Assessing Low Frequency Variability in North Atlantic Ocean Sea Surface
Temperatures in Global Climate Models
Nicholas G. HeavensCaltech
K.-F. Li, M.-C. Liang, L.-C. Lin, K.-K. Tung, and Y.L. Yung16 December 2009
AGU Fall Meeting Abstract # GC32A-08
NOAA/ESRLNOAA
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 byDelworth et al. (1993)before AMO identified(variability in AtlanticMOC)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 ofClimate of the 20th Century CMIP3 runs with ERA-40 and Kaplan SST
This study
• Stoner et al. (2009) concerned Climate of 20th century runs too short to assess multi-decadal variability like AMO
• Paleoclimate records indicate AMO pre-dates20th 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 (100-550 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 SpectraGray et al., 2004(1567-1870)
HadISSTBCCR-BCM2.0
GFDL CM 2.1GISS 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 NorthAtlantic may prove difficult; (2) period matching of
ECHO-G remarkable (given ENSO…)
AMO Relation to Atlantic MOCPrimary AMO-related change is intensity of sub-circulation controlling downwelling at 50°-60° N,
Mean
Streamfunction(m3s-1)solid line(+ correlationwith 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 transportfrom 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 bottomcurrents: sub-circulation intensityproxy?
Boessenkool et al. (2007)
2. Evaporation rates in LabradorSea: Salt content in downwind ice coresrelated to winds blowing over openwaters. 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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