Presentation on theme: "Thinking about the Arctic Oscillation"— Presentation transcript:
1 Thinking about the Arctic Oscillation Richard B. RoodUniversity of Michiganfor the National Park ServiceAugust 8, 2013
2 Outline Why am I giving this talk? Some aspects of climate variability The Arctic OscillationA heuristic: vorticesThe Arctic Oscillation and Climate ChangeSynthesisBig 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 ParkThe Arctic Oscillation emerged as an important weather-climate driverThe Arctic Oscillation has major impact on warm-cold, dry-wet, especially in winter and spring in the eastern half of North AmericaThe response of ecosystems to this variability can amplify the weather-climate variabilityWill 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 weatherRainy periodsFloodsDry periodsDroughtsDuring these times the weather for a region does not appear random – it perhaps appears relentless
5 An example of variability: Seasons ColdWarmColdTemperatureMessyMessyWinterSummerWinterMore 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 frontsRain comes in thunderstormsForced variability responding to solar heating
6 Internal Variability (Rood Class Lecture 2010) Weather – single “events” – waves, vorticesThere are modes of internal variability in the climate system which have global consequences.El Nino – La NinaWhat is El NinoNorth Atlantic Oscillation / Arctic OscillationClimate Prediction Center: North Atlantic OscillationAnnular ModeInter-decadal Tropical AtlanticPacific Decadal OscillationAre 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 sameLargest mode of variability in northern middle and high latitudesIt is present all year, we notice it more in winter and springEl Nino – La NinaArctic OscillationOcean – AtmosphereGlobal InfluencePredictableAtmosphereStrong Regional InfluenceDifficult to predict
9 Year-to-Year Changes in Winter Temperatures Differences Relative to AverageLate 1970sFrom 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 nextWinter 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 recordFebruary 1985 was the last month below twentieth century averageContiguous US winter temperatures have warmed ~2ºF since 1895Winter 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 Maps20102011
12 Side by Side Comparison Arctic Oscillation and North American Temperature Note: How cold is not obviously related to strength of indexNote: There has not been a below 20th century average month since Feb 1985Note: Negative Index is a Negative temperature anomalyDJF Temperature: AnomalyFrom Jim Hurrell
13 Some basic referencesHurrell, 1995: Decadal trends in the North Atlantic Oscillation: Regional Temperature and PrecipitationHurrell and Deser, 2010: North Atlantic climate variability: The role of the North Atlantic OscillationKuzmina et al. 2005: The North Atlantic Oscillation and Greenhouse Gas ForcingBai et al., 2012: Great Lakes ice and Arctic Oscillation and El Nino
14 What’s going on? A heuristic A conceptual physics-based foundationA vortex is circulating air.If there is a low pressure system on Earth in the northern hemisphere then air circulates counterclockwise around the low.
24 The Arctic Oscillation “Negative” PhasePressure systems weakerCold Arctic air spills intomiddle latitudesWarm middle latitude airmoves into Arctic“Positive” PhaseStrong low (high) air pressureat high (middle) latitudesExtremely cold air confinedto ArcticWarm middle latitudesStrong vortexWeak vortexFrom Jim Hurrell
25 Year-to-Year Changes in Winter Temperatures Differences Relative to AverageLate 1970sFrom Jim Hurrell
28 What Climate Processes Govern NAO Variability? 200 years of NCAR CAM withoutvariations in “external” forcingsBasic structure & time scale arisesfrom internal nonlinear atmosphericdynamicsEOF1 SLP(Dec-Mar)Random and UnpredictableVariationsSimulated NAO Indexr (1yr) = -0.07Simulated (Dec-Mar)Random and UnpredictableVariationsObservedr (1yr) = -0.03Observedr (1yr) = 0.4Except for the latter halfof the 20th centuryStrong 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 peaksThis 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 hypothesisA 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 modelsThese 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 atmosphereRadiative forcing due to change in colorHeat flux between ocean and atmosphereHeat flux between land and atmosphereMoisture flux between ocean-land-atmosphere
30 And in the end what might really matter – how storms propagate Edge of the vortexStrength of stormsDirection of stormsSpeed of stormsWarm at yyXCold at x
31 North Atlantic Oscillation (from Lamont-Doherty ) Positive PhaseU.S. East, Mild and WetEurope North, Warm and WetCanada North & Greenland, Cold and DryNegative PhaseU.S. East, Cold Air Outbreaks, Snow (dry)Europe North, Cold; South, WetGreenland, Warm
32 Some recent researchLiu 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 researchFrancis 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 researchGreene 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 synthesisWe are seeing, here, an instance of the “non-stationarity” of climate.Change in the surface, changes energy and moisture characteristics of weatherThe statistical distribution will changeThis is not a simple shift of the distribution functionEvidence that the variability will increaseBut, remember, we are in warming climateChanges in regional and seasonal heat and moisture budgetHeavy snow, fast melt, change of water supply and quality, winter and spring floodingPropagation of storms is likely to change to cause the accumulation of weather effects into more extreme eventsSlow moving storms are very good at, for instance, building up storm surgesSustained precipitation followed by sustained heat and dryIf 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 changePost et al., 2009: Population dynamics and hot spots of response to climate changeHurrell and Deser, 2010: North Atlantic Climate Variability (reference to other literature)
37 GLISAclimate.orgBig 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.