OCEANIC PRECIPITATION VARIABILITY ASSOCIATED WITH THE NORTH ATLANTIC OSCILLATION

1. OCEANIC PRECIPITATION VARIABILITYASSOCIATED WITH THE NORTH ATLANTIC OSCILLATION Phillip A. Arkin, Heidi Cullen and Pingping XieUniversity of Maryland, ESSIC, NCAR/ESIG and Climate Prediction Center, NOAA

2. OUTLINE • Background and questions to be addressed • Data • Around the Atlantic • Boreal Winter • Boreal Summer • Elsewhere • 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. Objectives/Questions • Describe oceanic precipitation anomalies associated with the NAO • What do they look like? In the Atlantic, tropical oceans? • What is the relationship to anomalies determined from gauge data? • Are they reasonable – can CMAP et al. be used outside the tropics? • How do the observed anomalies relate to circulation and storm track anomalies? • Are the features we find sensitive to the dataset (CMAP all/observation only; GPCP) used? • Are they sensitive to the time scale (monthly/pentad) examined?

5. Data Used • Indices from CPC web site • NAO (700 hpa Z), AO (1000 hpa Z) • Precipitation from CMAP • CMAP/O uses a combination of IR and microwave satellite estimates and gauge observations • CMAP/A uses reanalysis precipitation as well in regions without other data • Xie and Arkin (BAMS, 1997) • GPCP pentad • Same inputs as CMAP/O • Constrained to match GPCP monthly analysis • Xie et al. (J. Climate, accepted pending minor revisions) • Circulation from NCEP/NCAR Reanalysis • 1000 and 500hpa winds and geopotential heights • Kalnay et al. (BAMS, 1996)

6. Precipitation Climatology (CMAP) DJFM JJAS

7. Correlations with NAO Index (DJFM) CMAP precipitation REOF of 700mb Z January

8. Correlations with NAO Index (DJFM) 1000 hpa height 500 hpa height New CPC NAO index based on 500 hpa eof

9. DJFM Composites based on NAO Index (using monthly anomalies) CMAP precipitation High NAO quartile minus Low NAO quartile

10. How much difference do using the reanalysis precipitation make in these composites ? CMAP/O CMAP/A Not much – maybe provides a little continuity in high latitudes

11. How robust are these results? Low NAO High NAO Normalized CMAP composites Each month normalized by standard deviation of 1000 composites based on random time series

12. Conclusions - DJFM • CMAP et al. appear to be usable in extratropics • Winter precipitation signal is consistent with earlier results • Positive NAO associated with wet conditions in North Atlantic/northern Europe • Negative NAO associated with wet conditions in Atlantic between 30°-40°N extending across the Mediterranean into the Middle East • Oceanic precipitation anomalies strongest near eastern end of the main storm track – as much as 50% of the mean • Signal appears to extend into the tropics

13. Correlations with NAO Index (JJAS) CMAP precipitation REOF of 700hpa Z 1000 hpa height 500 hpa height

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

15. How “significant” is the NAO signal in Boreal summer? High NAO Low NAO Normalized CMAP composites

16. Conclusions - JJAS • Positive/negative NAO associated with dry/wet conditions in northern Europe with opposite anomalies farther north • Oceanic anomalies less prominent than during DJFM • Positive NAO associated with positive rainfall anomalies in western Atlantic between 15°-30°N – probably because of the positive SLP anomalies just to the north and their influence on tropical systems • Appears to be substantial dependence of the details of the results on the definition of the index

17. 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?

18. Composite DJFM Precipitation AnomaliesHigh NAO – Low NAO CMAP/A GPCP Pentad

19. CMAP normalized composite anomalies Based on NAO index DJFM Based on Arctic Oscillation index

20. Conclusions • Merged precipitation datasets good enough for descriptive diagnostic studies • CMAP, GPCP pentad give similar results • High latitude results look pretty reasonable • 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 (equivocal) evidence of a tropical signal in Indian and Pacific Oceans