Atmospheric cycles

In recent years not only sailors, but much of the general public has become aware of the cyclical El Niño/La Niña phenomenon. As the most well-known of the various oceanic/atmospheric weather and climate changing cycles, El Niño’s worldwide effects have become more and more apparent due to the related droughts, floods, and forest fires reported in the popular media.

As we now know, climatic cycles like El Niño produce major weather disruptions, including droughts, floods, changes in the prevailing winds, and changes in tropical cyclone patterns. Droughts caused by El Niño may occur in such widely separated areas as Indonesia, Australia, eastern Brazil, the U.S. West Coast, and sub-Saharan Africa. Floods may develop in Peru, Bolivia, and on the U.S. West Coast. During an El Niño, the trade winds decrease in force over the tropical Pacific Ocean, and hurricanes usually increase in frequency and ferocity over the tropical eastern Pacific and reduce in frequency and ferocity over the tropical Atlantic. Weaker upper-level winds over the eastern Pacific encourage tropical cyclone development while stronger winds at upper levels over the Atlantic discourage such development.

During La Niña the reverse occurs. Hurricane frequency and ferocity increase over the tropical Atlantic, along with a decrease in trade-wind velocity. A corresponding trade-wind increase is seen over the tropical Pacific, discouraging cyclone formation. Areas flooded by El Niño are usually dry during La Niña, and where El Niño brings drought, La Niña generally brings rain.

However, in addition to the El Niño Southern Oscillation (or ENSO cycle) several other, less well-known oceanic/atmospheric cycles have been discovered that cause major climate variability on very different time scales from that of El Niño, and they have major effects on a great many other geographic areas as well.

The ENSO cycles occur on an irregular schedule of two to seven years, with strong events showing up every nine to 11 years. The effects are felt not only over the tropical Pacific where it originates, but in tropical latitudes all around the planet, as well as varying distances into the middle latitudes both north and south.

The other less well-known atmospheric cycles that strongly affect worldwide weather and climate are:

  • Arctic Oscillation (AO)

  • North Atlantic Oscillation (NAO)

  • Pacific Decadal Oscillation (PDO)

  • Madden-Julian Oscillation (MJO)

    All of these oceanic and atmospheric climatic variations have alternating warm and cool phases that are termed positive and negative. They have all been discovered far more recently than ENSO; hence, we have far less detailed and reliable information about them. In addition, much of what information we do have is based on old observations and records made by observers and instruments far less sophisticated and accurate than those of today.

    From the information we have, it appears all of these oscillations differ as to the length of time required to complete a cycle, and except for the Madden-Julian Oscillation, their cycles are vastly longer than ENSO. It also appears likely that there are ways in which some of these oscillations are interrelated.

    The Arctic Oscillation

    The various major authorities are unclear as to whether the AO and the NAO are completely separate phenomena or whether they are in some way interconnected. Part of the reason for this confusion is that some of the weather changes produced by both the warm and the cool phases of both of these oscillating systems are very similar.

    The AO takes somewhere in the neighborhood of 80 years to complete an entire cycle. The warm phase of the AO brings a pattern of lower-than-usual sea-level atmospheric pressure over the Arctic Ocean. This produces strong westerly winds circling around the arctic in the upper atmosphere with relatively lighter surface winds. A result of this circulation pattern weakens the circumpolar ocean current, allowing comparatively warm, salty North Atlantic water to enter the Arctic Ocean through the gap between eastern Greenland and northwestern Europe. The strong upper-level westerly winds also keep cold arctic air from intruding southward into the northern United States. The consequence of this is relatively mild winters in the northern United States during the warm, or positive, phase. At the same time, over the central Atlantic Ocean, higher-than-usual sea-level atmospheric pressures produce strong westerly winds, bringing comparatively mild temperatures and wet conditions over northern Europe, while the Mediterranean area of southern Europe remains unusually dry.

    In the cool, or negative, phase of the AO, the sea-level atmospheric pressure over the arctic rises above normal with weak westerly winds in the upper atmosphere. Over the central Atlantic, pressures drop below normal. Strong surface winds maintain a powerful clockwise current circling the Arctic Ocean, which prevents the warmer Atlantic water from entering the Arctic Ocean.

    In a negative phase winter, cold arctic air intrudes over both Canada and the northern United States as well as northern Europe and Asia. Some authorities say that the AO was in its negative phase at the time of the German attack on Stalingrad. The winter of 1942-’43 was a particularly bitter one on the plains of Russia; thus, the AO in its cool phase may well have contributed to the decisive German defeat at Stalingrad. Lower-than-normal pressure over the Atlantic during the negative phase of the AO brings weak westerly winds over northern Europe and a wet, stormy winter over the Mediterranean.

    The North Atlantic Oscillation

    The NAO seems to follow a cycling pattern of approximately 40 years. The warm phase of the NAO produces a stronger-than-usual subtropical high-pressure center over the North Atlantic and a deeper-than-usual Icelandic low. The stronger high pressure over the North Atlantic already sounds suspiciously similar to the positive phase of the AO.

    With a greater-than-normal pressure differential across the North Atlantic at high latitudes, more and stronger winter storms blow across this area. Therefore, above-normal precipitation may be expected over northern Europe and Scandinavia, with below-normal precipitation over southern and central Europe. Northern Canada and Greenland become cold and dry. The eastern United States turns wet and mild. The NAO was in positive phase starting in the 1978-’79 winter and remained so through the 1994-’95 winter, causing stormy weather over northern Europe and drier winters over the Mediterranean. All of this sounds similar to the effects of the positive phase of the AO.

    By contrast, during the 1995-’96 winter, the phase reversed to the negative phase, bringing fewer storms to northern Europe and more storms farther south over the Med. The negative phase of the NAO shows a weak subtropical high — that means lower-than-usual pressures and a weak Icelandic low, meaning somewhat higher pressures than usual there. Higher-than-usual pressures bring weak winter storms over the Atlantic, wet stormy air to the Mediterranean and cold air to northern Europe. This again sounds quite similar to the cool phase of the AO.

    These similarities are why some authorities refer to the AO and NAO as a single combined event. Presumably, additional research and resulting data will eventually clarify this issue, but as of now (2002), you can call it either way, and you will find highly competent authorities ready to agree with you — and others, equally knowledgeable, ready to disagree.

    The NAO also appears to affect hurricane activity over the tropical Atlantic. When the NAO is in its negative phase, sea-surface pressures are lower than normal over the central subtropical area — this high-pressure area is often called the Bermuda High. Sea-surface temperatures are higher than normal, encouraging convection and hurricane formation in the easterly waves that originate in summer over the deserts of west Africa, and move westward with the trade winds. When the NAO reverses to its positive phase, higher-than-normal pressure returns to the central subtropical area, cooler sea-surface temperatures return, reducing convection, and hurricane formation is discouraged.

    Since the time periods required for complete cycles of each of these two oscillations are radically different — the AO requires about 80 years for a complete cycle, while the NAO cycles in about 40 years — the two cycles do not correlate with each other very well. At times, they appear to be on the same phase, as they were from 1955 to 1978, but then by 1979 to 1988, they seemed to be in opposite phases. Then from 1989 to 1999, they were back on the same phase again. Hopefully, with time we’ll get enough data to better understand what the effects of these two cycles are on each other and the oceanic and land masses they affect.

    Pacific Decadal Oscillation

    The PDO appears to complete a full cycle over periods ranging from 50 to 70 years. Here again, some authorities see a connection or relationship between the PDO and El Niño/La Niña, or the ENSO cycle, since both affect areas of the Pacific Ocean. Other authorities see them as totally separate and distinct entities.

    Here, the distinction between the two does seem considerably clearer than that between the AO and the NAO. The El Niño/La Niña cycle is definitely associated with increases or decreases in the sea-surface temperatures of the tropical Pacific Ocean. The PDO affects primarily a distinctly different oceanic area, specifically the North Pacific/North American area. In addition, the durations of complete cycles of these two phenomena are vastly different from each other.

    The PDO was definitively recognized and named as a separate entity only as recently as 1996 by a fisheries scientist named Steven Hare, who was studying, along with Nathan Mantua and others, the connection between salmon production in Alaska and variations in Pacific climatic conditions.

    Two major factors distinguish PDO events from ENSO events. The use of the word “event” in this case will apply to one phase of a complete cycle. That is, it will apply to either a warm phase or a cool phase. Firstly, during the 20th century, PDO events typically continued for 20 to 30 years. Cool PDO conditions dominated from 1890 to 1924. Although there were years when there were reversals, mostly cool conditions dominated during this period. Warm conditions dominated from 1925 to 1946. At that point, the cool phase again moved in from 1947 to 1976. From 1977 through the late 1990s, warm conditions returned until about 1998. By 1999, the cool phase apparently returned. Various authorities are not completely convinced that this most recent phase change is for real, but as of press time, enough time had passed to make it appear likely.

    The warm phase of the PDO produces anomalously warm sea-surface temperatures along the coasts of North and Central America and along the equator, as well as cool sea-surface temperatures over the vast central area of the North Pacific. In the cool phase, this pattern is reversed. The interior of the central North Pacific warms, while the American coastal and equatorial waters cool.

    The phase changes just noted coincided with major changes observed in the Pacific Ocean marine ecosystems. During the warm phases, coastal marine biological activity increased in Alaska and decreased along the West Coast of the continental United States. During cool periods, the opposite was observed.

    The second major distinction between the PDO and ENSO is that ENSO events persist for only six to 18 months. Thus, several complete ENSO cycles can pass during a single phase of the PDO. In addition, ENSO clearly is most visible in the tropics, with secondary effects elsewhere.

    We do have enough information by now to know what all of the climatic effects of the warm and cool phases of the PDO will normally be. Actually, the differences in sea-surface temperatures resulting in either positive or negative PDO phases are only 1° to 2° C. However, because the areas affected are so huge, the change in location of areas of warmer and cooler surface waters alters the path of the jet stream. This, in turn causes major changes in the North American climate.

    Recent studies also suggest that the effects of ENSO on the North American climate are related to the phase of the PDO. When ENSO and the PDO are in phase, they seem to reinforce each other. Thus, when the warm phase of the PDO is strong, the typical El Niño effect will be strong as well. And vice versa, when the cool PDO is strong, La Niña, or the cool phase of the ENSO, will be strong as well.

    The Madden-Julian Oscillation

    Of the various cyclical atmospheric/oceanic climatic variations, the MJO has the shortest duration by far. It completes a cycle in only 30 to 60 days. Since it is of such short duration, the MJO is also referred to as an intraseasonal oscillation.

    It is distinguished by the eastward movement of large areas of both enhanced and suppressed tropical rainfall seen first mainly over the Indian Ocean, then over the Pacific Ocean. Unusual rainfall is generally evident at first over the western Indian Ocean. It then moves eastward over the very warm ocean waters of the western and central tropical Pacific. As it reaches the cooler waters of the tropical eastern Pacific, the pattern of tropical rainfall then becomes spotty and uneven. It is then reestablished as it reaches the tropical Atlantic. This weather then continues on around to the western Indian Ocean. The complete cycle lasts between approximately 30 and 60 days.

    There are distinct patterns of lower- and upper-level atmospheric circulation anomalies accompanying MJO effects on tropical rainfall. These features circle the globe and show areas of ascending and descending air movements, particularly relating to areas of the tropics where rainfall is usually low or absent.

    There is considerable year-to-year variability in MJO activity. Long periods of strong activity are followed by periods when this oscillation is weak or non-existent. The MJO activity also varies with changes in the ENSO cycle. The MJO is often strong when the La Niña is weak or during years when ENSO is inactive. The MJO is usually weak when El Niño is strong.

    The major effects of the MJO on weather over the United States occur over the western part of the country during the winter. This is when the area receives the bulk of its rainfall. Records indicate a link between ENSO cycles and MJO variations in rainfall over this section. Particularly severe rains cause extensive flooding. These floods appear to develop most often in La Niña or ENSO’s neutral years. These are years that often result in increased MJO activity. A recent example was the winter of 1996-’97, when there was heavy flooding in California and the Pacific Northwest during a particularly active MJO season.

    A typical weather sequence over the Pacific Ocean preceding an MJO-enhanced heavy-rainfall event hitting the Pacific Northwest has been observed repeatedly. An area of MJO heavy tropical rainfall moves from the eastern Indian Ocean to the western tropical Pacific. At the same time, a strong blocking high develops over the Gulf of Alaska with a strong jet stream blowing around it to the north seven to 10 days prior to the event. A plume of moisture advances to the northwest ahead of the area of heavy rain.

    Three to five days ahead of time, the heavy tropical rain moves farther eastward and slackens. The blocking high weakens and moves west, while the jet stream splits. A moisture plume extends well to the northeast of the area of tropical rain, while a mid-latitude trough draws moisture from that plume extending up from the tropics.

    At the time of the heavy-rainfall event over the Northwest, the high has moved farther west, being replaced by a deep low close to the coast that brings several days of heavy rain with possible flooding. The jet stream extends across the North Pacific, entering the continent over the northwestern United States. These events are affectionately referred to on the West Coast as the Pineapple Express, since they cross Hawaii on their way in from the Pacific. The heaviest rain during these events falls on the U.S. Northwest, gradually becoming less intense as you move southward.

    The MJO also appears to influence weather over North America during the summer. However, the relative effects of ENSO and the MJO on the summer monsoons and summer rainfall are not well understood. Also, as the MJO moves eastward during the warmer season, the areas likely for tropical cyclones move eastward as well, from the tropical western Pacific to the Eastern Pacific and across to the tropical Atlantic. While observations indicate connection, the MJO is only one of many factors involved in the development of tropical cyclones.

    Hopefully, as these various long-term and short-term atmospheric/oceanic cycles become better understood and the relationships between them become clearer, both sailors and the general public will receive ever-more complete and accurate weather information. As understanding of the long-term cycles increases, so will the accuracy and usefulness of long-term weather prediction. A major benefit will be improved forecasting of oncoming severe weather events to prevent loss of life, minimize property damage and provide voyagers with better advance warning of bad weather at sea.

    Jeff Markell is a sailor and author of several weather books, including The Sailor’s Weather Guide, second edition, published by Sheridan House.

  • By Ocean Navigator