When the Typhoon Season Sleeps In

March 14, 2016

It is well known that there is no real typhoon season in the Pacific north of the equator and west of the Date Line—the Northwest Pacific basin. Unlike the North Atlantic, which has official start and end dates of June 1 and November 30, the Northwest Pacific basin is open for business all year round.

Does the continual typhoon season mean that activity in the Northwest Pacific basin starts early every year? Let’s take a look at the data.

Because we are coming off the heels of a very strong, record-breaking El Niño and expecting to go straight into a very strong La Niña, we will see what effect that kind of climate activity has had on season start dates.

In reviewing the genesis dates of the first storms named by the Japan Meteorological Agency since 1951, we see that in 24 out of 65 years the first storm occurred in January; the season was an early riser nearly 40% of the time. For 11 years, however, it slept in and started later, with genesis dates in May, June, and even July.

If we take a closer look at the years when the season really slept in—where the start date was May 15 or later—and compare those years with what is happening El Niño-wise, we see the statistics summarized in the table.

Northwest Pacific Basin Typhoon Season Start dates and El Niño activity
Year Start Date MaxENSOPrevFall ENSOSummer
1952 June 9 0.9 0.0
1964 May 15 1.2 -0.7
1973 July 1 2.0 -1.0
1983 June 25 2.1 0.3
1984 June 9 0.3 -0.3
1998 July 9 2.3 -0.7

The MaxENSOPrevFall column indicates the largest value of the El Niño 3.4 index in the previous fall and thus indicates the strength of the El Niño event going into the following year. A value greater than 1.0 indicates a moderate El Niño event; greater than 2.0 indicates a strong El Niño event. The ENSOSummer column indicates the value of the index during the June–August period.

It is easy to see that for years trending from moderate-to-strong El Niños into weak-to-moderate La Niñas (negative indices), the start date is late—the typhoon season sleeps in.  These are the rows highlighted in gray.
What happened in 1952, when the delta ENSO (MaxENSOPrevFall  - ENSOSummer) was not all that large but there was still a late start? Given that this was the pre-satellite era and that the previous year had a very busy autumn, it may have been that an earlier storm was not recorded.

The only real curiosity in the table occurred in 1984, when there was only a small delta ENSO but the first named storm did not occur until June. Although fully understanding that late start may involve more analyses, it shows that when a weak-to-moderate La Niña is in place by summer directly on the heels of a strong El Niño, that the Pacific Typhoon Season wakes up late.

After the record-setting El Niño event for 2015 (MaxENSOPrevFall = 2.3), we are projected to dive into a strong La Niña event later this year and the threshold for sleeping in will likely be met. If that happens, the odds are very high that the first storm named by the JMA will not occur before June 1.

What that will mean for total numbers, storm tracks, and where damage will occur will be a topic for a future blog.

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