The General Properties Of The Relationship Between Lower Stratospheric Water Vapor And Tropical Cold-point Tropopause Temperature And Their Impact Factors

Stratospheric water vapor plays an important role in the global radiation budget,energy balance,and stratospheric chemical processes.The tropical cold-point tropopause temperature(CPTT)is the main factor controlling the lower stratospheric water vapor(SWV).Generally,a higher(lower)CPTT value corresponds to a higher(lower)SWV value.However,there are still some mismatches between SWV and CPTT.Using satellite observations,soundings,reanalysis,and model data,a detailed analysis of the basic characteristics of their association and the impact factors is conducted.Moreover,the historical and future trends of SWV and CPTT are also analyzed.The main conclusions are as follows:(1)Firstly,using the satellite observation data,the climatological SWV distribution(altitude-latitude,altitude-longitude,and horizontal distribution)and the corresponding CPTT are investigated.The influence of CPTT on the SWV is quantified based on the water vapor mixing ratio values calculated from the CPTT.For the annualmean altitude-latitude distribution,there are significant low SWV values above the cold-point tropopause,which is the lowest in the equatorial region and increases slowly with latitude and corresponds to the CPTT distribution.For the annual-mean altitudelongitude distribution,there are significant low SWV values over the Indian-Pacific warm pool region,which is controlled by the low CPTT values over it.For the annualmean horizontal distribution,there are low SWV values over the equatorial region,caused by both the CPTT over the Indian-Pacific warm pool and the horizontal diffusion within the lower stratosphere.In addition to the annual-mean distribution,the SWV also shows an obvious seasonal cycle.In spring(March-May)and winter(December-February),the SWV is mainly affected by the dehydration of the CPTT,especially over the Indian-Pacific warm pool.In summer(June-August)and autumn(September-November),due to the impact of the Asian summer monsoon,the main feature of SWV is that the northern hemisphere is wetter than the southern hemisphere.(2)Furthermore,this paper explores the association between anomalous SWV and CPTT and the impact factors.The results show that the association between them shows significant meridional inhomogeneity,that is,the association is strong in the tropics and weak in the extratropics.This meridional inhomogeneity is related to Brewer-Dobson Circulation(BDC).In detail,the BDC uplifts in the tropics and sinks in the poles,and its upward branch can affect the CPTT and thus influencing the SWV.Therefore,the association between the anomalous SWV and CPTT is closer in the tropics.In addition,the association between the anomalous SWV and CPTT shows significant zonal inhomogeneity,that is,their correlation coefficient is weaker near 120°W than that in other tropical regions.Since the downward branch of the tropical Pacific Walker circulation is over there,which leads to a weaker condensation of water vapor.Moreover,near 120°W,the variability of horizontal motion is large and the horizontal diffusion of water vapor is significant.The association between anomalous SWV and CPTT is also stronger in the upwelling regions than that in the downwelling regions of the tropical Indian Walker circulation,but this is not evident in the tropical Atlantic Walker circulation region.The association between anomalous SWV and CPTT also indicates seasonal variability,which is weaker in summer since the main convective region shifts to the Tibetan Plateau in summertime.(3)This paper also analyzed the interannual variability of the connection between SWV and CPTT and the impact factors.The results show that El Ni?o and Southern Oscillation(ENSO)and the Quasi-Biennial Oscillation(QBO)could modulate the association between the SWV and CPTT at the interannual time scale.The maximum covariance analysis indicates that the first mode of the coupling between SWV and CPTT is related to the QBO,characterized by positive anomalies of SWV and CPTT over the equatorial Indian Ocean,the western Pacific,and the Atlantic Ocean.The second mode of the coupling between SWV and CPTT is related to ENSO with meridional-dipole-shaped negative anomalies straddling the central-eastern Pacific region and horseshoe-shaped positive anomalies over the Indian-Pacific warm pool.Using multiple linear regression analysis,it is found that ENSO and QBO can explain about 60% of the interannual variations of the SWV and CPTT.Considering the important influence of tropical sea surface temperature(SST)on SWV and CPTT and the significant warming of SST in the tropical Indian Ocean in recent years,the influence of SST over the Indian Ocean on the association between SWV and CPTT is further investigated.The results show that the two main modes of SST in the tropical Indian Ocean,i.e.,the Indian Ocean Basin Mode(IOBM)and the Indian Ocean Dipole(IOD),also have significant effects on SWV and CPTT.The effects of them on the interannual linkages of SWV and CPTT were comparable to ENSO.(4)Finally,this paper discussed the historical and future trends of SWV and CPTT.The results show that there is a significant decreasing trend between 1980 and 2000 and an increasing trend during 2000-2019 of SWV,with CPTT showing similar trends.Further analysis indicates that the variation in tropical zonal SST gradient has an important impact on this trend transition.In detail,both the zonal SST gradient from the west to the east of the tropical Pacific Ocean and the zonal SST gradient from the east to the west of the tropical Indian Ocean first increase and then decrease,which first strengthen and then weaken the intensity of the Walker circulation in the tropical Pacific and Indian Ocean.Changes in the tropical Walker circulation can lead to changes in vertical velocities over the warm pool and the central-eastern Pacific Ocean,and vertical velocities variations in these two regions can cause changes in waves near the cold-point tropopause,ultimately leading to trend reversals in the SWV and CPTT.It should be noted that the spatial distribution of CPTT trends exhibits heterogeneity due to tropical waves,i.e.,there are opposite trend changes in the central-eastern Pacific region(CEP)and non-CEP region,where the CPTT changes in the non-CEP region are the main contributing component of the whole tropical CPTT changes.This thesis also assesses the changes in SWV and CPTT under different emission scenarios(The Shared Socioeconomic Pathways,SSP)using the model results of the Coupled Model Intercomparison Project Phase 6,mainly considering three emission scenarios,SSP5-8.5,SSP2-4.5,and SSP1-2.6.The results show that there is an increasing trend of both SWV and CPTT under the high emission scenario(SSP5-8.5)and the medium emission scenario(SSP2-4.5),but the increasing trend is relatively weaker under the medium emission scenario(SSP2-4.5);under the low emission scenario(SSP1-2.6),the trend of SWV and CPTT is not significant.