Updated 12 October, 2003
Niño/La Niña and Hurricanes: Was 1999 a Preview of the Near
USGCRP Seminar, 16 December 1999
Dr. James Baker
Dr. Ants Leetmaa
Dr. Larry B. Crowder
Research on the Earth's climate system is beginning to provide answers to a host of important social and policy-relevant questions, as well as nurturing the development of tools for practical applications such as forecasts of El Niño/La Niña events. A major key to understanding and predicting climate variability lies with the improved understanding of the El Niño/Southern Oscillation (ENSO) phenomena. Coupled ocean-atmosphere interactions in the Tropics lead to multi-year fluctuations in large scale patterns of tropical ocean temperatures, and major changes in the geographic setting of significant rainfall in the Tropics, all of which lead to seasonal changes in the mid-latitude jet stream track and regional storminess.
The improved understanding of the ENSO phenomena has lead to a routine forecast capability for it and its regional impacts with respect to temperature and rainfall over the U.S. Forecasts made one to two seasons in advance, for the winters of 1997/1998 and 1998/1999 for example demonstrated remarkable skills given the unusual patterns of temperature and rainfall that occurred. During both winters the U.S. experienced record-breaking warmth. Although the forecasts were able to account for significant aspects of this warmth, it is clear that the climate of the U.S. is responding to more than ENSO variability alone. The experiences of the past two years are also demonstrating how regional changes in intense storminess are linked to circulation changes associated with climate variability. One aspect of this is their impact on seasonal variability in the numbers and intensity of hurricanes.
NOAA and Dr. Gray's research group from Colorado State University made seasonal forecasts for the 1999 hurricane season. Although the techniques underlying their forecasts are different, both forecasts suggested a significant likelihood of an above-average tropical storm and hurricane season. The season indeed was active with 12 named storms, 8 hurricanes, and 5 intense hurricanes being observed. By comparison, from 1950-1990, the average number of named storms in an Atlantic storm season was 9.3, while the average number of hurricanes was 5.8, and the average number of intense hurricanes was 2.2. Dr. Gray also noted in his forecast that "we have entered a new era of enhanced major hurricane activity". Indeed, observations show that a relatively abrupt change occurred in 1995 and since then hurricane activity has increased relative to the previous 20 years. Interestingly, as Dr. Gray also indicated, there was an abrupt drop in the number of Pacific storms at the same time. This latter observation suggests that global, rather than Atlantic-specific processes are responsible.
Abrupt shifts in climate regimes appear to be the norm rather than the exception over the past 100 years (and for probably much longer than that). Examples of such abrupt changes include changes in temperature, rainfall, and marine ecosystems in the Pacific Northwest. Research now points to the existence of two other climate oscillations: the Pacific Decadal Oscillation (PDO), and the Arctic Oscillation (AO), both of which contribute to abrupt shifts in climate regimes as described above. The manifestation of the AO in the Atlantic is usually referred to as the North Atlantic Oscillation (NAO). These decadal-scale climate oscillations have also been linked to long term climate trends in the U.S. and the globe. Research suggests that the long-term change in the behavior of the AO be possibly linked to changes in the stratosphere resulting from the long-term climate warming that has been observed to date.
Because of their resemblance to an El Niño or La Niña condition, but operating on decadal timescales as opposed to El Niño/La Niña time-scales (typically on the order of 2-5 years), PDOs are often thought of as weak but persistent El Niño- or La Niña-like states or conditions. As such, they also are associated with changes in global tropical ocean temperatures, rainfall, and modification of mid-latitude circulation patterns, but operate on longer time-scales. The summertime rainfall pattern associated with the PDO for the 1977 to 1995 period would tend to suppress Atlantic hurricane activity, enhance tropical storm activity in the Pacific, and bring above normal rainfall to the central U.S. However, there are indications that this PDO pattern may have shifted from an El Niño-like pattern to a La Niña-like pattern. The observed and anticipated regime change in hurricane activity in the Atlantic and Pacific is consistent with this notion, as is the development of drought in the eastern U.S. If this observation is correct, next summer's climate in the U.S. will be characterized by continued drought and a more active hurricane season.
With forecasters predicting additional, and possibly more intense hurricanes in the wake of the hurricanes of 1999, North Carolina is only beginning to fully assess the human and environmental impacts. Less than two weeks after Hurricane Dennis skirted the North Carolina coast and churned off Cape Hatteras for days, Hurricane Floyd cut a swath across the coastal plain dropping fifteen or more inches of rain in watersheds already saturated by Dennis. Huge floods resulted, devastating much of Eastern North Carolina. Floyd flooded farms and cities, washed away houses and cars and killed 51 people, ranking in the top 20 for hurricanes this century. Floyd also inundated sewage treatment plants, hog waste lagoons, and junkyards. The flooding grabbed national headlines and pictures conveyed the extent of human suffering. What these reports did not completely reveal was how the flooding flushed the by-products of human civilization––nutrients, human and livestock wastes, and toxic chemicals--downstream into Pamlico Sound. Just three weeks after Floyd, Hurricane Irene came ashore––the first time in nearly half a century that North Carolina sustained an assault from three major hurricanes in a single season.
In addition to the toll on human lives, 2.8 million chickens and turkeys, 880 cattle and over 30,000 hogs were killed. Thirty towns were completely submerged and 30,000 homes were damaged––nearly a third were destroyed. Initially, over 48,000 people sought refuge in shelters––over 2,000 families still live in temporary housing. Infrastructure damage included 650 closed roads and 14 bridges and culverts destroyed--22 dams failed and nearly 80 were damaged. Over 900 community drinking water systems and thousands of private wells were flooded and dozens of public school and university buildings were inundated. As of December 4th, state and federal recovery programs had provided over $630 million to disaster victims. Governor Hunt recently proposed to the state legislature an additional $830 million recovery package to aid homeowners, farmers and small businesses. Insured losses due to Dennis and Floyd alone top $863 million; claims to the national flood insurance program have jumped in the past six years, losing a cumulative $1.6 billion. Still FEMA (Federal Emergency Management Agency) says that over 115,000 North Carolina homes and businesses that should be insured for flooding are not. The total costs related to the hurricanes of 1999 are not yet in, but total losses are likely to exceed $5 billion, placing 1999's storms in the top three most costly this century.
During the floods, 50 animal waste lagoons and 24 municipal wastewater treatment plants spilled millions of gallons of organic waste into coastal rivers and sounds. Over 30,000 farms were flooded; washing away crops and soils from thousands of acres. Water quality problems plaguing the Neuse and Pamlico Rivers for decades have illustrated the connections between land use and water quality. Up to now, however, most scientists believed that these problems did not extend to Pamlico Sound. While many of our rivers (some wholly or partially polluted) still act as nurseries for 75-80% of recreational and commercial fishes, Pamlico Sound itself is doubly important as critical habitat and major fishing grounds. Many young fish, including gag grouper, Black Sea bass and summer flounder use the saltier waters of the sound, and nearly 75% of shrimp are landed there.
Pamlico Sound now suffers from a spate of additional stresses brought on by these recent storms. Freshwater from Floyd reduced salinity to half the pre-Floyd values, stressing or killing many marine invertebrates. Less dense freshwater floated over salty bottom waters isolating them from the wind and reducing oxygen levels. Bacterial populations soared in response to organic wastes, further depleting oxygen. A 40-square-mile low-oxygen zone persisted for three weeks, stressing juvenile fish, shrimp and crabs. Hurricane Irene however, mixed the oxygen back into Pamlico Sound preventing a massive fish kills.
The aftermath of these events also witnessed an increase in fish ailments. Menhaden, spot, croaker, pinfish and other fish were found with open, bleeding sores and deadly bacterial infections. Their skin and scales sloughed off and their stomachs swelled. Chances for survival are likely to be slim. Nutrients have increased potentially problematic algae ten fold; the blooms would have been larger but Pamlico Sound is now stained nearly black, blocking light that would otherwise have spurred even more algal growth. Cooling temperatures have temporarily calmed these troubled waters, but huge quantities of nutrients and organic wastes remain in Pamlico Sound, setting the stage for potentially serious problems next spring and summer.
The last time North Carolina's coast experienced so many damaging hurricanes was in 1954-55, when Hazel, Connie, Diane and Ione struck the Carolina coastline in less than one year. Based on correlations between climate variability and hurricane frequency, Dr. William Gray (Colorado State University) now suggests that we may be entering a new era of greater hurricane frequency. Such forecasts, coupled with staggering recovery costs, suggest that there are important lessons to be learned from the hurricanes of 1999 in order to make coastal communities of the U.S. more disaster-resistant. For example, in the case of North Carolina, limited hurricane track information, limited wind and flood forecasts, and poor land use practices may well have exposed North Carolina's communities, economy, and environment to excessive risk in the face of such storms.
Dr. Ants Leetmaa is the Director of the Climate Prediction Center (CPC) of NOAA's National Weather Service. Prior to his arrival at the Climate Prediction Center, Dr. Leetmaa was engaged in postdoctoral studies at MIT (1969-1972), sea-going oceanographic work at the Atlantic Oceanographic and Meteorological Laboratories of NOAA in Miami (1972-1986), and positions ranging from Oceanographer, Chief, and Senior Scientist at NCEP (National Center for Environmental Prediction) between 1986 and 1997. His professional interests have varied from studying large-scale ocean circulation to understanding ocean-atmosphere interactions that lead to natural climate variability, and the development and use of ocean-atmosphere models for seasonal climate forecasting. He has been, or is, a member or chairman of a number of national and international committees dealing with ocean observing systems, climate studies and forecasting, and the development of NOAA programs in these areas. The mission of the Climate Prediction Center is to make long range forecasts including 6-10 day averages, and monthly and seasonal forecasts out to a year in advance; to provide real-time climate monitoring and products for both U.S. and regions of the globe; and to study climate fluctuations on time scales from weeks to decades in order to incorporate these processes in the forecasts. Through its monitoring and prediction activities the CPC assists agencies both inside and outside the federal government in coping with climate related problems in food supply, water resources, energy allocation, and emergency management. The CPC products and services are constantly being refined to meet the needs of customers both in private and public sectors. Dr. Leetmaa received his B.S. degree in Physics from the University of Chicago in 1965 and a Ph.D. in Oceanography from the Massachusetts Institute of Technology in 1969.
Dr. Larry B. Crowder is the Stephen Toth Professor of Marine Biology at the Duke University Marine Laboratory, Nicholas School of the Environment, Beaufort, North Carolina. Dr. Crowder's research interests are in population and community ecology, fisheries ecology and conservation biology. He has been conducting fisheries ecology research in North Carolina for over 15 years and has published over 100 peer-reviewed papers, book chapters and reports. His recent research has examined the effects of nutrients and low oxygen on fish distributions, growth and survival in the Neuse River and Pamlico Sound. He also led a multi-institutional research program on environmental variation and fisheries recruitment in the Southeastern U.S., (the South Atlantic Bight Recruitment Experiment, funded by the National Oceanic and Atmospheric Administration. Dr. Crowder has served on many national science review panels including the National Science Foundation's Ecology and Biological Oceanography panels, various state Sea Grant programs, the steering committee of the U.S. Global Ocean Observing System, the NOAA Fisheries Oceanography Panel, and advisory committees such as the National Marine Fisheries Service's Sea Turtle Expert Panel, and the Scripps Institution of Oceanography's Institutional Review. He has twice won the outstanding paper of the year award from the American Fisheries Society and three of his students have been recipients of the J. Francis Allen Scholarship for the outstanding Ph.D. student in Fisheries. He has also served on editorial boards for Ecology and Ecological Monographs< Copeia, and Oecologia, in addition to conducting reviews for all the major journals in his field. Dr. Crowder received his B.S. degree in Biology and Mathematics at California State University-Fresno, in 1973, and his M.S. (1975) and Ph.D. (1978) degrees in Zoology at Michigan State University.