Milankovitch Forcing, Vegetation Feedback, and North Atlantic Deep-Water Formation

Abstract

An energy balance climate model with sea ice and seasonal land snow cover is used to investigate the climatic response to Milankovitch orbital variations. Model response to a change of obliquity is in phase in the Northern and Southern hemispheres and governed by summer insulation changes, whereas model response to a change of longitude of perihelion is 125° out of phase in the two hemispheres and more closely related to spring insulation. Temperature sensitivity is greater in the Northern Hemisphere (NH) for both orbital changes and temperature response in both hemispheres is 3?5 times greater for a change of obliquity from 22° to 25° than for a change of perihelion from 270° to 90° with an eccentricity of 0.04, even at low latitudes. High latitude tundra fraction is parameterized in terms of July land air temperature and low latitude forest and grassland fraction are parameterized in terms of summer land-sea temperature difference. Both vegetation feedbacks make a significant contribution to the local model response when orbital parameters are changed from those of 125 kyr BP to 114 kyr BP. The rate of formation of North Atlantic Deep Water (NADW) is parameterized in terms of summer to winter cooling of the ice-free mixed layer at 60°N and is assumed to be associated with the upward heat flux Fb to the base of the mixed lava at high latitudes in the Southern Hemisphere (SH). The feedback between NH inflation and SH Fb, gives a high latitude SH temperature response 50% or more larger than the high latitude NH temperature response, and could explain the tendency for SH temperature changes to lead NH temperature changes as peak NH temperature response would occur later, after the buildup of ice sheets. Land July temperatures fall by 6°C or more poleward of 60°N for this orbital change, but the adjacent mixed layer remains relatively warm, favoring the development of NH ice sheets. For all the experiments performed here, low latitude warming occurs in response to a change from 125 kyr BP to 114 kyr BP orbital parameters; low latitude cooling seems to be dependent on buildup of NH ice sheets.