Aerial Moisture Transport in the Earth Climate System: A Study of the Mean State and Perturbations Due to CO2-Doubling using Numerical Water Tracers and a Novel Linear Algebra Analysis Framework

Abstract:

Aerial moisture transport is fundamental to the climate system, and numerical water tracers (WTs) are a powerful tool for understanding this transport. Here, we present a novel matrix operator framework that permits systematic, rather than ad hoc, analysis of WT results. We use this framework to study moisture transport, from evaporation (or sublimation) to precipitation, in a state-of-the-art global climate model (GCM) which incorporates WTs. This framework separates moisture divergence over a given tagged region into local divergence (the export of locally-evaporated moisture) and remote convergence (the import of remotely-evaporated moisture). The remote convergence term may be further subdivided into zonal, meridional, intrabasin, and interbasin parts, and can be used to predict precipitation given a particular spatial pattern of evaporation (demonstrated in Figure 1a).

Findings from analysis of the preindustrial mean state concur with findings from earlier moisture transport studies: water evaporated at the equator and high latitudes tends to precipitate locally, whereas water evaporated in the subtropics and midlatitudes tends to precipitate remotely; water evaporated in the subtropics diverges both equatorward and poleward of its source region, while water evaporated in the midlatitudes mostly diverges poleward. New insights from the method reveal fundamental differences between the major ocean basins, with the Atlantic basin having the largest local divergence, smallest remote convergence, and greatest interbasin moisture export.

With quasi-equilibrium CO₂-doubling, we find that a greater fraction of locally-evaporated moisture is exported, moisture exchange between ocean basins increases (shown in Figure 2c), and moisture convergence within a given basin shifts towards greater distances between moisture source and sink regions. These changes can be understood in terms of a greater moisture residence time with warming, or, equivalently, a robust increase in the advective length scale of moisture transport. We conclude by discussing the effect of the increasing atmospheric moisture transport length scale on ocean state, and implications for the interpretation of water isotope records.