2013: Frequency domain multi-model analysis of the response of Atlantic meridional overturning circulation to surface forcing

• Main Authors: Douglas G. Macmartin, Eli Tziperman, Laure Zanna
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Labrador Sea
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.643.7786; http://www.seas.harvard.edu/climate/eli/reprints/MacMartin-Tziperman-Zanna-2013.pdf

The dynamics of the Atlantic meridional overturning circulation (AMOC) vary considerably among dif-ferent climate models; for example, somemodels show clear peaks in their power spectra while others do not. To elucidate these model differences, transfer functions are used to estimate the frequency domain re-lationship between surface forcing fields, including sea surface temperature, salinity, and wind stress, and the resulting AMOC response. These are estimated from the outputs of the Coupled Model Intercomparison Project phase 5 (CMIP5) and phase 3 (CMIP3) control runs for eight different models, with a specific focus on Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), and the Community Climate System Model, version 4 (CCSM4), which exhibit rather different spectral behavior. The transfer functions show very little agreement amongmodels for any of the pairs of variables considered, suggesting the existence of systematic model errors and that considerable uncertainty in the simulation of AMOC in current climate models remains. However, a robust feature of the frequency domain analysis is that models with spectral peaks in their AMOC correspond to those in which AMOC variability is more strongly excited by high-latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks. This explains why different models exhibit such different AMOC variability. These differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori as-suming a particular functional form. Finally, transfer functions are used to evaluate two proposed physical mechanisms for model differences in AMOC variability: differences in Labrador Sea stratification and ex-citation by westward-propagating subsurface Rossby waves. 1.