Ocean Feedback in Response to 6 kyr BP Insolation

Abstract

The role of the ocean in the 6 kyr BP climate change is investigated by comparing a coupled ocean?atmosphere simulation and a simulation with the same atmospheric component for which SSTs are kept to the modern ones. For these simulations, Earth?s orbital parameters have been changed from their current values to those valid 6 kyr ago. The resulting change in insolation strengthens the seasonal cycle in the Northern Hemisphere, and the summer African and Asian monsoons are more vigorous. The results show that the summer monsoon flow penetrates farther north into the Sahara when ocean feedbacks are included. The SST response lags by 2?3 months the insolation forcing. This delay has an impact on the timing of the monsoon change. In most coastal regions, the climate response is very different, and even of opposite sign, between the coupled and atmosphere-alone experiments, stressing the climatic impact of the ocean in these regions. The impact of the interactive ocean on the poleward heat transport is also analyzed. The 6 kyr BP insolation change creates seasonal perturbations of the zonal mean solar forcing, which in turn affects the meridional heat transport. The change in the ocean heat transport is as large as that of the atmosphere in most latitudes and seasons. The largest variations in the ocean occur in the Tropics and are dominated by changes in the Ekman transport. The coupled and atmosphere-alone simulations exhibit different changes in atmosphere heat transport, although they have almost the same change in net heat flux at the top of the atmosphere.