Geology Club Seminar

General circulation models (GCMs) suggest that much of the global hydrological cycle's response to anthropogenic warming will be caused by increased lower-troposphere water vapor and associated feedbacks. However, fingerprinting changes in the global hydrological cycle due to anthropogenic warming remains challenging. Held and Soden (2006) predicted that as lower-tropospheric water vapor increases, atmospheric circulation will weaken as climate warms to maintain the surface energy budget. Unfortunately, the strength of this feedback and the fallout for other branches of the hydrological cycle is difficult to constrain in situ or with GCMs alone.

This talk investigates the utility of stable hydrogen isotope ratios in atmospheric water vapor to quantitatively trace changes in atmospheric circulation and convective mass flux in a warming world. We compare water isotope-enabled GCM experiments for control (present-day) CO2 vs. high CO2 atmospheric simulations. We evaluate changes in the distribution of water vapor and vertical velocity between these experiments in order to identify spatial patterns of circulation change over the tropical Pacific (where vertical motion is strong) and map the δD of water vapor associated with atmospheric warming. We show that there are robust mechanisms that moisten the troposphere and weaken convective mass flux, and that these mechanisms can be tracked using the δD of water vapor. Further, currently available satellite missions measure δD in the atmospheric boundary layer, the free atmosphere, or total column; our study suggests that more accurate upper troposphere measurements (above 500hPa) may be needed to detect changes in convective mass flux using water vapor isotope ratios. Taken together, these findings provide a framework to develop new metrics for the detection of global warming impacts to the hydrological cycle.