A DIAGNOSTIC STUDY OF NORTH ATLANTIC OCEANIC PRECIPITATION VARIABILITY Phillip A. Arkin, Heidi Cullen and Pingping Xie University of Maryland, ESSIC, NCAR/ESIG and Climate Prediction Center, NOAA

OUTLINE Background Background Data Data Regional manifestations Regional manifestations –Boreal Winter –Boreal Summer Global associations Global associations Conclusions/Points for further study Conclusions/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 Used NAO Index NAO Index –EOF of 700 hpa geopotential height –From Barnston and Livezey (1987) Precipitation from CMAP Precipitation from CMAP –Combination of IR and microwave satellite estimates and gauge observations –Xie and Arkin (BAMS, 1997) Circulation from NCEP/NCAR Reanalysis Circulation from NCEP/NCAR Reanalysis –1000 and 500hpa winds and geopotential heights –Kalnay et al. (BAMS, 1996) Storm frequency from GISS Storm frequency from GISS –Number of cyclonic centers of circulation passing through a grid box per month –Automated tracking –Chandler and Jonas (

Boreal Winter Climatology (DJFM) Features similar north of 25°N Features similar north of 25°N Heaviest precipitation south and west of greatest storm frequency Heaviest precipitation south and west of greatest storm frequency Bifurcation of storm track/precip max across Northern Europe/Mediterranean Bifurcation of storm track/precip max across Northern Europe/Mediterranean Precipitation Storm Frequency

Correlations with NAO Index (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 NAO Index (using monthly 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 results? Precipitation correlations with NAO index computed from 92 months – absolute values of >0.3 clearly different from zero Precipitation correlations with NAO index computed from 92 months – absolute values of >0.3 clearly different from zero Tri-pole pattern in Atlantic precipitation appears robust Tri-pole pattern in Atlantic precipitation appears robust Circulation/storm frequency correlations stronger/weaker than precipitation Circulation/storm frequency correlations stronger/weaker than precipitation Monte Carlo calculation confirms this Monte Carlo calculation confirms this –Composite precipitation anomalies in terms of standard deviation of 1000 composites based on random time series Normalized CMAP composites High NAO Low NAO

Boreal Summer Climatology (JJAS) Storminess/precipitation similarity only north of 40°N or so Storminess/precipitation similarity only north of 40°N or so Precip max still south and west of greatest storm frequency Precip max still south and west of greatest storm frequency Strong tropical influence on precip maxima Strong tropical influence on precip maxima Storm track dwindles away over Northern Europe Storm track dwindles away over Northern Europe

Correlations with NAO Index (June - September) Circulation correlations still pretty strong Not so for precip/stormine ss Two centers in band of 500 hpa positive height anomalies 1000 hpa height 500 hpa height precipitation Storm frequency

Winter – summer shift in geopotential correlation pattern Positive anomaly at 1000hpa splits (no simple index possible) Summer correlations weaker, but still clear JJASDJFM Tripole pattern at 500hpa still present in summer Low latitude negative anomalies shift eastward 1000 hpa 500 hpa

JJAS Composites based on NAO Index (using monthly 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 signal in Boreal summer? Areas of significance in summertime correlations smaller Areas of significance in summertime correlations smaller Tri-pole pattern in Atlantic only at 500hpa Tri-pole pattern in Atlantic only at 500hpa Precipitation dipole displaced northward relative to winter Precipitation dipole displaced northward relative to winter Monte Carlo results suggest less symmetry during summer Monte Carlo results suggest less symmetry during summer Less oceanic signal in summer in precipitation Less oceanic signal in summer in precipitation Possible connection of positive NAO index to tropical storms (not found in storminess) Possible connection of positive NAO index to tropical storms (not found in storminess)

Does the NAO have manifestations outside the North Atlantic Ocean? Hoerling et al. (Science, 2001) found that increasing trend in tropical precipitation from was related to similar trend in NAO Hoerling et al. (Science, 2001) found that increasing trend in tropical precipitation from was related to similar trend in NAO Our record too short to compare directly, but maybe periods of high/low NAO index are characterized by coherent anomaly patterns away from the Atlantic Ocean Our record too short to compare directly, but maybe periods of high/low NAO index are characterized by coherent anomaly patterns away from the Atlantic Ocean Implication: high NAO index associated with greater Indian/Pacific Ocean tropical precipitation?

December - 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 - March Possible positive NAO index signal around Maritime Continent Normalized high composite Normalized low composite

How do these compare to a really 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

Conclusions Merged precipitation datasets good enough for diagnostic studies Merged precipitation datasets good enough for diagnostic studies The NAO has robust manifestations in Atlantic Ocean precipitation and circulation during Boreal winter The NAO has robust manifestations in Atlantic Ocean precipitation and circulation during Boreal winter Boreal summer signal present, but weaker and displaced northward Boreal summer signal present, but weaker and displaced northward Coherent precipitation/500hpa signal extends deep into tropics (maybe into SH for precip) Coherent precipitation/500hpa signal extends deep into tropics (maybe into SH for precip) Some evidence of a tropical signal in Indian and Pacific Oceans – maybe predictive (since tropical precipitation is coupled to SST) Some evidence of a tropical signal in Indian and Pacific Oceans – maybe predictive (since tropical precipitation is coupled to SST)

Questions How does the NAO signal depend on time scale? (investigate using pentad/daily data) How does the NAO signal depend on time scale? (investigate using pentad/daily data) Are results sensitive to precipitation data set used? Are results sensitive to precipitation data set used? Is the tropical “signal” (outside the Atlantic) real? Is the tropical “signal” (outside the Atlantic) real? What role does the ocean play? Is any memory (predictability) apparent? What role does the ocean play? Is any memory (predictability) apparent?