Exploring stability, variability, and regime transitions in a laboratory analogue of the ocean's thermohaline circulation

We investigate a laboratory analogue of the Atlantic thermohaline circulation, which is driven by horizontal gradients of thermal and haline forcing at the water surface. The system can exhibit different stable configurations, with a thermally driven overturning flow and a weakened or reversed flow with enhanced stratification driven by the salinity gradient.

A regime transition from the thermally driven to the weak state serves as analogue of a potential future collapse of the Atlantic meridional overturning circulation, and is likely also related to climate changes in paleoclimate history. By change of the surface salinity forcing (emulating increases in polar meltwater input) the system is moved towards and beyond the transition, and changes in the velocity field and tracers are monitored.

It is analyzed whether prior to the stability loss there are statistical early-warning signals in the variability of the turbulent up- and downwelling plumes, and it is determined what are the best observables to detect these. This helps shed light on whether such a regime transition can be viewed as a tipping point in the sense of a saddle-node bifurcation preceded by critical slowing down.