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El Niño Heads for Potential Record Strength; Japan Picks up Pieces from Etau’s Rains

By: Bob Henson 6:31 PM GMT on September 11, 2015

NOAA’s monthly update on El Niño, released on Thursday, held no big surprises: we are on the upswing of one of the strongest El Niño events--very possibly the strongest--of the past 65 years of recordkeeping. As of last week (see PDF), sea-surface temperatures across a key part of the eastern tropical Pacific called Niño3.4 were running 2.1°C above the long-term average for this time of year.

Figure 1. Anomalies (departures from average for this time of year) in sea-surface temperature across the northern and eastern Pacific show the distinct band of warmth in the eastern equatorial Pacific characteristic of El Niño, as well as several other large areas of unusual warmth over the Northeast Pacific Ocean. An excellent article in BayNature explains the persistent “Blob” in the Northeast Pacific and how it might intersect with El Niño in the coming winter. Image credit: Levi Cowan, tropicaltidbits.com.

Every El Niño is different, but the strongest events have some very distinct characteristics. Now that the atmosphere and ocean are in the mutually reinforcing pattern typical of strong El Niños, the course of the next few months is relatively predictable. Niño3.4 anomalies (departures from seasonal average) should continue to rise until peaking sometime around December or January, then subside early in 2016. (In its monthly update, NOAA gives 95% odds that El Niño will continue through the northern winter of 2015-16.) As evident in Figure 1, the odds of neutral conditions will rise dramatically toward spring, but this could represent the beginning of a transition toward La Niña. As the ocean rebounds from strong El Niño conditions, La Niña--a cooling of the eastern tropical Pacific--often, but not always, follows. (See the historical ENSO database for examples.) Since we’re now in the second winter of El Niño conditions (albeit borderline conditions in 2014-15), it’s very unlikely that we’ll see El Niño continue past next spring. I would expect to see gradually rising odds of La Niña in upcoming forecasts as they extend further into 2016. If a La Niña were to develop by mid-year, it would favor a more active Atlantic hurricane season than usual in 2016.

Figure 2. Probabilities of El Niño (red bars), neutral (olive bars), or La Niña (blue bars) conditions for each three-month period from August-October 2015 to April-June 2016, based on a forecaster-consensus outlook produced by NOAA’s Climate Prediction Center and the International Research Institute for Climate and Society. The thin red, olive, and blue lines show the long-term climatological likelihood of El Niño, neutral, or La Niña conditions for each period. Image credit: IRI.

A surge in El Niño strength this month?
In his WU blog, Steve Gregory observed earlier this week that temperatures in the Niño3.4 region appear to have risen by 0.3°C over a period of a week, with the rise not yet fully reflected in NOAA’s weekly El Niño updates (issued each Monday). Steve points out that the Niño3.4 values can vary quite a bit from one week to the next, and NOAA forecasters caution us not to obsess about minor week-to-week changes. That said, it’s worth noting that a weekly change of 0.3°C would fall in the top 5-10% of weekly changes observed from 1990 to mid-2015. If sustained, such a rise would also push the current El Niño event closer to record values.

NOAA’s weekly Niño3.4 values are based on a series of short-term analyses of sea-surface temperature called OISST (optimum interpolation SST). The OISST values include satellite-based measurements that can introduce biases when folded in with older data. To produce one-month and three-month statistics for the Niño3.4 area, NOAA uses a separate data set called the Extended Reconstructed Sea Surface Temperature (ERSST) to rank and classify El Niño events on a monthly and trimonthly basis. This monthly dataset goes back to 1854 and uses statistical techniques to fill in data gaps. The most recent three-month values, called the Oceanic Niño Index, were at 1.2°C above the long-term average for June through August. This is slightly below the JJA value of +1.4 observed during the pacesetting El Niño event of 1982-83. However, the 2015-16 event may now be overtaking 1982-83 in terms of its current strength, an outcome suggested by a range of international computer models (see Figure 3 below).

For more on the implications of El Niño for this winter, see our blog posts on the typical impacts for the U.S. in general and what might happen in the Northeast in particular. Jeff Masters will have a forthcoming post on the potential impacts beyond U.S. borders.

Figure 3. Forecasts by a suite of international computer models tracked by Australia’s Bureau of Meteorology have gotten progressively stronger on the 2015-16 El Niño. For the above outlooks, issued in August and valid in November, the model average (shown in the bottom bar) indicates a Niño3.4 anomaly (departure from the seasonal average) of just above 2.8°C for the month as a whole. If these model runs through winter 2015-16 were to prove accurate, the current El Niño would be the strongest in records going back to 1950. The red and blue shading denotes values above or below 0.8°C, which represent the threshold for El Niño and La Niña, respectively, as used by Australian forecasters. NOAA uses 0.5°C as the threshold, since lesser values can still produce U.S. impacts. More information on each models can be found here. Image credit: BOM.

Japan still recovering from floods, landslides associated with Tropical Storm Etao
Damage continues to mount in central Japan as a result of a persistent band of heavy rain that caused landslides and flooding over the area. The rainband formed on the southeast flank of Tropical Storm Etao as the storm moved northward across the island of Honshu, as the southerly inflow encountered Honshu’s high mountains. The city of Ikari recorded 21.69" (551 mm) of rain in just 24 hours, and the popular destination of Nikko received 26.30" (668 mm) from Sunday into Thursday (thanks go to Maximiliano Herrera for the Ikari rainfall info.) The 24-hour total at Ikari was larger than the city’s previous highest 48-hour total in more than 35 years of recordkeeping. The prolonged rains led to the collapse of a flood berm and the inundation of several areas, including parts of the Tokyo suburb of Joso (population 60,000). At least 3 people are dead and 23 missing, and at least 99 landslides have been reported. Nick Wiltgen at weather.com has much more on the flooding from Etao.

Figure 4. Local residents wait to be rescued on the roof of their home in a flooded area in Joso, Ibaraki Prefecture, on September 10, 2015. The Japanese city about 30 miles (50 km) northeast of Tokyo was flooded when Kinugawa river burst its banks, destroying homes and cars as desperate residents waited for help, and as thousands of people were ordered to evacuate. Image credit: Jiji Press/AFP/Getty Images.

Japan’s destructive rains from Etao may been influenced by the presence of weakening Tropical Storm Kilo to the southeast (see Figure 5). Although Kilo had weakened greatly over the last several days, it still had a large circulation and moisture envelope, and some of that moisture could have been ingested by the rainband located between the two storms. Along these lines, a recent open-access paper in the journal Advances in Meteorology documents the enhancement of a rainband in eastern China associated with Typhoon Fitow in early October 2013. Modeling of the event with and without the presence of Typhoon Danas, located well to the east of Fitow, suggests that moisture from Danas more than doubled the amount of rainfall produced in northern Zhejiang province, from about 220 mm (8.66”) in the non-Danas experiment to the 500-plus mm (more than 19 inches) that was actually observed as well as produced in the model when Danas was included. It’s also possible that the Fujiwhara effect, where two cyclones influence each other’s motion, contributed to Etau’s slowdown in forward speed, which also enhanced rainfall. (Thanks to Naoko Kitabatake, Japan Meteorological Agency, for pointing out the potential interactions between Kilo and Etau.)

Figure 5. Using imagery from Japan’s Himiwari-8 satellite with RealEarth software, this image shows Tropical Storm Etao northwest of Japan; the much larger circulation of Tropical Storm Kilo well to the east; and the intense rainband over eastern Honshu island (center). Image credit: UW-Madison CIMMS.

Elsewhere in the tropics
The Pacific is enjoying a rare moment of quiet in the tropical cyclone realm: both Kilo and Etau have expired as tropical cyclones, according to the Joint Typhoon Warning Center. The only other system of current interest is Invest 99E, which should remain weak as it spins well south of Mexico. The Atlantic is also relatively quiet, with Tropical Storm Henri zipping northeastward with top sustained winds of just 40 mph. Henri may nick southeast Newfoundland on Saturday as a weak post-tropical storm. The National Hurricane Center is tracking two areas with minimal odds of development--one near the northern Lesser Antilles and the other well southwest of the Azores. A wave that recently came off Africa is not expected to evolve significantly over the weekend, although NHC gives it 50-50 odds of developing into a tropical cyclone between Sunday and Wednesday. This system is worth monitoring later next week as it moves into the central tropical Atlantic.

Have a fantastic weekend, everyone--and happy birthday, Jeff Masters!

Bob Henson

Hurricane El Niño

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.