Unit 4: The Atmosphere and the Oceans Chapter 15: Physical Oceanography     Physical Oceanography Two ships, the Andrea Gail and the Titanic, are forever linked in the public's mind to two of the greatest hazards that face ocean-faring vessels: storm-generated waves and drifting icebergs. In October 1991, the Andrea Gail was lost on the open ocean after being hit by waves that towered more than 30 m high. The waves themselves were driven by the convergence of three powerful weather systems that together formed "The Perfect Storm." In 1912, the Titanic was struck by an iceberg in the Atlantic Ocean and the ship, hailed as unsinkable, went down along with 1500 people. Ocean Waves On a typical day, wave heights are no more than 5 m. Wave height, as you've learned, depends on three factors: wind speed, wind duration, and fetch, or the area over which the wind blows. Thus, wind plays the major role in determining how high waves get. As you might suspect, a large storm system with gusty winds generates large storm waves. Hurricanes can produce waves more than 10 m high, and waves from severe Antarctic storms can be twice that high. It takes days for a strong storm to build waves that are more than 10 m high. Consider the waves generated by a hurricane. Hurricanes are fairly compact systems with average diameters of 600 km. Their fetch, therefore, is only a few hundred kilometers, which is insufficient to raise truly gigantic waves. Large Antarctic storms, on the other hand, have diameters of thousands of kilometers, with a correspondingly larger fetch, and are capable of generating much higher waves than hurricanes can. And when several large storm systems collide, as in the case of the Andrea Gail, waves can reach unbelievable heights of 30 m. LINK-UP: Find out more information about ocean waves. Sea Ice As was mentioned earlier, large storm waves are not the only hazards found in ocean waters. In polar oceans, sea ice and icebergs are common. Sea ice begins to form when ocean surface temperatures reach -2°C. Most of the salt ions that are present in ocean water aren't incorporated into ice. Thus, sea ice has salinities of only a few parts per thousand and is similar, chemically, to freshwater ice. As you learned in your textbook, sea ice crystals first form a slush on the surface of the water. In the beginning stages of this process, wave motion prevents the ice from forming a continuous cover. Eventually, however, the thickening ice solidifies into individual pieces of pancake ice and finally into a continuous cover called pack ice. During Arctic and Antarctic winters, pack ice can be several meters thick and 1000 km wide. Icebergs Pack ice essentially brings shipping in polar seas to a standstill during winter. However, icebergs present the greatest hazard to ships. Icebergs are not sea ice. Rather, they originate from glaciers on land. In high latitudes, many glaciers in coastal regions flow into the sea and break apart in a process called calving. Icebergs are the large pieces of glacial ice that float away from calving glaciers. All North Atlantic icebergs come from Greenland glaciers. Many of these icebergs drift far to the south before they melt. An iceberg generally has a jagged, irregular shape and often rises tens of meters above sea level. However, only about one eighth of its mass is above water. The bulk of the iceberg is largely underwater and thus invisible. This huge, underwater bulk can cause tremendous damage to ships. One of the worst maritime disasters in history, the sinking of the Titanic in 1912, was caused when the ship collided with the underwater portion of an iceberg. This collision resulted in the deaths of more than 1500 people. Small icebergs produced by calving glaciers in Alaska also threaten shipping in some of Alaska's coastal waters. However, the Antarctic region has more icebergs than any other, and these icebergs tend to be the largest of all. Antarctic Giants The huge Antarctic ice sheet flows into the surrounding oceans and forms immense, floating ice shelves. The ice shelves are thousands of kilometers wide and approximately 600 m thick. Occasionally, a piece of an ice shelf breaks off and floats away as a flat-topped iceberg that may be larger than some countries. The visible part of the iceberg, known as the freeboard, can be as high as 80 m. Such icebergs may circle Antarctica for years before they drift into lower latitudes and finally melt. There have been reports of floating ice in tropical oceans south of the equator. This ice probably represents the last remnants of a gigantic Antarctic iceberg. Activity Research and write a report about how scientists track icebergs in the oceans. What technology do they use? How do they warn ship captains of approaching danger?