Historical Study of Loops in North Atlantic Tropical Cyclones

[Roger Smith]

Historical Study of Loops in North Atlantic Tropical Cyclones

1871 to 2017

Introduction

This is a study of the frequency, location and spatial parameters of loops that have been mapped for North Atlantic tropical cyclones in the period 1871 to 2017.

On the average, one storm every two to three years performs a loop somewhere in the North Atlantic tropical storm theater. A loop is defined for the purposes of this study as any motion that brings a named storm (in any level of its development) back to a point that it had earlier passed. Very small loops within 0.5 deg and a 12-hour time span are not included (examples include 1950 Easy northwest of Tampa as well as 1985 Elena in a similar location, and Diana 1984 on approach to landfall in NC). A very small-scale loop by 2012.P (Patty) near 25N 72W was of long enough duration but not greater than half a degree. Also excluded are near-loops where a storm meanders and almost completes a loop; there are at least ten examples, for example, see Gracie 1959, which returned to within 30 miles of its original position east of Florida after 2.5 days in a counter-clockwise meander, or Flora 1963 which made a similar clockwise meander around eastern Cuba. To be included in this study group, there must be a completed loop crossing the original path. Also the storm must be in a tropical phase, this excluded 1962-A which performed a clockwise loop off New England while extra-tropical as well as the 1991 "Perfect Storm" (1991.8) which produced a large counter-clockwise loop south of New England, but with the first half of that pre-tropical. 2012.C (Chris) started a loop in the central Atlantic as a hurricane but more than half of that one was completed while the storm was in its extratropical phase. Storms are allowed to become extratropical in the loop if that happens within 25% of the elapsed time from the end of the loop. This permitted 1967.D to be in the study group. Lili (1984) executed a large loop northeast of Bermuda in December 1984 but is not included because it was not always a tropical system during this time. The double loop of 1985 Juan south of and partly inland (LA) is counted as one complex loop, it included two CCW portions. The complex figure of eight track of 1999 Dennis is summarized as one complex loop. A small CCW loop was followed by a larger CW loop over four days.

The parameters that can be noted about such loops are the following, as well as perhaps others that some other researcher might wish to investigate. 

First, is the loop clockwise (CW) or counter-clockwise (CCW) in its orientation? 

Where does the loop begin and end (nearest lat/long in whole degrees), and from that point, what direction (360 degrees) is its central axis (a point joining the loop's start/finish point and its approximate furthest point displaced from its origin)? The entry in the table is the vector from the loop's start/finish point to that furthest point in the loop. If the loop is in the form of a squashed ellipse at right angles to that point, this "furthest displacement" is not the furthest point from the loop's origin but is the distance (in deg) to the opposite point.

What is the simplified shape that the loop performs? The analysis recognizes five types, circular, slightly elliptical along the displacement axis (ELL-1a), slightly elliptical with the long axis at right angles to the displacement axis (ELL-1b), highly elliptical along the displacement axis (ELL-2a), or highly elliptical with the long axis at right angles to the displacement axis (ELL-2b). A sixth possibility is "irregular" when the loop cannot be approximately matched to any of these. Only one example of ELL-1b was found and one example of ELL-2b (1990-J). Also, two loops were irregular enough to be so classified, 1967-D, as well as 1985.J and 1999.D which were complex double loop structures, although it should be noted that the paths in general are rarely perfect circles or ellipses. The irregular track of 1967.D was a highly elliptical CCW path oriented east-north-east and distended north on its outward half. It counts in the census as ELL-2a (with asterisk). The most extreme example of an ELL-2a was probably 2009.F (Fred) whose "loop" appears almost as a straight line extended and retracted (perhaps an ellipse of eccentricity .99). 

When the displacement is roughly southwest, northwest, northeast or southeast, the degrees of displacement are a compromise value based on pythagorean triangles, for example, 2 deg north and 2 deg west would be listed as 3 deg. 3 deg north and 4 deg east would be listed as 5 deg. 

Then, how long does it take to complete? This is listed in days and fractions based on a reasonable interpolation of 12-hourly positions available.

Finally, what dates define the start and end point, and (in the analysis section) is there any bias towards portions of the tropical season?

The author has examined maps of tracks of North Atlantic tropical cyclones for each year 1871 to 2017 (up to Jose which has just completed a loop, hence the interest in the subject). The following table summarizes the loops identified in terms of all of the above parameters. An analysis follows the table. 

 

2. Table of Loops performed by North Atlantic tropical cyclones 1871 to 2017 (0.5 d or greater, 1 deg or greater)

Note: Storms are identified by year and number up to 1950 when alphabetical lists began. From then on, if the storm had a name, that initial appears (the number may be somewhat different). 1953-7 did not receive a name at the time so that storm is noted as 1953.7. 1953.A, 1963.G and 2012.N had two separate loops as noted. Storm 1969-K had smaller loops at each end of its more significant loop so it actually looped three times but for the study this counts as just one loop episode. Storm 1971-G had a small CCW loop on the return portion of its much larger 10-day CW loop. That loop is not counted as it occurred within a loop.

 

n __ Storm __ CW or CCW _ Loop origin/end __ Displace__ Direction _ Shape __ Duration __ Dates

 

 1 _ 1884.2___ CW ________31 N _ 76 W ____ 3 deg ____ 190 ____ circular ___ 4.0 d __ 9-13 to 9-17

 2 _ 1887.15 ______ CCW __ 19 N _ 71 W ____ 4 deg ____ 260 ____ ELL-2a ___ 1.5 d __11-28 to 11-29

 3 _ 1896.6 _______ CCW __ 37 N _ 40 W ____ 3 deg ____ 340 ____ circular ___ 3.0 d __11-5 to 11-8

 4 _ 1897.4 _______ CCW __ 34 N _ 74 W ____ 3 deg ____ 350 ____ circular ___ 2.0 d __10-24 to 10-26

 5 _ 1900.2___ CW ________ 26 N _ 33 W ____ 3 deg ____ 070 ____ ELL-1a ___ 3.5 d __ 9-15 to 9-18

 6 _ 1903.7___ CW ________ 28 N _ 48 W ____ 2 deg ____ 180 ____ ELL-1b ___ 1.5 d __10-6 to 10-7

 7 _ 1904.3 _______ CCW __ 26 N _ 82 W ____ 1 deg ____ 180 ____ circular ___ 0.8 d __10-18

 8 _ 1906.6 _______ CCW __ 28 N _ 42 W ____ 5 deg ____ 260 ____ ELL-1a ____ 2.5 d __ 9-25 to 9-27

 9 _ 1908.2___ CW ________ 28 N _ 78 W ____ 3 deg ____ 150 ____ circular ___ 3.8 d __ 7-25 to 7-29

10 _ 1908.6 ______ CCW ___ 29 N _ 73 W ____ 4 deg ____ 070 ____ ELL-2a ___ 1.5 d __ 10-3 to 10-4

11 _ 1910.4 ______ CCW ___ 23 N _ 84 W ____ 2 deg ____ 300 ____ circular ___ 2.0 d __ 10-15 to 10-16

12 _ 1912.4 ______ CCW ___ 32 N _ 75 W ____ 2 deg ____ 350 ____ circular ___ 2.3 d __ 10-5 to 10-7

13 _ 1926.4___ CW ________ 40 N _ 60W ____ 2 deg ____ 180 ____ circular ___ 3.5 d ___9-18 to 9-21

14 _ 1926.8 ______ CCW ___ 36 N _ 29 W ____ 5 deg ____ 010 ____ ELL-1a ___ 3.5 d ___9-26 to 9-29

15 _ 1929.3 ______ CCW ___ 30 N _ 46 W ____ 3 deg ____ 270 ____ ELL-1b ___ 3.5 d __ 10-15 to 10-18

16 _ 1934.11__ CW ________ 28 N _ 67 W ____ 4 deg ____ 030 ____ELL-2a ___ 2.5 d __ 11-24 to 11-26

17 _ 1940.1 ______ CCW ___ 29 N _ 88 W ____ 1 deg ____ 190 ____ circular ___ 0.8 d __ 6-15

18 _ 1941.2___ CW ________ 25 N _ 88 W ____ 2 deg ____320 ____ ELL-1a ___ 2.5 d ___ 9-17 to 9-19

19 _ 1941.3___ CW ________ 30 N _ 71 W ____ 2 deg ____ 090 ____ELL-2a ___ 1.5 d ___ 9-20 to 9-21

20 _ 1941.5 ______ CCW ___ 30 N _ 74 W ____ 1 deg ____ 210 ____ ELL-1a ___ 1.0 d ___10-10

21 _ 1942.3___ CW ________ 35 N _ 58 W ____ 8 deg ____ 040 ____ ELL-2a ___ 6.5 d ___ 8-27 to 9-2

22 _ 1943.6 ______ CCW ___ 27 N _ 95 W ____ 1 deg ____ 320 ____ circular ___ 1.8 d ___ 9-16 to 9-18

23 _ 1947.7 ______ CCW ___ 31 N _ 83 W ____ 2 deg ____ 230 ____ circular ___ 0.8 d ___10-7

24 _ 1948.8___ CW ________ 32 N _ 48 W ____ 5 deg ____ 090 ____ ELL-2a ___ 5.0 d ___10-9 to 10-14

25 _ 1949.6___ CW ________ 35 N _ 44 W ____ 3 deg ____ 080 ____ ELL-1a ___ 2.5 d ___ 9-7 to 9-9

26 _ 1949.8___ CW ________ 26 N _ 94 W ____ 1 deg ____ 320 ____ circular ___ 0.7 d ___ 9-22

27 _ 1950.C___ CW ________ 33 N _ 57 W ____ 2 deg ____ 040 ____ circular ___ 1.0 d ___ 8-30

28 _ 1951.J ______ CCW ____ 32 N _ 73 W ____ 2 deg ____ 040 ____ ELL-1a ___ 2.5 d ___10-16 to 10-18

29 _ 1952.E___ CW ________ 15 N _ 51 W ____ 4 deg ____ 020 ____ ELL-2a ___ 5.0 d ___ 10-6 to 10-11

30 _ 1953.A(1) ___ CCW ____ 15 N _ 82 W ____ 4 deg ____ 270 ____ circular ___ 3.0 d ___ 5-26 to 5-29

31 _ 1953.A(2) ___ CCW ____ 23 N _ 84 W ____ 3 deg ____ 300 ____ ELL-1a ___ 2.0 d ___ 6-1 to 6-2

32 _ 1953.7___ CW ________ 25 N _ 88 W ____ 2 deg ____ 090 ____ ELL-1a ___ 2.0 d ___ 9-16 to 9-18

33 _ 1956.G _____ CCW ____ 26 N _ 72 W ____ 3 deg ____ 160 ____ ELL-2a ___ 1.6 d ___11-1 to 11-3

34 _ 1961.E___ CW ________ 41 N _ 70 W ____ 7 deg ____ 150 ____ ELL-1a ___ 4.3 d ___ 9-21 to 9-25

35 _ 1962.C___ CW ________ 30 N _ 65 W ____ 4 deg ____ 090 ____ ELL-1a ___ 2.4 d ___ 9-17 to 9-19

36 _ 1963.G(1)_CW ________ 34 N _ 74 W ____ 1 deg ____ 300 ____ ELL-1a ___ 1.5 d ___10-20 to 10-21

37 _ 1963.G(2)_CW ________ 33 N _ 74 W ____ 8 deg ____ 230 ____ ELL-1a ___ 5.5 d ___10-22 to 10-27

38 _ 1965.B___ CW ________ 28 N _ 75 W ____ 1 deg ____ 360 ____ ELL-2a ___ 1.5 d ___ 9-4 to 9-5

39 _ 1965.C___ CW ________ 35 N _ 42 W ____ 2 deg ____ 180 ____ ELL-2a ___ 3.5 d ___ 9-25 to 9-28

40 _ 1967.D _____ CCW ____ 33 N _ 76 W ____12 deg ____ 070 __Irr ELL-2a* __ 7.3 d ___ 9-10 to 9-17

41 _ 1969.I___ CW ________ 27 N _ 63 W _____ 3 deg ____ 030 ____ ELL-1a ___ 3.0 d ___ 9-30 to 10-3

42 _ 1969.K___CW ________ 34 N _ 70 W _____ 7 deg ____ 240 ____ ELL-2a ___ 6.0 d ___10-11 to 10-16 

43 _ 1969.L___ CW ________ 23 N _ 91 W _____ 5 deg ____ 060 ____ ELL-1a ___ 5.0 d ___10-20 to 10-25

44 _ 1971.G___ CW ________ 31 N _ 59 W ____10 deg ____ 080 ____ ELL-2a ___ 9.6 d ___ 10-12 to 10-22

45 _ 1972.B___ CW ________ 41 N _ 37 W _____ 4 deg ____ 090 ____ ELL-2a ___ 3.5 d ____ 8-28 to 8-31

46 _ 1977.C___ CW ________ 35 N _ 63 W _____ 2 deg ____ 190 ____ ELL-1a ___ 1.5 d ____ 9-9 to 9-10

47 _ 1980.C ______ CCW ___ 38 N _ 65 W _____ 2 deg ____ 320 ____ circular ___ 1.0 d ____ 8-22

48 _ 1980.I ______ CCW ____29 N _ 31 W _____ 1 deg ____ 180 ____ ELL-1a ___ 1.0 d ____10-6

49 _ 1980.L___ CW ________ 26 N _ 93 W _____ 3 deg ____ 280 ____ ELL-2a ___ 2.5 d ____11-13 to 11-15

50 _ 1981.E _____ CCW ____ 35 N _ 64 W _____ 2 deg ____ 320 ____ circular ___ 1.0 d _____9-4

51 _ 1984.H___ CW ________32 N _ 60 W _____ 2 deg ____ 030 ____ ELL-1a ___ 1.0 d _____ 9-28

52 _ 1985.J_____ 2 CCW ___ 29 N _ 90 W _____ 2 deg ____ W, N __ irregular ___ 2.5 d ____10-28 to 10-30

53 _ 1990.J___ CW ________ 30 N _ 30 W _____ 7 deg ____ 150 ____ ELL-2b ___ 3.5 d ____10-2 to 10-5

54 _ 1996.M _____ CCW ____ 16 N _ 77 W _____ 5 deg ____ 180 ____ ELL-1a ___ 4.0 d ____11-20 to 11-23

55 _ 1999.D___ CW+CCW ___34 N _ 72 W _____ 5,2 deg __ N, SW __ irregular __ 4.0 d ____ 8-29 to 9-1

56 _ 2000.A___CW _________37 N _ 45 W _____10 deg ____ 160 ____ ELL-1b ___7.5 d ____ 8-12 to 8-20

57 _ 2004.J ___CW _________26 N _ 70 W _____ 5 deg _____ 040 ___ circular ___ 3.5 d ___ 9-21 to 9-24

58 _ 2009.F___ CW ________ 17 N _ 34 W _____ 2 deg _____ 090 ___ ELL-2a ___ 2.0 d ___ 9-11 to 9-13

59 _ 2012.N(1)___ CCW ____ 31 N _ 26 W _____ 2 deg _____ 130 ___ ELL-2a ___ 1.5 d ___ 9-22 to 9-23

60 _ 2012.N(2)_CW ________ 34 N _ 37 W _____ 4 deg _____ 320 ___ ELL-1a ___ 2.5 d ___ 9-30 to 10-2

61 _ 2016.N _____ CCW ____ 27 N _ 65 W _____ 3 deg _____ 180 ___ ELL-2a ___ 3.5 d ___ 10-7 to 10-10

62 _ 2017-J___ CW ________ 27 N _ 69 W _____ 4 deg _____ 060 ___ circular ___ 3.5 d ___ 9-12 to 9-15 

 

ANALYSIS

The frequency of loops is 62 in 147 years or 1 every 2.4 years. If we add several near-loops and excluded cases the frequency is close to one every second year.

The location favored by loops seems to be a broad swath of the middle latitudes of the tropical theater from the central Gulf of Mexico across Florida, north and east of Bahamas, near Bermuda and the Azores. Further south than that, relative to the total frequency of storms, the number of loops drops away. It seems to maintain itself particularly well over the Gulf Stream. 

There is evidence that loops are more common in the second half of the season with relatively few in July and August. It seems that October and November run ahead of what you might expect at random. This is a frequency count of the 62 looping storms, when they straddle a monthly boundary each month counts 0.5. Those all rounded off to whole numbers eventually. The second row indicates frequency at random in the normalized season.

Frequency ____ MAY 1 JUN 2 JUL 1 AUG 5  SEP 25  OCT 22 NOV 6 DEC 0

Expected _____ MAY 0 JUN 2 JUL 6 AUG 16 SEP 22 OCT 14 NOV 2 DEC 0

diff   1 ___ 0 __ -5 __ -10 ____ 3 _____ 8 ____ 4 ____ 0

The frequency is clearly below expectation at random until about September 10th then above for the rest of the normal season. However, very early season storms also displayed some tendency to loop more often than later in June or in July-August. The first half of August is particularly under-represented.

The most likely configuration of a loop is a slight ellipse with its long axis in the direction of the loop but a few cases run opposite to this; however, almost one third are close to circular (18 of 62).

The most frequent duration of loops is 2 to 5 days. The mean is just under 3 days. Cases longer than 7 days are rare. It should be noted that the total time spent by all these storms looping is just about one normal hurricane season (six months) and that the total number of storms that looped is about 4% of the total number of storms that existed in the period. 

There is little to choose between the frequency of clockwise (36) and counter-clockwise (27) orientations. Once a loop begins, easterly displacement is more likely than other directions but anything seems possible over time. The frequency of counter-clockwise loops was running ahead until around 1960, since then clockwise loops have been the more common type.

Finally, very few of these storms ever hit land either before or after their loop (one looped over southern GA and northern FL), or became major hurricanes. The majority of these storms stayed out to sea by a ratio of about 4:1. This is partly because a large number of loops are observed in the eastern Atlantic where, if a storm has attained a latitude much higher than 25 deg N, it is unlikely to hit land anyway except possibly the Azores. 

This is a map showing the lat-long position where 62 identified storms began to loop. Land is shown on the diagram map by the dash symbol and water by the .. symbols but the letters Ber indicates position of Bermuda, and Azores are shown by their approximate range of lat-long. 

 

41N---90--------------80--------------- X..................60..................50..................40....X............30..................20

40N--------------------------------.....70...................X.........................................................................................

39N--------------------------------......................................................................................................................

38N--------------------------..----................X..........................................................................Azores..............

37N---------------------------..----............................................................X........X..............................................

36N------------------------------..--................................................................................................X.....................

35N-------------------------------......................XX.........X.............................X..X.................................................

34N -----------------------....XX..X..X..........................................................................X.....................................

33N------------------.......X..X........................................X..................................................................................

32N-----------------..........X...X..................Ber.....X..........................X................................................................

31N---------X----..........X..........................................X.....................................................................X................

30N-------------....................X....X..........X...........................................X..................................X.......................

29N..X X.....---.......................X............................................................................................X...........................

28N............----...X....X.............X....................................................X........X.......................................................

X.................---............................X......X..X.......................................................................................................

.XX.............X.---...............X....X...............................................................................................X.......................

25N...XX.......---..-...............................................................................................................................................

24N....................-............................................................................................................................................

23N..X.....XX.----................................................................................................................................................

22N..........--Cuba---------........-............................................................................................................................

21N.....................---------.....................................................................................................................

20N...---...................------.....--------.....................................................................................................

19N..----.............................X..---------....----...-.....................................................................................

18N.-------................---....................................-..................................................................................

17N------------...........---......................................-...............................................................X................

16N ------------...........X......................................-.................................................................................

15N-------------.X................................................-..............X.................................................................

 

SAME MAP but with 2 deg latitude per line, odd lats excluded and loops shown P

even lats are shown and loops on those are marked d.

42N--- 90 -------------- 80 -------------- 70 .................. 60 .................. 50 .................. 40 ................. 30 .................. 20

40N---- --- -------------- --- --------.......P. .. ..............d. .. ................... .. .................... .. ..P.........................................

38N----- --- ------------- --- --..----......... .. .....d............ .. ................... .. .................... .. .................A z ores.............

36N---- -- --------------- -- -------..--..... .. .................... .. ................... .. .........P.........P.. .. ................. .. ..X....................

34N --- -- --------------- -- -....d d..d..d.. .. .........P.P.. .. ...P............... .. .............P...P..... .. .......d........ .. .........................

32N---- -- -----------.. .. ......P.d P d..... .. .......Ber.....d. .. .P............ .. ........d............. .. ................. .. ...........................

30N--- -- -----P---..... .. ...P........d...d.. .. ......d................ P ..........................d............... .. .................d....P...................

28N P..P....----. .. d....d..........P..d........ .. ................... .. .....................d........d................ .. ......P.......... .. ................

Pdd.............d.---.. .. ..........d.. .. d...P....P.P............. .. .................. .. ...................... .. ...................d.. .. .....................

24N...PP............... . .-................ .. .............................. .. ..................... .. ........................ ... .......................... .. ............

22N..P .. ...PP--Cuba-- - ------..... .-. .. ........................ .. ...................... .. .......................................................................

20N...---...................------.....--------.....................................................................................................

18N.-- -- ---................---.......P.............................-..................................................................................

16N -- -- --------..........d. .. ............. .. ....................-.. .. ...................... ... .................... .. .............P...........

14N--- -- --------.P.... .. ................... .. ................. .. ..-..............P........ ... .................... ... ..........................

 

 

 

 

 

 

[salbers]

Interesting to see. I wonder how many of these loops were in the Atlantic for hurricanes that eventually made landfall?

[Roger Smith]

11 hours ago, salbers said:

Interesting to see. I wonder how many of these loops were in the Atlantic for hurricanes that eventually made landfall?

I will indicate the cases that either hit land before or after the loop, but would estimate the total numbers will be small. One that definitely qualified was 2004.J (Jeanne). 

The list is now complete and the analysis has been updated.