JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. C7, PAGES 11,299-11,321, JULY 15, 1992 Laboratory Simulation of ExchangeThrough Fram Strait KENNETH HUNKINS Lamont-Dohert• Geological Observator• of Columbia University, Palisades, New York J. A. WHITEHEAD Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy-drivenand wind-driven rotating fluids. In "lock-exchange"experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference,rotation rate, and assortedsidewall geometrieswas recorded. Volume flux of the fluid was also measuredand comparedwith a theory for lock-exchangeflow of a rotating fluid. In a separategroup of experimentswith a passiveupper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and thereforeoppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchangebetween the Arctic Ocean and the Greenland-Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock-exchange formulas are compared with observed ocean fluxes, and approximate agreement is found. INTRODUCTION ated with the term strait and cannot be expected to exert the same kind of control on exchange that, for example, The seasin high northern latitudes have a special interest the Strait of Gibraltar exerts on flow between the Atlantic for oceanographybecause they provide a major heat sink for Ocean and the Mediterranean Sea. Ice and Arctic waters the world ocean and are an important component of global flow southward through Fram Strait while Atlantic waters convectionand climate. In recognitionof this important role flow northward. Much of this transfer takes place in two there has been increasing attention in recent years to both narrow boundary currents. The East Greenland Current observationaland modeling programsfor the Arctic Ocean on the western side of the strait exports cold water of low and the Greenland Sea. The severe climate and the pres- salinity from the Arctic Ocean to the Greenland Sea and ence of sea ice, particularly in the Arctic Ocean, severely the West Spitsbergen Current on the eastern side carries restrict field operations, so oceanographicdata have accu- warm saline waters northward. Strong fronts in the strait mulated slowly and are still quite limited. It is necessaryto separate the two principal contrasting water masseswhich extract as much information as possiblefrom the data avail- have their origins, respectively, in high polar latitudes and able and to interpret them with theoretical models designed in the subtropics. Within these seas there is a wide range to test ideas about the physical processes. Simple labora- of mesoscale motion superimposed on the mean flow. In tory models of the Arctic Ocean and Greenland Sea will be Fram Strait, transient propagating eddies with diameters described here which have been developed with the object ranging from several kilometers to 100 km have frequently of clarifying ideas about their circulation and the exchange been observedalong the marginal ice zone both in remotely between them. These models explore idealized situations sensedimages and in hydrographicsurveys [Johannessen et based on relevant physical concepts but are not designedto al., 1987; ManIcy, et al., 1987a; Ginsbergand Fedorov,1989]. realistically mimic currents and hydrography. They are ex- Meanders of both the ice edge and the front along the East pected to contribute toward agreementon the nature of the Greenland Current are commonly seen, and one subice me- important physical processesin these oceans, to stimulate ander was seento developinto a cycloniceddy [ManIcy et more detailed modeling, and to aid in planning future field al., 1987b].Another type of feature is the nearly stationary exploration. eddy which has often been observed over the Molloy Deep in Before describing the model experiments we will review the northern central part of the strait. This cyclonic eddy is briefly the oceanography of the seas on which these stud- about 60 km acrossand seemsto be topographically trapped ies focus. The deep basins of the Arctic Ocean and the [Wadhamsand Squire,1983; Bourke et al., 1987]. Greenland Sea are connected by the gap between Green- Despite its lack of a sill or narrow constriction, Fram land and Spitsbergen known as Fram Strait. This opening Strait does appear to act as a barrier to exchange since is 450 km wide acrossits narrowest section with depths ex- strong contrastsexist between the water massesof the Arc- ceeding 2000 m for a distance of 100 km along that same tic Ocean and Greenland Sea. The Arctic Ocean is covered section(Figure 1). Thus, despiteits name, Fram Strait in its deep regionswith perennial drifting sea ice and an up- lacks the narrow width and shallow depth usually associ- per water layer about 100 m thick with salinity of only 31 to 32 ppt (Figure2). This upperlayer, freshened by river runoff Copyright 1992 by the American GeophysicalUnion. from surrounding continents, forms a cold, low-salinity lens which overlies a transition layer of increasing salinity and Paper number 92JC00735. density between 100 and 300 m. The lid of low-salinity 0148-0227]92 ]92 JC-00735$05.00 water on the Arctic Ocean prevents convective circulation 11,299 11,300 HUNKINS AND WHITEHEAD: SIMULATED EXCHANGETHROUGH FRAM STRAIT 180' - . CNUKCHI EAST . ß . SEA SIBERIAN ' .. - . SEA BEAUFORT SEA LAPTEV ARCTIC OCEAN 90* EURASIAN BASIN KARA SEA BAY BARENTS SEA GREEN LANi SEA ICELAND NORWEGIAN SEA o Fig. 1. Bathymetry of the Arctic Ocean and Greenland Sea. I I0O0 2000 3000 -05• 4000 -(a) I I • i • I I I I I ! I I I I I I I I I I I I I I ! I I ! I0O0 2OOO •000 4000 Fig. 2. Potential temperature and salinity sectionsacross the Arctic Ocean and the Greenland and Norwegian Seas(adapted from Aagaard,[1989]. in the upper layers. Weak stability prevails below 300 m cooled in winter to becomedenser than the underlying wa- where temperature and salinity are relatively uniform. In ter and convective overturn has been observed to depths the Greenland Sea, on the other hand, vertical stability is greater than 1000 m and may even penetrate to the bottom weak throughout the water column with pronouncedthin- during particularlysevere winters [GSP Group,1990]. The ning and even vanishing of the surface layer near the center ventilation of the Greenland Sea leads to formation of North of the cyclonicgyre (Figure 3). There surfacewaters are Atlantic Deep Water which contributes significantly to the HUNKINS AND WHITI•.HI•.AD:SIMULATI•.D EXCHANGI•. THROUGH FRAM STRAIT 11,301 24 21 20 õ0 48 47 winds and ice motion, Colony and Thorndike[1984] esti- mated currents as a residual of unexplained variance, while Coachmanand Aagaard [1974] used the densityfield with the assumptionof negligible currents at great depth. These results show that wind maintains the ice circulation, which in turn drives the upper ocean. The mean ice drift pattern may be interrupted in late summer when the polar anticy- clonic wind system weakens and even reverses. Ice drift in ...?.... both the gyre and the Transpolar Drift Stream may reverse 2000- for a period of severalweeks each summer [McLaren et ai., 1987; Serrezeet ai., 1989]. Althoughthere are no data yet on ocean currents during these reversals,theory indicates that . the baroclinic spindown time for the upper Arctic Ocean is 3000- ß . too long for any current changeto occurduring theseevents. The cyclonicgyre in the Greenland Sea is alsodriven mainly by winds, since calculations of Sverdrup transport based on POTENTIAL TEMPERATURE •C observed wind data bear a quantitative resemblance to the geostrophicsurface circulation determined from the density 24 21 20 õ0 48 47 field [Aagaard,1970]. The northwardSverdrup transport o I I • • I j -- :-•-- • -- 34.90.,.• must be balanced by a western boundary current which con- stitutes part of the East Greenland Current. The Greenland gyre did not reverseitself even when the wind stresscurl re- versed sign for a period of 5 months. I000• Within Fram Strait, wind is not the principal driving force for currents beneath the ice since it has been found that the S< 34.95 x10 -3 mean geostrophiccurrent, determined as a residual from ice and wind measurements, accounts for 2 to 5 times as 2000 much mean ice motion as local wind [Moritz and Colony, 1988]. Flow in the strait must thus be drivenby densitydif- ferences between the Arctic Ocean and the Greenland Sea, and an explanation for it in these terms was provided by 3000 . ß Wadhamset ai. [1979] using a rotating tank experiment. When a radial barrier, was introduced into a circular zonal front, the rotational constraint against meridional flow was SALINITY(xib -3) ß ' ' broken and a narrow boundary current rapidly transported fresh water toward the rim of the tank in an analog of the Fig. 3. Potential temperature and salinity sections extending East Greenland Current. The laboratory boundary current north-northwestacross the GreenlandSea [Carmack 1986]. closelyresembled the uniform potential vorticity model cal- culatedby Manley et ai. [1987a]on the basisof the section globalmeridional transport of heat. The physicaloceanog- shownin Figure 5. In that model the exponentialprofiles for raphy of theseseas has been reviewedby Carmack[1986, interfacedepth and velocity have e-foldingwidths scaledby 1990]and Aagaard[1989] and the physicaloceanography of the internal radiusof deformation(9.4 km for their data). Fram Strait by Hunkins[1990]. Details of the origin,distri- Using this and appropriate valuesfor other parametersthey bution, and transformation of the water masses in this re- found a volume transport for the East Greenland Current gion havebeen discussed by Coachmanand Aagaard[1974], of 1.1x10• ms s-•, which compareswell with the valueof Aagaardet al.