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Understanding the Atlantic Multidecadal Oscillation and Its Impact on Inter-American Rainfall Patterns

The research examines the Atlantic Multidecadal Oscillation (AMO), a major component of tropical climate variability, highlighting its connection to sea surface temperatures (SST) in the North Atlantic and its effects on rainfall patterns across the Americas. By analyzing historical data and climate models, the study reveals how variations in the AMO influence rainfall, hurricane frequency, and river flows, particularly in the U.S. The interplay between the AMO and ENSO-related rainfall is also explored, providing insights into future climate dynamics.

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Understanding the Atlantic Multidecadal Oscillation and Its Impact on Inter-American Rainfall Patterns

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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

  16. NOAA Atlantic Oceanographic & Meteorological Laboratory

  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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