2
The Four Winds and Waves

Refreshed by our midocean swim, we ate lunch. Midocean is an exaggeration; even though the water was around a mile deep where we were and there was no land in sight, we were barely on the fringes of the Pacific Ocean. Gradually the breeze picked up and the sea surface became rippled with capillary waves. Small wavelets appeared. Soon the sails stopped luffing and began to fill. Dreams gathered forward momentum. The wind had arrived.

Wind and current are important ingredients in the formation of extreme waves. At certain places in the world’s oceans, wind and current run amuck with regularity. Understanding their patterns is important for avoiding danger spots. No one knows this better than those who sail the Sydney–Hobart race.

BASTARDLY BASS STRAIGHT

“Thursday, April 19, 1770. Had fresh gales at SSW and cloudy squally weather with a large southerly sea. Saw land at a distance of 5 or 6 leagues. The southernmost point of land at latitude 37 degrees, 58 minutes south, longitude 149 degrees, 39 minutes east, I named Point Hicks, because it was Lieutenant Hicks who first discovered this land.”1



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 25
Extreme Waves 2 The Four Winds and Waves Refreshed by our midocean swim, we ate lunch. Midocean is an exaggeration; even though the water was around a mile deep where we were and there was no land in sight, we were barely on the fringes of the Pacific Ocean. Gradually the breeze picked up and the sea surface became rippled with capillary waves. Small wavelets appeared. Soon the sails stopped luffing and began to fill. Dreams gathered forward momentum. The wind had arrived. Wind and current are important ingredients in the formation of extreme waves. At certain places in the world’s oceans, wind and current run amuck with regularity. Understanding their patterns is important for avoiding danger spots. No one knows this better than those who sail the Sydney–Hobart race. BASTARDLY BASS STRAIGHT “Thursday, April 19, 1770. Had fresh gales at SSW and cloudy squally weather with a large southerly sea. Saw land at a distance of 5 or 6 leagues. The southernmost point of land at latitude 37 degrees, 58 minutes south, longitude 149 degrees, 39 minutes east, I named Point Hicks, because it was Lieutenant Hicks who first discovered this land.”1

OCR for page 25
Extreme Waves With these words, Captain James Cook recorded his discovery of the east coast of Australia in his sea journal. In his first voyage of exploration, he had crossed the Pacific to Tahiti and then, under secret British Admiralty orders, sailed due south to determine if there was a great southern continent south of latitude 40 degrees. Reaching this latitude, he continued west, disproving the notion of a mysterious continent, and more importantly, accurately charting the north and south islands of New Zealand. From New Zealand he continued west across the Tasman Sea until he discovered the southeast tip of Australia. Although he did not know it at the time, Cook was in Bass Strait, the 100-plus nautical mile wide channel between Australia and Tasmania. From this location west, no point of land intervenes until you track nearly two-thirds of the way around the world, back to the coast of Argentina. It is the notorious Southern Ocean, and in the vast distance between the Cape of Good Hope and Southern Australia, great storms build and pour their fury through the narrow and shallow waters of Bass Strait and into the Tasman Sea. Cook caught the tag end of one such storm; it propelled HM Bark Endeavor to the northeast, past Bermagui, Batesman Bay, Jervis Bay, and finally to a bay he described as “tolerably well sheltered from all winds, into which I resolved to go with the ship.” Due to the wealth of new and unusual plants discovered there, he named it Botany Bay. Later colonists would settle on Sydney as a more favorable location. In 1945, following the end of World War II, a small group of sailors decided to race from Sydney south to Hobart, the largest city on the island of Tasmania, a distance of around 628 nautical miles. The race effectively follows Cook’s course, but in the opposite direction, and then continues further south across Bass Strait and along the east coast of Tasmania into Hobart. Since then, every year on December 26 (Boxing Day), the race has been run. Some sailors consider the Sydney-Hobart race to be jinxed—every seventh year, they claim, the weather turns really nasty, as occurred in 1970, 1977, and 1984. The trend came to a halt in 1991, disproving the myth, but ironically returned with a vengeance in 1998. Peter Lewis, a veteran of six races or, as he puts it, “more accurately, five and one-fourth,” raced in 1981, 1982, and even in 1984, when 104

OCR for page 25
Extreme Waves out of 150 boats, including his, retired from the race due to extreme weather. Not discouraged, he raced again in 1985 and then, under the press of other obligations, did not race again until 1994 and then again in 1998. On race day, Saturday, December 26, 1998, the weather forecasts were not good. A low was developing, and the initial forecast was for gale force winds, 34 to 47 knots. By race time, some forecasts were predicting 50 to 55 knots, and a storm warning was issued. What concerned the meteorologists was a fast-moving low-pressure area, heading east through Bass Strait. If it developed, the racing fleet would be pounded as it came past the south end of Australia and into the open ocean. It is storms out of the southwest that create a problem in the race. When this occurs, storm waves interact with the Australia Current coming down from the north, creating a condition in which waves pile up into huge seas while traversing the shallow waters of Bass Strait. Sayonara, a boat owned by Larry Ellison of Oracle Corporation, was first out of the harbor, followed by Brindabella, owned by George Snow. Both were big, fast boats—fiercely competitive—and favored to place first overall. The balance of the fleet—113 boats—followed. Peter Lewis was on a 43-foot-long boat called Esprit d’ Corp. Initially the trip south was good, the fastest boats doing 20 knots. Lewis had gotten a private weather forecast and knew that they faced 50- to 55-knot winds that night—“Tough but to be expected in a Hobart,” was the way he put it to me. His recollection of the seas that first night as the barometer plunged and the weather got worse and worse was: “A long cold night on the wind, a lot of banging and crashing into 50 knots and a building sea. At daylight on a cold, blowing Sunday morning, the crew of Esprit d’ Corp noted that the mast had bent during the night—not a good sign, particularly with more bad weather coming. These weren’t ordinary waves,” Lewis said. “They had 4 or 5 feet of white water on top and no back to them. They’d leap up and the boat would rise and then drop like a stone for 20 feet. After much deliberation, and being very mindful of the weather, Esprit d’ Corp turned west and headed for shelter at Bermagui. At this point, we’d traveled about 160 nautical miles, a little less than a third of the race. We made Bermagui around 3:30 Sun-

OCR for page 25
Extreme Waves day afternoon, just as the first Maydays went out over the radio. The fleet and the storm had arrived simultaneously in Bass Strait, and all hell broke loose.” The faster boats were slightly ahead of the storm. Although Sayonara took a beating and suffered some damage, she finished the race Tuesday morning and took first place overall. Brindabella arrived a few hours later. Ellison is reported to have said after the race, “Never in a thousand years would I do this race again.” Only 43 vessels made it to Hobart. Others were not so fortunate, especially those sailboats that were in the middle of the fleet—they caught the worst of the storm. As conditions worsened and Mayday calls started being heard on the radio, many vessels—Esprit d’ Corp among them—made the wise decision to retire from the race.2 Plate 3 provides a sense of the sea conditions during the race. It shows Bobsled being blown sideways by winds estimated at 80 knots as she was entering Bass Strait. Breaking waves reached 80 to 100 feet, but the worst was yet to come. Boats ahead of Bobsled and behind her were capsized, broken up, and sunk by huge waves a little later in the day. The Sword of Orion was hit by a wave estimated to be 40 feet high at around 5:00 P.M. Sunday afternoon. The vessel was rolled completely and dismasted. Two crew members connected by safety lines were swept overboard. One managed to get back on board, but the helmsman’s safety line parted and he drowned. Stand Aside was hit by a huge wave that literally crushed the boat and then rolled it. The cabin roof was stove in, bulkheads failed, mast gone, hull leaking water, batteries submerged, engine inoperable. The crew launched two life rafts, but one failed to inflate and the wind broke the tether that held it to the boat. The crew—many of them injured—bailed frantically and threw everything overboard to lighten the boat. Photographer Richard Bennett took an extraordinary photograph (Plate 4) from a small plane flying 1,000 feet above Stand Aside. It shows Stand Aside, the remaining life raft trailing behind; a red smoke emergency flare has just been fired. Note the huge wave—80 feet high—that just broke to the right of the vessel. What started to be a routine photography assignment became a life-and-death rescue mission as Bennett and his pilot helped direct rescue aircraft to Stand Aside and other vessels in trouble. Miraculously, all 12 crew members from

OCR for page 25
Extreme Waves Stand Aside were plucked from the towering seas by helicopter. As the last man was being hoisted into the rescue helicopter, a giant wave could be seen towering over the boat; moments later Stand Aside was swept to destruction and sank. Winston Churchill was hit by a wave twice as high as the waves they had been experiencing; it was 80 feet high and heavily damaged the boat, but the crew managed to launch two life rafts before she sank from sight. In the stormy seas, several crew members were swept from the life rafts and lost; the others were eventually rescued. Sailing slightly behind Winston Churchill was Professor Peter Joubert’s boat Kingurra. The boat was hit by a large wave and rolled 140 degrees before righting itself. Joubert was seriously injured, with broken ribs, a collapsed lung, and ruptured spleen. Three crew members on deck with harnesses and safety lines were plunged into the ocean. Two were recovered, but as an attempt was made to haul the unconscious third man (John Campbell) aboard, he fell out of his safety harness and drifted out of reach. He was miraculously rescued around 40 minutes later by a Victoria police airwing helicopter. The helicopter was equipped with a radio altimeter. Professor Joubert later described to me how the helicopter hovered at 100 feet while crewman David Key was lowered into the water to rescue Campbell. As the helicopter hovered, the pilot, Senior Constable Darryl Jones, observed a wall of water approaching. He made an emergency climb to 150 feet to avoid having the helicopter swept into the sea. As the wave passed under them, the altimeter recorded 10 feet, meaning that the wave height was 140 feet.3 This may well be the highest wave for which any type of measurement has been made. In all, seven boats were abandoned; five were damaged, capsized, and sank. Six persons died, but altogether 55 were rescued, either lifted off their disabled boats or plucked from life rafts. The helicopter pilots and other rescuers did a truly heroic job of pulling people from seas in which the waves raged 70 to 80 feet high. At the time the race was running, an oil platform called Esso Kingfish B, located in Bass Strait, recorded significant wave heights in the range of 20 to 23 feet and an extreme height of 33 to 36 feet.4 By the time these storm-blown waves reached the Tasman Sea, they had more than doubled in height. At the end of my conversation with Peter Lewis, I asked if he

OCR for page 25
Extreme Waves planned to participate in the race again. He laughed. “Yeah,” he said. “Stupidity is another name for it.”5 REAP THE WHIRLWIND The sun’s energy penetrates the earth’s atmosphere, heating the earth’s surface and the oceans. Air receives nearly all of its heat from contact with the earth’s surface. More of this heating takes place at the equator, where the sun’s energy hits perpendicularly, and less at the poles, where it strikes obliquely. As the air heats up, it expands, rising into the upper atmosphere in the equatorial regions and sinking as a dense, colder stream at the poles. This process creates high- and low-pressure areas on the earth’s surface, causing winds to blow where air flows from high-pressure to low-pressure areas. The pattern of wind flow is complicated by the interference caused by landmasses and by the fact that the earth rotates counterclockwise when viewed from above the North Pole. In other words, a point on the earth’s surface in the northern hemisphere moves easterly when the earth rotates, as evidenced by the fact that sunrise in Los Angeles occurs after that in New York or London. The earth’s rotation gives rise to a force known as the Coriolis force that deflects fluids moving over the earth’s surface. In the northern hemisphere, an airstream flowing west will be deflected to the north, away from the equator, while an airstream flowing east will be deflected to the south, or toward the equator. In the southern hemisphere the resultant directions are just the opposite. The deflection is zero at the equator and strongest at the poles. As a result of these several effects, air circulates in complex patterns over landmasses and the oceans, generally following circular paths that are predominately east and west. An example of a wind pattern that is well known to sailors who cross the North Pacific is the Pacific High. Winds blow in a great circular flow from Alaska down the coast of California and from Mexico west across the Pacific to Hawaii. The return path swings far north, sometimes to Alaska, and then down the Pacific coast. The center of this circular pattern is a movable spot, roughly in the middle of the Pacific between the mainland and Hawaii. There the atmospheric pressure is at its highest and

OCR for page 25
Extreme Waves little or no wind is to be found, as many a sailor has learned to his or her dismay. A similar pattern exists in the South Pacific, off the coast of Chile, in the North and South Atlantic, and in the South Indian Ocean. As stated in Chapter 1, Columbus noted this pattern and gambled that the Northeast Trade Winds (blowing westerly) would carry him all the way to Japan and China. He was right about the wind, but underestimated the distance to Asia and did not know that the continents of North and South America blocked his course west. The winds that sailors sense are the surface winds—air currents that move along the bottom of the earth’s atmosphere. In addition, there are other circulating currents of air, some of which are vertical, moving down to or rising up from the surface. These are called Hadley cells, after George Hadley (1685-1768), the British meteorologist who discovered them in 1735. There are three such cells in the northern hemisphere and an equal number in the southern hemisphere. Warm air rises near the equator and descends at 30 degrees north and south latitudes. Warm air rises again around 60 degrees north and south latitude, to descend once again near the poles. The area near the equator where this occurs is known to meteorologists as the Intertropical Convergence Zone (but is known to sailors as the doldrums because of light and variable winds), and at 30 degrees it is called the northern subtropical divergence zone (known to sailors as the horse latitudes, similarly for lack of wind). The origin of the term “horse latitudes” is a reference to the difficulties faced by early voyagers. Vessels were often becalmed so long that the horses being brought to the New World became part of the sailor’s diet. High above the surface of the sea, the air flow again becomes horizontal and is characterized by the jet streams that blow from west to east and are familiar to all who have made cross-country airplane flights. The complex pattern of vertical and horizontal wind circulation is repeated in both the northern and the southern hemispheres. Weather patterns are similar, except for one difference that is important to those interested in extreme waves: the weather pattern of Antarctica. Because of its massive ice cover—9,000 feet thick at the South

OCR for page 25
Extreme Waves Pole—Antarctica is consistently 11 to 14 degrees Celsius (20 to 25 degrees Fahrenheit) colder than the Arctic. Because of this, the wind patterns described above are all pushed slightly to the north, meaning that warmer weather and thus warmer water are not centered around the equator but are found at some distance above the equator. Hence, there are no hurricanes in the South Atlantic and more hurricanes in the northern hemisphere than in the southern. However, strong westerly winds blow over the Southern Ocean all winter long.6 THE WINDS OF TRADE William Dampier (1652-1715), a British explorer and buccaneer, was the first to describe the trade winds and the fact that they blow consistently from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. Dampier made many other remarkable discoveries, recognizing that the equatorial currents flow in the direction of the trade winds and that tidal streams flowing near shore are not the same as ocean currents.7 In addition to the trade winds, which for centuries facilitated sail-driven trade and transport (hence the name), the global heat engine creates other winds, some beneficial but others detrimental to the navigator. Those that are detrimental include the kamsin wind, which originates in the Sahara, blowing southwest across Egypt from April to June; the mistral, a violent northwest wind that blows down into the Gulf of Lyons; the pampero, a dry northwest summer wind that blows from the Andes across the pampas to the sea; the sirocco, from the deserts of North Africa toward Italy; and the Santa Ana, warm air from the Mojave Desert blowing west to the Pacific Ocean to give just a few of the thousands of names for local winds. The circulation of the winds on the surface of the ocean and in the upper atmosphere is quite complicated, so a complete description is beyond the scope of this book, but one further distinction that is important to us is the variation of wind patterns with the seasons. From winter to summer there are notable shifts in the locations where some of the highs and lows of atmospheric pressure occur. These shifts have an important effect on weather patterns and therefore on wave forma-

OCR for page 25
Extreme Waves FIGURE 3 Ocean surface winds in August.8 tion. In keeping with the philosophy that “one picture is worth a thousand words,” see Figure 3, in which typical high- and low-pressure areas are shown. In January, the lows over Siberia and Canada become highs and the wind direction reverses in the North Pacific and Indian Oceans. The dashed line shows the Intertropical Convergence Zone. However, there is one final caveat: The figure shows the general trend of the surface winds over the oceans, averaged over a long period of time. Local disturbances and storms can at any time shift these patterns for a few days as local weather becomes extreme. OCEAN CURRENTS So, as in the earth’s atmosphere there are great circulating currents, the same is true in the oceans, where huge rivers of water, driven by similar wind and density differences and modified by the Coriolis force, circulate. Currents initially flow in the direction of the wind but, under the influence of the Coriolis effect, eventually deflect to the right in the northern hemisphere and to the left in the southern hemisphere. This deviation increases with depth, a phenomenon known as Ekman transport, after physicist V. W. Ekman who first described it. Another difference between currents and wind: When we speak of the direction of

OCR for page 25
Extreme Waves currents, it is the direction to which the water is flowing. This is not true of winds. Traditionally—and this has always driven me crazy—when meteorologists speak of wind directions, as in “it was a terrible nor’easter,” they are referring to the direction from which the wind blows. Throughout this book, I have endeavored to make it clear in which direction the wind is blowing, but beware the convention when you read weather reports. Currents are very important to sailors, not just because of their role in causing huge waves on occasion—which is my purpose in discussing them here—but also because of their significance to navigation. Santa Barbara Island, part of the Channel Islands National Park, is located about 40 nautical miles southwest of Marina Del Rey, California. It is home to elephant seal, sea lion, and harbor seal rookeries. I acquired a partnership in the sloop Karess with my friend Karl Bernstein before the advent of global positioning satellites. Our navigational aids were a compass and a radio direction finder. At that time, in 1987, Santa Barbara Island was a favorite destination for fishing and diving. On a good day, making 5 knots, it was an eight-hour trip over to the island, and we always left in the predawn hours. During the summer, the marine layer would often obscure the low-lying island until you were close to it; fog could obscure it entirely. (It is a small flyspeck of land, only 1 nautical mile wide.) Departing from Santa Monica Bay, Karess would be pushed sideways by the California Current, traveling south at a half to 1 knot. In navigational terms, we say that the current had a set (the direction toward which it is flowing) of 180 degrees and a drift (the speed of the current) of 0.5 to 1.0 knot. Unless adjustments were made, in just two hours of sailing, the current could carry the boat far enough to the south to completely miss the island—next stop, Hawaii, 2,200 miles away! In the Pacific Ocean, there are two westerly flowing currents. The North Equatorial Current flows westward, driven by the Northeast Trade Winds from the west coast of North America across the Pacific to the Mariana Islands, where it veers to the northwest past the Philippines and Taiwan. The South Equatorial Current flows westward from South America, eventually curving southward into the Coral Sea and

OCR for page 25
Extreme Waves then south along the east coast of Australia. There is a similar pattern in the Atlantic Ocean. The North Equatorial Current begins near the Cape Verde Islands and flows west at an average speed of 0.7 knots.9 The South Equatorial Current flows from the west coast of Africa to South America, commencing at a speed of around 0.6 knots but increasing to as much as 2.5 knots as it approaches Brazil. At the “hump” of Brazil (the state of Pernambuco), it divides—one portion going north; the rest, south. Differences in water density (due to either temperature or salinity) will also create a current. Water flows from areas of lower density (where water depth is slightly greater) to areas of high density, where depth is less). As in the atmosphere, there are both surface currents and deep-ocean currents, again modified by the presence of landmasses that sometimes constrict or channel the flow. Analogous to the patterns of the wind in the atmosphere, these great currents flow generally in circular patterns in the world’s oceans and are called gyres. Gyre circulation is of critical importance because it is a major mechanism for global heat transport. In addition to the major currents described above, there are dozens of other named currents that profoundly affect navigation and coastal weather in specific areas. For the purposes of this book, some of the more important ones are shown in Figure 4. The numbered currents in particular should be noted, because they have special significance for the purposes of this book, as described later. To best understand how huge waves can arise, I believe it is important to be able to visualize not only the sea in motion but also the various forces that act on it to create extreme waves, particularly in those areas where of necessity oceangoing vessels must pass to make their way from one ocean to another, from one port to the next. Oceanographers study current flow by using floating instruments with small position-indicating radio transmitters or by the simple expedient of dumping a lot of labeled bottles into the ocean. Recently, bad weather has expanded our knowledge of Pacific Ocean currents and provided data for comparison with computer models currently in use.

OCR for page 25
Extreme Waves FIGURE 4 Major ocean currents. Note: (1) Alaska, (2) California, (3) Peru, (4) Brazil, (5) Gulf Stream, (6) Benguela, (7) Agulhas, (8) Kuroshio. THOUSAND LEAGUE BOOTS AND RUBBER DUCKIES On any day, hundreds of container ships cross the oceans. Major trade routes lie between Asia and the west coasts of North and South America in the Pacific and between Europe and North America and South America in the Atlantic. A modern container ship carries hundreds of 40-foot-long steel containers on deck. When battered by rough seas or struck by large waves, containers can be knocked into the sea, where they are floating hazards to navigation.10 An incident of this type occurred in May 1990, when the container ship Hansa Carrier lost 21 containers overboard during a storm in the middle of the Pacific, roughly midway between the Aleutian and Hawaiian Islands. These containers were full of Nike brand athletic shoes, and according to estimates, 80,000 shoes were dumped into the ocean.11 Remarkably, they floated and were carried in an easterly direction by the prevailing North Pacific Current. About six months later they began washing ashore on the coasts of Vancouver Island and Washington State, roughly 1,400 nautical miles from the point where they had entered the water. They were in surprisingly good condition after their long voyage, and enterprising beachcombers began collecting them.

OCR for page 25
Extreme Waves The Internet and other means were used to advertise the availability of these shoes at bargain prices and also to trade with others in order to find a matching shoe in the same size. Curtis Ebbesmeyer, an oceanographer with Evans-Hamilton Inc., learned that beachcombers were finding free shoes and immediately recognized that fate had created a superb North Pacific Current measurement experiment. Announcements appeared in the local press for people to report shoe findings and to include in these reports the date, location, and an identification number from the shoe. From the records of hundreds of individual “thousand league boots,” researchers compiled a detailed profile of current speed and direction that showed reasonable agreement with the National Oceanic and Atmospheric Administration’s computer model of Pacific Ocean currents. A year or so later, some shoes reached the Big Island of Hawaii. At this point, they were truly “thousand league boots.”12 Ebbesmeyer predicted that if others remain intact in the ocean long enough, some will reach Asia and Japan. Other container spills have added further confirmation of ocean current behavior. Ebbesmeyer reported a spill of 12 more cargo containers in January 1992—one of which contained thousands of small floating bathtub toys. Small yellow ducks, green frogs, blue turtles, and others successfully “swam” across the North Pacific and reached the beaches near Sitka, Alaska. Eventually some may be carried north and then east by the Alaska Current, eventually becoming part of the Arctic ice pack. In 1994, thousands of hockey gloves were lost at sea; they are now following the course of the bathtub toys. And in December 2002, a container ship ran into heavy weather off Cape Mendocino, California, and lost several containers overboard, releasing more Nike shoes into the Davidson Current. A month later, these shoes started coming ashore along the Washington coast after a journey of some 450 nautical miles.13 WIND WAVES Knowing that the earth rotates around the sun, its surface largely covered with seawater in constant motion due to gravitational forces and

OCR for page 25
Extreme Waves temperature differences from the poles to the equator, it is not surprising that waves arise. When seas in constant motion come under the added influence of strong winds circulating above them, the stage is set for the creation of waves. Under the right circumstances, waves larger than you can imagine will occur, often with no prior warning, as Homer noted in the selection from the Odyssey quoted earlier, when “… East and South winds clashed and the raging West and North, / Sprung from the heavens, roiled heaving breakers up—.” It is at the interface of these four winds and water that waves are produced. Wind is not the only force that can produce waves, but it is the predominant one, and therefore it is important to understand how wind causes waves. As wind blows over the sea’s surface, there is some friction between the moving mass of air and the water beneath it. The effect lessens the farther away the wind is from the interface; thus, winds at high altitudes do not have much effect on the water. Likewise, a violent storm will scarcely be felt beneath the surface of the sea. Under the influence of the wind on still water, small water droplets are accelerated in the direction the wind is blowing. (You can observe this effect by blowing across a saucer filled with water.) In the sea, the wave direction may not be the same as the wind direction. Very light winds will produce small waves, a few millimeters high and a few centimeters long—known as capillary waves. These waves move at various angles to the direction of the wind, giving the sea’s surface a characteristic wrinkled, diamond-shaped appearance. As the wind increases slightly, small riffles—sometimes called cat’s paws by sailors—will be seen on the surface, traveling at an angle of about 30 degrees to the wind. Cat’s paws are indicative of winds with speeds in the range of 4 to 6 knots. Some say that the sea’s surface has a chicken wire appearance. If the wind dies, capillary waves fade away and the riffle smoothes out. Should the wind speed increase beyond a few nautical miles per hour, gravity waves are formed. If the wind continues to increase, they grow still larger. Now the waves move in the direction of the wind. As the waves become higher, the ocean surface becomes rougher, causing more wind turbulence and further increasing the wave height. This is also where fetch—the distance of unobstructed ocean over which the wind blows—becomes important. The longer the fetch, the higher the

OCR for page 25
Extreme Waves waves become. Long gravity waves are different from capillary waves in that if the wind dies, they keep traveling until they finally run into some obstacle, usually a shore. WIND AND WAVE SPEED IN KNOTS Nautical miles per hour is used universally to describe the speed of wind, waves, and boats (although meteorologists use meters per second). By long-standing tradition, nautical miles per hour is commonly abbreviated as knots. The word “knot” refers to the log line method sailors used to determine boat speed before the advent of instruments.14 Knowledge of a vessel’s speed was essential for navigation; the speed multiplied by the hours of sailing gave the distance traveled from the last known position. If this information was combined with the vessel’s direction (known from the compass heading) and corrected for any offset due to the set and drift of the current, the navigator could estimate the vessel’s new position using a process called dead reckoning. To determine the vessel’s speed, the navigator would periodically (say, once per hour under steady winds) throw a log line overboard and determine the vessel’s speed. At best this was an approximation, since it determined the vessel’s speed through the water but not over the ground. For example, if the vessel is making 6 knots into a current flowing 1 knot in the opposite direction, the true speed over ground is only 5 knots. After 24 hours of sailing, without correcting for the current, the navigator would erroneously believe his dead-reckoned position to be 24 nautical miles farther along his line of travel than it actually was. Thus, knowledge of currents was very important to the early explorers. As the wind speed increases, or as the wind blows for a longer time, the sizes of waves increase. Wind acts on water in several ways. The first is by means of friction, as described above. The second is by a direct push. As waves build up, they form a vertical surface (the back of the wave) upon which the wind can act in much the same manner as a sail. Only within the last 50 years or so has it been possible to develop

OCR for page 25
Extreme Waves mathematical theories that model the wind-wave interaction with any accuracy. For our purposes, it is sufficient to say that as wind first flows over the sea’s surface, a “resonant” effect occurs that causes small wave-lets to form. The resonant effect is not strong enough to cause waves to build. For this to occur, a second effect is required. As waves build, their very presence modifies the flow of air over the sea surface. Now, instead of being smooth, air is flowing over an uneven surface, and at the air-water interface, each fluid affects the other. To model this effect, analysts consider the sea surface “roughness,” which can be characterized by a drag coefficient. The drag coefficient is a measure of the energy transferred from the wind to the water. HOW WIND WAVES GROW When all of the theoretical considerations are combined, it is found that wave height basically depends on the wind speed, the length of time the wind blows, and the unobstructed sea distance over which the wind blows.15 For a given wind speed, duration, and fetch, a maximum wave will be produced. Let us assume that we are in the open ocean and the wind blows steadily at 30 knots for 24 hours. Waves will build gradually until they reach a significant wave height of 19 feet and a maximum of 34 feet, as indicated in Table 2. Waves cannot continue to build in height indefinitely because at some point they start breaking and their energy is dissipated. When this balance is reached, the condition is referred to as a fully developed sea. If high winds blow over a narrow body of water, the waves will not be as large as the theoretical maximum and are said to be fetch limited. Likewise, if the duration of the wind is insufficient, the maximum conditions will not be achieved. Plate 5 illustrates two wind-wave conditions. The top photograph shows waves resulting from Force 6 winds at 25 to 30 knots in the Gulf of Santa Catalina. The lower photograph shows waves during a Force 8 gale in the North Pacific. Usually seas consist of many different waves, different periods and different wave heights, and waves traveling in different and similar directions. For this reason, marine weather reports give the significant wave height (Hs), defined in Chapter 1 as the average height of the

OCR for page 25
Extreme Waves TABLE 2 Wave Height Versus Wind Speed, Duration, and Fetch Average Wind Speed (knots) Significant Wave Height (feet/meters) Maximum Wave Height (feet/meters) Significant Wave Speed (knots) Significant Wave Period (seconds) Minimum Fetch or Duration (nautical miles/ hours) 10 4/1.2 7.2/2.2 17 5.5 8.6/2.4 20 8/2.4 14.4/4.4 22.3 7.3 59/10 30 19/5.8 34.2/10.4 38.2 12.5 243/23 40 47/14.3 84.6/25.8 54.9 18.0 613/42 50 55/16.8 99/30.2 64.2 21.0 1,227/69 largest one-third of the waves, for a representative group of waves. In Southern California waters, a typical afternoon coastal wind forecast is “10- to 15-knot winds, swell 1 to 2 feet, wind waves 2 to 4 feet.” It is possible to estimate the maximum wave heights for a given wind speed, fetch, and duration.16 Table 2 shows that a 50-knot wind can produce an extreme wave 99 feet high—if it blows for almost three days across 1,227 nautical miles of open ocean. These are the extreme values—winds blowing for less time or over a reduced fetch would not normally be expected to produce waves this high. Here, interpret “normally” to mean “in the absence of other factors.” Extreme waves can result from interaction between moderate waves themselves, between waves and an opposing current, or when certain other conditions are satisfied. THE BEAUFORT WIND SCALE The earliest attempt to correlate sea conditions with wave height was done in the early 1800s by a British admiral, Sir Francis Beaufort (1774-1857). He produced a table in 1805 that was a masterpiece of clarity for ships in the days when shipboard instrumentation was uncommon. It was adopted by all the major seafaring nations. He divided the wind into 12 levels or “forces” as he called them, with Force 0 being “wind calm, sea like a mirror.” Other examples include Force 5, “Fresh breeze; moderate waves, many white horses are formed,” wave heights 6.5 to 8.2 feet, while Force 12 refers to hurricane conditions (winds greater than 64 knots) with seas 46 feet and higher.

OCR for page 25
Extreme Waves When storms arise, mariners are advised by maritime authorities or national weather services through periodic weather broadcasts and by signals in harbors. There are four levels of warnings: Small craft advisory: winds up to 33 knots; one red pennant Gale: winds from 34 to 47 knots; two red pennants Storm: winds from 48 to 63 knots; square red flag with black square in center Hurricane: winds 64 knots and above; two square red flags. During the day, flags are hoisted in harbors; at night a system of red and white lights is used to signal the warnings. However, in more remote areas there may be no local weather service and mariners must have the capability to access high-frequency radio weather faxes or Internet-based weather data via satellite. LOWS AND HIGHS Earlier I mentioned the Pacific High, a point at which there is little or no wind. Somewhere out in the middle of the Pacific Ocean—maybe at 145 degrees west, 40 degrees north—there is that spot where the barometric pressure (a measure of the weight of the earth’s atmosphere bearing down on land and sea) hits 1,028 millibars, or maybe 1,030, or about 30.4 inches of mercury.17 Virtually all of the weight of the atmosphere is concentrated in the first 19 miles (30 kilometers); above this altitude, the emptiness of space begins. The pressure exerted by the atmosphere was first measured in 1643 by Evangelista Torricelli (1608-1647), an Italian physicist. He constructed a mercury barometer, basically a long glass tube filled with mercury and closed at one end. The open end was placed in a bowl full of mercury, the closed end standing up vertically (Figure 5). The pressure of the air pressing on the surface of the bowl of mercury was sufficient to maintain the column of mercury in the closed tube to a height of 29.92 inches, or in modern terms a pressure of 1,013 millibars. This is considered standard atmospheric pressure at sea level and a temperature of 15 degrees Celsius. Mercury barometers would be impractical on a vessel in constant

OCR for page 25
Extreme Waves FIGURE 5 How a barometer works. motion, so for shipboard measurements an aneroid barometer is used. This is an evacuated metal box, normally round in shape, topped with a somewhat springy metallic diaphragm. The top is attached by a linking mechanism to a dial gauge. As atmospheric pressure increases or decreases, the evacuated box expands or contracts, and this movement is transferred to a dial calibrated in millibars and inches of mercury. Since air moves from high pressure to low pressure, where the atmospheric pressure gradient is steep there will be winds, so mariners pay close attention to their barometers. However, most squall lines, almost all thunderstorms, and many other weather systems can hit suddenly with no warning on the barometer. Only in middle latitudes is the weather formed by large-scale traveling weather disturbances. In much of the world, pressure changes are minor to nonexistent when local storms approach. But when the barometer starts rising or falling,

OCR for page 25
Extreme Waves it is a sure sign of weather change. If the rate of fall is 1 to 2 millibars (0.03 to 0.06 inches of mercury) per hour, it can signal danger. NOTABLE LOWS In June 1996, a group of sailboats set out from New Zealand to Tonga. It was an informal regatta that included all types of boats, from monohulls to catamarans. The atmospheric pressure was 1,020 millibars. Without warning, after the boats were under way, a tropical depression began forming between Vanuatu and Fiji. The atmospheric pressure dropped suddenly from 1,001 to 986 millibars and the next day to 979 millibars. When the atmospheric pressure in a tropical depression drops at the rate of 1 millibar per hour for 24 hours, the New Zealand Meteorological Service refers to it as a “meteorological bomb.” The gale from this depression moved southwest across the center of the fleet sailing for Tonga. Boats in the Force 12 storm’s path were brutalized by high winds and violent seas. Five boats were lost, a number of rescues were carried out, and miraculously only three persons drowned. New Zealand Air Force planes involved in rescue attempts reported surface winds at 70 to 80 knots and wave heights up to 100 feet. The boats at the center of the storm reported Force 12 conditions. Boats were knocked down, dismasted, rolled over—some several times. Crews frantically reported their condition and location by shortwave radio to Auckland. A freighter in the area, a French navy ship, and New Zealand naval and air force personnel all came to the rescue.18 The crew of one disabled vessel was rescued by a large cargo ship. As the captain skillfully maneuvered the huge vessel close to the small sailboat, the crew waited for the exact moment when the mountainous seas crested, leapt from the sailboat to cargo nets hanging from the side of the larger ship, and then climbed as fast as they could to avoid being swept away by a giant wave. The jump had to be perfect—there would be no second chance in those roiling waters. With each passing wave, the two vessels ground together with a horrific noise as the sailboat drifted toward the stern of the cargo ship. When all of the sailors were safely aboard, they watched in horror as their beloved boat passed under the stern of the cargo ship. As the larger ship slammed down into

OCR for page 25
Extreme Waves the trough of a wave, it crushed the smaller boat, which promptly sank, carrying a family’s belongings and memories of lengthy cruises to the bottom of the sea. AROUND THE WORLD IN (LESS THAN) 180 DAYS One area of the world where mountainous seas are routine is in the southern latitudes, 40 to 60 degrees south—known as the “roaring forties, furious fifties, and screaming sixties.” Here, the winds can literally blow clear around the world, because there are no landmasses to interfere, so the fetch can be large. Near the southern tips of the continents the ocean is shallower, so the waves pile up and can be huge. Boats rounding these strategic points face another hazard: It is dangerous to go too far south because of the polar ice. Consequently any vessel making this journey runs a gauntlet of dangers. Yet from Magellan and the other earliest navigators to modern sailors in boats as small as 33 feet or less, men and women have challenged the capes. These races, in which sailors single-handedly sail around the world, are the world’s most challenging athletic competition, with one’s life on the line for endless days. Among the most dreaded and difficult portions of the route are the southern passages, where sailors face terrible weather, collisions with icebergs, and above all huge waves. You can imagine the strain of battling waves such as these, day after day, with virtually no sleep. Ellen MacArthur, who came in second in the 2000-2001 Vendee Globe around-the-world solo race, experienced a knockdown during the race—one that snarled her mainsail near the top of the mast. She had to climb the mast while her 60-foot-long boat was surfing over 33-foot waves in a 40-knot wind. If the wind had increased before she could lower the sail, her boat could have suffered severe damage. But, having freed the sail, it took her more than an hour to climb back down, exhausted, freezing, at times swinging wildly and hitting the mast as the boat pitched and rolled.19 This experience served MacArthur well, though. In 2004 she took to the sea once again, this time in a 75-foot-long trimaran named B&Q (which she nicknamed Mobi). Her goal was to top the world record for

OCR for page 25
Extreme Waves a solo, round-the-world, nonstop voyage. Again, she experienced sail problems, and this time had to climb a 100-foot-tall mast to correct them. No problem; whatever it took, she did it—from hoisting a 300-pound sail, to repairing a generator, narrowly missing a collision with a whale, avoiding icebergs, and battling 50-foot-high waves. On February 7, 2005, at age 28, she completed the 27,000-nautical-mile trip in 71 days, 14 hours—a new world record.20