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VARIATIONSUS CLIVAR VARIATIONS CUS CLIVAR Lim Ity a Bil Te V Cta Ariability & Predi VARIATIONSUS CLIVAR VARIATIONS CUS CLIVAR lim ity a bil te V cta ariability & Predi Summer 2016 • Vol. 14, No. 3 Probing the Past for Recent natural variability of the Iceland Keys to the Future Scotland Overflows on decadal to Guest Editor: millennial timescales: Clues from the ooze K. Halimeda Kilbourne U. Maryland Center for 1 2 Environmental Science Ulysses S. Ninnemann and David J. R. Thornalley 1 The Atlantic Meridional Overturning University of Bergen and Bjerknes Centre for Climate Research, Norway Circulation (AMOC) has a profound 2University College London, United Kingdom impact on the climate system. But how AMOC has behaved in the past and how it will evolve in the future could be better addressed with longer observational records. For example multidecadal variability in Atlantic climate may be linked ow variable is the Atlantic Ocean’s Meridional Overturning Circulation (AMOC)? to AMOC intensity, but the observational record of AMOC and HThis question has become increasingly important in recent years due to the theoretically linked variables, such ocean’s influence on climate and potential impacts on future climate development. as the Inter-Tropical Convergence The northward flow of warm near-surface waters and southward flow of cooler Zone position or SST, are not long enough to establish the connection deep water comprising the AMOC redistributes a significant amount of heat within in the system. Evidence of these the Atlantic basin (Johns et al. 2011). This influences regional temperature and variables from natural archives of rainfall patterns (Enfield et al. 2001; Knight et al. 2006), including those on adjacent Earth’s past may provide a way forward without having to wait continents, and helps to ameliorate the human impacts of fossil fuel burning multiple decades or centuries for by absorbing CO2 from the atmosphere and transporting it into the deep ocean the observational record to become (Sabine et al. 2004). long enough. Natural archives, such as marine Despite its importance, our understanding of the overturning is far from complete, sediments, ice cores, cave deposits, including its natural variability on various timescales and its sensitivity to increased and biogenic calcium carbonate radiative forcing or surface warming and freshening—each of which appear (coral skeletons, bivalve shells, foraminifera tests, etc.), may contain quantifiable physical, chemical, or biological variables that respond to IN THIS ISSUE changes in the natural environment such that they can serve as a proxy Recent natural variability of the Iceland Scotland Overflows on decadal to millennial timescales: Clues for some desired variable. These from the ooze ........................................................................................................................................... 1 paleo proxies are widely used to Potential for paleosalinity reconstructions to provide information about AMOC variability ......................... 8 reconstruct Earth’s climate history The potential for the long-lived bivalve Arctica islandica to contribute to our understanding of past AMOC over geologic and historic timescales – ideally overlapping with and dynamics ................................................................................................................................................ 13 extending the instrumental record. The AMOC over decades to centuries: A workshop recap from May 2016 ............................................. 19 1 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 1 This issue of Variations contains imminent. This uncertainty is apparent when models are used to predict the AMOC articles describing how some response to future conditions. Collectively, the models show AMOC declining of these proxies have been and could be used to reconstruct past when forced by expected changes over the coming century, but there remains a AMOC variability. The goal is to large spread in the individual simulations—ranging anywhere from small changes share among the modern and paleo to more than a 50% reduction when forcing is strong (Schneider et al. 2007; Cheng climate communities the potential information available to address et al. 2013). Because of this, various communities are racing to better understand AMOC-related questions on and constrain AMOC. decadal to centennial timescales. A fundamental factor is time The best way to determine AMOC variability is through direct observation. And after resolution. Ocean sedimentation a decade of dedicated efforts, the RAPID program has shown that AMOC is highly rates are on average 1 mm/1000 variable across a number of timescales, including indications of a long-term decline years, providing little material to work with. One way around (Robson et al. 2014), even in the deeper southward flowing components (Smeed the resolution problem is to et al. 2014). Meanwhile numerous modeling studies have simulated multidecadal use sediment cores from high AMOC variability (Delworth and Mann 2000; Knight et al. 2005) and suggest a link deposition areas. Ninnemamn et to the multidecadal climate swings felt throughout the North Atlantic basin over al. describe some of the successes and difficulties of using coarser the 20th century, termed Atlantic multidecadal variability (AMV). AMV appears to grain sizes from sediment drift be a persistent feature of the climate system with evidence that it occurred over at deposits, which have relatively high least the last 1500 years (Gray et al. 2004; Mann et al. 2009; Svendsen et al. 2014), sedimentation rates, to provide estimates of bottom water flow rates if not the last 8000 (Knudsen et al. 2011). Empirical support for AMV-related ocean associated with Iceland-Scotland circulation changes has been missing. Could the recent AMOC decline be akin to Overflow Water. Thurmalai and what caused these past climate swings or is this something new—perhaps forced Richey propose several sites around the Atlantic basin where deposition by anthropogenic changes? Without extended records depicting the decadal rates are high and argue that variations in ocean circulation it is difficult to place these current trends in context reconstructing salinity fields from and test the idea that AMOC has played a role in generating or persisting lower a network of sites could provide valuable information about AMOC frequency climate swings. variability. Another way around the resolution problem is to glean In order to address these questions paleoceanographers have been coring into the information about past climates from long-lived, annually laminated seafloor mud and ooze that accumulates over time at the seabed. The recovered carbonate producing organisms. layers are analyzed to portray past ocean conditions and water mass properties, Wanamaker et al. describe the such as temperature, salinity, ventilation, nutrient contents, or geostrophic potential for Arctica islandica clam shells to provide information about transport and relative vigor of the currents. Traditionally, these archives have past AMOC circulation, through been used to study climate and ocean circulation changes over millennia, such 14C as a tracer of vertical mixing or as those associated with glacial cycles (cf. review by Stieglitz et al. 2007). In part through SST reconstruction. this is because the sediments normally only accumulate a few centimeters every thousand years, limiting the time resolution possible. The approach that best approximates the AMOC estimates provided by RAPID are the paleo-geostrophic US CLIVAR VARIATIONS estimates based on cross-Atlantic density reconstructions, which have been used Editors: Mike Patterson and to portray glacial interglacial changes in overturning (Lynch Stieglitz et al. 2007 and Kristan Uhlenbrock references therein). Therefore, ideally, a cross-basin geostrophic approach would be applied at even higher frequencies, but the need for a fairly dense network of 1201 New York Ave NW, Suite 400 USWashington, CLIVAR Project DC 20005 Office sites with appropriate resolution and time control represents a serious challenge 202-787-1682 | www.usclivar.org for depicting sub-centennial AMOC changes. © 2016 US CLIVAR 2 US CLIVAR VARIATIONS Given the interest in constraining the role of the ocean in Longer records tracking decadal variability in ISOW higher frequency climate variability, paleoceanographers bottom flow are now starting to emerge. These both are actively hunting for those rare archives capable extend and largely support the earlier results. Figure 1 of even higher fidelity to help bridge the gap between (panel b) shows a new six century long record of bottom the low frequency changes observed in many paleo flow produced by Mjell et al. (2016) together with the records and the shorter (and much more complete and two century record of Boessenkool et al. (2007). The dynamically better understood) changes captured by new record has higher mean grain sizes and greater modern observations. Bridging this gap may be beneficial variability suggesting the site, which is shallower and to to both communities—providing historical context for the north (Figure 1a), may be more strongly influenced modern changes, while the overlap with the modern by ISOW. Remarkably, despite their distant locations record can provide the necessary calibration period and independent age models, the two locations exhibit for moving toward a more quantified and mechanistic similarly timed multi-decadal variations in bottom flow understanding of the proxy signals.
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