DOI: 10.7291/d1fq18
title: Increased frequency of extreme precipitation events in the North Atlantic during the PETM: Observations and theory

description: 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.
description:             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 CO<sub>2</sub> values of 680 ppmv (hereafter referred to as LP) and PETM CO<sub>2</sub> 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<sup>-2</sup> 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.