The Study On The Mechanism Of The Midwinter Suppression Of The North Pacific Storm Track

Based on the NCEP/NCAR reanalysis data,this dissertation studied the climatological characteristics of the midwinter suppression of the North Pacific storm track(NPST),and analyzed the atmospheric circulation anomalies associated with the interannual variations of the midwinter suppression.Then,the influence of the East Asian trough(EAT)and the North Pacific atmospheric blocking(PB)on the interannual variation of the midwinter suppression was studied by composite analysis.Moreover,this dissertation analyzed the performance of CMIP6 model on the midwinter suppression and the heat transport by quasi-stationary wave,and pointed out the possible reasons for the midwinter suppression climatology.Furthermore,using a general circulation model FGOALS-f3-L,the impact of the mechanical and thermal effects of the Tibetan-Iranian Plateau(TIP)on the midwinter suppression was revealed.In addition,inconsistent variations between the northern and southern North Pacific storm track were discovered,which may provide a new perspective for understanding the midwinter suppression of the NPST.The main conclusions are as follows.The midwinter suppression of the NPST has distinct interannual variations.When the midwinter suppression does not occur,the upper-level subtropical jet weakens,the mid-latitude westerly wind strengthens,and the atmospheric baroclinicity increases significantly north of 40°N.In addition,the NPST anomalies in autumn and spring and the upper-level zonal wind anomalies are not significant,indicating that the focus of the interannual variation in the midwinter suppression of the NPST is still the anomalous atmospheric circulation during midwinter.Interannual variations in the midwinter suppression may related to the weakening of the EAT and the PB.However,when the midwinter suppression occurs,the atmospheric circulation anomalies are not significant.There is a significant interannual relationship between the EAT and the midwinter suppression of the NPST.When the EAT is weak,the seasonal cycle of the NPST is consistent with the atmospheric baroclinicity,and the midwinter suppression is not obvious.The enhanced baroclinic energy conversion and the intensified eddy available potential energy induced by the diabatic heating are favorable to the midwinter NPST.The meridional shear of the background flow is weakened,and there are significantly positive air temperature anomalies upstream of the midwinter NPST.Barotropic governor effect and weakened static stability can explain the enhancement of the midwinter NPST.However,the advection effect of the background flow and the upstream seeding effect cannot contribute to the EAT-related midwinter NPST anomalies.The interannual variation of the PB is significantly related to the midwinter suppression of the NPST.When the PB is weak,the midwinter suppression of the eastern NPST is not noticeable,but there is still a remarkable midwinter suppression of the western NPST.The atmospheric baroclinicity anomalies associated with the PB can well be responsible for the anomalous midwinter NPST.The upstream EAT can affect the downstream PB by changing the meridional gradient of the background potential vorticity,and the EAT and PB can coordinately affect the midwinter NPST.When they are weak at the same time,the midwinter suppression of the NPST is no longer marked.In addition,the midwinter NPST can establish positive feedback with the PB through eddy-induced geopotential height tendency.The CMIP6 model can well reproduce the climatology of the midwinter NPST.In the lower troposphere,some models,like IPSL-CM5A2-INCA,INM-CM4-8,GISS-E2-1-G and MIROC6,failed to reproduce the obvious midwinter suppression of the NPST.In the middle troposphere,in addition to INM-CM4-8 and MIROC6,other models can well reproduce the midwinter suppression of the NPST.In the upper troposphere,the midwinter suppression of the NPST can be faithfully reproduced by all these CMIP6 models.The climatological midwinter NPST has a significantly negative correlation with the climatological PB and heat transport by quasi-stationary wave,while it has a significantly positive correlation with the climatological upper mid-latitude westerly wind.When a certain climate state has weak PB or heat transport by quasi-stationary wave,or strong upper mid-latitude westerly wind,the climatological midwinter NPST tends to be strong,and it is not easy to observe the midwinter suppression of the NPST.The mechanical and thermal effects of the TIP can significantly affect the NPST.When the TIP-associated mechanical forcing is removed,the midwinter suppression of the NPST still existed,but it is not as obvious as before.The amplitudes of the zonal asymmetric wind field and temperature field are reduced,and their resultant“interference” effect is weakened,resulting in weakening of the heat transport by quasi-stationary wave.The weakening and widening upper-level subtropical jet can strengthen the midwinter NPST by barotropic governor effect.The weakened EAT and enhanced baroclinic energy conversion are conducive to strengthening the midwinter NPST.When the TIP-associated thermal forcing is absent,the seasonal cycle of the NPST does not change significantly,and the midwinter suppression is still distinct.The interannual,seasonal and climatological variations in the northern NPST are inconsistent with those in the southern NPST.In terms of the interannual variation,the northern NPST is closely related to the atmospheric circulation at middle and high latitudes,while the southern NPST is intimately bound up with the equatorial sea surface temperature.With respect to the seasonal variation,the seasonal cycle of the northern NPST shows a double peak structure,while the southern NPST has only one peak.The seasonal cycles of both the northern and southern NPST are consistent with the zonal wind field,which accords with the expectation of baroclinic instability theory.In terms of the climatological variation,when a climate state has a stronger northern NPST,the southern NPST does not significantly tend to be stronger.