Responses of Westerly Jets over Asia versus North America in the Last Glacial Maximum and Their Attributions

Abstract

Abstract By using the Community Atmosphere Model version 4 (CAM4) coupled to a slab ocean model, the total responses of westerly jets (WJ) over Asia and North America during the Last Glacial Maximum (LGM), as well as the relative influences of different individual LGM forcing on the changes in WJ, are evaluated in this study. The results show that the seasonal variation of Northern Hemisphere WJ in LGM is characterized by distinct regional discrepancies. The WJ is intensified and shifts southward in North America, while it generally weakens and slightly displaces northward in central Asia. Over Japan, the WJ is enhanced and moves southward in summer but is attenuated and shifts northward in winter. The change of WJ has a close relationship with the midtroposphere temperature anomalies which result from the different external forcings. In summer, the ice sheet albedo plays a leading role in wind fields at mid-to-high latitudes of the Northern Hemisphere by causing a zonal wave–like response of temperature. In winter, the ice sheet topography dominates the wind fields across North America to North Atlantic by inducing a northwest–southeast-oriented tripole temperature response. The effects of orbital parameters and greenhouse gases largely contribute to the alteration of upper wind fields over Asia and North Pacific by causing a dipole response of temperature.