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

Abstract

The Madden‐Julian oscillation (MJO) is the principal source of tropical intraseasonal variability, yet challenges remain to accurately simulate its observed convective behavior and eastward propagation. There is specific need for evaluating the role of water within the MJO, including evaporation, vertical transport, precipitation, and latent heating of the coupled atmosphere‐ocean system. Hydrogen isotope ratios are particularly useful for investigating these aspects of the water cycle. This study complements previous characterization of MJO joint distributions for water vapor and isotopologue concentrations (δD) with consideration of moist entropy in the tropical zonal overturning circulation framework of the MJO. The goals are to distinguish the roles of convective and large‐scale dynamic processes through the life cycle of the MJO and identify shortcomings for modeling MJO humidity, clouds, and convection. From MJO composite analyses, wet equivalent potential temperature (θq) anomalies are largest at 500 hPa and tilt westward with altitude. A positive θq anomaly co‐occurs with the precipitation maxima, and negative θq anomalies co‐occur with subsidence both trailing and leading the convective center. Out of phase with θq, δD anomalies are positive east and negative west of the convective center, coherent with the regional zonal overturning stream function. These results point to a decoupling in the MJO between midtroposphere water vapor, which is tied to convective processes, and the isotopologue ratios, which are tied to the large‐scale circulation. A conceptual model is presented to describe the physical processes that explain the MJO life‐cycle for joint distributions of humidity (q) and water vapor δD.

The Intra‐Seasonal Oscillation of Precipitation δ18O Over the Asian Equatorial and Monsoon Regions

“…The vapor δD in the convective phase is 40‰ to 90‰ lower than in the non‐convective phase. The activity of MJO leads to the east‐west contrast pattern for vapor δD between the convective active and inactive regions from the equatorial Indian Ocean to the Pacific Ocean in our results, which is also found in the study of TES data (Hurley et al., 2019).…”

Water isotopes, climate variability, and the hydrological cycle: recent advances and new frontiers

“…Studies leveraging tropics-wide isotopic retrievals from satellites consistently identify four stages of distinct moisture regimes (Berkelhammer et al 2012, Tuinenburg et al 2015, Hurley et al 2019a. These stages occupy the four quadrants on a plot of isotope ratio versus specific humidity, indicating that the isotopic observations contribute unique information in the identification of moistening and dehydrating processes (Hurley et al 2019a). During the first stage of the MJO lifecycle, large-scale vertical mixing isotopically enriches the lower free troposphere.…”

“…Modern precipitation and water vapor isotope observations provide an opportunity to evaluate the dynamical processes regulating higher frequency variability, including seasonal (Wright et al 2017, Feng et al 2009, intraseasonal (Berkelhammer et al 2012, Tuinenburg et al 2015, Hurley et al 2019a, and synoptic variability (Noone et al 2011). In the tropics, the dominant mode of intraseasonal variability is the MJO, which is characterized by the eastward propagation of mesoscale convective systems (MCSs) across the Indian and tropical Pacific Oceans over a period of 30-90 days (Madden and Julian 1994).…”

“…However, efforts to describe moisture's role theoretically are more recent (Adames and Kim 2016). Isotopic investigations have contributed to our understanding mainly by identifying the moist processes that define the distinct stages of the MJO (Kurita et al 2011, Berkelhammer et al 2012, Kurita 2013, Tuinenburg et al 2015, Conroy et al 2016, Hurley et al 2019a, thus helping pinpoint the GCM physics critical for MJO simulation.…”

The Influence of Convective Aggregation on the Stable Isotopic Composition of Water Vapor

“…Hurley et al. (2019) linked the changing slope of the δD − q relationship during different phases of the Madden‐Juilan Oscillation to transitions in convection from shallow (with detrainment) to deep (with rain evaporation). Similar results were found by Diekmann, Schneider, Knippertz, et al.…”