Sensitivity of the Atlantic Meridional Overturning Circulation to Model Resolution in CMIP6 HighResMIP Simulations and Implications for Future Changes

Abstract

A multimodel, multiresolution ensemble using Coupled Model Intercomparison Project Phase 6 (CMIP6) High Resolution Model Intercomparison Project (HighResMIP) coupled experiments is used to assess the performance of key aspects of the North Atlantic circulation. The Atlantic Meridional Overturning Circulation (AMOC), and related heat transport, tends to become stronger as ocean model resolution is enhanced, better agreeing with observations at 26.5°N. However, for most models the circulation remains too shallow compared to observations and has a smaller temperature contrast between the northward and southward limbs of the AMOC. These biases cause the northward heat transport to be systematically too low for a given overturning strength. The higher‐resolution models also tend to have too much deep mixing in the subpolar gyre. In the period 2015–2050 the overturning circulation tends to decline more rapidly in the higher‐resolution models, which is related to both the mean state and to the subpolar gyre contribution to deep water formation. The main part of the decline comes from the Florida Current component of the circulation. Such large declines in AMOC are not seen in the models with resolutions more typically used for climate studies, suggesting an enhanced risk for Northern Hemisphere climate change. However, only a small number of different ocean models are included in the study.

Interannual Variability of the South Atlantic Ocean Heat Content in a High‐Resolution Versus a Low‐Resolution General Circulation Model

“…This suggests that the highly intermittent Agulhas leakage is not properly captured by the low‐resolution simulation and the interocean exchange is through a rather laminar flow, which could reduce its impacts on OHC variability on interannual time scales. The findings of this work might be of particular importance for the adequate choice of model resolution in studies that are strongly linked to OHC variability and its influence on climate and they are in agreement with a recent study by Roberts et al (2020) who demonstrated that a high model resolution might be essential to represent MHT according to observations. It demonstrates the need for the application of high‐resolution climate models when investigating processes such as rainfall pattern variability and monsoon modifications or predictions.…”

“…resolution in studies that are strongly linked to OHC variability and its influence on climate and they are in agreement with a recent study by Roberts et al (2020) who demonstrated that a high model resolution might be essential to represent MHT according to observations. It demonstrates the need for the application of high-resolution climate models when investigating processes such as rainfall pattern variability and monsoon modifications or predictions.…”

Resolving and Parameterising the Ocean Mesoscale in Earth System Models

“…In CMIP6 HighResMIP and OMIP simulations (see Fig. 1a, b), the AMOC transport more often than not becomes stronger at higher ocean resolution [20], and in coupled models, this tends to be driven by enhanced convection in the Labrador Sea [73,75]. For both CMIP5 models [76] and CMIP6 HighResMIP [73], there tend to be too deep mixed layers in the Labrador Sea in order for the AMOC strength to be comparable with observations (noting that this is particularly true in NEMO models, suggesting that the model structure as well as resolution may be a factor).…”

“…For both CMIP5 models [76] and CMIP6 HighResMIP [73], there tend to be too deep mixed layers in the Labrador Sea in order for the AMOC strength to be comparable with observations (noting that this is particularly true in NEMO models, suggesting that the model structure as well as resolution may be a factor). When projecting future climate, this convection tends to reduce more quickly than that in the Nordic Seas [73], which means that the higher-resolution models have a stronger AMOC decline in the future. The effect of convection changes in the Nordic Seas is linked to the AMOC via the overflows.…”

“…d An illustration of the impact of ocean model resolution on the ACC-AMOC simulation. Model simulations are from CMIP6 OMIP experiments [25] and from CMIP6 HighResMIP control-1950 experiments [73] (plus additional data not yet archived, see 'Model Data' for details). AMOC/NHT observations [67,68] are shaded with the annual mean range between 2004 and 2017.…”