from late 1970s to present
Change in NAO has lead to
higher mean SLP over
Europe and lower over the
pole. This, not surprisingly,
looks a lot like the AO.
Also, a general pattern of
cooling over oceans and
warming over continents
(not shown).
NAO Perspective: Large-Scale Climate Changes
Pacific
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Deepened and eastward-shifted Aleutian low
Southward shift in storm tracks and synoptic eddy activity
Changes in sensible and latent heat fluxes
Most likely these changes are related to ENSO-like decadal variability in the
Pacific governed by tropical Pacific SSTs.
Atlantic
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Northward shift of jet stream and storm track
Warm over Northern Europe, cold over Greenland and Eastern Mediterranean
Continental scale changes in European rainfall
Atlantic changes are not related to tropical SSTs, in the Pacific or Atlantic.
NAO Perspective: Moisture Flux and
Divergence in Atlantic Sector
High NAO index winters: Strong
jet stream and storm activity in
northern Europe
Low or normal NAO index
winters: Weaker jet stream,
further south
Coherent changes in the moisture
flux divergence and rainfall over
Europe. High NAO favors more
rainfall in northern Europe and drier
around the Mediterranean.
AO Perspective: Index Trends
Using various definitions of
the AO based on surface air
temperature, precipitation,
and sea level pressure,
basically same trends as
Hurrell’s NAO index.
Again note the dramatic
increase in the AO after
approximately late 1970s.
WHAT IS GOING ON HERE??!!
AO Perspective:
Wintertime Trends in
SLP and SAT
As with NAO index:
Strong warming over continents
Decrease in SLP near pole
Most trends explained by AO
index.
Residual reflects ENSO-like
decadal variability in the Pacific.
NOTE: Look at winter (active)
season of the AO here because that
is when trends are most statistically
significant.
AO Perspective:
Precipitation Trends
Same trends as observed for the
NAO in Europe, except now note the
changes over North America.
The North American rainfall trends
show drying in Canada and northern
US and wetter in the southern US.
These trends are consistent with
ENSO-like decadal variability in the
Pacific and are not related to shift in
AO.
AO Perspective:
Stratosphere Trends
Stratosphere trends show an
equivalent barotropic anomaly.
AO trend explains 40% of
stratospheric cooling and 70% of
geopotential height falls poleward
of 60 N.
Thompson et al. also use a AO*
index based on leading PC of zonal
mean geopotential height from 60-90
N, and they essentially get same
results as with AO index.
AO Perspective:
TOMS Ozone and
Tropopause Pressure
TOMS Ozone and tropopause
pressure are closely related
enough that one may be used as a
proxy for the other.
AO index trends associated with
decreased ozone concentration
overall in the northern hemisphere
over recent decades.
Regionally, increases in ozone
where tropopause sinking
(Greenland) and decreases where
rising (Siberia).
AO Perspective: Long Term Ozone Trends
The plot of inverted ozone
concentration at a long term Swiss
observing site versus trends in the
AO and European surface air
temperature.
Ozone decreases are being
observed in the most populated,
industrialized areas of the world
over the last few decades, not
just Antarctica. The same trend
presumably exists in the US.
AO Perspective: Latitudinal
Depth Profiles of Wind,
Temperature Trends
Strengthening of westerly flow
poleward of 45 N which extends all
the way to the surface. Weakening
of westerlies south of 45 N.
Cooling maximized in the lower polar
stratosphere, warming in subtropical
upper tropopshere. Suggests
weakening of meridional circulation
in lower stratosphere.
MMC changes in the same sense as
the Ferrel Cell.
Hadley cell strengthened and
extends farther north.
AO Perspective: Summer Trends (SLP and Ozone)
Summer trends are of the same sign,
but not as strong or statistically
significant. Changes in summer ozone
reflect the memory of the previous
winter season.
SAM Perspective: Latitudinal Depth Profiles of
Temperature and Geopotential Height Trends
Similar trend as with NAM with some important differences:
Strongest signal is not in winter but (SH) summer, corresponding the time
when flow allows for propogation of waves from the troposphere (recall earlier
Thompson et al. 2000 paper). A weaker secondary maximum in fall.
Trends of strengthened polar vortices in periods of stratospheretroposphere coupling are reducing the sensitivity to breakdown of the
polar vortex in both hemispheres.
SAM Perspective: Ozone
Variation in spring ozone (when photochemical decay most rapid) is
felt through the summer months through consistent variation in the
SAM.
SAM Perspective: Geopotential Height and
Temperature Trends
As in NH, majority of
atmospheric trends in
recent decades explained
by variation in the
annular mode.
These trends are related to
coherent climate changes
observed in the southern
hemisphere—in particular
Antarctica and southern
South America.