Mauka Showers

By now, many of you have heard that an El Niño is in the process of developing in the Pacific. As of May 14th, NOAA’s Climate Prediction Center (CPC) still had an El Niño Watch in effect, and that, “El Niño is likely to emerge soon (82% chance in May-July 2026.” The main question appears to be not if an El Niño forms but how strong will it be at its peak. CPC’s latest ENSO intensity probabilities, also from May 14th, projected a 2 out of 3 chance that the El Niño will peak at either strong or very strong (a.k.a. “super El Niño”) levels (see bar graph below). An update should be released by CPC next week Thursday, June 11th. I wouldn’t be surprised if they say “El Niño is here.”

NOAA CPC’s 3-month ENSO strength probabilities for El Niño (red shades), La Niña (blue shades), and ENSO-neutral (gray). The x-axis shows 3-month seasons from  April-May-June (AMJ) through December-January-February (DJF). The maximum probability for a strong or very strong El Niño event is 67%.

I think many of you also know that the presence of an El Niño event means that the central North Pacific (CNP) tropical cyclone basin (north of the Equator, between 140W and the Dateline) is more likely to have above normal activity during the hurricane season. The high probability of El Niño was cited as a key factor in the National Weather Service’s (NWS) forecast of a 70% chance for 5 to 13 tropical cyclones (TC) in the CNP this year. The normal range is 4 to 5 TCs. Note that the term tropical cyclone includes tropical depressions, tropical storms, and hurricanes.

Based on CPC’s Relative Oceanic Niño Index (RONI), there have been 18 El Niño events since 1970. Out of 18 events, 10 became strong (RONI >= +1.5C), and 5 became very strong (RONI >= 2.0C). See the tables below for RONI values since 1970. Looking at the 10 strong El Niño events, 5 reached strong intensity during the peak TC activity months. These events are in  1972, 1982, 1987, 1997, and 2015. I thought it would be interesting to show what the CNP TC seasons were like during the years where the RONI reached strong intensity. Based on data from the International Best Track Archive for Climate Stewardship (IBTrACS) database, I plotted the tracks for the 5 years.

CPC’s RONI table values. RONI values in bold red font are El Niño events, bold blue values are La Niña events, and the remainder are ENSO-neutral. The x-axis shows 3-month seasons starting with December-January-February (DJF) on the left to November-December-January (NDJ) on the right. The table was split and cropped for easier viewing. I highlighted the peak seasons for tropical cyclone activity (JJA to ASO) in light red, and El Niño events that reached strong intensity during the peak of tropical cyclone season in yellow.

In 1972, the TC season occurred during the development of a very strong El Niño event, which  peaked in the Northern Hemisphere’s winter. The CNP had 7 TCs, 5 of which were well south and southwest of the Hawaiian Islands. Tropical Storm Diana got close to the island chain but dissipated near Maui County.

 Track graphic of tropical cyclones in 1972 in the CNP basin. Portions of the tracks east of 140W longitude and west of the International Dateline are not included. Plot points are in Coordinated Universal Time (UTC), which is 10 hours ahead of Hawaiian Standard Time (HST). Thus, 0000 UTC is 2 PM HST of the previous day, and 1200 UTC is 2 AM HST of the same day. Data for the graphic are from the International Best Track Archive for Climate Stewardship (IBTrACS) database hosted by the NOAA National Centers for Environmental Information (NCEI).

The strongest El Niño event in the 1950-present RONI dataset developed in 1982. This was an interesting event because it developed slowly in the first half of the year, then strengthened quickly in the fall to peak at “Super El Niño” intensity. It was a very active season with 10 TCs, 3 of which had their centers pass close to the main Hawaiian Islands. The most notable was Hurricane Iwa (Category 1 on the Saffir-Simpson Hurricane Wind Scale). Iwa’s center passed just north of Kauaʻi and produced significant impacts on Kauaʻi and Oʻahu. Tropical Depression Daniel passed just west of Kailua-Kona on the Big Island before dissipating just south of Maui. Gilma passed just south of the Big Island as it moved toward the west-northwest.

Same as the above map, but for the 1982 season.

The 1987 season occurred in the 2nd of consecutive strong El Niño years. The 1986-1987 El Niño weakened in the spring, intensified to strong levels and peaked in summer 1987, then weakened in the fall. The summer peak was unusual since most El Niño events have an intensity peak in the fall or winter months. Despite the strong El Niño, there were only 4 TCs in the CNP that year. All were well south or southwest of the Hawaiian Islands.

Same as above, but for the 1987 season.

In 1997, El Niño was already in place during the spring and on its way to becoming one of the strongest events on record. It had the highest RONI values during the peak CNP TC season. There were 9 TCs that year, 5 of which came from the East Pacific. The remaining 4 developed well southwest of the state. One of these became Tropical Storm Paka, which eventually hit Guam as a super typhoon. No TCs threatened the Hawaiian Islands that year.

Same as above, but for the 1997 season.

And then we have 2015…THE BIG ONE. The 2015-2016 El Niño was also one of the strongest on record. It got a head start from a failed El Niño the previous year with the ocean already primed, and reached El Niño status in late winter to early spring. Similar to 1997, the RONI was already at a strong level entering the peak of the TC season. An incredible 16 TCs occurred in the 2015 season. I think the seasonal number is really 17 because one system formed in January 2016, which is probably a long extension of the 2015 season. Despite all the TC activity, no TCs seriously threatened the main Hawaiian Islands, though we were sweating through some of the model projections during the development phase of what would become Hurricane Kilo. Guillermo got close to the island chain, but was weakening on its approach and dissipated north of Oʻahu. I thought it was interesting that a lot of the TCs coming in from the East Pacific had a northwestward motion instead of a west or west-northwest motion. Also noteworthy was Oho, which had an eastward component of motion for nearly all of its lifecycle. Oho formed within a belt of gale westerlies (!!) south of the Hawaiian Islands. This is insane! I can’t remember any other time when there were gale westerlies in the tropical CNP during the summer so it would be interesting to look in the reanalysis datasets for other occurrences of these conditions.

 Same as above, but for the 2015 season.

One thing that stands out is that each season is very different, even though El Niño is considered to be strong in each case. As a result, there was a wide range in the number of TCs, from 4 to 16. This variation is not surprising since every El Niño event has different details while sharing similar broad characteristics. Another issue is that El Niño is not the only game in town. There are other climate scale features, such as the Pacific Decadal Oscillation (PDO) that have a say in the matter (Chu and Murakami, 2022). In any case, it’s important to remember that regardless of the number of forecast TCs, or how TCs behaved in previous seasons, you always need to be prepared for the possibility of an impact during every season.

Reference

Chu, P.S., and H. Murakami, 2022: Climate Variability and Tropical Cyclone Activity. Cambridge University Press, 303 pp.