wpac seasonal ACE v season start

WPAC Seasonal ACE v Start of the Season

does the slow start to the 2016 season presage below normal activity?

Mike Fiorino

NOAA ESRL Boulder CO

14 JUN 2016

michael.fiorino@noaa.gov

1.0 The first storm (01W) of the 2016 WPAC Season

The 2nd 90W of 2016 has been reanalyzed by JTWC as the first numbered TC in WPAC.   Although there is no fixed minimum max wind that defines a TC, reports of winds >= 25 kts prompted the forecasters to reclassify the 90W pre-potential TC (pTC) as a the first TC (tropical depression) of the season as it made landfall over China in Fig. 1 below:

Figure 1. track plot of the 2nd 90W (B0W) in WPAC 2016052400-052712

I've commented on unusual periods of NO TC (or pTC) activity in May and June, but what also surprised me was low activity in WPAC in 2016 and the 'late' start the season...

2.0 Start date of the first storm (01W) in WPAC

I reprocessed the JTWC ATCF data files ('a' and 'b' decks) to calculate the first TC position of the first storm (01W) in WPAC from 1970-2015 (46 years).  The observing system has undergone huge changes in this period and that early storms would be 'missed' in the a/c recon era (1970-1987) with limited satellite coverage.  However, of the 12 earliest season starts with dates < 9 JAN, 5 were from the 1970s.  Conversely, of the 12 latest seasons, 2 come from the 1970s and 5 from the early 1980s.  Thus, the early a/c recon period are not obviously biased.

Fig. 2 below shows the cumulative probability distribution of start dates:

Figure 2. Cumulative histogram of start dates, the median is 19 JAN and 75% start before 09 MAR

WPAC is an early-season basin with 50% of the first storms starting <= 19 JAN and 75% <= 09 MAR.  In contrast, the official atLANTic season is 1 JUN.  A start data of around 27 MAY would make 2016 one of the latest season openers ever ( probability < 10%).

3.0 Seasonal ACE v season start

Using the JTWC best tracks for 1970-2014 (2015 is not yet complete), I calculated the seasonal ACE but scale by 65*65 kt^2 (typhoon) so that the units are days and I call this 'sACEd' for scaled ACE days.  the plot below shows sACEd v start date:

Figure 3.  WPAC seasonal scaled ACE days (sACEd) versus start date of first storm (01W).  Red bars are for the lowest/weakest 25% of seasons and Green for the 25% strongest season.

The most remarkable obsevation about Fig. 3 is that all top 25% sACEd seasons started before 15 MAR, whereas half of the 25% weakest seasons started after 15 MAR.  The implication is that 2016 is very likely to be a below-normal season in WPAC