Periods of Global TC INactivity
The Recent Period of 14.5 d is the longest since 2007 and maybe ever...
Mike Fiorino
NOAA ESRL Boulder CO
29 June 2016
michael.fiorino@noaa.gov
1.0 TC INactivity
The recent period of global tropical INactivity was particularly noteworthy for its long duration -- a remarkable 14.5 day period from 2016042700-2016051106 (00Z 27APR16 - 06Z 11MAY16). The tropics were simply 'dead' quiet -- no TCs and no disturbances...
While TC existence is well known and tracked (pun intended), pre-storm disturbances are less so and are to some degree forecaster/operations dependent. However, in the current observing system it is now highly unlikely that a cloud cluster with TC-formation potential will go undetected. In fact, it has been over 10 years since a TC warning/advisory was issued with no tracking of the pre-genesis disturbance. These disturbances go by many names, but I call them 'pTCs' -- 'p' for pre/potential.
2.0 ATCF TC numbered scheme
Because both JTWC and NHC use the ATCF system (http://www.nrlmry.navy.mil/atcf_web/docs/ATCF-FAQ.html), TCs are always numbered from 01-50 (NN) appended with a subbasin code, e.g., 01E would be the first TC in the Eastern north Pacific.
'INVESTS' or pTCs are numbered from 90-99 or 9X. The first INVEST would be 90, but the 11th would also by 90. Thus, in operations, pTCs do not have unique identifiers and the actual track data is located in two (really three) different files that change during the monitoring period. Therefore, special data management is needed to keep the 9X complete and unique, but that begs the question: why bother? The first reason is to reproduce operations for more realistic testing of forecast tools, e.g., model tracking. The second and more important reason is for TC genesis studies/forecasting as all TCs originate from a weaker pTC disturbance...
The two main purposes of analyzing pTCs in operations are: 1) initiate storm-specific satellite reconnaissance and 2) begin storm tracking in the models, i.e., to 'warm up' the forecast/warning process. In fact, the mean time between when a pTC is started and when the time of the first warning/advisory for a TC is about 72 h (more about pTCs in a future paper). The input to the recon/tracking process is called the 'TCvitals' and consists of a 'card' for each pTC/TC with data on position, intensity, motion and wind radii. Thus, a period of INactivity is a time (6 h) with no TCvitals...
However, a basic problem with pTCs is that they are NOT best tracked -- they have no operational value, but they are essential for TC genesis applications and reproducing operations. Since 2006, I have maintained a global pTC data set using the ATCF files from JTWC and NHC. I also have some salvaged 9X files from 1998-2009 courtesy of Buck Samson, NRL Monterey, but the record is incomplete during the early years... However, the data for the 10-y period 2007-2016 are solid.
3.0 Periods of global pTC and TC INactivity
The pTC and TC data are saved as python objects for rapid listing of all TCs by date-time-group (DTG -- YYYMMDDHH used in section 1.0 above) or by storm id. One application is to calculate the TCvitals (e.g., http://ftp.nhc.noaa.gov/atcf/com/) as done in operations. The other obvious application is verification.
Global TC INactivity or quiet periods are defined as NO TCs or pTCs anywhere around the globe. Another way to define INactivity is no TC vitals -- no systems to track in the models.
In general, quiet periods are quite rare and last less than 2 days.
Here is a list of events with the beginning and ending MMDDHH period followed by length in days:
2007 events
042512-042612 1.25
050606-050606 0.25
052306-052318 0.75
060806-060818 0.75
061506-061518 0.75
2008 events
031506-031518 0.75
040800-040812 0.75
052012-052012 0.25
060806-060818 0.75
103006-103112 1.50
2009 events
051606-051712 1.50
060712-060712 0.25
062900-062906 0.50
063018-070306 2.75
072218-072218 0.25
122906-122906 0.25
2010 events
041500-041506 0.50
043018-050412 4.00
120806-120806 0.25
2011 events
011806-011906 1.25
042512-042612 1.25
2012 events
011418-011618 2.25
040906-040906 0.25
041118-041406 2.75
050312-050318 0.50
2013 events
042100-042112 0.75
2014 events
012600-012600 0.25
050212-050218 0.50
051018-051500 4.50
053000-053000 0.25
2015 events
030118-030218 1.25
041806-041900 1.00
120218-120518 3.25
2016 events (so far)
030806-030912 1.50
031118-031200 0.50
032512-032518 0.50
040800-040900 1.25
042700-051106 14.50
052200-052318 2.00
061012-061418 4.50 (tie with 2014)
062118-062118 0.25
062918-062918 0.25
Of the top four longest events (bold/italics) in the 10-y period, this year's is the longest by far (14.5 d v 4.5 d) and note how the top three all occurred in the late April - mid May period... 2016 not only has the longest period on record since 2007, but has the greatest number of events.
The following plot displays all events of length >= 1 day for the years 1999-2016 by month of occurrence:
First note the far greater number of events for the period 1999-2006 (76) v 2007-2016 (18). The pTC data prior to my management is clearly limited and not representative of current operations, but I should also point out that the process for analyzing pTCs in operations has also improved with more and better satellite data. Thus, the 2007-2016 period is probably more representative of the actual climatology.
Regardless, the transition from the end (April-June) of the southern Hemisphere season (SHEM) to the beginning (May-June) of the northern Hemisphere (NHEM) season has the most INactivity for one rather obvious reason -- about 70% of all TCs occurs in the NHEM with virtually no quiet periods in the peak of the NHEM season (August-October).
4.0 Relationship with seasonal ACE
From the listing in Section 3.0, we can see substantial interannual variability in the yearly total of significant quiet periods (>= 1 day) which suggests a potential relationship to seasonal TC activity. For example, in 2013, there were NO significant quiet periods, the NHEM ACE was 246 days, the the previous SHEM ACE was 117 days (I scale ACE by 65*65 kts^2 to make the units days vice energy) and a NHEM-SHEM difference of 129 d. In contrast, 2015 had 5.5 quiet days and a NHEM-SHEM ACE difference of 291 d. The basic idea is that few quiet days in the transition from the SHEM to NHEM season might imply less contrast in the inter-hemisphere ACE.
The plot below shows the yearly quiet days v the NHEM-SHEM ACE difference for 2007-2016 (the SHEM 2016 just concluded and we do not have the 2016 NHEM ACE):
A faint correlation between quiet periods and inter-hemisphere ACE can be seen using your SpongeBob SquarePants imagination -- more quiet periods => greater contrast between NHEM and SHEM seasonal activity.
By this reasoning, the 2016 NHEM season would be a big year ACE wise, but this year's INactivity is so large and anomalous that the opposite may happen...and that's my forecast...
Comparing the yearly total of quiet periods with seasonal ACE showed a potential relationship to the inter-hemisphere change -- few quiet periods, a weaker difference in NHEM-SHEM seasonal ACE.
2016 is very unusual in the number and length of quiet periods -- over 23 days so far whereas the previous record was 6 days in 2015.
The plot below shows the yearly quiet days v the NHEM-SHEM ACE difference for 2007-2016 (the SHEM 2016 just concluded and we do not have the 2016 NHEM ACE):
By this reasoning, the 2016 NHEM season would be a big year ACE wise, but this year's INactivity is so large and anomalous that the opposite may happen...and that's my forecast...
5.0 Summary
We have analyzed a different aspect of TC activity -- periods of global INactivity defined as no TCs and no pTCs (no TCvitals) based on JTWC/NHC operational data for 2007-2016. These quiet periods are meteorologically curious because they imply little convection in the ITCZ and/or unfavorable conditions for tropical cyclone formation, i.e., a benign and 'uninteresting' global tropics.
Comparing the yearly total of quiet periods with seasonal ACE showed a potential relationship to the inter-hemisphere change -- few quiet periods, a weaker difference in NHEM-SHEM seasonal ACE.
2016 is very unusual in the number and length of quiet periods -- over 23 days so far whereas the previous record was 6 days in 2015.
