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Thinking about the Arctic Oscillation
Richard B. RoodUniversity of Michigan
for the National Park ServiceAugust 8, 2013
Outline• Why am I giving this talk?• Some aspects of climate variability• The Arctic Oscillation• A heuristic: vortices• The Arctic Oscillation and Climate Change• Synthesis
• Big Thanks to Jim Hurrell for consultation and some slides.
• This talk and backup material at GLISAclimate.org
Why am I giving this talk?• Worked on an adaptation planning activity with Isle
Royale Park• The Arctic Oscillation emerged as an important
weather-climate driver• The Arctic Oscillation has major impact on warm-
cold, dry-wet, especially in winter and spring in the eastern half of North America
• The response of ecosystems to this variability can amplify the weather-climate variability
• Will the behavior of the Arctic Oscillation change?
Some Aspects of Climate Variability• One of the ways to think about climate variability
is to think about persistent patterns of weather– Rainy periods
• Floods
– Dry periods• Droughts
• During these times the weather for a region does not appear random – it perhaps appears relentless
An example of variability: SeasonsTe
mpe
ratu
re
Winter WinterSummer
Cold ColdWarm
Rain comes in fronts
Rain comes in thunderstorms
Mes
sy
Mes
syForced variability
responding to solar heating
Internal Variability(Rood Class Lecture 2010)
• Weather – single “events” – waves, vortices• There are modes of internal variability in the climate
system which have global consequences.– El Nino – La Nina
• What is El Nino
– North Atlantic Oscillation / Arctic Oscillation• Climate Prediction Center: North Atlantic Oscillation
– Annular Mode– Inter-decadal Tropical Atlantic– Pacific Decadal Oscillation
Definition: CPC Climate GlossaryArctic Oscillation (AO) - The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. The North Atlantic Oscillation is often considered to be a regional manifestation of the AO.
Some Attributes of Arctic Oscillation• For our discussion Arctic Oscillation, North Atlantic Oscillation,
and Northern Annular Mode are related – essentially the same• Largest mode of variability in northern middle and high
latitudes• It is present all year, we notice it more in winter and spring
El Nino – La Nina
• Ocean – Atmosphere• Global Influence• Predictable
• Atmosphere• Strong Regional Influence• Difficult to predict
Arctic Oscillation
Year-to-Year Changes in Winter TemperaturesDifferences Relative to 1961-1990 Average
Late 1970s
2006-2011
From Jim Hurrell
Winter (Dec-Feb) Surface TemperatureDifferences Relative to 1961-1990 Average (32.6ºF)
United States (lower 48)
• Lots of variability including very large changes from one winter to the next
• Winter of 1979 was the coldest in the US record (5.3ºF below average)
• Contiguous US winter temperatures have warmed ~2ºF since 1895
• Winter of 2010 was 15th coldest since 1895 (1.4ºF below average), while winter 2011 was the 39th coldest (0.35ºF below average).
• Most winters since 1985 have been warm; colder winters early in the record• February 1985 was the last month below twentieth century average
From Jim Hurrell
Arctic Oscillation
1895 – 2011
2011
Daily Index
Period used in Previous Maps
2010
Side by Side ComparisonArctic Oscillation and North American Temperature
DJF Temperature: Anomaly 1961-91
From Jim Hurrell
Some basic references• Hurrell, 1995:
Decadal trends in the North Atlantic Oscillation: Regional Temperature and Precipitation
• Hurrell and Deser, 2010: North Atlantic climate variability: The role of the North Atlantic Oscillation
• Kuzmina et al. 2005: The North Atlantic Oscillation and Greenhouse Gas Forcing
• Bai et al., 2012: Great Lakes ice and Arctic Oscillation and El Nino
What’s going on? A heuristic• A conceptual physics-based foundation• A vortex is circulating air.• If there is a low pressure system on Earth in
the northern hemisphere then air circulates counterclockwise around the low.
Heuristic: A vortex
http://commons.wikimedia.org/wiki/File:Rotational_vortex.gif
fast
Low
Heuristic: A vortex and a ballfast
http://commons.wikimedia.org/wiki/File:Rotational_vortex.gif
http://i206.photobucket.com/albums/bb194/Dantejfisher/RollingBall.gif
Low
Try to roll the ball towards the vortex
Heuristic: A vortex and a ballfast
http://commons.wikimedia.org/wiki/File:Rotational_vortex.gif
Try to roll the ball towards the vortex
http://i206.photobucket.com/albums/bb194/Dantejfisher/RollingBall.gif
Low
• Vortices are boundaries or separators. • Air inside of vortices often takes on distinct
characteristics.
Ball rolling experiment
Ball roller and friend
http://24.media.tumblr.com/tumblr_m3q2hkeyJV1r3a6jho1_500.gif
Weak vortex surrounded by warm air
X
y
Warm at y
Cold at xConcept of Blocking
The Arctic Oscillation “Positive” Phase
• Strong low (high) air pressure at high (middle) latitudes
• Extremely cold air confined to Arctic
• Warm middle latitudes
“Negative” Phase
• Pressure systems weaker
• Cold Arctic air spills into middle latitudes
• Warm middle latitude air moves into Arctic
Strong vortex Weak vortexFrom Jim Hurrell
Year-to-Year Changes in Winter TemperaturesDifferences Relative to 1961-1990 Average
Late 1970s
2006-2011
From Jim Hurrell
What Climate Processes Govern NAO Variability?
Random and Unpredictable Variations
Simulated (Dec-Mar)
A role for external forcing?
• 200 years of NCAR CAM without variations in “external” forcings
• Basic structure & time scale arises from internal nonlinear atmospheric dynamics
EOF1 SLP(Dec-Mar)
Random and Unpredictable Variations
Simulated NAO Index
r (1yr) = -0.07
Observed
r (1yr) = -0.03
Observed
r (1yr) = 0.4
Except for the latter half of the 20th century
What’s the future?• The indication from model simulations prior to 2012
are that the positive phase of the Arctic Oscillation will become more prominent …
• But … these models don’t have the loss of sea ice and northern snow cover … – Huge changes in the forcing of the atmosphere
• Radiative forcing due to change in color • Heat flux between ocean and atmosphere• Heat flux between land and atmosphere• Moisture flux between ocean-land-atmosphere
And in the end what might really matter – how storms propagate
X
y
Warm at y
Cold at x
Edge of the vortex• Strength of storms• Direction of storms• Speed of storms
North Atlantic Oscillation(from Lamont-Doherty )
Positive PhaseU.S. East, Mild and WetEurope North, Warm and WetCanada North & Greenland, Cold and Dry
Negative PhaseU.S. East, Cold Air Outbreaks, Snow (dry)Europe North, Cold; South, WetGreenland, Warm
Some recent research• Liu et al., 2012:
Impact of declining Arctic sea ice on recent winter snow fall“ some resemblance to the negative phase of the winter Arctic oscillation. However, the atmospheric circulation change linked to the reduction of sea ice shows much broader meridional meanders in midlatitudes and clearly different interannual variability than the classical Arctic oscillation.”
Some recent research• Francis and Vavrus, 2012:
Evidence linking Arctic amplification to extreme weather in mid-latitudes “Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.”
Some recent research• Greene et al., 2013: Superstorm
Sandy: A series of unfortunate events?
“However, there is increasing evidence that the loss of summertime Arctic sea ice due to greenhouse warming stacks the deck in favor of (1) larger amplitude meanders in the jet stream, (2) more frequent invasions of Arctic air masses into the middle latitudes, and (3) more frequent blocking events of the kind that steered Sandy to the west”
Some synthesis• We are seeing, here, an instance of the “non-stationarity” of climate.
– Change in the surface, changes energy and moisture characteristics of weather– The statistical distribution will change– This is not a simple shift of the distribution function
• Evidence that the variability will increase– But, remember, we are in warming climate
• Changes in regional and seasonal heat and moisture budget– Heavy snow, fast melt, change of water supply and quality, winter and spring
flooding• Propagation of storms is likely to change to cause the accumulation of
weather effects into more extreme events– Slow moving storms are very good at, for instance, building up storm surges– Sustained precipitation followed by sustained heat and dry
• If I were a planner, then I would be expecting more variability with increasing extremes associated with storms surges, heat, air quality, drought and flood
Some Ecological References• Walther et al., 2002:
Ecological responses to climate change• Post et al., 2009:
Population dynamics and hot spots of response to climate change
• Hurrell and Deser, 2010: North Atlantic Climate Variability (reference to other literature)
GLISAclimate.org• Big Thanks to Jim Hurrell for consultation and some slides.
• Material and more in project on Arctic Oscillation at GLISAclimate.org . Please join project, write comments, re-use material, correct mistakes, ask questions, and add more.
• http://glisaclimate.org/project/arctic-oscillation%3A-climate-variability-in-great-lakes
