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Updated 13 August, 2004
Climate Oscillations of Short Duration and the Long Term Climate Warming
- Sorting Out the Climate System
USGCRP Seminar, 20 March 2000
Dr. J. Michael Hall
Dr. John M. Wallace
Dr. Ants Leetmaa
Some component of the observed climate variability is natural, while it is quite possible that some part of it may be human induced.
Suggested modes of natural climate variability, dubbed with acronyms such as ENSO, PDO, IPO, NAO, AO, NAM and SAM, abound in the climate literature, but when grouped together appropriately, only two classes of phenomena seem to figure prominently in global and U.S. climate variability. One of these oscillations is rooted in the Pacific sector (the PDO or Pacific Decadal Oscillation) and the other is a pair of ring-like modes encompassing the entire circulation of their respective hemispheres, but focused on the polar regions. Although known for quite some time, meteorologists have only recently begun to understand these oscillations sufficiently in order to incorporate them in climate diagnostics and prediction.
El Niño and El Niño-Like Phenomena of Longer Duration (the PDO)
El Niño has important impacts on global and U.S. climate on time scales ranging from seasons to a year or two. Instrumental records of El Niño events extend back about 140 years, and historical evidence dates back to the Spanish conquest of Peru. The El Niño cycle has exhibited a wide variety of different kinds of behavior in the past, but it is difficult to find convincing evidence of long-term trends. The El Niño events of the late 19th century appear to have been remarkably similar to those of the past few decades.
The irregular cycling back and forth between warm (El Niño) and cold (La Niña) phases of the ENSO (El Niño Southern Oscillation) cycle in the equatorial Pacific results from a complex interplay between the strength of the surface winds that blow westward along the equator and the subsurface currents and temperatures. El Niño events like the very strong one that prevailed from mid-1997 through mid-1998, are marked by above normal sea surface temperature across a broad expanse of the equatorial Pacific. Abnormally heavy rainfall over these warm waters extends into coastal Ecuador and northern Peru, while drought conditions often prevail in parts of Australia, Malaysia, Indonesia, and Micronesia, Africa, northeast Brazil, central America and tropical Africa. The impacts of El Niño over the United States vary from one event to the next and they tend to be restricted to winter and early spring. On average, Hawaii tends to be dry; Alaska, the Pacific Northwest and the Northern Plains, mild and dry, and the South, wet. Cold years in the equatorial Pacific, popularly know as a 'La Niña' years, tend to be marked by the opposite kinds of climate phenomena in the above regions.
El Niño-like variability is also evident on a decadal time scale, particularly over the North Pacific. For example, the shift toward more El Niño-like conditions over the North Pacific around 1976-77 has had a modest but significant influence on wintertime temperatures and rainfall over much of the U.S. Since 1977 winters have been wetter in the South than in previous decades, milder and drier in the Pacific Northwest, and much warmer in Alaska (typical of conditions associated with El Niño years). These changes have had important impacts on water supply and salmon recruitment in the affected areas. With the benefit of hindsight, several such 'regime shifts' can be identified in the climate record. Evidence of such shifts is also found in tree-ring records that date back centuries. This distinctive pattern of decadal-scale variability has been dubbed the "Pacific Decadal Oscillation". Whether it is distinct enough from El Niño to deserve a name of its own remains to be seen.
The circulation in both Northern and Southern Hemispheres exhibit ring-like (or 'annular') modes of variability encircling the poles. These modes fluctuate in time scales ranging from a week to decades, and may be viewed as oscillating back and forth between 'high-index' (or positive) phase and 'low index' (or negative) phase. High index conditions are marked by below normal barometric pressure over the polar cap regions, above normal pressure in mid-latitudes, enhanced subpolar (55 degrees latitude) westerlies, enhanced subtropical tradewinds, and above normal temperatures over most continental regions including the contiguous United States. The fluctuations in the Northern Hemisphere annular mode are most pronounced during late winter. In Europe, where this mode is commonly referred to as the North Atlantic Oscillation (NAO), there are related fluctuations in precipitation. High index conditions tend to be wet in Scotland and Scandinavia and dry in Mediterranean countries, and vice versa - mild in northern Europe and cool in the Middle East and North Africa. Here in the contiguous U.S., where this mode is sometimes referred to as the Arctic Oscillation (AO) in recognition of its symmetry about the North Pole, high index conditions tend to be mild. For reasons that are not entirely clear at this point, both annular modes have exhibited a trend over the past 30 years toward lower pressure over the poles and stronger subpolar westerlies extending upward into the stratosphere. The annular mode has contributed substantially to the strong trend toward milder winters over the U.S. (except Alaska) and over most of Eurasia in over the past 30 years.
There has been speculation that the observed trends in both the Northern Hemisphere and the Southern Hemisphere annular modes toward the high index state might be a response to human activities: either CFC-induced ozone depletion and/or greenhouse gas induced stratospheric cooling. There has also been speculation that global warming could shift the balance between the frequency of occurrence of El Niño and La Niña events or change their intensity. However, if there is such a trend in the statistics, it is not obvious in the historical record.
The background variability that remains after the effects of these various "climate oscillations" are removed from the climate record is largely attributable to: (1) random chance; (2) poorly understood episodic phenomena like the 'dust bowl' of the 1930s, which may or may not be inherently random; and (3) a long term global warming.
The Climate Prediction Center has been making official seasonal forecasts of temperature and rainfall for the U.S. out to a year in advance, since 1995. The initial forecast tools were heavily biased towards projecting recent trends forward into the immediate future, taking into account some impacts of the ENSO (consisting of an El Niño or warm phase, and a La Niña or cold phase) oscillation.
Since 1997, forecasts began to be primarily based on the impacts of ENSO. A successful forecast was made for the winter of 1997/98, based on the anticipated impacts of a major El Niño. However, forecasts for the past two years have been mostly influenced by the onset of a La Niña. In addition, the expected impacts were further modified to account for long term trends in temperature and rainfall since 1970. Although these forecasts showed considerable skill for the wintertime, the spatial extent of the warming was underestimated both for El Niño and La Niña winters. As a result, it became clear that even during major ENSO events certain features may have been underestimated and/or other factors may have been important in determining U.S. seasonal variability.
The improved understanding of natural climate oscillations such as the PDO and the ring-like modes (e.g., the Arctic Oscillation), and the impacts of these on seasonal variability in U.S. temperature and rainfall, provide the key for unlocking the origins of the systematic errors in short-term forecasts. Estimates can now be made as to how each of these oscillations contributes regionally to the recent wintertime warming trend. For example, average wintertime temperatures for the period 1988 through 1997 have been warmer than normal for the whole U.S., with anomalies of over 1 degree C for much of the country, and anomalies on the order of 1.5 degrees C for the upper Midwest. The predominately positive (warm) phase of the AO during this period contributes to the warming over much of the country, with the strongest impacts located in the upper Midwest. The PDO contributes to warming in the northwest, but causes cooling in the southeast. Averaged over this ten-year period the impacts of El Niño and La Niña are relatively insignificant.
Subtraction of the contribution of the PDO and the AO from the overall warming for this decade gives what is referred to as the "residual" warming, or that part of the warming not accounted for by the PDO and/or the AO. This "residual" warming represents the effects of the long term warming trend and any plausible yet presently unknown natural modes of variability. This "residual" warming is comparable to the contributions of these natural oscillations, and is largest in the southern and southwest U.S.
A similar computation can be done for the trend in global mean temperatures over the past 50 years. Whereas the overall temperature trend shows areas of cooling and warming attributable to natural climate oscillations such as the PDO and AO, there is a large component of the warming not accounted for by these oscillations (a "residual" warming), that occurs largely over land areas with weaker warming over the ocean, as one might anticipate as a result of the ocean's resistance to warming (its thermal inertia).
Dr. John M. Wallace is a professor in the Department of Atmospheric Sciences, and co-director of the Program on the Environment, at the University of Washington, Seattle. From 1981-98 he served as director of the (University of Washington/NOAA) Joint Institute for the Study of the Atmosphere and Ocean. His research interests and expertise include the study of atmospheric general circulation, El Niño, and global climate. He is a member of the National Academy of Sciences; and a fellow of the American Association for the Advancement of Science, the American Geophysical Union, and the American Meteorological Society. He is also a recipient of the Rossby medal of the American Meteorological Society and the Roger Revelle medal of the American Geophysical Union. He has served on numerous panels and committees of the National Research Council.
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 a variety of positions ranging from Oceanographer, Chief, and Senior Scientist at NCEP (National Center for Environmental Prediction) between 1986 and 1997. His professional interests include 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 served as chair or member 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.
Dr. Leetmaa received a B.S. degree in Physics from the University of Chicago in 1965, and a Ph.D. degree in Oceanography from the Massachusetts Institute of Technology in 1969.