Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes

Abstract

The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere-ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv ϵ 10 6 m 3 s Ϫ1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…For example, while all models show a shutdown of the thermohaline circulation under a 1.0 Sv hosing (Stouffer et al, 2006), they differ in response when the forcing is closer to what is currently estimated for Heinrich events, i.e. 0.1 to 0.4 Sv (Hemming, 2004).…”

“…Model inter-comparisons show that not all models are sensitive to freshwater hosing in the same manner (Stouffer et al, 2006;Kageyama et al, 2010). For example, while all models show a shutdown of the thermohaline circulation under a 1.0 Sv hosing (Stouffer et al, 2006), they differ in response when the forcing is closer to what is currently estimated for Heinrich events, i.e.…”

“…In numerous modelling studies, meltwater fluxes have been applied to perturb the Atlantic Meridional Overturning Circulation (AMOC) (Stommel, 1961;Bryan, 1986;Stouffer et al, 2006) and the associated northward heat transport, thus creating abrupt climate events (Paillard and Labeyrie, 1994;Ganopolski and Rahmstorf, 2001). Often, the North Atlantic has been used as the region where the freshwater flux was applied since it is: (i) advectively close to the main north Atlantic convection sites (where deep sinking of denser surface water 54 occurs), thus ensuring an effective response of the simulated oceanic system; and (ii), where the main marine deposits of IRD are found (see Section 2.7) i.e.…”

“…Consequently, most models have been run with an input of freshwater, termed 'hosing experiments', Ganopolski and Rahmstorf, 2001;Stouffer et al 2006) in the Ruddiman belt, with different sets of magnitudes used to evaluate the sensitivity of the models to a freshwater perturbation (see Kageyama et al, 2010, for a review).…”

The Influence of the AMOC Variability on the Atmosphere in CCSM3

“…An influence of the Atlantic multidecadal oscillation (AMO), which has been suggested to largely result from AMOC fluctuations, on summer precipitation in North America and Europe, Atlantic hurricane activity, and other climatic features has been suggested by several empirical and modeling studies (e.g., Enfield et al 2001;Sutton and Hodson 2005;Knight et al 2006). Changes in the atmospheric circulation and a strengthening of the North Atlantic storm track have been attributed to the sustained weakening of the AMOC in most hosing and anthropogenic gas forcing experiments with climate models (e.g., Stouffer et al 2006;Brayshaw et al 2009;Woollings et al 2012), but control simulations provide a cleaner setup to detect the influence of the AMOC variability on the atmospheric circulation (Msadek and Frankignoul 2009;Teng et al 2011;Gastineau and Frankignoul 2012, hereafter GF12). However, because of the natural variability of the atmosphere, the signal-to-noise ratio is small and the predictability of the atmospheric signals associated with the natural variability of AMOC seems weak, albeit less so on decadal scales (Teng et al 2011).…”