Weak oceanic heat transport as a cause of the instability of glacial climates

Abstract

International audienceThe stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean--atmosphere--sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard--Oeschger events, may be internal instabilities of the climate system

A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies

“…Weaver et al (1991) and Winton and Sarachik (1993) found that it is possible to make the AMOC oscillate in an ocean-only model with steady, mixed surface boundary conditions. Later work by de Verdi ere and Raa (2010) found that such oscillations were still a robust feature of an Earth system model of intermediate complexity (EMIC); they also found that the magnitude of the oscillations was damped by the sea ice response. A large increase in ocean heat transport to the high latitudes and a salinitydriven jump in the equator to pole density gradient are both features of the AMOC switch mechanism in this model.…”

The interaction between sea ice and salinity-dominated ocean circulation: implications for halocline stability and rapid changes of sea ice cover

“…However, paleoclimatic reconstructions have shown active formation of North Atlantic Deep Water also during stadials (Yu et al 1996). Latitudinal shifts in the North Atlantic Deep Water formation site is also hypothesized to account for the D-O events (Ganopolski and Rahmstorf 2001;Arzel et al 2010;Colin de Verdiere and Raa 2010;Sevellec and Fedorov 2015). Freshwater forcing has typically been the agent to force the different states of the circulation, and the duration of the cold and the warm states generally depend on the duration of the freshwater forcing (Ganopolski and Rahmstorf 2002).…”

Geostrophic Closure of the Zonally Averaged Atlantic Meridional Overturning Circulation

“…The solution of (27a) reinforces, a posteriori, the use of such parameterization extensively applied in zonally averaged ocean models Stocker 1991, 1992;Sévellec et al 2006Sévellec et al , 2010Sévellec and Fedorov 2011;Colin de Verdière et al 2006;Colin de Verdière and te Raa 2010). This suggests that the zonally averaged circulation is actually driven by the meridional pressure gradient in the northern region.…”

“…We start from the 3D set of equations typical of the planetary-geostrophic regime [geostrophic regime of type 2 in Phillips (1963), Colin de Verdière (1988, and Salmon (1998)]: the geostrophic balance together with a Rayleigh friction, the hydrostatic, the nondivergence, and the time evolution of density. This corresponds to a low Rossby number (Ro ( 1, measuring the ratio of inertial terms to Coriolis terms in the horizontal momentum equations).…”

“…As suggested by Colin de Verdière and Tailleux (2005), the ocean dynamics have two regimes depending on the role of the vertical advection versus the horizontal advection in the tracer(s) evolution. If the former dominates, the regime is more inclined to free propagation of a Rossby wave.…”