High-latitude oceanic regions

High latitude oceanic regions, also known as oceanic polar regions, refer to sections of the oceans near either pole, especially the regions within either the Arctic Circle or the Antarctic Circle. The Northern Hemisphere's high latitude area is between the Arctic Circle, which is at 66 degrees 33 minutes north latitude, and the North Pole, sitting at 90 degrees north. Parts of Alaska, Canada, Europe, Russia, and Asia are within the Arctic Circle in this northern high latitude area. The Southern Hemisphere's high latitude area is located between the Antarctic Circle, at 66 degrees 33 minutes south latitude, and the South Pole, at 90 degrees south latitude. Antarctica is located at the South Pole.

The difference between these two high latitude oceanic regions is that Antarctica has a land mass that has ocean currents circulating around it, while the Arctic has no land mass, but a year-round existing ice cover. High winds, high sea state, extreme cold temperatures, seasonal sea ice, and the remoteness of the regions make them unique, harsh places. There is now an increased awareness of the importance of polar regions in the Earth system, as well as their vulnerability to anthropogenically derived change, including global climate change. Both high latitude oceans have sets of currents, each of which influences weather, upwelling—and therefore food webs—and seasons, among many other factors in their corresponding hemisphere. Although these two regions share many commonalities, they are also very different.

The Northern Hemisphere's high latitude Arctic region is a landlocked ocean, covered by pack ice that can persist for several years. The Arctic has large areas of tundra and permafrost and several very large river systems. It contains Greenland, covered by the massive Greenland ice sheet, which is on average 1.24 miles (2 kilometers) thick. In contrast, the Antarctic is made up of a land mass almost entirely covered by the huge east and west Antarctic ice sheets 1.24–2.48 miles (2–4 kilometers thick) that are separated by the Transantarctic Mountains.

The Antarctic continent is completely surrounded by the Southern Ocean, of which 6,177,634 million square miles (16 million square kilometers) freeze over every year, effectively doubling the area of the frozen Antarctic continent. Whereas the Arctic has land connections with other climate zones, the Antarctic is effectively cut off from the rest of the world because of the barrier of the Southern Ocean (the shortest distance is the 503-mile (810-kilometer) wide Drake Passage between South America and the Antarctic Peninsula). Together, the Greenland and Antarctic ice sheets account for more than 90 percent of the Earth's freshwater, and if they were to melt, sea level would increase by about 223 feet (68 meters), 200 feet (61 meters) from the Antarctic ice sheets and 23 feet (7 meters) from the Greenland ice sheet. These differences in land and ice also account for the varied biodiversity in each region.

Despite their very different characteristics and the vast expanses covered by the oceans of the world, they are all interconnected by a large-scale movement of water that is referred to as the meridional overturning circulation (MOC), global thermohaline circulation, or global ocean conveyor belt. The basis of themohaline circulation is that a kilogram of water that sinks from the surface into a deeper part of the ocean displaces a kilogram of water from the deeper waters. As seawater freezes in the Arctic and Antarctic and ice sheets consolidate, cold, highly saline brines are expelled from the growing ice sheet, increasing the density of the water and making it sink. In the conveyor-belt circulation, warm surface and intermediate waters (0–3,281 feet, or 1,000 meters) are transported toward the northern North Atlantic, where they are cooled and sink to form the North Atlantic Deep Water that then flows southward. In southern latitudes, rapid freezing of seawater during ice formation also produces cold high-density water that sinks down the continental slope of Antarctica to form Antarctic bottom water. These deepwater masses move into the South Indian and Pacific Oceans, where they rise toward the surface. The return leg of the conveyor belt begins with surface waters from the northeastern Pacific Ocean flowing into the Indian Ocean, and then into the Atlantic Ocean.

It is not just the temperature and salinity of the deepwater formation in the polar regions that is crucial to the ocean circulation. These water masses are rich in oxygen, and so are fundamental for transporting oxygen to the ocean depths, where respiration by deep-sea organisms consumes oxygen. The transport of dissolved organic matter and inorganic nutrients is also governed fundamentally by this transportation system, increasing the nutrients that are remineralized during the transfer of the deepwater masses. Therefore, water rising at the end of the conveyor belt in the northeastern Pacific has higher nutrient loading and lower oxygen concentrations than North Atlantic waters at the beginning of the conveyor belt. The ocean circulation in high latitude waters has been determined only relatively recently, using a somewhat different method from that employed for the more temperate ocean currents.

The principal difficulty in studying the polar currents is that large portions of the oceans are covered by ice for a large part or all of the year. There is still a very small database. Whereas the positions of the Gulf Stream and the Agulhas Current have historically been determined by trading ships during their passage through these areas, the general circulation of the Weddell Sea and the Arctic Ocean were initially determined using more dramatic means. Unlike other oceans, the general circulation of polar waters was determined by temperature and salinity analysis carried out with relatively few measurements.

Northern High Latitude Ocean Currents

The Arctic Ocean is a deep basin, almost completely surrounded by continental land masses, with just one deep entry and exit point called the Fram Strait. The Arctic Ocean is permanently covered in ice up to 16.4 feet (5 meters) thick, and this ice cover has made its circulation difficult to determine. Toward the end of the 19th century, several renowned scientists began to believe that there might be a current from the Bering Strait across the North Pole to the Fram Strait, based on the discovery of items from a shipwreck, as well as Siberian fir trees on the coast of Greenland. This hypothesis was not tested until Fridtjof Nansen led the carefully designed Fram (Norwegian for “forward”) to the Arctic Ocean, where she drifted across the basin from 1893 until 1896, in what we now know as the Transpolar Drift.

Exploration of the Arctic Basin really began in earnest in the 1930s, with the drift of the Sedov and the start of the Russian North Pole annual drifting ice stations, which began in 1937 and continued until 1990. At any one time, there were up to three North Pole camps on the ice, each studying various physical and biological processes. Position data from these drifting ice camps and the addition of modern satellite-tracked drifting buoys deployed on the surface of the ice revealed that the speed of the Transpolar Drift can vary between .39 and 1.57 inches (10 and 40 millimeters). The other main feature of the Arctic Ocean circulation is a large, slow anticyclonic (referring to a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere) circulation over the Canadian Basin called the Beaufort Gyre.

At cold temperatures, salinity has a strong effect on controlling the density of seawater. Coupled with the Arctic halocline, this means that most of the transport in the Beaufort Gyre (approximately 80 percent) is in the upper 984 feet (300 meters) of the gyre. The two ocean currents have a significant effect on ice conditions within the Arctic Ocean. The Transpolar Drift moves a large volume of ice from the Siberian Coast toward the Fram Strait. The open water caused by the removal of the ice allows significant ice growth and salt rejection, and may contribute to the maintenance of the halocline. The Beaufort Gyre is responsible for the piling up (ridging) of large volumes of ice on the north Greenland and Canadian coasts, making the mean ice thickness in these places up to as much as 26 feet (8 meters). There is still much we do not know about the circulation within the Arctic Ocean; satellite-tracked drifting buoys have revealed that the Beaufort Gyre has been known to slow down and stop for periods of several months. There are many large research programs planned in order to investigate these features.

As the transpolar drift exits through the Fram Strait, it is responsible for transporting large quantities of sea ice into the Nordic Basin. This ice travels down the east coast of Greenland within the East Greenland Current, which is one of the major sources of water entering the Nordic Basin. Nansen first correctly suggested, and then proved with data from the Fram, that the origin of the flow was from the rotation of the Earth and the operation of buoyancy forces along the coast. The other major current of the Nordic Basin is the remnant of the North Atlantic drift, which first becomes the Norwegian Current along the coast of Norway, and then becomes the West Spitsbergen Current at roughly 78 degrees north. The flow of the warm, salty, Atlantic-derived waters is complex at the Fram Strait. A certain fraction enters the Arctic Ocean, while another fraction of this water turns westward and then southward at approximately 79 degrees north to join the flow of the East Greenland Current in the Return Atlantic Flow.

The source waters of the cold fresh Arctic water and the warm salty Atlantic water form a cyclonic gyre, which is closed in its southern section at approximately 72 degrees north by the eastward-flowing Jan Mayen Current. This current is cold and relatively fresh. In winter, it is covered by a famous ice feature called the Odden, which may be an important factor in the driving of the ocean conveyor belt. The cold East Greenland Current leaves the Nordic Basin through the Denmark Strait and turns north at the southern tip of Greenland to become the West Greenland Current. As this current heads north, it collects icebergs from some of the most active glaciers in the world. At the Nares Strait, the current then turns south again to become the Labrador Current. Past Newfoundland, the current, now iceberg-laden, can be a major hazard to shipping.

Southern High Latitude Ocean Currents

The current structure in the southern high latitudes is less complex than in the northern high latitudes because of the absence of land. Antarctica is bounded on all sides by deepwater, and the most significant current in the high latitudes is the Antarctic Circumpolar Current (ACC), or West Wind Drift, which lies between 40 and 60 degrees south. The current flows around the Antarctic continent in a clockwise direction, uninterrupted by land, with surface speeds that can range between 1.64 and 4.92 feet (0.5 and 1.5 meters) per second, and can be considered the ocean analogue of the atmospheric jet stream.

The passage of the ACC around Antarctica is heavily determined by topography, but this is not well defined in the high latitudes because of sea ice cover. The ACC is especially strong below the Agulhas Current and in the Drake Passage, where recent measurements have determined the transport as 130 × 106 cubic meters per second, with an uncertainty factor of roughly 10 percent; the instantaneous flow may, however, differ by as much as 20 percent from this figure. The ACC derives from the Ekman transport, which is to the left of the prevailing wind (the Roaring Forties) and raises the sea surface to the north. The surface slope generates a current that is stable and balanced by the Coriolis force.

The ACC delineates two major frontal regions: at the northern boundary, the Sub-Antarctic Front; and at the southern boundary, the Antarctic Front. The fronts are highly variable, and changes in position of 62 miles (100 kilometers) in 10 days have been observed. Larger meanders in the fronts can form eddies similar to those formed in the Gulf Stream. The greater of the two fronts is the Antarctic Front (southern), which separates the warmer surface waters of northern origin from the cold waters of southern origin. This front is historically identified as the Antarctic Convergence, given that water is converging in this region, but recent measurements have shown that the ACC between the two fronts is zonal and complex in structure throughout its north–south extent, and there is a series of convergences.

Consequently, the region is now termed the Antarctic Polar Frontal Zone. It is important for biological processes because deepwater rich in nutrients is brought to the surface, resulting in an area of high localized primary production. The primary production is then grazed by zooplankton, and in particular by krill, which is a staple food for many species of birds, seals, fish, squid, and whales. Close to the Antarctic continent, there is a narrow and westward-flowing current at approximately 65 degrees south, which is called the Antarctic Coastal Current, or occasionally the East Wind Drift. This current has not been well studied, but it has a speed typically of .33 feet (0.1 meter) per second. The current is not, however, continuous around Antarctica, and it is absorbed in the two large gyre systems of the Weddell Sea and the Ross Sea.

The Weddell Sea is one of the few areas of the Antarctic to retain a permanent ice cover. A gyre was first suspected from the drift of the Deutschland, which was trapped in the southeast Weddell Sea in 1911, and drifted for nine months before escaping from the pack ice close to the Antarctic Peninsula. A worse fate struck Ernest Shackleton's Imperial Trans-Antarctic Expedition in the Weddell Sea when the Endurance was beset just off the coast of the Filchner Ice Shelf in January 1915, and sank in November of the same year. Shackleton and his men drifted within the northward portion of the Weddell Gyre until April 1916, when they landed on Elephant Island at the edge of the peninsula and went on to complete one of the most famous self-rescues in polar history. The existence and continuity of both the Weddell Sea and Ross Sea gyres have been subsequently confirmed from hydrographic measurements and satellite-tracked drifting buoys. The region at the north edge of the Antarctic Peninsula marks the Weddell–Scotia Confluence, a region that has been identified as extremely biologically important.