The Atlantic Multidecadal Oscillation (AMO)

(1)

The Atlantic Multidecadal Oscillation (AMO)

The AMO is an important multi-decadal oscillation that impacts climate variabilty over much of the World, especially in the Atlantic basin.

It is important to be aware of the AMO when investigating the possible Effects of anthropogenic climate change.

The AMO – proxy evidence

Delworth and Mann, 2000

- Instrumental observations capture only two full cycles of the AMO

- Northern hemisphere temperature reconstruction (Mann et al., 1999) does show multidecadal variability in North Atlantic

-Spectrum of proxy- reconstructed surface temperature patterns

over N. Atlantic (0°-60°N, 1650-1980) show highly significant peak near

70 year period

- suggests this mode has been present in the climate system over at least the last 300 years

(8)

A tree-ring index of the AMO

Gray et al., 2004

12 tree ring sites

(1567-1990), detrended PC-Analysis

Multiple regression

Calibration period (1922- 1990)

Verification period (1856-1921)

(9)

(10)

- Slowly evolving mode of natural variability, difficult to observe and quantify based on observations (records are mostly too short), but:

-modeling studies document and reproduce this oscillation -oscillation is clearly documented in proxy records

- Time scale too long for atmospheric forcing; Ocean is main component driving this variability

- Suspected culprit: The thermohaline (THC) or Atlantic meridionally overturning circulation (AMOC)

The AMO -Characteristics

(11)

Global Ocean Circulation

In order to balance the excess heating in the Tropics, the oceans transports heat (in the from of warm, salty water) from low to high latitudes

(12)

Global Ocean Circulation The Conveyor Belt

current mode:

-warm water (red) flows northward along the East Coast of the U.S. toward Iceland

-warm water exchanges heat with the cooler air, becoming cooler and saltier - near Iceland, water becomes more dense (cool and salty) than the water below and sinks, flowing southward along the floor of the Atlantic = North Atlantic Deep Water Formation

(13)

(14)

AMO dynamics

Long (~1400 year) model integrations with HadCM3 able to simulate the observed pattern and amplitude of the AMO (Knight et al., 2005) Model did not include any fluctuations in external forcing

→ suggests AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries

→ Model hints that AMO results from variability in the oceanic THC

Link between AMO dynamics and THC first suggested by Delworth and Mann, (2000)

Reasonable hypothesis as mean THC transports sufficient heat

northward (1.2 PW (10¹⁵W) at 30°N) to warm the Northern Hemisphere by several °C

(15)

Spatial patterns and magnitudes of observed and simulated

variability are comparable

Evolution of AMO cycle in HadCM3

→Fluctuations in THC appear to be responsible for multidecadal SST signal

Knight et al. (2005)

Repeating this study with a slab (50m deep) ocean does not generate an AMO,

→ deep ocean is necessary to produce the AMO.

-phase 0° (THC at max.): coherent large-scale temperature pattern, widespread warm anomalies in N.

hemisphere

- phase 60° and 120°: signal weakening

Phase 180° (THC at min.): signal out of phase, much of N.

Hemisphere anomalously cool

(16)

Enfield, 2001

(17)

Lagged correlations show both large in-phase covariability and anti-

correlations at leads and lags of about 50 years.

Thus the simulated THC-AMO

variability is quasi-periodic, evolving coherently for typically half a cycle; 50 years after a peak (trough) in the THC, statistically a cold (warm) phase would be anticipated.

The THC in the HadCM3 model

Knight et al. (2005)

Predictability of the simulated AMO:

Seeking analogues to evolution of past 100 years and then tracking its

subsequent evolution.

Ensemble of 8 segments representing possible THC strength for next 35

years

Results suggest that the THC is currently at or near a peak and likely to diminish thereafter (in the absence of external forcings).

Solid line: global T Dotted line: N. Hem. T

(18)

The AMO- a role for external forcing?

(19)

Pacific Decadal Oscillation

Warm phase Cold Phase

See http://jisao.washington.edu/pdo

(20)

Note that PDO and AMO operate on different time scales! McCabe et al., 2004

(21)

(22)

McCabe et al., 2004

More than half (52%) of spatiotemporal variance in multidecadal drought frequency over US attributable to combined PDO / AMO influence

Recent US droughts (1996, 1999–2002) associated N. Atlantic warming (positive AMO) and NE and tropical Pacific cooling (negative PDO)

→ Much of the long-term predictability of drought frequency may reside in the multi-decadal behavior of the N. Atlantic

AMO+: much of US under drought conditions, regardless of PDO state

PDO vs. AMO impacts in the US

(23)

Schubert et al., 2004

The AMO and the ‘great dust bowl’

During 1930s, US experienced one of the most devastating droughts of the past century.

- drought affected ~2/3 of US, parts of Mexico and Canada

- generated numerous dust storms in the southern Great Plains.

Warm N. Atlantic, Cold Pacific

(24)

AMO Impacts on US and European climate (warm phase):

- plays an important role in modulating boreal summer climate on multidecadal time scales in US and Europe

-low pressure centers over SE US and UK

-enhanced PPN western Europe, Sahel and N. Africa -reduced PPN central US and Mexico

-warm T anomalies over US and central Europe

-may affect not only mean climate but also frequency of extreme events (US droughts, hurricanes, heat waves)

-phase change of AMO around 1960 may have caused summertime cooling in US and Europe

- Most recent phase change (around 1990) may have contributed to rapid warming

(25)

Enfield, 2001 AMO+ phase: large warm pool

AMO- phase: small warm pool