The Atlantic Meridional Overturning Circulation in High‐Resolution Models

Abstract

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC. Key Points:• Observations and high-resolution models have changed view on the AMOC pathways • High-resolution models suggest the presence of previously unknown high-frequency AMOC variability • High-resolution models allow to estimate the intrinsic/chaotic component of the AMOCCorrespondence to:

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

“…3, left). Similar results are found in Hirschi et al (2020). However the GIN seas overturning is weaker in MR in both coordinates.…”

“…The higher resolution models mostly (with one exception, EC-EARTH-HR) have a more westerly location of the North Atlantic current at 50 • N. The high resolution models also have a stronger subpolar gyre strength (Fig. 11c), consistent with a multi-model study of ocean only models (Hirschi et al 2020). These circulation differences are likely to be responsible for a greater transport of North Atlantic water Fig.…”

“…One factor thought to affect the AMOC strength is the horizontal resolution in the ocean. Many studies have found a stronger AMOC in higher resolution models (Roberts et al 2016(Roberts et al , 2019aSein et al 2018;Menary et al 2018;Docquier et al 2019;Hirschi et al 2020), though other studies have found a weaker AMOC in higher resolution models (Yokohata et al 2007;Delworth et al 2012;Winton et al 2014). Increasing resolution from non-eddying to eddy-permitting (or even eddy-resolving) has been shown to improve biases in the North Atlantic (Small et al 2014;Menary et al 2018;Roberts et al 2018Roberts et al , 2019aCaldwell et al 2019).…”

Impact of horizontal resolution on global ocean-sea-ice model simulations based on the experimental protocols of the Ocean Model Intercomparison Project phase 2 (OMIP-2)

“…This is due to the large seasonal variability of the AMOC, associated with the migration of the Intertropical Convergence Zone (ITCZ) and the changes in wind patterns and Ekman transport (Xu et al, 2014). The standard deviations based on annual means does not show such a maximum near the Equator (see Figure 7 in Hirschi et al 2019 for results based on different averaging scales). The variability is much weaker beyond the equatorial region.…”

Eddies in the North Greenland Sea and Fram Strait From Satellite Altimetry, SAR and High‐Resolution Model Data

“…During recent decades, satellite and in situ observations with high spatial resolution (1-10 km) and high temporal resolution (days to weeks) have made a significant contribution to a better understanding of the importance of mesoscale eddies in the large-scale ocean dynamics (Golivets & Koshlyakov, 2003;Lozier, 2010;Maze et al, 1997;Wunsch & Ferrari, 2004). Most frequently developed as a result of dynamic instability of the mean flows, eddies play a major role in the cross-frontal exchange (Dutkiewicz et al, 2001;Fratantoni, 2001;Spall & Chapman, 1998) and influence the along-front advection intensity (Danabasoglu et al, 1994;Hirschi et al, 2020;Luo & Lu, 2000). Having horizontal dimensions from tens to hundreds of kilometers and vertical scales of hundreds of meters (Chelton et al, 2011;Jacobs et al, 2001;Kamenkovich et al, 2012;Shoosmith et al, 2005), a single eddy may carry trillions of tons of water and dozens of terajoules of heat (Bashmachnikov et al, 2015;Jayne & Marotzke, 2002;Wunsch, 1999).…”