For roughly the past year, North America has experienced El Nio conditions. This El Nio has provided varying weather throughout the United States, which included a mild winter for New York. Recently, the equatorial Pacific has transitioned to a cool La Nina. This La Nina is expected to last into the Northern Hemisphere spring of 2011.
El Nio is a name given to a predetermined set of oceanic/atmospheric changes, typically when surface temperatures in the Equatorial Pacific experience a rise of .5C from average temperatures. Additionally, we often see a high surface air pressure in that region. But now the surface temperatures in the Pacific are experiencing a cooling of approximately 3-5C. We are now experiencing the opposite phase of El Nio, which is La Nia.
So what is La Nia , and what are the conditions necessary for it to happen? La Nia is the cooling of surface temperatures by about .5C from average temperatures. Also of note, La Nia is often preceded by a strong El Nio.
You may be wondering why we are talking about a condition that is predominantly noticeable in the eastern Pacific. Well, winter is right around the corner and El Nio and La Nia are most obvious in the winter months in the high latitudes. That puts us in the La Nia corridor. With this we often (but not always) see a harsher winter than normal. We won TMt be the only area hit hard by La Nia though. It is expected that in the Pacific Northwest, winter will be more rainy and snowy, in the desert Southwest, winter is often drier than normal with an increase in tornadic activity in the spring and summer.
By the way, we are still in hurricane season, which officially runs from June 1 through November 30. La Nina has been know to influence not only the length of the season (making it longer), but other factors related to tropical development as well.
Though El Nio and La Nia conditions are generally not stated in weather forecasts since it is more climatological, you can track sea surface temperatures and anomalies through NOAA.
For recent La Nia findings and the full article, click here .