Younger Dryas Experiments

Abstract

Probably the best documented climate event in which ice played a major role is the Younger Dryas. This event interrupted warming over the North Atlantic and surrounding regions during the last termination for a period of about 1000y. Broecker and colleagues suggest that the event was due to a temporary weakening or cessation of the overturning circulation in the Atlantic Basin, which presently carries more than a Petawatt of heat into this region. The change in ocean circulation may have been initiated by the input of meltwater to the North Atlantic, but the event has not been successfully modelled.

An important contribution to our knowledge of the Younger Dryas event is provided by a reconstruction of the glacial melting rates by Fairbanks (1989) who demonstrates that the Younger Dryas event occurred during a minimum in meltwater discharge. He notes the possible contradiction with the idea that melt-water caused a shut-down of the Atlantic overturning circulation.

Here, we examine the influence of meltwater discharge on a steady state solution of a simple, coupled, global ocean-atmosphere-sea ice model. If an initial state for the model experiments is obtained by spinning up to present-day conditions, then the response to meltwater input is consistent with past studies: the Atlantic overturning circulation collapses and never recovers. However, if the hydrological cycle of the initial state is modified such that there is a modest increase in net fresh water loss from the Atlantic basin, then the response to meltwater input is more consistent with reconstructions of the Younger Dryas event. Results indicate that the initial pulse of meltwater resulted in a fresh water cap over the high latitude North Atlantic which caused a shut-down of the overturning circulation in this basin. During the ‘collapsed’ state, air temperatures decrease and sea ice extends southward over the northern North Atlantic, qualitatively consistent with observations. With appropriate model parameters, after the meltwater pulse terminates, the fresh water cap is slowly eroded away, convection is eventually re-initiated, the sea ice melts, and the overturning circulation rapidly resumes. The second meltwater pulse described by Fairbanks has relatively little influence on the overturning circulation or the climatic conditions.

Sensitivity of results to model parameters and idealizations are examined, and limitations of the model and the solution are identified.