Climate model simulations of the PETM (Paleocene-Eocene Thermal Maximum) warming have mainly focused on replicating the global thermal response through greenhouse forcing, i.e. CO₂, at levels compatible with observations. Comparatively less effort has gone into assessing the skill of models to replicate the response of the hydrologic cycle to the warming, particularly on regional scales.  Here we have assembled proxy records of regional precipitation, focusing on the Mid-Atlantic Coasts of North America (New Jersey) and Europe (Spain) to test the response of the hydrologic system to greenhouse gas forcing of the magnitude estimated for the PETM (i.e., 2x). Given evidence that the PETM initiated during a maximum in eccentricity, this includes the response under neutral and extreme orbital configurations. Modeled results show excellent agreement with observations in Northern Spain, with a significant increase in both mean annual and extreme precipitation resulting from increased CO₂ levels under a neutral orbit. The Mid Atlantic Coast simulations agree with observations showing increases in both overall and extreme precipitation as a result of CO₂ increases. In particular, the development of sustained atmospheric rivers might be significantly contributing to the extremes of the eastern Atlantic, whereas extratropical cyclones are likely contributing to the extremes in the western Atlantic. With an eccentric orbit that maximizes insolation during boreal summer, there is a suppression of precipitation in the eastern Atlantic and an amplification in the western Atlantic which may account for observations in the relative timing of the sedimentary response to the carbon isotope excursion associated with the PETM.

            A series of experiments simulating the PETM warming have been conducted (Kiehl et al., in prep; Shields et al., in prep) utilizing the high resolution (0.25°) CAM5, Version 5.3, with fixed sea surface temperatures and finite volume dynamical (FV) core, with 30 levels in the vertical for the atmosphere component (Neale et al., 2010; Park et al., 2014). The land component is the Community Land Model, Version 4 (CLM4) (Lawrence et al., 2011), also at 0.25° resolution, with the river transport model (RTM) at 1° resolution. Organic aerosol emissions were produced by running MEGAN (Model of Emissions of Gases and Aerosols) approximated from PETM biomes using DeepMIP protocols (Guenther et al., 2012; Lunt et al., 2017). The boundary conditions and sea surface temperatures from this model were obtained from a fully coupled LP and PETM FV 2° CESM1.2.2 (Community Earth System Model, Version 1.2) with output taken at a monthly temporal resolution over 1800 years.

Output was obtained from CAM5 at 6 hourly, daily, and monthly temporal resolution for over 20 years. The model was run with late Paleocene CO₂ values of 680 ppmv (hereafter referred to as LP) and PETM CO₂ values of 1590 ppmv (hereafter referred to as PETM). Methane was held at 16 ppmv in all runs. Additionally, in order to test the impact of orbital forcing, the model was run with both a neutral orbit and a configuration that maximized solar insolation over the northern hemisphere (i.e. High eccentricity, perihelion NH summers), hereafter referred to as OrbMax. Solar forcing was calculated based on a solar constant of 1355 Wm^-2 consistent with Kiehl et al. (2018). The four runs are therefore referred to as LP, PETM, LP OrbMax, PETM OrbMax. Paleocoordinates for each location were set over a 2° by 2° area and were taken from the DeepMIP protocols (Lunt et al., 2017). EMA was set to 34.5°-36.5°N, 0°-2°E. WMA was set to 41°-43°N, 49°-51°W. In order to account for the time required for the model to reach equilibration, data was trimmed to the final 15 years of the 20-year model run.

The parameters of interest include median and 1st and 3rd quartile monthly precipitation and runoff to track both annual and seasonal variation, and exceedance frequency to track storm intensity and to track changes in frequency of storm events. Exceedance frequency is calculated as P=m÷(n+1), wherein P is the exceedance frequency, m is the rank of a given event, and n is the total number of events.