![]() ![]() ![]() The influence of the monsoon on summer precipitation decreases from south to northwest (after Douglas et al. The effects of El Niño tend to be even stronger during the positive phase of the PDO, and the effects of La Niña tend to be even stronger during the negative phase of the PDO (Sheppard et al. When the temperatures are warmer, known as the positive phase of the PDO, the Southwest experiences wetter winters. Winter precipitation is also influenced by the Pacific Decadal Oscillation (PDO), a climate pattern that can last for several decades when the temperatures in the northeastern Pacific Ocean are warmer or cooler than usual. The opposite is true of La Niña years (when the sea surface temperatures of the eastern Pacific Ocean near the equator are cooler than normal). During El Niño years (periods when the sea surface temperatures of the eastern Pacific Ocean near the equator are warmer than normal), the Southwest experiences wetter winters. The amount of winter precipitation that the Southwest receives in a given year strongly depends on the sea surface temperatures in the Pacific Ocean. Winter storms, in contrast to monsoonal storms, cause widespread precipitation over a large geographical area, affecting distinct locations similarly. The strength of the monsoon varies from year to year, but researchers are currently unsure why (Adams and Comrie 1997). North of this line, precipitation is spread evenly over the year (Comrie and Glenn 1998). Between this line and the line just north of Tucson, precipitation peaks in both the winter and the summer. The mean northern extent of the monsoon forms a diagonal line through far northwestern Arizona up into Utah and Colorado (Mitchell 1976). Areas south of an imaginary line that runs just north of Tucson and east into New Mexico receive more than 50% of their annual precipitation during July, August, and September (Douglas et al. The monsoon tends to be strongest in northwestern Mexico and weakest towards northern Arizona and New Mexico (Adams and Comrie 1997). The influence of the monsoon on total annual precipitation depends upon the location. During the months between these peaks, very little precipitation falls. July through September, precipitation comes from monsoonal storms that derive their moisture from both the Pacific Ocean and the Gulf of Mexico, and November through March, the precipitation comes from winter storms originating in the Pacific Ocean (Sheppard et al. However, because of its location near two major moisture sources-the Pacific Ocean and the Gulf of Mexico-the Southwest has two noticeable peaks in precipitation every year. This positioning leads to year-round warm temperatures, low annual precipitation, and clear skies. The climate of the Southwest is most influenced by its geographic location between the mid-latitude and subtropical atmospheric circulation regimes. Current climateĮven small changes in temperature and precipitation can affect sensitive desert plants and animals. Read on for a sweeping overview of climate change in the Southwest, or visit the Science of the American Southwest Climate Change webpage to learn about specific climate change projects. Using past and present climate records, scientists have been modeling how increases in greenhouse gases will affect the earth’s climate in the future and how a climate shift might affect ecosystems and human society. The build-up of these gases over time is leading to a rapid global rise in temperature. Human activity, such as the burning of fossil fuels, is releasing greenhouse gases like carbon dioxide and methane into the atmosphere. Now another shift in climate is happening, but this time most climate scientists think that people, not natural processes, are driving the shift. Annual temperatures and precipitation tend to be very different from one year to the next, and the Southwest has also seen major climate shifts that have lasted for centuries for example the Southwest dried out significantly over the entire period between AD 10 in a series of epic droughts known as the medieval megadroughts. But records of past climate show that southwestern climate is also highly variable, both across the landscape and over time. The overall climate of the Southwest-its weather patterns over a long period of time-tends to be warm and dry. ![]()
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