Mean State AMOC Affects AMOC Weakening through Subsurface Warming in the Labrador Sea

Abstract

While most models agree that the Atlantic Meridional Overturning Circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the 21st century, they disagree on the projected magnitudes of AMOC weakening. In this work, CMIP6 models with stronger climatological AMOC are shown to project stronger AMOC weakening in both 1 percent ramping CO2 and abrupt CO2 quadrupling simulations. A physical interpretation of this result is developed. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO2 increase, surface warming is mixed to the deeper Labrador Sea in models with stronger upper ocean mixing. This subsurface warming and corresponding density decrease drives AMOC weakening through advection from the Labrador Sea to the subtropics via the Deep Western Boundary Current. Time series analysis shows that most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in the subtropics by several years. Also, idealized experiments conducted in an ocean-only model show that the subsurface warming over 500-1500 m in the Labrador Sea leads to stronger AMOC weakening several years later, while the warming that is too shallow (<500 m) or too deep (>1500 m) in Labrador Sea causes little AMOC weakening. These results suggest that a better representation of mean state AMOC is necessary for narrowing the inter-model uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.

Asymmetry of AMOC Hysteresis in a State‐Of‐The‐Art Global Climate Model

“…This weaker AMOC is also reflected in the decreased meridional potential density gradient across the Atlantic Ocean (Figures S3e and S3f in Supporting Information ). The vast expansion of the Arctic sea‐ice pack after AMOC collapse (van Westen et al., 2023) limits the AMOC recovery to about 5 Sv as sea ice strongly suppresses deep water formation (Lin et al., 2023) by limiting air‐sea fluxes (details are discussed in the next subsection). The resulting effect of sea ice is that there is a relatively cold and fresh surface layer near the deep water formation regions at the higher latitudes (Figure S2 in Supporting Information ) and this strongly increases the vertical stratification preventing deep water formation.…”

“…We have shown here that the Northern Hemisphere sea‐ice distribution has a substantial effect on the recovery of the AMOC. The fact that sea ice inhibits air‐sea fluxes, and hence convection, is important (Lin et al., 2023) for the recovery of the AMOC in the CESM and hence for the hysteresis width. The sea ice is the main reason for the strong asymmetry in the hysteresis, where the recovery is a factor six faster than the collapse.…”

Challenges simulating the AMOC in climate models

“…One important mechanism controlling AMOC weakening proposed previously by [88] and supported by [13] involves sea ice: models with a greater sea ice extent have greater sea ice retreat as the climate warms and a hence greater increase in area exposed to the atmosphere. This enables more heat loss from the ocean, offsetting the reduction in heat loss from atmospheric warming which is driving the AMOC weakening.…”

“…Correlations of sea surface salinity (SSS) in the North Atlantic with the AMOC across several CMIP6 models show similar findings (figure 1 a ). A recent study [13] has also shown that other aspects of the mean state in the Labrador sea are related to AMOC strength: those models participating in CMIP6 that have a stronger mean AMOC strength have a warmer, more saline Labrador sea, with weaker stratification and less sea ice extent. However, it is not clear what the causality of the biases is, since a model with a stronger AMOC might be expected to have a greater northwards transport of heat and salt, resulting in a warmer and more saline Labrador sea, and hence less sea ice and a weaker stratification with deeper convection.…”

“…The mean state in the Labrador sea has also been shown to have a strong influence on AMOC projections in CMIP6 models [13], with those models with a strong AMOC weakening having a stronger mean AMOC and a warmer, more saline Labrador sea, with weaker stratification and less sea ice extent than those models with less AMOC weakening. Parallel analysis of CMIP6 models here also shows correlations between salinity in the Labrador sea and both AMOC mean strength and the AMOC weakening trend in the 1pctCO 2 experiments (figure 1 c , d ).…”