Sylvia G. Dee

PRYSM: An open‐source framework for PRoxY System Modeling, with applications to oxygen‐isotope systems

Paleoclimate observations constitute the only constraint on climate behavior prior to the instrumental era. However, such observations only provide indirect (proxy) constraints on physical variables. Proxy system models aim to improve the interpretation of such observations and better quantify their inherent uncertainties. However, existing models are currently scattered in the literature, making their integration difficult. Here, we present a comprehensive modeling framework for proxy systems, named PRYSM. For this initial iteration, we focus on water-isotope based climate proxies in ice cores, corals, tree ring cellulose, and speleothem calcite. We review modeling approaches for each proxy class, and pair them with an isotope-enabled climate simulation to illustrate the new scientific insights that may be gained from this framework. Applications include parameter sensitivity analysis, the quantification of archive-specific processes on the recorded climate signal, and the quantification of how chronological uncertainties affect signal detection, demonstrating the utility of PRYSM for a broad array of climate studies.

SPEEDY-IER: A fast atmospheric GCM with water isotope physics

The interpretation of variations in the global isotopic composition of precipitation and water vapor can be strengthened using an isotope-enabled atmospheric general circulation model (AGCM). Here we present a fast-physics atmospheric circulation model suitable for long ensemble integrations: the efficient AGCM Simplified Parameterizations, Primitive Equation Dynamics (SPEEDY), with newly added water isotope physics. The model (SPEEDY-isotope-enabled reconstructions (IER)) simulates the hydrological cycle and isotope ratios in atmospheric water at a fraction of the computational cost of Intergovernmental Panel on Climate Change (IPCC)-class GCMs. Despite its simplified physics, SPEEDY-IER captures many key features of the observed range of tropical, subtropical, and midlatitude isotope variability when compared to the Global Network of Isotopes in Precipitation, Stable Water Isotope Intercomparison Group (SWING2) simulations, and satellite observations of isotopes in vapor. The incorporation of water isotopes in SPEEDY required two updates to the models physics: postcondensational exchange associated with falling rain and soil hydrology. It is evident that these physical processes are essential for a skillful simulation of isotopes in precipitation and vapor. We conduct a suite of sensitivity tests to constrain effective parameters in the rain exchange and land models and assess the impact of the new physics to isotope simulations. The strong sensitivity to parameter choice in these components reaffirms the importance of land-atmosphere interactions and rain-vapor exchange on stable water isotope ratios in the atmosphere and thus on the interpretation of paleoclimate records. The utility of SPEEDY-IER for climate applications is discussed.

El Niño increases the risk of lower Mississippi River flooding

Mississippi River floods rank among the costliest climate-related disasters in the world. Improving flood predictability, preparedness, and response at seasonal to decadal time-scales requires an understanding of the climatic controls that govern flood occurrence. Linking flood occurrence to persistent modes of climate variability like the El Niño-Southern Oscillation (ENSO) has proven challenging, due in part to the limited number of high-magnitude floods available for study in the instrumental record. To augment the relatively short instrumental record, we use output from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) to investigate the dynamical controls on discharge extremes of the lower Mississippi River. We show that through its regional influence on surface water storage, the warm phase of ENSO preconditions the lower Mississippi River to be vulnerable to flooding. In the 6–12 months preceding a flood, El Niño generates a positive precipitation anomaly over the lower Mississippi basin that gradually builds up soil moisture and reduces the basin’s infiltration capacity, thereby elevating the risk of a major flood during subsequent rainstorms. Our study demonstrates how natural climate variability mediates the formation of extreme floods on one of the world’s principal commercial waterways, adding significant predictive ability to near- and long-term forecasts of flood risk.