[NOAA/NESDIS Sea Surface Temperature Anomaly chart for September 4, 2014]
Summary El Nino, which refers to warmer-than-normal waters in the central and eastern tropical Pacific Ocean, can have an important effect on global weather patterns. There were some forecasts earlier this year for a “super strong El Nino” to form, but we argued against that back in April here at "thesiweather.com" providing several reasons as to why that did not appear likely (http://thesiweather.com/2014/04/22/1100-am-el-nino-on-the-way-but-odds-are-against-a-super-one/) and it now appears very unlikely. There are signs, however, that a weak El Nino is starting to form and even a weak El Nino can have important implications on global weather patterns for the upcoming fall and winter seasons.
El Nino signs The first sign that suggests an El Nino is indeed developing in the Pacific Ocean is the noticeable area of warmer-than-normal sea surface temperatures (SST) now seen just off the west coast of South America. The latest SST anomaly map from NOAA shows this area of warmer-than-normal temperatures (circled area above; reds, oranges, yellows) which actually were first in evidence during the spring and, in fact, this fueled El Nino speculation; however, there was then was a temporary reversal earlier this summer in that particular temperature trend. In the last few weeks, however, the warmer-than-normal sea surface temperature trend has become re-established and is likely to persist for awhile.
A second signal that suggests a weak El Nino is forming has to do with the southern oscillation index. The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin. Sustained negative values of the SOI below −8 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean. The last three full months have seen an increasing “negative” trend in the SOI providing evidence that an El Nino is indeed forming (June: -0.8, July: -4.0, August: -10.1).
Potential Winter Implications One final note, a wintertime weak El Nino can have important implications to winter weather patterns in the Northeast US. For example, a weak El Nino can help to strengthen the southern branch of the jet stream across the southern US during the winter season and this could add to storminess (and snowfall) along the east coast. Not only is the magnitude (weak, moderate or strong) of an El Nino important in determining its potential impact on winter weather here in the Northeast US, but its location in the Pacific Ocean is critical as well. For example, a weak El Nino based in the central part of the Pacific Ocean tends to help generate more snowfall in the Northeast US compared with an El Nino centered in the eastern Pacific. Odds favor a centrally-based weak El Nino as we approach the upcoming winter season, but stayed tuned on that one.