Is Arctic Warming Causing Extremes in Mid-Latitude Weather?

Florence Chen, Joseph Lanzillo, Upasna Sharma

Why is “Waviness” Important?

http://www.whitehouse.gov/photos-and-video/video/2014/01/08/polar-vortex-explained-2-minutes

Rossby Waves and the Jet Stream

• The Jet stream: Westerly air current that separates cold, low pressure polar air from warmer high pressure air in mid-latitudes

Winds Associated with Rossby Waves

https://www.youtube.com/watch?v=FNnIrIuNbeA

Outline: Francis & Vavrus believe AA causes mid-latitude extremes

Arctic Amplification

Decreased Thickness Gradients

Decreased Zonal Wind Speed

Increased Wave Amplitude

Mid-Latitude Weather Extremes

?

??

Defining Arctic AmplificationDifferential warming of the pole relative to lower latitudes: Twice as large rise in Arctic near-surface air temperatures compared to global average in recent decades

• Driven by: Ice-albedo feedback, atmospheric H2O vapor content, increased greenhouse gases

• Strongest in Fall and Winter

• Potentially alters mid-latitude weather patterns

Source: http://jeb.biologists.org/content/213/6/855/F1.expansion.html

Change in Annual Surface Temperature from 1958-2008

Z500 = 500 hPa Height Contour

500 hPa height = height at which pressure is 500 hPa

Z500= daily mean height of 500 hPa contour

Defining the Thickness GradientThickness: vertical distance between two

pressure surfaces at a given location

Thickness gradient: change in thickness between two locations

PV=nRT

P = pressureV = volumen = moles of gasR = gas constantT = temperature

Recent Changes in Thickness

Decreased thickness gradient leads to decreased temperature gradient

Increases in Amplitude caused by Thickness Changes

• Thickness gradient decreases

• Rossby wave expands North/South to cover the same vertical height

• Larger amplitude corresponds to slower westerlies

Francis and Vavrus (2012)

Ridge North/South Elongation resulting from Arctic Amplification

AA Causes Weakened Zonal Winds

Zonal latitudinal

Zonal flow is generally west to east

Increased AA Weaker Poleward Thickness Gradients Slower Zonal Winds

Geostrophic Balance

Outline of Francis and Vavrus (2012)

• Methods• Thickness Anomalies• Amplitude

Gridpoints = Number of Ridge Peaks above 50°N

– Northward Movement of Gridpoints– Number of Gridpoints– Changes in Gridpoints with Time and

Latitude• Zonal Wind

– Decreases in 1000-500hPa Thickness– Decreases in Zonal Wind Speed

Jennifer Francis, Rutgers

Stephen Vavrus, U. Wisconsin-Madison

Francis and Vavrus (2012): Methods

• Looked for mid-latitude patterns at 500 hPa height fields (NCEP/NCAR post-satellite data)

• Assessed if changes in AA caused changes in atmospheric circulation

• Investigated effects of high-latitude change on mid-latitude patterns at 500 hPa

Red Asterisks are examples of selected range of 500 hPa heights used in analysis

Regions: N. America and Atlantic Ocean basin

North Pole

Measuring Zonal Thickness Anomalies•Graph measures thickness between 500-1000 hPa•OND=Oct-Nov-Dec means•Anomalies seen in 2000-2010 mean thickness compared to 1970-1990 mean thickness•White spots: Statistically significant (95% confidence) anomalies *F and V do not specifically mention statistical test*

Conclusion: Thickness anomalies apparent in areas that experienced warming

Northern Hemisphere Thickness Anomalies from 40-90° N

Northward Progression of Ridge Peaks

Conclusion: northward movement of peaks supports hypothesis that AA contributes to ridge elongation

Increase in Max. Latitude of Ridge Peaks 1979-2010

SeptSea Ice area

Lat.

Maximum latitude of daily wave peaks increasing – 99% confidence in trends

Sea Ice Decreasing on reversed scale

(OND)

Increased Wave Amplitude and Number of Ridge Peaks North of 50o N

(Longitude °E)

Increase in wave amplitude (deg/decade)

Increase in number of ridge peaks north of 50oN over past 30 years (Gridpoints/decade)

Red asterisks indicate significance at 90% confidence interval

Conclusion: support for hypothesis that there is now a larger number of bigger waves

(OND)

Time/Longitude Variations in Gridpoints

F&V conclusion: In 2010, more peaks occur North of 50° N than did in 1980, indicating increase in wave amplitude.

More gridpoints (darker red) indicates that more waves peak farther North than previously

°E240

260

280

300

340

# of Peaks

4

44

84

125

165

205

Decreased 1000-500 hPa ThicknessTime Series of Seasonal 1000-500 hPa thickness differences between 80-60oN and 50-30oN Why post-1980 data?

Rapid ice lossEnhanced warming

Observed: ~10% decrease in poleward thickness differences.

Decrease especially during winter (OND) and fall(JAS, >95% confidence)

Zonal Speed of 500hPa Winds is Decreasing

• Possible effects– Rossby wave slows– Amplitude increase?

• Natural variability problem– Trend of steady decrease

starting from 1990– 20 years may not be enough

to distinguish between “real” trends and variability

500 hPa Zonal Wind Speed Change Over Time

Francis and Vavrus 2012

14% decrease from 1979-2010 in OND wind speed (95% confidence)

Outline: How Does Barnes question F and V?

• Methods– SeaMinMax and DayMinMax– Z500, v250, and u500

• No North/South Wave Amplitude Change– Seasonal and Daily Method Comparison– Change in Elevation not Magnitude– No Trend in Maximum Wave Extent

• Are Wind Speeds Changing?– Zonal Wind– Z500 Phase Speed– Meridional Wind Speed

Elizabeth Barnes, Colorado State

Barnes et al. (2013): What are SeaMaxMin and DayMaxMin?

Used to calculate North-South geopotential height extents

Examples of SeaMaxMin (left) and DayMaxMin (right) meridional wave extent metrics

DayMaxMin: daily wave extent for single wave.

SeaMaxMin: Seasonal

s: Seasond: Dayλ: longitudeΘ: various latitudes

Seasonal and Daily Wave Extents Are Not Increasing

Conclusion: Trend detection sensitive to methodology, and no significant increase in wave extentContrast with F and V’s conclusion: increase in North/South wave size

North/South Extent of Z500 Isopleths

“Small, nonsignificant” daily trend

“Large but nonsignificant” seasonal trend (two-sided t-test; 90% confidence)

Compare U500, Z500 and v250

U500: monthly mean zonal wind speed at 500 hPa

Z500: daily mean height of 500 hPa contour

V250: mean daily North-south wind speed at 250 hPa

Elevation of the Jet Stream is Changing, not Magnitude!

Elevation of 500 hPa surface (SeaMaxMin) based on longitude

Since 1980, change in meridional extent and elevation of seasonal average 500 hPa level have been small BUT…

Lat (oN)

Latitude difference between time series

Elevation difference between time series

Elevation of the Jet Stream is Changing, not Magnitude!

Elevation of 500 hPa surface (DayMaxMin) based on longitude

…Daily average 500 hPa layer has seen greater elevation change over time than Seasonal average

Lat (oN)

Latitude difference between time series

Elevation difference between time series

Maximum Extent of Waves is Not Increasing

No significant trends in wave extents observed for any season Significant slopes

defined as differing from 0 at 90% confidence

Maximum North-South Extent of 500hPa Pressure (z500)

Zonal Wind Speed Does Decrease – Agreement with F and V

Trends in zonal wind speed between 30-70° N

-3 independent data analyses shown

-Circles indicate 95% confidence that trends are significant

Chan

ge in

zona

l win

d sp

eed

(m/s

per

de

cade

)

These trends confirm F & V (2012) argument that zonal wind speed is decreasing.

Z500 Phase Speed Is Not Decreasing

Barnes concludes: Relationship between Z500 and u500 is complexContrast with F and V’s conclusion: Z500 Decrease in Fall is important because OND is key season

Trends in Z500 Phase Speeds in Each Season

Decrease in phase speed – different from 0 at 95% confidence

Positive and nonsignificant phase speeds

Meridional Wind Speed Not Decreasing when Viewed on 30-70N Latitude Range

Seasonal trend of v250 phase speeds for Atlantic NA region

No significant trend in wave phase speeds in any reanalysis in any season.

Agreement in v250 and Z500 averaging (domain dependent) Positive significant

OND trends seen only when speeds averaged between 40oN and 60oN

Daily Mean 250hPa meridional wind speed (c250, 30-70N)

How does Wallace question F and V?

Despite any extreme cold weather trends, global warming still prevails

Arctic Warming

Mid-latitude weather

Wallace claims this connection is too weak

Streamlines of 500 hPa winds – Jan 7, 2014 (Wallace et al. 2014)

Natural Variability

• Screen’s t-test model shows 50+ runs (years) needed to distinguish significant change (95% confidence) from atmospheric variability

• Francis and Vavrus and Barnes include 30 year data sets

Variables from Francis and Vavrus (2012)

Number of Runs Needed to Detect Significant Changes (Nmin) in Atmospheric Variables

Variable from Barnes 2013

Screen et al. (2013)

Lag Between Ice Melt and Weather Extremes

Late summer decrease in

sea ice

Change in atmospheric circulation at mid latitudes in other

seasons

This observed lag contradicts theory of Francis and Vavrus

Despite some trends between Arctic warming and mid-latitude weather changes, one pattern is not explained by theory:

Thinner Sea Ice Doesn’t Always Mean Colder Winters

More severe cold air outbreaks in past when Arctic sea ice was thicker & more extensive

Longer time scales (50-100 yrs) show:

Conclusion: Longer time scales more indicative of future.

Rate at which low temperature records are being set

Rate at which high temperature records are being set<

Implications for the Scientific Debate

“The research linking summertime Arctic sea ice with wintertime climate over temperate latitudes

deserves a fair hearing. But to make it the centerpiece of the public discourse on global warming is inappropriate and a distraction.” –

Wallace et al. (2014)

• Theory of “waviness” far from airtight and up for debate• We can still focus debate on the idea and its implications• Precautionary principle: we should act to prevent a

potentially harmful result even if full scientific consensus does not exist

Key Considerations

• Links between thickness gradient-amplitude and amplitude-zonal wind speed not well understood

• Counting northern ridge points may not be effective measure of wave amplitude

• Barnes does detect increases in N/S wave extent, decreases in zonal wind speed and decreases in Z500 phase speed

• Important to consider natural variability and methods of measuring significance

• We can and should discuss theories that are not fully substantiated