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El Nino no longer weak could make winter milder than many have been predicting
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El Niño conditions have strengthened in recent weeks, crossing the threshold from "weak" to "moderate", according to data compiled by NOAA's Climate Prediction Center. NOAA defines "moderate" El Niño conditions as existing when sea surface temperature (SST) departure from average in the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region") warms above 1.0°C. According to the latest time-series plot of "Niña 3.4 region" SSTs we crossed that threshold. Monthly average SSTs will have to remain above 1.0°C for five consecutive months in order for this to be considered a "moderate" El Niño event. The ongoing intensification of El Niño could have major impacts on this winter's weather.
Departure of sea surface temperature (SST) from average for the past two years along the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). Moderate strength El Niño conditions occur when the Niña 3.4 anomaly exceeds 1.0°C, which occurred last week (red arrow). Weak El Niño conditions (Niña 3.4 anomaly between 0.5 - 1.0°C) were present from early June to mid-October.
The intensification of El Niño is due to a combination of events in the ocean and the atmosphere. In the ocean, a slow-moving wave of water more than 5°C (9°F) warmer than average is progressing from west to east along the Equator. This wave, known as a "Kelvin" wave, is focused at a depth of 150 meters, but also affects surface waters. The Kelvin wave was at 175W on October 1, and is now near 140W, so it is traveling east at about 4 mph (100 miles/day). At the ocean surface, a burst of west-to-east winds near the Date Line has weakened the trade winds, which blow the opposite direction--east to west. The trade winds have weakened by 1 - 2 m/s over the past few weeks, allowing the Kelvin wave to push warm water eastward towards South America. The result of this interplay between ocean and air is an intensification of El Niño conditions from weak to moderate.
Not all El Niño winters follow the usual pattern. For instance, the El Niño winter of 2004 - 2005 had above-average temperatures over the entire U.S., and drier than average conditions across Florida, which are the opposite of the typical pattern observed during most El Niño winters. There are other natural variations in the weather and climate, such as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) that can act to counteract or re-enforce the typical El Niño pattern.
Wintertime (December - February) temperatures for the U.S. over the past nine years. The past three winters with El Niño conditions (yellow highlighted years) were 2003, 2005, and 2007. The 2003 El Niño was moderate strength, and 2005 and 2007 were weak.
Wintertime (December - February) precipitation for the U.S. over the past nine years. The past three winters with El Niño conditions (yellow highlighted years) were 2003, 2005, and 2007. The 2003 El Niño was moderate strength, and 2005 and 2007 were weak.
The 2008-2009 winter season was characterized by a "near La Nina" pattern as the criteria for an actual La Nina was just not quite reached. However, sea surface water temperatures in the Pacific were below normal and showed La Nina qualities, but just fell short of meeting the length of time needed to be classified officially as a La Nina. Now we are quite confident that an El Nino will be ongoing during the 2009-2010 winter season.
Based on this current pattern of going from a La Nina type pattern to an El Nino pattern, we looked for past years since 1950 when such a pattern existed. We call these "Analog" years, because they represent a pattern similar to what we are currently experiencing. Scientists use analog years to see if there are any similarities in the weather, and use that information to help shape future forecasts. A large number of analog years can give better clues to patterns compared to just a few analog years.
Since 1950, there have been 18 El Nino episodes (1951-52, 1957-58, 1963-64, 1965-66, 1968-69, 1969-70, 1972-73, 1976-77, 1977-78, 1982-83, 1986-87, 1987-88, 1991-92, 1994-95, 1997-98, 2002-03, 2004-05, 2006-07). Eleven of these El Nino episodes were preceded by winters with La Nina or near La Nina conditions (1951-52, 1957-58, 1963-64, 1965-66, 1968-69, 1972-73, 1976-77, 1986-87, 1997-98, 2002-03, 2006-07).
These 11 years will serve as our El Nino "analog" years since they are similar to the pattern we have been in and the pattern we are expecting this winter. Remember my PRELIMINARY winter forecast was published back in September, my FINAL OFFICIAL CALL will be published no later than December first.
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