The overarching goal of our research is to understand the mechanisms causing abrupt onset of megadroughts, and to understand the processes that maintain them. In this proposal, we focus on paleo megadroughts as seen in the speleothem records, and explore the role of the jet stream in linking hydrologic changes across the Northern Hemisphere.

We hypothesize that abrupt hydroclimate transitions in mid-latitudes are related to the seasonality, position, and strength of the jet stream and that megadroughts are maintained by particular combinations of climate forcing and natural variability. This project focuses on the hydroclimate for the past 250 thousand years, as the growing network of high-resolution records of calcite d18O in speleothems provides critical information for testing our hypothesis with the Earth System Model. Speleothems are unique paleo-climate proxies because calcite d18O values are direct in situ records of precipitation d18O, and because they are highly resolved and precisely dated, with an average resolution of 70 years between 5.0 and 80.0 kyBP. For the proposed work, we will focus on several aspects shown in the two-time series. Firstly, paleo-hydrology behavior is very different from the classic saw-tooth view of ice-age cycles, with rapid warming followed by a slow descent into the glacial period. The ascent into wet periods and descent into dry periods may be abrupt, suggesting thresholds of climate forcing. Secondly, there are periods (e.g. between 240 and 250 kyBP) when droughts occurred with increasing insolation. We hypothesize that the drought periods in East Asia are connected by the jet stream to the rest of the Northern Hemisphere, so that the triggers for the droughts could be remote and that droughts in East Asia could be a regional manifestation of hemispheric changes in hydroclimate.

The principal tool in this study is the DOE-NCAR Community Atmospheric Model (CAM), a component of the Earth System Model (EaSM) with a thermodynamic “slab” ocean. A version of CAM has a water isotope module embedded in it. A series of CAM experiments will be carried out, with high and low climate forcing (insolation and greenhouse gases), and with freshening of the North Atlantic Ocean. For each series, various sea surface temperature (SST) anomalies associated with El Niños and La Niñas will be added to the slab ocean SSTs. The experiments will be carried out at successively high resolution to study the influence of complex topography on precipitation and hydrodynamic processes in the atmosphere and to resolve frontal precipitation. The analysis of model results will focus on whether one combination of climate forcing and natural variability could lead to threshold behavior and megadroughts, as well as on the role of the jet stream in linking hydroclimate changes in the Northern Hemisphere.

A novel feature of the proposed work is the development of a connected component labeling algorithm in CAM to determine jet streams, storm tracks, and frontal precipitation. The algorithm will be useful for evaluating the spatial persistence of these rainbands, their direction of propagation, and their links to large-scale circulation of the atmosphere.