Presentation on theme: "Teleconnections & The Zonal Index Part I: The Big Picture Brian Griffith & Jason Furtado AT750 – Climate Variability Lecture 1."— Presentation transcript:
Teleconnections & The Zonal Index Part I: The Big Picture Brian Griffith & Jason Furtado AT750 – Climate Variability Lecture 1
Quick Outline ► Teleconnection Works Walker and Bliss (1932) Pacific-North American Pattern ► The Zonal Index Cycle & Atmospheric Circulation Rossby & Willett (1948) ► Zonally Symmetric Seesaw Lorenz (1951) ► Onto Wallace & Gutzler (1981) and Wallace & Hsu (1985)
The Motivation ► What prompted scientists of the early 20 th century to focus on finding teleconnection and circulation patterns? Understand the overall general circulation of the planet. Expand the predictability of future anomalies and weather patterns (extend long range forecasts). Gain more insight into the dynamical principles of the atmosphere.
Some Key Definitions ► Teleconnection: A relationship between changes in variables between two locations ► Zonal Index: A measure of the strength of the mid-latitude westerlies. ► Zonal Symmetry: Equally distributed signal at all longitudes along a latitudinal belt. In this discussion, we look at zonally symmetric see-saws, which means anti- correlations between two latitudinal belts.
Early Teleconnection Work ► Walker & Bliss (1932) investigated three (3) “connected relationships” in the world’s circulation. North Atlantic Oscillation (NAO) North Pacific Oscillation (NPO) Southern Oscillation (SO) ► Used correlation coefficients in T, P, and rainfall measurements between certain stations to define each pattern mathematically
The NAO NAO+NAO- Figure from J. M. Wallace, U. of Washington
More On The NAO ► Walker and Bliss (1932) defined the NAO Index as: ► OK, but what does it fundamentally reflect? Changes in SLP between the Icelandic Low and Azorean High Affects temperatures in North America and Europe, as seen by van Loon and Rogers (1978)
The NPO ► The NPO has negatively correlated centers of action between Hawaii and an extended region from Siberia to the Canadian Rockies (Walker and Bliss 1932). ► Originated from observations of storm tracks based on pressure in Alaska and the Intermountain West of the US. From Walker and Bliss (1932)
The Pacific-North American Pattern From Pacific Marine Environmental Lab http://www.pmel.noaa.gov/~miletta/images/pna.gif Aleutian Low Hawaii “High Plateau” High Gulf Coast *Note: Wave-train signal in the PNA Pattern. Often recognized by SST changes in the Pacific. Another center?
A Circulatory Argument ► Walker and Bliss’s discussion depended solely on surface measured variables. (Why?) ► Enter the MIT Extended Forecasting Project (MIT EFP) in the early 40s. The ultimate goal – Make longer, more accurate long range forecasts (several days to two weeks) ► But how can one hope to make longer range forecasts without understanding a simple general circulation of the planet? ► One theory – Hadley’s 1735 theory of the thermal circulation that produces the tropical trade winds. From Ahrens (1994)
Toward A Zonal Theory ► Rossby & Willett (1948) investigated upper troposphere / lower stratosphere circulation ► They noticed that the polar jet stream had a tendency to oscillate between 35 and 55 N. ► This oscillation coincided with expansion and contractions of the polar vortex. ► In particular, Rossby & Willett noticed that a 3-8 week period accompanied these jet stream and polar vortex changes. Mean zonal V g Jan. – Feb. (Rossby & Willett 1948) Mean zonal V g July-Aug. (Rossby & Willett 1948)
The Zonal Index Cycle ► With these observations, Rossby & Willett (1948) defined the zonal index cycle. High Index: Stronger zonal flow in the higher latitudes, decreasing storminess and meriodional air mass advection, Low Index: A weaker polar vortex, allowing the jet stream to shift further south, allowing for a period of increased storminess. ► Rossby (1948) theorized that zonal index cycle changes could be predicted at natural intervals based on radiational cooling effects at higher latitudes. More importantly, Rossby (1948) formulated a realistic circulation model of the atmosphere.
Rossby’s Circulation Model Hadley Cell Ferrell Cell Polar Cell (?) ? Nothing? From Rossby and Willett (1948) *Note: This entire circulation model was derived based on the zonal index cycle. According to Rossby (1948), fluctuations in the polar vortex strength and size caused the movements of the jet streams and hence develop these cells.
Lorenz’s Seesaw ► Lorenz (1951) investigated variations in NH SLP data, seasonal and anomalous. ► Findings summarized on the right – Lorenz found two (2) zones in NH SLP. +r within the zone, and –r with the other zone. ► The connection: Lorenz linked the zones to the fluctuations in the index cycle (top) NH mean SLP profiles connected with the index cycles and (bottom) normal SLP profiles for each month From Lorenz (1951).
What Do We Have? ► Three (3) “relationship” (teleconnection) patterns The NAO – SLP differences between points in the Atlantic basis. The NPO – SLP differences between subtropical Pacific and Siberian-Alaskan-Rockies pressure system. The PNA – Wave train pattern in the central and eastern Pacific extending into N. America ► The Zonal Index Cycle Linked to expansions and contractions of the polar vortex ► The Zonally Symmetric Seesaw Mass transport between middle and higher latitudes Linked to zonal index changes.
An Emerging Interest ► Teleconnection interests waned after the early ’50s but picked up significantly in the late ’60s and early ’70s. ► Why a renewed interest in teleconnection patterns? Early ’60s into the ’70s - anomalously snowy & cold winters plagued the Eastern US as well as parts of W. Europe (Hurrell and van Loon 1997). ► Linked to a long-lasting negative phase of the NAO. Dynamics of the proposed patterns not understood or only speculated. ► Forcing? Changes in ocean circulations? ► Ocean-atmosphere coupling?
From Theory to More Objective Analyses ► The earlier works were instrumental in laying the foundations of the existence of these patterns, but much of the work was “subjective.” ► Later works by Kutzbach (1970), van Loon & Rogers (1978) & Wallace & Gutzler (1981) attempted to use contemporary methods and data to recreate or disprove the proposed theories. First EOF of January SLP from 1899 – 1969 (Kutzbach 1970).
More On Teleconnection Work There is a robust negative correlation between temperatures in Greenland & Oslo. (van Loon and Rogers 1978) Correlations between IL mean position SLP and all other points on the globe. Notice the north-south dipole in the Atlantic as well as the strong negative correlation in the Aleutians. (van Loon and Rogers 1978) c
A Look Ahead ► Wallace and Gutzler (1981) One-point correlation maps to identify the individual patterns and then centers of action of each pattern. Composite maps to understand oscillations in planetary wave action. ► Wallace and Hsu (1985) Eddy component maps vs. composite maps of the zonal index ► How should they compare if the zonal index cycle theory were true?
Objective Analysis ► Consider teleconnections emerging from objective treatment of data ► Monthly mean SLP and 500mb GPH for 15 winter data set ► Test reproducibility with independent 13 winter data set
Zonal Index ► Objectively examine relationship btwn ZI, mean square eddy amplitude and standing mode amplitude ► If ZI hypothesis true: should see strong negative correlation btwn ZI and eddy amplitude and composite high ZI maps should resemble composite high eddy amplitude maps
Analysis ► ZI = avg SLP(35N) - avg SLP(55N) ► Eddy = average squared departure from avg polar 500mb height ► Standing mode = normalized projection of transient eddy component of 500mb GPH on climatological mean
Zonal Index? ► Fig 3 looks like Fig 4, not like Fig 1 ► Implies: Variations in ZI not accompanied by major changes in eddy activity Variations in S account for much of observed variability in eddy activity ► Correlation btwn e and S = 0.64 ► Correlation btwn e and ZI = -0.27 ► Cross-spectrum analysis shows no significant peaks btwn e and ZI
The Unanswered… ► Wallace and Gutzler (1981) ends with many unanswered and unexplored issues. Connections with the Southern Hemisphere? ► Symmetry! Extent of the teleconnection patterns? ► Upper troposphere? Stratosphere? Are all of these patterns realized exclusively? ► Which ones are “parent” patterns, and which are “children”? Can the extratropics affect the tropics?
The Zonal Index Theory Falls… ► Wallace & Hsu (1985) confirmed relocations of mid-latitude pressure systems based on zonal wind changes. But no zonally symmetric T changes and a lack of negative correlation in the zonal index and eddy component discredits Zonal Index Theory ► What about the zonally symmetric see-saw? Composite difference (high – low) of 1000 -500 mb thicknesses. The figure is proportional to mean temperature changes. (Wallace and Hsu 1985)
From Zonal to Regional ► Essentially, Wallace and Hsu argue that “high index” and “low index” cycles may be more regional. E.g., NAO, PNA phase changes Topographic/orographic features and patterns ► The pair also looks at pressure gradients in different latitudinal belts (20-40 N and 45-65 N) Slight deterioration in signal in 20-40 N 45-65 N have index differences in Atlantic sector and at high latitudes