THE WORLD OCEAN CIRCULATION EXPERIMENT

• Introduction
• Goals
• B.I.O. Contributions to the WOCE Field Program
• Some WOCE Results From B.I.O.
• The Future of WOCE Related Work

INTRODUCTION

The World Ocean Circulation Experiment (WOCE) is by far the largest, most ambitious oceanographic experiment ever carried out. To-date, it has involved the efforts of more than 40 nations over a period of more than 10 years, making use of several different satellites, dozens of ships and thousands of instruments. These observations have already improved our knowledge of the global ocean, including its present state, its spatial and temporal variability, and its participation in the global climate system. The total contribution of this experiment will not be realized until researchers around the world have had time to digest the enormous amount of information which it has supplied.

The origins of WOCE go back to the late 1970s. Meteorologists have been able to use maps of sea level pressure to infer the strength of the surface wind fields. Maps of sea level elevation would allow oceanographers to infer surface currents in the same way; however, there was no way to measure such fields in the open ocean. The shortlived Seasat mission demonstrated that both sea surface elevation and surface wind stress from space. At the same time rapid advances in computer capability promised to facilitate the ability to model the ocean globally. In 1983, a Scientific Steering Group (SSG) was established to design and implement a global scale oceanographic program -- the World Ocean Circulation Experiment.

GOALS

The formal goals of international WOCE are:

1. To develop models useful for predicting climate change and to collect the data necessary to test them.
2. To determine the representativeness of the specific WOCE data sets for the long-term behaviour of the ocean, and to find methods for determining long-term changes in ocean circulation.

Canada made significant contributions to WOCE both in the North-East Pacific (west coast) and the North-West Atlantic and Labrador Sea as well as improving our understanding of the role of the thermohaline circulation in the global ocean circulation.

Within Canada, funding for WOCE related activities have come primarily from within DFO, the climate change component of Canada's Greenplan Program, the Panel on Energy Research and Development (PERD) and from the Natural Sciences and Engineering Research Council (NSERC). Canadian WOCE activities have been coordinated by the Canadian National Committee for WOCE (CNC-WOCE). CNC-WOCE coordinated the writing of two successful Collaborative Special Project Proposals to NSERC, through which Canadian University participation in WOCE was supported.

BIO CONTRIBUTIONS TO THE WOCE FIELD PROGRAM

Researchers at the Bedford Institute of Oceanography have occupied the repeat hydrography section AR7W nine times (July 1990, May 1991, May 1992, June 1993, May 1994, May 1996, October 1996 and May 1997 and June 1998). This section extends across the Labrador Sea from Labrador to Greenland. The and the repeated occupations are being used to estimate the changes in the temperature, salinity and relative volumes of all the intermediate and deep waters of the North Atlantic. This information is being used to better understand how the thermohaline circulation of the North Atlantic operates and varies.
(Click here for more information on the Labrador Sea)

During 1996/97, the international WOCE community conducted an Atlantic Circulation Experiment in which the sub-polar gyre of the North Atlantic was mapped during the fall, winter and spring of that cooling season. In addition, a large number of autonomous floats and surface drifters as well as moorings were deployed to observe the temperature, salinity and circulation fields between research expeditions. The BIO team mapped the entire Labrador Sea during the October '96 and May '97 expeditions. In addition, the fall '96 expedition deployed a large number of floats, drifters and moorings as part of US-DFO and German-DFO collaborative programs. These deployments set the stage for the winter expedition of RV Knorr to the Labrador Sea.

In the winter of 1996/97 Canadian scientists participated in an international joint meteorological/air-sea interaction/oceanographic experiment in the North Atlantic and the Labrador Sea (FASTEX/Labrador Sea Deep Convection experiments) on board RV "Knorr" of the Woods Hole Oceanographic Institution. The principal objectives were to document the driving mechanisms and the resulting oceanic deep convection in the winter Labrador Sea. As luck would have it a major convection event occurred and the atmosphere and ocean were documented from top to bottom and from shore to shore, using a wide variety of sounding instruments in air and sea and at the interface. It was one of the extremely rare coincidences of a full complement of functioning meteorological and oceanographic and air-sea interaction field measurements combined with an active and interactive modelling effort in both ocean and atmosphere being in place to observe an important "event". The results are only beginning to appear and have generated great excitement in the marine physics and climate research community.

Further south, east of the Grand Banks of Newfoundland, BIO scientists led Canadian, US and German efforts to estimate the heat, salt and volume transports of the Gulf Stream and North Atlantic Current in the region where water is exchanged between the sub tropical and sub polar gyres of the North Atlantic. The Canadian contribution to the project consisted of an array of current meter moorings extending from the eastern continental slope of the Grand Banks of Newfoundland to the centre of the Newfoundland Basin as well as three hydrographic/tracer surveys of the Gulf Stream / North Atlantic Current system from the Tail of the Banks to Flemish Cap. The hydrographic/tracer sections were located, when possible, along the Topex/Poseidon satellite altimeter ground tracks and formed four closed boxes that were designed to allow one to estimate how much water moved from the Gulf Stream and Labrador Current into the North Atlantic Current and its various branches. The eight current meter moorings were deployed in a line extending 390 km offshore from the 2000m isobath off the Grand Bank, cutting across the North Atlantic Current near 43N (WOCE Current Meter Array ACM6) in August 1993 and recovered in June, 1995.

In addition to directly measuring the heat and mass transports of the North Atlantic Current the current meters also measured the transports of deep and intermediate waters carried southward by the Deep Western Boundary Current (DWBC). The University of Rhode Island contributed some additional current meters to the array and also deployed an array of inverted echo sounders (IES) to determine whether these instruments could be used as a much less expensive means of monitoring these transports. During this same period, the Bundesamt füür Seeschiffahrt und Hydrographie (BSH), in Hamburg carried out three occupations of a trans-Atlantic section from the mouth of the English Channel to the Grand Banks of Newfoundland. The western end of this section follows the current meter array. The sections plus the current meter results provide estimates of the total meridional heat and salt flux carried by the North Atlantic circulation across a nominal latitude of 48° N.

A joint BIO-Dalhousie field experiment contributed hydrographic and microstructure measurements on two expeditions to the WOCE control volume AR10 as part of dedicated tracer release experiment. A purposeful tracer was injected into the pycnocline and measured four times over a period or two years by a team from the US and the UK ( Ledwell, Watson and Law) to determine its horizontal and in particular its vertical dispersion. The purposeful tracer release experiment gives a single value of vertical diffusivity but does not examine the physical processes which cause vertical mixing. To this end, measurements were made by the BIO-Dalhousie team (Oakey and Ruddick) to estimate diapycnal diffusivity from small-scale turbulence and microstructure and to compare these results with the integral of mixing in the dye diffusion study and make them applicable to models. The microstructure field program consisted of two experiments, a one month mission on the R/V Oceanus in October- November, 1992 and a second six week mission on the CSS Hudson in April - May, 1993. In both the tracer dispersion studies and the microstructure studies consistent estimates of vertical diffusivity of order 2x10-5 m2/s was observed which is about one fifth of the values traditionally inferred from vertical advective diffusive models.

SOME WOCE RESULTS FROM BIO

The Labrador Sea is one of the few places in the world where water is transformed through cooling into denser water. The dense water sinks, mixing with deeper levels through "deep convection", and forms "Labrador Sea Water". The newly formed Labrador Sea Water flows out and spreads into the North Atlantic Ocean at mid-depths, contributing to the global ocean circulation and the sequestering of gasses from the atmosphere into the ocean. Any greenhouse gasses that are removed from the atmosphere into the ocean will not contribute to global warming until they are returned to the atmosphere - an event which will be delayed by sequestering into the deep ocean. An understanding of the processes associated with deep convection and water mass spreading is thus crucial to an understanding of climate change.
(Click here for more information on the Labrador Sea)

Another part of the Labrador Sea program relates to the Greenhouse Gas question, specifically the buildup of anthropogenic carbon dioxide in the atmosphere and the climate changes predicted as a result of that buildup. The ocean absorbs a large fraction of the carbon dioxide emitted as a result burning fossil fuels and other processes. One of the major sinks of anthropogenic carbon dioxide was suggested to be the Labrador Sea. Profiles of concentrations anthropogenic carbon dioxide in the central region of Labrador Sea mimic those of CFC-11 (Figure 3), whose concentration also has been building up in the atmosphere and hence in the surface of the ocean. When deep convection occurs, these are transported to deeper regions and sequestered for periods of decades to centuries. An initial estimate suggests that the Labrador Sea is sequestering up to 40% of the total amount of anthropogenic carbon dioxide entering the deeper waters of the world's oceans. It is a significant sink, though further work is required to verify this large estimate.
(Click here for more information on CFC-11 in the Labrador Sea)

The Newfoundland Basin work, including the current meter array, has provided good estimates of the oceanic heat and salt flux carried by the Atlantic across 48 N. These estimates are important tests of the ability of coupled ocean - atmosphere climate models to simulate the large scale transports of heat and freshwater under current climate conditions.

Diapycnal mixing alters the temperature and salinity of water masses and this affects the pole-ward transport of heat by the circulation. Numerical modelers of ocean circulation realize that uncertainties about diapycnal mixing remains a major problem with present models. Unless we understand the mechanisms causing mixing in order to properly model the vertical fluxes of heat, salt and other quantities like nutrients in ocean circulation, the changes in circulation which occur as a result of climatic change cannot be confidently assessed. The results of the WOCE AR10 dye and microstructure experiment (called the North Atlantic Tracer Release Experiment, NATRE) have made important contributions to our understanding of the intensity of mixing and its dependence on several different driving mechanisms. The mean rate of mixing in the main pycnocline integrated over two years by the dye injection study was confirmed to be approximately 2x10-5m2/s, nearly an order of magnitude smaller than values typically used in models. (This rate of mixing is extremely small and corresponds to an energy dissipation of about 1 kilowatt per cubic km of seawater.) Microstructure and turbulence measurements conducted twice during the first year of the experiment were consistent with the estimates from the vertical dispersion of the dye. Progress has been made in understanding how to paramaterize the mixing rates so that the results for the tracer can be applied appropriately and confidently to heat, salt, and density, and extrapolated from the experimental sites to the global ocean. We have also shown that vertical diffusivity is responsible for the major source of nitrate which balances biological uptake of nitrate in new production in oligotrophic regions of the ocean such as the are in which the NATRE experiment was carried out.

THE FUTURE OF WOCE RELATED WORK

A major WOCE conference addressing 'Ocean Circulation and Climate' was held in Halifax during the week of May 25-29, 1998. The conference focused on the results obtained from initial analyses of the massive data set obtained during the field program of WOCE, spanning the period 1990-1997. Most of the data from this program was made available to participants through a set of some 18 CD-ROMS. The conference also addressed what can be learned from these initial results regarding planning the next generation of climate experiments. Although the field phase of WOCE is over, there is no doubt that the atmosphere of this conference was one of a program that was entering a new and exciting stage of development. A solid foundation has been laid and researchers (many of whom were not yet members of the oceanographic community when WOCE was first conceived) are anxious to build on it.

WOCE has entered its Analysis, Interpretation, Modelling and Synthesis (AIMS) phase. Formally, this phase is to last for 3-5 years under the WOCE banner, but the data set that has been collected will continue to be used long after this period. Numerous results can be anticipated, including the following.

• A better understanding of the strengths and weaknesses of various numerical ocean models. This is at the heart of WOCE objectives.
• A better understanding of the surface fluxes of heat and salt that drive the climatically important thermohaline circulation. Such improvements in understanding are essential for the further development of coupled ocean-atmosphere-cryosphere-biosphere models that are needed for climate research, and particularly for climate change prediction.
• A better understanding of the mixing across density surfaces. The processes involved are still being unravelled. Parameterizations will be developed/refined for use in climate models which cannot resolve the small scales involved. This is a requirement for the development of reliable ocean models to be used in climate research and climate change prediction.
• A better description of the present state of the global ocean, including tracer fields and dynamical balances.
• Improved documentation of the variability of the water masses of the NW Atlantic and NE Pacific over the last three decades.
• Ocean models and coupled ocean-atmosphere climate models that can be used by the Canadian research community to provide advice to decision makers on the nature and reliability of climate change predictions.

The accomplishments to-date of the World Ocean Circulation Experiment are very substantial and much more is clearly expected over the next few years, but these represent only a few steps along a long road. The distant future is impossible to predict with any accuracy, but there are some obvious possibilities for the immediate future. First, there is a need to continue to work towards a truly global ocean observing system for use in the assessment of global climate change and for the prediction of future change in much the same manner that weather prediction is now routinely performed (albeit with larger uncertainties). Improvements in technology have certainly made this much more feasible than it was just a decade ago. Second, the oceanographic expertise built up during WOCE needs to be carried over into the next stage of understanding and predicting climate change. In particular, we must contribute to a new international program, CLIVAR, focused on improved description, understanding and modelling of climate variability.