Teleconnections

By

Dr. Gerry BellClimate Prediction Center (CPC)

NOAA/NWS/NCEP

Outline•General concepts

•Teleconnection Patterns: What they are• CPC Monitoring •Use care with teleconnection indices•Setting the stage: Average winter conditions

•Some Northern Hemisphere teleconnection patterns•Pacific/ North American Pattern (PNA)•North Atlantic Oscillation (NAO)•Arctic Oscillation (AO)

•Tropical teleconnections•El Niño and La Niña•Tropical Multi-decadal Signal

•Summary

Teleconnection Patterns: What They Are

Definition: Recurring and persistent, air pressure and circulation patterns spanning vast geographical areas. Also called “leading modes of variability, “ or “circulation regimes.”

Impacts: • Anomalous weather over seemingly vast distances: entire ocean

basins, continents, some are global. • Strong seasonality.

Monitoring Northern Hemisphere Teleconnections at CPC

Climate and Weather AAO,AO,NAO,PNADailyhttp://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/teleconnections.shtml

Daily indices, time series, model forecasts and forecast verifications of PNA, NAO, AO, AAO.

Monitoring and Data Index

Northern HemisphereTeleconnection Patterns

Monthly

http://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.shtmlMonthly Indices and time series, Calculation procedures, Descriptions of the ten leading patterns, Circulation maps, Temperature and precipitation departures.

Use Care with Teleconnection IndicesSeveral different indices are often available, with differing calculation procedures (grid point, area averaging, rotated EOF or RPCA).

Problems with indices based on grid points or area averaging:•Don’t recognize that patterns and strengths vary seasonally (e.g. Winter PNA index used to assess summer conditions).

•Independently calculated for each teleconnection pattern (PNA, NAO).•Some patterns overlap spatially. •Techniques cannot isolate pattern for which anomalies belong.•Same pressure anomaly can be reflected in several different indices

CPC’s indices are based on rotated EOF analysis (Barnston and Livezey 1987):•Above problems avoided.•Indices for all patterns are computed simultaneously. •Indices reflect the combination of patterns/ strengths that best explain the observed monthly (or daily) anomaly pattern.

Air Pressure Patterns and Jet Streams

Wet

Jet CoreDry

Dry

WetJet Stream

Jet Streams produce a 4-celled pattern of wet/dry

Jet CoreJet StreamStorms

Form Here

Storms Decay Here

H

Winds flow clockwise around areas of High Pressure

L

Winds flow counter-clockwise around areas of Low Pressure

Setting the Stage: Average Winter Conditions

L

L

H

H

L H

LH

Main Regions Where Storms FormEast Asian Jet Stream Atlantic Jet Stream

High pressure (H) and low pressure (L) areas, and jet streams strongly influence regions of storm formation, precipitation and temperature.

These patterns vary substantially. Preferred patterns are called teleconnections.

Air Pressure Pattern at Jet Stream Level (35,000 ft)

Some Northern Hemisphere Teleconnection Patterns

1. Pacific/ North American Pattern (PNA)• A main teleconnection pattern affecting the North Pacific

and North America.

2. North Atlantic Oscillation (NAO)• A main teleconnection pattern affecting eastern North

America to Europe.

PNA and NAO are continental scale wind/ pressure patterns linked to recurring jet stream patterns.

3. Arctic Oscillation (AO)Hemispheric pattern linked to polar-mid latitude mass exchanges– Features aspects of PNA and NAO patterns.

Pacific/ North American Pattern (PNA)The PNA is a main teleconnection pattern affecting the North Pacific and North America.

PNA Teleconnection Pattern Air Pressure Correlations (x100)

Maps also depict air pressure departures during positive phase of PNA pattern.

PNA pattern changes between winter and summer.

Higher pressure Lower pressure

January July

PNA Pattern Time Series: 3-Month Running Means

CPC index based on Rotated EOF analysis.

Negative Phase

•Weaker high / low pressure systems

•Jet stream and region of storm formation shifts west toward central Pacific.

PNA: January Air Pressure Departures from NormalComparing Positive and Negative Phase

Higher pressure Lower pressure

Winds Storm Formation region

•Stronger high / low pressure systems

•Jet stream and region of storm formation shift eastward

Positive Phase

Pacific/ North American Pattern (PNA)Surface Temperature Correlations (x100)

PNA pattern influences N.A. surface temperatures mainly during cool-season.

Surface Air Pressure and Jet Stream Showing Blocking During Negative Phase of PNA Pattern

Negative phase of PNA pattern favors blocking and strong cold-air outbreaks into western North America.

COLDLH

H

Pacific/ North American Pattern (PNA)Precipitation Correlations (x100)

The PNA influence on precipitation varies during the year.

North Atlantic Oscillation (NAO)The NAO is a main teleconnection pattern affecting eastern North America to Europe.

•Weaker high / low pressure systems

•Jet stream and region of storm formation shift to southern Europe

Negative Phase

NAO: January Air Pressure Departures from NormalComparing Positive and Negative Phase

Higher pressure Lower pressure

Storm Formation region

•Stronger high / low pressure systems

•Jet stream and region of storm formation shift north and east

Positive Phase

Winds

Winter NAO Index

Year 50 55 60 65 70 75 80 85 90 95 00 03

The NAO pattern can persist in one phase for decades at a time.

North Atlantic Oscillation (NAO) Surface Temperature Correlations

Surface Air Pressure Pattern Showing Blocking During Negative Phase of NAO

LH COLD

Negative phase of NAO pattern favors blocking and strong cold-air outbreaks into northern Europe.

North Atlantic Oscillation (NAO) Precipitation Correlations

The main precipitation signals for the NAO pattern are seen in Europe.

Arctic Oscillation (AO)•AO reflects mass exchange between polar region and middle latitudes

•Affects Pacific and North Atlantic storm tracks, jets

•Features aspects of PNA and NAO pattern

Wintertime Arctic Oscillation (AO): Positive Phase

Higher pressure Lower pressure

Maps show air pressure departures from normal.

Surface Air Pressure and Winds

Weaker Aleutian Low Stronger Icelandic Low

LL

Stronger Aleutian Low Weaker Icelandic Low

L L

Arctic Oscillation (AO): Significant Winter Impacts

Positive AO

Negative AO

Warmer

Colder

FewerNor’easters

More Nor’eastersMore cold-air outbreaks

Increased Snowfall

More heavy rain events

Frequent warm-ups

More IceInland snow

Less rainand clouds

More rainAnd clouds

Stronger Winds,Waves

El Niño and La Niña

•El Niño and La Niña are naturally occurring climate phenomena, and represent extremes in the El Niño/ Southern Oscillation (ENSO).

•El Niño: a periodic warming (every 3-6 years) of the central and eastern equatorial Pacific .

•La Niña: a periodic cooling (every 3-6 years) of the central and eastern equatorial Pacific.

Climate & Weather

El Niño /La Nina

Monitoring El Niño and La Niña (ENSO)

•Current Conditions—Weekly Update (.ppt)•Animations•Weekly and Monthly Analyses•Indices•Diagnostics Discussion•Tutorial

ENSO Diagnostics Discussion

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

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Normal Ocean Surface Temperatures (°C)January

Equator

Date Line

Equatorial ocean temperatures are normally warmest in the western Pacific and coolest in the eastern Pacific.

In the central and eastern Pacific, there is a lot of year-to-year variability. Some years are much warmer and wetter (El Niño) and some years are much cooler and drier (La Niña).

Warm, Wet

Cool, Dry

SST(°C) and Departures

28 28

El Niño is present. Expected to strengthen and last into the spring. A moderate strength event is most likely.

ENSO Alert System Status: El Nino Advisory

Niño 3.4 Region

Temperature Departures (oC): Last 30 Days

Defining El Niño and La Niña Real-time: El Niño or La Niña conditions are present when the monthly SST departures in Niño 3.4 region meet or exceed +/- 0.5°C along with consistent atmospheric features. Anomalies must be forecasted to persist for 3 consecutive months.

Strength Thresholds:Weak: Monthly Niño 3.4 index is 0.5° to 1°C.Moderate: Monthly Niño 3.4 index is 1.0° to 1.5°C.Strong: Monthly Niño 3.4 index exceeds 1.5°C

2009: Niño 3.4 Sea Surface Temperature Index (°C) Ja

n

MarFeb

Apr

May Ju

l

Jun

Aug

Sep

Oct

Nov

The current El Niño developed in June

Niño 3.4 SST Forecasts (oC)

-1

0

0.5

1

1.5

-0.5

2

ON

D

ND

J

DJF

JFM

FMA

MAM AM

J

MJJ

JJA

Expected Peak Strength

Continue through Spring

El Niño and La Niña: 1950-PresentClassified using Oceanic Niño Index (ONI, oC): Three-month

running-mean SST departures in the Niño 3.4 region.

Historical Classification:

El Niño: ONI ≥ +0.5°C

La Niña: ONI ≤ -0.5°C

Must persist for five consecutive overlapping 3-month seasons.

Most recent ONI value (August – October 2009) is +0.9oC.

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Wintertime Tropical Rainfall (Inches)

•Tropical convection and jet stream extend across central and eastern Pacific.

•Triggers positive PNA pattern

Jet Stream

WetterDrier

Strong El Nino

4 8 12 16 20

Jet Stream

Wetter Drier

Strong La Nina

•Tropical convection and jet stream confined to western Pacific.

•Triggers negative PNA pattern

4 8 12 16 20

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Global El Niño Impacts

•El Niño impacts are strongest and most extensive in winter.

•Some impacts are positive, some are negative

December-February

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El Niño:•Pacific jet stream, storm track are south of normal•Polar jet stream well into Canada•Fewer arctic outbreaks

January-MarchTypical ConditionsWintertime Impacts in North America

Pacific jet stream, storm track

Polar jet streamWarmer

Wetter

El Niño

Cooler

La Niña:•Pacific jet stream, storm track are variable •Periods with strong polar jet stream•More arctic outbreaks

Polar jet stream Cooler

DrierPacific jet stream, storm track more variable

La Niña

Warmer

CPC: Dec.-Feb. 2009-10 Seasonal Outlooks

Temperature Precipitation

These outlooks largely reflect El Niño and long-term trends.

Combined Climate Impacts

1. Understanding combinations of signals is key to understanding observed climate variability and improving seasonal predictions.

2. El Niño impacts can vary depending on • Other teleconnections: NAO, AO• Tropical rainfall and temperature patterns that

can last for decades: called multi-decadal signal– affects hurricanes

3. Care must be taken when performing regression analysis on individual climate factors.

El Niño and Negative NAO

El Niño Only

oC

Winter El Niño – NAO Temperature Composites

El Niño and Positive NAO

40

Influence on Atlantic Hurricane Activity

El Niño increases the westerly winds, over the Atlantic, increasing the wind shear and suppressing hurricane activity.

More Shear

Increased Wind Shear, Fewer Hurricanes

Warm, Wet

El Niño

Winds at 35,000 ft

La Niña decreases the westerly winds over the Atlantic, reducing wind shear and enhancing hurricane activity.

Less Shear

Increased Wind Shear, Fewer Hurricanes

Cool, Dry

La Niña

Historical Atlantic Hurricane Season Strength

ACE index shows the overall strength of the hurricane season.

Multi-decadal fluctuations in season strength are clearly evident.

Tropical Multi-Decadal SignalCurrent High Activity Era

DrierDrier

WarmerWetWet

Warmer

High-activity eras are associated with the above climate conditions.Low-activity eras have opposite departures from normal.

Conditions For Active Atlantic Hurricane Era

Atlantic Hurricane Activity

Above Near BelowNormal Normal Normal 3 4 2

El NinoHigh Activity Decades

Above Near BelowNormal Normal Normal 0 2 7

Low Activity Decades

Above Near BelowNormal Normal Normal 6 0 1

La Nina

High Activity Decades Above Near BelowNormal Normal Normal 1 2 2

Low Activity Decades

Summary

PNA and NAO Teleconnection Patterns: • Seasonally dependent, continental scale, linked to recurring jet

stream patterns, Vary months to seasons to decades (NAO).• EOF-based indices are better (CPC)

Arctic Oscillation (AO): • Combines parts of PNA and NAO • Hemispheric impacts linked to polar-mid latitude mass exchange.

Tropics• El Niño/ La Niña: • Niño 3.4 and ONI indices• Global teleconnections, strongest in winter hemisphere• El Niño is present and will last into the spring.

• Multi-decadal signal: Atlantic SSTs and west African monsoon.

• Combinations of climate signals for understanding observed climate variability and improving seasonal predictions.

Jet Stream

Wet

Jet CoreDry

Dry

WetJet Stream

Jet Streams produce a 4-celled pattern of wet/dry

Jet CoreJet StreamStorms

Form Here

Storms Decay Here

Jet streams strongly influence regions of storm formation

H

Winds flow clockwise around areas of High Pressure

L

Winds flow counter-clockwise around areas of Low Pressure