Improved clouds over Southern Ocean amplify Antarctic precipitation response to ozone depletion in an earth system model
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Improved clouds over Southern Ocean amplify Antarctic precipitation response to ozone depletion in an earth system model David P. Schneider1,2 · Jennifer E. Kay2,3 · Jan Lenaerts3 Received: 8 January 2020 / Accepted: 21 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Increasing precipitation on the Antarctic Ice Sheet (AIS) in a warming climate has the potential to partially mitigate Antarctica’s contribution to sea level rise. We show that a simple, physically motivated change to the shallow convective cloud phase in the Community Earth System Model (CESM)—improving a long-standing bias in shortwave cloud forcing over the Southern Ocean—leads to an enhanced response of precipitation when the model is forced with realistic stratospheric ozone depletion, with other radiative forcing remaining constant. We analyze two ozone-forced ensemble experiments with the CESM version 1.1: one using the standard version of the model and the other using the cloud-modified version. The standard version exhibits a precipitation increase on the AIS of 34 gigatons y ear−1; the cloud-modified version shows an increase of 109 Gt year−1. The cloud-modified version shows a more robust, year-round poleward shift in the westerly jet and storm tracks, which brings more precipitation to the AIS, compared to the standard version. Greater surface warming and larger-amplitude stationary waves further increase the Antarctic precipitation response. The enhanced warming in the cloud-modified version is explained by larger positive shortwave cloud feedbacks, while the enhanced poleward jet shift is associated with a stronger meridional temperature gradient in the upper troposphere—lower stratosphere. These results illustrate (1) the sensitivity of forced changes in Antarctic precipitation to the mean state of a climate model and (2) the strong role of atmospheric dynamics in driving that forced precipitation response. Keywords Antarctic ice sheet · Precipitation · Shortwave cloud feedbacks · Structural uncertainty
1 Introduction An estimated 6 mm of sea level equivalent, or over 2000 Gt, falls as snow on the grounded Antarctic ice sheet (AIS) and its major ice shelves each year. As such, any significant change in snowfall on the AIS, the dominant component of the surface mass balance (SMB), will significantly affect global sea level. In a warming climate, Antarctic snowfall is expected to increase due to the higher moisture holding capacity of the atmosphere with warmer air * David P. Schneider [email protected] 1
Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
2
Cooperative Institute for Research in Environmental Sciences, University of Colorado – Boulder, Boulder, CO, USA
3
Department of Atmospheric and Oceanic Sciences, University of Colorado – Boulder, Boulder, CO, USA
temperatures, and the accelerated hydrologic cycle. Despite a modest warming trend over the Antarctic continent since the 1950s (e.g. Steig et al. 2009; Nicolas and Bromwich 2
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