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Subtropical to Subpolar Atlantic Model Biases Addressed through Process-Level Diagnostics (Sub2Sub)

The proposed work is a collaborative project between the University of Wisconsin Madison and
the National Center for Atmospheric Research (NCAR) to understand how the North Atlantic
Current (NAC) path influences the Atlantic meridional overturning circulation (AMOC) in climate
models of varying configurations and resolutions. The goal of this project is a thorough model
comparison of NAC pathway biases – a longstanding issue in ocean and climate models – and their
relationship with thermohaline biases from the subtropics into the subpolar North Atlantic. As part
of this proposal, a set of process-oriented diagnostics (PODs) will be applied to output from the
Coupled Model Intercomparison Project version 6 (CMIP6), Ocean Model Intercomparison
Project (OMIP), High-Resolution Model Intercomparison Project (HiResMIP), and to experiments
from an in-development version of the Community Earth System Model (CESM) coupled to the
Modular Ocean Model version 6 (MOM6). The most novel of the PODs to be implemented are
two NAC pathway identification algorithms, one applying image processing techniques on
currents and the other drawing from the atmospheric thermal front identification literature.
Analysis will focus on combining the NAC pathway POD with surface-forced water mass
formation, allowing for a dynamically-defined decomposition of surface formation between
subtropical and subpolar regions, as delineated by the NAC. Decomposing formation further by
individual surface fluxes and by the overlap of density classes with surface fluxes will allow
process-based characterization of ocean and climate model biases. Sources of pathway-related
biases will be identified through comparisons of models from the multimodel ensembles described
above, focusing on the effect of horizontal resolution in models with MOM. An expected outcome
of this project is a clearer understanding of sources of NAC pathway and subtropical-to-subpolar
formation model biases, identifying potential directions for ocean model development.
Relevance: Through its analysis of ocean and coupled model simulations of historical climate,
this proposal supports both NOAA’s mission to “advance our understanding of the Earth’s climate
system” and the CPO MAPP program’s overall mission to “enhance the Nation’s and NOAA’s
capability to understand, predict, and project variability and changes in Earth’s climate system.”
The proposed work aligns with the MAPP program’s primary objectives of “improving Earth
System models”, “supporting an integrated Earth System analysis capability” and “improving
methodologies for global to regional scale climate analysis, predictions, and projections” by
identifying sources of ocean model biases in the subtropical to subpolar North Atlantic. The current
Model Development Task Force (MDTF) diagnostic software have a large gap in open ocean
model diagnostics. The North Atlantic Current pathway identification and water mass
transformation PODs described in this proposal would partly fill this gap. Through close
collaboration with NCAR, opportunities for MOM6 improvement will be identified in CESM-
MOM6 development simulations. Because MOM6 is also the ocean component model in NOAA
ESMs, the CESM-MOM6 diagnosis described here will indirectly contribute to the improvement
of NOAA models. In collaboration with NCAR, POD software will be contributed to MDTF for
further dissemination and to CESM community diagnostic packages.

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