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Abrupt Cold Events in the North Atlantic Ocean in a Transient Holocene Simulation

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Abrupt Cold Events in the North Atlantic Ocean in a Transient Holocene Simulation Clim. Past, 14, 1165–1178, 2018 https://doi.org/10.5194/cp-14-1165-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Abrupt cold events in the North Atlantic Ocean in a transient Holocene simulation Andrea Klus1, Matthias Prange1, Vidya Varma2, Louis Bruno Tremblay3, and Michael Schulz1 1MARUM – Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Bremen, Germany 2National Institute of Water and Atmospheric Research, Wellington, New Zealand 3Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada Correspondence: Andrea Klus ([email protected]) Received: 25 August 2017 – Discussion started: 25 September 2017 Accepted: 16 July 2018 – Published: 14 August 2018 Abstract. Abrupt cold events have been detected in nu- 1 Introduction merous North Atlantic climate records from the Holocene. Several mechanisms have been discussed as possible trig- Holocene climate variability in the North Atlantic at differ- gers for these climate shifts persisting decades to centuries. ent timescales has been discussed extensively during the past Here, we describe two abrupt cold events that occurred dur- decades (e.g., Kleppin et al., 2015; Drijfhout et al., 2013; ing an orbitally forced transient Holocene simulation us- Hall et al., 2004; Schulz and Paul, 2002; Hall and Stouffer, ing the Community Climate System Model version 3. Both 2001; Bond et al., 1997, 2001; O’Brien et al., 1995; Wanner events occurred during the late Holocene (4305–4267 BP and et al., 2001, 2011). North Atlantic cold events can be accom- 3046–3018 BP for event 1 and event 2, respectively). They panied by sea ice drift from the Nordic Seas and the Labrador were characterized by substantial surface cooling (−2.3 and Sea towards the Iceland Basin as well as by changes in the −1.8 ◦C, respectively) and freshening (−0.6 and −0.5 PSU, Atlantic Meridional Overturning Circulation (AMOC). The respectively) as well as severe sea ice advance east of New- sea ice proxy IP25 (Belt et al., 2007) and diatom-based sea foundland and south of Greenland, reaching as far as the surface temperature (SST) reconstructions from a sediment Iceland Basin in the northeastern Atlantic at the climaxes core north of Iceland show evidence for abrupt sea-ice and of the cold events. Convection and deep-water formation in climate changes (Massé et al., 2008). During the Little Ice the northwestern Atlantic collapsed during the events, while Age (AD 1300–1850) several cold intervals at multi-decadal the Atlantic Meridional Overturning Circulation was not sub- timescales have been identified in the Northern Hemispheric stantially affected (weakening by only about 10 % and 5 %, SST records (Crowley and Lowery, 2000), associated with respectively). The events were triggered by prolonged phases the Dalton (AD 1790–1820) and the Late Maunder solar min- of a positive North Atlantic Oscillation that caused substan- ima (AD 1675–1715). Wanner et al. (1995) showed that the tial changes in the subpolar ocean circulation and associated Late Maunder Minimum has been a relatively cool and dry freshwater transports, resulting in a weakening of the subpo- period of approximately 40 years with a larger-than-normal lar gyre. Our results suggest a possible mechanism by which sea ice extent. However, not all North Atlantic cold phases abrupt cold events in the North Atlantic region may be trig- during the Holocene are related to external forcing. For in- gered by internal climate variability without the need of an stance, Camenisch et al. (2016) report that the 1430s has external (e.g., solar or volcanic) forcing. been one of the coldest decades during the last millennium in northwestern and central Europe with a stronger-than-usual seasonal cycle in temperature neither related to anomalous solar nor volcanic activity. Several mechanisms for the development of abrupt cold events in the North Atlantic have been discussed (e.g., Crow- Published by Copernicus Publications on behalf of the European Geosciences Union. 1166 A. Klus et al.: Abrupt cold events in the North Atlantic Ocean ley, 2000; Alley, 2005). These include anomalous input of freshwater (Hawkins et al., 2011; Rahmstorf, 1996), volcanic activity (Sigl et al., 2015), solar forcing (Jiang et al., 2005; Steinhilber et al., 2009; Gray et al., 2010), or a combina- tion of these factors (Büntgen et al., 2011; Jongma et al., 2007). Other causes for abrupt events that have been con- sidered are associated with internal atmosphere–ocean vari- ability (Hall and Stouffer, 2001), sea ice transport (Wanner et al., 2008, and references therein), and sea-ice–atmosphere interactions (Lehner et al., 2013; Li et al., 2005, 2010). An expansion of sea ice could trigger a sudden and extensive change in air temperature by switching off the heat exchange between ocean and atmosphere (Kleppin et al., 2015). Fur- Figure 1. Areas of investigation: northwest Atlantic (NWA) in thermore, an abrupt climate shift can be forced by a “Great green, Irminger Sea (IS) in pink, Iceland Basin (IB) in purple, and Salinity Anomaly” (GSA) – a term coined by Dickson et Nordic Seas (NS) in blue. al. (1988) – describing an event with a major input of fresh- water to the Nordic Seas in the 1960s and 1970s with a in a transient Holocene simulation with the Community Cli- freshening of ∼ 0:3 PSU and a cooling of 1.5 ◦C off the cen- mate System Model version 3 (Varma et al., 2016). Focusing tral Greenland coast (Dickson et al., 1988; Häkkinen, 1999) on multi-decadal timescales we aim at improving our under- impacting the AMOC (Ionita et al., 2016). Hall and Stouf- standing of the mechanisms and feedbacks associated with fer (2001) described a similar event associated with an inten- Holocene cold events and their complex spatiotemporal pat- sification of the East Greenland Current triggered by a high tern. pressure anomaly centered over the Barents Sea that lasted ∼ 40 years. Moreover, internal variability such as a positive phase of 2 Model description and experimental design the North Atlantic Oscillation (NAO) can lead to a tripolar SST pattern in the North Atlantic (Deser et al., 2010; Hur- A low-resolution transient simulation of Holocene climate rell et al., 2013). The surface above the subpolar gyre (SPG) change has been performed with the comprehensive Com- loses energy while the mid-latitudes gain energy as a result of munity Climate System Model version 3 (CCSM3; Varma the associated wind anomalies. The duration of the positive et al., 2016). Detailed information about the model includ- NAO phase is crucial for the response of the ocean (Lohmann ing the source code is available at http://www.cesm.ucar. et al., 2009; Visbeck et al., 2003). During a short positive edu/models/ccsm3.0/ (last access: 23 August 2017). The NAO phase the SPG strengthens due to fast processes asso- fully coupled global climate model consists of four com- ciated with surface fluxes, while it weakens if the phase con- ponents representing the atmosphere, ocean, land, and sea tinues over a longer period because of slow processes (e.g., ice (Collins et al., 2006a). The atmospheric component of transport of saline water or freshwater). To assess potential the CCSM3 is the Community Atmosphere Model version 3 predictability of the NAO and to understand its interaction (CAM3; Collins et al., 2006b). In this model setup we used with the ocean, documentation of the past variability is re- the T31 resolution (3.75◦ transform grid) with 26 unevenly quired. Commonly used single proxy-based NAO reconstruc- distributed layers in the vertical (Yeager et al., 2006). The tions have severe shortcomings since regional teleconnec- ocean component is the Parallel Ocean Program (POP; Smith tions can be nonstationary (e.g., Raible et al., 2014; Schmutz and Gent, 2004) which has a nominal resolution of 3◦ with et al., 2000). Therefore, a multi-proxy NAO reconstruction as a refined meridional resolution of 0.9◦ around the Equator presented by Ortega et al. (2015) can be helpful to adequately and 25 vertical levels. The sea ice component of CCSM3 is represent not just one but multiple typical NAO-related pat- the Community Sea Ice Model version 5 (CSIM5; Briegleb terns, for example, in temperature and precipitation. et al., 2004), which runs on the same horizontal grid as POP. So far, our understanding of climate variability on multi- The model run has been performed at the North-German decadal to millennial timescales is limited and cannot explain Supercomputing Alliance (HLRN2) in Hanover. From a pre- if and to what extent specific regions would be affected and industrial equilibrium simulation (Merkel et al., 2010), the whether rapid cold events are coupled to a substantial weak- model was integrated for 400 years with orbital forcing con- ening of the AMOC. The understanding of multi-decadal ditions representing 9000 years BP (before present) to reach cold events and their triggers will not only lead to a better a new quasi-equilibrium. Afterwards, a non-accelerated understanding of paleoclimatic aspects but could potentially transient Holocene simulation was carried out by forc- also improve predictions of climate change. ing the model with changing orbital parameters until the In the following, we study two spontaneous cold events year 2000 BP (Varma et al., 2016). Greenhouse gas con- in the northern North Atlantic and the Nordic Seas detected centrations and aerosol and ozone distributions were kept Clim. Past, 14, 1165–1178, 2018 www.clim-past.net/14/1165/2018/ A. Klus et al.: Abrupt cold events in the North Atlantic Ocean 1167 at pre-industrial values (CH4 D 760 ppbv, CO2 D 280 ppm, h). In the following, all anomalies are described in compari- N2O D 270 ppbv; Braconnot et al., 2007).
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