Southwest Climate Overview
The Jet Streams
Jet Stream, Image from NASA
El Niño and La Niña (El Niño-Southern Oscillation)
El Niño-Southern Oscillation (ENSO) is a global climate phenomenon involving temperature changes in the surface temperature of the tropical Pacific Ocean. ENSO cycles from warm to cold conditions on a two-to-seven-year pattern with three phases. El Niño and la Niña are the extreme phases of the ENSO cycle. During El Niño or the warm phase of ENSO, the southwestern US experiences an increased winter and spring precipitation. In contrast, during La Niña or the cold phase of the ENSO cycle, the Southwestern US experiences increased temperature conditions and reduced precipitation during the winter and growing season. The effects of ENSO are often referred to as teleconnections since changing conditions across the tropical Pacific Ocean can impact weather patterns in far geographic regions.
During normal conditions (the neutral phase of the ENSO cycle), trade winds blow East to West along the Equator driving warm surface water westward across the surface of the tropical Pacific Ocean. In the East, the warm water is replaced by dense cooler water from the deep ocean (upwelling). In the West, the warm surface temperature causes the western Pacific sea level to be higher than in the eastern Pacific, and the the transition zone between warm surface water and cold deep water (thermocline), to be deeper. Warm surface waters add heat and water vapor to the atmosphere. The rising warm air in the West (atmospheric convection), responsible for rains, produce relative low pressure. Air masses tend to move from high to low pressure thus, trade winds move east to west in normal conditions. During el Niño, easterly trade winds weaken or reverse allowing warm masses of water to move from the western Pacific towards the Americas, shifting the prevailing rain pattern from the normal Western Pacific to the Central Pacific. This is accompanied by a drop in sea level in the Western Pacific and the rise in the thermoclime. On the other hand, during la Niña, the eastward trade winds intensify, shifting the prevailing rain pattern further to the West. Image from NOAA.
The ENSO fire teleconnection refers to the synchronicity between ENSO events and variations in fire regimes. Years of widespread fires in the southwestern US occur in drought years (La Niña conditions) preceded by two to three years of above average wet conditions (El Niño conditions). The above average moisture during el Niño conditions leads to fuel build-up through both increased plant productivity and decreased fire activity. Subsequent la Niña conditions result in drying of the accumulated fine fuels, leading to more successful fire ignitions and widespread burning.
Pacific Decadal Oscillation
Global expression of the PDO and ENSO, Image from Deser et al. (2016)
The Greenhouse Effect
Observed Changes and Impacts in the Southwestern US