Modelling the younger dryas

Abstract

An energy balance climate model (EBCM) having land-sea, surface-air and latitudinal resolution, and which simulates the seasonal growth and decay of sea ice in terms of a number of explicit physical processes, is used to test the following hypotheses for the cause of the Younger Dryas cold episode: (1) a low salinity meltwater lens from ablation of continental ice sheets, which would have increased the freezing point and stability of the oceanic mixed layer, thereby allowing more extensive sea ice; (2) a flood of icebergs into the North Atlantic from collapse of continental ice sheets or of a hypothetical Arctic Ocean ice shelf; (3) a reduction or cessation in the rate of formation of North Atlantic Deep Water (NADW), which would have led to a reduction in the northward oceanic heat flux in the North Atlantic Ocean; and (4) a reduction in atmospheric CO 2 concentration. A low salinity meltwater lens leads to zonally averaged atmospheric cooling over land of up to 2.5°C in winter but warming of up to 1.5°C in summer. Temperature effects outside the latitudes of hypothesized low salinity meltwater are negligible. An iceberg flood covering 30% of the ocean between latitudes 45° and 70°N causes a winter atmospheric cooling over land of up to 8°C and a summer cooling of up to 4°C in the Northern Hemisphere, but maximum winter cooling at high Southern Hemisphere latitudes of only 1.3°C. If the icebergs are initially 100 m thick and allowed to ablate without replenishment, then the icebergs completely disappear in only 20–50 years and negligible temperature response occurs in the Southern Hemisphere. A reduction in the northward meridional oceanic heat flux by 0.6 × 10 15 W at 45°N leads to maximum winter and summer atmospheric coolings over land of 5–6°C and 1–2°C, respectively, at high latitudes in the Northern Hemisphere, but a warming at low latitudes and in the Southern Hemisphere. However, a reduction in the rate of NADW formation would also lead to a reduction in the upward heat flux at the base of the mixed layer at high latitudes in the Southern Hemisphere. This is a parameter of the model used here, and reducing its value by 20% causes a mean annual temperature cooling of up to 3.3°C in the Southern Hemisphere. A CO 2 decrease from 300 ppmv to 250 ppmv causes a global mean cooling of about 0.7°C and maximum mean annual atmospheric temperature responses of about 1°C in both hemispheres, with greater response in winter. This study identifies a number of specific types of paleoclimatic data which can help in discriminating among different hypothesized causes of the Younger Dryas, in particular: (a) seasonal patterns of temperature change; (b) differences between high and low latitude and between Northern Hemisphere and Southern Hemisphere temperature changes, as well as regional patterns of climatic change; (c) phase lags of temperature change between different regions; and (d) a more precise determination of the duration of the Younger Dryas in various localities.