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Quantitative, observational constraints on lower-tropospheric mixing are crucial for improved models of low-cloud feedbacks, and analysis of water vapor isotopic composition provides an independent means for generating such constraints. In situ measurements of water vapor isotopic composition from the Chajnantor Plateau, in northern Chile, are merged with sounding data from Antofagasta and satellite measurements of cloud fraction (CF) from the SE Pacific to show an inverse relationship between the estimated inversion strength (EIS) and water vapor export from the marine boundary layer into the free troposphere. When merged with results from the subtropical northern Pacific, the relationship between EIS and water vapor transport is found to be exponential across EIS values ranging from 0 to 15.6 K. The data from Chile are stratified into terciles of EIS with average EIS values of 9, 12.6 and 15.6 K. We show positive relationships between EIS, cloud fraction, and the mixing diagnostic delta D-v - delta D-r and negative relationships between EIS and the observed mixing ratios and water vapor delta D values at Chajnantor, all of which are consistent with an inverse relationship between inversion strength and water vapor export from the marine boundary layer. Inverse modeling of the isotopic data using a simple process model shows that the average mixing ratios at Chajnantor derived from the marine boundary layer are estimated to be 2.1, 1.15, and 0.84 g/kg, respectively, for the lowest, middle, and highest terciles of EIS. These results can be used to constrain convective parameterizations and models of low-cloud feedbacks.

Plain Language Summary Water vapor plays a major role in Earth's climate, especially as it relates to the formation of clouds. The potential for cloud amounts in the lower atmosphere to change as the climate warms has major implications for the magnitude of anthropogenic climate change and is one of the most important unresolved questions in climate science. Changes in low-cloud fractions are related to mixing of water vapor in the lower atmosphere. Stable isotopes in water vapor from the southeast Pacific provide new constraints on this mixing process in a region of especially large low-cloud fractions. When merged with recent results from the subtropical northern Pacific, we identify an exponential relationship between the strength of the inversion that caps the boundary layer throughout the subtropics and the transport of water vapor into the lower atmosphere. These results provide a new and independent way to test climate models and improve estimates of anthropogenic climate change.