Mechanisms for Late 20th and Early 21st Century Decadal AMOC Variability

Alex Megann,Adrian New,Simon Josey,Bablu Sinha,Adam Blaker

doi:10.1029/2021jc017865

Alex Megann, Adrian New + Show 3 more

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https://doi.org/10.1029/2021jc017865

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Abstract

AbstractRecent studies using data from the OSNAP observational campaign and from numerical ocean models suggest that the Iceland Basin and the Irminger Sea may be more significant for formation of upper North Atlantic Deep Water than the Labrador Sea. Here, we present a set of hindcast integrations of a global 1/4° NEMO simulation from 1958 until nearly the present day, forced with three standard forcing data sets. We use the surface‐forced stream function, estimated from surface buoyancy fluxes, along with the overturning stream function, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the Atlantic meridional overturning circulation (AMOC). We use the stream functions to demonstrate that watermasses in the simulations are transformed to higher densities as they propagate around the subpolar gyre from their formation locations in the north‐east Atlantic and the Irminger Sea, consistent with the picture emerging from observations. The surface heat loss from the Irminger Sea is confirmed to be the dominant mechanism for decadal AMOC variability, with the heat loss anomaly from the Labrador Sea having about half the magnitude. A scalar metric based on the surface‐forced stream function, accumulated in time, is found to be a good predictor of changes in the overturning strength. The AMOC variability is shown to be related to that of the North Atlantic Oscillation (NAO), primarily through the surface heat flux, itself dominated by the air‐sea temperature difference, but also with some local feedback from the SST to the surface fluxes.

Highlights

The projections of the models included in the Coupled Model Intercomparison Project Phase 5 (CMIP5) indicate a reduction in the strength of the Atlantic meridional overturning circulation (AMOC) into the 21st century as a result of anthropogenic greenhouse gas emissions (Cheng et al, 2013), leading to a substantial cooling over northern Europe resulting from the associated reduction in ocean heat transport (Liu et al, 2020)
Recent studies using data from the OSNAP observational campaign and from numerical ocean models suggest that the Iceland Basin and the Irminger Sea may be more significant for formation of upper North Atlantic Deep Water than the Labrador Sea
If the cold air outbreaks over the North Atlantic subpolar gyre typical under positive North Atlantic Oscillation (NAO) conditions are associated with high heat loss over this region and if they, as we have shown, lead to a stronger AMOC over the following few years, there should be a strong correspondence between the NAO index and the SAT and heat flux indices presented in Figures 7 and 8

Summary

Introduction

The projections of the models included in the Coupled Model Intercomparison Project Phase 5 (CMIP5) indicate a reduction in the strength of the Atlantic meridional overturning circulation (AMOC) into the 21st century as a result of anthropogenic greenhouse gas emissions (Cheng et al, 2013), leading to a substantial cooling over northern Europe resulting from the associated reduction in ocean heat transport (Liu et al, 2020). Recent reports of an apparent decline in the AMOC have created more than a little interest, with a reduction of about 3 Sv at 26°N since 2008 being reported in the RAPID time series (Smeed et al, 2018), which is significantly more rapid than the projected decrease from increasing CO2 concentrations To place this variation in the context of longer time scales, estimates of the AMOC strength based on proxy data such as temperature and surface elevation have been used to extrapolate the AMOC strength before the RAPID era: Worthington et al (2020) used the 26°N RAPID data to create an empirical model which they applied to earlier hydrographic data sets, deriving an AMOC time series that has a maximum in the late 1990s, followed by a gradual reduction toward the start of the RAPID campaign in around 2005.

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