Detecting shifts in tropical moisture imbalances with satellite-derived isotope ratios in water vapor

As global temperatures rise, regional differences in evaporation (E) and precipitation (P) are likely to become more disparate, causing the drier E-dominated regions of the tropics to become drier and the wetter P-dominated regions to become wetter. Models suggest that such intensification of the water cycle should already be taking place; however, quantitatively verifying these changes is complicated by inherent difficulties in measuring E and P with sufficient spatial coverage and resolution. This paper presents a new metric for tracking changes in regional moisture imbalances (e.g., E-P) bydefining δD_q—the isotope ratio normalized to a reference water vapor concentration of 4 mmol mol^-1—and evaluates its efficacy using both remote sensing retrievals and climate model simulations in the tropics. By normalizing the isotope ratio with respect to water vapor concentration, δD_q isolates the portion of isotopic variability most closely associated with shifts between E- and P-dominated regimes. Composite differences in δD_q between cold and warm phases of El Niño–Southern Oscillation (ENSO) verify that δD_q effectively tracks changes in the hydrological cycle when large-scale convective reorganization takes place. Simulated δD_q also demonstrates sensitivity to shorter-term variability in E-P at most tropical locations. Since the isotopic signal of E-P in free tropospheric water vapor transfers to the isotope ratios of precipitation, multidecadal observations of both water vapor and precipitation isotope ratios should provide key evidence of changes in regional moisture imbalances now and in the future.