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
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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
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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. One initial target has during the period of overlap. This consistency points been the sediments that accumulate rapidly due to the toward a common influence on bottom water flow— lateral transport and focusing by bottom currents. The inferred by both studies to be related to changes in the North Atlantic sediment drifts that accumulate under the flow of ISOW. influence of the deep overflows from the Nordic Seas have generated intense interest, since here the sediment With records spanning many centuries, comparison with influx is itself related to key constituents of the deep limb extended AMV reconstructions becomes possible. The of the AMOC. variations in ISOW have a similar pacing to changes in basin wide climate. At first glance, it appears that periods Natural variability in Iceland Scotland Overflow Water of Atlantic warmth occur when ISOW is strong and cooling Waters overflowing the ridges to the east and west of occurs when ISOW is weak—much as one might expect Iceland are the source of the densest waters contributing if components of AMOC, and associated heat transports, to the deep limb of the AMOC. Approximately half of were spinning up and down on these timescales. this overflow occurs east of Iceland as Iceland-Scotland Mjell et al. (2016) note that ISOW may lag the AMV by Overflow Water (ISOW). Previous reconstructions based 0-20 years, but caution that the uncertainties in the on the mean grain size of the sediments sortable silt ( ), available reconstructions (e.g., in age models) make this reflecting vigor of near bottom flow (McCave et al. 1995), determination of the phasing between ocean circulation document multi-millennial to centennial variability along and climate highly tentative. Determining this phasing will the western boundary guided flow path of ISOW (e.g., help delineate the nature of any climate and circulation Bianchi and McCave 1999; Hoogakker et al. 2011; Kissel et linkage but will require a concerted effort, using the full al. 2013; Thornalley et al. 2013). However, it is difficult to toolbox of conventional dating approaches, and likely achieve higher resolution and close the time gap with the need to be further refined by absolute age markers such period of modern observations. One notable exception as tephra layers and a better understanding of regional is the work of Boessenkool et al. 2007, whose record radiocarbon reservoir corrections through time. revealed, on decadal timescales spanning the past two centuries, subtle changes in bottom flow that seemed Regardless of absolute phasing, at least one major to respond to decadal changes in the North Atlantic component of the deep overturning circulation (ISOW) Oscillation (NAO) index—the major mode of atmospheric may have been persistently varying on multi-decadal variability and an important forcing in the North Atlantic. timescales over the past 600 years. Yet there is growing evidence that AMV could be an intrinsic feature of the
3 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 3 Atlantic climate for at least the past 8,000 years (Knudsen et al. 2011). Thus, if deep water circulation (e.g., ISOW) is linked intrinsically to AMV, one would expect to see evidence for similar persistence in ISOW variability over this same period. New reconstructions at decadal resolution, spanning several millennia (Mjell et al. 2015; Moffa-Sanchez et al. 2015), suggest that multidecadal variability may indeed be part and parcel of ocean circulation during the current warm interglacial (Mjell et al. 2015). Although, the frequency of this variability may vary through time, warranting further investigation into their robustness and possible climate dependence.
Concerted efforts are also being made to understand the role of ISOW in lower frequency climate oscillations. Recent results have revealed a possible coupling between regional climate conditions, deep water formation in the Nordic Seas, and the strength of the ISOW over the Holocene (Thornalley et al.
Figure 1 : (a) Top panel is a bathymetric location map of the ISOW reconstructions referred to 2013; Mjell et al. 2015; Hoogakker in the text (bathymetry from Ryan et al. 2009 using http://www.geomapapp.org). The dense et al. 2011). An important outcome overflows from the Nordic Seas, Iceland Scotland Overflow Water (ISOW), and Denmark of this work is the realization that Strait Overflow Water (DSOW) are schematically illustrated with red and blue arrows. Red and green dots show sites used to reconstruct recent multidecadal changes in near bottom the flow of ISOW migrated vertically flow (bottom panel), after Mjell et al. 2016 and Boessenkool et al. 2007, respectively. Blue through the Holocene (Figure 2), dots mark sites used to reconstruct vertical changes in ISOW through the Holocene (see with the deepest flow occurring Figure 2; Thornalley et al. 2013). (b) Bottom panel shows the reconstructed multidecadal variability in the near bottom flow ( ; μm ) over the past ~600 years indicated by the during the mid-Holocene climatic green (Boessenkool et al. 2007) and red (of Mjell et al. 2016) curves plotted together with optimum, when sea-ice cover in the a proxy reconstruction of Atlantic Multidecadal Variability (black curve, 20-year smooth of Nordic Seas was at a minimum, thus detrended AMV from Gray et al. 2004). While similar variability exists, the exact phasing enabling vigorous ocean convection between bottom flow and AMV is difficult to determine precisely due to the current uncertainties (e.g., age model) in the reconstructions. producing a strong and dense overflow.
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Figure 2: Vertical migration of ISOW. Time-depth contour plot of SS variability (μm) on the South Iceland Rise/Bjorn drift (left panel; modified after Thornalley et al. 2013). The progressive delay in the timing of peak inferred flow speed with increasing water depths has been used to infer a gradual deepening of the main flow path of ISOW during the early-mid Holocene, as sketched in the cartoon (right panel).
Challenges If such vertical migrations accompanied higher frequency used to build confidence when identifying spatial shifts ISOW variations, as a recent modeling study suggests they in water masses. Continuing progress in the calibration could as the density and intensity of overflow at the ridge of the proxy, such as the current effort led by Nick changes (Langehaug et al. 2016), this presents a sobering McCave (University of Cambridge), will also enable more caveat for attempts to metric ISOW based on a single site. quantitative reconstructions to be made (e.g., Thornalley Different locations will have different sensitivities, and et al. 2013). potentially even a different sign of response, to vertical changes in ISOW. Further work will ultimately be required Despite these promising advancements in our to fully elucidate the role of ISOW in multi-decadal, understanding of past ISOW and how to better reconstruct as well as multicentennial, climate events such as the its variability, it is worth remembering that ISOW is Medieval Climate Anomaly and the Little Ice Age (Bianchi only one constituent of the deep limb of AMOC, and and McCave 1999; Oppo et al. 2003; Hall et al. 2004). compensating rerouting of deep flow between different Moving forward it will be critical to move from single interior pathways could occur without a change in total site characterizations to highly resolved, well-aligned, overturning. There are hints that such compensation, at and long depth transects to portray the non-stationarity least in the vigor of the eastern and western overflows in bottom flow. In addition, the newly ventilated deep across the Greenland Scotland Ridge, may have occurred waters in the North Atlantic have a distinct characteristic on centennial timescales (Moffa-Sanchez et al. 2015). in tracers such as δ13C (Olsen & Ninnemann 2010) Modeling and paleo-reconstructions also suggest the relative to ambient deep waters. Co-registered signals of direct coupling between major constituents of the deep current dynamics and water mass ventilation could be limb of the AMOC, which is likely to complicate efforts
5 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 5 to identify the causal mechanisms for the reconstructed underline the importance of long, well-resolved paleo variability. Furthermore, the controls on and response records for understanding the full spectrum of ocean of the individual deep AMOC constituents are likely variability. Existing records have served to demonstrate to vary with timescale, different phase relationships the importance of ocean variability in the climate system; between North Atlantic physical properties (heat/salt), future, more sophisticated efforts, will strive to quantify and atmospheric forcing on annual to decadal timescales its sensitivity to climate and reveal precise forcing versus those occurring on longer timescales (e.g., greater mechanisms. than decadal). These competing influences on ISOW, and the potentially varying timescale of their dominance,
References
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International WCRP/IOC Conference
Regional Sea Level Changes and Coastal Impacts
July 10-14, 2017 | The Earth Institute, Columbia University, New York, NY
The WCRP, jointly with the Intergovernmental Oceanographic Commission of UNESCO, is organizing an international conference on sea level research that will address the existing challenges in describing and predicting regional sea level changes and in quantifying the intrinsic uncertainties.
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7 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 7 Potential for paleosalinity reconstructions to provide information about AMOC variability
Kaustubh Thirumalai1 and Julie Richey2
1University of Texas at Austin 2US Geological Survey, St. Petersburg Coastal and Marine Science Center
he Atlantic Meridional Overturning Circulation Oscillation. Reconstructions of surface and deepwater T(AMOC) is a fundamental mechanism for poleward circulation from highly-resolved sedimentary deposits heat and salt transport, with a critical influence on the stand to overcome the deficit of data required to fully climate patterns of the Western Hemisphere. Climate understand variability in the AMOC system. models forecast a reduction in AMOC in response to increased greenhouse gas levels, however, both Reconstructions of AMOC variability historic and future simulations of general circulation Relatively few paleorecords exist that document models (GCMs) disagree on the magnitude of decadal- decadal-to-centennial scale AMOC variability. Most to-centennial scale AMOC variability (Cheng et al. 2013). paleoceanographic records inferring past AMOC The most comprehensive and continuous observational variability have focused on glacial-interglacial and data that are available to understand AMOC variability millennial timescales (McManus et al. 1999, 2004; Böhm come from recent measurements as part of the Rapid et al. 2014; Henry et al. 2016) due to sample availability Climate Change program (RAPID) array of observing and relatively larger signal-to-noise ratios. These studies systems (Srokosz and Bryden 2015). These observations have utilized proxies including benthic foraminiferal 18 13 demonstrate that variability on sub-seasonal, seasonal, shell chemistry (δ Ocalcite, δ Ccalcite, Cd/Ca, etc.) (Oppo et and interannual timescales as well as the decade- al. 1995; Marchitto and Broecker 2006; Lynch-Stieglitz et long trend is larger than previous observations and al. 2011) and radiogenic isotopes (εNd, 14C, etc.) to trace modeling simulations (Srokosz and Bryden 2015). New past water mass composition (Wei et al. 2016; Jonkers et data continue to kindle debate on natural versus forced al. 2015; Xie et al. 2012; Freeman et al. 2016), and have AMOC variability (c.f. Haine 2016). The brevity of this also used sedimentary ratios of selectively scavenging observational record, spanning slightly over a decade, particles (231Pa/230Th) to infer past circulation changes limits our understanding of AMOC variability on longer (Negre et al. 2010; Lippold et al. 2016). Records that timescales. Thus, there is an imminent need to turn resolve decadal and centennial-scale AMOC variability towards the paleoceanographic record to characterize over the last few millennia are less common because natural, long-term AMOC variability and its links with marine sediments that allow for the application of the the Atlantic Intertropical Convergence Zone (ITCZ), the aforementioned proxies at such sample resolutions are North Atlantic Oscillation, and the Atlantic Multidecadal rare.
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Most inferences regarding past AMOC variability on (Krebs and Timmermann 2007; Wang and Zhang 2013). decadal-to-centennial timescales are generated from Patterns of annual mean surface salinity across the proxies that are potentially sensitive to changes in Atlantic Ocean (Figure 1, left) reveal imprints of prominent deepwater formation in the higher latitudes of the mixed-layer currents, including the Gulf Stream, North Atlantic Ocean (Moffa-Sanchez et al. 2015; Mjell et al. Atlantic current, and the subtropical gyres (Krebs and 2016; Thornalley et al. 2009). However, observations over Timmermann 2007; Mignot and Frankignoul 2004; Curry the past decade from the RAPID Program suggest that the et al. 2003; Reid 1994). Thus, paleoceanographic studies overall net AMOC cannot be reconstructed from a single can seek to exploit this linkage between salinity and spatial component (Srokosz and Bryden 2015). AMOC is flow (Dickson et al. 2002; Curry et al. 2003) in order to driven by both geostrophic and buoyancy components, understand past variability of currents that are integral which may vary in their relative influence on different to Atlantic Ocean surface circulation. timescales (Polo et al. 2014). In other words, the surface geostrophic flow does not necessarily co-vary with Importantly, these spatial patterns can help synthesize deepwater formation on all timescales. This underscores paleosalinity reconstructions from seemingly disparate the importance of also looking at processes upstream of regions into a perspective relevant to circulation and past deepwater formation in the North Atlantic when inferring AMOC variability. Figure 1 (right) depicts the correlation changes in past AMOC variability, including variability in coefficient between zonal surface salinities at 26.5oN mixed-layer currents. (black box; chosen as a measure of meridional SSS
Surface salinity and circulation One approach to investigate surface ocean circulation upstream of deepwater formation in the North Atlantic is to look at patterns of sea-surface salinity (SSS) variability across the Atlantic Ocean (Figure 1). Regional SSS variability is mainly controlled by freshwater fluxes, evaporation, precipitation, and lateral advection (Krebs and Timmermann 2007; Mignot and Frankignoul 2004). In many regions in the Atlantic Ocean however, advection, and hence meridional circulation, controls SSS variability over decadal and longer timescales (Frankignoul et al. 2009; Polyakov et al. 2005). Figure 1: (left) Mean annual sea-surface salinity (SSS) in the Atlantic Ocean; (right) Correlation For example, multidecadal between 10-year-filtered monthly mean SSS at 26.5ºN (black box) and global oceanic SSS, where variability in ocean circulation stippling indicates signficance at the 5% level. SSS data are from the ORA-S4 reanalysis dataset (Balmaseda et al. 2012). By correlating observed (or modeled) surface salinities in the Atlantic can manifest as characteristic Ocean on long timescales, patterns that can help understand past variability in ocean circulation spatial changes in regional SSS may be developed (Goff et al. 2015).
9 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 9 advection) and global oceanic SSS on multidecadal increasingly sophisticated numerical and geochemical timescales (see caption for details) from the ORA-S4 methods that can augment uncertainty quantification 18 dataset (Balmaseda et al. 2012). Here, monthly SSS at each and facilitate robust δ Osw estimates (Tierney et al. point is filtered (10-year-lowpass) to remove interannual 2015; Khider et al. 2015; Thirumalai et al. 2016; Marino 18 variations prior to correlation analysis so as to isolate et al. 2013). The reconstructed δ Osw signal can be long-term variability. A tripole pattern is observed across converted into salinity estimates using a regional salinity- 18 the Atlantic basin, with significant positive correlations δ Oswequation (LeGrande and Schmidt 2006), although (black dots in Figure 1, right) north of 26.5°N and from 15- general practice involves inferring relative paleosalinity 18 25°S, whereas a significant negative correlation occurs variability using the δ Osw itself due to potential non- 18 from 10°S to 26.5°N. The large-scale patterns depict the stationarity of the salinity-δ Osw relationship over relationship between the Atlantic ITCZ and poleward time (Rohling 2000; Rohling and Bigg 1998; Thirumalai salt advection towards the centers of deepwater et al. 2016). For the purpose of decadal-to-centennial formation. Here, the northern tropical Atlantic freshens timescales, stationarity is a reasonable assumption, due to the northward (southward) position of the ITCZ depending on the region of interest (Leduc et al. 2013; whereas salt advection in the north Atlantic indicates Singh et al. 2010; Holloway et al. 2016). Thus, paired Mg/ 18 stronger (weaker) poleward transport and larger (lesser) Ca-δ Ocalcite measurements in planktic foraminifera can interhemispheric heat transport (Schneider et al. 2014; yield insights into past SSS variability throughout the Krebs and Timmermann 2007). This blueprint can be Atlantic Ocean. This methodology can be widely applied used to understand past surface circulation changes, if towards high-resolution investigations of paleosalinity. an appropriate spatial network of paleosalinity records are generated that span the same interval. Confidence There are many regions in the Atlantic Ocean that can in hypotheses centered around the state of meridional potentially yield highly-resolved (decadal-to-centennial circulation during these intervals can be significantly resolution) paleosalinity records including the Gulf of bolstered by the covariance of paleosalinity records. Mexico, Caribbean seas, northern South American slope, Such correlation analyses can be performed on model the Gulf of Guinea, the Carolina slope, and the western output from paleoclimate simulations as well. flank of Europe to name a few. Developing coeval paired 18 Mg/Ca-δ Ocalcite records from these locations can provide Reconstructing paleosalinity information about the surface circulation state of the Direct proxies for salinity are still being developed and Atlantic and hence, the AMOC. For example, a network scrutinized, but paired measurements of Mg/Ca and of paleosalinity records spanning the Little Ice Age or 18 δ Ocalcite of foraminifera have been widely used to infer the Medieval Climate Anomaly, even from these limited 18 18 changes in the δ O of seawater (δ Osw), which varies regions where sedimentation rates are high, can yield as a function of salinity. Using empirically-derived insights into past AMOC variability depending on their 18 relationships that detail foraminiferal δ Ocalcite variability covariance (or lack thereof). One caveat to this approach 18 with temperature and δ Osw variability (e.g., Bemis et al. is the need for sufficient and strict age control when 1998), and another empirical equation that describes comparing phasing between relative high and low salinity Mg/Ca as a function of paleotemperature (e.g., Anand events amongst a suite of sedimentary records spanning 18 et al. 2003), the paired Mg/Ca-δ Ocalcite data can be the Atlantic Ocean, especially on decadal-to-centennial simultaneously deconvolved to obtain exclusive records timescales. 18 of temperature and δ Osw variability (e.g., Thirumalai et al. 2016). Though there are caveats to this methodology Recommendations (namely concerning preservation, sampling, and Though there is a need to develop direct proxies of 18 calibration uncertainties amongst others), there are paleosalinity, paired Mg/Ca-δ Ocalcite measurements in
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planktic foraminifera can provide information on past control points. Despite not being proximal to the centers 18 SSS variability by reconstructing past δ Osw variations. of deepwater formation, each paleosalinity record can There are many regions in the Atlantic basin with high be placed into an Atlantic-wide context by using long- sedimentation rates that can be targeted for paired Mg/ term correlation analyses in observational, as well as 18 Ca-δ Ocalcite paleosalinity reconstructions. Developing modeled, SSS datasets. These correlation maps can be long-term calibration campaigns in these regions, such used as a template to perform model-data comparisons 18 as salinity-δ Osw monitoring and/or utilizing sediment for past circulation changes as well. Furthermore, as traps to build empirical transfer functions, will greatly isotope-enabled GCMs become more common, long- aid in the quantitative refinement of the generated term correlation analyses can be performed directly 18 paleosalinity records. There are also several studies on simulated δ Osw signals. Thus, generating new that have been performed on extant high-resolution paleosalinity records, synthesizing published records, sediment cores that have only investigated either and performing additional measurements on cores 18 downcore Mg/Ca or δ Ocalcite variability in foraminifera from high-sedimentation regions in the Atlantic that (and not both together). Revisiting these cores to can resolve decadal-to-centennial variability will greatly perform the additional paired measurement can improve our understanding of past circulation and increase the spatial coverage of paleosalinity records in AMOC variability. the Atlantic, though some may require additional age
References
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The potential for the long-lived bivalve Arctica islandica to contribute to our understanding of past AMOC dynamics
Alan D. Wanamaker Jr., Madelyn J. Mette, and Nina M. Whitney
Iowa State University
Context and rationale What is Arctica islandica and why is it an excellent Multiple modeling studies suggest that the Atlantic proxy archive? Meridional Overturning Circulation (AMOC) will weaken The marine bivalve A. islandica is the longest-lived in the next 100 years with anthropogenic climate change solitary animal known, with a maximum lifespan of as the result of warming and freshening waters in the greater than 500 years (Figure 1; Butler et al. 2013). Also North Atlantic (e.g., IPCC 2013). This weakening could known as the “Ocean Quahog” or “Mahogany Clam,” this have significant consequences for regional and global sedentary benthic species is widely distributed across the climates. However, these consequences are hard to assess as little is known about the extent to which AMOC strength varies on different timescales, particularly semiannual to decadal, due to limited availability of instrumental records (Srokosz and Bryden 2015).
Recent developments in the field of sclerochronology (the study of accretionary hard tissues of organisms and the temporal context in which they form; see https://isc16. las.iastate.edu/ for conference proceedings from June 2016) have brought the opportunity for more successful paleoclimate reconstructions. High-resolution records from the long-lived, marine bivalve Arctica islandica provide proxies with many advantages over terrestrial Figure 1: (A) The left valve of an Arctica islandica shell. Dashed line and other marine-based proxies. Here we describe the shows maximum growth axis. (B) Photomicrograph of the acetate peel of an A. islandica shell sectioned along the maximum growth axis. applicability and future potential of proxy records from A. (C) Photomicrograph of boxed area in (B), showing annual growth islandica for understanding surface AMOC dynamics on lines and increments (depicted by black bars). (D) Photograph of an annual to centennial timescales over the last millennium. A. islandica shell, embedded in an epoxy block and positioned under a micromill. (E) Photograph, in reflected light, of an A. islandica shell, partially milled for oxygen isotopes. This photograph depicts the boxed region in (B) and (D). Annual growth lines are marked by blue triangles.
13 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 13 continental shelves of the extratropical North Atlantic live-collected shells to be crossdated with dead-collected Ocean, including near locations thought to be influenced shells where there is substantial temporal overlap over by AMOC variability (Figure 2). the lifespans of the individuals, enabling the extension of the growth chronology back in time in the same manner as in dendrochronology (Butler et al. 2013). The absolutely dated growth chronology then serves as a template for geochemical sampling of the aragonitic shell material, which can be sampled at annual to subannual resolution using an automated micromill or sampled directly (i.e., through laser ablation) to obtain various geochemical records (see Schöne et al. 2013 for a review).
The widths of annual shell growth increments in A. islandica are influenced by the environmental conditions in which the organism lived. Seawater temperature and/ or food availability and food quality can explain much of the variance in shell growth (Witbaard et al. 2003; Wanamaker et al. 2009;
Figure 2: Map of the North Atlantic showing modern Arctica islandica distribution Mette et al. 2016). In addition, A. islandica (orange shading; based on Dahlgren et al. 2000). Major North Atlantic surface grows in oxygen isotopic equilibrium with currents are shown with red arrows indicating northward flowing currents and seawater. The oxygen isotopic signature blue arrows indicating southward flowing currents (GS=Gulf Stream, NAC=North 18 Atlantic Current, LC=Labrador Current). Green markers indicate the location of (δ O) of the shell material reflects both the previously published A. islandica master chronologies or growth series (chronol- seawater δ18O (which covaries with salinity) ogies only containing a couple of shells). Pink markers indicate the location of A. and the seawater temperature during islandica master chronologies not yet published. Boxed locations designate the two locations highlighted in the case studies (see text). shell formation (Weidman et al. 1994). A. islandica shells also record ambient seawater radiocarbon (14C) content as they calcify, a measurement that has been used as a Shell growth and geochemical records from A. islandica water mass tracer, allowing inferences of ocean mixing have been applied as paleoclimate proxies since the dynamics (e.g., Weidman and Jones 1993; Scourse et al. early 1980s (Jones 1980). Much like tree-rings, A. islandica 2012; Wanamaker et al. 2012). Records of radiocarbon produces annual growth lines in its shell (Figure 1), with a and oxygen isotopes measured in A. islandica shells can similar ontogenetic trend of decreasing growth rate with thus provide information about ocean current dynamics increasing age. Through a process called crossdating, the and seawater temperature over the length of the shell pattern of the detrended widths of growth increments chronology. can be matched between shells within a population to establish an error-free chronology (Black et al. 2016), The above methods encompass the most established provided that the calendar year of the outermost growth ways A. islandica is used for paleoclimate reconstructions. increment is known. The so-called unique “bar code” However, several emerging techniques using A. of growth at a particular oceanographic site allows for islandica shells, which have the potential to contribute
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to reconstructing additional aspects of paleoclimate, Case Study #1 - Radiocarbon in A. islandica shells are currently being developed. Clumped isotope reveals water mass variability measurements (Δ47) in A. islandica shells have been Wanamaker et al. (2012) was one of the first studies shown to reflect seawater temperatures (Eagle et al. to use the radiocarbon signature in A. islandica shells 2013; Henkes et al. 2013). However, limitations remain to infer surface AMOC dynamics. Using the 1,357-year due to relatively large errors associated with the transfer master shell growth chronology established by Butler et function (±2°C). Ba/Ca ratios measured in A. islandica al. (2013), which included 29 crossdated shells collected shells show potential as a salinity proxy (Gillikin et al. 2006, from 80 meters water depth on the North Icelandic Shelf, 2008; Poulain et al. 2015) as well as an aid in crossdating, 41 radiocarbon measurements were used to estimate the as synchronous peaks are observed within populations ΔR (the local radiocarbon offset from the modeled global (Marali et al. 2015). Recent work also suggests potential ocean) through time. In general, water masses at the for compound-specific δ15N measurements in A. islandica surface are in close contact with the atmosphere for long shells, as is done with corals (Sherwood et al. 2011). δ15N periods (e.g., the Gulf Stream and North Atlantic Current) may contain information pertaining to changes in food and have relatively more 14C content compared to deep source and/or changes in water mass source, providing or ice covered water masses (e.g., North Atlantic Deep insight into ocean current dynamics. It should be noted Water and some Arctic derived currents) that are more that Mg/Ca and Sr/Ca ratios, often used to reconstruct isolated from the atmosphere. These “disconnected” temperature from carbonate material, have produced water masses have relatively less 14C content and appear conflicting results when measured in A. islandica shells. to be older with respect to radiocarbon. Mg/Ca and Sr/Ca are therefore not proven to be a viable technique for reconstructing temperature using this The North Icelandic Shelf is at the forefront of the species (Schöne 2013). convergence between relatively young Atlantic sourced water and relatively old Arctic sourced waters (Figure Why are SST and water mass proxies useful for 2). Wanamaker et al. (2012), therefore, proposed that AMOC? increased (decreased) AMOC strength would bring The above discussion highlights how records from A. more (less) radiocarbon young Atlantic water to the islandica shells can be used to reconstruct seawater region. Because the A. islandica shells are assigned temperatures and ocean circulation dynamics. While absolute calendar ages using crossdating techniques, ΔR much more work is needed, current research provides measurements in these shells can provide information some insight into how the AMOC influences these on the age of the water mass in which the shell was hydrographic properties (e.g., Zhang 2008; Rahmstorf precipitated, as described above. The authors were et al. 2015), guiding the application of A. islandica-based able to reconstruct ages of water masses in the region reconstructions as proxies for various components over the last 1,357 years (the length of the chronology) of AMOC variability. These high-resolution proxy and infer changes in surface AMOC strength. The A. records would greatly contribute toward the goal of islandica ΔR record indicated decreased surface AMOC reconstructing AMOC variability through time. The two strength during the transition between the Medieval case studies below highlight instances where A. islandica Climate Anomaly and the Little Ice Age and a relatively shells have the potential to reconstruct aspects of AMOC weak surface AMOC throughout the Little Ice Age. These variability. Identifying additional locations along the finding are consistent with estimates of Florida Current North Atlantic shelves where seawater temperatures transport (Lund et al. 2006) but contrary to the AMOC and ocean current dynamics are most strongly coupled findings of Rahmstorf et al. (2015) during this interval, to AMOC would provide future targets for A. islandica highlighting the need to establish a more complete sampling and chronology development. spatial network of highly resolved marine proxy records.
15 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 15 Case Study #2 - Reconstructing seawater temperature showed a robust, inverse relationship between annual in the Gulf of Maine shell oxygen isotope values and the Boothbay Harbor To illustrate the potential that A. islandica-based seawater sea surface temperature record (1905-2003; r = -0.71; temperature reconstructions have for investigating p<0.05), as well as the -8 m temperature record from aspects of AMOC variability, particularly on semiannual February to May (1989-2012; r = -0.76; p<0.05; Figure 3). to centennial timescales, we now highlight ongoing work on an A. islandica record from the Gulf of Maine, a semi- How do these proxy relationships with seawater enclosed sea on the western North Atlantic shelf. A 253- temperature in the Gulf of Maine potentially relate to year A. islandica master shell growth chronology was AMOC dynamics? Gulf of Maine hydrographic properties, constructed from 34 shells collected at 38 meters water including seawater temperatures, are influenced by the depth near Seguin Island, located in the western Gulf proportion of relatively warm Warm Slope Water (WSW) of Maine (Figures 2 and 3). This site was chosen in part formed adjacent to the North Wall of the Gulf Stream due to its close proximity to one of the longest seawater to relatively cold Labrador Slope Water (LSW) from the temperature records in North America, from Boothbay Labrador Current that enter the Gulf of Maine through Harbor, Maine. Previous work (Wanamaker et al. 2008) the Northeast Channel (Pershing et al. 2001). Modeling studies suggest that decreasing AMOC strength is associated with a northward shift in the path of the Gulf Stream (bringing it closer to the Gulf of Maine) resulting from the decrease in the transport of the southward flowing Deep Western Boundary Current (a bottom component of AMOC; Pena-Molino and Joyce 2008; Zhang 2008, Joyce and Zhang 2010), thus affecting hydrographic conditions in the Gulf of Maine. Strong correlations between the Gulf of Maine shell growth record with the Florida Current, one of the longest instrumental records of surface AMOC dynamics, support this hypothesis (r = 0.78; p < 0.05; 2-year average, shell growth record lagged by 2-years; Figure 3). Reconstructing Figure 3: (A) The master shell growth chronology (Detrended Growth Index; DGI) is shown with seawater temperatures in the Gulf a bold black line. Individual detrended shell growth records that make the DGI are shown in of Maine using A. islandica shell- color. (B) Biplot between -8 m seawater temperature averaged from February, March, April, and May at Boothbay Harbor and the DGI is shown. (C) Biplot between annual Florida Current based records, therefore, has the Transport and the DGI is shown. Both records are smoothed using a 2-year filter and the DGI potential to reveal multi-decadal record is lagged by 2-years. Only years with every month of Florida Current data were used to centennial variability in AMOC (data available at http://www.aoml.noaa.gov/phod/floridacurrent/data_access.php). strength.
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Future efforts and concluding remarks spring inflow of North Atlantic waters into the North Sea The last decade has seen tremendous growth in the through the Fair Isle Channel, ocean current dynamics field of sclerochronology. A. islandica shell growth that again are potentially related to the AMOC. and geochemical records are now available and still being developed and extended across nearly its entire In order to reconstruct various components of the AMOC geographic range (Figure 2). Many of these regions are on multiple timescales and at multiple locations, other largely influenced by surface AMOC dynamics, including high-resolution, marine-based proxies in the extratropical the Gulf Stream and its extension, the North Atlantic North Atlantic may also complement A. islandica records. Current, the Irminger Current, the Faroe Current, and Glycymeris glycymeris is another long-lived, bivalve other surface branches. The major limitation is the lack mollusk with a wider geographic range than A. islandica. of lengthy instrumental records of such currents with This bivalve has been used for similar paleoclimate which to calibrate the proxies. Perhaps some opportunity reconstructions in UK waters and elsewhere (Reynolds may arise from ongoing work with the RAPID and OSNAP et al. 2013; 2016b). Crustose coralline algae are shallow observing system arrays, which seek to elucidate the marine benthic calcifiers with global distribution and also complexity and connectivity of the AMOC between high- produce annual growth increments. Proxies from this and low-latitudes. archive have successfully been used in reconstructing Arctic sea ice dynamics (Halfar et al. 2013) and Labrador In addition to the research highlighted here, several Current dynamics (Gamboa et al. 2012). other recent A. islandica studies offer the potential to reconstruct aspects of the AMOC. Work by Reynolds et While a strong consensus on the relationship between al. (2016a) uses a millennial-length A. islandica oxygen the AMOC and North Atlantic sea surface temperatures isotope record from the Butler et al. (2013) master shell (i.e., the Atlantic Multidecadal Oscillation; AMO) has yet chronology to examine the influence of external forcing to be reached (Zhang et al. 2016; Clement et al. 2016), coupled with ocean circulation dynamics, potentially basin-wide proxy reconstructions of North Atlantic sea linked to the AMOC, on local hydrographic conditions on surface temperatures may provide the data necessary for the North Icelandic Shelf. The same authors have also hypothesis testing of such relationships when combined compared networks of sclerochronologies to infer AMOC with more established AMOC proxies or model data. variability, a strategy that will become more prominent Once the relationship between the AMOC and the AMO is as the field develops. Chronologies are also currently clarified, sclerochronology-based proxy records of North being developed on the Faroese shelf (Bonitz et al. 2016), Atlantic sea surface or subsurface temperatures will be a region particularly sensitive to large-scale ocean and extremely useful in deducing surface and subsurface atmospheric circulation relevant to AMOC dynamics. AMOC dynamics. Indeed, one of the most established Mette et al. (2016) have observed strong correlations and widely used applications of A. islandica and similar between a far northern Norway combined shell growth proxies, discussed above, is for sea surface and near- and isotope record and North Atlantic sea surface surface temperature reconstruction. Therefore, resolving temperatures, with a spatial pattern mimicking the this problem is of paramount importance. surface current pathway of the North Atlantic Current, a surface component of AMOC. Work by Estrella-Martinez In summary, A. islandica, ideally distributed along the et al. (2016) seeks to employ proxy comparisons with continental shelves in the high-latitudes of the North model data to overcome the lack of instrumental records Atlantic Ocean, is a promising tool to gain insight into near the Fladen Ground in the North Sea. They find strong ocean circulation dynamics, including the AMOC. Proxy relationships between growth increments and winter to records derived from A. islandica can be absolutely-dated
17 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 17 and span centuries to millennia in length. The high- Acknowledgments resolution (sub-annual to annual) nature of this archive and its sensitivity to hydrographic conditions lends This work has been supported by the National Science Foundation (grants 1003438 and 1417766 to ADW). All authors itself to better understanding components of the AMOC contributed equally to this article. system with great detail.
References
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Conference Program and Abstracts, Portland, ME, USA, [Available gca.2012.12.020. online at https://isc16.las.iastate.edu/]. IPCC, 2013: Climate Change 2013: The Physical Science Basis. Butler, P. G., A. D. Wanamaker Jr., J. D. Scourse, C. A. Richardson, Contribution of Working Group I to the Fifth Assessment Report of the and D. J. Reynolds, 2013: Variability of marine climate on the Intergovernmental Panel on Climate Change. Stocker, T. F., D. Qin, North Icelandic Shelf in a 1357-year proxy archive based on G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. growth increments in the bivalve Arctica islandica. Palaeogeogr., Bex and P.M. Midgley, Eds., Cambridge University Press, 1535 pp. Palaeoclima., Palaeoecol., 373, 141-151, doi:10.1016/j. Jones, D., 1980: Annual cycle of shell growth increment formation in palaeo.2012.01.016. two continental shelf bivalves and its paleoecologic significance. 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Reynolds, D., and Coauthors, 2016a: Annually-resolved North Atlantic Wanamaker, A. D., and Coauthors, 2009: A late Holocene paleo- marine climate over the last millennium: The ULTRA series. 4th productivity record in the Western Gulf of Maine, USA, inferred International Sclerochronology Conference Program and Abstracts, from growth histories of the long-lived ocean quahog (Arctica Portland, ME, USA, Available online at https://isc16.las.iastate. islandica). Int. J. Earth Sci., 98, 19-29, doi:10.1007/s00531-008-0318-z. edu/. Wanamaker, A. D., Jr., P. G. Butler, J. D. Scourse, J. Heinemeier, J. Reynolds, D. J., C. A. Richardson, J. D. Scourse, P. B. Butler, P. Hollyman, Eiriksson, K. L. Knudsen, and C. A. Richardson, 2012: Surface A. Román-González, and I. R. Hall, 2016b: Reconstructing changes in the North Atlantic meridional overturning circulation North Atlantic marine climate variability using an absolutely- during the last millennium. Nat. Comm., 3, 899, doi:10.1038/ dated sclerochronological network. Palaeogeogr., Palaeoclima., ncomms1901. Palaeoecol., in press, doi:10.1016/j.palaeo.2016.08.006. Weidman, C. R., and G. A. Jones, 1993: A shell-derived time history of Schöne, B. R., 2013: Arctica islandica (Bivalvia): A unique bomb 14C on Georges Bank and its Labrador Sea implications. J. paleoenvironmental archive of the northern North Atlantic Geophy. Res., 98, 14,577-588, doi:10.1029/93JC00785. Ocean. Glob. Planet. Change, 111, 199-225, doi:10.1016/j. Weidman, C. R., G. A. Jones, and K. C. Lohmann, 1994: The long-lived gloplacha.2013.09.013. mollusc Arctica islandica: A new paleoceanographic tool for the Scourse, J. D., A. D. Wanamaker, C. Weidman, P. Heinemeier, P. Reimer, reconstruction of bottom temperatures for the continental R. Butler, R. Witbaard, and C. A. Richardson, 2012: The marine shelves of the northern North Atlantic Ocean. J. Geophy. Res., 99, radiocarbon bomb pulse across the temperate North Atlantic: 18305-18315, doi:10.1029/94JC01882. A compilation of delta C-14 time histories from Arctica islandica Witbaard, R., E. Jansma, and U. Klaassen, 2003: Copepods link quahog growth increments. Radiocarbon, 54, 165-186, doi:10.1111/j.1502- growth to climate. J. Sea Res., 50, 77-83, doi:10.1016/S1385- 3885.2010.00141.x. 1101(03)00040-6. Sherwood, O. A., M. F. Lehmann, C. J. Schubert, D. B. Scott, and M. Zhang, R., 2008: Coherent surface-subsurface fingerprint of the D. McCarthy, 2011: Nutrient regime shift in the western North Atlantic meridional overturning circulation. Geophy. Res. Lett., 35, Atlantic indicated by compound-specific delta15N of deep- doi:10.1029/2008gl035463. sea gorgonian corals. Proc. Nat. Acad. Sci. , 108, 1011-1015, Zhang, R., Sutton, R., Danabasoglu, G., Delworth, T. L., Kim, W. M., doi:10.1073/pnas.1004904108. Robson, J., and S. G. Yeager, 2016: Comment on “The Atlantic Srokosz, M. A., and H. L. Bryden, 2015: Observing the Atlantic Multidecadal Oscillation without a role for ocean circulation.” Meridional Overturning Circulation yields a decade of inevitable Science, 352, 1527, doi:10.1126/science.aaf1660. surprises. Science, 348, doi:10.1126/science.1255575.
The Atlantic Merdional Overturning Circulation over decades to centuries: A workshop recap from May 2016
K. Halimeda Kilbourne
University of Maryland Center for Environmental Studies
o make more accurate projections of future climate, provide information about past AMOC behavior that Twe must improve our understanding of the Atlantic could address questions about AMOC processes on these Meridional Overturning Circulation (AMOC), its drivers timescales and put recent changes into historical context and its impacts. Modern oceanographic observations (Alley 2007; Denton and Broecker 2008). Doing so most of AMOC have limited record lengths, making it difficult effectively requires coordination and cooperation across to address important processes with timescales of traditional disciplinary boundaries. decades to centuries (Buckley and Marshall 2016). Paleoceanographic data and techniques applied to Two different scientific communities—modern physical Earth’s recent past have a demonstrated potential to oceanographers and paleoceanographers—came together
19 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 19 to work on understanding AMOC over decades to and models provide a tool to explore how such behavior centuries in Boulder, Colorado in May 2016. We had arises. Conversely, if different models have different approximately 60 attendees from nine countries, mechanisms, we can use observations to constrain which including 18 early-career scientists. One third of model might have a more-realistic simulation of the participants indicated in their meeting applications that process. The last 1000 years is a key target period because they were working on the modern system, half were of (i) relatively abundant existing paleoclimate data working on paleoceanographic questions, and the rest to provide information about background climate; (ii) defied such simple characterization. During the meeting, it became clear the group represented four communities—modelers and observationalists working on the modern AMOC system, and modelers and OSNAP Subpolar Array observationalists working on the AMOC Installed 2014 system of the past. Sometimes the lines 53 Moorings were blurry, but there were clear differences 60N in vocabulary, assumptions, and scientific priorities. The discussions led to four main recommendations for progressing toward a better understanding of AMOC.
A consistent framework between models 30N and observations RAPID/MOCA/WBTS A factor limiting progress is the ability 26.5˚N Array Installed 2004 to make valid comparisons between 18 Moorings paleoclimate data, model data, and modern observations. AMOC in models is often EQ based on the zonally integrated circulation in the Atlantic, and modern observational networks have been set up with this in mind. SAMOC/SAMBA 34.5˚S Array Paleoclimate proxy records, on the other Installed 2013 21 Moorings hand, usually reconstruct an AMOC-related 30S variable, such as the vertical mixing in the Proxy Archive Type Labrador Sea. Priority needs to be given Sclerosponge to research and activities that move the Coral communities toward a common standard Sediment core Bivalve for comparisons between observational and 60S model data. 80W 60W 40W 20W 0 A denser network of AMOC and AMOC- related variables Location of some existing paleoceanographic and modern oceanographic AMOC Iterating between observational data and observing systems. Paleoceanographic sites represent the locations of samples/ models can be a powerful tool for improving cores with paleotemperature information archived in the PAGES2k proxy mechanistic understanding of AMOC. temperature dataset version 2.0.0 (PAGES2k Consortium 2016). Observations tell us how the system behaves
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reasonably constrained climate forcing variables; (iii) the forcing factors used to drive models, and in data-model availability of annual or better resolution proxy archives, comparisons. and (iv) the potential to overlap with the instrumental record to provide quantitative proxy calibrations. Such Encourage coordination between scientific communities data-model comparisons require an improved network Coordination between communities may be of proxy records guided by process-based information accomplished at the level of individual researchers or from models, and can be used to reliably characterize larger organizations. Examples of the latter include the past behavior of AMOC, including the frequency and adding a paleoceanography-specific team to the US amplitude of decadal to centennial variability, as well as AMOC or UK RAPID programs; reviving something similar the response in associated environmental variables. to the former PAGES-CLIVAR Intersections program to Improved understanding and communication of focus on targeted workshops; and strengthening links uncertainties between CMIP and PMIP. Cross-disciplinary coordination and cooperation could be facilitated if an effort is made to better quantify and This workshop report is being distributed in both the report the uncertainties of our research. This issue came PAGES and US CLIVAR newsletters as part of this effort up repeatedly in reference to proxy reconstructions to better coordinate between the physical oceanographic and calibrations, data assimilation projects, climate and paleoceanographic communities.
References
Alley, R. B., 2007: Wally was right: Predictive ability of the North Denton, G. H. and W. S. Broecker, 2008: Wobbly ocean conveyor Atlantic "Conveyor Belt" hypothesis for abrupt climate change. circulation during the Holocene? Quat. Sci. Rev., 27, 1939-1950, Ann. Rev. Earth Planet Sci., 35, 241-272, doi:10.1146/annurev. doi:10.1016/j.quascirev.2008.08.008. earth.35.081006.131524. PAGES2k Consortium, 2016: A global multiproxy database for Buckley, M. W. and J. Marshall, 2016: Observations, inferences, and temperature reconstructions of the Common Era. Sci. Data, mechanisms of the Atlantic Meridional Overturning Circulation: A submitted. review. Rev. Geophys., 54, 5-63, doi:10.1002/2015RG000493.
2016 AMOC SCIENCE TEAM REPORT ON PROGRESS AND PRIORITIES
JUST RELEASED!
21 US CLIVAR VARIATIONS • Summer 2016 • Vol. 14, No. 3 21 ANNOUNCEMENTSUS CLIVAR VARIATIONS
Variations Webinar Series: Probing the Past for Keys to the Future
Wednesday, October 12 12:00 PM EDT / 9:00 AM PDT
Featuring authors from this edition: K. Halimeda Kilbourne, U. Maryland Center for Environmental Studies Kaustubh Thirumalai, U. Texas at Austin Alan Wanamaker, Iowa State U.
Call for new US CLIVAR workshops and working groups Workshops
The US CLIVAR program annually sponsors open community workshops, conferences, and science meetings to coordinate, develop, plan, and implement new or focused activities for the benefit of the scientific community and relevant to the goals of US CLIVAR. Workshops can serve as an initiation point in the planning process for future initiatives within the community. Funding is limited and not all submitted workshop requests may be supported.
Working Groups
The US CLIVAR program establishes limited-lifetime, action-oriented Working Groups of scientists to coordinate and implement focused activities for the benefit of the scientific community. This year's call supports the possibility of up to one new Working Group, which will be initiated around January 2017 and undertake actionable and measurable tasks over a 2-3 year period.
Read the full announcement here.
Requests are due by October 7, 2016.
www.usclivar.org US CLIVAR acknowledges support from these US agencies: [email protected]
CUS CLIVAR lim ity twitter.com/usclivar a bil te V cta ariability & Predi US Climate Variability and Predictability (CLIVAR) Program 1201 New York Ave. NW, Suite 400 This material was developed with federal support of NASA, NSF, and DOE (AGS-1502208), and NOAA (NA11OAR4310213). Washington, DC 20005 Any opinions, findings, conclusions, or recommendations expressed in this material are (202) 787-1682 those of the authors and do not necessarily reflect the views of the sponsoring agencies.
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