Climate

Southwest Climate Overview

The response of the ecosystems to climate and climate change depends on the interactions with non-climatic factors at the local scale. How we manage resources, e.g., water, agriculture, wildlife, invasive plants and animals, the population density exploiting these resources, and the laws and policies implemented.

The Jet streams

Jet streams are fast-moving air currents that meander in a wavelike pattern influencing the world's weather. The jet streams form when warm air masses rise and cool and dense air masses descend replacing the warm air masses. In the winter, the polar jet stream moves south, entering North America at the US Canadian border, guiding storms from west to east and drying the southern region. In the summer, the polar and the subtropical jet streams migrate to the north. The Southwest gets storms and moisture that enter from the Southeast, the North American Monsoons, affecting mostly the four corners. Meanwhile, the west coast states, California, Nevada, Oregon, and Washington tend to have dry summers.

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 Southwesterm US experiences increased temperature conditions and reduced precipitation during the winter and growing season. The effects of ENSO are often refer 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 climate system includes the atmosphere, land surfaces, and snow-covered land and how it interacts with the incoming solar energy, the atmosphere and the ocean circulation, and the way the ocean and the atmosphere interact.

The greenhouse effect

In the Earth's atmosphere, there is a thin layer of gases with properties that allow the energy and heat from the sun to pass through the atmosphere as short-wave radiation. This energy heats the surface of the Earth and then it is re-radiated back to the atmosphere as long-wave radiation. Those same gases that let the short-wave radiation pass throw the atmosphere, trap the long-wave radiation, keeping the Earth's temperature at a level for which human life is possible. However, we are adding more heat-trapping gases through burning fossil fuels that trap more long-wave radiation, heating up the Earth's lower atmosphere. This shifts the Earth's energy causing an imbalance. Short-term regional cycles and long-term global shifts happen at the same time.

We can measure the increase in GHGs by looking at gas bubbles trapped in the ice cores. The rise of the heat-trapping gases has increased rapidly in recent time compared to almost the last million years, matching the recent combustion of fossil fuels and the effect on the increase in global temperatures. Other indicators of the effects of the increase in GHGs are the increase in the ocean surface temperature, in the atmosphere, and in the land surface. Also, melting sea-ice and glaciers and snow cover and the increase in sea level.


Observed Changes and Impacts in the Southwestern US

Mistletoe

Bark beetles

Tree die-off

Phenological mismatch

Wildfires

Water supply

Snowpack


Projected Changes and Impacts


Representative Concentration Pathways (RCPs)

Projections such as the ones presented in the Intergovernmental Panel on Climate Change Report (IPCC) provide plausible scenarios of climate change based on assumptions on human behavior.



RCP8.5 Increased temperatures in the other of 4-6 degrees by the middle of the century and 6-12 degrees by the end of the century based on the assumptions that humans will do business as usual (BAU)