Abstract
The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.
Highlights
The westerlies are the planetary wind belts between the subtropical high-pressure belts and the subpolar low-pressure belts in the northern and southern hemispheres
Four high-resolution models participated in comparison Project phase 5 (CMIP5)/PMIP phase 3 (PMIP3) were used to investigate
Four high-resolution models participated in CMIP5/PMIP3 were used to the characteristics and mechanisms of the position and intensity changes of the northern hemisphere (NH) upper-level investigate the characteristics and mechanisms of the position and intensity changes of the NH upperwesterly jet in summer during the mid-Holocene
Summary
The westerlies are the planetary wind belts between the subtropical high-pressure belts and the subpolar low-pressure belts in the northern and southern hemispheres. Atmosphere 2020, 11, 1193 of maximum wind speed near the tropopause in the westerlies. Even the location and intensity of the jet stream change slightly, it will have a great impact on the climate in the middle latitudes [5,6]. The meridional position and zonal position of the jet stream could affect the intensity and seasonal variation of the East Asia monsoon [7,8,9,10]. The meridional movement of the jet stream is closely related to the interannual variation of precipitation in China [5,8,9,11].
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