Summer Westerly Jet In Northern Hemisphere In Typical Period During The Holocene: A Multi-model Study

In this study by using the simulation data of multiple models in the Paleoclimate Modelling Intercomparison Project Phase 3 and Coupled Model Intercomparison Project 3 were used in this paper Multiple mode simulation data in Project Phase 5(PMIP3/CMIP5),and averages the simulation results of multiple models,the characteristics of jet stream and related mechanism in the four typical periods,i.e.midHolocene(MH),Medieval Warm Period(MCA),Little Ice Age(LIA)and Present Warm Period(PWP),are analyzed by using the results of simulations.It shows the cause and mechanisms of the change of the westerly jet stream in the northern hemisphere under different climate background,especially the warming background,so as to better understand the change situation of the westerly jet under the future global warming,which is of great scientific significance to improve the prediction of future climate change.The conclusion of the paper is as follows:1.By using four high-resolution models in PMIP3/CMIP5,the changes of position and intensity of the northern hemisphere westerly jet at 200 h Pa 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.From the thermodynamic perspective,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 dynamic perspective,two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the northern shift,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.2.Based on the simulation results of several models in PMIP3/CMIP5,as well as the Last Millennium ensemble simulation results in CESM-LME,the changes of position and intensity of the northern hemisphere westerly jet at 200 h Pa in summer during the past millennium---Medieval Warm Period(MCA),Little Ice Age(LIA),as well as the related mechanisms,are investigated.Although MCA and LIA have great differences in temperature from the result of simulation,the meridional position,zonal position and intensity of the westerly jet in MCA and LIA are not significantly changed compared with the mean modes of the past millennium.From the thermal perspective,during MCA,Since the temperature rise at high latitude is stronger than at low latitude;during LIA,the temperature drop at high latitude is stronger than at low latitude.Therefore,there is no significant difference in the change of meridional temperature gradient between the two periods.However,the meridional pressure gradient shows that the anomalous pressure gradient over North America and the North Atlantic during MCA and the anomalous pressure gradient over Eurasia and the Pacific during LIA,it will cause a northerly movement of the westerly jet stream.From the dynamic perspective,the maximum centers of eddy kinetic energy over the Pacific Ocean during MCA weak and no change in position,during LIA the maximum centers of eddy kinetic energy over Eurasia and Pacific Ocean strength slightly and expansion,inspired the transient vorticity compensation mechanism,so as to offset the thermodynamic factors result in the north of phenomenon.In terms of the anomalous intensity of the jet stream,the reason about the jet stream not change obviously in the two periods is that the thermal effect and dynamic effect are opposite in the Pacific Ocean and North America and North Atlantic.3.The present warm period(PWP)is mainly formed by human activities,while MCA is mainly formed by natural external forcing.By using models in PMIP3/CMIP5,the changes of position and intensity of the northern hemisphere westerly jet at 200 h Pa in summer during the Modern Warm Period(PWP)compared to the MCA,as well as the related mechanisms,are investigated.At the hemispheric scale,the simulated westerly jet has an equatorward shift and a strengthen intensity during the PWP compared to the MCA.Due to the inhomogeneity of temperature increase in the northern hemispheric during PWP compared with MCA,which leads to the strengthening of temperature gradient,thus strengthening the meridional pressure gradient in the three study areas in the northern hemisphere,and making the westerly jet intensify and move southward.From the dynamic perspective,the strengthening of jet stream is also related to the strengthening of the ascending branch of Hadley circulation.Specifically for the three study areas,the thermal effect over Eurasia and Pacific region has a weakening effect on jet stream,the dynamic factor is greater than that of thermal factor is the reason for the enhancement of the jet stream.The enhancement of jet over North America is mainly caused by the dynamic process.