A DIAGNOSTIC STUDY OF NORTH ATLANTIC OCEANIC PRECIPITATION VARIABILITY

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

2. OUTLINE • Background • Data • Regional manifestations • Boreal Winter • Boreal Summer • Global associations • Conclusions/Points for further study

3. The North Atlantic Oscillation (NAO) • 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 Images courtesy Martin Visbeck

4. Data Used • NAO Index • EOF of 700 hpa geopotential height • From Barnston and Livezey (1987) • Precipitation from CMAP • Combination of IR and microwave satellite estimates and gauge observations • Xie and Arkin (BAMS, 1997) • Circulation from NCEP/NCAR Reanalysis • 1000 and 500hpa winds and geopotential heights • Kalnay et al. (BAMS, 1996) • Storm frequency from GISS • Number of cyclonic centers of circulation passing through a grid box per month • Automated tracking • Chandler and Jonas (http://www.giss.nasa.gov/data/stormtracks/)

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

6. Correlations with NAO Index(December – March) 1000 hpa height precipitation Ocean-centered dipole near surface Tripole in precip – strongest in eastern Atlantic/Europe Stronger dipole (tripole?) at 500hpa 500 hpa height Storm frequency Storminess (or storm data) decoupled from precip in low latitudes

7. 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

8. How robust are these results? Normalized CMAP composites High NAO • 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 • Circulation/storm frequency correlations stronger/weaker than precipitation • Monte Carlo calculation confirms this • Composite precipitation anomalies in terms of standard deviation of 1000 composites based on random time series Low NAO

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

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

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

12. JJAS Composites based on NAO Index (using monthly anomalies) 1000 hpa height precipitation • 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 500 hpa height Storm frequency

13. How “significant” is the NAO signal in Boreal summer? • Areas of significance in summertime correlations smaller • Tri-pole pattern in Atlantic only at 500hpa • Precipitation dipole displaced northward relative to winter • Monte Carlo results suggest less symmetry during summer • Less oceanic signal in summer in precipitation • Possible connection of positive NAO index to tropical storms (not found in storminess)

14. Does the NAO have manifestations outside the North Atlantic Ocean? • Hoerling et al. (Science, 2001) found that increasing trend in tropical precipitation from 1950-2000 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 Implication: high NAO index associated with greater Indian/Pacific Ocean tropical precipitation?

15. December - March Composites impressive, but not when normalized Correlation Correlations weak outside the Atlantic except maybe convergence zones and western Indian Ocean Difference between H/L composites

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

17. How do these compare to a really robust signal? (ENSO) DJFM JJAS High SSTA High SSTA • 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) Low SSTA Low SSTA

18. Conclusions • Merged precipitation datasets good enough for diagnostic studies • The NAO has robust manifestations in Atlantic Ocean precipitation and circulation during Boreal winter • Boreal summer signal present, but weaker and displaced northward • 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)

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