Assessing Low Frequency Variability in North Atlantic Ocean Sea Surface Temperatures in Global Climate Models Nicholas G. Heavens Caltech K.-F. Li, M.-C. Liang, L.-C. Lin, K.-K. Tung, and Y.L. Yung 16 December 2009 AGU Fall Meeting Abstract # GC32A-08 NOAA/ESRL NOAA

AMO Should be Simulation Priority Obscures or enhances global temperature trend attributed to anthropogenic forcings Affects climate of North America, Europe, and West Africa Goldenberg et al., 2001 Janet Nye, NOAA NEFSC

Simulated AMOs are elusive First found by Delworth et al. (1993) before AMO identified (variability in Atlantic MOC) More recent work: 1. Atmosphere-ocean vs. ocean alone 2.Hierarchy of model complexity 3. Surveys of IPCC models Stoner et al. (2009) Stoner et al. (2009) comparison of Climate of the 20 th Century CMIP3 runs with ERA-40 and Kaplan SST

This study Stoner et al. (2009) concerned Climate of 20 th century runs too short to assess multi-decadal variability like AMO Paleoclimate records indicate AMO pre-dates 20 th century (last millennium or more) To what extent do global climate models simulate the Atlantic Multidecadal Oscillation (AMO) without secular forcing?

Procedure Find longest pre-industrial run for 22 CMIP3 models with daily data ( yr. runs) Calculate AMO Index just like the real ocean Correlate detrended, deseasonalized annual mean local time series with AMO Index to get spatial pattern Power spectrum analysis. Amplitude based on variance of ten year running mean Compare with both modern instrumental and Gray et al. (2004) AMO reconstruction.

Results: Power Spectra Gray et al., 2004 ( ) HadISST BCCR-BCM2.0 GFDL CM 2.1 GISS AOM ECHO-G (MIUB) CGCM2.3.2 (MRI) PCM1 (NCAR) HadCM3

Results: Spatial Patterns

Summary Only seven CMIP3 models simulate multi- decadal variability ECHO-G has: (1) variability with spectrum similar to Gray et al. (2004); (2) reasonable amplitude; (3) qualitatively similar spatial pattern to modern (in-family with other models); (4) minimal global SST drift Take-home: (1) decadal predictability in North Atlantic may prove difficult; (2) period matching of ECHO-G remarkable (given ENSO…)

AMO Relation to Atlantic MOC Primary AMO-related change is intensity of sub-circulation controlling downwelling at 50°-60° N, Mean Streamfunction (m 3 s -1 ) solid line (+ correlation with AMO Index) dashed (-) Explanations from previous modeling work, circulation is driven by positive salinity anomaly shut down by: 1.Atmospheric feedback with NAO produces weak evaporation in sinking regions (Timmermann et al., 1998): salinity primarily produced in situ 2. Feedback with eddy salinity transport from south through sub-polar gyre water temperatures by atmospheric feedback/water accumulation (Dong and Sutton, 2005; Frankignoul and Msadek, 2008) (studies disagree about NAO role) salinity produced elsewhere The period sensitivity arises through timing of various processes: phase lag

Validation beyond SST? 1.Grain size sorting by bottom currents: sub-circulation intensity proxy? Boessenkool et al. (2007) 2. Evaporation rates in Labrador Sea: Salt content in downwind ice cores related to winds blowing over open waters. Evaporation proxy? Roethlisberger et al. (2009) Take-home: Collection of high-resolution proxies related to deep circulation or salt budget priority for validation of model AMOs

Questions?

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