RECENT PROJECTS

DEVELOPMENT OF A HIGH-RESOLUTION GLOBAL OCEAN TRACER TRANSPORT MODEL FOR SOLVING OCEANOGRAPHIC, BIOLOGICAL AND ENVIRONMENTAL PROBLEMS

Aims:

Background:

Results:

One important initial task was to run the global tracer transport model in validation experiments using CFCs and bomb radiocarbon. Predicting future climate change and quantifying the ocean carbon cycle requires high-quality global ocean circulation models. Chemical tracers such as CFCs and radiocarbon provide the most stringent test of ocean model skill [England and Maier-Reimer, 2001; England, 1995a]. CFCs have become one of the most widely used benchmarks for ocean climate model performance. Figure 1 below shows the model simulated distribution of CFC-laden North Atlantic Deep Water (NADW) compared to an observed section at a density layer intersecting the core of NADW outflow. The model exhibits excellent agreement with the observed, demonstrating that the off-line model thermohaline circulation is well-resolved in the North Atlantic. Similar comparisons have been made in the Southern Ocean, with model and observed overturning pathways showing good agreement [see Sen Gupta and England, 2004].

An animation of CFC uptake can be viewed by clicking here [warning: .avi file size is 10 Mbytes]. The test of skill using CFCs has enabled the model’s use in studies of the ocean’s thermohaline circulation, which is tied directly to climate via the ocean’s capacity to store and transport heat. Variability and abrupt change in the ocean’s thermohaline circulation has been linked to major re-organisations of the global climate system.

Once the model was assessed for its skill in reproducing observed ocean circulation pathways, numerical model simulations have been conducted for a variety of marine applications, including:-

Simulations of the pathways of flow of Antarctic Intermediate Water and Subantarctic Mode Water have also just been analysed and completed [Sen Gupta and England, 2006]. A preliminary version of the model was also employed to estimate the uptake of anthropogenic CO2 by the oceans [Thomas, England, and Ittekkot, 2001].

Applications:

1. Global distribution of Aurelia aurita jellyfish: a natural or introduced species?

Dispersal of marine larvae by ocean currents has been examined using the global ocean model. This particular study involved an investigation of the global genetic profile of the Aurelia aurita jellyfish by collaborator Mike Dawson, and later an attempt to reconcile this with ocean currents and larval advection pathways. The study appeared in Proceedings of the National Academy of Sciences [Dawson, Sen Gupta, and England, 2005]. Genetic data provided evidence that the species originated in Japan. Genetic data from other sites were less conclusive; suggesting either multiple introductions via ship’s ballast water or natural migration. To investigate this, dispersion of Aurelia larvae was simulated using the off-line tracer model, modulated by biological parameters such as mortality, settlement, and spawning. The model resolves transport across ocean basins and/or around coastlines, the mixing effects of eddies, near-shore processes such as tidal mixing and transient currents. The methodology can be applied to other exotic species, and so the model can be made freely available to interested researchers. The extent that natural oceanic migration can account for the species distribution of the Aurelia aurita jellyfish is shown below in Fig. 2.

2. The potential fate of radioactive leakage from Moruroa Atoll

A second application of the global tracer transport model has been to study the fate of radioactive waste dispersion from Moruroa Atoll (located in the South Pacific near 220°E, 20°S) in the event of a leakage from nuclear weapons testing shafts [Hazell and England, 2003]. A series of experiments were undertaken with hypothetical releases at different times of the year, and during normal, El Niño, and La Niña years. These scenarios were examined because Pacific Ocean circulation varies seasonally, and can vary dramatically during different phases of El Niño, and La Niña, which would affect the pollutant’s dispersal. Radioactive tracer was released into the model’s uppermost level, corresponding to a venting of contaminated material through the atoll structure into the lagoon, or a geological event such as a rock-slide affecting the upper level of the atoll structure. Another scenario examined involved a release over the depth range 360 – 510-m, corresponding to the location of the karst layer - the interface between the basalt foundation of the atoll and the carbonate upper layer. This layer has been suggested as a potential conducting medium for the escape of radionuclides coming from the volcanic formations in which weapons tests were conducted. An example of the simulation of the spreading of radioactive waste from a surface leakage is shown in Fig. 3 [from Hazell and England, 2003], for both an instantaneous and a gradual release (using long-term mean circulation fields). Both release scenarios see the highest concentration of tracer migrate toward Chile, although in the instantaneous scenario significant quantities appear across much of the South Pacific, including around the Australian continental margin. For further interpretation and details, see the publication by Hazell and England [2003] or click here for animations of the tracer dispersion.

3. Pathways of deep and abyssal water spreading in the ocean’s thermohaline circulation

One of the key goals in large-scale oceanography is to determine the rate at which the various layers of the ocean interior are renewed or ventilated by water originating from the sea surface. This is because the ocean’s overturning can act to limit future climate change, either via the uptake of anthropogenic CO2, or by absorbing and redistributing heat from the atmosphere. The tracer transport model has been used to examine ocean circulation over a range of time-scales using model tracers of water-mass “age” and by tagging surface waters with an effective dye tracer that varies geographically [e.g., Sen Gupta and England, 2004; England, 2001; England, 1995b]. The age tracer tracks how long it takes surface waters to ventilate the ocean interior, which, among other things, is relevant to the detection of climate change. The age tracer also gives insight into how long the ocean takes to overturn and renew its deep waters completely. An example of the age diagnostic from the global ocean model is shown in Figure 4, alongside radiocarbon estimates. The diagram shows the age of the ocean’s abyssal waters in the model and estimated from observed radiocarbon. This quantity is central to the detection and attribution of climate change in the ocean’s interior. Seawater age is now widely used to diagnose overturning rates in ocean models and to estimate the uptake of anthropogenic CO2 by the oceans.

4. Pathways of NADW spreading in the ocean’s interior

A transient dye tracer tracking North Atlantic Deep Water (NADW) in a global high-resolution off-line model is shown below in Fig. 5 [from Sen Gupta and England, 2004]. It shows that after 1000 years the Atlantic Ocean is ~ 80-95% ventilated with NADW, in contrast to the slowly ventilated Pacific (~10-20%). This information on ocean ventilation time-scales is critical for understanding present-day measurements. For example, Pacific Ocean deep water sampled today has clearly been formed many thousands of years ago. Using a surface dye tracer we have tracked the gradual penetration of surface waters into the most remote ocean regions, yielding a model estimate of the time taken to ventilate the entire World Ocean: approximately 5000 years.

5. Pathways of Antarctic Bottom Water spreading in the ocean’s interior

Another application of the ocean model is a study of the pathways of Antarctic Bottom Water flow in the abyssal oceans [Sen Gupta and England, 2004; refer to Fig. 6 below]. In the diagram, implied pathways of bottom water flow derived from the time evolution of the 1% tracer concentration front in the global eddy-resolving off-line model are shown. Measurements of bottom water flow are sparse, so that some of the abyssal circulation patterns indicated here are shown for the first time.

References:

Dawson M.N., A. Sen Gupta, and M.H. England, 2005: Coupled biophysical global ocean model and molecular genetic analyses identify multiple introductions of cryptogenic species. Proc. Nat. Acad. Sciences, 102, 11968-11973. Reprint.

Hazell, D., and M.H. England, 2003: Prediction of the fate of radioactive material in the South Pacific Ocean using a global eddy-resolving model, J. Environmental Radioactivity, 65, 329-355. Full article or further details and movies

Sen Gupta, A., and M.H. England, 2004: Evaluation of interior circulation in a high resolution global ocean model, Part I: Deep and Bottom Waters, J. Phys. Oceanogr., 34, 2592-2614. Reprint.

Sen Gupta, A., and M.H. England, 2006: Evaluation of interior circulation in a high resolution global ocean model, Part II: Mode and Intermediate Waters, J. Phys. Oceanogr., manuscript under review.