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By Matt Weiser
mweiser@sacbee.com
Spring passed California by, and summer remains in hiding.
Nine tornadoes have torn up the Sacramento Valley from Oroville to Fairfield. A giant Sierra snowpack, still frozen fast, has put innumerable summer adventures on hold.
The Golden State's weather has gone haywire.
And it's not over yet: Sacramento can expect as much as another 1.4 inches of rain this weekend and temperatures 20 degrees below normal, with more mountain snow.
"It's what I call global weirding," said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in Pasadena. "This has been a very strange year all over the planet."
What's going on?
First of all, this spring's weather is not unprecedented, just uncommon. California has had wet, cold spring weather before, notably in 1983, a year that produced record Sierra snows.
This year, the blame falls on a complex interaction between La Niña and another phenomenon called a negative Arctic oscillation, Patzert and others said.
(CUT)
Read more: http://www.sacbee.com/2011/06/04/3676297/researcher-says-climate-change.html#ixzz1OJobi43D
--- In californiadisasters@yahoogroups.com, Kim Noyes <kimnoyes@...> wrote:
>
> 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
>
> --
> Check out http://groups.yahoo.com/group/californiadisasters/
> Read our blog at http://eclecticarcania.blogspot.com/
> Visit me on Facebook at http://www.facebook.com/derkimster
>
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