Increased frequency of extreme precipitation events in the North Atlantic during the PETM: Observations and theory

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.

Methods
            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.

针对PETM（古新世-始新世极热事件，Paleocene-Eocene Thermal Maximum）升温的气候模式模拟，既往研究多聚焦于通过温室强迫（即二氧化碳）在匹配观测结果的浓度下复现全球热响应。相较而言，针对模式复现水循环对升温响应能力的评估工作则较少，在区域尺度上尤为如此。本研究收集了区域降水的代用记录，以北美中大西洋沿岸（新泽西州）与欧洲（西班牙）为研究区域，用以检验水文系统对PETM量级（即两倍当前浓度）温室气体强迫的响应。鉴于有证据表明PETM始于轨道偏心率极大值时期，本研究同时考察了中性与极端轨道构型下的响应情况。

模式结果与西班牙北部的观测数据吻合极佳：在中性轨道构型下，二氧化碳浓度升高会使得年平均降水与极端降水均显著增加。中大西洋沿岸的模拟结果同样与观测一致，即二氧化碳浓度升高会导致总降水与极端降水均有所增加。具体而言，持续大气河的形成可能是东大西洋极端降水的主要成因，而西大西洋的极端降水则可能由温带气旋所致。当轨道偏心率处于北半球夏季日照最大化的构型时，东大西洋的降水会受到抑制，而西大西洋降水则会增强，这一现象可解释与PETM相关的碳同位素异常的沉积响应相对时间的观测结果。

## 研究方法
本研究开展了一系列PETM升温模拟实验（Kiehl等，待发表；Shields等，待发表），采用分辨率为0.25°的CAM5（Community Atmosphere Model Version 5.3，第五代社区大气模式5.3版本），其大气模块采用有限体积动力（finite volume dynamical, FV）核心，垂直方向共30层（Neale等，2010；Park等，2014），海表温度为固定值。陆面模块采用分辨率同样为0.25°的CLM4（第四代社区陆面模式，Community Land Model Version 4）（Lawrence等，2011），径流传输模式（River Transport Model, RTM）分辨率为1°。有机气溶胶排放通过运行MEGAN（气体与气溶胶排放模式，Model of Emissions of Gases and Aerosols）生成，该模式基于DeepMIP协议从PETM生物群系中近似得到参数（Guenther等，2012；Lunt等，2017）。本模式的边界条件与海表温度取自全耦合的LP与PETM FV 2°分辨率CESM1.2.2（Community Earth System Model Version 1.2，社区地球系统模型1.2版本）的输出结果，该结果的时间分辨率为月尺度，时长为1800年。

CAM5模式的输出结果时间分辨率涵盖6小时、日尺度与月尺度，模拟时长超过20年。本实验分别采用古新世晚期二氧化碳浓度680ppmv（下文简称LP）与PETM时期二氧化碳浓度1590ppmv（下文简称PETM）两组参数进行模拟。所有模拟实验的甲烷浓度均固定为16ppmv。此外，为检验轨道强迫的影响，本研究分别采用中性轨道与北半球夏季日照最大化的偏心率构型（即高偏心率、近日点位于北半球夏季，下文简称OrbMax）开展模拟。太阳强迫的计算采用1355 Wm⁻²的太阳常数，与Kiehl等（2018）的设定一致。因此本研究的四组模拟实验分别记为LP、PETM、LP OrbMax与PETM OrbMax。

各研究区域的古地理位置设定为2°×2°的范围，参数取自DeepMIP协议（Lunt等，2017）。东大西洋区域（EMA）设定为北纬34.5°~36.5°、东经0°~2°；西大西洋区域（WMA）设定为北纬41°~43°、西经49°~51°。为扣除模式达到平衡所需的时间，本研究仅选取20年模拟时长的最后15年数据进行分析。

本研究关注的参数包括月降水与径流的中位数、第一及第三四分位数，用以表征年际与季节变化；以及超阈值频率，用以表征风暴强度与风暴事件发生频率的变化。超阈值频率的计算公式为P=m/(n+1)，其中P为超阈值频率，m为某一事件的排序位次，n为总事件数。