Transcript The NAM,Blocks and the Sun Lecture

Annular Modes, Blocks and
the Solar Influence
A look at three separate, but
arguably important influences on
climate prediction
Annular Modes
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A different view of the zonal index
– global sea level pressure at 55N/S
– positive index: westerlies are strong
– negative index: more meridional flow
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averaged by day, week, month, season
most significant in the winter (NH)
Annular Mode
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NAM averaged for
Jan-Feb-Mar (high)
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NAM averaged for
Jan-Feb-Mar (low)
Annular Mode
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NAM averaged
Jan-Feb-Mar (high)
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NAM averaged
Jan-Feb-Mar (low)
Annular Mode
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Blue shading
indicates ratio of
cold days (T<1.5sd)
to low values of
NAM
Annular Mode
Annular Mode
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The Polar Night Jet (PNJ) oscillates in
the stratosphere
– develops each autumn at about 20km and
reaches speeds of 50m/s
– Northern Hemisphere PNJ is asymmetric
due to thermal contrasts of land/oceans
– PNJ is perturbed by upward propagating
velocity waves (due to baroclinic waves)
Annular Mode

PNJ Dissipation
– In Northern Hemisphere, deformation of
the polar vortex plus upward propagating
planetary waves leads to rapid weakening
of the jet
– lower polar stratosphere can warm by 50C
as upper stratospheric air replaces cold
core jet.
Annular Mode
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Stratosphere Temps - Feb 22 2002
Annular Mode
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Global Warming and the NAM
– Many GCM predict warming in troposphere
and cooling in stratosphere
– This causes a steep thermal gradient in the
lower stratosphere due to tilted tropopause
– In turn, leads to stronger than average PNJ
which locks coldest air near the Pole
– Explains warmth of most winters since
1980’s and patches of frigid air in far north
Blocking Patterns
Stagnation at High Latitudes
Blocking Patterns

Definition:
– a ridge of high pressure aloft that bridges
up to 45 degrees latitude and persists for
more than 10 days. This causes a split in
the jet stream leading to a meridional flow.
– First studied by D.F Rex (rex block) and
later by Austin, et al.
Blocking Patterns
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Example of 500 mb flow
Blocking Patterns
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Result of uneven
distribution of land
and ocean
– leads to intense
baroclinic waves
– disrupts upper air
flow due to
anomalous thermal
pools
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Result of dynamic
flow on a rotating
annulus
– normal disruption of
flow pattern that has
axis of rotation at it
pole
– numerous model
simulations lead to
blocking patterns
without topography
Blocking Patterns
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“Chicken and the Egg” complex
– NAO, ENSO, PNA - NAM all describe to
some extent blocking patterns
– Do these effects cause or are they the
result of blocking patterns?
– Once again, ocean temperature departures
seem to be at the core of this conundrum
Solar Contributions
Sol - the weather’s best friend
Solar Contributions

Solar Flux - largest impact in the Tropics
or middle latitudes during their
summers
– Must be considered for polar latitudes
during winter because of ‘ionization’ effect
of fewer neutrons when solar flux is high.
– The ‘Svensmark’ effect has a purported
role
– Fewer neutrons, less clouds, more warmth
Solar Contributions
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Svensmark Effect:
Solar Contributions
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The connection:
Solar Contributions
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Svensmark claims that concentration of
neutrons explains much of the variance
in satellite integrated cloud cover:
Solar Contributions
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This effect seems to explain drift
upward in global temperatures:
Solar Contribution
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http://cloudws.web.cern.ch/cloudws/do
cuments_talks/Henrik_Svensmark/index