PAGES Magazine articles

Hubertus Fischer1 and Eric W. Wolff2

European Science Foundation Conference - Ötztal, Austria, 27 May - 1 June 2012

Figure 1: Schematic diagram showing the major modes of climate variability and how they are likely to change in the future. The high-latitude modes have already undergone significant change over the past century. Trends in the tropical modes (ENSO, Indian Ocean Dipole IOD) have been detected in the more recent climatological record (from England 2011).

Assessing the ongoing climate warming demands a detailed understanding of global and regional climate variations as well as the capacity to forecast future changes using climate models. While greenhouse gas forcing affects climate globally, it is the regional changes that have a direct impact on individual welfare and societies. Moreover, spatially representative climate modes and teleconnection patterns offer the means to contrast rather coarsely-resolved climate models to point-wise paleoclimate data.

Accordingly, modes of climate variability and their biogeochemical impact were the subject of this conference. The first session provided the background on the definition of modes and teleconnection patterns based on observational evidence. Moreover, the physical context of atmospheric and oceanic teleconnections by advection and wave propagation was provided.

Session 2 focussed on tropical climate variability. Latest results show that the ENSO impact is not confined to the tropical Pacific but that hydrological changes and wave propagation transfer energy to other ocean basins. However, representation of ENSO in global climate models is still not satisfactory, hampering progress in making predictions. Past monsoon intensity was another discussion point. It appears to be highly controlled by the Atlantic Meridional Overturning Circulation (AMOC) and concurrent changes in the Intertropical Convergence Zone.

Completely different boundary conditions for modes of climate variability prevailed during the last glacial, which was the topic of session 3. Model studies show that the topography of ice sheets has a strong control on the teleconnection patterns in the North Atlantic and also likely in the Southern Ocean. Moreover the phase relationship between ice sheet retreat, greenhouse gases and ocean circulation shows that climate changes in the Southern Ocean slightly preceded the CO2 increase over the transition, while increasing CO2 accelerated ice sheet loss in the northern hemisphere and led climate changes in the North.

Extratropical teleconnection patterns were the subject of session 4. Reconstructions show that the North Atlantic Oscillation (NAO) also has far-field connections to adjacent regions. The discussion showed that field reconstructions are more desirable for NAO analysis compared to two-point indices. For the Southern Ocean region the Southern Annular Mode (SAM) is the most important teleconnection pattern. Latest results show that both ozone and greenhouse gases can change the location of the southern jet stream and the Southern Ocean westerlies. The lack of a sufficient representation of the stratosphere/troposphere coupling in many models is a major caveat for the model response of the SAM to climate changes.

Session 5 raised the discussion that changes in the ocean circulation modes such as the AMOC and the Antarctic Circumpolar Current can also cause strong changes in inter-hemispheric heat transport and atmospheric CO2 levels. During the glacial, proxy evidence shows that a less-ventilated, carbon enriched water mass prevailed in the Southern Ocean extending into the deep North Atlantic as well as into intermediate waters. Changes in both the southern westerly winds and in the AMOC are able to disrupt this water mass, bringing old CO2 back to the surface. Model studies also show that changes in the AMOC lead to rapid hemispheric responses in climate and the hydrological cycle, which are essentially synchronous with the shut-off of the AMOC.

Changes in the AMOC also have strong impacts on tropical and boreal wetlands and, thus, methane emissions as discussed in session 6. Another influence of changing modes on biogeochemical cycles is the control of the southern westerly wind belt on dust mobilization and transport in the Southern Ocean region, which also strongly affects marine bioproductivity. Ecological studies show that export production can be significantly enhanced by iron fertilization. However, this is not always the case – it depends on complex ecological interaction within the trophic chain and the competition between calcareous and silicious plankton groups.

In addition to invited lectures and poster sessions, several interactive discussions were organized to identify the major gaps and stumbling blocks in research on modes and teleconnection patterns and potential solutions. The conference demonstrated unequivocally that trans- and interdisciplinary research is required to move forward in this field of strong societal importance.

acknowledgements

This Conference was co-sponsored by PAGES, the EPICA Descartes Prize, the Oeschger Centre for Climate Change Research (University of Bern, Switzerland), and the German Research Foundation through the project INTERDYNAMIK (University of Bremen, Germany).

affiliations

1Climate and Environmental Physics and Oeschger Centre for Climate Change Research, University of Bern, Switzerland; hubertus.fischerclimate.unibe.ch

2British Antarctic Survey, Cambridge, UK

references

England MH (2011) In: Richardson K, Steffen W, Liverman D (Eds) Climate Change: Global Risks, Challenges and Decisions, Cambridge University Press, 33-35

Polychronis C. Tzedakis1, J.F. McManus2, D. Raynaud3, D.A. Hodell4, L.C. Skinner4 and E.W. Wolff5

Department of Earth Sciences, University of Cambridge, Cambridge UK, 2-5 July 2012

Examination of the paleoclimate record reveals a large diversity among interglacials in terms of their intensity, duration and internal variability, but a general theory accounting for these differing characteristics remains elusive (Tzedakis et al. 2009). This has provided the impetus to attempt a comprehensive comparison of interglacials of the last 800 ka BP within the context of a PAGES Working Group on Past Interglacials (PIGS).

An initial PIGS workshop held at Bernin, France in October 2008, laid out the themes to be addressed at each of three subsequent workshops. A second workshop, held at the University of the Aegean on the Island of Lesvos, Greece in August 2009, examined intra-interglacial variability and the deglacial onset of interglacials. The third workshop, held at Lamont-Doherty Earth Observatory of Columbia University, USA in October 2010 focused on the duration of interglacials and the ensuing glacial inception. The fourth workshop, held in Cambridge, UK in July 2012, focused on how well we can explain the diversity of interglacials from the forcing and feedbacks. The meeting brought together 35 scientists from 10 countries (including nine postdoctoral investigators and two PhD students), representing the marine, ice core, terrestrial and modeling communities.

Figure 1: Differences in interglacial intensities in relation to the Mid-Brunhes Event (MBE; ~430 ka BP). (A) Percent biogenic silica in composite sequence BDP-96 from Lake Baikal, SE Siberia (Prokopenko et al. 2006); (B) Planktonic δ18O record from ODP Site 983, North Atlantic (Channell et al. 1998; Channell and Kleiven 2000); (C) Atmospheric CO2 concentration in Antarctic ice cores (Lüthi et al. 2008); (D) δD composition of ice in the EDC ice core (Jouzel et al. 2007); (E) Mg/Ca deep-water temperatures from ODP site 1123 on the Chatham Rise, SW Pacific (Elderfield et al. 2012).

The first theme of the workshop was on interglacial intensities, and more specifically, whether the distinction between "cooler" and "warmer" interglacials before and after the so-called Mid-Brunhes Event (MBE) ~430 ka BP, respectively, identified in Antarctic temperatures, CO2 concentrations and benthic δ18O records (Jouzel et al. 2007; Lüthi et al. 2007; Tzedakis et al. 2009; Fig. 1C-D) was evident in other records. A review of the evidence showed that a pre- and post-MBE distinction was also observed in deep-water temperature reconstructions (Elderfield et al. 2012), but not in some regional marine and terrestrial records, including speleothems, faunal and floral temperature reconstructions and lake sediment records (Fig. 1A-B). This raises questions about the global significance of the MBE and the extent to which Antarctic temperatures are representative of global temperatures during the pre-MBE interglacials.

The second theme of the workshop revolved around the duration of interglacials. More specifically, it examined the sequence of climatic events at glacial terminations and inceptions and considered whether any emerging patterns could be identified. This was followed by a discussion of whether differences in the duration of interglacials can provide insights into climate forcings and feedbacks that are relevant to the onset and end of interglacials. It was proposed that the broad duration of interglacials may be determined by the phasing of precession and obliquity and the history of insolation, rather than the instantaneous forcing strength at inception (Tzedakis et al. 2012).

The third theme considered whether models are capable of “explaining” the diversity of interglacials, and what they imply in terms of rules and processes. Finally, interglacials were placed within the wider context of glacial-interglacial cycles and the extent to which these are deterministic was debated.

The meeting ended with a session summarizing the discussions and planning a community paper that will develop the major themes considered over the course of the project. To this end, a writing-workshop, involving a focus group is taking place in March 2013.

affiliations

1Department of Geography, University College London, UK; p.c.tzedakisucl.ac.uk (p[dot]c[dot]tzedakis[at]ucl[dot]ac[dot]uk)
2Lamont-Doherty Earth Observatory of Columbia University, New York, USA
3Laboratoire de Glaciologie et Géophysique de l’Environnement, Grenoble, France
4Department of Earth Sciences, University of Cambridge, UK
5British Antarctic Survey, Cambridge, UK

selected references

Full reference list online under: pastglobalchanges.org/products/newsletters/ref2013_1.pdf

Elderfield H et al. (2012) Science 337: 704-709

Jouzel J et al. (2007) Science 317: 793-796

Lüthi D et al. (2008) Nature 453: 379-382

Tzedakis PC et al. (2009) Nature Geoscience 2: 751-755

Tzedakis PC et al. (2012) Climate of the Past 8: 1473–1485

Anne de Vernal1, E.W. Wolff2 and R. Gersonde3

1st Sea Ice Proxies (SIP) Working Group workshop, Montréal, Canada, 7-9 March 2012

Figure 1: Sea ice edge as a productive environment (photograph from the Southern Ocean and provided by Claire Allen, British Antarctic Survey, UK).

Sea ice is a complex parameter that is difficult to reconstruct from indirect observations. While climate scientists often refer to sea ice as a purely physical parameter, geoscientists reconstruct past sea ice assuming it plays a role in the biogeochemistry of seawater, and thus on primary productivity and trophic structure of the planktonic populations (e.g. Meier et al. 2011). Moreover, whereas climatologists and modelers examine sea ice at hemispheric scale, geoscientists make reconstructions from coring sites where small-scale processes may obscure larger-scale sea ice behavior relevant to the climate system. Nevertheless, geoscientists have unique tools to contribute to the understanding of long-term sea ice dynamics by providing pictures of past sea ice states. This is the overarching objective of the PAGES Sea Ice Proxies (SIP) working group, which was created in 2011.

To achieve the objective of documenting sea ice in the paleoclimate system with the best possible coverage and accuracy, an assessment of each proxy and the development of multi-proxy approaches are both necessary. During the first workshop, scientists with physical, chemical, and biological backgrounds met to assess the reliability and use of sea ice indicators recovered in marine sediments and ice cores, and the robustness of calibration with instrumental data. The geographical and temporal ranges of application of the different proxies were also considered.

Sea ice proxies include chemical tracers in ice cores such as methanesulfonic acid and sea salt, which relate to regional circum-ice-cap sea ice extent (Röthlisberger and Abram 2009). Most sea ice proxies, however, consist of biogenic remains recovered from marine sediment such as diatoms, foraminifers, ostracods and dinocysts, as well as the IP25 biomarker (a C25 mono-unsaturated hydrocarbon). Because productivity in sea ice environments mostly occurs close to the ice edge in spring and summer, most biogenic proxies relate to the occurrence of seasonal sea ice. It is more difficult to quantify the seasonality of the ice extent, although diatom and dinocyst assemblages yield information about the yearly extent of the sea ice cover in the Southern Hemisphere (e.g. Crosta et al. 2004) and Northern Hemisphere (e.g. de Vernal et al. 2008), respectively. IP25 and related biomarker indices offer great promise for reconstruction of sea ice (e.g. Belt et al. 2007; Müller et al. 2011), but large-scale calibrations are still needed and the available data suggest primarily regional relationships. Another difficulty is the identification of multiyear ice because of the extremely low productivity of such environments. However, the occurrence of an ostracod species, parasitic of amphipods living in perennial sea ice environments, may lead to inferences about multiyear ice (Cronin et al. 2010). The shell of Neoquoboquadrina pachyderma, which is the only planktonic foraminifer species found in sea ice environments, may yield an isotopic signature providing clues on sea ice production rates (Hillaire-Marcel and de Vernal 2008).

Each sea ice proxy has limitations and uncertainties. Diatoms have allowed circum-Antarctic sea-ice extent reconstructions, but limitations remain where the signal is affected by opal dissolution. Other uncertainties come from the relationship to sea ice that is often indirect, as in the case of dinocyst, foraminifer and ostracod assemblages. In addition, taxonomical heterogeneity of populations in space may be related to endemism or to the development of genotypes having different ecological affinities, which make each biogenic proxy applicable mostly at a regional scale. Hence the Arctic-subarctic and circum-Antarctic have to be considered as distinct sea ice ecosystems with very different biogenic characteristics.

Reconstructing past sea ice is a challenge, which has to be addressed based on proxies offering complementary local to regional information on sea ice occurrence. The SIP Working Group will publish a special issue of Quaternary Science Reviews entitled “Sea ice in the paleoclimate system: modeling challenges and status of proxies” in 2013. The next step is to combine results with their respective uncertainties for multi proxy data integration and hemispheric scale sea ice reconstructions of Holocene and Last Glacial Maximum time slices. This will be the focus of the July 2013 rendezvous of the SIP Working Group in Cambridge, UK.

affiliations

1GEOTOP, Université du Québec à Montréal, Canada; devernal.anneuqam.ca

2British Antarctic Survey, Cambridge, UK

3Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven, Germany

references

Belt ST et al. (2007) Organic Geochemistry 38: 16-27

Cronin TM et al. (2010) Quaternary Science Reviews 29: 3415-3429

Crosta X, Sturm A, Armand L, Pichon JJ (2004) Marine Micropaleontolology> 50: 209-223

de Vernal A et al. (2008) In: E. Weaver (Ed) Arctic Sea ice Decline: Observations, Projections, Mechanisms, and Implications, AGU Monograph Series 180: 27-45

Röthlisberger R and Abram N (2009) PAGES news 17(1): 24-26

Takeshi Nakatsuka1, Ryuji Tada2 and Kenji Kawamura3

Despite its long tradition, Japanese paleoscience was not well represented in the scientific world until recently. This may be partly due to Japan’s geographical location at the eastern rim of the Eurasian continent, far away from the scientific hotspots of Europe and North America. In addition, numerous publications were written in Japanese and have not yet been translated.

To strengthen the contact with the Japanese paleoscience community and to highlight the diversity of Japanese paleoresearch, a PAGES Regional Workshop was held in Nagoya, Japan in 2010, alongside the PAGES Scientific Steering Committee meeting. That workshop resulted in the idea of a dedicated special section of the PAGES newsletter. The following thirteen science highlights showcase to the global community a cross-section of Japanese contributions to paleoscience.

The North Pacific Ocean around Japan was for a long time one of the most under-researched areas in paleoceanography due to the very deep waters and the resulting scarcity of calcareous microfossils in the sediments. However, by taking advantage of recent progress in sediment coring technology (e.g. through the IMAGES program), Japanese scientists now have access to many new sediment cores. For example, Okazaki et al. used cores from sea mounts and continental slopes to demonstrate that deep-water ventilation occurred in the North Pacific during the deglacial as it does today in the North Atlantic. Recent studies in the western North Pacific Region (Harada et al., Nagashima and Tada, and Yamamoto) also show that the western North Pacific and its marginal seas are well suited to reconstruct millennial-scale climate variability.

Climate in East Asia, including Japan, is characterized by the strong Asian summer and winter monsoon, resulting in a meridional “green” belt extending from the equator to the subarctic uninterrupted by any major mid-latitude desert. The monsoon signal is well preserved in the sediments of Lake Biwa, one of the most studied lakes in Japan (Takemura). At the northwestern coast of Japan, the strong seasonality intrinsic to the monsoon has led to the formation of distinct varve layers in the brackish lake sediments of Lake Suigetsu. Detailed counting of varve layers up to 150 ka by Nakagawa et al. revealed details of climate variations over the entire last glacial-interglacial cycle. Moreover, with several hundreds of 14C data of plant fragments in the dated varve layers, Lake Suigetsu is now becoming a new international standard for 14C calibration.

Asian summer monsoon often favors dense forests where it is difficult to extract significant paleoclimate signals from tree-ring width. Alternatives are presented by Sano et al. and Watanabe et al. They indicate that oxygen isotope ratios of tree-ring cellulose and oxygen and carbon isotope ratios in stalagmites are good proxies for past hydroclimate in the humid tropical-subtropical regions of Asia. Finally, data from glacial ice cores in high mountains (Fujita and Sakai) and from coral cores in subtropical islands (Suzuki) help elucidate past changes in Asian monsoon dynamics.

Japan has a very long history and a unique cultural heritage. Japanese paleoscientists have been utilizing many precious cultural artifacts, such as documentary information, to reconstruct past variations in climate and environment. One of the significant features in Japanese culture is that, until the 19th century, most buildings in Japan were made of wood. The construction timber can be recovered from old buildings or excavated at archeological sites, and utilized for high-resolution climate reconstructions. When coupled with Δ14C and δ18O analyses, these studies can even help in elucidating impacts of changes in solar activity on Earth’s past climate (Miyahara et al.).

In Japan, the quantity of historical private and governmental documents and their conservation is remarkable, reflecting the high literacy rates in pre-modern Japan. By assembling numerous weather descriptions in national diary archives, daily meteorological conditions in Japan, including weather charts, have been quantitatively reconstructed for the last 400 years (Zaiki). Historical documents not only describe daily weather but often also local environmental conditions such as deforestation and animal extinction, together with the population’s (political) reactions to those environmental changes. Yumoto has led a unique research project involving historians and archeologists, and reports how the Japanese environment has been managed in the past and what lessons can be learned for the sustainable management of ecosystems.

Compared to Europe and North America, past climatic and environmental changes in the western North Pacific and East Asian regions, and their role in the global system, are still poorly understood. Many interesting challenges remain for Japanese and Asian paleoscientists, together with colleagues from elsewhere, to elucidate the unique climate and environment in this region.

affiliations

1Graduate School of Environmental Studies, Nagoya University, Japan; nakatsuka.takeshif.mbox.nagoya-u.ac.jp

2Department of Earth and Planetary Sciences, The University of Tokyo, Japan

3National Institute of Polar Research, Tokyo, Japan

Yusuke Okazaki1,2, A. Timmermann3, L. Menviel4, M.O. Chikamoto3, N. Harada2 and A. Abe-Ouchi2,5

Sedimentary and modeling evidence presume a major deglacial switch between the sites of deepwater formation in the North Atlantic and North Pacific. These results suggest that the North Pacific may have played a more prominent role in organizing the global ocean circulation and shifting climate regimes than previously thought.

The North Pacific is considered a terminal region of the “Ocean’s Conveyer belt circulation”. Abyssal waters from the south flow into the North Pacific, upwell to mid-depth, mix with surrounding waters, and return south (Schmitz 1996). In this configuration and as a result of mixing, the present North Pacific is characterized by high concentrations of surface nutrients, thus promoting high biological productivity. Today, no deep water forms in the North Pacific in response to surface buoyancy fluxes because the surface water of the North Pacific is not saline and dense enough to trigger deep convection and downwelling (Warren 1983). However, in certain areas, such as the Okhotsk Sea, surface conditions are still favorable to form North Pacific Intermediate Water (NPIW) to depths of about 300 to 800 m (Talley 1993).

Last Glacial Maximum

The glacial Pacific Ocean had two water masses: well-ventilated and nutrient-depleted glacial NPIW above ~2000 m and less-ventilated and nutrient-enriched deep water below ~2000 m (Keigwin 1998; Matsumoto et al. 2002). Compared to today, the NPIW volume under glacial conditions was significantly higher extending down to about 2000 m. Microfossil (Ohkushi et al. 2003) and neodymium isotope data (Horikawa et al. 2010) suggest that the glacial NPIW possibly originated from the Bering Sea. This “stratified” water mass structure of the glacial North Pacific prevented upwelling of nutrient-rich deep waters. Thus, biological productivity in the glacial North Pacific was relatively low (Narita et al. 2002; Jaccard et al. 2005; Galbraith et al. 2007; Brunelle et al. 2010).

Last Glacial Termination

Figure 1: Ventilation age changes based on published radiocarbon data in the western North Pacific between 900 and 2800 m water depths (Okazaki et al. 2010). Reconstructed ventilation change based on the 14C age offset between co-existing benthic (BF) and planktic foraminifers (PF; open diamonds), projection ages (considering atmospheric 14C change; gray circles), and smoothed spline interpolation of averaged BF-PF age offsets and projection ages (blue line). H1: Heinrich event 1.

Figure 2: Zonally averaged simulated radiocarbon age anomalies in the North Pacific between a collapsed Atlantic MOC state and the preindustrial control simulation. Squares indicate projection age anomalies for the deglacial H1 period reconstructed from western North Pacific sediment cores (Okazaki et al. 2010).

Major reorganization of water-mass structure in the North Pacific occurred during the last glacial termination, when a stratified glacial mode transformed to an upwelling interglacial mode. During the early period of the termination between 17.5 and 15 ka BP, the Meridional Overturning Circulation (MOC) in the Atlantic substantially weakened (McManus et al. 2004) due to freshwater forcing by melting icebergs in the North Atlantic (Heinrich event 1; H1). A compilation of sedimentary radiocarbon ventilation records in the North Pacific and freshwater perturbation experiment mimicking a Heinrich event performed with the earth system model of intermediate complexity, LOVECLIM, suggest that deep water formation in the North Pacific extended to a depth of ~2500 to 3000 m during H1 (Fig. 1 and 2; Okazaki et al. 2010; Menviel et al. 2011). The establishment of the Pacific MOC during times of Atlantic MOC weakening could have played an important global role in regulating poleward oceanic heat transport during H1.

During the Bølling-Allerød period (15-13.0 ka BP) and after the Younger Dryas (13-11.5 ka BP), the ocean circulation in the North Pacific resumed to an interglacial mode without deep-water formation, similar to the modern condition. At the beginning of the Bølling-Allerød, productivity in the subarctic Pacific rose rapidly (Crusius et al. 2004; Galbraith et al. 2007; Brunelle et al. 2010) in association with enhanced upwelling and breakdown of the glacial stratification.

The Atlantic MOC was weakened during the Younger Dryas event, but not as much as during H1 (McManus et al. 2004). Oceanic ventilation in the North Pacific during the Younger Dryas appeared to be stronger than that of the Bølling-Allerød possibly responding to the MOC weakening in the Atlantic. However, it is unclear whether an MOC was established or not in the Pacific during the Younger Dryas. 

Role of the Bering Strait

Modeling studies demonstrate that a closed Bering Strait (sill depth 50 m) is required for the build-up and maintenance of higher surface salinity in the North Pacific during Heinrich events (Saenko et al. 2004; Hu et al. 2007; Okazaki et al. 2010), which is a precondition for establishing MOC in the Pacific. The role of the final opening of the Bering Strait between 11 and 12 ka BP (Keigwin et al. 2006) in the transition from the glacial to the modern NPIW regime is still not well understood. 

Perspectives

Different thrusts have to be pursued to further elucidate the effects of North Pacific Ocean circulation changes on global climate change. 

1. Model intercomparison

The establishment and extent of a Pacific MOC following an Atlantic MOC weakening are model-dependent (Chikamoto et al. 2012; Hu et al. 2012). Further model intercomparison studies should be performed to test the robustness of the proposed mechanism for the Pacific MOC set up as well as its extent in the North Pacific. 

2. Error reduction for reconstructed ventilation records

As shown in Fig. 1, reconstructed ventilation data still have substantial errors. This is mainly caused by large uncertainties of the marine reservoir effect in the conversion from radiocarbon age to calendar age. In order to constrain the regional marine reservoir effect, precise age dating for the targeted sample is fundamental. High-resolution magnetostratigraphy and tephra chronology are potential tools for evaluating past regional marine reservoir ages.

3. Reconstruction of the Bering Strait gateway history

The Bering Strait opened and closed numerous times during the last glacial cycle (e.g. Brigham-Grette 2001). However, the detailed history is not reconstructed yet, but would provide new insights on the impact of this gateway on past global ocean circulation and climate change.

affiliations

1Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan; yokazakigeo.kyushu-u.ac.jp

2Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

3International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, USA

4Climate Change Research Centre, University of New South Wales, Sydney, Australia

5Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Chikamoto MO et al. (2012) Deep-Sea Research II 61-64: 114-126

Matsumoto K, Oba T, Lynch-Stieglitz J, Yamamoto H (2002) Quaternary Science Reviews 21: 1693-1704

Menviel L, Timmermann A, Timm OE, Mouchet A. (2011) Quaternary Science Reviews 30: 1155-1172

Okazaki Y et al. (2010) Science 329: 200-204

Saenko OA, Schmittner A, Weaver AJ (2004) Journal of Climate 17: 2033-2038

Naomi Harada1, A. Timmermann2, M. Sato1, O. Seki3, Y. Nakamura1, K. Kimoto1, Y. Okazaki4, K. Nagashima1, S.A. Gorbarenko5, L. Menviel6, M.O. Chikamoto2 and A. Abe-Ouchi1,7

Statistical multivariate analysis of a new compilation of alkenone-derived sea surface temperatures from the western North Pacific indicates a coherent mode of millennial-scale variability that is closely linked to the deglacial changes of the Atlantic Meridional Overturning Circulation.

The mid- to high-latitude region of the western-central North Pacific including its marginal seas is an essential area for understanding paleoclimate change across Asia. Surface seawater conditions such as temperature (SST) and salinity (SSS) in this region play a key role in controlling the sinking branch of the Pacific Meridional Overturning Circulation. For instance, the formation of Dense Shelf Water (Martin et al. 1998) on the continental shelf of the northwestern Okhotsk Sea is affected by autumn SST (Ogi et al. 2001), SSS, and the sea-ice extent (Sakamoto et al. 2005). Sea-ice extent has a direct effect on the volume and characteristics of Okhotsk Sea Intermediate Water, a key component of the North Pacific Intermediate Water (Tally and Nagata 1995). Moreover, large-scale changes of temperature and salinity can weaken the stratification in the Bering Sea and lead to the formation of Pacific deep water, as described in Okazaki et al. (2010).

Understanding the drivers of North Pacific surface density changes during the large-scale climate transition of the last deglaciation may provide important insights into an often-overlooked branch of the global ocean conveyor belt circulation and its effect on climate and the global carbon cycle.

Here, we focus on the SST contribution to the surface density changes in the North Pacific. Despite previous efforts to synthesize compilations of deglacial SST (Kiefer and Kienast 2005), gaps still remain in our understanding of SST variations in sinking and subduction regions in the mid- to high-latitude western-central North Pacific. We present a new compilation of SST reconstructions for the last deglaciation derived from the UK’37 -index of alkenones (Brassell et al. 1986; Prahl and Wakeham 1987) from the western-central North Pacific. We discuss how these SST responded to millennial-scale variability in the North Atlantic during the last deglaciation, including Heinrich Event 1 (H1, 17.5-14.6 ka), the Bølling-Allerød period (B-A, 14.6-12.8 ka), and the Younger Dryas (YD, 12.8-11.5 ka) through associated changes in the atmospheric circulation.

Interpretation of alkenone SST

Figure 1: Sediment core locations in the western–central North Pacific and the Okhotsk and Japan Seas. Numbers indicate the coring sites as in Harada et al. (2012). Arrows show the average direction of flow of surface waters, with red and blue arrows indicating warm and cold currents, respectively. EKC, East Kamchatka Current; WSAW, Western Subarctic Water; ESC, East Sakhalin Current; SWC, Soya Warm Current; TSWC, Tsushima Warm Current; OKIZ, Oyashio–Kuroshio interfrontal zone; WSG, Western subarctic gyre.

Alkenone-SSTs were reconstructed from 19 sites in the North Pacific (Fig. 1). Alkenone-SSTs are likely to exhibit seasonal biases towards early summer to autumn in the Okhotsk Sea (Seki et al. 2007) and western-central North Pacific (Harada et al. 2004; Minoshima et al. 2007) but represent more evenly weighted near-annual mean SST in the Sea of Japan (Ishiwatari et al. 2001). In a sediment-trap study in the western North Pacific (40-50°N), Harada et al. (2006) found that the season of the maximum alkenone export flux varied from the beginning of summer to late autumn, and the export period corresponded to the period when stratification had developed in the surface-subsurface layer. The light-limitation depth is also critical for alkenone producers (Harada et al. 2006). Thus calm conditions and high surface–subsurface light intensity are important for alkenone producers, and their main growing season might shift depending on when adequate conditions for their active growth occur. The high adaptability of alkenone producers might have often caused seasonal biases for alkenone-SST depending on the conditions at the coring locations. 

Pacific-Atlantic SST linkages

An empirical orthogonal function (EOF) analysis was conducted for alkenone-SSTs of the interval 22-8 ka BP at 14 sites (Figs. 2A-2D; Harada et al. 2012) to extract the common dominant features of spatio-temporal variability from all the records.

Figure 2: Empirical orthogonal function (EOF) analysis of alkenone-derived temperatures throughout 22-8 ka BP. A) First principal component [°C] corresponding to EOF1 pattern. B) Second principal component [°C] corresponding to the EOF2 pattern, along with estimates of North Atlantic meridional overturning circulation (McManus et al. 2004). C) EOF1 and (D) EOF2 spatial patterns [unitless]. The color scale indicates the loading of the EOF pattern.

The dominant EOF mode (EOF1) is characterized at most core sites in the northwestern Pacific by a cooling trend from 20 to 14 ka BP and a subsequent warming trend from 14 to 8 ka BP (Fig. 2A). Note that the EOF contribution to the SST evolution at any core site is obtained by multiplying the principal components with the EOF pattern loading (Fig. 2C) at this site. A possible scenario is based on an independent EOF analysis from longer, but spatially more constrained alkenone-SST datasets (Harada et al. 2012), suggesting that the first EOF follows the precessional cycle of autumn insolation at 45°N, with increased insolation during the LGM and minimum insolation around 13 ka BP. These results suggest that the alkenone data track autumn temperature variations throughout the entire analysis period, and that warming during the fall season can be explained by strengthened surface stratification.

The second EOF mode (EOF2) clearly captures the main deglacial warming signal at most of the core sites as well as the millennial-scale variability associated with temperature and ocean circulation changes in the North Atlantic. It shows a distinct minimum during H1, a rapid increase, concomitant with the B–A transition, as well as the YD. The correspondence to a North Atlantic ventilation proxy, based on 231Pa/230Th isotope ratios (Fig. 2B) demonstrates a clear linkage between changes in the North Atlantic overturning circulation and Pacific climate variations. The spatial EOF2 pattern is dominated by two cores (from sites 6 and 11, purple and deep red colors, respectively in Fig. 2D) having an opposite pattern. During H1 and according to the EOF2 reconstruction the waters near the location of site 11 would have cooled and those near the site 6 would have warmed. This opposite temperature pattern between southern and northern sites may relate to a south-north migration of the Kuroshio/Oyashio front. This pattern may be evidence of the heat convergence toward the north by the intensified Pacific Meridional Overturning Circulation (Okazaki et al. 2010). Wind-induced transport near the western boundary affected by an intensification or shift of the Aleutian Low might be another possible factor. An intensified Aleutian Low causes also a northward migration and a strengthening of the subtropical gyre, thereby warming the waters overlying site 6 and turning heat away from the area of the site 11.

Our analysis revealed a distinct pattern of millennial-scale variability in the western North Pacific that correlates well with millennial-scale climate variations in the North Atlantic. Overall, our study suggests that multivariate data analysis of core compilations can help to identify the dominant patterns of variability and provide important insight into the driving mechanisms of variability on a range of timescales.

affiliations

1Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Japan; haradanjamstec.go.jp

2International Pacific Research Center (IPRC), University of Hawaii, USA

3Institute of Low Temperature Science, Hokkaido University, Japan

4Institute for Advanced Study, Kyushu University, Japan

5V.I.II'ichev Pacific Oceanological Institute, Far East Branch of Russian Academy of Science, Russia

6Climate Change Research Centre, University of New South Wales, Australia

7Atmosphere and Ocean Research Institute, The University of Tokyo, Japan

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Brassell SC et al. (1986) Nature 320: 129-133

Harada N et al. (2012) Deep-Sea Research II 61-64: 93-105

McManus JF et al. (2004) Nature 428: 834-837

Okazaki Y et al. (2010) Science 329: 200-204

Prahl FG , Wakeham SG (1987) Nature 330: 367-369

Kana Nagashima1 and Ryuji Tada2

We present evidence for millennial-scale changes of the westerly jet path over East Asia during the last glacial period and suggest that the westerly jet plays a critical role in a millennial-scale climate teleconnection between Asia and the North Atlantic.

Masanobu Yamamoto

Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan; myamaees.hokudai.ac.jp

A Holocene temperature record from the Japan margin shows a significant 1500-year period, suggesting the existence of a persistent cycle since the last glacial period.