This week, I’m going to talk about the difference between El Niño and La Niña, and have a look at the different kinds of weather the Lake Tahoe area experiences when the West Coast is in the grips of one versus the other during the northern hemisphere winter. El Niño, La Niña, and Tahoe Winter Weather | Unofficial Networks

El Niño, La Niña, and Tahoe Winter Weather

El Niño, La Niña, and Tahoe Winter Weather

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El Niño, La Niña, and Tahoe Winter Weather

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This week, I’m going to talk about the difference between El Niño and La Niña, and have a look at the different kinds of weather the Lake Tahoe area experiences when the West Coast is in the grips of one versus the other during the northern hemisphere winter.

Before I get too deep into the science though, let’s let the late, great Chris Farley set the scene. What exactly is El Niño?

Now that we have that covered, let’s get down to business.

In actuality, El Niño translates to “the child,” and is the name given to the temporary weather-influencing phenomenon of warmer-than-average surface water in the equatorial regions of the Pacific Ocean. Where did the name “El Niño” come from? According to George Philander of Princeton, who wrote the book “El Niño, La Niña and the Southern Oscillation” (Academic Press, 1990), the name is derived from observations Peruvian fisherman made about abnormal currents in ports in Peru:

In the year 1891, Senor Dr Luis Carranza, President of the Lima Geographical Society, contributed a small article to the Bulletin of that Society, calling attention to the fact that a countercurrent flowing from north to south had been observed between the ports of Paita and Pacasmayo. The Paita sailors, who frequently navigate along the coast in small craft, either to the north or the south of that port, name this countercurrent the current of “El Niño” (the child Jesus) because it has been observed to appear immediately after Christmas.

What causes an El Niño? Normally, trade winds in the regions north and south of the equator blow from NE to SW in the northern hemisphere, and SE to NW in the southern hemisphere, bringing cooler upwelling water towards warmer water at the equator. When these winds slacken, the water warms in the equatorial pacific as the winds are no longer pushing cold water north and the sea surface warms.

Diagram of global wind circulation patterns, showing trade winds bringing warm air and water to the equator. Image from 2b.blogspot.com

El Niño conditions are thought to occur every three to seven years, and are declared when the average temperature is at least 0.5 degrees Celsius above average for three months in a row. During an El Niño, the average temperatures of the surface of the Pacific can vary up to 4 degrees above average. Four degrees might not seem like a lot, but in terms of average ocean surface temperature, that rise in temperature is enormous.

In 1997, a powerful El Niño resulted from an average rise of 1.62 Celsius (2.9 Fahrenheit) across the Pacific. The amount of energy required to raise sea surface temperatures this much is staggering. A one-megaton Hydrogen nuclear device releases 4.18×10^15 joules. One joule is the energy released as heat by a resting person every 1/60 th of a second, or the amount of energy expended when raising an apple up almost 4 feet. An H-bomb is 4,180,000,000,000,000 times that.

If you could blast off an H-bomb in the ocean and manage to convert all its energy into heat without vaporizing any of the water or moving it around, one bomb would heat 1 cubic kilometer of seawater (0.24 cubic miles) 1 degree C (~1.8 degrees F). This seems like a large amount of water, but think about just how much water is in the upper part of the Pacific ocean around the equator. If you consider just the upper 200 meters (656 feet) of the region from 10 degrees north latitude to 10 degrees south, that’s about 6 million cubic kms (1.4 million cubic miles). During the 1997 El Niño, the temperature of the sea warmed by 2.9 F, so following from the H-bomb analogy, to heat this amount of seawater one would need the equivalent heat from  17.9 million 1-megaton H bombs. The amounts of energy required to change the global climate are truly staggering.

El Niño and it’s opposite phenomenon La Niña have significant impacts on the weather in the Western Hemisphere. The west coast of northern South America experiences increased amounts of rainfall from April to October, which can lead to devastating floods. Further south in Brazil and northern Argentina, wetter conditions also prevail, but arrive earlier in the year during the Austral spring and early summer. Further west, El Niño often means periods of extreme drought in Australia which can lead to catastrophic brush fires and hazards to livestock.

What does El Niño mean for Western North America and Lake Tahoe? Winters in the Pacific Northwest are typically warmer and drier than average. Further south in Southern California,  northwest Mexico and east into Arizona and New Mexico, wetter than average winters occur during El Niño, bringing flash floods to desert landscapes. El Nino is also known to supress hurricane seasons in the Gulf of Mexico.

Typically, El Niño events mean wetter winters in Tahoe, which translate into epic snowfall events and great skiing here. Take a look at this graph from the Donner Summit snow study plot:

Snowfall on Donner Summit, from El Dorado County

Major winters on Donner Summit (>40 feet total) on Donner Summit were recorded in the years 1982-1983, 1983-1984, and 1997-1998, both of which were strong El Niño years.

What happens when the sea surface temperatures in the equatorial Pacific go the other direction and cool? The opposite of an El Niño is La Niña. Tahoe experienced a relatively weak La Nina last winter, and the effects here were seemingly never-ending monster dumps. According to NOAA, we are heading into another La Nina winter, and it is reasonable to expect another deep and heavy winter. During La Niña events, South America experiences prolonged drought across coastal regions in Peru and Chile.

Pacific sea-surface temperature fluctuations, showing El Niño years (+ trend, red) versus La Niña years (- trend, blue). Image from NOAA.

The Tahoe area, however, lies right in the geographic transition zone between wetter and drier effects of El Niño and La Niña to our north and south. Because of our location, external forcings from other atmospheric events such as the strength and frequency of cold Arctic storms that form in the Bering Sea and ride the jet stream right into Squaw can have a higher impact on the winter weather here than El Niño conditions.

Long-term forecasts are reliably unreliable though, so it’s really hard to say with any degree of certainty what kind of winter we’re looking at this winter. We could see a repeat of having to tunnel into our houses in Truckee, or we could also be looking at dry rocks and manzanita bushes into January just like Tahoe suffered through in the winter of 1977. Only time will tell.

The scene in my driveway in April 2011

 

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