Strongest MJO Event on Record Boosts El Niño Odds

The Madden Julian Oscillation (MJO) is a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, and has many important impacts on weather patterns world-wide. For example, when the area of increased thunderstorms associated with the MJO is located in a particular ocean basin, the odds of tropical cyclone formation increase there. Scientists use the Wheeler-Hendon MJO index to monitor how strong the MJO is, and this week, the amplitude of the MJO set a new all-time record for the strongest MJO event observed since record keeping began in 1974 (with no data available from 3/17/1978-12/31/1978 due to satellite problems). The MJO index hit 4.09 on March 15, 2015, beating the old record of 4.01 set on February 14, 1985.  On March 16, 2015, the MJO index set an even higher mark--4.67. That was likely the peak of this record MJO event, as the MJO index fell to 4.51 on March 17, and is forecast to drop significantly over the coming week. Thanks go to CSU's Phil Klotzbach for alerting me to the MJO record.


Figure 1. Wind flow diagram from March 13, 2015, when counter-rotating tropical cyclones on each side of the Equator in the Pacific created strong westerly winds along the Equator. Image credit: http://earth.nullschool.net/.

This record-strength MJO event began increasing in intensity last week, and aided in the formation of twin tropical cyclones, one on each side of the Equator (an event that tends to happen at least once per year.) The Southern Hemisphere storm that formed was Tropical Cyclone Pam, which intensified to Category 5 strength and devastated the island nation of Vanuatu. The Northern Hemisphere tropical cyclone, Tropical Storm Bavi, was not as strong since it had cooler waters to navigate, but still managed to attain peak winds of 60 mph before decaying to a tropical depression this Wednesday. These two tropical cyclones were counter-rotating--the Northern Hemisphere storm spun counter-clockwise, while the Southern Hemisphere storm spun clockwise. As a result of having two counter-rotating storms on either side of the Equator, very strong west-to-east blowing winds formed along the Equator, opposing the usual east-to-west blowing equatorial trade winds. The westerly winds from the counter-rotating tropical cyclones were strong enough and persistent enough that they boosted the odds of the current weak El Niño event staying on through the summer and into fall, since these "westerly wind bursts" tend to move warm water from the Western Pacific into the Eastern Pacific (the presence of warmer-than-average water in the Eastern Pacific is needed for an El Niño event to occur.) According to an email I received from University at Albany El Niño expert Paul Roundy, he expects an 80% chance that there will be a strong El Niño late this year, based on the latest model forecasts, and the evolution of the present event--which is being boosted by the current record-strong MJO, and was helped along by the counter-rotating tropical cyclones in the Pacific last week. He cautions, though, that "failure of El Niño could occur this year if sufficient warm water volume is expelled up and down the coasts of the Americas before June, without maintenance of westerly wind burst activity into June (when the equatorial easterly winds typically amplify with the seasonal cycle and bring the cold tongue back.) "


Figure 2. Projections of sea-surface temperature in the Niño 3.4 region (5°S - 5°N, 120°W - 170°W), where SSTs must be at least 0.5°C above average for five consecutive months (each month being a 3-month average) for an El Niño event to be declared. These forecasts are for August 2015 in terms of departure from average (degrees C), as compiled from eight international climate models and released by the Australian Bureau of Meteorology in a March 17 update. Each model’s projection on the graph above is based on the average of multiple runs in an ensemble. The threshold for El Niño conditions for Australia is 0.8°C above average, as shown on the chart, while the NOAA threshold (not shown) is 0.5°C. Image credit: Australia Bureau of Meteorology .

In a March 17 outlook, Australia's Bureau of Meteorology bumped up their El Nino odds. They stated, "The past fortnight has seen unusual conditions in the tropical Pacific, which may increase the chance of El Niño in 2015. In the western Pacific, severe Tropical Cyclone Pam and Tropical Storm Bavi straddled the equator, producing one of the strongest reversals in the trade winds in recent years. This change is expected to increase the already warm sub-surface temperatures currently observed in the tropical Pacific Ocean, which may in turn raise tropical Pacific Ocean surface temperatures in the coming months. All eight international models surveyed by the Australian Bureau of Meteorology project that water temperatures will increase by August to at least 1.0°C above average in the Niño 3.4 region (see Figure 2), which would suggest at least a moderately strong El Niño event by NOAA’s definition.

NOAA's Climate Prediction Center is saying that there is an approximately 50-60% chance that El Niño conditions will continue through Northern Hemisphere summer 2015. This outlook was issued before the recent record-strength MJO and counter-rotating Pacific tropical cyclones emerged, so it will interesting to see how their outlook changes in the next update on April 9.

If El Niño is present this fall, it increases the odds that we will have a quiet Atlantic hurricane season. El Niño typically brings high wind shear to the tropical Atlantic, disrupting hurricanes and tropical storms as they try to form or intensify. Conversely, El Niño favors a more active than usual Eastern Pacific hurricane season, by lowering wind shear there and increasing water temperatures. If we get a strong El Niño this winter, it would increase the odds of breaking California's streak of four consecutive dry winters, and put a decent dent in their crippling drought. As we saw this past winter, California can still get low precipitation and drought conditions during a weak El Niño event. If El Niño builds and stays through the summer, warmer waters in the Pacific could lead to a wide-scale coral bleaching event, resulting in massive die-off of coral.

Jeff Masters