1 / 20

Tropical SST-Dipole forcing of NAO

Tropical SST-Dipole forcing of NAO. Fred Kucharski, Annalisa Bracco and Franco Molteni Physics of Weather and Climate Section The Abdus Salam ICTP. Outline:. Motivation Observed NAO SST-Dipole definition and time-series Tropical dipole forcing of observed NAO

jamesspeterson
Download Presentation

Tropical SST-Dipole forcing of NAO

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Tropical SST-Dipole forcing of NAO Fred Kucharski, Annalisa Bracco and Franco Molteni Physics of Weather and Climate Section The Abdus Salam ICTP

  2. Outline: • Motivation • Observed NAO • SST-Dipole definition and time-series • Tropical dipole forcing of observed NAO • SPEEDY results forced with HadISST • Conclusions

  3. Motivation What causes the decadal variations of the NAO? Year-to-year variations of NAO are likely to be primarily an atmospheric internal phenomenon (Czaja & Frankignoul, 2002) Some papers suggest that recent decadal trends in the NAO are forced by tropical Pacific SSTs (e.g. Hoerling et. al., 2001), extratropical and subtropical Atlantic SSTs (Rodwell et. al., 1999), others blame internal Variability (e.g. Schneider et. al., 2003); Tropical SST likely to be influenced by GHG increase (Tett et. al., 2002)

  4. Observed NAO Hurrell NAO index (red) and 11-year running mean (black) SLP diff Lisbon, Reykjavik DJFM mean

  5. Observed NAO NAO Regression onto NCEP 500Z Filtered NAO regression onto 500Z

  6. Observed NAO Filtered NAO regression onto HadISST Dipole Filtered NAO regression onto SPEEDY precip SPEEDY Ensemble (10) 1870 to 2002 Forced by HadISST

  7. Dipole definition Regression filtered NAO onto HadISST Southern: 150E to 190E 20S to 5N Northern:150E to 190E 10N to 20N Index=average SST south-north

  8. SST-Dipole Dipole index (red) versus Hurrell NAO; 11-year filtered (corr=0.82)

  9. Nino3.4 index (170W-120W,5S-5N) NINO3.4 and Dipole index are highly correlated on a year-to-year basis (corr=0.71) NINO3.4 index (red) versus Hurrell NAO, 11-year filtered (corr=0.36)

  10. Dipole and NINO3.4 regression (11-year filtered) Dipole regression onto NCEP 500Z Nino3.4 regression onto NCEP 500Z

  11. Dipole and NINO3.4 regression Filtered Dipole regression onto HadISST Filtered NINO3.4 regression onto HadISST

  12. Forcing SPEEDY with HadISST 1870 to 2002 • SPEEDY is forced with the HadISST SSTs from 1870 to 2002 • Hor. Res. T30 (3.75x3.75), 8 vertical levels • Ensemble of 10 members • No other forcing included • Results from ensemble mean

  13. Forcing SPEEDY with HadISST Regression Dipole index SPEEDY 500Z 500Z SPEEDY trend 77/01-52/76 Define alternative NAO

  14. Filtered NINO3.4 regression Filtered Nino3.4 regression onto SPEEDY 500Z

  15. SPEEDY alternative NAO Filtered Dipole index (red) versus SPEEDY altern NAO index (black) (corr=0.79)

  16. SPEEDY alternative NAO Filtered SPEEDY altern NAO (red) versus Hurrell NAO index (black) (corr=0.64)

  17. Another SPEEDY experiment: • SPEEDY forced with SST only in the Dipole region (1950-2002) • 10 member ensemble • Is there an Atlantic response?

  18. Another SPEEDY experiment Regression of filtered Dipol index onto SPEEDY 500Z Full SSTs Dipole SSTs Similar Atlantic structure and amplitude

  19. SPEEDY NAO time-series (Dipole forced only) Filtered SPEEDY (red) versus Hurrell index (black) (corr=0.56)

  20. Conclusions • An equatorial west Pacific SST Dipole seems to govern the observed decadal NAO behaviour • On decadal time-scale distinct from ENSO • SPEEDY confirms potential predictability of NAO-like mode in the Atlantic region on decadal time-scale due to Dipole • This gives an idea why some models can simulate decadal NAO changes and others not

More Related