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Atlantic Multidecadal Variability & Inter-American Rainfall

Atlantic Multidecadal Variability & Inter-American Rainfall. D. B. Enfield 1 & A. M. Mestas-Nuñez 2 1 NOAA-AOML, Miami, Florida, USA 2 U. Miami/CIMAS, Miami, Florida, USA Research supported by NOAA OGP/PACS & IAI/CRN Enfield et al., 2001: Geophys. Res. Lett. What is the AMO?.

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Atlantic Multidecadal Variability & Inter-American Rainfall

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  1. Atlantic Multidecadal Variability & Inter-American Rainfall D. B. Enfield1 & A. M. Mestas-Nuñez2 1NOAA-AOML, Miami, Florida, USA 2U. Miami/CIMAS, Miami, Florida, USA Research supported by NOAA OGP/PACS & IAI/CRN Enfield et al., 2001: Geophys. Res. Lett.

  2. What is the AMO? • Documented by Schlesinger & Ramankutty (1994) • Regionally strong component of SSA analysis • 65-70 year oscillation in N. Atlantic SST • Related to shallow overturning circulation • Other observations • Multi-taper SVD analysis of Kushnir et al. • Gyre transport variations (Curry et al.) • Model replication • GFDL coupled model (Delworth & Mann) ==> Overturning circulation implicated NOAA Atlantic Oceanographic & Meteorological Laboratory

  3. Global warming model w/ greenhouse gases & solar forcing • …residual low-pass observations not explained • …residual behaves like AMO NOAA Atlantic Oceanographic & Meteorological Laboratory

  4. Other evidence … • Supported by paleoclimate data, e.g.: • 1650-1983 reconstruction of global air     temperatures by Mann et al. (Nature,    1998) • 1500-1999 European reconstruction by    Luterbacher et al. NOAA Atlantic Oceanographic & Meteorological Laboratory

  5. 1650-1983 tree rings & ice cores Mann et al. (Nature, 1998) NOAA Atlantic Oceanographic & Meteorological Laboratory

  6. 1500-1999 European trees Luterbacher et al. (GRL, 1999) Wavelet Power Spectrum, NAOI winters, 1500-1999 2 4 8 16 Period (years) 32 64 128 256 1500 1600 1700 1800 1900 2000 NOAA Atlantic Oceanographic & Meteorological Laboratory

  7. Our work … • Define AMO index, correlate with global SST • Association with hemispheric rainfall • Associated changes in hurricane frequency • AMO slow covariation with US rainfall • Effects on river flows (Okeechobee) • Examine change in ENSO rainfall impacts NOAA Atlantic Oceanographic & Meteorological Laboratory

  8. SSTA: global distribution • Correlate AMO index with annual mean gridded SST • …10-year running means • …correlation significance by Monte Carlo •          (random phase method of Ebisuzaki, 1997) • …pattern like global rotated EOF •         (Mestas & Enfield, JC, 1999) NOAA Atlantic Oceanographic & Meteorological Laboratory

  9. Correlation of AMO with global SSTA

  10. Western Hemisphere rainfall associations • Which season is primarily responsible for the U.S. associations we see? • What rainfall associations exist on a larger, hemispheric scale? NOAA Atlantic Oceanographic & Meteorological Laboratory

  11. Correlation of AMO vs. Eischeid gridded rainfall (JAS) NOAA Atlantic Oceanographic & Meteorological Laboratory

  12. Correlation of AMO with U.S. divisional rainfall (1895-1999)

  13. Lake Okeechobee inflow vs. AMO NOAA Atlantic Oceanographic & Meteorological Laboratory

  14. Q: How are Atlantic hurricanes affected? • The total numbers of tropical storms are not affected by the AMO phase. However, • The intensity *IS* affected: • A significantly larger number of MAJOR    HURRICANES occurs during AMO warm    phases • We have been in a cool phase from 1970    through 1994 and have moved back into    another warm phase • Goldenberg et al. (Science 2001) NOAA Atlantic Oceanographic & Meteorological Laboratory

  15. NOAA Atlantic Oceanographic & Meteorological Laboratory

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  17. Q: How is ENSO-related rainfall affected by the AMO? • Previous studies ==> ENSO-PDO interaction Gershunov & Barnett; Dettinger et al. • 20-year running correlations (R-vs-AMO) • Two 30-year correlations: AMO- , AMO+ • DJF NINO 3.4 vs. JFM rainfall (unsmoothed) • AMO- ==> 1965-1994 • AMO+ ==> 1930-1959 NOAA Atlantic Oceanographic & Meteorological Laboratory

  18. 20-year running correlations NOAA Atlantic Oceanographic & Meteorological Laboratory

  19. AMO- ==> 1965-1994 NOAA Atlantic Oceanographic & Meteorological Laboratory

  20. AMO+ ==> 1930-1959 NOAA Atlantic Oceanographic & Meteorological Laboratory

  21. Principal Results • AMO+ ==> less rain, Mississippi basin, most of U.S., Mississipi outflow ==> 10% peak-to-peak • AMO+ ==> more rain in Florida, NE & Pacific NW, Okeechobee inflow ==> 40% peak-to-peak • Pattern is dominated by N.H. summer (JAS),  Also N. Europe+, NW Africa+ and NE Brasil- (summer) • Major hurricanes are >2x more frequent for AMO [+] • ENSO-rainfall patterns change with AMO phase (DJF) NOAA Atlantic Oceanographic & Meteorological Laboratory

  22. Further research needed… • How does the AMO work? • Usual questions RE mechanisms, feedbacks, etc • Role of the Atlantic overturning circulation? • Atlantic-Pacific connection ==> NH annular mode? NOAA Atlantic Oceanographic & Meteorological Laboratory

  23. Further research needed… • Slow co-variation of rainfall (mainly summer?) • WH warm pool effects on moisture transport to     U.S. • N. Atlantic influence on prevailing summer     circulation patterns over North America • Possible interaction with the North American     Monsoon NOAA Atlantic Oceanographic & Meteorological Laboratory

  24. Further research needed… • AMO-ENSO interaction (mainly winter) • North Atlantic influence on prevailing winter     circulation over North America • Hypthesis: AMO(+) ==> deepened trough     over eastern U.S. NOAA Atlantic Oceanographic & Meteorological Laboratory

  25. Relevant documentation • Relevant publications: • Enfield & Mestas-Nuñez (J. Clim., 1999) • Mestas-Nuñez & Enfield (J. Clim., 1999) • 1999 & 2000 reprints are PDF files at <http://www.aoml.noaa.gov/phod/docs.html> • This paper submitted to Geophys. Res. Lett. Email request to <enfield@aoml.noaa.gov> NOAA Atlantic Oceanographic & Meteorological Laboratory

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