Atlantic Multidecadal Variability and Its Climate Impacts in CMIP3 Models and Observations Mingfang Ting With Yochanan Kushnir, Richard Seager, Cuihua Li June 8, U.S. AMOC Annual Meeting Miami, FL

Observed AMO (AMV) Indices North Atlantic SST Index (7.5W-75W, 0-60N, ocean only) for Models and Observations Linear de-trending Global mean SST as forced signal 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 Forced Ts Precip.  (Stippled regions are for values significant at or above 95% confidence level)

What are the spatial and temporal characteristics and climate impacts of AMV in CMIP3 models for both the 20 th and 21 st Centuries? Hoe do they compare to 20 th Century observations? Is the signal to noise maximizing EOF method successfully separating AMV from the externally forced component? Questions to be Addressed:

In this study… Apply signal-to-Noise Maximizing EOF Analysis to IPCC multiple model, multi-ensemble members of the 20 th and 21 st simulations First apply EOF analysis to deviations from multi- model 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 20 th Century 21 st Century

Obs. Comparison of 20 th and 21 st Century AMV in CMIP3 Models 20 th Century21 st Century

(Stippled regions indicate at least 15 out of 19 models have the same sign regression) ForcedInternal Regression of Ts and Precip on Forced versus Internal NASST Variability (Annual Mean 20 th Century) Ts Precip Ts

(Stippled regions indicate at least 15 out of 19 models have the same sign regression) ForcedInternal Regression of Ts and Precip on Forced versus Internal NASST Variability (Annual Mean 21 st Century) Ts Precip Ts

Ts Regression (Annual) Forced Internal (AMV) Obs. 20 th 21 st Obs. 20 th 21 st

Precip Regression (Annual) ForcedInternal (AMV) Obs. 20 th 21 st Obs. 20 th 21 st

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?

AMV Amplitude Pre-Industrial 20 th Century 21 st Century

AMV Surface Temperature Regression (Annual Mean) Stippling indicates sign agreement for 15 out of the 20 models or 5% significance (obs.) Pre-Industrial 21 st Century 20 th CenturyObservations

AMV Precipitation Regression (Annual Mean) Stippling indicates sign agreement for 15 out of the 20 models or 5% significance (obs.) Pre-Industrial21 st Century 20 th Century Observations

AMV and AMOC Atlantic Meridional Overturning Streamfunction

Summary A robust spatial pattern of AMV is identified for observations, CMIP3 models’ pre-industrial, 20 th and 21 st Century simulations, despite the differing temporal scales between models and observations. The AMV spatial pattern is characterized by a comma- shaped 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 20 th Century model simulations, and to some extent for the 21 st 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.