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admin | Category: Improving Erections | 09.09.2014
We never realized that El Nino and La Nina are principally caused by weakening or strengthening of the east blowing equatorial winds.  Why didn’t someone (NOAA?) just tell us that before!?  So simple! A satellite image shows the sea surface temperature in October 2015, with the orange-red colors indicating above-normal temperatures that are indicative of El Nino. As the warm water pools it works in tandem with intense solar rays to heat the surrounding air. As the air ascends it cools and condenses, forming cotton-ball clouds that burst with rain. In this way, a large circular pattern known as the Walker circulation is completed (Figure 1). During El Niño events, the trade winds slacken, enabling an eastward migration of warm water.
The transition from one event to another is a natural example of a climate system regulating itself. ENSO events are commonly defined by how high or low sea surface temperatures are compared to the average. To reveal how ENSO events alter atmospheric circulation, the Southern Oscillation Index, or SOI, evaluates the pressure difference in the central and western Pacific Ocean.
La Niña events, on the other hand, often bring dry conditions to Arizona and New Mexico. Critical winter precipitation for the Southwest also falls as snow in the headwaters of Arizona’s most important river, the Colorado.
The impact of ENSO on summer weather is not as clear-cut as the changes that occur in winter.
There is currently debate about whether the character of ENSO events will change as the world warms in response to increased levels of greenhouse gases.


The sibling events, born thousands of miles away in the tropical Pacific Ocean, can deliver copious rain and snow to the region or cause widespread drought. The rotation of the Earth causes trade winds from the east to converge on the equator in the tropical Pacific, pushing warm surface water into the western tropical Pacific Ocean like a snow plow. The center of rain follows the warm water, moving east to the middle of the Pacific Ocean near Tahiti.1 Rainfall also increases further east in the typically dry region off the coast of Ecuador and Peru. The SOI is calculated based on the differences in air pressure anomalies between Tahiti and Darwin, Australia.
As a result, changes to atmospheric circulation, and therefore weather, are most prominent in the winter. During El Niño years, all climate divisions in Arizona and New Mexico tend to receive above-average winter precipitation.
During this same period, the Arizona portion of the Lower Colorado River Basin experienced wet conditions with rainfall greater than 115 percent of average about half of the time. El Niño events often are associated with two phenomena that have opposite effects on precipitation in the Southwest. A more La Niña-like future could strain already limited water resources, and more intense events could deliver more frequent floods or droughts. How can changes off the coast of Peru ripple across the globe to steer rain away from Arizona? Sinking air piles on the Earth’s surface, forming a high pressure zone that acts like a vice.
In a La Niña, for example, a self-reinforcing cycle would begin with strong trade winds, which would enhance the pressure difference between the east and west. A negative SOI, which is characteristic of El Niño events, indicates air pressure over Tahiti is less than at Darwin.


However, when the southern regions of the Southwest are wet, precipitation in the Upper Colorado River Basin is often average or below average. Values represent the percentage of December-March precipitation compared to non-El Niño years for the period 1895-1996.
On one hand, an El Niño can stifle summer rains in Arizona and New Mexico because it can weaken and reposition the subtropical high that guides moisture into the Southwest.
Why does El Niño soak southern Arizona during the winters but reduce snowfall in the parts of the Rocky Mountains?
The pressure difference squeezes air in the east toward the west, where it fills the void created by the hot, rising air.
Both the sea surface temperature index and the SOI are used together to help evaluate the early and late stages of an ENSO event when conditions are not obvious, the duration of the events, and their strength. On the other hand, El Niño events also can foment a higher number of tropical storms, some of which deliver copious summer and fall rains to the region.
In Colby, BG and Jacobs, K (eds), Arizona Water Policy: Management Innovations In An Urbanizing, Arid Region. Will climate change cause more El Niño or La Niña events, and if so, what does this mean for Arizona and New Mexico? A memory of departed winds is expressed in the ocean as a wave that develops and propagates along a temperature boundary called the thermocline, which separates deeper, cold water from warmer, surface water (Figure 1).



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