Sensitivity of a coupled atmosphere/mixed layer ocean model to changes in orbital forcing at 9000 years B.P.

Abstract

Experiments were made with an atmospheric general circulation model coupled to a 50‐m mixed layer ocean in order to study the sensitivity of the climate to the changed seasonal and latitudinal distribution of solar radiation for 9000 years ago. At 9000 years B.P. (before present), when perihelion was in July (compared to January at present) and the axial tilt was greater than at present, the July (January) solar radiation in the northern hemisphere was increased (decreased) by about 7% compared to that at present. The coupled atmosphere/ocean model simulated warmer continents in summer and intensified northern summer monsoons at 9000 years B.P., compared to the present, that were similar to previous results with an atmospheric model. Owing to small positive feedbacks, the coupled atmosphere/ocean model produced a slightly greater intensification of the 9000‐year B.P. monsoon than the atmospheric model. The ocean temperature changes were less than 1 K and lagged the solar radiation changes; the 9000‐year B.P. ocean was warmer in September and colder in March than the control simulation. The annual‐average ocean temperature was slightly lower in the tropics because the annual‐average incoming solar radiation was lower at 9000 years B.P. In middle latitudes the annual‐average ocean temperature was decreased because of feedbacks associated with seasonality. In high northern latitudes, sea ice thickness was reduced at 9000 years B.P. in response to increased summer and annual‐average solar radiation. The reduced high‐latitude sea ice caused somewhat warmer wintertime conditions over the northwestern portions of northern continents.