Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 3.2. Waves in the Westerlies, Weather, and Climate AnomaliesMaps of winds in the upper atmosphere (e.g., at 30,000 feet), shown daily as the jet-stream in newspaper, television,and web-based accounts of current weather, typically show a meandering pattern of air flow with three to five“waves” in the westerly winds that circle the globe in the mid-latitudes. These “Rossby waves” in the westerliesconsist of sets of ridges, where (in the Northern Hemisphere) the flow pattern in the upper atmosphere broadlycurves in the clockwise direction, and troughs, where the curvature is in a counter-clockwise direction. Rossbywaves are ultimately generated by the temperature and pressure gradients that develop between the tropics andhigh-latitude regions, and in turn help to redistribute the energy surplus of the tropics through the movement ofheat and moisture from the tropics toward the middle and high latitudes. Over North America, an upper-levelridge is typically found over the western third of the continent, with a trough located over the region east of theRocky Mountains.Distinct surface-weather conditions can be associated with the ridges and troughs. In the vicinity of the ridges, airsinks on a large scale, becoming warmer as it does so, while high pressure and diverging winds develop at the surface,all acting to create fair weather and to suppress precipitation. In the vicinity of troughs, air tends to convergearound a surface low-pressure system (often bringing moisture from a source region like the subtropical NorthPacific or Atlantic Oceans or the Gulf of Mexico) and to rise over a large area, encouraging precipitation.From one day to the next, the ridges and troughs in the upper-level circulation may change very little, leadingto their description as stationary waves. Meanwhile, smaller amplitude, shorter wavelength waves, or eddies,move along the larger scale stationary waves, again bringing the typical meteorological conditions associated withclockwise turning (fair weather) or counter-clockwise turning (precipitation) air streams, which may amplify ordamp the effects of the larger scale waves on surface weather. Standard weather-map features like cold and warmfronts develop in response to the large-scale horizontal and vertical motions. Although uplift (and hence cooling,condensation, and precipitation) may be enhanced along the frontal boundaries between different airmasses, frontsand surface low- and high-pressure centers should be thought of as the symptoms of the large-scale circulation asopposed to being the primary generators of weather. The persistence or frequent recurrence of a particular wavepattern over weeks, months, or seasons then imparts the typical weather associated with the ridges and troughs,creating monthly and seasonal climate anomalies.The particular upper-level wave pattern reflects the influence of fixed features in the climate system, like theconfiguration of continents and oceans and the location of major mountain belts like the cordillera of westernNorth America and the Tibetan Plateau (which tend to anchor ridges in those locations), and variable features likesea-surface temperature patterns, and snow-cover and soil-moisture anomalies over the continents.drying comes to pass, it will impact the futureeconomic, political, and social development ofthe West as it struggles to deal with decliningwater resources.2.1.4 Impacts of Change in the AtmosphericBranch of the HydrologicalCycle for Ground Water and River FlowThe nature of these impacts ranges fromreductions in surface-water supplies affectingreservoir storage and operations, and deliveryand treatment of water, to drawdown of aquifers,increased pumping costs, subsidence, and reductionsof adjacent or connected surface-waterflows. A multitude of water uses, including irrigatedand unirrigated agriculture, hydroelectricand thermoelectric power (cooling), municipaland industrial water uses, transportation, andrecreation (National Assessment, 2000), canbe severely impacted by rapid hydroclimaticchanges that promote drought. Reductions inwater supplies that affect these uses can haveprofound impacts on regional economies. Forexample, drought in the late 1980s and early1990s in California resulted in a reduction inhydropower and increased reliance on fossil81