A DIAGNOSTIC STUDY OF A DIAGNOSTIC STUDY OF NORTH ATLANTIC NORTH ATLANTIC

OCEANIC PRECIPITATION OCEANIC PRECIPITATION VARIABILITYVARIABILITY

Phillip A. Arkin, Heidi Cullen and Phillip A. Arkin, Heidi Cullen and Pingping XiePingping Xie

University of Maryland, ESSIC, University of Maryland, ESSIC, NCAR/ESIG and Climate Prediction NCAR/ESIG and Climate Prediction

Center, NOAACenter, NOAA

OUTLINEOUTLINE

• BackgroundBackground

• DataData

• Regional manifestationsRegional manifestations– Boreal WinterBoreal Winter– Boreal SummerBoreal Summer

• Global associationsGlobal associations

• Conclusions/Points for further studyConclusions/Points for further study

• Dominant mode of climate variability in the Atlantic in winter (van Loon & Rogers, 1972)• Seesaw of atmospheric mass between subtropical high and subpolar low (Walker and Bliss, 1932)• Controls the path and intensity of storm track (Hurrell, 1995)• Spectral density of NAO weakly exists at 2-3 years (QBO), 7-10 years, also an increasing trend (Hurrell and van Loon, 1997) • Significant impact on marine and terrestrial ecosystems

The North Atlantic Oscillation (NAO)

Images courtesy Martin Visbeck

Data UsedData Used• NAO IndexNAO Index

– EOF of 700 hpa geopotential heightEOF of 700 hpa geopotential height– From Barnston and Livezey (1987)From Barnston and Livezey (1987)

• Precipitation from CMAPPrecipitation from CMAP– Combination of IR and microwave satellite estimates and Combination of IR and microwave satellite estimates and

gauge observationsgauge observations– Xie and Arkin (BAMS, 1997)Xie and Arkin (BAMS, 1997)

• Circulation from NCEP/NCAR ReanalysisCirculation from NCEP/NCAR Reanalysis– 1000 and 500hpa winds and geopotential heights1000 and 500hpa winds and geopotential heights– Kalnay et al. (BAMS, 1996)Kalnay et al. (BAMS, 1996)

• Storm frequency from GISSStorm frequency from GISS– Number of cyclonic centers of circulation passing Number of cyclonic centers of circulation passing

through a grid box per monththrough a grid box per month– Automated trackingAutomated tracking– Chandler and Jonas Chandler and Jonas

(http://www.giss.nasa.gov/data/stormtracks/)(http://www.giss.nasa.gov/data/stormtracks/)

Boreal Winter ClimatologyBoreal Winter Climatology(DJFM)(DJFM)

• Features similar Features similar north of 25°Nnorth of 25°N

• Heaviest Heaviest precipitation south precipitation south and west of greatest and west of greatest storm frequencystorm frequency

• Bifurcation of storm Bifurcation of storm track/precip max track/precip max across Northern across Northern Europe/MediterraneaEurope/Mediterraneann

Precipitation

Storm Frequency

Correlations with NAO IndexCorrelations with NAO Index(December – March)(December – March)

Ocean-centered dipole near surface

Stronger dipole (tripole?) at 500hpa

Tripole in precip – strongest in eastern Atlantic/Europe

Storminess (or storm data) decoupled from precip in low latitudes

1000 hpa height

500 hpa height

precipitation

Storm frequency

DJFM Composites based on DJFM Composites based on NAO Index (using monthly NAO Index (using monthly anomalies)anomalies)

Based on monthly anomalies from months with NAO index in highest/lowest quartiles

Precip/storminess consistent north of 25°N; maxima poleward of max wind anomaly

How robust are these How robust are these results?results?

• Precipitation correlations Precipitation correlations with NAO index computed with NAO index computed from 92 months – absolute from 92 months – absolute values of >0.3 clearly values of >0.3 clearly different from zerodifferent from zero

• Tri-pole pattern in Atlantic Tri-pole pattern in Atlantic precipitation appears robustprecipitation appears robust

• Circulation/storm frequency Circulation/storm frequency correlations stronger/weaker correlations stronger/weaker than precipitation than precipitation

• Monte Carlo calculation Monte Carlo calculation confirms this confirms this – Composite precipitation Composite precipitation

anomalies in terms of anomalies in terms of standard deviation of standard deviation of 1000 composites based 1000 composites based on random time serieson random time series

Normalized CMAP compositesHigh NAO

Low NAO

Boreal Summer ClimatologyBoreal Summer Climatology(JJAS)(JJAS)

• Storminess/precipitation Storminess/precipitation similarity only north of similarity only north of 40°N or so40°N or so

• Precip max still south and Precip max still south and west of greatest storm west of greatest storm frequencyfrequency

• Strong tropical influence Strong tropical influence on precip maximaon precip maxima

• Storm track dwindles Storm track dwindles away over Northern away over Northern EuropeEurope

Correlations with NAO IndexCorrelations with NAO Index(June - September)(June - September)

•Circulation correlations still pretty strong

•Not so for precip/storminess

•Two centers in band of 500 hpa positive height anomalies

1000 hpa height

500 hpa height

precipitation

Storm frequency

Winter – summer shift in Winter – summer shift in geopotential correlation geopotential correlation

patternpattern

•Positive anomaly at 1000hpa splits (no simple index possible)•Summer correlations weaker, but still clear

JJAS DJFM

•Tripole pattern at 500hpa still present in summer•Low latitude negative anomalies shift eastward

1000 hpa

500 hpa

JJAS Composites based on NAO JJAS Composites based on NAO Index (using monthly Index (using monthly

anomalies)anomalies)•Results confirm correlation patterns

•European centers of action emphasized – possibly because of the index used

•Storm track and precip features displaced well poleward relative to winter

•Connection to tropical storms?

•Storm track composites similar over ocean and land, unlike precip

1000 hpa height

500 hpa height

precipitation

Storm frequency

How “significant” is the NAO How “significant” is the NAO signal in Boreal summer?signal in Boreal summer?

• Areas of significance in Areas of significance in summertime correlations summertime correlations smallersmaller

• Tri-pole pattern in Atlantic Tri-pole pattern in Atlantic only at 500hpaonly at 500hpa

• Precipitation dipole displaced Precipitation dipole displaced northward relative to winternorthward relative to winter

• Monte Carlo results suggest Monte Carlo results suggest less symmetry during less symmetry during summersummer

• Less oceanic signal in Less oceanic signal in summer in precipitationsummer in precipitation

• Possible connection of Possible connection of positive NAO index to positive NAO index to tropical storms (not found in tropical storms (not found in storminess)storminess)

Does the NAO have Does the NAO have manifestations outside the manifestations outside the North Atlantic Ocean?North Atlantic Ocean?• Hoerling et al. (Science, 2001) found that increasing Hoerling et al. (Science, 2001) found that increasing

trend in tropical precipitation from 1950-2000 was trend in tropical precipitation from 1950-2000 was related to similar trend in NAO related to similar trend in NAO

• Our record too short to compare directly, but maybe Our record too short to compare directly, but maybe periods of high/low NAO index are characterized by periods of high/low NAO index are characterized by coherent anomaly patterns away from the Atlantic coherent anomaly patterns away from the Atlantic OceanOcean

Implication: high NAO index associated with greater Indian/Pacific Ocean tropical precipitation?

December - MarchDecember - March

Correlations weak outside the Atlantic except maybe convergence zones and western Indian Ocean

Correlation

Difference between H/L composites

Composites impressive, but not when normalized

December - MarchDecember - March

Possible positive NAO index signal around Maritime Continent

Normalized high composite

Normalized low composite

How do these compare to a How do these compare to a reallyreally robust signal? (ENSO) robust signal? (ENSO)

•Based on monthly CMAP anomalies and quartiles of Niño 3.4 SST anomaly•Core anomalies very clear•Teleconnections evident, but don’t get every one that we expected to see •Results suggest that NAO tropical manifestations may be real (at least worth a further look)

DJFM JJAS

High SSTA

Low SSTA

High SSTA

Low SSTA

ConclusionsConclusions• Merged precipitation datasets good enough for Merged precipitation datasets good enough for

diagnostic studiesdiagnostic studies• The NAO has robust manifestations in Atlantic The NAO has robust manifestations in Atlantic

Ocean precipitation and circulation during Boreal Ocean precipitation and circulation during Boreal winterwinter

• Boreal summer signal present, but weaker and Boreal summer signal present, but weaker and displaced northwarddisplaced northward

• Coherent precipitation/500hpa signal extends deep Coherent precipitation/500hpa signal extends deep into tropics (maybe into SH for precip)into tropics (maybe into SH for precip)

• Some evidence of a tropical signal in Indian and Some evidence of a tropical signal in Indian and Pacific Oceans – maybe predictive (since tropical Pacific Oceans – maybe predictive (since tropical precipitation is coupled to SST)precipitation is coupled to SST)

QuestionsQuestions

• How does the NAO signal depend on time How does the NAO signal depend on time scale? (investigate using pentad/daily scale? (investigate using pentad/daily data)data)

• Are results sensitive to precipitation data Are results sensitive to precipitation data set used? set used?

• Is the tropical “signal” (outside the Is the tropical “signal” (outside the Atlantic) real?Atlantic) real?

• What role does the ocean play? Is any What role does the ocean play? Is any memory (predictability) apparent?memory (predictability) apparent?