|From Ocean Navigator #87 |
Will the latest El Nino event mean more heavy weather for ocean-going mariners in the Pacific?
Oceanographers and meteorologists have found that unusually warm West Coast water is only one aspect of the El NiÃ±o phenomenon. Directly involved are unusual sea surface temperatures, unusual air temperatures, changes in normal wind velocities and directions, and changes in the locations and amounts of rainfall.
Whether the El NiÃ±o/Southern Oscillation, or ENSO as it is now known, is a case of the atmosphere influencing the ocean or whether it’s the ocean affecting the atmosphere has not been settled. In addition, the jury is still out regarding the connection of the many side-effects that seem to be related to the main event. Some of these are: severe droughts in Australia, Indonesia, Malaysia, northeastern Brazil, South Africa, and the Sahel of West Africa; unusually heavy rains causing flooding in Peru, Ecuador, and the southeastern U.S.; unusually severe winter storms on the U.S. West Coast; unusually cold summers that ruin crops in northeastern China; and abnormal hurricane patterns over both the Atlantic and Pacific.
The global extent of the El NiÃ±o/Southern Oscillation was not recognized until very recently. Only since WW II has enough atmospheric and oceanographic data been collected on a worldwide scale to reveal ENSO’s full scope.
The first known historical references to the El NiÃ±o phenomenon appear in old ship captain’s logs. The earliest one we have dates from 1795, another appeared in 1822-’23, and still another in 1891. From all indications, the 1891 event was a particularly intense one. Other strong events occurred 1925, 1934, 1957-’58, 1965, 1972-’73, 1982-’83, and, most recently, 1991-’92. El NiÃ±os occur at irregular intervals and vary greatly in intensity. The more powerful ones appear to recur on an average cycle of somewhere between nine and 11 years. Whether ENSO events are becoming more frequent now than formerly is unclear. Attempts to trace recurrences through analyses of ancient coral deposits in tropical waters have thus far proved inclusive.
The name El NiÃ±o is believed to have originated with fishermen from the port of Paita, Peru, back in the late 1800s. They used the name Corriente del NiÃ±o (Current of the Christ Child) to describe a warm, south-flowing current in the coastal waters off Peru that appeared periodically shortly after Christmas. To the fishermen it was very bad news, indeed. It seriously reduced their catches by driving away the cold-water species that are normally found in those waters. In contrast, for farmers ashore, the El NiÃ±o was the herald of an aÃ±o de abundancia (year of abundance). In these years the usually dry and barren deserts of Peru received heavy rains that quickly produced favorable conditions for both farm crops and livestock forage.
Sometimes it brought too much rain. This frequently resulted in widespread and destructive flooding. Michael Moseley, an archeologist at the Chicago Museum of Natural History, has found evidence that a very strong El NiÃ±o produced floods in A.D. 1100 that destroyed the elaborate Indian irrigation system then existing on the arid plains of Peru. It is his theory that the catastrophic famine that then followed so weakened the Chimu Indian civilization that it caused their downfall.With regard to El NiÃ±o, we now know an important fact that the Peruvian fishermen did not know: The abnormal warming of the coastal waters off Peru is merely the eastern end of a widespread warming of the entire tropical Pacific Ocean, an event now known as the El NiÃ±o/Southern Oscillation. The Southern Oscillation part of the name originated with Sir Gilbert Walker, who was Great Britain’s director-general of observatories in India in the early 1900s. He had observed irregular fluctuations from year to year in atmospheric pressure over the South Pacific and Indian Oceans. Specifically, when pressure is high over the tropical Pacific Ocean it tends to be low over the Indian Ocean from Africa to Australia, and vice versa. He called this the Southern Oscillation.
He was trying at the time to find a way to predict the fluctuations in the annual monsoons in India, which he theorized were a part of a global phenomenon. He was able to correlate his Southern Oscillation with changes in rainfall and wind patterns in the tropical Pacific and Indian Oceans. Also, he found correlations with temperature changes in such widely scattered areas as southeastern Africa, southwestern Canada, and the southern U.S. Unfortunately for him, during his lifetime his statistical run was too short to prove his point to skeptical colleagues. It was a good many years later before enough data was compiled to fully prove his theory. The direct connection between the El NiÃ±o of the eastern Pacific and Sir Gilbert’s Southern Oscillation was not definitely made until the major El NiÃ±o of 1957-’58. This one happened to be particularly well documented since it occurred during the International Geophysical Year. During this El NiÃ±o, measurements showed that the warm surface waters off Peru extended west all the way to the International Date Line. The Pacific trade winds were unusually weak, and heavy rainfall was observed in the central Pacific in an area that is normally dry.
The distinguished meteorologist, Prof. Jacob Bjerknes of UCLA, then noted that the coincidence of these unusual oceanographic and meteorological conditions across the Pacific Ocean were not unique to the 1957-’58 event. They had occurred repeatedly in connection with El NiÃ±o events along the South American coast. Since the phenomena Walker had found were now definitely linked to it, El NiÃ±o was renamed the El NiÃ±o/Southern Oscillation.
Researchers at the Scripps Institution of Oceanography have recently discovered an Indian Ocean parallel to the El NiÃ±o in a warming of ocean waters that move from the African coast eastward toward India. This Indian Ocean warming occurs simultaneously with the Pacific Ocean El NiÃ±o and appears to be related to the drop in rainfall that causes droughts in India, Indonesia, and western Australia during El NiÃ±o years.ENSO cycle What starts an ENSO is still wide open, but by now there is no doubt that the weakening of the trade winds in the atmosphere, and the warming of the surface waters of the tropical Pacific are linked together to produce an ENSO event. Which comes first? Oceanographers tend to say it is atmospheric changes, meteorologists prefer to blame it on oceanic changes. So far no one knows for certain which it isor if there is some other, thus far unknown, force that affects them both.
Exploring this possibility, a geophysicist at the University of Hawaii, Daniel A. Walker, has developed a theory that is completely unrelated to either atmospheric or oceanographic causes. Looking at the Pacific Ring of Fire, and specifically at underwater volcanic eruptions, he sees these as a possible cause for the vast heating of Pacific Ocean waters necessary to produce an El NiÃ±o. As yet there is no adequate data as to how much heat such eruptions infuse into the Pacific Ocean. There is also the question of the seeming cyclical nature of El NiÃ±o recurrences. How do volcanic eruptions relate to the apparent 10- to 11-year cycle of strong events? Most students of the El NiÃ±o/Southern Oscillation phenomenon tend to dismiss this theory, but so far they have no other that is convincing.
We do know that the atmosphere responds to influences that produce warming, cooling, and related pressure changes far more rapidly than do the oceans. Consequently, there seems to be a lively possibility that an atmospheric change such as the weakening of the trade winds might precede and produce changes in the ocean. So, if a satisfying explanation for the periodic weakening of the trade winds over the eastern tropical Pacific were to come forward, the nagging question of how an ENSO starts might be solved.
Looking at conditions when there is no ENSO in effect, strong trade winds drive the warm South Equatorial Current westward across the tropical central Pacific Ocean. This allows upwelling of the cold water the Paita fishermen normally find along the South American coast to the east. Meanwhile, from the Date Line on to the west lies a large equatorial area of very warm water above which active convection produces frequent heavy rains. To the east of this area ocean water temperatures become substantially cooler. And, lacking the active atmospheric convection that takes place over warmer water, rainfall in this eastern zone is quite scarce. As the trade winds weaken during an ENSO, the westward-flowing Equatorial Current weakens as well. The area of warm surface water now spreads far to the east and with it the area of strong convection and rains. What causes an El NiÃ±o to break down and become its opposite is as puzzling as what starts the cycle in the first place. As La NiÃ±a develops, the trade winds strengthen, the cold surface waters reappear in the eastern Pacific, the warm surface waters recede to the west, and with them the area of heavy equatorial rains.
In August 1997 sea-surface temperatures in the eastern tropical Pacific were well above normal while somewhat farther north off San Diego in mid-September sea-surface temperatures of 74° were recorded where they are usually about 68°. During the same month, gourmet fish restaurants in San Francisco were serving fish dinners featuring tropical species such as mahi-mahi caught locally. One of the most prominent authorities on El NiÃ±o, S. George Philander, indicates that he feels there really is no such thing as a “normal” condition over the Pacific. Rather he feels there is a continuous cycling from El NiÃ±o to La NiÃ±a and back again.
When Linda, the most powerful hurricane ever recorded in the eastern tropical Pacific, developed in September of 1997, it was already the second Category 5 storm of the year to form in the region. Then in early October Hurricane Pauline, the strongest hurricane to actually strike the Mexican coast in decades, came ashore to devastate Acapulco. While these storms might have occurred anyway, the unusually warm surface waters of the strong ENSO certainly contributed to intensifying them, and the weakened trade winds allowed them to turn northward rather than to follow the usual westward hurricane path out over the central Pacific.
As further evidence of the active tropical conditions fostered by ENSO over the Pacific, during the same period that Linda was active, a second tropical storm was churning its way over the eastern Pacific and two typhoons raged over the western Pacific. The entire tropical Pacific Ocean area was active with tropical storms. An ENSO does not by itself generate tropical storms. These form in non-ENSO years as well, but the abnormally warm ocean surface waters of an ENSO provide favorable conditions for storm development and intensification.
While hurricanes and typhoons were howling over the tropical Pacific, the tropical Atlantic was quiet. Tropical storm activity there was well below normal.
ENSO and the sailor
Clearly when an ENSO is in effect, the conditions you will encounter at sea will usually disagree quite radically from what appears on the Pilot Chart for that month. This will be true not only if making a transpacific passage, but also if cruising through many areas along the U.S. coasts, as well as Mexico, Central America, the Caribbean, and the Atlantic.
As with hurricanes, no two ENSOs are alike; they vary in extent and intensity. We still have a great deal to learn about the mechanisms affecting the development of the ENSO in the Pacific, let alone all its various side-effects around the world. Our recognition of the magnitude of its scale is so recent that only incomplete records presently exist. Therefore, predictions of wind and sea conditions over the oceans as an ENSO develops are not yet adequate. In spite of the sketchiness of available data one would be silly, knowing that an ENSO is developing, to put to sea ignoring such data as does exist.
For example, spring and summer trade winds weaker than usual mean slower passages to the south Pacific islands from the U.S. West Coast, while east of the date line rain squalls and headwinds may develop instead of the usual following easterly trade winds.
Summer tropical storms in the Eastern Pacific normally start a few degrees north of the Equator and west of Central America and move westward with the trades across the tropics before curving north. Due to trade winds weaker than normal an ENSO may allow them to turn northward earlier than usual as hurricanes Linda and Pauline did this year. The abnormally warm surface waters of the eastern tropical Pacific during an ENSO, along with the weakened trade winds, tend to cause more numerous and more powerful hurricanes than normal to form south and west of Mexico.
During an ENSO, the normal summer northwesterly winds along the U.S. West Coast are likely to be lighter than usual, and along the Southern California coast the winds may remain from the west to southwest. This is exactly what has happened during the summer of 1997. When winter comes, severe storms are likely to blow down from the Gulf of Alaska or up from the tropics or, worse yet, blow in from both directions at once and meet over Southern California. The northwest coast of the U.S. is likely to experience unusually strong winter storms as well.
Farther out over the Pacific, droughts have previously been associated with ENSO events. By mid-September typical ENSO-associated droughts have been reported in the past in Malaysia, Indonesia, and the Philippines and developing in Australia. The 1997 event has been no exception.
Over the tropical Atlantic during an ENSO the northeast trades are stronger than usual, making for a fast passage westward from Europe and North Africa toward the Caribbean, but it’s likely to be a rough beat to get from Florida and the Bahamas out to Puerto Rico, the Virgin Islands, and the Lesser Antilles. In August of this year, while cruising for a couple weeks in the British Virgin Islands, we encountered trade winds that were stronger than usual for this month and only one very mild easterly wave. The stronger Atlantic trades tend to have one beneficial effect: to sharply decrease the number of Atlantic hurricanes during the season from August to November. For example, 1997 has been another extremely quiet year over the tropical Atlantic. By the end of October there had been only one full-scale Atlantic hurricane.
The continuing extensive studies of El NiÃ±o, his kid sister La NiÃ±a, and their counterparts in the Atlantic will eventually reveal how and why they occur. More accurate prediction of their intensities and side-effects will then be possible, allowing the voyager to better prepare for the conditions that will be encountered.
Jeff Markell is the author of several books on marine weather, the latest being Coastal Weather Guide for Southern California and Western Mexico, published by ProStar Publications in Marina Del Rey, Calif.