Changes In Global Hydrological Cycle Under Different Climate Conditions And Its Mechanisms

Earth's hydrological cycle can maintain the dynamic balance of global water resources,and the water vapor is the material condition for cloud formation.As moisture flux is one of the most crucial section of earth's hydrological cycle,its changes in strength,pathway and divergence/convergence can influence the intensity and range of precipitation,playing an important role in regional moisture budget.In particularly,changes in large-scale hydrological cycle plays a key role in extreme weather events,such as regional droughts and floods,as well as climate change.From the perspective of decomposed moisture budget equation,in this study,we use the reanalysis data set and model output data to explore the features of large-scale hydrological cycle under different climate conditions,as well as its potential mechanism.The main conclusions are as follows:?1?Changes in global summer monsoon precipitation in recent decadesChanges in global monsoon precipitation are primarily thermodynamically caused by changes in specific humidity and dynamically caused by changes in circulation,as well as by changes in moisture transport that are driven by transient eddies and local evaporation.The results show that the increasing trends in global summer monsoon precipitation from 1979 to 2016 is dominated by contributions from the southern Asian,South African,and North American monsoon regions and are driven by the response of atmospheric circulation to the enhanced east–west thermal contrast in the tropical Pacific.We find that changes in the sea surface temperature?SST?patterns promote increased low-level vertical velocities in monsoon regions,leading to moisture convergence through divergent circulation anomalies when combined with the climatological humidity field.Hence,the processes result in increasing trends in monsoon precipitation.In addition,evaporation makes increasing contributions to monsoon precipitation throughout the world,except in the South American monsoon region.Moreover,subtropical regions in the East Pacific?the Atlantic?may be major sources of water vapour that have supported increases in precipitation over the southern Asian and North American monsoon regions?the South African monsoon region?in recent decades.?2?Changes in moisture sources over the Tropical Indian Ocean under global warmingThe moisture sources?evaporation minus precipitation;EmP?in the tropical central-eastern and southwestern Indian Oceans experienced significant increasing trends during boreal summer from 1979 to 2016.The increased EmP in the tropical central-eastern Indian Ocean is due to the enhanced dynamic divergence?account for approximately 51%?,while a stronger dynamic advection contributes more moisture supply to the southwestern Indian Ocean?account for approximately 34%?.We find that during recent decades,the enhanced east–west thermal gradient in the Pacific strengthens the Walker Circulation,which leads to a movement in convection over the Indian Ocean warm pool,resulting in weakened convection and descent over the tropical central-eastern Indian Ocean.The weakened convection leads to an anomalous low-level atmospheric divergent circulation,which intensifies the dynamic divergence contributing to the enhanced EmP over the tropical central-eastern Indian Ocean.Additionally,the warming climate during recent decades also increases the land–sea thermal contrast in the vicinity of the Indian Ocean,which enhances the southeastern wind in the low-level troposphere and leads to an enhanced EmP over the southwestern Indian Ocean.?3?The changes in ENSO-induced tropical Pacific precipitation variability in the past warm and cold climatesPliocene epoch?3.264 to 3.025 Million years ago?is the most recent warm period in the past climate,which CO₂ level is similar to present anthropogenic forcing level.Last Glacial Maximum?LGM,21 000 years ago?is the most recent glacial interval in past cold period.The results of the European community Earth-System Model?EC-Earth?show that the changes in ENSO precipitation are intensified?weakened?over the tropical western Pacific but weakened?intensified?over the tropical central Pacific in Pliocene?LGM?,compared with the pre-industrial?PI?simulation.Based on the decomposed moisture budget equation,these changes in ENSO precipitation patterns are highly related to the dynamic effect.The mechanism can be understood as follows:the zonal gradient of the mean sea surface temperature?SST?over the tropical Indo-Pacific is increased?reduced?during the Pliocene?LGM?,leading to the strengthening?weakening?of Pacific Walker Circulation as well as a westward?eastward?shift.In the Pliocene,the westward shift of Walker Circulation results in an increased?decreased?ENSO-induced low-level vertical velocity variability in the tropical western Pacific?central Pacific?,and,in turn,favoring convergent?divergent?moisture transport through a dynamic process,and then causing intensified?weakened?ENSO precipitation there.The opposite mechanism exists in LGM.These results suggest that changes in the zonal SST gradient over tropical Indo-Pacific under different climate conditions determine the changes in ENSO precipitation through a dynamic process.?4?The features of large-scale hydrological cycle of Pliocene climate:PlioMIP2 modelsThe ensemble of PlioMIP2 shows there exists“wet gets wetter,dry gets drier”phenomenon in the past warm climate?Pliocene?compared with PI simulation.That is,the changes in precipitation minus evaporation?PmE?are increased over the tropics and mid-to-high latitude,but decreased over subtropics.On the one hand,the changes in PmE are due to the increased specific humidity in the past warm climate,leading to“wet gets wetter,dry gets drier”mechanisms through thermodynamic process.In details,when combined with the climate mean convergence?divergence?circulation,the increased specific humidity can lead to more moisture transporting from northern subtropics to tropics through meridional thermodynamic transport?MTT?term.On the other hand,the intensity of Hadley circulation becomes stronger in the northern hemisphere,but becomes weaker in the southern hemisphere during Pliocene compared with PI simulation.These anomalous circulations can induce more moisture transporting from tropics to northern subtropics through meridional dynamic transport?MDT?term,which offsets the MTT term,leading to“wet gets drier,dry gets wetter”mechanisms.On the contrast,PlioMIP2 shows more moisture transport from southern subtropics to tropics through MDT term,inducing“wet gets wetter,dry gets drier” mechanisms.