MIDWEST CLIMATE: EL NIÑO 2015-2016
DIVERSITY OF EL NIÑO
Location and Strength of the Warmest Waters During El Niño
While El Niño is broadly defined as a warm anomaly in the equatorial Pacific Ocean with an atmospheric coupling, not all El Niño events are created equal. Sea surface temperature anomalies during an El Niño event can differ greatly from one El Niño event to another depending on ocean temperatures, the location of the warmest waters, and timing of the warmest waters. Yet, the most important factor that drives El Niño diversity is usually the location of the warmest waters in the equatorial Pacific Ocean.
Depending on ocean circulations and the atmospheric feedback patterns during an El Niño event, the warmest waters can be confined to different areas of the equatorial Pacific Ocean. This can lead to differing impacts across the globe. When the warmest ocean waters are confined to the central Pacific Ocean, the impacts on weather in the United States are often less while the impacts on weather in India and Australia are often much higher. When the warmest ocean waters extend to the western coast of South America, the opposite is often the case, with the United States experiencing more impacts on weather patterns. During the 1997-98 event for example, warm Pacific waters extended to the western coast of South America and impacted temperature and precipitation patterns across North America.
Sea surface temperature anomalies during three recent El Niño events and the current El Niño. The 1997-1998 (top left) and 2009-2010 (top right) events both had warm waters stretch to the coast of South America while 2002-2003 (bottom left) was confined more to the central Pacific. The 2015-2016 El Niño (bottom right) has abnormally warm waters in the North Pacific, making it harder to directly compare to other El Niño events.
Image credit: http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/.
The relative strength of an El Niño event (weak, moderate, strong) can also impact the effect El Niño has on weather in North America. Stronger events can have more of an effect on global climate patterns while weaker El Nino events can be overpowered by other factors. Winters during strong El Niño events were much warmer across the northern half of the country, while temperatures were closer to normal during moderate and weak El Niño winters. Combined with other climate factors in play, there can be quite a diversity of scenarios during an El Niño event.
Comparing winter (December-February) temperature departures from normal (1981-2010) during El Niño events: (A) Composite of strong, moderate & weak El Niño events combined. (B) Composite of weak El Niño events. (C) Composite of moderate El Niño events. (D) Composite of strong El Niño events. Image Credit: NOAA Earth System Research Laboratory. http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl
Other Climate Factors in Play
The El Niño Southern Oscillation is only one of many oscillations that naturally occur on the globe. These oscillations, called teleconnections, can have both direct and indirect effects on the weather in North America. They can influence the location of the jet stream and storm tracks, which in turn influences temperature and precipitation. Like El Niño, these teleconnections can change phases and can be stronger or weaker during different time periods.
Above, Left: an image of the December, January, and February temperature departure from normal for El Niño events 1950-1999 as classified by the Climate Prediction Center in 2000. Right: images of temperature departures from normal when an El Niño was present and the Arctic Oscillation was in a positive phase (top right), a neutral phase (middle right), and a negative phase (bottom right). While El Niño events are said to bring mild winter temperatures to the Midwest during the winter, temperatures will remain much closer to winter time normal during the negative phase. From: http://www.cpc.ncep.noaa.gov/products/outreach/research_papers/ncep_cpc_atlas/8/toc.html.
Depending on the phase of these teleconnections, the effects of El Niño on weather can change. The phases of the Arctic Oscillation, for example, can affect winter weather in North America during an El Niño. When the Arctic Oscillation is in the negative phase, the jet stream over North America weakens, allowing more cold air to sink southward and reducing the warming effects of El Niño during the winter. In the Arctic Oscillation’s positive phase, the jet stream is stronger across North America, keeping colder air in the Arctic and increasing the warming effects of an El Niño during the winter. Precipitation can also be affected based on the phase of the Arctic Oscillation, where the positive phase leads to normal or slightly above normal precipitation compared to the typical El Niño winter.
Above, Left: an image of the December, January, and February precipitation departure from normal for El Niño events 1950-1999 as classified by the Climate Prediction Center in 2000. Right: images of precipitation departures from normal when an El Niño was present during the positive phase of the Arctic Oscillation (top right), the neutral phase (middle right), and the negative phase (bottom right). While El Niño events are said to bring a drier winter to the Midwest during the winter, precipitation remains closer to winter time normal during the positive phase. From: http://www.cpc.ncep.noaa.gov/products/outreach/research_papers/ncep_cpc_atlas/8/toc.html.
What Does this all Mean for the 2015-2016 El Niño?
While the 2015-16 El Niño is expected to be similar to the 1997-98 El Niño in both strength and location of the warmest waters in the equatorial Pacific, it may not be a good idea to directly compare the two. Unlike 1997-98, the North Pacific Ocean is abnormally warm. No well-documented El Niño event has unfolded with the North Pacific Ocean as warm as it has been during the development of the 2015-16 El Niño. It is also unclear what effect, if any, a warm North Pacific will have on weather in North America during this El Niño. Other teleconnections that are not as easily predicted will also likely play a role in how weather evolves this winter. Even if the 2015-16 El Niño results in the same impacts as other strong El Niño events, it may not be for the same reasons.
Antonietta Capotondi, Andrew T. Wittenberg, Matthew Newman, Emanuele Di Lorenzo, Jin-Yi Yu, Pascale Braconnot, Julia Cole, Boris Dewitte, Benjamin Giese, Eric Guilyardi, Fei-Fei Jin, Kristopher Karnauskas, Benjamin Kirtman, Tong Lee, Niklas Schneider, Yan Xue, and Sang-Wook Yeh, 2015: Understanding ENSO Diversity. Bull. Amer. Meteor. Soc., 96, 921–938.
National Weather Service’s Climate Prediction Center. http://www.cpc.ncep.noaa.gov/