[californiadisasters] La Niña and its Affects on Southern California Weather

La Niña and its Affects on Southern California Weather

By Sean Klein ~ NWS Los Angeles/Oxnard

La Niña, like its sibling El Niño, occurs about every 3-7 years. While El Niño results in above average Sea Surface Temperatures along the Equator in the Central and Eastern Pacific Ocean, La Niña results in below average Sea Surface Temperatures in rough-ly the same region. Both variations of ENSO (El Niño Southern Oscillation: essentially the seesaw that occurs between El Niño and La Niña) result in changes in global weather patterns, but unlike El Niño, La Niña gets very little attention from media in Southern California. This lack of La Niña coverage is well found-ed. While El Niño increases the potential for damaging winter-time storms in South-ern California, La Niña often results in less frequent and weaker winter-time storms. These effects combined with slight-ly below average air temperatures make La Niña benign, some-what predictable, and on the whole: a rather lackluster news story. Issues of excitement aside, the remarkable consistency of south-ern California rainfall during La Niña makes the cold ENSO sister extremely interesting from a meteorological standpoint.

During an El Niño, unusually strong high pressure forms high in the atmosphere over the tropical Eastern Pacific in response to low pressure near the ground (because the ocean is warmer, the air above it is warmer. Warmer air is less dense, hence lower air pressure). Not surpris-ingly, the opposite occurs during La Niña where unusually weak low pressure forms high in the atmosphere over the tropical East-ern Pacific in response to high pressure near the ground (because the ocean is cooler, the air above it is cooler. Cooler air is more dense, hence higher air pressure) (Fig. 1). The pressure however does not change as intensely to continued ocean cooling during a La Niña event as it does to ocean warming during an El Niño. This suggests that the strength of an El Niño (weak, moderate, strong) has more of an effect on our weather than the strength of La Niña.
The significance of these pressure differences is their relation to the strength and position of the Pacific Jetstream. While these pressure changes themselves are not likely the cause of the reposi-tioned Jet, they are representative of the pressure imbalance between the tropics and the poles. High-er pressure in the tropics means greater pressure difference between the equator and poles, lower pressure in the trop-ics means lower pressure difference between the equator and the poles. The stronger this pres-sure difference, the stronger the Jetstream. The Jetstream of course does not flow from south to north, but from west to east, and this is because of the spin of the Earth (Coriolis Force) (Fig. 2). Because the Jetstream occurs at the location of greatest north-south pressure difference, it will form at the intersection between the two most differing air masses. During El Niño, this location is in between the upper level high pressure discussed above and the Aleutian Low. During La Niña, this location is in between a Cen-tral Pacific High and the easterly repositioned Aleutian Low (Fig. 3).

The change in position of the Jetstream means colder, drier winters during La Niña, and warmer, wetter winters during El Niño. That said, the above state-ment is in refer-ence to the aver-age climate dur-ing an El Niño or La Niña. Vari-ous factors can cause the above statement to be untrue for any one particular El Niño or La Niña event. Furthermore, these changes are usually confined to the winter months (January through March). For example: during the fall months (Oct-Dec) of La Niña, downtown Los Angeles on average receives 103% of normal precipi-tation (Just about aver-age). Just under half of La Niña's (43%) pro-duced above average precipitation during the same (Oct-Dec) time frame. This is followed by a sharp drop off in precipitation (as a per-cent of average) during the winter months, (January, February, March) where La Niña Precipitation drops to just 72% of normal. La Niña's effect on precipi-tation follows the same trend as El Niño's effect on precipitation where strong differences from normal precipitation do not occur until the win-ter months.

Similarly, average minimum temperatures during a La Niña (Dec-March) tend to be slightly below average: rang-ing from about ¾ of a degree below normal during November and March to 1.5 degrees below average during January (Fig. 4). This is likely the result of less latent heating (condensation warms the air) due to less rain days. Not only are there (as one would expect) fewer days with precipitation during La Ni-ña, but there are also less days with high precipitation amounts. During February for example, the rate of occurrence of 1 in. or greater precipitation days is more than 3 times higher during an El Niño than during a La Niña. (2.06 times/month vs. 0.6 times/month).
All in all, La Niña (on average) whether weak or strong, results in slightly cooler winter temperatures, near aver-age fall precipitation and below average (though not significant-ly below average) pre-cipitation from January through March. Precip-itation and temperatures vary from one La Niña to the next, but the ma-jority of La Niña years (52%) have produced seasonal totals (Nov-Apr) between 8.7 in. and 13.7 in for the CQT, providing moderate confidence in winter/spring precipitation totals.

Source: http://www.wrh.noaa.gov/lox/spotter/documents/Newsletter_april2011.pdf

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