Atlantic Multidecadal Variability and Its Climate

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Atlantic Multidecadal Variability
and Its Climate Impacts in CMIP3
Models and Observations
Mingfang Ting
With
Yochanan Kushnir, Richard Seager, Cuihua Li
June 8, 2010
2010 U.S. AMOC Annual Meeting
Miami, FL
Observed AMO (AMV) Indices
Linear de-trending
Global mean SST as
forced signal
North Atlantic SST Index
(7.5W-75W, 0-60N, ocean only)
for Models and Observations
Signal to noise EOF PC1 as
forced signal (Ting et al,
2009)
Forced
Atlantic SST
in models
Observed
AMV
Climate Impacts of Forced versus Internal
NASST Variability (Regression based on Annual Mean)
Internal
Precip.
Ts
Forced
(Stippled regions are for values significant at or above 95% confidence level)
Questions to be Addressed:



What are the spatial and temporal
characteristics and climate impacts of AMV
in CMIP3 models for both the 20th and 21st
Centuries?
Hoe do they compare to 20th Century
observations?
Is the signal to noise maximizing EOF
method successfully separating AMV from
the externally forced component?
In this study…




Apply signal-to-Noise Maximizing EOF Analysis to
IPCC multiple model, multi-ensemble members of
the 20th and 21st simulations
First apply EOF analysis to deviations from multimodel average to determine the spatial structure
of the internal modes of variability (noise)
Apply a spatial pre-whitening transformation
based on the internal EOFs to remove the spatial
correlations in the internal atmospheric
variability (i.e., “climate noise”) contained in the
multi-model average
Apply EOF analysis to the signal to get forced
component
S/N Maximizing EOF 1 for 19 IPCC Model
Simulations
20th Century
21st Century
Comparison of 20th and 21st Century AMV in CMIP3 Models
20th Century
Obs.
21st Century
Regression of Ts and Precip on Forced versus
Internal NASST Variability (Annual Mean 20th Century)
Forced
Ts
Precip
Internal
Ts
Precip
(Stippled regions indicate at least 15 out of 19 models have the same sign regression)
Regression of Ts and Precip on Forced versus
Internal NASST Variability (Annual Mean 21st Century)
Forced
Internal
Ts
Ts
Precip
Precip
(Stippled regions indicate at least 15 out of 19 models have the same sign regression)
Ts Regression (Annual)
Internal (AMV)
Forced
Obs.
Obs.
20th
20th
21st
21st
Precip Regression (Annual)
Forced
Internal (AMV)
Obs.
Obs.
20th
20th
21st
21st
AMV in Pre-Industrial
CMIP3 Runs



Can we reproduce AMV spatial patterns
and climate impacts in a model without
radiatively forced changes?
If so, what are the typical time scales and
amplitude of the AMV?
What are the circulation features
associated with the AMV?
20th Century
AMV Amplitude
Pre-Industrial
21st Century
AMV Surface Temperature Regression
(Annual Mean)
Pre-Industrial
20th Century
21st Century
Observations
Stippling indicates sign agreement for 15 out of the 20 models or 5% significance (obs.)
AMV Precipitation Regression
(Annual Mean)
Pre-Industrial
20th Century
21st Century
Observations
Stippling indicates sign agreement for 15 out of the 20 models or 5% significance (obs.)
Atlantic Meridional Overturning Streamfunction
AMV
and
AMOC
Summary



A robust spatial pattern of AMV is identified for
observations, CMIP3 models’ pre-industrial, 20th and
21st Century simulations, despite the differing temporal
scales between models and observations.
The AMV spatial pattern is characterized by a commashaped SST pattern over the North Atlantic with the
largest amplitude in the sub-polar region which
extends to the tropical Atlantic along the east side of
the basin.
The precipitation patterns associated with AMV are
remarkably similar for the pre-industrial and 20th
Century model simulations, and to some extent for the
21st Century simulations and observations. The
positive phase of AMV (warm North Atlantic) is
associated with northward shifted Atlantic ITCZ,
increased rainfall over Sahel and eastern tropical
Pacific, and dry condition over tropical south Atlantic,
North America and Australia.
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