Nature of Climate Variability
Devils Lake responds directly to climate variability across the region. This climate variability generally can be regarded as the movement of the jet stream from season to season and from year to year. The jet stream, which is a ribbon of high-velocity air located about 30,000 feet above the Earth's surface, exists because of temperature differences between air masses at the Earth's poles and at the equator. The movement of weather systems along the jet stream determines the distribution of precipitation about the globe. Climate variability results from a long-term shift in circulation patterns of the jet stream. As the circulation patterns shift, precipitation and temperature patterns also shift. Devils Lake has an enhanced sensitivity to long-term shifts in global circulation patterns as the level of the lake depends on many years of antecedent precipitation, runoff, and evaporation. If at any time precipitation, runoff, or evaporation is dominant, a corresponding dramatic response occurs in the lake level.
Global atmospheric circulation patterns are driven predominantly by variations in sea-surface temperatures The most noticeable of these variations, known as El Nino, occurs in the equatorial Pacific and accounts for the dramatic variations in precipitation patterns along the western equatorial regions of South America. Across the plains of the northern United States and southern Canada, El Nino and its cold counterpart--La Nina--produce variations in precipitation and temperature patterns primarily in the winter months (Montroy, 1997). However, long-term variations in annual precipitation and temperature patterns across the region also occur as a result of variations in the tropical sea-surface temperatures. These long-term variations often span decades and are instrumental in the occurrence of flood and drought conditions across the Devils Lake Basin and elsewhere.
When the position of the jet stream across the western United States shifts to the southwest, strong storm systems move predominantly from the southwest at upper levels. These storm systems, typically referred to as Colorado Lows, cause unstable conditions across the Devils Lake Basin because warm, moist air from the Gulf Coast interacts with cool, dry air from Canada. The net result is a higher frequency of both warmer and wetter conditions across the Devils Lake Basin than during more stable periods. When the position of the jet stream shifts to the northwest, the Devils Lake Basin experiences a high frequency of Alberta Clippers, which are associated with cold, dry conditions in the basin. When the position of the jet stream shifts to a more westerly flow, referred to as a zonal pattern, the Devils Lake Basin generally experiences more normal precipitation and temperature patterns (patterns close to long-term seasonal average precipitation and temperature patterns).
This page originally published at: http://nd.water.usgs.gov/devilslake/science/climatology/climatevar.html