What do moisture recycling estimates tell us? Exploring the extreme case of non-evaporating continents

摘要

Moisture recycling estimates are diagnostic measures that could ideally beused to deduce the response of precipitation to modified land-evaporation.Recycling estimates are based on moisture-budget considerations in whichwater is treated as a passive tracer. But in reality water is athermodynamically active component of the atmosphere. Accordingly, recyclingestimates are applicable to deduce the response to a perturbation only ifother mechanisms by which evaporation affects climate do not dominate theresponse – a condition that has not received sufficient attention in theliterature. In our analysis of what moisture recycling estimates tell us, wediscuss two such additional mechanisms that result from water's active role.These are (I) local coupling, by which precipitation is affected locally viathe thermal structure of the atmosphere, and (II) the atmosphericcirculation, by which precipitation is affected on a large spatial scale.We perform two global climate model experiments: One with and another withoutcontinental evaporation. By this extreme perturbation we test the predictiveutility of a certain type of recycling measure, the "continental recyclingratio". Moreover, by such a strong perturbation the whole spectrum ofpossible responses shows up simultaneously, giving us the opportunity todiscuss all concurrent mechanisms jointly.The response to this extreme perturbation largely disagrees with thehypothesis that moisture recycling is the dominant mechanism. Instead, mostof the response can be attributed to changes in the atmospheric circulation,while the contributions to the response by moisture recycling as well aslocal coupling, though noticeable, are smaller. By our case study it is notpossible to give a general answer to the question posed in the title, but itdemonstrates that recycling estimates do not necessarily mirror theconsequences of land-use change for precipitation.