1. Thinking about the Arctic Oscillation
Richard B. Rood
University of Michigan
for the National Park Service
August 8, 2013

2. 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

3. 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?
I am not an expert on the Arctic Oscillation.
I synthesize information trying to make it more usable

4. 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

5. An example of variability: Seasons
Cold
Warm
Cold
Temperature
Messy
Messy
Winter
Summer
Winter
More than hot and cold, weather type is different. The transitional seasons are messy, sort of bouncing back and first.
This is “forced” variability due to the seasonal cycle of the Sun.
Rain comes in fronts
Rain comes in thunderstorms
Forced variability responding to solar heating

6. 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
Are these the noise that the signal must over come? Not only are they noise, but they might, too, change.

7. Definition: CPC Climate Glossary
Arctic 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.

8. 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
Arctic Oscillation
Ocean – Atmosphere
Global Influence
Predictable
Atmosphere
Strong Regional Influence
Difficult to predict

9. Year-to-Year Changes in Winter Temperatures
Differences Relative to Average
Late 1970s
From Jim Hurrell

10. Winter (Dec-Feb) Surface Temperature
Differences Relative to 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)
Most winters since 1985 have been warm; colder winters early in the record
February 1985 was the last month below twentieth century 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).
From Jim Hurrell

11. Arctic Oscillation 1895 – 2011 Daily Index Period used in
Previous Maps
2010
2011

12. Side by Side Comparison Arctic Oscillation and North American Temperature
Note: How cold is not obviously related to strength of index
Note: There has not been a below 20th century average month since Feb 1985
Note: Negative Index is a Negative temperature anomaly
DJF Temperature: Anomaly
From Jim Hurrell

13. 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

14. 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.

15. Heuristic: A vortex fast Low

16. Heuristic: A vortex and a ball
Try to roll the ball towards the vortex
fast
Low

17. Heuristic: A vortex and a ball
Try to roll the ball towards the vortex
fast
Low
Vortices are boundaries or separators.
Air inside of vortices often takes on distinct characteristics.

18. Ball rolling experiment
Ball roller and friend

19. Cold vortex
Imagine air isolated over the pole in winter, with no sun.

20. Cold vortex surrounded by warm air
Cold at y y X
Warm at x

21. Strong vortex and weak vortex

22. Weak vortex surrounded by warm air
Warm at y y X
Cold at x
Concept of Blocking

23. Bring it back to the atmosphere

24. The Arctic Oscillation
“Negative” Phase
Pressure systems weaker
Cold Arctic air spills into
middle latitudes
Warm middle latitude air
moves into Arctic
“Positive” Phase
Strong low (high) air pressure
at high (middle) latitudes
Extremely cold air confined
to Arctic
Warm middle latitudes
Strong vortex
Weak vortex
From Jim Hurrell

25. Year-to-Year Changes in Winter Temperatures
Differences Relative to Average
Late 1970s
From Jim Hurrell

26. Wave Motion and Climate

27. What about climate change?

28. What Climate Processes Govern NAO Variability?
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
Simulated (Dec-Mar)
Random and Unpredictable
Variations
Observed
r (1yr) = -0.03
Observed
r (1yr) = 0.4
Except for the latter half
of the 20th century
Strong evidence that the basic structure of the NAO arises from the internal, nonlinear dynamics of the atmosphere, in particular interactions between the time mean flow and the departures from that flow (the so-called transient eddies) comes from AGCM simulations with climatological annual cycles (no interannual variations) of all forcings “external” to the atmosphere, like insolation, SST, sea ice, snow cover, and land surface moisture, as well as fixed atmospheric trace‑gas composition.
The results from one such integration (200 yr) are illustrated in here, and it shows the the observed spatial pattern and amplitude of the NAO are well captured by such a run.
Moreover, the time series of Dec-Mar values exhibits little temporal coherence, consistent with a stochastic (Markov, or first-order autoregressive) process with a fundamental time scale of about 10 days.
This is mostly consistent with the observed NAO behavior: indeed, the NAO exhibits no preferred time scale, and the power spectrum is slightly red, -- power increasing with period--, with no significant peaks
This then means that observed interannual and longer time scale NAO fluctuations (Figure 12) could entirely be a statistical remnant of the energetic weekly variability – the so-called climate noise paradigm which serves as a good null hypothesis
A possible exception to this reference is the enhanced NAO variability over the latter half of the 20th century including the apparent upward trend in the boreal winter NAO index.
Several recent studies support this: for instance, it has been shown that over the last 50 years or so, something like 60% of the NAO interannual variability exceeds the noise (the variability expected if it was entirely due to intraseasonal stochastic processes), and the trend appears to be significant compared to an appropriate red noise model.
Moreover, the observed trend is outside the range of internal variability generated in multi-century integrations with at least 7 state-of-the-art coupled models
These findings give support to the view that “external forcing” could be playing a role, and the focus here is on the role of the ocean.
A role for external forcing?

29. 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

30. And in the end what might really matter – how storms propagate
Edge of the vortex
Strength of storms
Direction of storms
Speed of storms
Warm at y y X
Cold at x

31. North Atlantic Oscillation (from Lamont-Doherty )
Positive Phase
U.S. East, Mild and Wet
Europe North, Warm and Wet
Canada North & Greenland, Cold and Dry
Negative Phase
U.S. East, Cold Air Outbreaks, Snow (dry)
Europe North, Cold; South, Wet
Greenland, Warm

32. 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.”

33. 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.”

34. 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”

35. 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

36. 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)

37. 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.