The freshwater balance of polar regions in transie

Authors: Flavio Lehner Christoph C Raible Dominik Hofer Thomas F Stocker

Publish Date: 2011/10/05

Volume: 39, Issue: 1-2, Pages: 347-363

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Abstract

The ocean and sea ice in both polar regions are important reservoirs of freshwater within the climate system While the response of these reservoirs to future climate change has been studied intensively the sensitivity of the polar freshwater balance to natural forcing variations during preindustrial times has received less attention Using an ensemble of transient simulations from 1500 to 2100 AD we put presentday and future states of the polar freshwater balance in the context of low frequency variability of the past five centuries This is done by focusing on different multidecadal periods of characteristic external forcing In the Arctic freshwater is shifted from the ocean to sea ice during the Maunder Minimum while the total amount of freshwater within the Arctic domain remains unchanged In contrast the subsequent Dalton Minimum does not leave an imprint on the slowreacting reservoirs of the ocean and sea ice but triggers a drop in the import of freshwater through the atmosphere During the twentieth and twentyfirst century the buildup of freshwater in the Arctic Ocean leads to a strengthening of the liquid export The Arctic freshwater balance is shifted towards being a large source of freshwater to the North Atlantic ocean The Antarctic freshwater cycle on the other hand appears to be insensitive to preindustrial variations in external forcing In line with the rising temperature during the industrial era the freshwater budget becomes increasingly unbalanced and strengthens the high latitude’s Southern Ocean as a source of liquid freshwater to lower latitude oceansThe model used for the simulations is the Community Climate System Model version 301 CCSM3 which was developed by the National Center for Atmospheric Research NCAR Collins et al 2006 The model includes four components namely the atmosphere ocean sea ice and land surface all linked through a coupler which exchanges fluxes without flux corrections and state information We selected the lowresolution version 375° by 375° in the atmosphere 26 vertical levels and on land and a sea ice and ocean resolution of about 36° in longitude and 06° to 28° in latitude with finer resolution in the tropics and around Greenland allowing for an open passage through the Canadian Arctic Archipelago The ocean has 25 levels and a rigid lid The land model does not include a dynamic ice sheet model but is able to lose and grow snow and ice up to 1000 kg m−2 Additional mass above this threshold is inserted into runoff We discuss the implications of a missing ice sheet model for our study in Sect 5For this and other studies Yoshimori et al 2010 Hofer et al 2011 a series of model simulations was conducted In this paper we use the following two setups a 1500 AD equilibrium simulation with perpetual 1500 AD conditions hereafter CTRL1500 and an ensemble of four transient simulations from 1500 to 2100 AD TRa1 to TRa4 as in Hofer et al 2011Natural and anthropogenic forcing used in the simulations the combined radiative forcing from GHGs and solar irradiance in W m−2 calculated as in IPCC 2001 with reference to wrt 1990 solid line assuming a global average albedo of 03 and the volcanic forcing as optical depth dashed The inset highlights the changes in forcing by stretching the vertical scale however the curve is the same as in the main plot Further details on the forcing dataset can be found in Hofer et al 2011 The time periods as used throughout the study are indicated by the vertical bars and labeled at the top Initial Conditions IC 1500–1530 AD Maunder Minimum MM 1685–1715 AD Dalton Minimum DM 1800–1830 AD Preindustrial PI 1840–1870 AD PresentDay PD 1960–1990 AD and Future Projections FP 2068–2098 ADOur analysis focuses on six 31year periods with different forcing levels the quasiequilibrium period just after branching from CTRL1500 which corresponds approximately to the beginning of the Little Ice Age Mann et al 2009 when the solar forcing generally is lower than today averaged over 1500–1530 AD hereafter Initial Conditions the period of the pronounced solar forcing minimum of the Maunder Minimum 16851715 AD the Dalton Minimum 18001830 AD when a solar forcing minimum coincides with two large eruptions of tropical volcanoes a period when GHGs start to increase 18401870 AD Preindustrial a period of presentday climate 19601990 AD PresentDay and finally a period at the end of the twentyfirst century when the average radiative forcing is projected to be approximately 5 W m−2 larger than 1990 AD 20682098 AD Future Projections For the twentyfirst century we apply the IPCC SRES A2 scenario IPCC SRES 2000The CCSM3 has been thoroughly tested for various resolutions Hack et al 2005 Yeager et al 2006 Kiehl et al 2006 Bryan et al 2006 Holland et al 2006a b Generally the lowresolution version differs more strongly from observations than the middle or highresolution version This is largely due to the fact that the middle and highresolution versions both run with the higher resolved 1° ocean/sea ice component This has implications for properties such as salinity heat flux sea ice extent and volume as will be discussed herea Arctic and b Antarctic domain’s surface air temperature anomaly with reference to wrt 1961–1990 AD The small panels on the right show mean anomalies for the different time periods Initial Conditions IC 1500–1530 AD Maunder Minimum MM 1685–1715 AD Dalton Minimum DM 1800–1830 AD Preindustrial PI 1840–1870 AD PresentDay PD 1960–1990 AD and Future Projections FP 2068–2098 AD The time series are smoothed by a 30year Gaussianweighted filter Bekryaev et al 2010 updated the GISS dataset Hansen et al 2010 in the Arctic region 20CR is Compo et al 2010 ERA40 data is available from ECMWF in 25° × 25° resolution

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