La Niña returns

La Niña has returned and is forecast to strengthen this upcoming winter, according to a report by The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC).

La Niña typically brings warmer and drier conditions across the southern United States, and an increase in precipitation for the Pacific Northwest and Ohio Valley. With another round of La Niña developing, more extreme weather could continue for the remainder of the year. Last year, we saw a fairly strong La Niña develop, which brought a lot of moisture across portions of the Ohio Valley, later contributing to a lot of flooding around these regions.

What is La Niña, and what can we expect?

In La Niña phases, dry conditions are possible for the southern portions of the United States. Image Credit: NOAA

First of all, La Niña and El Niño phases are all associated with the El Niño/Southern Oscillation, which we call the ENSO.  According to the CPC, ENSO refers to the year-to-year variations in sea-surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. La Niña and El Niño are the opposite extremes in the ENSO cycle.  During La Niña phases, ocean surface temperatures in the equatorial Pacific are colder than normal.  In El Niño, they are warmer than normal. La Niña episodes are characterized by lower than normal pressure over Indonesia and Australia and higher than normal pressure over the eastern tropical Pacific.  During this past spring (2011), we were in an ENSO-neutral state, which meant ocean temperatures across the equatorial Pacific were near average.  La Niña phases can contribute to more tropical activity across the Atlantic Ocean during the summer and fall, while the eastern Pacific sees below-average tropical systems.  For El Niño phases, the exact opposite happens – fewer storms in the Atlantic and an increase in tropical activity for the eastern Pacific.

Typical pressure changes from El Niño and La Niña. In La Niña events, dry and warmer conditions are possible for the southern United States, which explains the increase in pressure readings. Lower the pressure, more storms. Image Credit: NOAA

There are many other factors that determine our weather in short-term model runs.  For instance, it was a very active winter for the majority of the United States in late 2010 and early 2011.  Parts of the Southeast saw a major snowstorm, but we cannot completely blame La Niña for this. La Niña weather has been added up over decades, and the average pattern associated with this phase shows wetter conditions for the Pacific Northwest and drier conditions for the southern portions of the United States.  Unfortunately, that could spell danger for Texas, which desperately needs rainfall.

Mike Halpert, deputy director of the Climate Prediction Center, said:

This means drought is likely to continue in the drought-stricken states of Texas, Oklahoma and New Mexico.

According to NOAA, La Niña typically occurs every three to five years, and back-to-back episodes occur about 50 percent of the time. Sometimes, a back-to-back La Niña can cause even more dry conditions in areas that have already seen drought from the previous La Niña.  The drastic changes in the ENSO, such as El Niño and La Niña, typically become stronger during the months of December through April.  By May and June, it typically weakens.

Bottom line: NOAA Climate Prediction Center has announced that La Niña has returned.  La Niña typically provides warmer and drier weather across the southern United States in the winter, and wet and cooler conditions for the Pacific Northwest and the Ohio Valley.  In general, this is the last thing people in Texas want to hear. Perhaps this La Niña will be different.  Only time will tell.

September 15, 2011

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Matt Daniel

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