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The Atlantic Multidecadal Oscillation: A signature of

persistent natural thermohaline circulation cycles in observed

climate

Jeff Knight, Rob Allan, Chris Folland, Michael Vellinga and

Michael Mann*

Hadley Centre for Climate Prediction and Research*Dept of Environmental Sciences, University of

Virginia, USA

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Introduction Observed multidecadal variations in surface climate HadCM3 Control Simulation: THC variability Simulated THC links with climate What do instrumental surface temperature records show

with regard to the modelled multidecadal THC-climate mode?

Relationship of global and hemispheric mean temperatures with the modelled THC.

Links to regional climate phenomena, e.g. NE Brazil and Sahel rainfall.

Multidecadal modulation of North Atlantic hurricanes. Reconstruction the 20th century THC from historical

observations of climate? Forecast of natural THC evolution over the next few

decades.

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Variations in historical instrumental SSTs

HadISSTLow-pass (> 13.3y)

EOFs 1911-200240ºS - 70ºN

Projections 1870-2002

After Folland et al., 1999

Variations, but are they modes?

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Model THC in the Frequency Domain

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THC-Climate mode: model vs observations

0°

60°

120°

180°

70-180 Year band 25-125 Year band

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THC-temperature cross-correlation functions

NorthernHemisphere

SouthernHemisphere Global

0.05°C Sv-1 (0.59)0.09°C Sv-1 (0.55) 0.01°C Sv-1 (0.13)

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NE Brazilian Rainfall

Station data from Todd Mitchell’s site http://jisao.washington.edu/data/brazil/

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MAM Temperature, Winds and Precipitation

0°

60°

120°

180°

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Sahel Rainfall

Data from Climatic Research Unit, U. East Anglia

10°N-20°N 15°W-30°E

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JJA Temperature, Winds and Precipitation

0°

60°

120°

180°

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Atlantic HurricanesGoldenberg et al., Science, 2001

Major Atlantic Hurricanes (>50 m s-1) 1944-2000

1st Rotated EOF of SSTENSO Removed. 1870-2000

SST Reconstruction (Northern Box)

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200-850 hPa Shear in the Atlantic Hurricane Development Region

Peak to peak -

model: ~6 ms-1

NCEP: ~3 ms-1

Decadal Mean Shear: Model - black, NCEP - Red

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Regression of HadISST SST with Mean Shear in Development Region

Significance level ±0.3

Significance level ±0.04

K (ms-1)-1

IPO-AMO correlation 0.06

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North Atlantic SST Index

N Atlantic mean 35°-80°NRunning decadal mean from HadISST,

detrended using a quadratic fit.

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THC Reconstruction from Atlantic SST

North Atlantic region with local signal to noise ratio

THC model based on area and s/n ratio weighted

model SSTRunning decadal THC

reconstruction 1870-2002

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THC Forecast - model analoguesLook for episodes where model THC rises through the

reconstructed level of modern day THC.

Take the 50 years after each

intersection to form an ensemble of

outcomes for the next 33 years.

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Conclusions (1)

The model shows observed Atlantic Multidecadal Oscillation (AMO) variability is likely to be a long-lived climate mode linked to natural THC variability.

There is a strong influence on Global and Northern Hemisphere mean temperatures.

The quasi-periodic nature of the model AMO implies some predictability of climate for up to ~50 years.

Influences on regional climate e.g. NE Brazil, Sahel and Atlantic Hurricanes are similar to those observed.

A reconstruction of the THC based on the model and observed SSTs shows likely significant THC changes, including THC strengthening over the past 25 years.

Forecast of downturn in natural component of THC in coming decades.

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Conclusions (2)

Shows how coupled models can be used to study 20th century climate phenomena – long simulations can be a surrogate for the ‘observations we would like to have’.

Highlights need for a range of C20C sensitivity experiments with imposed AMO anomalies eg. AMO- (like 1970s), AMO+ (like 1940s) to simulate climate anomalies across the range of seasons.