Impact of ocean resolution and mean state on the rate of AMOC weakening

Abstract

We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO 2 at different horizontal resolutions in a state-of-the-art climate model and in a small ensemble of models with differing resolutions. There is a strong influence of the ocean mean state on the AMOC weakening: models with a more saline western subpolar gyre have a greater formation of deep water there. This makes the AMOC more susceptible to weakening from an increase in CO 2 since weakening ocean heat transports weaken the contrast between ocean and atmospheric temperatures and hence weaken the buoyancy loss. In models with a greater proportion of deep water formation further north (in the Greenland-Iceland-Norwegian basin), deep-water formation can be maintained by shifting further north to where there is a greater ocean-atmosphere temperature contrast. We show that ocean horizontal resolution can have an impact on the mean state, and hence AMOC weakening. In the models examined, those with higher resolutions tend to have a more westerly location of the North Atlantic Current and stronger subpolar gyre. This likely leads to a greater impact of the warm, saline subtropical Atlantic waters on the western subpolar gyre resulting in greater dense water formation there. Although there is some improvement of the higher resolution models over the lower resolution models in terms of the mean state, both still have biases and it is not clear which biases are the most important for influencing the AMOC strength and response to increasing CO 2 .

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

“…Changes to the atmosphere resolution alone give generally smaller changes and are more model dependent. These increases in AMOC and NHT with HR ocean tend to make the models agree better with transport characteristics of RAPID-MOCHA observations at 26.5°N, while as shown in Jackson et al (2020) some properties in the subpolar North Atlantic can be better in LR models.…”

“…This work complements that of Jackson et al (2020), in which there is more of a focus on the mechanisms of AMOC decline in the CMIP6 simulations using the HadGEM3-GC31 models. That study makes use of a subset of the HighResMIP multimodel ensemble discussed here to provide additional evidence of robustness of the proposed mechanisms.…”

“…(2) errors in the transport of saline water into the Labrador Sea, as described in Jackson et al (2020);…”

“…(3) inherent issues with the NEMO-based models, perhaps including bathymetry and steering of boundary currents affecting the connectivity with the subtropics; (4) errors in surface fluxes contributing to biases in water mass properties; and (5) lack of eddy activity to export the densest Labrador Sea Water Georgiou et al, 2019). Reconciling the results of Jackson et al (2020) in which both resolutions of HadGEM3-GC31 have reasonable agreement with the OSNAP observations (Lozier et al, 2019), with the disagreement between modeled and observed DMV (and with observed mixed layer depths in the Labrador Sea), suggests that there is something more to understand with regard to how and where the water is transformed and feeds into AMOC.…”

“…In terms of DMV, these observed values are only consistent with a small subset of models, most of which (apart from CNRM-CM6-1) having relatively weak AMOC. For some models (HadGEM3-GC31 and ECMWF-IFS) the HRs have markedly more activity in the Labrador Sea than in the Nordic Seas, which becomes significant in future projections-see later and Jackson et al (2020). There is a strong relationship between DMV in both regions and the AMOC strength across models, with those models producing more dense water in the Labrador or GIN Seas having a greater AMOC strength.…”