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

Abstract

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) by defining δDq—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, δDq isolates the portion of isotopic variability most closely associated with shifts between E‐ and P‐dominated regimes. Composite differences in δDq between cold and warm phases of El Niño–Southern Oscillation (ENSO) verify that δDq effectively tracks changes in the hydrological cycle when large‐scale convective reorganization takes place. Simulated δDq 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.

Tracking the Strength of the Walker Circulation With Stable Isotopes in Water Vapor

“…Figures d, e, and S6 show the difference in water isotopes in vapor in TES satellite retrievals for composite ENSO events spanning 2004–2011 and compares changes in ω to observations of water isotope ratios and iCAM5‐simulated δ D V . These observations, though limited in time, suggest circulation changes with ENSO leave an isotopic fingerprint (Bailey et al, ) and that a decelerated Walker tends toward El Niño‐like conditions (in agreement with Vecchi & Soden, ). Further, Figures d and e show that δ D V estimated from TES satellite retrievals for El Niño versus La Niña are consistent with the models' future prediction of a shoaling W−E gradient in δ D V with mean warming.…”

“…Currently, available satellite missions measure δ D V in the atmospheric boundary layer, the free atmosphere, or total column; our study suggests that more accurate midtroposphere measurements (above ∼700 hPa) may be needed to detect changes in convective mass flux using water vapor isotope ratios. Recent work using TES observations across El Niño‐Southern Oscillation events show variations of 20–40‰ in δ D V suggesting that it is possible to distinguish between warmer and colder phases in the tropical Pacific and related circulation changes (Bailey et al, ). Figures d, e, and S6 show the difference in water isotopes in vapor in TES satellite retrievals for composite ENSO events spanning 2004–2011 and compares changes in ω to observations of water isotope ratios and iCAM5‐simulated δ D V .…”

“…Better diagnostics are needed for the many processes impacting isotopes in vapor related to circulation (e.g., impacts of residence time, Aggarwal et al, ; divergence, Bailey et al, ; Lee et al, ; precipitation efficiency, Bailey et al, ; cloud physics, Moore et al, ; and updraft strength on the isotopic signal Bolot et al, ), and we have yet to fully address the impact of horizontal transport, potentially using water tracer/tagging experiments. Further, recent modeling work has challenged the relationship between global convective mass flux M and the strength of the Walker circulation, which is formally defined by SLP gradients in the tropical Pacific (Sandeep et al, ).…”

Moist Entropy and Water Isotopologues in a Zonal Overturning Circulation Framework of the Madden‐Julian Oscillation

“…This general result agrees with conventional interpretation of the joint distributions of tropical q ‐ δ D. The convention considers middle troposphere humidity as largely a product of large‐scale horizontal mixing. In regions of large‐scale convection, however, Rayleigh fractionation, moisture convergence, rainfall evaporation, and intensity are more dominant processes (Bailey et al, ; Lee et al, ; Moore et al, ). This finding, which highlights the transport characteristics of the δ D associated with the MJO, contrasts with the bulk analysis of Duan et al (), who showed that δ D has many similarities with relative humidity for the tropical mean.…”

Patterns of Evaporation and Precipitation Drive Global Isotopic Changes in Atmospheric Moisture

“…Indeed, precipitation and ice cores in Alaska sometimes show stronger correlations between water isotope ratios and moisture source region than temperature (Fisher et al, 2004;Putman et al, 2017), and modern precipitation measurements in the tropical Pacific suggest that isotope ratios respond more sensitively to variations in the intensity and organization of convection regionally than to the amount of local precipitation (Conroy et al, 2016;Kurita, 2013;Kurita et al, 2009;Moerman et al, 2013). More recently, studies have shown correlations between isotope ratios and regional imbalances in E and P (Feng et al, 2009;Lee et al, 2007;Moore et al, 2014) or associated variations in atmospheric residence time and moisture transport (Aggarwal et al, 2012;Bailey et al, 2017;Putman et al, 2017). Nevertheless, to decipher changes in hydrologic balance from isotope ratios, a strong theoretical framework that can explain these correlations is necessary.…”