Marine Boundary Layer Decoupling and the Stable Isotopic Composition of Water Vapor

Decoupling of the subcloud layer in stratocumulus-topped marine boundary layers (STMBL) influences low-cloud cover by limiting the supply of water vapor from the surface. However, the relative importance of mixing between surface fluxes and other reservoirs of water vapor as a function of the degree of decoupling is poorly understood. Water vapor transport within the STMBL and its response to decoupling is explored using surface measurements of water vapor isotopic composition that were obtained from Graciosa Island, Azores, during the summer and fall of 2018. The data show an inverse relationship between the decoupling metric, Delta q, and the degree of drying from an isotopically depleted water vapor source. The isotopic data require some degree of drying with an isotopically depleted source for coupled conditions, and likely require a small amount of mixing even for strongly decoupled conditions. The data are consistent with mixing between purely local reservoirs of water vapor, subjected to a small amount of condensation and fractionation, and do not require the invocation of large-scale transport of water vapor aloft or additional cloud-formation effects aloft. Plain Language Summary The transport of water vapor from the surface of the ocean to the upper part of the marine boundary layer helps to maintain low marine clouds that play a crucial role in the climate system. Under certain conditions, the water vapor evaporating from the ocean surface is decoupled from the cloudy layer, resulting in reduced cloud cover. The details of water vapor transport within the marine boundary layer under these decoupled conditions are poorly understood, but the stable isotopic composition of water vapor can provide important constraints on the mixing between different reservoirs of water vapor. Surface measurements of water vapor isotopic composition from the Azores shows how the degree of decoupling governs mixing between surface fluxes and water vapor reservoirs aloft, and that some amount of mixing occurs even under strongly decoupled conditions. These results will help scientists to better understand the potential for changes in marine low-cloud frequencies within the current climate and, potentially, as the climate warms.