World Ocean Circulation Experiment Problems of Interbasin Exchanges and Marginal-Sea Overflows John M. Toole1 Submitted as part of a continuing series of reports on WOCE planning activities in the United States 1. Introduction interbasin exchanges, marginal-sea overflows, and technolo- gies thought valuable to the success of WOCE. A summary One of the classic oceanographic problems that the World of the meeting proceedings and recommendations are given Ocean Circulation Experiment (WOCE) will be able to ad- here, the full proceedings will appear as part of United States dress concerns the formation mechanisms of the oceans' WOCE Planning Report No. 5. dominant water masses and their subsequent spread through- out the world's oceans. The formation sites for water-mass 2. Interbasin exchange end members are often mediterranean or partly enclosed seas where distinct water-mass property characteristics are ef- Owing to the different geometries of the world's three oceans ficiently produced because of the relatively small volumes of and the spatial variation in the air-sea exchanges, water-mass these seas and restricted communication with the ocean in- conversions are not identical in the Atlantic, Pacific, and In- terior. The distributions of individual marginal-sea water- dian basins. Global balance of the ocean's heat and fresh- mass characteristics in ocean interiors have proven to be valu- water budgets appears to depend upon active exchange of able diagnostic for the general circulation. Despite the water properties between basins. There exist three avenues variety of water-mass-formation sites on the globe, there ex- for interbasin exchange of waters and/or water-mass proper- ists a strong commonality in the world's oceans water-mass ties. Bering Strait provides a conduit for North Pacific-North characteristics. The relative homogeneity is attributable to Atlantic exchange via the Arctic Ocean. Passages in the In- the active interchange of waters between the oceans. Such in- donesian Archipelago connect the Indian and Pacific oceans terbasin exchange may be thought of as the global adjust- at low latitude. Finally, broad passages are found in the ment of the oceans to differential thermohaline forcing. Southern Ocean between Antarctica and the Southern Hemi- A workshop was convened 2-4 April 1986, at the Woods spheric continents. Quantification of the exchanges through Hole Oceanographic Institution to discuss WOCE plans for these passages and determination of the mechanisms respon- addressing the problems of interbasin exchange and marginal- sible are achievable goals during WOCE. sea outflow.2 Background to the meeting was provided by the series of ocean sector workshops held in 1985 (United States a. The role of model studies WOCE planning report number 2). Attendees were asked to address whether the mix of physical, chemical, and numeri- In spite of the large error bars in present measurements of cal measurements discussed previously were optimal on a heat transport in the ocean, it is well established that ocean global basis for studying the exchange and outflow prob- currents play a significant role in global heat balance. Does lems. Given a meeting of limited size and duration, it was not heat transport by ocean currents vary with time or does it possible to discuss the detailed oceanography of each basin remain static? It is difficult to find experimental design cri- and adjacent marginal sea. Reflecting the interests of those in teria to address this question since an extensive data base for attendance, the workshop focused primarily on the Atlantic diagnosing the thermohaline circulation does not exist. sector. Presentations were made in three general categories: Model results may be useful as "proxy data." Results from an eddy-resolving model with a thermohaline circulation were given as examples. Instantaneous poleward heat trans- port was sampled as a function of time in the model and 1 Woods Hole Oceanographic Institution, Woods Hole, MA compared to the known time-mean transport. Given results 02543. like this, an optimal sampling plan can be designed for which 2 Participants and Contributors. Knut Aagaard, U/Wash/NOAA; the magnitude of errors can be assessed. Kirk Bryan, GFDL, Princeton; R. Allyn Clark, BIO; Lee-Lueng Fu, JPL; Norman Guinasso, US/WOCE; Terrence Joyce, WHOI; Ste- phen Murray, LSU; John Toole, WHOI; D. Randolph Watts, URI; b. Bering Strait Thomas Whitworth III, TAMU; Melbourne Briscoe, WHOI; Harry Bryden, WHOI; Curtis Collins, NSF; Arnold Gordon, LDGO; Wil- The North American and Asian continents are separated by liam Jenkins, WHOI; Michael McCartney, WHOI; James Swift, a narrow ocean passage, Bering Strait, through which recent UWASH; Bruce Warren, WHOI; John A. Whitehead, WHOI. studies indicate a net northward flow of low-salinity water © 1987 American Meteorological Society exists. The transport of this current is so small, however, it 136 Vol. 68, No. 2, February 1987 Unauthenticated | Downloaded 10/04/21 12:56 PM UTC Bulletin American Meteorological Society 137 appears to have little significant contribution to the global d. Southern Ocean budgets of even its most extreme property transports. The The passages between Antarctica and the three Southern principal role of the Bering Strait flow on the large-scale cir- culation appears to be its contribution to the Arctic Ocean Hemisphere continents constitute the largest avenue for inter- stratification, possibly supplemented by its production of basin exchange. Here in the Southern Ocean, the Antarctic dense brines in winter. Given the weakness of the Bering Circumpolar Current (ACC) transports approximately 135 X 6 3 1 Strait throughflow and the apparent insensitivity of the 10 m • s" between the Atlantic, Pacific, and Indian oceans. Atlantic and Pacific circulations and budgets to the through- Anomalous water-mass properties injected into this stream flow, WOCE should not devote extensive resources to moni- in one basin are efficiently carried into the other ocean sec- toring this transport. tors. Measurements of absolute property transports through the three ACC choke points, Drake Passage, south of Africa, c. Indonesian Straits and south of Australia-New Zealand, may provide quantita- tive estimates of these exchanges, shed light on the mecha- The only tropical-latitude link between the world's ocean nisms of ACC interbasin exchange and, furthermore, pro- basins is via the Indonesian Seas and archipelago. The possi- vide information about ACC dynamics. bility of a significant flux of mass, salt, and heat from the Pa- The narrowest constriction of the ACC is Drake Passage cific Ocean to the Indian Ocean basin through this route, between South America and the Antarctic Peninsula. Trans- bears crucially on balances of heat, fresh water, and mass in port monitoring in this passage could be accomplished with a both the Pacific and the Indian oceans. It is generally be- moored-instrument array similar to that used during the In- lieved that the predominant transport pathways into the In- ternational Southern Ocean Studies (ISOS) program. The dian Ocean are the deep (1500 m) passages north and south monitoring scheme employs bottom-moored precision pres- of Timor. Consideration of the hydrologic data indicates sure gauges on each side of the ACC at depths such that they that as much as 15-20 percent of the global fresh-water flux span the current with no intervening bathymetric obstruc- enters the Indonesian Seas across the section between Singa- tions. Estimation of absolute current requires that the gauges pore and Borneo and apparently transits the Indonesian Seas be "leveled" relative to a geopotential surface. This can be into the Indian Ocean. Available temperature and salinity accomplished by having an independent estimate of the cur- data suggest that other straits closer to the South China Sea rent speed at the pressure-gauge depths. During ISOS, a line (e.g., Sunda, Malaka, and Lombok) may carry a significant of current meters across Drake Passage provided reference fraction, disproportionate to their size, of this fresh water speeds for hydrographic measurements to determine the spa- and heat into the Indian Ocean. tially averaged speed at 500 m. An efficient method of ob- The Lombok Strait, located east of Java between the is- taining an independent reference speed would be to make lands of Bali and Lombok, is second with respect to trans- acoustic-Doppler measurements in conjunction with re- port capacity only to the deep Timor passages. Current-meter peated hydrographic surveys across the ACC. Care must be data from the Lombok Strait during the northern winter in- exercised, however, since at Drake Passage large transport dicate a persistent southward-directed net flow into the In- changes are known to occur within the time frame it would dian Ocean interrupted by four-to-seven-day pulses of north- take to make two surveys. ward flow. A rough estimate of net transport through the Volume-transport monitoring and water-mass-property Lombok Strait alone (excluding cyclone phases) from this in- flux monitoring requires information about the vertical and itial data set of 1.5 X 106 m3 -s_1 appears consistent with a latitudinal structure of the current. This may be supplied by substantial total throughflow for the entire archipelago moored hydrographic stations placed across and within the (0[10 X 106 m3 • s-1]). ACC. These "transport moorings" would be heavily instru- Given this evidence of significant interbasin exchange mented vertically with temperature, pressure, and conductiv- through Indonesian Straits and the associated impact on ity sensors from which density and dynamic-height profiles water-mass budgets of the Indian-Pacific oceans, measure- may be calculated. Because of the harsh Antarctic environ- ment of the throughflow should be attempted. A variety of ment, it is not practical to instrument shallow depths. In- technical difficulties are present, however. The number of verted echo sounders (IES) could fill this void by providing passages is large, tidal flows and internal waves are strong, independent estimates of the dynamic height.