Historical AMOC in an Eddy-resolving CESM2 Simulation

Stephen G Yeager1,2, Gokhan Danabasoglu2,3, Ping Chang4,5, Alper Altuntas1,2, Frank Bryan3, Frederic S Castruccio6,7, Who M Kim7, Elizabeth Maroon1,8, Justin Small1,2, Lixin Wu5,9 and Shaoqing Zhang2,10, (1)National Center for Atmospheric Research, Boulder, CO, United States, (2)International Laboratory for High-Resolution Earth System Prediction (iHESP), College Station, TX, United States, (3)National Center for Atmospheric Research, Climate and Global Dynamics, Boulder, United States, (4)Texas A&M University, Department of Oceanography, College Station, United States, (5)International Laboratory for High-Resolution Earth System Prediction (iHESP), College Station, United States, (6)NSF National Center for Atmospheric Research, Boulder, United States, (7)National Center for Atmospheric Research, Boulder, United States, (8)University of Wisconsin Madison, Madison, WI, United States, (9)Qingdao National Laboratory for Marine Science and Technology, Qingdao, China, (10)Ocean University of China, Qingdao, China
Abstract:
How robust are historical AMOC and northward heat transport (NHT) reconstructions generated from forced ocean simulations? This question is examined using a pair of CMIP6-OMIPv2 simulations using the CESM2 model: one at coarse resolution (nominal 1°; LR) and the other at eddy-resolving resolution (nominal 0.1°; HR). The simulations are both initialized from observed climatology and forced with JRA55-do atmospheric state fields and fluxes (1958-2018). A long spinup through consecutive cycles of the 61-year forcing (6 cycles for LR; 3+ cycles for HR) allows for an assessment of how AMOC variability at various timescales is impacted by resolution-dependent model drift. The long HR simulation affords some ability to quantify the relative magnitudes of intrinsic vs. forced AMOC variability in a strongly eddying ocean. Direct comparison to observed time series (including RAPID, MOVE, OSNAP) reveals the extent to which LR/HR differences can be interpreted in terms of simulation fidelity. We assess whether the mechanistic understanding developed in LR studies, including the relative contributions of wind and buoyancy forcing in driving interannual-to-decadal AMOC variability, carries over to AMOC in HR. Particular attention is given to evaluating the robustness of thermohaline processes believed to underpin high decadal predictability in the subpolar North Atlantic.
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