Potential causes of abrupt climate events: A numerical study with a three‐dimensional climate model

Abstract

A multi‐millennia simulation performed with a three‐dimensional climate model under constant forcing shows abrupt climate events lasting for several centuries caused by a spontaneous transition to an infrequently visited state of the oceanic thermohaline circulation. This state is characterized by a more southern location of the main area of deep ocean convection in the North Atlantic and implies a large cooling in the mid and high latitudes of the northern hemisphere. This transition of the thermohaline circulation occurs spontaneously less than once in 5000 years in the model, but such transitions can also be triggered by a reduction of the solar irradiance.

Low-frequency oscillations of the Atlantic Ocean meridional overturning circulation in a coupled climate model

“…Abrupt changes in the strength of the AMOC at millennial timescales were also found in an earlier version of the ECBilt-CLIO model (Goosse et al, 2002). The authors describe a spontaneous weakening of the overturning strength that occurred twice within an approximately 10 000-year long experiment.…”

“…1, top). Spectral analysis reveals that the variability of the AMOC shows a slight tendency towards characteristic timescales of approximately 12 and 200 years (not shown; see Goosse et al (2003) for a discussion of the variability in ECBilt-CLIO). Since our main interest is in low-frequency variability, i.e., in timescales of at least a few hundred years, we turn to smoothed AMOC time series in the following.…”

“…Although the resulting time series bear some similarity with the results of our 5-mSv experiment, the underlying mechanisms are different. In the model version employed by Goosse et al (2002) deep convection in the North Atlantic Ocean occurred only in the Nordic Seas and not in the Labrador Sea. In their model, the changes in deep-water production rate were associated with spontaneous shifts in convection depth and location within the Nordic Seas.…”

The stationarity of global mean climate

“…An exception to this situation is the multi-millennial simulation by Goosse et al (2002), in which two noticeable anomalies were obtained in the annually averaged global mean surface-temperature time series. These were associated with perturbations in the deep convection in the North Atlantic Ocean.…”

“…Interestingly, compensating positive temperature anomalies were found in the Southern Hemisphere. Although the physical processes Goosse et al (2002) describe are plausible, the departure from climatic stationarity may be attributable to the simplicity of their atmospheric model, i.e. a three vertical-level quasi-geostrophic formulation with simplified parameterizations for diabatic heating.…”