La Nina Back for Round Dos
After a year of record precipitation in some parts of the country and blistering drought in others, everyone was hoping that this would be a neutral year. But weather forecasters show that a second La Nina began forming in August and could be at least as severe as last year.
For those who floated to work on flooded Vermont streets or who dug their cars out of ten-foot snowdrifts, another record-breaking weather year is not necessarily better. And in drought-ravaged Texas where fires burned much of 2011, more is definitely bad news.
It is rare but not unheard of for their to be two La Ninas in a row. When La Ninas come in back-to-back years, the second has been weaker than the first in the three of the five cases since 1950. But the National Centers for Environmental Prediction says the current La Nina may become as strong as last year’s.
La Nina forms when cooler than normal temperatures are found in the sea surface of the central and eastern equatorial Pacific Ocean. For the first two weeks of November, the mean temperature was almost one degree Celsius below normal, indicating a weak to moderate La Nina on the horizon.
La Nina tends to push the U.S. storm track further north, creating flooding in the northern plains and drought in the south.
In the month since the U.S. Climate Prediction Center issued its first report about this years El Nino Southern Ocscillation (ENSO) state, La Nina has already strengthened. the center says that the new La Nina will continue to strengthen into 2012. But other factors will determine what weather you experience at your house.
The Pacific Decadal Oscillation (PDO) is setting up with warm water in the central and northern Pacific with cool water pooling off the coast of Alaska and Canada. This is the same pattern as last fall. Based on that, forecasters are comfortable saying that there will be frequent cold and wet storms in the western half of the country.
But the Arctic Oscillation presents a weather wild card that scientists struggle to predict. The AO changes multiple times per winter fluctuating between positive and negative. A strong weak AO can overpower a warm El Nino and create a snowy winter in the Mid Atlantic. Last year the AO was predominately negative and the same is expected for this year.
The World Meteorological Organization says that expansion and contraction of cold air over the North Pole “could produce dramatic short-term swings in temperatures this winter.”
La Ninas occur every three to five years and last from nine to 12 months.