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 technologies 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 indonesian 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 transport 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

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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 circulation 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 quantitative 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 report 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. Finally, the and the net flow may be modulated on seasonal and inter- deep flow (below the level of bathymetric obstruction) can be annual El Nino-Southern Oscillation time scales. A variety measured with near-bottom-mounted current meters. of techniques for sampling the throughflow were discussed The two remaining ACC passages south of Africa and including Australia-New Zealand present greater logistical sampling • arrays of moorings and pressure gauges within the difficulties owing foremost to their width. Nevertheless, end- major passages; station monitoring arrays could yield time series of volume- • an array of tall moorings in the Timor Sea on a line be- transport fluctuations through these constrictions. Instru- tween Java and Australia; menting the flanks of the islands found in these passages and • closely spaced hydrographic stations with reference ve- repeated section work could provide latitudinal information locities determined by acoustic-Doppler techniques in needed to estimate property transports and their variability. combination with inverse computations; and South of Africa the Agulhas retroflection area must be con- • induced electromotive force (EMF) in telephone-tele- sidered. Aside from introducing technical complications, graph cables stretched across the Timor Sea, Java to this also may be the site for direct warm-water flow from the Australia. Indian to the Atlantic oceans. Since this warm-water exchange

Unauthenticated | Downloaded 10/04/21 12:56 PM UTC 138 Vol. 68, No. 2, February 1987 could have profound consequences in the global heat budget Such data might reveal interannual variations in the proper- and thermohaline circulation, its study is integral to the glob- ties of the overflow that would provide valuable tracer in- al interbasin-exchange problem. A sampling line extending formation for study of the interior circulation. It is important southwest of the Cape of Good Hope would be situated to to recognize, however, that the so-called Med water in the capture this flow. Given the observations of an energetic North Atlantic thermocline is not pure overflow water, water eddy field in this region, a moored-current-meter array may that has sufficient density to sink to the bottom of the Atlan- be required. Extension of the line to Antarctica might include tic. Vigorous mixing and entrainment of lighter waters down- sensors on Bouvet Island. stream from the strait result in the mid-thermocline water The South Pacific Ocean Sector Report recommended the type. What role these mixing processes might play in moder- third ACC monitoring line be situated south of New Zealand ating or enhancing interannual variability, is not known. to take advantage of the narrow passage for deep and bottom waters found there. The drawback is that the Indian to Pa- c. Arctic Ocean cific exchange estimate must be corrected for flow between Until very recently, it has been thought that the role of the Australia and New Zealand. Assuming a South Pacific west- Arctic Ocean in the larger circulation is primarily an estua- ern boundary current program is implemented, sampling the rine one, in which it exports cold, low-salinity water to the ACC south of New Zealand appears preferable to an Aus- North Atlantic through Fram Strait and the Canadian Ar- tralia-Antarctica line. Here also, instrumentation of island chipelago, the amount being several Sverdrups in each case. flanks may permit property transport monitoring. In addition to this estuarine conceptualization, a new view has developed in the last few years according to which a significant thermohaline circulation is driven within the Arctic 3. Marginal sea outflows Ocean forcing an outflow of saline deep water through Fram Strait that contributes to Norwegian Sea Deep Water. It is therefore appropriate to consider the Arctic Ocean as a mar- Marginal seas are the formation sites for a variety of water ginal sea contributing dense water to the Atlantic sector of types found in the oceans' interiors. These waters contribute the world ocean. Certainly sufficient new questions have to the structure of the thermocline and are the chief compo- been raised about the role of the Arctic Ocean that for pur- nent of the colder water masses in the ocean. Thus the pro- poses of WOCE, it would seem worthwhile to, at a minimum, cess of lateral injection of waters formed in marginal seas is monitor the exchange through the Fram Strait. Beyond this, the complement to the ocean interior process of subduction. however, the Arctic Ocean is currently the subject of process- To what extent should WOCE treat marginal seas as "black oriented studies by both physical oceanographers and geo- boxes" by focusing strictly on the waters transiting the pas- chemists, and it is conceivable that new information may sages or straits connecting the marginal seas to the oceans? warrant a more direct WOCE interest in process studies Discussions during the workshop made clear that those within the Arctic Ocean. scientists who are interested in marginal-sea processes will not be content to focus on the overflows. The overflows are d. Labrador Sea better understood in the broader context of the marginal-sea processes responsible for them and their effects on the adja- The Labrador Sea-Labrador Current is part of the pathway cent ocean circulation. by which the low-salinity outflows of the Arctic Ocean are consolidated in water-mass space and move southward and a. Indonesian Seas downward toward denser surfaces. The process responsible for this is winter cooling with the associated vertical convec- The role of the Indonesian Seas in interbasin exchange was tion producing deep-surface mixed layers. The circulation of discussed above. Not to be overlooked is the water-mass the resulting low-salinity waters constitutes an important conversion that occurs in these basins. These marginal seas component of the North Atlantic fresh-water balance. Win- to the Indian and Pacific oceans are the source for a low- ter convection, particularly in the Labrador Sea, appears to salinity water type clearly seen in upper-ocean salinity maps be highly sensitive to the time history of air-sea interaction. in the Indian Ocean. The production mechanism of this Observations at Station Bravo in the Labrador Sea and near water is unique among the basins discussed, in that deep con- the Newfoundland coast indicate considerable fluctuations vection does not play a role. Rather, vertical mixing appears of salinity of the upper waters on both a seasonal and an in- responsible for lowering the salinity of the waters entering terannual time scale. It would appear from a number of stud- these seas from the Pacific. Study of this process will proba- ies of the historical data sets that the formation or non- bly involve examination of boundary mixing, perhaps re- formation of water masses, such as the Labrador Sea water, lated to tidal stirring. does not affect the circulation significantly. These conditions do, however, result in changes in water properties over de- b. Mediterranean Sea cades or faster, which may provide valuable time informa- The Mediterranean Sea is the source for the warm-saline tion to the spatial distributions of water properties. A North water type found in the thermocline of the North Atlantic. Atlantic observing program for WOCE should have a means Presently, an Office of Naval Research program is underway of measuring the heat and salt transport of the Arctic out- in which the exchange of waters through the Strait of Gibral- flow, either in the East Greenland Current or the Labrador tar is being investigated. These results indicate it is possible Current. Good-quality hydrographic measurements should to obtain time series of the Mediterranean (Med) overflow. be obtained in the West Labrador, Irminger, and Iceland seas

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in early summer each year to characterize the previous win- Land, are far denser than the waters traditionally defined as ter's convection products. Antarctic Bottom Water. Is the entire process of bottom- water formation in the Southern Ocean a local phenomenon e. Deep overflows to the North Atlantic unrelated to the rest of the world ocean? Or does newly formed bottom water mix with something else to form a lighter The dense waters that are formed in the Greenland and Ice- product, which then flows out of the Southern Ocean? If so, land seas and outflow from the Norwegian and Iceland seas what is the "something else", and where does it come from? constitute the chief source of deep and bottom waters in the Is newly formed deep water a fairly "pure" constituent of North Atlantic. Thus, the formation of these waters at the Antarctic Bottom Water? And, most mysterious, what is the winter sea surface between Greenland and Norway is integral origin of the lighter component of Antarctic Bottom Water to WOCE. Important related problems include the dynamics that enters the South Atlantic through Drake Passage? of anthropogenic substances, such as bomb-generated ra- These questions need to be answered not merely to deline- dioactive tracers and fossil-fuel-derived C02, and the propa- ate Southern Ocean kinematics but also to determine the gation into the oceans of the effects of climatic changes. Five quantitative role of the Antarctic in the deep circulation of regional sources for North Atlantic Deep Water (NADW) the world ocean and in the global climatology. The general have been identified: subject of Antarctic Bottom Water seems very unclear at pres- 1) Derivatives of the salty Mediterranean Sea outflow; ent and so it is difficult to consider design of observational 2) Products of open-ocean convection in the Labrador programs with which to address them. Instead, the commu- Sea; nity needs first to sort through the complexities of the known 3) Iceland-Scotland Overflow Water from the Norwegian water-property distributions in the light of simple physics, in Sea; order to construct a selfconsistent, physically sensible story 4) Denmark Strait Overflow Water from the Iceland and about Antarctic Bottom Water. That story ought then to Greenland seas; point toward specific observational programs and modeling 5) Remnants of deep water from the Antarctic circum- ventures that could test, quantify, and otherwise enlarge it— polar region. as appropriate elements of WOCE.

Because the formation of each of these waters may be viewed as a response to a complex series of events and interactions, it is difficult to examine the sensitivity of NADW to environ- 4. Technologies mental fluctuations. Nevertheless, major deep-ocean re- sponses have occurred; between 1972 and 1981 the northern Several ocean-measurement techniques were discussed at the North Atlantic (north of 50°N) freshened by about .02 psu workshop. Some systems are presently available, others and cooled by about 0.15°C below 2000 meters. await development or refinement. Bottom pressure gauges Long-term studies of NADW variability have to date fo- have recently become viable sensors for long-term deep- cused only upon fluctuations in the characteristics of the var- ocean observation programs. Drift errors that seriously con- ious water masses and not variations in the coupled circula- taminated previous measurements appear to have been tion. Thus it simply is not known whether the problem of greatly rectified in the latest generation of gauges. Bottom source "0-S" variability is one of a passive tracer-like pro- pressure gauges will be an important complement to satellite cess, or whether important "rate" fluctuations must also be altimetric measurements of surface-height fluctuations dur- addressed. An interrelated program of measurements is ing WOCE. This is particularly true at high latitudes that probably required to work out the connection between air- TOPEX/Poseidon satellite will not sample. Measurements sea exchange, water-mass properties, and the deep circula- in these regions may also come from combining the Earth Re- tion. This program might include long-term observation of sources Satellite-1 (ERS-l)and the Navy Remote Ocean Sens- fluctuations in sea-surface topography across the northern ing System (NROSS) observations with those from TOPEX. North Atlantic and Greenland, Iceland, and Norwegian seas, Acoustic-Doppler velocity profiling from ships equipped measurements appropriate for determining fluctuations in with the Global Positioning System offers great promise for the wind torque and to at least crudely estimate the air-sea advancement past the reference-level problem endemic to heat exchanges, occasional (ca. 8 times in 5 years?) CTD/hy- hydrographic section work. Induced-electromagnetic-field drographic sections across the northern North Atlantic and measurement using submarine cables may offer an efficient the Iceland and Greenland gyres to establish fluctuations in means of monitoring integrated transport. Another tool the average characteristics of the source and product water needed for long-term measurements is a moored-conductiv- masses, and long-term observation of fluctuations in the in- ity sensor with stable calibration characteristics. Finally, de- flows and outflows over the Greenland-Scotland ridges. velopment of telemetry systems for recovery of in situ data in near real time should continue. f Antarctic Bottom Water

The connection between the climatologically important 5. Summary of Recommendations "Antarctic Bottom Water" and deep sinking around Antarc- tica is very murky at present. Newly formed dense Antarctic The recommendations for WOCE resulting from the meeting waters, with the possible exception of the deep waters origi- include develop liaisons with programs planned for Bering nating in the Weddell Polynya and from the shelf off Wilkes Strait; encourage exploratory efforts to improve our under-

Unauthenticated | Downloaded 10/04/21 12:56 PM UTC 140 Vol. 68, No. 2, February 1987 standing of the mechanisms of Pacific-Indian Ocean ex- pressure gauges spanning ocean currents to monitor trans- change; stimulate analysis of model results and existing data port variability; continue the study of ways to utilize the pro- sets for development of practical Antarctic circumpolar- posed altimetric data for examination of interbasin and mar- current—volume monitoring scheme and property-trans- ginal-sea exchanges; exploit the technologies of ship-mounted port monitoring schemes; promote the examination of the acoustic-Doppler velocity profilers and the Global Position- unique water-mass conversions in the Indonesian Seas where ing System for estimation of the absolute ocean-velocity convection is not playing a role; establish water-mass moni- field; explore the use of induced EMF measurements ob- toring stations in important marginal-sea sites of water-mass tained from cables for estimating transports through ocean conversion like the "Bravo" station in the Labrador Sea; de- passages; encourage development of moored conductivity velop methods for monitoring the outflows from marginal sensors and data-telemetry systems. seas equally as important, the inflowing source waters and the air-sea exchange rates of heat and fresh water; study existing data sets to discover the Antarctic connection to bot- References tom-water production; consider the Arctic a site for WOCE process studies of eddies, boundary current, and coupled U.S. Scientific Steering Committee for WOCE, Ocean Sector Meet- wind-thermohaline-driven circulations; promote liaisons ings: South Pacific, North Pacific; South Atlantic, and Indian between WOCE and existing or planned programs within in- Oceans. U.S. WOCE Planning Report Number 2, U.S. WOCE dividual marginal seas; consider extensive use of bottom Planning Office, College Station, Texas, 1985. •

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