OCEANIC PRECIPITATION OCEANIC PRECIPITATION VARIABILITYVARIABILITY

ASSOCIATED WITH THE ASSOCIATED WITH THE NORTH ATLANTIC NORTH ATLANTIC

OSCILLATIONOSCILLATION

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

• Background and questions to be Background and questions to be addressedaddressed

• DataData

• Around the AtlanticAround the Atlantic– Boreal WinterBoreal Winter– Boreal SummerBoreal Summer

• ElsewhereElsewhere

• 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

Objectives/QuestionsObjectives/Questions• Describe oceanic precipitation anomalies Describe oceanic precipitation anomalies

associated with the NAOassociated with the NAO– What do they look like? In the Atlantic, tropical What do they look like? In the Atlantic, tropical

oceans?oceans?– What is the relationship to anomalies determined What is the relationship to anomalies determined

from gauge data?from gauge data?– Are they reasonable – can CMAP et al. be used Are they reasonable – can CMAP et al. be used

outside the tropics?outside the tropics?

• How do the observed anomalies relate to How do the observed anomalies relate to circulation and storm track anomalies?circulation and storm track anomalies?

• Are the features we find sensitive to the Are the features we find sensitive to the dataset (CMAP all/observation only; GPCP) dataset (CMAP all/observation only; GPCP) used?used?

• Are they sensitive to the time scale Are they sensitive to the time scale (monthly/pentad) examined?(monthly/pentad) examined?

Data UsedData Used• Indices from CPC web siteIndices from CPC web site

– NAO (700 hpa Z), AO (1000 hpa Z)NAO (700 hpa Z), AO (1000 hpa Z)

• Precipitation from CMAPPrecipitation from CMAP– CMAP/O uses a combination of IR and microwave CMAP/O uses a combination of IR and microwave

satellite estimates and gauge observationssatellite estimates and gauge observations– CMAP/A uses reanalysis precipitation as well in regions CMAP/A uses reanalysis precipitation as well in regions

without other datawithout other data– Xie and Arkin (BAMS, 1997)Xie and Arkin (BAMS, 1997)

• GPCP pentad GPCP pentad – Same inputs as CMAP/OSame inputs as CMAP/O– Constrained to match GPCP monthly analysisConstrained to match GPCP monthly analysis– Xie et al. (J. Climate, accepted pending minor revisions)Xie et al. (J. Climate, accepted pending minor revisions)

• 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)

Precipitation Climatology Precipitation Climatology (CMAP)(CMAP)

DJFM

JJAS

Correlations with NAO Index Correlations with NAO Index (DJFM)(DJFM)

CMAP precipitation

REOF of 700mb Z

January

Correlations with NAO Index Correlations with NAO Index (DJFM)(DJFM)

1000 hpa height

500 hpa heightNew CPC NAO index based on 500 hpa eof

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

CMAP precipitation

High NAO quartileminus

Low NAO quartile

How much difference do using the How much difference do using the reanalysis precipitation make in reanalysis precipitation make in

these composites ?these composites ?

CMAP/A CMAP/O

Not much – maybe provides a little continuity in high latitudes

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

Normalized CMAP composites

High NAO Low NAO

Each month normalized by standard deviation of 1000 composites based on random time series

Conclusions - DJFMConclusions - DJFM• CMAP et al. appear to be usable in extratropicsCMAP et al. appear to be usable in extratropics• Winter precipitation signal is consistent with earlier Winter precipitation signal is consistent with earlier

resultsresults• Positive NAO associated with wet conditions in North Positive NAO associated with wet conditions in North

Atlantic/northern EuropeAtlantic/northern Europe• Negative NAO associated with wet conditions in Negative NAO associated with wet conditions in

Atlantic between 30°-40°N extending across the Atlantic between 30°-40°N extending across the Mediterranean into the Middle EastMediterranean into the Middle East

• Oceanic precipitation anomalies strongest near Oceanic precipitation anomalies strongest near eastern end of the main storm track – as much as eastern end of the main storm track – as much as 50% of the mean50% of the mean

• Signal appears to extend into the tropicsSignal appears to extend into the tropics

Correlations with Correlations with NAO Index (JJAS)NAO Index (JJAS)

1000 hpa height 500 hpa height

CMAP precipitation

REOF of 700hpa Z

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

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

High NAO Low NAO

Normalized CMAP composites

Conclusions - JJASConclusions - JJAS• Positive/negative NAO associated with dry/wet Positive/negative NAO associated with dry/wet

conditions in northern Europe with opposite conditions in northern Europe with opposite anomalies farther north anomalies farther north

• Oceanic anomalies less prominent than during DJFMOceanic anomalies less prominent than during DJFM• Positive NAO associated with positive rainfall Positive NAO associated with positive rainfall

anomalies in western Atlantic between 15°-30°N – anomalies in western Atlantic between 15°-30°N – probably because of the positive SLP anomalies just probably because of the positive SLP anomalies just to the north and their influence on tropical systemsto the north and their influence on tropical systems

• Appears to be substantial dependence of the details Appears to be substantial dependence of the details of the results on the definition of the index of the results on the definition of the index

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?

Composite DJFM Precipitation AnomaliesComposite DJFM Precipitation AnomaliesHigh NAO – Low NAOHigh NAO – Low NAO

CMAP/A

GPCP Pentad

CMAP normalized composite CMAP normalized composite anomaliesanomalies

Based on NAO index

Based on Arctic Oscillation index

DJFM

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

descriptive diagnostic studiesdescriptive diagnostic studies– CMAP, GPCP pentad give similar resultsCMAP, GPCP pentad give similar results– High latitude results look pretty reasonableHigh latitude results look pretty reasonable

• The NAO has robust manifestations in Atlantic Ocean The NAO has robust manifestations in Atlantic Ocean precipitation and circulation during Boreal winterprecipitation and circulation during Boreal winter

• 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 (equivocal) evidence of a tropical signal in Some (equivocal) evidence of a tropical signal in Indian and Pacific OceansIndian and Pacific Oceans