By Zack Guido | The University of Arizona | September 14, 2008
The climate system is dynamic. Forceful winds and ocean currents circumnavigate the globe, redistributing the energy of the sun. The sun provides the external energy that drives the climate from the outside. The sun also stimulates internal changes that cause, for example, ocean currents to weaken or strengthen and trade winds to migrate.
Although the climate system is always in motion, scientists have been able to recognize key patterns in winds, currents, temperatures and pressure. Understanding the nature and cause of these patterns is essential for fingerprinting natural climate variability and human-caused climate change, both globally and in the Southwest.
Figure 1. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation.
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Credit: Intergovernmental Panel on Climate Change, 2007
Energy from the Sun powers the climate system. The balance between the incoming solar radiation and energy leaving the Earth’s atmosphere is called the energy balance. On average, about 30 percent of the Sun’s energy (shortwave radiation) is immediately reflected back to space by clouds, light-colored land such as glaciers, and tiny particles floating in the air called aerosols.1 The remaining 70 percent heats the atmosphere and land surface and is then re-radiated to space as longwave radiation (Figure 1).
The distribution of the Sun’s energy across the globe is not even. More energy strikes the equatorial regions than the polar areas because the sunlight enters the atmosphere at lower angles at higher latitudes. This causes temperatures to be cooler in polar regions compared to tropical regions. The uneven distribution of heat helps drive atmospheric circulation, which in turn influences oceanic circulation patterns by the force of winds and by altering the oceans’ surface temperatures and salinities.
In a state of equilibrium, the amount of energy entering the Earth’s atmosphere balances the amount of energy escaping from the atmosphere. Natural processes and human actions can change the total amount of energy within the atmosphere in three ways. They can increase or decrease the amount of incoming solar radiation by changing the earth’s orbit around the Sun; change the fraction of solar radiation that is reflected by altering the cloud cover, amount of atmospheric particles, or type of land cover; and alter the amount of longwave radiation that escapes to space by changing atmospheric concentrations of greenhouse gases, such as carbon dioxide and methane.1
Figure 2. The Hadley cell circulation illustrates how rising air in the superheated tropics descends in the subtropics. This creates high-pressure zones in subtropical regions, including the U.S. Southwest.
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Credit: Barbara Summey, NASA Goddard Visualization Lab
Key atmospheric circulation patterns such as the Hadley and Walker Cells help redistribute energy around the globe. The Hadley Circulation is the movement of air in a north-south motion that arises from differences in solar heating (more at the equator and less toward the poles). The Walker Circulation is an east-west movement of air in the tropical Pacific region near the equator and is both the cause and result of changing sea surface temperatures in that region.
The Hadley Circulation helps transport energy from the equatorial regions toward the north and south mid-latitudes (Figure 2). Hot, moist air rises near the equator, moves toward the poles high in the atmosphere, sinks in the subtropics around 30° latitude, and then returns to the equator near the surface (Figure 2). The descending air in the poleward arm of the Hadley Cell, stripped of moisture after leaving the tropics, is responsible for the arid climate of the Southwest and other deserts of the world.
Figure 3. The Walker circulation.
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Credit: The University Corporation for Atmospheric Research
The Walker Circulation is created as the trade winds in each hemisphere converge on the equator in the tropical Pacific. The resulting westward blowing winds at the equator push warm surface waters west, forcing cooler waters from deeper depths to rise to the surface in the eastern tropical Pacific. This creates what is called a ‘cold tongue’ of sea surface temperatures in the eastern Pacific. Warm surface waters pile up in the western Pacific, creating some of the warmest waters on the planet. These warm ocean temperatures then warm the atmosphere, causing warm, moist air to rise. This air then diverges aloft, flowing east and losing most of its moisture before descending over the eastern Pacific Ocean (Figure 3). Changes in the strength of this circulation cause a see-saw in surface pressures: when pressure is high in the South-Central Pacific Ocean, it tends to be low in the Western Pacific Ocean. This, in turn, causes changes in the strength of the atmospheric circulation, completing a circular cause and effect chain that is also known as the Southern Oscillation (see below).
The Walker Circulation affects precipitation patterns in many locations, including the Southwest, and influences the easterly trade winds, oceanic upwelling, and ocean biological productivity. It also provides the background against which El Niño-Southern Oscillation (ENSO) events take place.
Jet streams are meandering rivers of fast-moving air in the upper atmosphere that flow from west to east. These currents form about six miles above the land surface due to the rotation of the Earth and the unequal distribution of the Sun’s energy, which causes air to move from hotter, tropical regions to cooler, polar regions. The jet streams form at the boundary between two air masses with strong differences in temperature, and migrate seasonally. The Northern Hemisphere jet stream impacts the Southwest as it dips south during the winter, steering cooler, moisture-laden air from the North Pacific into the arid Southwest.
North American Monsoon
The North American monsoon occurs in the Southwest during the summer months. As summer progresses, the Sun’s solar radiation warms the land and Pacific Ocean at different rates. This sets up a pressure difference that alters the flow of air and rainfall patterns.
Read more about monsoon in the Southwest in Zack Guido's feature article Understanding the southwestern monsoon.
Monsoon storms in the Southwest typically begin in early July after several complex and dynamic weather phenomena collide. By July, the Four Corners region, where Arizona, New Mexico, Colorado, and Utah meet, has baked in the sun for months. Air has risen, creating a low pressure trough in the lower atmosphere. Off the coast of Baja California, the Sun’s energy has also boosted ocean temperatures to around 85°F. But the ocean has a moderating effect on the air temperatures, keeping them lower than those over the deserts of the Southwest. This temperature imbalance becomes large enough that a change in the high and low altitude atmospheric movement occurs. The winds high over the Southwest, near an altitude of 30,000 feet, take a U-turn westward, opposite their trajectory for the previous nine months. The winds carry moisture from the Gulf of Mexico. At approximately the same time, the near-surface air over the Gulf of California rushes northward into Arizona and New Mexico, also carrying moisture.
The monsoon season in the Southwest typically brings short, intense thunderstorms that can produce flash flooding. Approximately half of the yearly precipitation in Arizona and New Mexico falls during this period. Lightning strikes are also common, and the Southwest fire season reaches its peak during the early monsoon seasons when lightning ignites the parched vegetation. In Arizona, the monsoon season officially begins on June 15 and ends on September 30.
Often the dynamics of climate are a result of phenomena that occur in one region but cause changes in the climate and weather in distant regions. For the Southwest, the climate is shaped in part by changes in sea surface temperatures in the tropical Pacific and north Pacific Ocean and by the summer temperature difference between the baked Southwest land and the cooler oceans.
El Niño-Southern Oscillation
El Niño-Southern Oscillation (ENSO) involves a change in ocean sea surface temperatures and atmospheric wind strength in and above the equatorial Pacific Ocean. These two events are entwined, each causing and reinforcing the other. El Niño events involve the warming of the tropical Pacific Ocean surface waters from the region near Tahiti to the west coast of South America. La Niña events, on the other hand, occur when the ocean waters between Tahiti and the west coast of South America cool. Changes in sea surface temperatures alter atmospheric pressures which, in turn, change the strength of the trade winds. The Southern Oscillation is characterized by these changes in the trade winds and also by modifications to precipitation and atmospheric heating.
Figure 4. During El Niño years, all climate divisions in Arizona and New Mexico tend to receive above-average winter precipitation. Values represent the percentage of December-March precipitation compared to non-El Niño years for the period 1895-1996.
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Credit: National Oceanic and Atmospheric Administration, Climate Prediction Center
ENSO events cause changes in weather patterns all over the globe. During an El Niño event, the jet stream over the north Pacific Ocean is pulled farther south, and this air flow carries storms into the southwestern U.S. and northern Mexico. As a result, the region is more likely to experience increases in precipitation, particularly in the winter (Figure 4). During La Niña events, on the other hand, it is common for the Southwest to have reduced precipitation, particularly in the winter and spring.
Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is a natural pattern of climate variability that is observed in Pacific Ocean sea surface temperatures north of 20°N latitude. The pattern switches between a cool and warm phase about every 20 to 30 years. During the warm, or positive phase, the western Pacific Ocean cools, while the eastern ocean warms; during a cool, or negative phase, the opposite pattern occurs.
Research suggests that these PDO phases can combine with El Niño and La Niña conditions to influence precipitation in the West, particularly in winter. The warm, or positive, phase of the PDO tends to enhance El Niño conditions and weaken the effects of La Niña events; the cool, or negative, phase has the opposite effect.2 When the PDO is in its positive phase, the Southwest tends to experience wetter El Niño winters and relatively average La Niña winters.3
- Le Treut, H., et al. 2007. Historical overview of climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Mantua, N.J., et al. 1997. A Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society, 78 (6): 1069–1079.
- Climate Assessment for the Southwest. 2008. Pacific Decadal Oscillation “PDO.” http://www.climas.arizona.edu/learn/pdo/index.html (last accessed on November 13, 2008).