El Niño: Why Strength Matters

Posted: October 1, 2015, 12:44 pm by bmarmo

As we enter the first week of October, we continue to watch the strengthening El Niño in the Tropical Pacific. At this point, it looks nearly certain that El Niño will stick around for the entire upcoming winter. However, when assessing the impact it will have on our winter in the Midwest, Northeast and Mid-Atlantic, it is vital to also know the strength of the event.

When measuring the strength of an El Niño, we generally use the Oceanic Niño Index (ONI). These events are broken down into weak (at least five consecutive overlapping 3-month periods of SST anomalies between +0.5°C and +0.9°C), moderate (SST anomalies between +1.0°C and +1.4°C) or strong (SST anomalies greater than +1.5°C) events. Typically, forecasting the strength of an El Niño is quite challenging. However, we do not have that problem this year. The main question right now is “will it be the strongest one ever?”

Figure above shows ECMWF ensemble model forecast for the Niño 3.4 region. All of the members (red lines) predict that strong El Niño conditions will continue into 2016.

So why is the strength of an El Niño so important? The reason is (with all other things being equal) a strong El Niño will promote a different weather pattern than a weaker event. Overall, El Niños (regardless of strength) are often marked by an enhance split flow pattern and a stronger than average southern jet stream. This will favor wetter and cooler conditions across a good portion of the Southern US. Conversely, the Pacific Northwest will generally experience milder than normal temperatures during an El Niño.

Idealized weather pattern and associated temperatures departures during a strong El Niño.

The images above and below portray the idealized pattern and associated temperatures anomalies for a strong and weak El Niño respectively. While the strong split flow is present in both of them, there are important differences in the orientation of the polar jet stream. A weaker El Niño (below) will favor a ridge of high pressure across the western North America and a trough of low pressure over the eastern part of the continent. As a result, the Midwest, Ohio River Valley and East Coast generally see colder than normal temperatures. In contrast, the polar jet stream is displaced further north into Canada during strong events (above). This promotes above average temperatures anomalies across our region, as milder air is able to spill across the whole northern tier of the country. This was the dominate pattern during the winters of 1982-83 and 1997-98, the two strongest events on record.

Idealized weather pattern and temperatures anomalies during a weak El Niño.

While winters in the Midwest usually end up milder and drier than usual, it is important note that the correlation between strong El Niño becomes weaker as you move into the Northeast and Mid-Atlantic. While snowfall totals average out below normal overall during strong El Niños, they have been quite variable from event to event. Also we stress that there are other factors to consider when preparing a seasonal forecast, such as the sea surface temperatures across the Gulf of Alaska. In addition, teleconnections such as the Arctic Oscillation have a profound impact on how much snow we get in the Eastern US.

As we move through early October, we are now putting the finishing touches on the 2015-16 winter outlook. For a complete and in-depth forecast, including monthly breakdowns, keep an eye out for the WeatherWorks Official Winter Forecast, due out during the middle of October.

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