Polynyas in the Southern Ocean They Are Vast Gaps in the Sea Ice Around Antarctica

Polynyas in the Southern Ocean They are vast gaps in the sea ice around Antarctica. By exposing enormous areas of seawater to the frigid air, they help to drive the global heat engine that couples the ocean and the atmosphere by Arnold L. Gordon and Josefino C. (omiso uring the austral winter (the In these regions, called polynyas, the largest and longest-lived polynyas. It months between June and Sep surface waters of the Southern Ocean is not yet fully understood what forces Dtember) as much as 20 million (the ocean surrounding Antarctica) are create and sustain open-ocean polyn square kilometers of ocean surround bared to the frigid polar atmosphere. yas, but ship and satellite data gath ing Antarctica-an area about twice Polynyas and their effects are only ered by us and by other investigators the size of the continental U.S.-is cov incompletely understood, but it now are enabling oceanographers to devise ered by ice. For more than two cen appears they are both a result of the reasonable hypotheses. turies, beginning with the voyages of dramatic interaction of ocean and at Captain James Cook in the late 18th mosphere that takes place in the Ant n order to understand the specific century, explorers, whalers and scien arctic and a major participant in it. forces that create and sustain po tists charted the outer fringes of the The exchanges of energy, water and Ilynyas and the effects polynyas can ice pack from on board ship. Never gases between the ocean and the at have, one must first understand the theless, except for reports from the mosphere around Antarctica have a role the Southern Ocean plays in the few ships that survived after being major role in determining the large general circulation of the world ocean trapped in the pack ice, not much was scale motion, temperature and chemi and in the global climate as a whole. known about the ice cover itself. cal composition of the ocean and at Because the Southern Ocean has ex Since 1973, however, satellites bear mosphere throughout the globe. Po tensive open connections to the rest ing passive microwave sensors have lynyas accelerate these processes by of the world ocean, and because it is made it possible to survey the ice exposing the surface of the Southern much larger than the polar ocean in cover routinely from space, and these Ocean directly to the atmosphere. the Arctic region, the Antarctic plays a observations have brought about a There are two general kinds of po larger role in the global ocean circu marked change in investigators' view lynyas: coastal polynyas and open lation than the Arctic does. of the Antarctic ice. One of the most ocean polynyas. Coastal polynyas de When solar radiation warms the surprising findings of the satellite era velop when strong local winds origi tropical and subtropical oceans, the is that the ice cover is not at all a nating on the Antarctic continent blow resulting heat diffuses downward into continuous blanket. Within its borders ice away from the shoreline as it freez deeper water. Heat from the sun also there are many small breaks, on the es, leaving an ice-free area between causes relatively high rates of evapo scale of from one to 10 kilometers, the coast and the ice pack. Even before ration, which raises the salinity of the called leads. More surprisingly, there satellite data were available, investiga surface waters; the excess salt, like the are sometimes vast regions-up to tors had known about the existence of heat, diffuses into the depths. The 350,000 square kilometers in area coastal polynyas. They were very sur warmth and salinity of the deep water that are completely free of ice. prised, however, to learn from satellite are also enhanced by deep-reaching observations how widespread coastal mixing events in the North Atlantic, polynyas are and how large they can in which relatively warm, high-salinity ARNOLD L. GORDON and JOSEFINO C. COMISO combine sea- and satellite become. (In some coastal polynyas the surface water is mixed downward. The based research in their study of polyn distance from the shoreline to the resulting mass of warm, salty deep yas. Gordon is professor of physical border of the ice pack is as much as 50 water gradually spreads toward the oceanography at Columbia University to 100 kilometers.) Southern Ocean. In the Antarctic this and is on the senior staff of Columbia's Open-ocean polynyas form within water, now called the Circumpolar Lamont-Doherty Geological Observato the body of the ice cover, far from the Deep Water, rises toward the surface. ry. His research is focused on large-scale coast. Some of these are by far the There it gives up some heat to the circulation and mixing in the ocean and their relation to the global climate. Co miso is a physical scientist at the Na tional Aeronautics and Space Adminis WEDDELL POLYNYA, an enormous ice-free region amid the ice cover near the Weddell tration's Goddard Space Flight Center. Sea, formed during three consecutive Southern Hemisphere winters. In these satel He went to Goddard in 1979 after doing lite images, which were made in September of 1974 (top), 1975 (middle) and 1976 research in experimental particle phys (bottom), the white region represents the Antarctic landmass and colored regions ics at various institutions. At Goddard he has helped to demonstrate the utility represent ocean areas covered by various concentrations of sea ice. Pink and purple of passive-microwave and infrared data regions are almost completely covered by ice and light blue regions are ice-free. In from satellites in the study of oceanic summer the polynya disappeared with the melting of the ice cover. At its largest and atmospheric processes. the polynya measured about 350 by 1,000 kilometers. It had measurable effects on the temperature of the underlying ocean at depths as great as 2,500 meters. 90 SCIENTIFIC AMERICAN June 1988 © 1988 SCIENTIFIC AMERICAN, INC 1974 45'W 45"£ I I SCIENTIFIC AMERICAN June 1988 91 © 1988 SCIENTIFIC AMERICAN, INC - MERIDIONAL CIRCULATION PATTERN of the Southern Ocean termediate Water. Another fraction of the surface water moves (the ocean surrounding Antarctica) is dominated by the up toward the pole; it eventually sinks and flows northward as welling of a warm, salty water mass called the Circumpolar Antarctic Bottom Water. The circulation is influenced by polyn Deep Water and its transformation into Antarctic Surface Wa yas. In so-called open-ocean polynyas (center) heat is vented to ter, which ultimately sinks to become Antarctic Intermediate the atmosphere rapidly because there is no insulating layer of Water and Antarctic Bottom Water. The circulation is driven by ice. Convection currents, in which warm water rises as cold wind and the exchange of heat and fresh water between the water sinks, accelerate the movement of warm water toward ocean and the atmosphere. The upwelled water is converted the surface. So-called coastal polynyas (right) form when ice is into cold, relatively fresh surface water as it loses heat to the blown away from the continent as fast as it freezes. The re atmosphere and gains fresh water from precipitation and the sulting open area vents heat rapidly, and the salt that is re melting of ice. Some of the surface water moves northward and jected when sea ice forms increases the density of the surface gradually sinks, contributing to the formation of Antarctic In- water and accelerates the formation of Antarctic Bottom Water. atmosphere and becomes a cold, ocean, by drawing water to the sur heat is transferred from the Equator dense mass of water, which sinks face, allowing it to approach equilibri to the pole helps to determine the toward the sea floor. This so-called um with the temperature and compo degree to which temperature varies Antarctic Bottom Water then moves sition of the atmosphere and forcing it with latitude on a global scale. This northward along the ocean floor, trav downward again. The process cools in turn has a part in driving many of eling well beyond the Equator; it is the deep ocean, lowers its salinity and the processes that govern the earth's the major mass of bottom water in restores oxygen levels that have been weather and climate. the world. As it moves north it grad depleted by organisms. The deep ually mixes upward, and the cycle of ocean overturning may also be signifi erhaps the most important for warming and cooling begins again. cant in establishing a rough balance ces driving overturning in the The Southern Ocean is thus one part between the oceanic concentrations of PSouthern Ocean are buoyancy of an enormous heat engine that such dissolved gases as carbon diox and the wind. Buoyancy gives the drives the motions of much of the ide and the levels of those gases in the ocean a layered structure, in which world ocean. In addition, the massive atmosphere. Hence the overturning is masses of water are stratified accord overturning of water that takes place an important factor in such events as ing to their density (which in turn is in the Antarctic provides a mechanism the "greenhouse warming" that may determined by the water's tempera by which the chemistry of the atmos take place as carbon dioxide gas is ture and salinity). The wind, on the phere can affect the chemistry of the added to the atmosphere. other hand, tends to disturb this strat deep ocean and vice versa. The over The heat engine's effects do not ification. In addition, the exchange of turning in a sense ventilates the deep stop there.

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