La Niña typically means a drier-than-normal winter for southern Arizona. Here's why!
Trade winds naturally occur along the equator, blowing from east to west.
For some reason (one which meteorologists aren't certain about) every few years there is a change in those winds. Sometimes they get weaker (El Niño) and sometimes they get stronger (La Niña).
Since the equatorial region gets so much sunlight (very long days!), the ocean water there is typically very warm.
However, when the trade winds are extra-strong, they blow the warmest top layer of the ocean westward toward Indonesia.
Something has to replace the water that has been pushed westward by the strong wind. When something is displaced in nature, creating a void, something else has to move in to fill that void.
To fill the void, ocean water from well below the surface moves upward. This is called "upwelling." Since the water came from a place deep in the ocean, where the sun doesn't reach it, it is very cold.
Voilà! We have colder-than-normal water now off the west coast of South America.
Cold water makes the air above it cold.
Cold air is dense (heavy) and sinks. Sinking air creates an area of high pressure in the atmosphere that translates upward to the upper-levels where the jet stream resides.
The high pressure shoves the jet stream, the "steering wheel for storms", northward into southwest Canada and the Great Northwest of the USA.
That leaves us high and dry.
It doesn't mean we won't see rain and mountain snow during a La Niña winter. It just means that, when the season is all finished, we'll likely have less precipitation than in a "normal" year.
Now you know all about La Niña!
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