Impact of ocean resolution and mean state on the rate of AMOC weakening
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Impact of ocean resolution and mean state on the rate of AMOC weakening L. C. Jackson1 · M. J. Roberts1 · H. T. Hewitt1 · D. Iovino2 · T. Koenigk3 · V. L. Meccia4 · C. D. Roberts5 · Y. Ruprich‑Robert6 · R. A. Wood1 Received: 9 December 2019 / Accepted: 21 June 2020 © The Author(s) 2020
Abstract We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO2 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 CO2 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 CO2. Keywords Climate · AMOC · Resolution · Weakening
1 Introduction Climate model projections show a weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increased greenhouse gases, however there is a wide * L. C. Jackson [email protected] 1
Met Office Hadley Centre, Exeter, UK
2
Centro Euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy
3
Swedish Meteorological and Hydrological Institute, Norrköping and Bolin Centre for Climate Research, Stockholm, Sweden
4
Institute of Atmospheric Science and Climate (ISAC-CNR), Bologna, Italy
5
ECMWF, Reading, UK
6
Barcelona Supercomputing Center, Barcelona, Spain
range in the weakening rates. In particular, the Fifth report of the Intergovernmental Panel on Climate Change (IPCC) concluded that a weakening of the AMOC before 2100 was very likely, but with a range of weakening of 12–54% for a high-range scenario (Collins et al. 2013). This spread in AMOC response in climate models has been shown to be responsible for some of the model spread in projections of surface temperature (Drijfhout et al. 2012), winter storms over Europe (Woollings et al. 2012) and sea level changes (
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