Ocean‐Only FAFMIP: Understanding Regional Patterns of Ocean Heat Content and Dynamic Sea Level Change

Oleg Saenko,Jonathan Gregory,Kewei Lyu,Duo Yang,Xuebin Zhang,Matthew Couldrey,John A. Church,Alexander Todd,Riccardo Farneti,Quran Wu,Laure Zanna,René Navarro‐Labastida

doi:10.1029/2019ms002027

Abstract

AbstractThere is large uncertainty in the future regional sea level change under anthropogenic climate change. Our study presents and uses a novel design of ocean general circulation model (OGCM) experiments to investigate the ocean's response to surface buoyancy and momentum flux perturbations without atmosphere‐ocean feedbacks (e.g., without surface restoring or bulk formulae), as part of the Flux‐Anomaly‐Forced Model Intercomparison Project (FAFMIP). In an ensemble of OGCMs forced with identical surface flux perturbations, simulated dynamic sea level (DSL) and ocean heat content (OHC) change demonstrate considerable disagreement. In the North Atlantic, the disagreement in DSL and OHC change between models is mainly due to differences in the residual (resolved and eddy) circulation change, with a large spread in the Atlantic meridional overturning circulation (AMOC) weakening (20–50%). In the western North Pacific, OHC change is similar among the OGCM ensemble, but the contributing physical processes differ. For the Southern Ocean, isopycnal and diapycnal mixing change dominate the spread in OHC change. In addition, a component of the atmosphere‐ocean feedbacks are quantified by comparing coupled, atmosphere‐ocean GCM (AOGCM) and OGCM FAFMIP experiments with consistent ocean models. We find that there is 10% more AMOC weakening in AOGCMs relative to OGCMs, since the extratropical North Atlantic SST cooling due to heat redistribution amplifies the surface heat flux perturbation. This component of the atmosphere‐ocean feedbacks enhances the pattern of North Atlantic OHC and DSL change, with relatively stronger increases and decreases in the tropics and extratropics, respectively.

Highlights

A rise in global mean sea level is a robust feature of projected anthropogenic climate change from state‐of‐ the‐art atmosphere‐ocean general circulation models (AOGCMs) (Church et al, 2013; Slangen et al, 2014)
This study has examined the ocean's response to surface momentum and buoyancy flux perturbations in an ensemble of OGCMs and AOGCMs
A novel, ocean‐only Flux‐Anomaly‐ Forced Model Intercomparison Project (FAFMIP) experimental design is presented, where high temporal frequency surface fluxes are prescribed to OGCMs in order to simulate a control state, faf‐ passiveheat, with minimal drift and without surface restoration or bulk formulae

Summary

Introduction

A rise in global mean sea level is a robust feature of projected anthropogenic climate change from state‐of‐ the‐art atmosphere‐ocean general circulation models (AOGCMs) (Church et al, 2013; Slangen et al, 2014). Part of the large disagreement in regional sea level projections stems from model structural differences, such as the variety of spatial grids and subgrid‐scale parameterizations. These structural differences contribute to a spread in model performance at simulating regional ocean processes. Multimodel ensembles such as CMIP5 allow for a better quantification of the uncertainty associated with regional sea level change. Air‐sea buoyancy and momentum flux changes are coupled to ocean dynamic and thermodynamic changes and play an important role in modulating regional sea level change (Bouttes & Gregory, 2014; Lowe & Gregory, 2006)

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