# Presentation: 546620

## 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?

- 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

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

**...****residual low-pass observations not explained**
**...****residual behaves like AMO**

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## 1650-1983 tree rings & ice cores

- Mann et al. (Nature, 1998)

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 1500-1999 European trees

- Luterbacher et al. (GRL, 1999)

**1500**

**1600**

**1700**

**1800**

**1900**

**2000**

**4**

**2**

**8**

**16**

- Period (years)

**32**

**64**

**128**

**256**

**Wavelet Power Spectrum, NAOI winters, 1500-1999**

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## 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

## Correlation of AMO with global SSTA

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## Correlation of AMO vs. Eischeid gridded rainfall (JAS)

- NOAA Atlantic Oceanographic & Meteorological Laboratory

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

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## Lake Okeechobee inflow  vs. AMO

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## Slide 15

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## Slide 16

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## 20-year running correlations

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## AMO-   ==>  1965-1994

- AMO**-**   ==>  1965-1994

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## AMO+   ==>  1930-1959

- NOAA Atlantic Oceanographic & Meteorological Laboratory

## 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

## 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

## 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

## 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

## 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