Leon F. Osborne, Jr.

Professor, Department of Atmospheric Science

Located in the center of the North American continent, the climate of North Dakota is predominantly semiarid with characteristic cold winters and warm summers. The amount of precipitation on average fluctuates significantly from one year to the next. The dramatic swings in temperature and precipitation are controlled by a variable weather pattern owing its dependence on the mean position and intensity of the polar jet stream. The polar jet stream, located approximately 42,000 feet above sea level, exists because of the larger-scale north-south temperature differences throughout the atmosphere. As a result when the jet stream is south of the state, temperatures are generally colder. When it is north, temperatures are warmer. Because the polar jet stream winds always blow with a westerly component across North America, storm systems associated with the polar jet stream move in a general west to east direction.

Air flowing across North America must confront the Rocky Mountains as it moves across the continent. With the Rocky Mountains serving as a barrier to low-level moisture transport into North Dakota from the Pacific Ocean, the only significant source of moisture is supplied by the Gulf of Mexico. The transport of this Gulf moisture is dependent upon a southwest to northeast orientation of the polar jet stream. This promotes the formation of extratropical cyclones (low pressure centers) in the lee of the Rocky Mountains and with their cyclonic (counterclockwise) rotation, these extratropical cyclones supply low-level moisture that often moves quickly across the nearly 1,500 miles from the Gulf coast to the Northern Plains. As the centers of these extratropical cyclones move northeastward under the influence of the polar jet stream, the movement of cold air southward behind the storm’s center and warm air lifting over and around the cold air ahead of the storm’s center results in cloud and precipitation formation. While these southwesterly storms generate of the majority of the annual precipitation across the North Dakota, they constitute only a small fraction of the total extratropical systems that cross the region. The remainder of the extratropical systems that cross the state may produce minor precipitation amounts, but largely provide nothing more than a change in temperature and cloud cover. The frequency and intensity of these southwesterly storms largely determine the variability in the annual precipitation that occurs across the state. During years where there are higher frequencies of these extratropical cyclones, the region receives excessive precipitation. However, in some years these cyclones fail to occur and the region undergoes a dramatic decrease in precipitation. Successive seasons with a low frequency of southwesterly cyclones often result in drought conditions across the Central and Northern Plains.

The determination of the position and orientation of the polar jet stream is partly due to the geology and geography of North America. The presence of the Rocky Mountains as a broad north-south mountain massif results in frequent development of long-term alterations of the upper-level winds known as persistent flow anomalies. These persistent flow anomalies may exist for periods ranging from two weeks to as long as an entire season. A high amplitude ridge, a form of a persistent flow anomaly, that often establishes itself with an axis centered over and parallel to the crest of the Rockies, will typically result in colder than normal temperatures and dry conditions in North Dakota and western Minnesota. On the other hand, some orientations of persistent flow anomalies can result in warm, wet conditions across the state. An omega block, a persistent flow anomaly named for its shape in the form of the capital Greek letter omega, sometimes forms along the Appalachian Mountains in the eastern United States. This results in strong southwesterly flow across the western half of the United States and produces extratropical cyclones that become trapped in the Upper Midwest often producing heavy precipitation across the state, particularly in the Red River Valley.

While persistent flow anomalies owe part of their existence to the lay of the land, they are also heavily influenced by the global circulation pattern about the Northern Hemisphere. The global circulation in the Northern Hemisphere is the result of the atmosphere attempting to achieve a balance between an excess of solar radiation near the equator and a lack of solar radiation at the poles. This difference in solar radiation produces a north-south temperature contrast between the equator and poles that drive the winds around the planet. Excursions of warm air northward and cold air southward in the vicinity of extratropical cyclones is a response of the atmosphere attempting to achieve a temperature balance across the hemisphere. Eventually, the continual occurrence of extratropical cyclones brings the atmosphere into an approximate temperature balance. However, as the balance is achieved there is no reason to mix warm low-latitude air with cold high-latitude air and the north-south temperature imbalance returns as the frequency and intensity of extratropical cyclones diminishes. Approximately every four to six weeks the Northern Hemisphere goes through this cycle of balancing the hemispheric temperature differences.

Of course the intensity and frequency of occurrence of extratropical cyclones crossing North Dakota is directly dependent upon not only this processes of balancing the contrast in temperature across the hemisphere, but also on the orientation of the polar jet stream across North America. Unfortunately, this is where the atmosphere truly becomes complex as the position of the polar jet stream is influenced not only by geography i.e., locations of mountain ranges, coastlines, but it is influenced by planetary-scale circulations found hidden within the background of the global circulation. Most of these planetary-scale circulations are quasi-permanent in there locations. In the Western Hemisphere, two such circulations dominate at higher-latitudes. The Pacific-North American (PNA) oscillation is most commonly found in the vicinity of the Gulf of Alaska. The North Atlantic Oscillation (NAO) is most commonly found in the vicinity of Greenland. Although they seemingly move little from year to year, only minor changes in location can result in a significant modification in the polar jet stream location and hence affect the precipitation and temperature distribution across the Northern Hemisphere including the Northern Plains. While the PNA and NAO appear to move independently of one another, it is still not well understood what causes the movements. One major event that appears to be a significant factor in the movement of the PNA is the strength of the El Niño – Southern Oscillation.

Since water can store energy from solar radiation more effectively than land, the excess in solar radiation in the tropics is often not immediately transferred into a warming of the tropical atmosphere. Further, the sea-surface temperatures warm considerably more in the tropics than at higher latitudes. Left alone from any oceanic circulation the tropical Pacific Ocean surface temperatures would be fairly uniform from west to east. However, there is a periodic redistribution of sea-surface water temperatures from east-to-west across the tropical Pacific Ocean associated with unusually warm waters along the coast of Peru. This El Niño or "The Christ Child", named for its appearance near Christmas, results in not only redistribution in precipitation in the tropics, it also alters the north-south gradient in temperature in an east-west direction across the Pacific Ocean. This in turns results in a modification of the upper-level winds positioned around the Northern Hemisphere. A similar event occurs when the upwelling of cold water occurs in the eastern tropical Pacific resulting in La Niña or "The Little Girl". As would be expected, the north-south temperature gradient across the Pacific also reverses resulting in a different pattern of upper-level winds across the Northern Hemisphere. A cycle of El Niño events occurs on average every 4 to 7 years with La Niña events frequently falling between the peaks in El Niño. Further, studies now reveal that the east-west redistribution in tropical Pacific sea-surface temperatures also results in a fluctuation in the oceanic currents found throughout the Northern Pacific Basin. Evidence now suggests that the resulting variation in ocean currents produces long-term cycles in the position of the Pacific-North American oscillation from 20 to possible over 100 years in duration. These fluctuations in the PNA induced by El Niño/La Niña along with variations in the strength and position of the NAO can be linked to longer-term cycles in positions of persistent flow anomalies across western and central Canada. These in turn represent changes in the climatic conditions across the Northern Plains.

It is apparent that understanding the controlling factors on the polar jet stream’s strength and position will provide us with a greater knowledge of how and why extremes in North Dakota’s annual precipitation occur. Excessive precipitation over the past six years has been a direct result of a more persist polar jet stream aligned from southwest to northeast across the Central and Northern Plains. Winter snowfall amounts this decade are double the thirty-year average across North Dakota and western Minnesota. Meanwhile summer precipitation has been more persistent, when it occurs, resulting in summer rainfall amounts as high as thirty inches or more across parts of the North Dakota for each year since 1993. While spring flooding of the Red River has been a frequent occurrence since records have been kept in the Valley, the present string of dramatic flooding during the 1990s appears to only have been matched once before during the 1820s. Unfortunately, atmospheric records necessary to compare trends in weather during the 1820s with the present weather conditions do not exist. However, anecdotal evidence gathered from diaries and logs of early Red River settlers indicate a period of winter and spring weather conditions that appear to be more similar to conditions of recent years than to conditions found anywhere else within the twentieth century. Clearly, a continuation of a favorable El Niño/La Niña condition coupled with the present location of the Pacific-North American oscillation points to further wet conditions across the Northern Plains in the years to come.

But what about when the polar jet stream diverts for an extended period to a position away from the moist southwesterly direction? Recent studies have shown that periods of intense drought are not a rarity in the Northern Great Plains climatic history. Lake salinity and tree ring data reveal that a series of "Megadroughts" occurred prior to1200 AD. These droughts encompassed decades and in some instances centuries at a time with extremely dry conditions. Although nothing near being megadroughts, two of the most severe droughts to affect the Northern Plains in the recent past occurred from 1988 to 1992 and during the "Dust Bowl" in the 1930s. As with the study of the wet climatic conditions of the 1800s, we can say little directly about the structure of the polar jet stream during the 1930s. However, because of the detailed news accounts documenting the persistence of the dry westerly surface winds and the low relative humidity, we can reconstruct upper-level weather patterns that were similar to the upper-level wind conditions of the late 1980s. During this latter time period the polar jet stream actually split into two parts with each part flowing around a persistent flow anomaly centered over the Rocky Mountains across Colorado through Montana. The converging upper-level winds as the two branches of the polar jet stream passed around the persistent flow anomaly resulted in subsiding air across the Central Plains states. This drying of the atmosphere not only resulted in clear skies, it blocked any attempt by extratropical cyclones to bring Gulf of Mexico moisture northward into North Dakota and Minnesota. While lasting only three years, this drought devastated crops and rangeland and placed a stress on available water supplies. Should the upper-level winds have persisted in a drought-supporting manner for another five years, an economic disaster unprecedented in the Northern Plains would have occurred.

Much is still to be learned about the controlling influences on the polar jet stream. However, we do understand the importance of the polar jet stream upon the annual precipitation occurring in the Northern Plains. Until it is possible to accurately predict future positions and strengths of this ribbon of upper-level winds seasons to years in advance, the management of water resources important for crop production will require cautious over preparation to mitigate the worst nature has to provide – either from flood or drought. Anything less will create an opportunity for disaster that might exceed the greatest that has already occurred. Of course, as financial resources will always be a critical factor in preparation, the benefit to risk evaluation will need to be made by decision makers from political leaders to individual producers. Because of the magnitude of the threat potential and the limitation of resources to develop and implement mitigation plans, understanding of the climate possibilities should lead to region wide cooperation in planning and preparation.

It is also important that a bridge of communication be constructed between resource managers and the atmospheric science community as the latter expands its knowledge of why and how climate variations occur. This dialog with those charged with developing an understanding of the atmosphere’s circulation could provide insight to what has recently occurred and what shortly may happen. Turning this dialog into planning and preparation will require close cooperation from both groups.

Understanding nature is elusive, for when you believe you have a grasp on the processes involved, you suddenly find a new and unexpected problem to solve. Such is our quest to understand our climate and what it will yield in everyday weather. Atmospheric science is but a young discipline scarcely a century old. By the end of the next millennium we will look back and marvel at our primitive understanding of the atmosphere. Maybe then, too, the questions in water resource management will be simpler as well.