# Presentation: 521166

## OCEANIC PRECIPITATION VARIABILITYASSOCIATED WITH THE NORTH ATLANTIC OSCILLATION

- Phillip A. Arkin, Heidi Cullen and Pingping XieUniversity of Maryland, ESSIC, NCAR/ESIG and Climate Prediction Center, NOAA

## OUTLINE

- Background and questions to be addressed
- Data
- Around the Atlantic
- Boreal Winter
- Boreal Summer
- Elsewhere
- Conclusions/Points for further study

## 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

## 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?

## 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)

## Precipitation Climatology (CMAP)

- DJFM

- JJAS

## Correlations with NAO Index (DJFM)

- CMAP precipitation

- REOF of 700mb Z

- January

## Correlations with NAO Index (DJFM)

- 1000 hpa height

- 500 hpa height

- New CPC NAO index based on 500 hpa eof

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

- CMAP precipitation
- High NAO quartile
- minus
- Low NAO quartile

## How much difference do using the reanalysis precipitation make in these composites ?

- CMAP/A

- CMAP/O

- Not much – maybe provides a little continuity in high latitudes

## How robust are these results?

- Normalized CMAP composites

- High NAO

- Low NAO

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

## 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

## Correlations with NAO Index (JJAS)

- 1000 hpa height

- 500 hpa height

- CMAP precipitation

- REOF of 700hpa Z

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

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

- High NAO

- Low NAO

- Normalized CMAP composites

## 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

## 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?

## Composite DJFM Precipitation AnomaliesHigh NAO – Low NAO

- CMAP/A

- GPCP Pentad

## CMAP normalized composite anomalies

- Based on NAO index

- Based on Arctic Oscillation index

- DJFM

## 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