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FRINATEK-Fri mat.,naturv.,tek

North Atlantic Ocean-Climate Variability in a Warmer World

Awarded: NOK 4.9 mill.

In order to better understand possible future ocean changes NOCWARM used the geologic record to establish how variable ocean circulation and climate were during past periods that were warmer than today. NOCWARM published three studies covering the curren t interglacial (Thornally et al., 2013) and the last interglacial (Irvali et al. , 2012; and Galaasen et al., 2014). All three articles focused on the oceans role in natural climate variability and the mechanisms and drivers of the major ocean currents in fluencing climate in the North Atlantic and Europe. In Thornally et al., (2014 ), published in the open access journal Climate of the Past Discussions, the team involving all NOCWARM partners collaborated on a synthesis article that compared paleoclimate reconstructions from the North Atlantic with model simulations to study the variation in the deep water that flows between Iceland and Scotland over the last 11700 years. While the results of previous studies had found conflicting results, this synthesis used an array of 13 core localities to show that a coherent pattern of circulation changes was evident along Gardar sediment drift south of Iceland. Deep ocean circulation was weak prior to 7000 years ago due to freshwater from wasting remnants of glacial ice from the last ice age. After this deep circulaion peaked and then gradually declined due gradual cooling and increased Arctic sea ice influence in the Nordic Seas. Although sea ice is projected to decline the paleoclimatic results, together with mo del simulations, show that under future conditions with high greenhouse gas emissions ocean warming may still result in less deepwater formation and weaker currents. Nil Irvali et al.,(2012) published a second study in Paleoceanography. This study, invo lving all NOCWARM partners, shows that during the last interglacial the subpolar North Atlantic temperatures were up to 5 degrees warmer than today. Furthermore Irvalis work reveals that this warm period was interrupted by an unstable situation with abrup t reorganization in currents that led to cooling and southward transport of cool fresh polar waters. The work also shows close coupling between these circulation changes and the influx of Ice Rafted material suggesting Greenland Ice sheet melting may have played a role in triggering surface ocean changes. These results may have relevance for the future as IPCC scenarios show increased melt from Greenland. Taken together, Thornally et al (2013) and Irvali et al., (2012) suggest that both surface and deep circulation in the North Atlantic are sensitive to warming and freshening: both of which are predicted in the future. Building on this, Galaasen et al ., (2014), published in the renowned journal Science this year, showed that deep ocean circulation in A tlantic underwent several major and rapid reorganization during the last interglacial period when the climate in the North Atlantic was similar to what we expect within this century. This study, which again included the NOCWARM partners, pointed out that we may soon approach the climatic conditions necessary to trigger major changes in the deep sea, which could have consequences for climate and sea level over vast areas. Ocean circulation plays an important role in shaping the current climate and these re sults force us to re-evaluate long standing notions how the ocean works during warm periods. While we used to think that deep circulation was relatively stable during warm periods, we now see that this is not the case with large and rapid changes repeate dly occurring when conditions were warmer than today. Galaasens work, which contributes to a better understanding of how ocean circulation varies, is potentially of great societal relevance. The article received over 20 reports in the Norwegian and intern ational mass media. Finally, the project organized a workshop on the west coast in the spring of 2013, with participation from all project teams. The workshop was a success and generated ideas for new articles. There are 5 articles now submitted / revis ed for publication. Four of the articles (Irvali et al. TBS , Galaasen et al. TBS , Mjell et al., in review and Kleiven et al. TBS) are paleoclimatic proxy studies, while one article (Mjell et al . TBS) uses a coupled model to investigate the mechanisms b ehind changes in the deep Nordic Seas overflows. Synnøve Kolstø delivered her master's thesis in June 2013 , Eric V. Galaasen defended his PhD 24.04.2014 and Tor Lien Mjell will deliver his PhD thesis in the end of May. As planned in NOCWARMs disseminatio n plan the project and the results were popularised by H. Kleiven through Generation Green Climate courses for teachers organized by the Ministry of Environment's Climate Awareness campaign. Kleiven, who is a popular outreach speaker, used the research-ba sed popular science stories from NOCWARM research to communicate the ocean 's role in natural climate variability to all levels of society

NOCWARM will provide the extended high-resolution proxy reconstructions of ocean state variables necessary to characterize the fundamental behavior of deep ocean circulation and ventilation on multi-decadal to centennial time scales over the current (Holo cene) and previous interglacial (MIS 5e) periods. These periods represent the most recent and best characterized, yet distinctly different, examples of warmer-than-present conditions in the North Atlantic. Hence, they offer the possibility to compare a nd contrast the climate-ocean system behavior under a range of boundary conditions, including many found in simulations of future climate, such as warmer and fresher surface ocean conditions, increased regional radiative forcing, reduced sea ice, and rapi d Greenland ice sheet wasting. Utilizing recently recovered sedimentary sequences we will reconstruct past variations in the transport and properties of the lower branches of the Atlantic Meridional Overturning Circulation (AMOC) at key locations in the North Atlantic during the Holocene and MIS5e. Initial results reveal both, 1) that the proposed sedimentary archives and proxies capture multidecadal variability in circulation and deep water properties, and 2) that ocean ventilation in warm climate st ates is much more variable than previously appreciated. In addition, we will use these same sediments to reconstruct surface ocean hydrographic variability (SST, SSS, and stratification) and determine its phasing relative to deep ocean changes. Togeth er, these constraints will be used to elucidate the relationship between climate and the lower limb of AMOC over a range of timescales and background states. Finally we will synthesize the results with results from ongoing and available transient GCM mode l experiments, in order to validate model performance and investigate the dynamics of interglacial ocean-climate variability.

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FRINATEK-Fri mat.,naturv.,tek