Publication Abstract

This paper is Part I of a two-part paper that presents the GFDL-CM4X (Geophysical Fluid Dynamics Laboratory Climate Model version 4X) coupled climate model hierarchy. The primary application for CM4X is to investigate ocean and sea ice physics as part of a realistic coupled Earth climate model. CM4X builds from the GFDL-CM4.0 climate model, yet with notable modifications. In particular, CM4X utilizes an updated MOM6 (Modular Ocean Model version 6) ocean physics package relative to CM4.0, and there are two members of the hierarchy: one that uses a horizontal grid spacing of 0.25deg (referred to as CM4X-p25) and the other that uses a 0.125deg grid (CM4X-p125). CM4X also refines its atmospheric grid from the nominally 100 km (cubed sphere C96) of CM4.0 to 50 km (C192). Finally, CM4X makes some simplifications to the land model to allow for a more focused study of the role of ocean changes to global mean climate.  Both CM4X configurations are identical in their physical and numerical setup, differing only through their ocean/ice horizontal grid spacing and bottom topography. 

The refined ocean used in CM4X-p125 leads to a climate model that reaches a global ocean area mean heat flux imbalance of $-0.02 W m^-2 within O(100) years in a pre-industrial simulation, and retains that thermally equilibrated state over the subsequent centuries.  This 1850 thermal equilibrium is characterized by roughly 400 ZJ less ocean heat than present-day, which corresponds to estimates for anthropogenic ocean heat uptake between 1850 and present-day. In contrast, CM4X-p25, with its coarser ocean model, approaches its thermal equilibrium only after more than 1000 years, at which time its ocean has roughly 1100 ZJ more heat than its early 21st century ocean initial state. Furthermore, the root-mean-square sea surface temperature bias for historical simulations is roughly 20% smaller in CM4X-p125 relative to CM4X-p25 (and CM4.0). 

We define mesoscale dominant ocean circulation models as those with the following three properties: (I) an accurate rendering of ocean mesoscale transport (via explicit representation or a parameterization), (II) an accurate parameterization of diapycnal (small scale turbulent) mixing, and (III) negligible spurious numerical diapycnal mixing. We hypothesize that mesoscale dominant ocean models are necessary to realize a thermally equilibrated pre-industrial ocean state with volume mean temperature cooler than present day, and with a thermal equilibration time on the order of centuries rather than millennia.  

Stephen M. Griffies, Alistair Adcroft, Rebecca L. Beadling, Mitchell Bushuk, Chiung-Yin Chang, Henri F. Drake, Raphael Dussin, Robert W. Hallberg, William J. Hurlin, Hemant Khatri, John P. Krasting, Matthew Lobo, Graeme A. MacGilchrist, Brandon G. Reichl, Aakash Sane, Olga Sergienko, Maike Sonnewald, Jacob M. Steinberg, Jan-Erik Tesdal, Matthew Thomas, Katherine E. Turner, Marshall L. Ward, Michael Winton, Niki Zadeh, Laure Zanna, Rong Zhang, Wenda Zhang, Ming Zhao