Drivers of the enhanced decline of land near-surface relative humidity to abrupt 4xCO 2 in CNRM-CM6-1
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Drivers of the enhanced decline of land near-surface relative humidity to abrupt 4xCO2 in CNRM-CM6-1 Hervé Douville1 · B. Decharme1 · C. Delire1 · J. Colin1 · E. Joetzjer1 · R. Roehrig1 · D. Saint‑Martin1 · T. Oudar1 · R. Stchepounoff1 · A. Voldoire1 Received: 14 December 2019 / Accepted: 21 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Projected changes in near-surface relative humidity (RH) remain highly model-dependent over land and may have been underestimated by the former generation global climate models. Here the focus in on the recent CNRM-CM6-1 model, which shows an enhanced land surface drying in response to quadrupled atmospheric C O2 compared to its CNRM-CM5 predecessor. Atmosphere-only experiments with prescribed sea surface temperature (SST) are used to decompose the simulated RH changes into separate responses to uniform SST warming, pattern of SST anomalies, changes in sea-ice concentration, as well as direct radiative and physiological CO2 effects. Results show that the strong drying simulated by CNRM-CM6-1 is due to both fast C O2 effects and a SST-mediated response. The enhanced drying compared to CNRM-CM5 is partly due to the introduction of the physiological CO2 effect that was not accounted for in CNRM-CM5. The global ocean warming also contributes to the RH decline over land, in reasonable agreement with the moisture advection mechanism proposed by earlier studies which however does not fully capture the contrasted RH response between the two CNRM models. The SST anomaly pattern is a significant driver of changes in RH humidity at the regional scale, which are partly explained by changes in atmospheric circulation. The improved land surface model may also contribute to a stronger soil moisture feedback in CNRMCM6-1, which can amplify the surface aridity induced by global warming and, thereby, lead to a non-linear response of RH. Keywords Climate change · Relative humidity · Land · Drying · Drivers
1 Introduction Under enhanced atmospheric CO2 concentration, relative humidity (RH) is expected to remain approximately constant on global and climatological time scales (Held and Soden 2006). Yet, global climate models show regional disparities in projected changes in near surface RH, especially over land where such changes may have policy-relevant impacts on human health (e.g., heat stress) and terrestrial ecosystems (e.g., wildfires). RH is also a key variable for understanding the projected land-sea warming contrast (Byrne and O’Gorman 2013), regional changes in surface evaporation (Laîné et al. 2014), land-ocean shifts in tropical precipitation * Hervé Douville [email protected] 1
Centre National de Recherches Météorologiques, CNRM/GMGEC/AMACS, Université de Toulouse, Météo-France, CNRS, 42 Avenue Gaspard Coriolis, 31057 Toulouse Cedex 01, France
(Lambert et al. 2017), and global changes in vegetation growth (Yuan et al. 2019). According to the fifth assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), there i
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