Water Isotopes Provide Insight into Tropical Convective Processes
Isotope analysis of samples from the ARM Manus site shows that links between isotopic variability and convective processes are more complicated than previously thought.
Understanding the movement of water in all its phases throughout the climate system is important for understanding current climate and projecting future climate changes. Examining isotopic composition of water vapor and precipitation can provide insight into factors controlling the water and energy balance of the atmosphere, including constraints on specific convective cloud processes.
Observations from a measurement campaign at the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility site on Manus Island, Papua New Guinea, are used to study the variability in the stable isotopic composition of precipitation and water vapor to provide insights into tropical convection processes and interpretation of paleoclimate proxies.
Multiple stable isotopes of hydrogen and oxygen occur naturally in water. Due to the mass differences between the isotopes, physical processes such as transitions between the vapor and condensed phases of water can change the relative proportion of various isotopes. Thus, examining isotope ratios in samples of precipitation and water vapor can provide insight into hydrological cycle processes that affected the samples. Understanding controls on the stable isotopic composition of precipitation and vapor in the tropics can provide important constraints on the representation of convective processes in models and correct interpretation of isotope-based paleoclimate proxies. The stable isotopic composition of water vapor, precipitation, and seawater was measured at the ARM facility on Manus Island, Papua New Guinea. The results demonstrate variability in the stable isotopic composition of precipitation and vapor in individual precipitation events and over a 10-day period. Isotope ratios progressively increased throughout the period of measurement, coincident with a transition from high to low regional convective activity. Vapor isotope ratios approached equilibrium with seawater during the quiescent period and likely reflected downwind advection of distilled vapor and re-evaporation of rainfall during the period of regional convection. In individual storms, isotope ratios in precipitation were strongly correlated with isotope ratios in surface vapor; however, they were not strongly correlated with surface meteorological data, including precipitation rate, in all storms. Yet across all events, precipitation deuterium excess was negatively correlated with surface temperature, sea level pressure, and cloud base height and positively correlated with precipitation rate and relative humidity. Results from the short campaign support the interpretation that isotope ratios in precipitation and vapor in the western tropical Pacific are indicators of regional convective intensity at the timescale of days to weeks. However, a nonstationary relationship between rain rate and stable isotope ratios in precipitation during individual convective events suggests that condensation, rain evaporation, moisture recycling, and regional moisture convergence do not always yield an amount effect relationship on intra-event timescales. Apart from aiding in understanding modern convective processes, such metrics hold important implications for interpreting archives of past isotopic variability. For example, these results suggest that interpretation of deuterium excess from low-latitude ice cores as reflective of evaporative conditions at the moisture source may be oversimplified; more investigation is required to understand how the signal evolves across the differing timescales represented in various isotope archives. Together, these findings offer observationally based interpretive guidance for proxies that reflect isotope ratios of precipitation in terms of precipitation characteristics in the tropics.
University of Illinois
Funding sources for this work include NSF-AGS-PF 1049664, NSF ATM-0645291, NNX08AR23G, and 07-NEWS07-0020. This research was supported by the U.S. Department of Energy, Office of Biological and Environment Research, Atmospheric Radiation Measurement Climate Research Facility.
Conroy, J. L., D. Noone, K. M. Cobb, J. W. Moerman, and B. L. Konecky. 2016. “Paired Stable Isotopologues in Precipitation and Vapor: A Case Study of the Amount Effect Within Western Tropical Pacific Storms,” Journal of Geophysical Research Atmospheres 121(7), 3290-3303. DOI: 10.1002/2015jd023844.