A Modelling Study On The Response And Attribution Of The Northern Hemisphere Westerlies-Monsoon Climate During The Last Glacial Maximum

The Northern Hemisphere westerlies-monsoon circulation holds an important position in the Earth’s climate system,which exerted a profound impact on the formation and development of global climate on modern or even longer time scales.During the Last Glacial Maximum(LGM),influenced by the change of external boundary conditions,the Northern Hemisphere westerlies-monsoon circulation underwent great changes,regulated the evolution of glacial cycles to some extent.However,the variation of Northern Hemisphere westerlies-monsoon climate in LGM and the contributions and driving mechanisms of different external forcings are still unclear.Based on numerical simulation methods,this study systematically evaluates the responses and regional differences of Northern Hemisphere westerlies-monsoon climate during LGM,and investigates the different contributions and mechanisms of individual external forcing in the variation of westerlies-monsoon climate through a series of sensitivity experiments.In addition,the anomalous heating of Tibetan Plateau in LGM and its potential influence on Asian westerly jet were discussed.The main conclusions were summarized as follows:(1)During the LGM,the average surface temperature of Northern Hemisphere generally decreased,with the largest cooling existed in mid-to-high latitudes of North America and around Scandinavian Peninsula.The cooling in North America was violent in summer,while the surface temperature of Scandinavian Peninsula decreased more in winter.Both precipitation and area of the Northern Hemisphere terrestrial monsoon regions(NHTMR)were significantly reduced.The maximum reduction of precipitation happened in South Asian and North African monsoon regions,and the contraction of Asian monsoon region is the most significant.In the Northern Hemisphere terrestrial arid regions(NHTAR),the precipitation was decreased except for the southwestern North America,and the drought in Arabian Peninsula and Iranian Plateau were significantly intensified.Meanwhile,the total area of NHTAR expanded dramatically,which mainly due to the increase of arid regions in Central Asia and central North America.(2)During the LGM,the East Asian,South Asian and North African summer monsoons all weakened significantly,while the North American summer monsoon had no obvious change.The presence of ice sheet blocked the horizontal movement of low-level flow,making the low-level westerly winds flow around the ice sheet and shift southward.In summer,an zonal difference wave train existed in the 200 h Pa wind fields of Northern Hemisphere at mid-to-high latitudes.However,in winter,the variation of 200 h Pa wind fields over North America to North Atlantic was featured by a northwest-southeast orientated triode pattern,while appeared as a meridional dipole structure over Eurasia to North Pacific.Moreover,the westerly jet(WJ)over North America was apparently enhanced and moved southward throughout the whole year.Over Central Asia,the WJ was generally weakened and shifted southward from January to March and in August but placed northward in the rest of the year.Over Japan,the WJ was strengthened and southward in summer while weakened and northward in winter.(3)The effects of sea surface temperature and ice sheet albedo prominently contributed to the reduction of precipitation and area in NHTMR.For the NHTAR,the declined precipitation was owed to the effects of ice sheet albedo and its topography,and the expanded area was also associated with the sea surface temperature.In summer,the 200 h Pa wind fields in the mid-to-high latitudes of Northern Hemisphere were dominated by the ice sheet albedo and its topography.In winter,the ice sheet topography mainly affected the 200 h Pa wind fields from North America to North Atlantic,while the orbital parameters and greenhouse gas were closely related to the change of 200 h Pa wind fields from Eurasia to North Pacific.Correspondingly,the seasonal characteristics of WJ over North American were mainly influenced by the ice sheet.For the variation of WJ over Central Asia and Japan,the ice sheet albedo contributed the most in summer,while the orbital parameters and greenhouse gas were the major participators in winter.(4)The cooler sea surface temperature and the ice sheet albedo weakened the Asian,Arabian and African summer monsoon,which were prominently responsible for the reduction of precipitation and area in the NHTMR.For the NHTAR,the effect of ice sheet albedo exacerbated the aridity in northern North Africa by enhancing the northeasterly trade winds,while the effect of ice sheet topography limited the low-level westerly winds to flow into the western Eurasia and Central Asia,thus potentially resulting in the aridity in Central Asia.In summer,the ice sheet albedo and ice sheet topography induced zonal difference wave train on temperature deviation fields in the middle troposphere of Northern Hemisphere at mid-to-high latitudes significantly contributed to the similar response of upper wind fields.In winter,the ice sheet topography influenced the 200 h Pa wind fields from North America to North Atlantic by inducing a northwest-southeast orientated triode difference pattern on temperature deviation fields in middle troposphere.While the variation of 200 h Pa wind fields over Eurasia to North Pacific are mainly attributed to a meridional dipole difference on temperature deviation fields in middle troposphere,which motivated by orbital parameters and greenhouse gases.(5)During the LGM,the anomalous warming(cooling)center over the Tibetan Plateau(TP)became stronger in summer(winter).The Asian westerly jet changes are well explained by meridional temperature gradients in middle troposphere,which is closely linked to seasonal thermal anomalies over the TP.Derived from the heat budget equation,the stronger heating(cooling)center is mainly caused by the weaker adiabatic cooling(heating)generated from ascending(subsidence)motion over south of the TP.