Is Arctic Warming Causing Extremes in Mid-Latitude Weather? Florence Chen, Joseph Lanzillo, Upasna Sharma

Why is “Waviness” Important? 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 A

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 Amplification Differential 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 H 2 O vapor content, increased greenhouse gases Strongest in Fall and Winter Potentially alters mid-latitude weather patterns Source: Change in Annual Surface Temperature from

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 Gradient Thickness: vertical distance between two pressure surfaces at a given location Thickness gradient: change in thickness between two locations PV=nRT P = pressure V = volume n = moles of gas R = gas constant T = 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 hPa 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 hPa OND=Oct-Nov-Dec means Anomalies seen in mean thickness compared to 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 Sept Sea 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 50 o N (Longitude °E) Increase in wave amplitude (deg/decade) Increase in number of ridge peaks north of 50 o N 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 °E # of Peaks

Decreased hPa Thickness Time Series of Seasonal hPa thickness differences between o N and o N Why post-1980 data?  Rapid ice loss  Enhanced 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 % decrease from 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: Season d: Day λ: longitude Θ: various latitudes

Seasonal and Daily Wave Extents Are Not Increasing Conclusion: Trend detection sensitive to methodology, and no significant increase in wave extent Contrast 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 ( o N) 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 ( o N) 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 Change in zonal wind speed (m/s per decade) 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 complex Contrast 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 40 o N and 60 o N 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 ( 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