sign in sign up
<<
Home , Polar climate

THE FUTURE OF OCEANOGRAPHY

NUMERICAL SIMULATION OF SEA- AND OCEAN CIRCULATION

By David M. Holland

CURRENTLY THERE IS considerable in- rated by the Lomonosov Ridge, which (1987) removed the relaxation constraint terest in improving our understanding extends from across the Arctic of Hibler and Bryan and simulated an an- of the interactions between the compo- Ocean to . The waters of the ticyclonic circulation at all depths in the nents of the 's system. The basin communicate with the Pacific Canadian Basin while obtaining a cy- polar regions, which form an integral Ocean through the narrow and shallow clonic flow below 200 m depth in the yet distinct part of this system, are be- and with the Atlantic Eurasian Basin. This anticyclonic flow lieved to be particularly sensitive to an- Ocean via the relatively wider and pattern in the Canadian Basin is inconsis- thropogenically induced global warm- deeper Fram Strait as well as the wide tent with observations that suggest a cy- ing and therefore are worthy of special opening between Spitsbergen and Nor- clonic flow in that basin. study. The polar climate subsystem way. There is also a flow from the The Oberhuber model used here is a consists of the atmosphere, the oceans, western Arctic through the Canadian general circulation model based on the and the . The presence of sea ice into Baffin Bay. following ideas: isopycnals are used as on the ocean surface drastically alters Freshwater is received along the periph- Lagrangian vertical coordinates, a real- the interaction between the atmosphere ery of the basin from numerous rivers. istic equation of state is included, the and the ocean at high latitudes. The is well stratified primitive equations together with the Our understanding of the interac- because of the annual cycle of sea-ice hydrostatic approximation are applied, tions in the polar regions has been growth and melt and also because of and a surface mixed layer and a hampered by a shortage of basin-wide river input. There is a strong pycnocline and sea-ice model are coupled to the observations due to the harsh climate. at -300 m depth, which separates the interior ocean. The sea-ice model in- Those that do exist up to the late 1980s cold, fresh surface waters from the rela- corporates both thermodynamics and are reviewed by Carmack (1990). Nu- tively warm, salty deeper waters. The dynamics. A fundamental difference merical modeling thus provides an im- general circulation of the waters within from other sea-ice models is that Ober- portant tool for investigating the com- this basin is not well known. It is as- huber uses a different numerical plex mechanisms and interactions that sumed that the surface mixed layer is scheme (i.e., implicit) and also solves control polar climate. The northern and dragged along by the moving ice sur- the sea-ice equations written in spheri- southern polar regions are very differ- face and thus follows an anticyclonic cal coordinates. The work presented ent from one another. The Arctic con- path in the Canadian basin and then here is the first application of the Ober- sists of an ocean surrounded by conti- exits the basin through Fram Strait. The huber model to the Arctic Ocean; Ober- nental land masses, whereas the deeper waters are thought to flow in the huber (1993) has previously applied the Antarctic has a continent surrounded by opposite (i.e., cyclonic) sense. The model to the North Atlantic. a high-latitude ocean. This study fo- deeper waters may be strongly con- The so-called Atlantic layer of the cuses on a numerical simulation of the strained by topography and thus flow Arctic Ocean is the result of warm, Arctic using the recently developed along the contours of bottom topogra- saline water penetrating into the Arctic Oberhuber (1993) ocean model. phy. Ocean via the Fram Strait (and also The Arctic Ocean is essentially con- The present modeling work was moti- partly from the Barents Sea). This tained within a closed basin. It has the vated in part by two previous modeling water is heavier than the surface Arctic largest continental shelves in the world studies of the Arctic Ocean, namely those waters and subducts as it travels north oceans. These shallow and broad mar- of Hibler and Bryan (1987) and Semtner of Fram Strait, occupying the water gins cover about one-third of the sur- (1987). Both of these studies used a cou- column between 300 and 1,000 m face area of the Arctic Ocean. The pled sea-ice-ocean model with specified below the surface. The model simula- basin is divided into two subbasins, atmospheric forcing to simulate the gen- tion of the velocities and temperatures namely the Canadian Basin and the eral circulation of the Arctic sea-ice and at a depth of 500 m are shown in Fig- Eurasian Basin. These basins are sepa- ocean. Hibler and Bryan were not able to ure 1. The velocity pattern indicates investigate the deeper circulation because that the waters there are basically con- D.M. Holland, Department of Atmospheric and they had imposed an artificial damping of strained to follow the topography. The Oceanic Sciences and Center for Climate and the salinity and temperature fields back Global Change Research, 805 Sherbrooke Street modeled circulation consists of three West, McGill University, Montreal, Quebec, to long-term averages (i.e., a relaxation principal cyclonic gyres (see the num- H3A 2K6; Ph.D. 1993, McGill University constraint) at all depths below the top bers 1, 2, and 3 in Fig. 1), which sim- (supervisor: Lawrence A. Mysak). level of their ocean model. Semtner ply mimic the pattern of the ocean

OCEANOGRAPHY'VoI. 7, No. 1"1994 27 topography. This circulation can be the Lomonosov Ridge near where the tions in the global thermohaline circu- compared with the circulation inferred ridge intersects the continental slope. lation could have impacts on the Arctic from earlier temperature and salinity Furthermore, the modeled temperature Ocean and vice versa. Secondly, a measurements, which show a single is consistent with the pattern of the ob- major limitation of the model is that it basin-wide gyre as opposed to the served field; however, the absolute consists only of a coupled ice-ocean multigyre structure shown here. How- temperatures in the simulation are model with the atmosphere specified. In ever, an analysis of recent data col- everywhere -0.5°C too warm. reality, the polar climate system in- lected during the 1991 Oden cruise Overall, the study has validated the cludes three freely interacting systems, suggests the existence of a multigyre usefulness of the model for simulating namely the atmosphere, sea-ice, and structure (Rudels et al., 1993). the general circulation of the sea-ice ocean, with various feedback processes. The modeled temperature shows a and ocean in the Arctic. Although not Only the sea-ice and ocean are coupled pattern consistent with the circulation. discussed here, the simulation of the together here; consequently, there are It shows the warm, saline Atlantic sea-ice thickness, compactness, and ve- no feedbacks operating between the at- water entering via Fram Strait. The locities is in good agreement with ob- mosphere and the sea-ice or ocean. temperature decreases as the water cir- servations (Holland, 1993). There are The results of this work, which are culates cyclonically in the Eurasian limitations of the model that prevent a further described in Holland (1993), Basin. This cooling of the Atlantic direct application of the results pre- indicate that further studies of the layer water is due to mixing and diffu- sented here to that of the global climate polar regions can be profitably carried sion of the Atlantic layer water with system. First, a global model domain out using the Oberhuber model. Future the waters above and below it. Simi- was not used; rather, the simulation work will involve studying interannual larly, the temperature pattern suggests was performed using only the Arctic variability of the Arctic sea-ice cover a cyclonic circulation in the Canadian Ocean and the North Atlantic Ocean. In and simulating the general circulation Basin. The Atlantic layer water enters reality, the Arctic Ocean is intricately of the Arctic Ocean and its connection the Canadian Basin by flowing over connected to the global ocean. Varia- to the global circulation. The model will likewise be used to investigate the circulation in the Antarctic to make comparisons with observations and other modeling studies. Acknowledgments This project was carried out with the assistance of Prof. Lawrence A. Mysak and Dr. Josef M. Oberhuber. Computing resources were provided by Cray Research Inc. and the Arctic Region Supercomput- ing Center of the University of .

References Carmack, E.C., 1990: Large-scale physical ocean- ography of polar oceans. In: Polar Ocean- ography, Part A, Physical Science, W.O. Smith, ed. Academic Press, New York, 171-222. Hibler, W.D. III and K. Bryan, 1987: A diagnostic ice-ocean model. J. Phys. Oceanogr., 17, 987-1015. Holland, D.M., 1993: Numerical simulation of the Arctic sea ice and ocean circulation. Ph.D. thesis, McGill University, 203 pp. Oberhuber, J.M., 1993: Simulation of the Atlantic circulation with a coupled sea ice-mixed layer-isopycnal general circulation model. 1.1 1.2 1.3 1,4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 Part I: model description. J. Phys. Ocean- UNIT! Ce,!#(us ogr., 23, 808-829. Rudels, B., E.P. Jones, L.G. Anderson, and G. Fig. 1: Simulated temperature and circulation of the Atlantic layer (i.e., depth of 500 Kattner, 1994: On the origin and circulation of the Atlantic Layer and the intermediate m) in the Arctic Ocean Orbr January). The underlying contours give the temperature depth waters of the Arctic Ocean. In: The field, which ranges from hot temperatures in excess of 2.4°C in Fram Strait to cold Role of the Polar Oceans in Shaping the temperatures less than 1 °C in the center of the Canadian Basin. The overlying vectors Global Environment, R. Muench and O.M. illustrate a multigyre cyclonic circulation with maximum speeds of-3 cm/s. The solid Johannesen, eds. Amer. Geophys. Union, green color indicates land. The thin white lines represent lines of constant longitude Washington D.C. In Press. Semtner, A.J., Jr., 1987: A numerical study of sea spanning out.from the , which is located near the center of the image. The ice and ocean circulation in the Arctic. J. numbers' 1, 2, and 3 in the figure show the centers of three cyclonic gyres. Phys. Oceanogr., 17, 1077-1099. {_]

28 OCEANOGRAPHY'VoI. 7, No. 1"1994