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494 assessment of climate change in the southwest united statesa clear example of the dipole is during El Niño winters, when the Pacific Northwesttends to be dry and the Southwest tends to be wet.North American monsoon – a shift in the large-scale atmospheric circulation that bringsmoisture originating from the Gulf of Mexico, Gulf of California, and Pacific Ocean intothe Southwest from around July to Septemberone-hundred-year events – climatic events that have a 1% probability of occurrence inany given year and a 100% chance of occurring in 100 yearsorographic precipitation – precipitation that occurs on the windward side of a mountain,caused by drafts of moist air forced upward along the ridgeozone – a form of oxygen that forms naturally in the stratospheric portion of the atmosphere,where it absorbs most of the sun’s UV radiation. In the lower atmosphere it isconsidered an air pollutant, where it is produced through chemical reactions betweennitrogen oxides and hydrocarbons typically produced by the burning of fossil fuels.Ozone in the lower atmosphere is an important factor that worsens some respiratoryailments.Pacific-Decadal Oscillation – a pattern of climate variability in the Pacific Ocean thatshifts phases approximately every twenty or thirty years; the positive phase exhibitsmore El Niño-like conditions; the negative phase brings on more La Niña-like conditionspaleoclimate – (from IPCC AR4 WGI): “Climate during periods prior to the developmentof measuring instruments, including historic and geologic time, for which onlyproxy climate records are available. Proxy climate records include tree rings, pollencores, and ice cores.”paleoclimate reconstruction – reconstruction of paleoclimate from indirect (proxy) evidencesuch as tree rings, pollen, and sediment layerspaleodrought – drought that occurred before humans began collecting instrumentalmeasurements of weather, as determined through environmental proxy data such astree rings and lake sedimentsparameter – a constant in a mathematical equation (itself part of a model) that must bedefined; a parameter is usually representative for some inherent property of the systemdescribed by the model. Sometimes the name of a type of data (e.g., precipitation, temperature,soil moisture) is referred to as a parameter of interest.particulate – a small amount of solid matter suspended in a gas or liquid; air pollutionstudies often refer to dust in the atmosphere as particulate matterPaso del Norte – the border region that includes the Ciudad Juárez municipality in Chihuahua,Mexico, El Paso County in Texas, and Doña Ana County in New Mexicopathogen – a microorganism that causes disease in its host; can include viruses, bacteriaand fungipeak flow, peak pulse – the maximum instantaneous discharge of a stream or river at agiven location. It usually occurs at or near the time of maximum amount of flow.peaks over threshold approach – analysis of climatic conditions using location-specificdefinitions (or thresholds) of extreme temperature, precipitation, humidity, or wind