Tropical Cyclone Variability in the PAR in relation to ENSO : An initial Study for
the October to December Season
Abstract
The variability of tropical cyclone occurrences during El Niño events in the last 58 years
has been examined. Tropical cyclones were analyzed using the best track data of the
Weather Branch, PAGASA (Weather Bureau) occurring over the Philippine Area of
Responsibility (PAR) from 1948–2005. Recorded tropical cyclones were examined and
categorized according to the intensity of the El Niño events. These events were
characterized based on corresponding sea surface temperature anomalies (SSTA) in the
Niño 3.4 region. An examination of the variability of tropical cyclones in the last 58 years
showed that tropical cyclone is generally below the historical average during El Niño
events. Below average number of tropical cyclones are manifested during the first year
(1951, 1957, 1963, 1968, 1972, 1973, 1997 and 2002) and others are apparent in the
second year of the El Niño event. (1958, 1969, 1973,1983, 1987, 1992, 1998).
This paper
presents the initial results of a study on tropical cyclone tracks in the PAR
during various intensities of El Niño for the October to December season .
Introduction
The destruction caused by tropical cyclones can be immensely great. The Philippines,
due to its geographical setting, is situated in a region were the most number of tropical
cyclones form than anywhere else in the world. Nearly every year, an average of 20
tropical cyclones enter the PAR with about 7 to 9 making landfall and causing massive
destruction to properties. Tropical cyclones spawned in this region generally track
westward and may later turn northwest, first affecting the Philippines and other parts
of Asia.
Annual Variability of Tropical Cyclones in the PAR
The Northwest Pacific Ocean region is the most active basin in the world, accounting for
one third of all global tropical cyclone activity. This region usually sees an average of 23
storms in a normal season and an average of 28 storms for the entire year. Typhoons
that form in the Northwest Pacific tend to pass through a standard route towards
Southeast Asia guided by high-pressure systems that steer the storms towards the
Philippines. Once it entered the PAR (25 ºN, 120 ºE, 25ºN, 135 ºE, 5ºN, 115ºE,
15ºN,115ºE,21ºN, 120ºE and 25ºN, 120ºE), a Philippine name is given to the tropical
cyclone.
Figure 1 shows the monthly variability of tropical cyclone occurrences in the PAR,
wherein no month is free from tropical occurrence. June to December is cons idered the
typhoon season where the monthly average is more than 1 and the peak of the typhoon
season is July through September (JAS), when nearly 70% of all typhoons develop.
Average typhoon during the season (JAS) is 3 or more.
Monthly Average Frequency of Tropical Cyclone
in the PAR
No. of Tropical Cyclones
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
J
F
M
A
M
J
J
A
S
O
N
D
Month
Figure 1: Monthly Average Frequency of Tropical Cyclone in the PAR
In the last 58 years, there were 17 El Niño and 14 La Niña events. (based on the
classification of NOAA). An examination of the variability of tropical cyclones in the last 58
years showed that tropical cyclone is generally below the historical average during El Niño
events. Below average number of tropical cyclones are noted during the first year (1951,
1957, 1963, 1968, 1972, 1973, 1997 and 2002) and others are manifested in the second
year of the El Niño event (1958, 1969, 1973,1983, 1987, 1992, 1998. Analysis of the
average number of tropical cyclones from October to November (OND), showed that
there are about 6.5 in neutral years, 6.7 in La Nina years and 4.4 during El Niño years
(Figure 2). Tropical cyclone activity during neutral and La Niña years is almost equal for
the same period (OND).
It can therefore be inferred that during El Niño events, the
number of tropical cyclone occurrences in the PAR are generally below average and the
same is true during the last quarter (OND) as can be seen in Figure 2.
Average number of Tropical Cyclones for the 4th
quarter (October-November-December) 19482005
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
6.5
6.7
Nuetral
La Niña
El Niño
4.4
Nuetral
La Niña
El Niño
Figure 2: Average number of Tropical Cyclones for the 4th quarter (1948-2005)
Seasonal variations of tropical cyclone activity depend upon changes in one or more of
the necessary environmental conditions for tropical cyclone development. Figure 3
shows the monthly variability of tropical cyclones occurrences in El Niño years during the
last quarter. As shown in Figure 3, there are months with no tropical cyclones in the
PAR. This was noted in 2002 where no tropical cyclone entered the PAR during the last
quarter. In other El Niño years (1957, 1963, 1965, 1968, 1969 and 1976), there are
months which are free from tropical cyclones. This is possibly due to the shift in the
location of formation of tropical cyclones during El Niño years. When the equatorial
Pacific SSTs are higher than normal (El Niño events), more typhoons originate east of
160° E, and there is a tendency for typhoons to recurve toward higher latitude (Elsner, et
al, 2003). There is strong evidence showing that El Niño shifts the favorable place for
tropical cyclone formation eastward, and also induces the northward movement of the
storms.
Tropical Cyclone Tracks during El Niño Years in the PAR
Globally, tropical cyclones are affected dramatically by the El Niño-Southern Oscillation
(Landsea, 2000). The slight change in SST distribution brought by El Niño does have an
impact on the location and movement of tropical cyclones over the area. It may be noted
that the displacement of warm pool towards east of the dateline during El Niño events
could suppress tropical cyclone formation over its normal breeding ground. Recorded
tropical cyclone were examined and categorized according to the intensity of the El Niño
events.
Monthly Tropical Cyclone occurrences during El Niño
for the months of October, November and December
1948-2005
5
October
4
November
3
December
2
1
0
51 57 63 65 68 69 72 76 77 82 86 87 91 94 97 02 04
Figure 3: Monthly Tropical Cyclone occurrences during El Niño for OND (1948-2005)
Figure 4 shows the actual tropical cyclone tracks during El Niño events. When El Niño
occurs, convective activity over the western North Pacific and the South China Sea is
reduced because of the slightly-lower-than-normal SSTs in these areas. The result of a
previous study (Jose, et al, 1997) reveals that the effect of tropical cyclones tend to take
on a more northerly track earlier in their lifespan in El Niño years.
Figure 4: Actual tropical cyclone tracks during El Niño (1948-2005)
Figure 5: Actual tropical cyclone tracks during El Niño Weak El Niño (1948-2005)
( 0.5 to 1.0º C SSTA)
Figure 6: Actual tropical cyclone tracks during El Niño Moderate El Niño (1948-2005)
(1.0 to 1.5º C SSTA)
> 1.5 SSTA
SSTASSTA
Figure 7: Actual tropical cyclone tracks during Strong El Niño (1948-2005) ( >1.5 oC)
Figures 5,6, and 7 show the different tropical cyclone tracks during various intensities of
El Niño. In a weak El Niño (0.5 C to 1.0 SSTA) more tropical cyclones are recurving
northward (Figure 5) while during a moderate El Niño (1.0 C to 1.5 C) there are more
tropical cyclones crossing the PAR than during a weak El Niño. The effect on tropical
cyclone activity is very evident during a strong El Niño (SSTA >1.5). Very few tropical
cyclones entered the PAR and if ever, they recurve and move northward (Figure 7).
Below average tropical cyclone occurrences were noticeable in the following El Niño
years 1972 (13), 1983 (14), 1992(15), 1998 (11) and 2002(13).
These numbers
reflected the fact that most tropical cyclones over the western North Pacific in the years
mentioned above, tended to form farther east and move northward and recurve before
reaching the Philippines.
Conclusion
The ultimate goal of this initial study is to provide information on tropical cyclone activity
in the PAR in relation to ENSO during the October the November season. Its main focus
is to study the variability of tropical cyclone occurrences during El Niño events.
Some significant findings in the study are the observed trends in the number of tropical
cyclone occurrences during El Niño years. Below average tropical cyclones have been
noted in 11 El Niño events out of the 17 El Niño events classified by NOAA. Tropical
cyclone occurrences during the various El Niño years have fluctuated from 17 to 11,
tropical cyclones
compared to the historical average of 20 per year. As discussed
above, the SST changes associated with El Niño events have different impacts on the
tropical cyclone activity in the PAR.
Analysis done on the tropical cyclone tracks
showed that during a strong El Niño very few tropical cyclones entered the PAR. What
seems to be noticeable during El Niño years are the typhoon tracks that move northward
or those that recurve, sparing passages of tropical cyclones over the PAR.
The above study shows encouraging results although there are a lot of things that
should be done. Tropical cyclone occurrences during the JFM, AMJ, JAS will also be
studied in the next report. Due to time constraint the main emphasis of the study is the
tropical cyclone occurrences during the last quarter. For a more comprehensive study,
tropical cyclone activities during various phases of ENSO will also be examined.
REFERENCES
Camargo et al.2004, Western North Pacific Tropical Cyclone Intensity and
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Landsea, Christopher, 2000. Climate Variability of Tropical Cyclone: Past, Present,
Future, Storms 2000
Jose, et al. 1997. A study on Potential Indicators/Indices of Tropical Cyclone
Variability/Changes in the Philippine Vicinity (1948-1997)
Wang, Bin et al. 2000, How strong ENSO Events Affect Tropical Storm Activity over the
Western North Pacific, Journal of Climate, December 2000