Optimal Estimates of Regional Moisture Exchange Using Isotopic Ratios of Water Vapor From the Tropospheric Emission Spectrometer

Further understanding of the variations in the seasonal sources of mid-tropospheric moisture, including the contributions from surface evaporation and the re-evaporation of falling rain, can provide refined knowledge of the global hydrological cycle. Stable water isotope measurements from the Tropospheric Emission Spectrometer (TES) are useful in this regard since isotopic fractionations occurring during evaporation and condensation give rise to measurable variations in the isotopic composition of water vapor, which can be used to estimate the strength of regional hydrologic processes. The present study employs an optimized Lagrangian isotopic exchange model to estimate the regional supply of water vapor from surface evaporation and rainfall evaporation, the loss of moisture via precipitation, and the regional isotopic signatures of the source water for the 500-825 hPa layer. Given the advection pathways formed from NCEP/NCAR Reanalysis wind fields, the Lagrangrian model uniquely constrains these moisture exchange processes by employing the changes in isotopic composition found en route over a one to three day time frame. The estimates show vigorous moisture exchange occurring over land surfaces in monsoonal regions, where the timescale for complete replacement of the moisture in the parcels by the evaporative supply of boundary layer moisture is found to be on the order of two to four days during the seasonal monsoons. Conversely, over areas of oceanic upwelling in the subtropics, our estimates reveal a much slower timescale for moisture replacement on the order of ten to fifteen days. The timescale for the complete loss of initial moisture in the parcels via precipitation varies from one or two days in the mid-latitudes to three or four days in the tropics. The estimates of the isotopic composition of the source waters found from the model show that recently evaporated oceanic water enters the 500-825 hPa layer primarily over land surfaces during the seasonal monsoons, as well as over the western Pacific warm pool. Additionally, rainfall recycling is found to be an important part of the hydrology directly upstream of the monsoonal regions of northern Australia, the Amazon Basin, and southeastern Asia. Using the isotopic measurements from TES, this study provides unique diagnostics of mechanisms that control the seasonal distribution of water vapor.