atlantic multidecadal variability & inter-american rainfall

Atlantic Multidecadal Variability & Inter-American Rainfall

D. B. Enfield1 & A. M. Mestas-Nuñez2

1NOAA-AOML, Miami, Florida, USA2U. Miami/CIMAS, Miami, Florida, USA

Research supported by NOAA OGP/PACS & IAI/CRN

Enfield et al., 2001: Geophys. Res. Lett.

NOAA Atlantic Oceanographic & Meteorological Laboratory

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


Global warming model w/ greenhouse gases & solar forcing …residual low-pass observations not explained …residual behaves like AMO


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.


1650-1983 tree rings & ice coresMann et al. (Nature, 1998)


1500-1999 European treesLuterbacher et al. (GRL, 1999)

1500 1600 1700 1800 1900 2000

4

2

8

16

Peri

od (

years

)

32

64

128

256

Wavelet Power Spectrum, NAOI winters, 1500-1999


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


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)

Correlation of AMO with global SSTA


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?


Correlation of AMO vs. Eischeid gridded rainfall (JAS)

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


Lake Okeechobee inflow vs. AMO


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)


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


20-year running correlations


AMO- ==> 1965-1994


AMO+ ==> 1930-1959


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)


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?



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



AMO-ENSO interaction (mainly winter)

North Atlantic influence on prevailing winter circulation over North America

Hypthesis: AMO(+) ==> deepened trough over eastern U.S.


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 <[email protected]>

atlantic multidecadal variability & inter-american rainfall

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