Responses of westerly jet over Asia versus North America in Last Glacial Maximum and their attributions

Abstract

<p>The Northern Hemisphere westerly jet (WJ) during the Last Glacial Maximum (LGM) exhibits significant anomalies from present day. However, the total responses of Northern Hemisphere WJ, as well as the relative influences of different individual LGM forcings are still unclear. Therefore, this study analyzed the seasonal variation of Northern Hemisphere WJ during LGM through a series of sensitivity experiments, and evaluated the relative contributions and dynamic mechanisms of different forcing factors, with a special focus on ice sheet topography and albedo. Our results show that the WJ is intensified and shifts southward in North America, while generally weakens and displaces northward in central Asia. Over Japan, the WJ is enhanced and moves southward in summer but attenuated and shifts northward in winter. The change of WJ has a close relationship with the mid troposphere temperature anomalies which are resulted from different external forcings. In summer, the ice sheet albedo plays a leading role in wind fields at mid-to-high latitudes of Northern Hemisphere by causing a zonal wave-like response of temperature. While in winter, the ice sheet topography dominates the wind fields across North America to North Atlantic by inducing a northwest-southeast orientated tripole temperature response in this region. The effects of orbital parameters and greenhouse gases largely contribute to the alteration of upper wind fields over Asia and North Pacific by motivating a dipole response of temperature.</p>