The main source of summer moisture is the North American monsoon, which transports moisture into the region from sources in the subtropical Pacific and Gulf of Mexico. This annual phenomenon brings drama to the southwestern desert skies, but only occasionally does it provide enough precipitation to contribute appreciably to hydrologic supplies. For the mainstem Colorado River and its major tributaries, the bulk of the precipitation that contributes to water supply falls during the winter months, primarily in the form of snows at high elevation. Summer months comprise the period of higher water demands and, except in extreme weather years, will provide at best only modest additions to mainstem reservoir water supplies. If a season of winter precipitation and water storage is “lost” because of drought conditions, there will be little opportunity to replenish supplies until the following winter.

The Tropical Pacific and ENSO

Ocean temperature patterns that have the greatest influence on Colorado River basin climate are in the tropical Pacific in a band that straddles the equator between Peru and the International Date Line. At irregular intervals of typically 2-7 years, sea surface temperatures (SSTs) in this region warm above climatological averages.1 This phenomenon, called El Niño, is part of a complex ocean-atmosphere oscillation. El Niño has a climatic counterpart called La Niña that is characterized by below-average SSTs (La Niña events usually have smaller departures from average SST than do El Niño events). The terms El Niño and La Niña refer only and exclusively to ocean temperatures in this geographic domain and not to their effects elsewhere.

Another atmospheric feature relates to barometric pressure gradients in the South Pacific. In the 1920s, British meteorologist Sir Gilbert Walker published his seminal work describing the inverse relationship in atmospheric surface pressure between Tahiti and Easter Island in the tropical Pacific, and over Darwin in northern Australia (Walker, 1925). That is, when atmospheric pressure is high in one of these locations it tends to be low in the other region, and vice versa. Walker termed this phenomenon the Southern Oscillation. It refers only to the atmosphere. The Darwin-Tahiti pressure difference (nor-


Tropical Pacific SSTs are 1-3°C above average in modest El Niño events, 3-5°C above average in major episodes.

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