from Climate Alert Volume 10, No. 2 May-June 1997

Scientists Suggest an Enhanced Role for Ocean Oscillations in Understanding Extreme Weather

After realizing the crucial influence of the El Niño/Southern Oscillation on the world's weather, scientists are now discovering other oceanic influences may play a vital role in extreme weather events. The connection of these events to climate change is an object of uncertainty and intense research.

Extreme weather events seem to be on the rise- very heavy rainfall, and with it the chance of floods; record temperatures, bringing sudden snow melt or unexpected blizzards; heat waves; and the antithesis of flooding: drought.

Since we last reported on extreme events in 1994, the Pacific Northwest and California have been hit by severe flooding for the third consecutive year, there has been a "500-year flood" in the Northern Plains, an unusually arid winter and early spring in the UK and France, heavy rains and drenching floods in Bolivia. Summer floods in 1995 in North Korea were the worst natural disaster of the nation's history, leaving half a million homeless, washing away schools, crops and livelihood in three-fourths of the country and costing the lives of 68 people. The floods reappeared in 1996 and have been followed by famine of still-to-be-confirmed dimensions.. Are these excessive occurrences within the realm of natural variation? possible indications of global warming? swings in phenomena which could become more exaggerated later?

Clinton Speculates on Global Warming Connection "We do not know for sure that the warming of the earth is responsible for what seems to be a substantial increase in highly disruptive events; but many people believe it is," said President Climate, speaking before his departure to survey the damages inflicted by the North Dakota floods. "We have to find the best scientific evidence... and we have to keep searching for the answers to this. I think every American has noticed a substantial increase in the last few years of the kind of thing we're going to see in North Dakota today. And if there is a larger cause ... we ought to go after that solution as well."

The North Atlantic Oscillation is one of the factors yielding new clues to the cold winter and drought in Europe and chilly spring in northeastern US. Dr. Michael McCartney, senior scientist at the Woods Hole Oceanographic Institution in Massachusetts, and his colleagues have focused on variability in the North Atlantic and its impacts for several years. The NAO, a shift that occurs periodically in winds and atmospheric pressure, is partially related to the more intense storms over the past ten years, says Tom Karl of the National Climatic Data Center.

The previous phase of the NAO has made Europe and North America warm and Greenland cold for the last three decades, and now the pattern is reversed"The Oscillation's new pattern is likely to persist for a few years, with continued cool weather" in Europe and North America, says Goddard Institute of Space Studies Director James Hansen.

We asked Dr. Hansen whether the greenhouse-induced warming is likely to change the NAO and if so how. He replied that this is a tough question; nobody knows the answer. He speculated that "the 30-year trend toward Greenland cool, North american and Eurasia warm may be driven by global warming, especially tropical worming - if this is correct, then the reversal of North Atlantic patterns of the past two years may be temporary - and as an El Nino is no starting, we may revert to the Greenland cold pattern."

Recent Extreme Events Around the World A freeze in Holland allowed skating on the canals for the first time in years. Heavy rains brought the worst flooding in 30 years to Brazil, and heavy rains in Peru triggered deadly mudslides. Brutally cold weather hit southern Russia and Scandinavia and many sections of continental Europe with temperatures averaging 6 degrees C below normal. Excessive precipitation fell in southern Spain, Gibraltar and Morocco. Tropical cyclones drenched Northern Australia, with some sections recording 200-400 mm of rain, and six week precipitation surpluses up to the 660 mm range.

In the past decade scientific knowledge about the role of the ocean in climate change has grown significantly. NOAA Administrator D. James Baker announced increasing attention to critical ocean concerns in his agency in late April, as the role of the ocean in climate change has attracted mounting attention.

The interaction of the ocean and atmosphere has become an essential element in models, and the coupled changes of these two variables improves predictability. The ocean has a much greater capacity to store heat and carbon dioxide than the atmosphere. It also has the ability to move these components around horizontally over large distances as the atmosphere does.

The ocean and atmosphere exchange momentum, heat and water vapor, in the process forming ocean currents. Ocean circulation stores, redistributes and releases the heat and vapor. In the ocean, sea surface temperatures, salinity and sea ice vary, heat and salt are distributed internally, the patterns and intensities of circulation are shifted. In the atmosphere the intensity and location of pressure centers and the storms they produce also vary, and heat and water are dispersed. The intense rain and drought, heat and cold, and storms are the product of these "coupled" changes.

The El Nino/Southern Oscillation is an example of this coupled variability. A pool of warm water appears in the western Pacific, moves east across the tropical ocean to South America, and exerts a tremendous effect on worldwide weather patterns. Research on ENSO has enabled climate predictions to become a great deal more accurate. While ENSO has received a great deal of attention, the scientific community, including meteorologists and ocean scientists, want to explore the connection of the tropics to mid-latitude variability and to extend predictability to the Atlantic and Indian Oceans.

Dr. Michael McCartney, Senior Scientist at the Woods Hole Oceanographic Institution in Massachusetts, and his co-workers have focused on variability in the North Atlantic and its impacts for several years. Sea surface temperatures of a basin of water off Newfoundland vary from cold to warm in periods about a decade long; near surface winds vary over the same time scale. In normal atmospheric circulation over the North Atlantic there is a high pressure cell near the Azores and a low pressure cell near Iceland and Greenland. Like the Southern Oscillation associated with El Nino, the North Atlantic Oscillation is a seesawing of barometric pressures between these two, with the westerlies blowing from North America toward Europe in the intervening space. North of Iceland and south of the Azores, the high winds are easterlies. This phenomenon has a strong influence on weather in Europe and North America

When there is a high NAO (intense westerlies) cold continental air from North America is warmed by heat from the ocean water it crosses as it continues to Europe. This has been the phase for the last decade. When there is a low NAO as in the last two years, the Icelandic low pressure cell moves far to the south off Newfoundland, a high pressure center moves over northern Greenland, cold dry polar air blows across northern Europe and west towards North America, warming on the way by heat released from the ocean to the atmosphere. Northern Europe experiences much cooler summers and more severe winters, Labrador is much warmer, and the northeast US has more nor'easters. Spring on the East Coast is windy, chilly, stormier as it has been this year.

The differing winds and contrasting warm and cool periods over North America and Europe contribute significantly to the distribution of global temperature change. Looking back historically over 50 years, McCartney also reports that the amplitude of the NAO appears to greater now, the variations from average have become larger. It is possible that, he speculates, that greenhouse gases may be modulating the oscillation. The NAO normally operates in the troposphere, but as the stratosphere is very sensitive to changes in greenhouse gases, it is possible that the boundary conditions for the NAO are changing.

A comparison of the NAO with a similar oscillation, the Pacific-North American, indicates that, over decades, there may be coordinated variations throughout the northern hemisphere or even the whole globe.

Unusual appearances of fresh water driven by ocean circulation could be an important key to the puzzle of the poorly understood NAO. One of the possible causes of the oscillation over a decade is the transformation of water from warm to cold. Winter winds cool the surface waters, they sink and mix more deeply into the cooler waters below. A mix of homogenized water, called mode water, is created. In summer, the sun heats the surface, forming a cap of warm water which isolates the mode water from the atmosphere. Surface cooling during the following winter removes the cap and the mode water is cooled again, growing thicker. It cools and thickens over several years while moving around the subpolar gyre, a massive system of current flowing counterclockwise in the North Atlantic between Labrador and the UK.. The trip around the gyre takes about a decade, with the water entering east of Newfoundland at a temperature of about 12 - 14û C and emerging from the pipeline in the Labrador basin ten years later at about 4û C.

The heat released from ocean to atmosphere by water flowing along this pipeline acts as a regional radiator for northern Europe where the westerlies carry the warm air. The effect of changes in surface salinity, the impact of an unusually large discharge of ice from the Arctic Ocean in 1967, and fluctuations in sea surface temperatures are properties of the ocean which need to be more clearly understood. How are they linked? asks McCartnery. Which are determining elements? Is the ocean a passive participant, responding to changes forced by the atmosphere? Or do feedbacks from ocean to atmosphere force the evolution of the atmospheric climatic?

Recent Extreme Weather Events in the US The month of December 1996 and the first week of January 1997 brought tremendous amount of rain and snow to parts of Washington, Oregon, Idaho, California, Nevada and Montana. Combined with unusually warm temperatures, there was enormous melting of snow, producing some record floods. The Truckee River in the Sierra Nevada Mountains reached its highest level on record. Lake Tahoe was higher than it has been since 1917. The Sacramento River flooding was at a 50-year level, according to a preliminary Army Corps of Engineers estimate. Mudslides were a severe problem. As of January 20, the death total was 36. It was the third consecutive year for severe flooding on the west coast, attributable to El Nino in 1995, but not in 1996. In early March the deadliest outbreak of tornadoes in three years hit Arkansas, Mississippi and Tennessee, accompanied by heavy rainfall and flooding over an even larger area. At the end of the month, the Northeast was hit by a major nor'easter, producing snowfalls of one to three feet. Boston had its heaviest April snowfall on record and third heaviest for any month. And a little more than two weeks later, the Red River in the Northern Plains broke its 100-year flood crest record of 39.1 feet at Fargo and went on to a 500-year record with a crest of 54 feet.

It makes sense, as McCartney notes, that changes in warm water ought to lead to changes in the atmosphere. But the link between sea surface temperature anomalies and their possible forcing of climate change signals has been elusive and "is one of the primary unresolved issues in climate change research." Continual measurements are necessary to further progress in unraveling the signals and the physics underlying them, to monitor evolution of the system and sharpen our understanding. Salinity is an important element, but monitoring of salinity is difficult in the vastness of the ocean and it cannot be done by satellite.

Another oscillation, the Madden-Julian, was surmised to be a factor in the heavy rains and flooding that began in the Pacific Northwest in late December. The Madden-Julian, first confirmed in 1971, is a 40-50 day fluctuation in the tropical atmosphere and ocean, one of the dominant variabilities within a season. In a strong MJO, an increased upward current of warm air shifts from the Indian Ocean to the western Pacific, strengthening the jet stream over the mid- latitudes of the North Pacific. It shows up in the troposphere and in the surface and upper layers of tropical oceans, in surface winds and flow of heat, in sea surface temperatures and in ocean currents. Satellite imagery confirmed that in late December it was directly related to an abnormally strong flow of warm moist air reaching from the western tropical Pacific to the west coast of North America. This flow, according to a special climate summary of the Climate Prediction Center was "strongly connected" to increased tropical convection associated with an MJO.

Like the NAO, no present theory fully explains the basic characteristics of this phenomenon which one scientist calls "one of the most important unexplained components of current climate."

There are other relationships between warming and extreme events which need investigation, although as Tom Karl of the National Climatic Data Center, warns, it is very difficult to assign any causes. In addition, he says, the deficiencies of models are "most apparent" in trying to reproduce extreme events. But, "it now seems probable that regional changes in severe weather and climate extremes will accompany warming,"

Intense Rain

Precipitation has been increasing -it has grown as much as 50 percent in high latitudes - and there is six percent more precipitation than a half century earlier. As the climate warms, the atmosphere is expected to change; it will hold more water vapor as temperature rises. In a warmer world there is likely to be more precipitation in each event even if the episodes of rain or snow do not change much.

It is estimated that flooding has taken the lives of nearly 120,000 people during the last 25 years, according to the British Institute of Hydrology, and has caused distress and disruption to over 750 million people. Such numbers are a strong incentive for any preventive actions we can take.

In the US, about 10 percent of total annual precipitation falls during very heavy downpours, defined as at least 50 mm (two inches) in a single day. At the beginning of the century, this share was 8 percent; and so the amount has grown 20 percent.

Drought

Paradoxically, soil in North America, southern Europe and other places is expected to become drier as higher temperatures dry the soil by raising evaporation rates and transpiration by plants. Several models project an increase in drought severity. In the US, there was a six-year drought in California in the early 90s, and drought lingered in the Southwest. A May 1996 headline reads, "Worst Drought Since 30's Grips Plains," and the accompanying article describes, how "from Kansas south to Texas one of the worst droughts on record has pushed thousands of farmers on the Great Plains to the edge of financial ruin." In Nevada in February 1996, another article reports, "The last eight years have marked the longest drought on record." New York and New Jersey had severe water shortages in 1995 "after a mild, largely snowless winter and a hot, dry summer." Around the world, there was a severe drought in 1996 in Bolivia that threatened the existence of the Chipaya tribe. Brazil's cocoa crop dropped by half during the five years preceding 1995, and "plantation owners have been walking away from their farms, hammered by drought among other reasons."

"Five years of unrelenting drought in southern and central Spain, the worst of the century, have sapped water from the streams and rivers and drawn the life from the region's rich groves and orange and olive trees." A dispatch from Bangladesh in May 1995 reports that, "More than 25 million Bangladeshis, almost a quarter of the country's population, are on the verge of becoming 'refugees' in their own land as the once-mighty Ganges River dries up, turning vast green rice paddies into sandy beaches." [Find info on recent drought in Africa.] A "Drought Watch" communiqué by the British Institute of Hydrology Reports in April 1977, "The arid start to the spring, coming on the back of an outstanding long term rainfall deficiency, has produced widespread and severe drought conditions. River flows and groundwater levels are exceptionally depressed and the very parched soils are producing difficulties for the farming community." An IPCC study concluded last June that Mediterranean wetlands are under intense pressure from drought. Spain has lost two-thirds of its inland wetlands since 1965. (Worldwatch, State of the World 1997, p. 82)

However, in examining drought records back to 1900, we find no confirmation of an increase in drought frequency and intensity. It is possible, Karl speculates, that at least in the early stages of global warming, other factors have overwhelmed the drying effects of warmer weather. It may be that increases in cloud cover in the US and Russia have led to a decline in evaporation. In fact in western Russia, soil moisture has actually increased.

Warmer Temperatures

Heavy rains and drought are not the only potential hazards of a warming climate; temperatures also fluctuate in unexpected ways. The summer of 1995 scorched much of the US, reaching a crescendo in the killer heat wave in Chicago. Although central and eastern US were relatively cool in 1996, the year still ranks as the fifth warmest worldwide, according to GISS researchers, and this is surprising they point out because it occurred despite the cooling effects of aerosols from the Pinatubo volcanic eruption and the record depletion of ozone. Some scientists believe the relatively cool US weather was caused by a reversal in the NAO.

We know that the Earth's surface has, on average, been warmer for the last 20 years than the preceding 20, although not uniformly so. Is this a forewarning of a crisis which may continue through the next century? asks McCartney. Or is it a phase of a natural oscillation lying on top of a less severe warming? Or is it the warming phase of a still longer oscillation? "There is a preponderance of scientific judgment, as carefully compiled and described by the IPCC" says McCartney, that the answer is somewhere between the first two possibilities, and that it is caused by human impact on the climate system "For statistical reasons," says Tom Karl, "even small increases in mean temperature or variability can make a big difference in the frequency or severity of extremes.... Temperature extremes can be very sensitive to unusual circulation/air masses. Sometimes trends in extremes do not coincide with trends in the mean." With a 6û F increase in mean July temperature, the probability of the heat index reaching 120 degrees F increases from 1 in 20 to 1 in 4 during July. There may be a reduction in early and late-season freezes, but little change in the risk of damage to vegetation; warmer days encourage an earlier and longer growing season, so the risk to tender vegetation may not differ.

More Stormlines

Because cyclones form only if sea surface temperatures are at least 26û C, early work on global warming suggested the number of cyclones would be likely to increase. But recent research, says Tom Karl, suggests this is simplistic. Other factors may play a role in storm development: atmospheric buoyancy, instabilities in wind flow, differences in wind speed at various heights. It is impossible to establish a record of the history of tropical cyclones in the 20th century because of changes in observing systems (satellites were introduced in the late 60s) and population changes in tropical areas.

In the North Atlantic, good records (by aircraft) since the 40s, show a decline in intensity and in the total number of hurricanes. From 1991 to 94, it was extremely quiet in terms of the number of storms and strong hurricanes. Even the unusually intense 95 season did not reverse the downward trend, Karl has written. But the number of storms in the northwest Pacific, on the other hand, seem to have risen. "Overall it seems unlikely that tropical cyclones will increase significantly on a global scale," Karl concludes. However, a recent report by Drs. Mark Sanders and Andrew Harris of University College London using statistical studies of Atlantic basin hurricanes shows that surface warming was the main influence on the 1995 hurricane season (more important than El Nino or rainfall in the Sahel), suggesting sea warming should be a factor in prediction calculations.

The outlook for the future depends on the trends in greenhouse gas emissions and aerosols. Or North Atlantic currents could suddenly shift causing fairly rapid change in Europe and eastern North America. The climate system is complex, and "the chance always remains that surprises will come about." (In a later issue we will have a report on potential climate surprises.)

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