Rapid and dramatic changes
In order to evaluate the possibility of future rapid climatic change due to changes in the strength of the thermohaline circulation it is important to understand under what circumstances past rapid changes occurred. The Northward (meridional) heat transport in the North Atlantic based on WOCE sections is estimated to be 0.65 PW (MacDonald, 1998). In a general sense it is recognised that this is connected with the thermohaline circulation in which NADW is formed in Polar and Sub-Polar regions and subsequently flows southward as a deep western boundary current in the Atlantic. Paleoceanographic observations indicate that overturning in the North Atlantic/Nordic Seas has been close to the modern strength for the last 10.000 years, although possibly punctuated by millennial scale perturbations of moderate strength (Bianchi and McCave, 1999). Beyond this period the paleoceanographic record indicates the possibility of rapid reorganisations, shifting modes of deep overturning (Dokken and Jansen, 1999), which were accompanied by high amplitude changes in SST and atmospheric temperature in the Circum-Atlantic and probably globally. Models also support this possibility. While the present overturning is strong, leading to mild climates over the North Atlantic and Northern Europe, the North Atlantic could also be dominated by cold and fresher water masses akin to the present North Pacific. Different regimes of this kind have been reported from experiments with a full suite of ocean and climate models ranging from simple models to complex fully coupled ocean-atmosphere GCMs (e.g. Stocker et al, 1992, Marotzke and Willebrand 1991, Rahmstorf, 1995, Mikolajewicz et al. 1997, Manabe and Stouffer, 1988, 1997). The advection of salty waters from the subtropics and the meridional density gradient which drives the deep southward flow and the near surface poleward heat flow can be perturbed by changes in the surface freshwater balance and temperature. Such changes may arise from variations in precipitation, evaporation, runoff and sea ice formation, advection and melting. All these processes are likely to change in the future. These processes are also possibly influenced by different teleconnections. Model results indicate that the THC has multiple equilibria, implying the existence of threshold values and possible irreversible behaviour. Major changes in overturning strength may be triggered by changes in the surface buoyancy balance, and there are strong indications that such thresholds may be within the range of simulated climate changes of the 21st century. Although abrupt THC changes have been modelled and described and documented from paleo-observations, the processes and mechanisms involved are to a large extent still unknown. The thresholds appear to be highly model dependant, the paleo-records are inconclusive as to the degree of buoyancy forcing which triggered them and the degree to which the changes were forced by changes in low to high latitude fluxes or by the freshwater balance of the high latitude alone. A better understanding of the mechanisms behind rapid changes depend critically on improved detailed knowledge of how the elements of the system interact before, during and after the changes. Recently, it has been documented that the Northern and Southern hemispheres were out of phase during major re-organisations of climate. Both from ice cores and ocean records, there are strong indications that the climate at high latitude Southern Hemisphere leads the changes observed at the Northern Hemisphere by as much as 1,000 to 2,000 years. The millennial scale asynchrony of the Northern and Southern hemispheres during the last glacial period implies that the climate system acts as a bipolar seesaw (Broecker, 1998). Stocker (1998) suggests a simple mechanical analogue that depicts the ocean behaviour as a seesaw driven by either high-latitudinal, or near-equatorial sea surface perturbations, or by both these disturbances. Recent modelling suggest that the leads and lags between the hemispheres can be explained entirely by the variability of the meridional overturning and by the corresponding change in the oceanic heat transport (Seidov and Maslin, submitted). We here propose a combined observation and modelling study, which is aimed at solving some of the problems in our present understanding of rapid changes in the thermohaline circulation. From the observational side we aim by a detailed study of two episodes of abrupt climate change to establish how processes in the surface and deep ocean are linked during rapid changes and how low and high latitude areas are linked. This aims at understanding what the critical forcings are which determine the off and on switches of thermohaline overturning, and the important question whether the switching is driven by changes in low to high latitude changes in heat flux or salinity. As a second step in the second phase of the project we will study mechanisms of thermohaline changes in a coupled ocean-atmosphere model with focus in the N-Atlantic/Nordic Seas. The model will be forced with realistic forcing as evaluated from the observation elements with the aim of investigating key elements of signal propagation, threshold values and critical regions. The two above mentioned intervals in the paleo-record selected as targets for the study in task 1 carries important information which in turn may improve our understanding how climate may be triggered to shift from one mode to another. The oldest interval to be studied, Heinrich 4 (H4), took place about 40,000 years before present. H4 belongs to a set of major climatic cold reversals identified in the North Atlantic region during the last glacial, referred to as Heinrich events. These cold reversals appear to be organised into quasiperiodic cycles following a gradual cooling from moderately warm climate. The cooling cycle into a final H-event is disturbed by frequent perturbations of less dramatic cooling periods, which is culminating into the so-called Heinrich events lasting for about 1,000years. The H-events, representing extreme cooling events with high flux of fresh water added to the ocean, and a dramatic drop in temperature, are followed by a rapid transit to a new phase of relatively warm climate. The second interval to be studied is the Younger Dryas (Y.D) cold period, which is very similar to a H-events, however, the warming prior to Y.D, called the Bølling/Allerød, is higher than in the period before a Heinrich layer, almost representing interglacial condition. The return to cold condition of the Y.D from the incipient interglacial warming, about 13,000 years ago took place within decades, where SST temperatures dropped by 6-8oC. Also the transit out of the Y.D, into the Preboreal (c. 11,500 years ago), was very rapid and took place within less than 5-20 years (Taylor et al. 1993). Although, a weakening of the THC, or a re-organisation, or even a complete stop in thermohaline overturning have been suggested to be the main cause bringing the climate to shift rapidly from one mode to another, the results are somewhat contradictory, especially for the Y.D cold period. During H-events, and especially during H4, there is strong evidence from the North Atlantic and the Southern Ocean that the THC was dramatically reduced or even completely shut off. Some records may even prove that the reduction in thermohaline overturning started long time prior to the H-events. What kind of circulation that existed during the Y.D is more controversial? Some Deep Sea proxy data from the North Atlantic show some reduction in deep water flux during the Y.D (e.g. Boyle and Keigwin, 1996), while other records strongly suggest that the flow operated with full strength (Jansen and Veum, 1992). Results from the Atlantic sector in the Southern Ocean indicate that the NADW flux during Younger Dryas was operating in a mode similar to present day circulation (Charles et al. 1996). If the global THC was perturbed to its glacial mode during the Y.D one might expect to see some anomaly in the records from the Southern Ocean. This suggests that the THC was strong during Y.D while the northward flux of heat to Greenland and the Atlantic sector was not. If the circulation style during H4 and the Y.D was fundamentally different, but still giving somewhat similar global response in climate, these two intervals may give valuable insight in how the climate is responding to changes in ocean circulation. Recent advances in dating specific events, and in the ability of high temporal resolution makes it possible to link rapid changes and the various sub-systems, surface and deep ocean in high detail Our studies will via integration with material and work provided by our international co-workers enable us to link high latitude regions of the Nordic Seas, and the Northern North Atlantic with low latitude regions, and a possibility to study variability of surface water with changes in the vigour of deep water flow and ventilation of the deep North Atlantic along the flow axis of NADW.
The overall objective is to investigate in high detail i.e. (multi)-decadal resolution two transitions where subsystems can be tied together with dating and where low to high latitude linkage and phasing can be studied. Specific objectives to be studied:
·What are the forcing, linkages, and feedbacks which produce abrupt millennial-scale climate change?
·What is the magnitude of forcing which produce switches of the thermohaline circulation
·What are the surface to deep water linkages and phasing?
·Do the abrupt climatic shifts represent a response to the same forcing, and involve the same climate processes?
·Which are critical regions? Are the abrupt shifts initiated at high northern latitudes, at low latitudes, or at high southern latitudes?
·What is the coupling between ice sheet dynamics and ocean circulation?
·What processes allow abrupt changes to be transmitted from the northern to southern hemisphere or vice versa?
The second phase of the project will involve model experiments aimed at studies of mechanisms of thermohaline circulation changes in a coupled ocean-atmosphere model with focus in the N-Atlantic/Nordic Seas.
The project is conducted by a consortium of Norwegian institutions, with international collaboration.
Principal investigator for task 1: Trond Dokken, UNIS-Svalbard. Responsible participants from other Norwegian institutions: Prof. Eystein Jansen (UiB), Prof. Morten Hald (UiT), Dr. Nalan Koc (NP), Dr. Helge Drange (NERSC).
UK collaborators: M. Chapman, School of Environmental Sciences, University of East Anglia; J. Dowdeswell, Bristol Glaciology Centre, University of Bristol; I. N. McCave, Dep. of Earth Sciences, Cambridge University; N. Shackleton, Dep. of Earth Sciences, Cambridge University; R. Zahn, Dep. of Earth Sciences, Univ. of Walses, Cardiff .
Other international collaborators: C. Laj, C. Kissel, L. Labeyrie, all
from Laboratoire de Sciences du Climat et lEnvironment (LSCE), Gif
sur Yvette, France; J. McManus, L. Keigwin, Woods Hole Oceanographic institution
(WHOI), USA; E. Boyle, Massachusetts Institute of Technology (MIT), USA;
J. Adkins, CALTEC, USA.
Describe changes in surface water properties and deep water flow across two major phases of abrupt change (Y. Dryas and H-layer 4)
- Estimate the timing and rapidity of abrupt climatic changes
- Resolve possible phase differences between sea surface, deep ocean, atmosphere and ice sheet dynamics during abrupt changes.
- Estimate the leads and lags between low and high latitude changes in the North Atlantic, estimate signal propagation during thermohaline switches.
- Quantify the buoyancy forcing associated with rapid changes.
All core material is available from the start of the project, and no additional coring is required. The Table below indicates the set of cores that is available for this study. Some of the cores in this list have already been studied in great detail, but these cores and others have to be further examined with regard of different proxies to obtain the level of resolution which is required for solving the objectives. We have chosen cores with sedimentation rates higher than 20cm/1000years. In most cases the sedimentation rates are far higher, which enable us to study climatic changes with a resolution less than 50 years, but often with a resolution down to 3-5 years.
Table – A selection of available cores for the study organised by region.
Dating, time control and variation in the sedimentation rate
Precise dating is prerequisite for high resolution climate studies. This study will depend on a high number of AMS 14C dates (more than 100) to establish a reliable chronology. However, radiocarbon ages are not giving absolute ages in the sense of enabling an exact match of events from one region to another.
To be able to study the phasing of signals between records from different regions and from different climatic systems (air-sea, reflected by marine cores and ice cores), we need to rely on different stratigraphical markers that tie different records to each others. Such markers may be ash-layers and certain physical and chemical properties that are monitored in the record. We will use paleomagnetic properties in the record to establish the paleointensity in the cores. Laj and co-workers in press) have recently established a paleointensity record for the North Atlantic and the Nordic Seas. They found a marked intensity low at 40ka, corresponding to the directional anomaly of the Lachamp event, and another low at 34ka, corresponding to the Mono Lake event. These two intensity low periods are well defined within the North Atlantic and the Nordic Seas. These events and the specific peaks of cosmogenic isotopes (Be, Cl) that are associated with these lows in the Earth´s magnetic field are important stratigraphical markers that can be used to tie all the records together regardless of other dating techniques.
Sea surface temperature (SST)
Sea surface temperature will be estimated by a number of methods. The multi-method approach will help resolve limitations based on a single method. Foraminiferal based SSTs will be estimated by the SIMMAX, RAM, MAT-techniques. In some cores, diatom SSTs estimates will be performed based on WAPL and CABFAC techniques. Also available for some of the core material are SSTs based on alkenones.
Sea Surface Salinity (SSS)
Sea surface salinity (SSS) estimates will be performed by using the SST estimates to subtract the temperature influence on the planctonic oxygen isotope data, and estimate salinity from the residual.
Sea ice extent, fresh water flux and Arctic and Polar Front placements
Map changes in sea surface gradients, sea ice distribution and fresh water flux based on combined information from planktonic foraminifera, diatoms, dinoflagellates and stable oxygen isotope measurements.
Track Deep Water dynamics, changes in the thermohaline circulation
- Studies of the fine fraction to identify changes in the dynamics of deep ocean currents, i.e. deep current flow strength on sediment drift sites along the flow paths (Gardar Drift).
- d13C studies to identify changes in mode of deep water circulation and ventilation of the deep water
- Based on cadmium to calcium ratio (Cd/Ca) measurements in selected intervals the nutrient content of the deep water masses will be estimated to better be able to evaluate changes in the d13C record and changes in the mode of deep water formation.
Ice Rafted Debris (IRD) ice sheet dynamics.
- Count ice rafted debris (IRD) and calculate flux rate changes, based on U-series measurements in the sediment to identify signals that may be used to track the variability and dynamics of the surrounding ice sheets.
The first phase of the project (2000-2002) deals only with paleo observations of rapid changes. In the second phase a series of model experiments are planned. This phasing is due to the need for establishing a solid observational base which can be used to compare with the modelled responses. The project plans to base these experiments on the newly developed fully coupled global Ice-Ocean-Atmosphere model with stretched coordinates developed at NERSC and GFI/UoB. This model has a focus on the Nordic Seas due to the stretched co-ordinates, and includes an isopycnal ocean model (MICOM), the Arpege atmosphere model, the NERSC sea ice model and the OASIS coupling tool. The model is at the time of writing running in a test run without flux corrections, with very limited climatic drift, and is expected to provide production runs at the end of year 2000. As the second phase of the project commences the model will already have been tested, validated, and long control runs will be in existance. At this stage the plans are tentative, and will be further developed in the proposal and evaluation phase prior to phase 2 of NOClim. Preliminary plans are to compare observed and modelled responses and behaviour during rapid changes of thermohaline circulation by: 1: Forcing model with freshwater anomalies, 2: Force model with extra low to high latitude heat flux, 3. Run model with PMIP (Paleoclimate Model Intercomparison Project) fields and freshwater forcing. 4. Model-data comparisons of timing, leads, lags and modelled paleo-proxy fields.
Subtask 1.1: Sampling, dating and analytical work (2000-2001)
· Selection and evaluation of cores. A variety of marine cores from the IMAGES program and national cores (UiT, UiB, UNIS) are available for the purpose of our study, in addition to cores made available from international partners (see table above). There is no need for further coring to implement the project.
· An early focus on dating (AMS 14C measurements, paleomagnetism, U-series measurements and identification of ash-layers) is necessary for the purpose of this study were we want to put focus on the linkage and phasing of climatic signals and how they are distributed in space. AMS 14C measurements will be made by the Gif AMS laboratory and the Kiel Leibniz AMS laboratory. The magnetic properties in the cores will be measured using the U-channel facilities at the Gif laboratory (C. Laj and C. Kissel). Measurements of Uranium and Thorium for the purpose of sediment flux change calculations will be performed using the ICP-MS facilities at CALTEC, USA.
· Early evaluation of selected cores, age models and preliminary paleo-proxies after 6 months for selection of cores to be studied in ultra-high resolution. (Workshop together with all the national partners)
· Analytical work: Stable benthic and planktonic isotope measurements (UiB, Cardiff, Cambridge, WHOI), IRD analyses (UiT, UNIS, UiB), planktonic foraminifera analyses (UiT, UNIS, UiB), Cd/Ca-measurements (MIT, USA), diatom analyses (NPI), fine fraction analyses (Cambridge, UK).
Subtask 1.2: Parameterisation, correlation and compilation of data (2001-2002)
· Calculation of SST and SSS (responsibility: UiT, NPI, UNIS, UiB)
· Map temperature gradients and sea ice extent (UiT, NPI, UNIS, UiB)
· Flux estimates of IRD, determined the sources and duration of the cold intervals, study potential leads and lags of surges from the Laurentide-, Greenland-, Iceland- and Fennoscandia-ice sheets (Bristol, UiB, UiT, UNIS)
· Study timing and phasing of meltwater plumes associated to H4 and the Younger Dryas. (UiT, UNIS, UiB)
· Map the deep water changes, ie. the mode of deep water formation, ventilation and the dynamics of the deep water flow (UiB, UNIS, Cambridge, Cardiff, WHOI, MIT)
· establishing the phase relationship between the surface and deep-water signals (UiB, UNIS)
· Time synchronous paleo-maps of different paleo-proxies (UiT, UiB, UNIS)
· Map temperature and salinity gradients (Polar Fronts, Arctic Fronts) (UiT, UiB, UNIS, Cardiff, Cambridge).
Table: Summary of main milestones during the project period. Also included are plans for a continuation of the project in 2003 and 2004. This is the period we want to implement the modelling study based on the incoming data during the first period.