Statistical Characteristics of Cyclones in the Southern Oceans
during the Summer of 2004 - 2007
ABSTRACT
Characteristics of cyclones and explosively developing cyclones (or “bombs”)
over the Southern Oceans in the summer months (December, January, February) from
2004 to 2007 are analyzed using the Final Analysis (FNL) data issued by the National
Centers for Environmental Prediction (NCEP). Statistical results show that both
cyclones and explosively developing cyclones frequently develop in January, and
most of them occur within the latitude zone 55°S-70°S. Those cyclones gradually
approach to the Antarctic Continent from December to February. In most cases,
cyclones and bombs move east-southeastward with some exceptions moving
northeastwardly. The lifetime of cyclones is around 2-6 days, and the horizontal scale
is about 1000 km. For explosive cyclones, however, the lifetime is about 1 week, and
the horizontal scale can reach up to 3000 km. Compared with cyclones developed in
the northern hemisphere, cyclones over the southern oceans have much higher
occurrence frequencies, lower central pressures and larger horizontal scales, which
may be caused by the unique geographical environment of the southern hemisphere.
Key Words： cyclone, explosive cyclone, southern hemisphere, lifetime, horizontal
scale.
1. Introduction
Explosive cyclones, referred to as meteorological “bombs,” are usually
accompanied with rapid central pressure reduction and extreme intensity. These
cyclones are hard to predict and can pose serious threats to human life and property
when they approach coastal regions, or intersect with shipping lanes. Explosive
cyclones are also considered to be an important contributor to global climate change
and energy transportation.
Over the last two decades explosive cyclones have received considerable attention
from the research community, much research are directed toward the exploration of
the mechanisms that trigger rapid pressure reduction and to documenting some of the
mean characteristics of NH explosive cyclones. According to the landmark study of
Sanders and Gyakum (1980), explosive cyclone events are predominantly maritime
and cold-season phenomena. Moreover, in the Northern Hemisphere it has been
observed that the maximum frequency of bombs occurs in the westernmost portions
of both the Atlantic and Pacific Oceans within, or just north of the warm currents of
the Gulf Stream and the Kuroshio, respectively (Sanders and Gyakum 1980; Chen et
al. 1992). Chen et al. (1992) showed that in the northwestern Pacific region, explosive
cyclones often appeared over the Sea of Japan and over the Kuroshio Current. They
also suggested that two peaks of monthly frequency existed in early winter (January)
and late winter (March). Yoshida and Asuma (2003) reported the structures and
environment of explosively developing extratropical cyclones in the northwestern
pacific region.
In contrast, there have been only a few studies about cyclones over the southern
hemisphere. Sinclair (1995) presented a compilation of rapidly developing cyclones
using 7 yr (1980-86) of operational European Centre for Medium-Range Weather
Forecasts (ECMWF) analyses. Lim and Simmonds (2002) showed a compilation of
Southern Hemisphere explosively developing cyclones which assembled based on the
National Centers for Environmental Prediction-Department of Energy reanalysis-2
data over the 21-yr period form 1979-1999. They also discussed the characteristics of
bombs in the SH and a comparison with NH events, and pointed out that summer
explosive cyclones in the SH are six times more numerous than those in the NH,
although there are about twice as many bombs appear in the NH as in the SH every
year. Chen et al (2000) found that there were much more explosive cyclones
developed during summer time over the southern hemisphere.
The purpose of the present paper is to examine the statistical characteristics of
the entire cyclones and explosive cyclones developed during the summer over the
Southern Ocean. In particular, we describe the geographical and monthly variations
and the statistical properties of these events. Data sources are described in section 2.
2. Data and methods
This study is based on the Final Analysis (FNL) dataset provided by the National
Centers for Environmental Prediction (NCEP). It is a global gridded dataset with a
1o×1o latitude-longitude resolution and 26 vertical levels from the surface to 10 hPa at
6 hour intervals. In this study, we examine the cyclones that occur during three
summers, including the following months: December 2004 , January 2005, February
2005, December 2005, January 2006, February 2006, December 2006, January 2007,
February 2007. The spatial coverage is between 40°S and 90°S in latitude and
between 0° and 360° in longitude (Fig. 1). Synchronous GOES-9 infrared satellite
images obtained from Kochi University are also used in this study.
Sanders and Gyakum (1980) defined an explosively developing extratropical
cyclone as a cyclone that had a central sea level pressure decrease normalized at
60 N over 24 hPa in a day (24h). The cyclone deepening rate (Bergeron) is calculated
from the following definition for each cyclone:
Cyclone deepening rate
=[
pt 6  pt  6
sin( 60)
][
]
t 6  t  6
12hPa
sin
2
(1)
where t is analysis time in hours, p is the sea level pressure at the cyclone center, and
φ is the latitude at the cyclone center. A 12-h pressure change is used in this paper to
find an instance of most rapid deepening in a cyclone’s life. An explosively
developing cyclone is defined as cyclones having a deepening rate of at least 1
Bergeron. Cyclones that disappear within 24 h after initial appearance are excluded
from the analysis.
3. Results
3.1 Larger-scale environment
Previous studies show that environmental baroclinicity affects the geographical
distribution of explosive cyclones and cyclone structures may change under the
influence of atmospheric and geographical environments. The Antarctic continent is
often surrounded by a distinct low-level baraclinic zone. Cyclones in the summer with
high intensity and low central pressure (lower than 985 hpa) are found over the entire
region south of 40oS with averaged moving speeds over 100 km/h. Much of the
anticyclones are located in the subtropical area and along the Antarctic coast.
Baroclinicity plays an important role in SH explosive cyclone occurrence as in the NH.
However, the SH baroclinicity is less affected by the seasonal cycle with a mean
circumpolar trough existing (where?) associated with the mean sea level pressure. The
maximum propagation speed of migrating cyclones in SH is about twice as fast as
those in NH, and the situation is similar for the mean westerly (Qu et al. 2001). From
December to February, sea surface temperature increases and land temperature
decreases gradually in SH, which result in an increasing temperature difference
between ocean and land. [Kuo: Do we have plots to show that?]
3.2 Number of cyclones and bombs
The frequency of occurrence for summer cyclones and explosive cyclones from
Dec 2004 to Feb 2007 is shown in Table 1. In all, a total of 734 cyclones occurred
during these months, and among which 135 explosive cyclones were found, which
represent 18.4% of the all cyclones over the Southern Ocean. There were 234
cyclones and 47 explosive cyclones during summer 2004~2005, 247 cyclones and 38
explosive cyclones for 2005~2006, 252 cyclones and 50 explosive cyclones for
2006~2007. January is the month with most frequent explosive cyclones, and most
cyclones also develop in January except for year 2006~2007. January 2006 and 2005
are the two special moths when cyclones and explosive cyclones develop with highest
frequency respectively. There were about 75 cyclones and 13 explosive cyclones on
average with a little difference for December 2006. [Kuo: Your table says 92 total
cyclones and 17 explosive cyclones. They are not consistent with your statement.]
3.3 Cyclone and bomb characteristics
Because much of the SH landmass (except for Antarctica) is located in the
subtropical region, which is different from the NH, cyclones in the SH often display
the characteristics of high occurrence frequency, low central pressure, and large
horizontal scale. Statistical analysis was used to study the locations of all the cyclones
every time they occurred. [Kuo: What does this mean? Do you mean the “genesis”? or
the same cyclone would be counted multiple times during its life time?] In order to
show the spatial distributions of cyclones clearly, we divided the cyclones into four
types according to the central pressure: Type A with the central pressure lower than
970 hPa, Type B between 970 hpa and 980 hpa, Type C between 980 hpa and 990 hpa,
Type D higher than 990 hpa. [Kuo: Why would this be a good way to characterize
cyclones? Is the FNL analysis reliable enough for such categorization?]
a. Locations of entire cyclone
Figure 2 shows locations of all the cyclones from their initial stage to their
dissipated stage for a specific month. In December 2004 (Fig. 2a), cyclones always
occurred within the zone between 50°S and 70°S in latitudes. Between 40°S and 50°S,
the distribution of cyclones was not homogeneous, most cyclones appeared around
the New Zealand. A small number of cyclones of which the central pressures are
about 980 hpa, also existed in the areas between 0°~25°W and 120°~150°W in
longitude. The west of Antarctic peninsula between 50°S and 70°S, north of Rose sea
and around of 20°E and 90°E between 50°S and 65°S is a favored place of type A.
[Kuo: Frankly, I have problem with this type of classification. As you know, in the
past, scientists define different types due to different forcing mechanism. Here, we
define it purely on central pressure. As a cyclone would go through its life cycle, with
varying central pressure, this is not “physically” representative of forcing
mechanisms.] The loci of cyclones for Dec 2005 did not differ much from those of
Dec 2004. However, the amount, density and the spatial scales of cyclones between
40°S and 50°S were all much higher in Dec 2005 than those of Dec 2004. They
occurred not only around the New Zealand, but also expanded to the other areas.
Cyclones in Type A still occurred mostly in the Antarctic Peninsula, Rose sea and
20°E, 90°E but the amount and density were also much larger than the previous year.
The distributions of cyclones in Dec 2006 is similar to those of Dec 2005, except that
more cyclones in Type A clustered around the Antarctic Peninsula
Cyclones mainly stayed in the latitude zone between 55°S-75°S in Jan 2005, but it
is much higher number than that in Dec 2004 (Fig. 2b). Between 40°S and 50°S, there
were still a few cyclones, but the density became much lower and most of them
belonged to Type D. The latitudinal distributions of Type A are more dispersive than
that in Dec 2004, and most cyclones converged towards the higher latitude area.
Compared with Jan 2005, many cyclones in Jan 2006 moved a little toward lower
latitude areas and there were also a lot of Type C and Type D cyclones existing in the
mid-latitude areas. However, in comparison with Dec 2005, most cyclones still
accumulated towards higher latitude area in Jan 2006. Different from the former two
years, the amount of cyclones became lower and positions were highly dispersed, but
the trend of cyclones converging toward the higher latitude as the season progressed
didn’t change.
Figure 2c shows that the trend that cyclones moved to higher latitude from
December 2004 to February 2005 was even more clear. There were some areas where
almost no cyclones existing within the latitude zone 40°S~50°S (like 90W~120W and
60E~90E) in February 2005. Latitudinal distributions of type A cyclones became
more even and most of them concentrated in the latitude zone 60°S-70°S. Compared
with the other months of the same year, positions of cyclones moved to the relatively
higher latitude in Feb 2006 and Feb 2007, but the spatial distributions became much
dispersed. Many cyclones still appear in the low- and mid-latitude, and Type A and
Type B were more evenly centered on the broader latitude zone of 50°S-70°S.
As is described above, we can see that most cyclones accumulate towards the
higher latitude area from December to February, and the latitude range of 55°S-70°S
is the area where most cyclones exist. Type A cyclones are evenly distributed along
the latitude range, except for the west to the Antarctic Peninsula where Type A
cyclones often have high concentration. Distinct differences among these three
summers are found that is the amount of cyclones is larger, the density is higher, and
the horizontal distribution is broader in the second and the third year than those in the
first year.
b. Entire cyclone tracks
Figure 3 shows the monthly distributions of cyclone tracks (plotted with 6-h
segments over the 24-h of cyclones, the dots denote the locations of formation)
compiled for the 6 month of record. [Kuo: Actually, I think identifying the “genesis”
and “dying” locations of these cyclones are interesting. We will find “where” will the
cyclone form, develop and die. We can make the plots without the cyclone tracks.
Another important point is “where” does explosive cyclogenesis take place. To me,
the more interesting result is to see “where” does cyclone experience more than 1
bergeron deepening. I am sure that will be different from just the distribution of
cyclones.] From these figures, one can find that cyclones form over the east of South
America and surround the Antarctic Peninsula at all the nine months. A chain of
cyclones are found to extend from these regions to the south of Australia and Tasman
Sea. The tracks of the entire cyclone population are very dense near the circumpolar
trough.
Most of cyclonic systems have a strong southeastward component and some
northeast movement of cyclones was observed clearly around 50°S. It is noticed that
most of these cyclones move to avoid the sea ice covered regions. [Kuo: It would be
useful to present a figure of sea ice for each month. How do they change, and how
they affect the cyclone track.] Carleton and Fitch (1993) observed a number of
meso-cyclones moving northward in Antarctic and sub-Antarctic latitudes in winter
and associated it with the Antarctic sea ice expansion and katabatic outflow.
The tracks of summer cyclones in these three years all move towards the high
latitude region, which is similar to the distributions of their positions. However, the
important difference among these three years is that activities of the cyclones in the
first year always exist in the area closer to the Antarctica than the second and the third
year. [Kuo: Are there significant differences in the sea ice extend between these three
years? We should find out.]
c. Distributions and tracks of bomb
Figure 4 shows the tracks of bombs, locations of formation, maximum deepening,
and minimum center pressure. The distribution of bomb tracks is much like that of the
entire cyclone population and both of them show spiral bands stretching from South
America and Australia (Jones and Simmonds 1993). From these figures, one can see
that most explosive cyclones form on the mid-latitude. Southwest of Australia,
northeast of Antarctic Peninsula, and the vicinity of 20°E and 60°E are still the
favored places for the bombs formation for all nine months. [Kuo: Are these areas
different from the regular cyclones?] The maximum deepening almost always
occurred over the ocean at latitudes zone 45°S-60°S, and the locations of minimum
center pressure are centered on the high-latitude zone 60°S-70°S. Most of the tracks
move east-southeastward and some specials are mostly found south of the South
America and around of the Antarctic Peninsula.
d. Intensity of entire cyclones
The central pressure of cyclones in the SH is much lower than in the NH, of which
the lowest can reach 940hPa and the explosive cyclone can reach 946hPa in this study.
[Kuo: It is strange to have the regular cyclone be deeper than an explosive cyclone. Is
there something wrong?] Table 2 shows the statistics of each type of cyclones based
on 6-h FNL analysis. About 67% of the SH cyclones are with central pressures lower
than 980 hPa. The number of type A and type B increases gradually from summer to
autumn with decreasing number of type C and type D, indicating the increasing
intensity of SH cyclones during this seasonal transition. [Kuo: I suggest that we don’t
use “type” to indicate cyclone of different intensity. May be “category” would be
better than “type”. When people use “type” they usually mean significant different
forcing mechanisms for cyclones.]
e. The life-time and horizontal scales
The life-time of each cyclone can be estimated by the analysis dataset. Results show
that the average life-time of a cyclone is about 2-6 days and the maximum life-time
can reach 10.5 days. The life time of explosive cyclones is about one week.
The diameter range of a SH cyclone is about 1000Km, and the maximum can reach
5000-6000Km. Most explosive cyclones are extratropical cyclones which is always
associated with a front in the structures. [Kuo: Are you saying regular cyclones are
not associated with a front? It seems cyclones are always associated with fronts. I
would be interested in seeing cyclones that are not associated with fronts.] The
average diameter range of a SH explosive cyclone is about 3000 km.
f. Satellite images
Figure 5 shows the GOES-9 infrared satellite image over the southern oceans at 18
UTC 31 January 2005. Symbols A, B and C indicate ordinary cyclones, while D and E
represent the explosive cyclones. The explosive cyclone E whose deepening rate is
1.85 Bergeron and central pressure is 954 hPa, and is reaching its maximum intensity.
The deepening rate fits the “strong intensification category” (Table 3). The explosive
cyclone D whose deepening rate is 1.01 Bergeron and central pressure is 992 hPa, is
in its initial development stage. The deepening rate fits the weak intensification
category (Table 3). Cyclones A, B, and C are all mature ordinary cyclones, with
central pressures of 971 hPa, 970hPa, and 960 hPa, respectively. SH cyclones rotate
clockwise which is opposite to NH cyclones, and they are often connected with long
horizontal fronts (E is over 3000 km, and C can reach 5000 km) and with no obvious
eyes found in some NH explosive cyclones (Kuo et al. 1992).
g. Intensity of bombs
Followed by Sanders (1986), cyclones identified as explosive developing cyclones
can further be classified into three categories of intensification: strong intensification
(ST) cyclone (deepening rate larger than 1.8 Bergeron), moderate intensification (MO)
cyclone (deepening rate between 1.3 and 1.8 Bergeron), and weak intensification (WE)
cyclone (deepening rate is less than 1.3 Bergeron). The occurrence frequency of each
type is summarized in Table 3. During the three summers from Dec 2004 to Feb 2007,
WE cyclones occurred most frequently (79 cases), followed by MO cyclones (44
cases), and ST cyclones (12 cases). The average deepening rate of these cyclones is
1.32 Bergeron, and the strongest deepening rate is about 2.35 Bergeron. Cyclones
occurred in the first summer are the weakest of the three, and the average deepening
rate is within 1.2-1.25 Bergeron. The last summer cyclones are middle, and the second
summer cyclones are the strongest. So, there are also noticeable interannual
variability.
4. Conclusions
To better understand the characteristics of cyclones and explosively developing
cyclones (or “bombs”) over the Southern Oceans in the summer (December, January,
February), statistical analysis are used in this study by analyzing the Final Analysis
(FNL) data provided by the National Centers for Environmental Prediction (NCEP)
from 2004 to 2007. Our results reveal that cyclones and explosively developing
cyclones frequently occurred in January. During the seasonal transition from summer
to autumn, with the increasing land-ocean temperature differences, cyclones gradually
move to the Antarctic Continent, and most cyclones accumulate within the latitude
zone 55°S-70°S. [Kuo: I think we need to look at the sea-ice and cyclone location
connection.] Type A cyclones are evenly distributed along the latitude, except for the
west to the Antarctic Peninsula where type A cyclones occur a lot. Cyclones often
form over the east of South America and around the Antarctic Peninsula area, move
across the Atlantic Ocean and Indian Ocean, result in a formation of a long chain of
cyclones between the south of Australia and South America. [Kuo: Are these cyclone
triggered by mountains of Antarctic Peninsula? Are they orographic cyclogenesis?] In
general, SH cyclones move east-southeastward, with some exceptions moving
northeast mostly found near 50°S and around the Antarctic Continent. Distributions of
cyclone tracks are often found around the persistent circumpolar trough. Most
explosive cyclones form on the mid-latitude, specifically the west south of Australia,
east north of Antarctic Peninsula, the vicinity of 20°E and 60°E are the favored places
for the bombs formation. The maximum deepening of explosive cyclones always
occur within latitude range 45°S-60°S [Kuo: Are these some special “longitude
zones” that favor explosive cyclogenesis? If so, why? Can we speculate as to why
explosive cyclones have their deepening over these regions? Actually, I am thinking
that one can plot the distribution of the “deepening rate” horizontally for each month,
and see if we can identify some clear pattern.], while the locations of minimum center
pressure are found within the high-latitude zone 60°S-70°S. Most of the summer
explosive cyclones move east-southeast, while some moving northeast are mostly
found south of the South America and around of the Antarctic Peninsula. The
lifetimes of cyclone over the southern oceans are around 2-6 days, and their
horizontal scales are around 1000 km. For explosive cyclones, the lifetimes are about
1 week and horizontal scales are nearly 3000 km.
Overall comments:
This is a very interesting study. However, a review may ask the following questions:
1. What are the same and different from previous studies of Antarctic cyclones?
Previous work cover the entire year, here you only cover the summer.
2. Did the previous study also identify the migration of cyclones toward the
Antarctic continent as the season progresses?
3. Are there preferred locations for “explosive development”? Why?
4. Are explosive cyclones over the SH distinctly different from the regular
cyclones? Or, are they just a little bid stronger? Remember that in the Sanders
and Gyakum paper, the fundamental question is: “Are explosive cyclones
fundamentally different types of cyclones from the non-explosive cyclones?”
5. It seems that if you want to pursue numerical simulations, we should look at
one “ordinary cyclone” and one “explosive cyclone”, and study their
differences.
6. Are there ways we can examine the precipitation (and latent heat releases)
associated with these explosive cyclones? Are they stronger because of
stronger “latent heating?”
7. Are the distribution of cyclones affected by the extend of sea ice? Can you
quantify the relationships?
Acknowledge
References:
Carleton, A.m., and M. Fitch, 1993: Synoptic aspects of Antarctic mesocyclones. J.
Geohpys. Res., 98, 12 997-13 018.
Chen, J.-N., K.-T. Le, C.-M. Jia, and Y. Peng, 2000: The change of the frequency of
cyclone occurrence in the southern hemisphere influence on SST in the East
Pacific and Southern Oscillation. Acta. Oceanologica. Sinica., 22(3), 86-93.
Chen, S.-J., Y.-H. Kuo, P.-Z. Zhang, and Q.-F. Bai, 1991: Synoptic climatology of
cyclogenesis over east Asia, 1958-1987. Mon. Wea. Rev., 119, 1407-1418.
Kuo, Y.-H., R. J. Reed and S. Low-Nam, 1992: Thermal structure and airflow in a
model simulation of an occluded marine cyclone. Monthly Weather Review, 120,
2280-2297
Lim, E.-P., and I. Simmonds, 2002: Explosive cyclone development in the Southern
Hemisphere and a comparison with Northern Hemisphere events. Mon. Wea.
Rev., 130, 2188-2209.
Qu, W.-Z., X.-R. Chen, R.-S. Shen, and H.-G. Wang, 2001: Some Characteristics of
General Circulation at Sea Level of the Southern Hemisphere. Journal Of
Oceanography Of Hanghai & Bohai Seas, 19(1), 9-16.
Sanders, F., and J. R. Gyakum, 1980: Synoptic-dynamic climatology of the ‘bomb’.
Mon. Wea. Rev., 108, 1589-1606.
Simmonds, I., K. Keay, 2000: Variability of Southern Hemisphere extratropical
cyclones behavior, 1958-97. J. Climate, 13, 550-561.
Simmonds, I., K. Keay, 2000：Mean Southern Hemisphere extratropical cyclones
behavior in the 40-year NCEP-NCAR reanalysis. J. Climate, 13, 873-885.
Simmonds, I., K. Keay, and E.-P. Lim, 2003: Synoptic activity in the seas around
Antarctica. Mon. Wea. Rev., 131, 272-288.
Sinclair, M. R., 1995: A Climatology of Cyclogenesis for the Southern Hemisphere.
Mon. Wea. Rev., 123, 1601-1919.
Yoshida, A., and Y. Asuma, 2003: Structures and environment of explosively
developing extratropical cyclones in the Northwestern Pacific region. Mon. Wea.
Rev., 132, 1121-1142.
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Figure 2: Location distributions of cyclones (symbol ●denotes type A，▼is type B，□is type C，╋is type D）。
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Figure 3: Tracks of cyclones over the southern oceans (black dot
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o
70 o
S
2004-12
6 0W
80 o
S
50 o
S
o
60
E
2005-01
0o
0o
0o
8 o
0S
8 o
0S
80 o
S
7 o
0S
7 o
0S
70 o
S
60 o
S
6 o
0S
6 o
0S
o
a
6 0W
oE
120
o
2005-12
d
6 0W
oE
120
oE
120
6 0 oE
o
6 0 oE
6 0W
oE
120
oW
60
60 o
S
oW
60
oE
120
18
0o
W
5 o
0S
5 o
0S
50 o
S
18
18
0oW
0oW
18
0o
W
120 oW
o
120
W
o
120
W
b
o
60
E
o
120
W
c
2005-02
o
120
W
o
60
E
2006-02
f
o
120
W
o
120
W
0oW
18
5 o
0S
6 o
0S
o
6 o
0S
7 o
0S
7 o
0S
8 o
0S
8 o
0S
70 o
S
80 o
S
g
o
60
E
2007-01
h
0o
0o
0o
oE
120
oE
120
2006-12
o
oW
oE
120
6 0 oE
6 0W
60
60 o
S
6 0W
50 o
S
5 o
0S
18
0o
W
18
0oW
120 oW
o
60
E
i
2007-02
Figure 4：Tracks of explosive cyclones over the southern oceans (black diamond ◆ indicates the initial position of cyclone, star ★ denotes the
position of maximum surface pressure depressing, and triangle ▼represents the location of lowest central pressure).
B
A
E
C
D
Figure 5: GOES-9 infrared satellite imagery over the southern oceans at 18 UTC 31 January 2005. Symbol A, B and C indicate ordinary
cyclones, while D and E represent the explosive cyclones.
Table 1 Monthly numbers of cyclones and explosive cyclones in the summer over the Southern Ocean from 2004 to 2006
2004~2005
Dec
Jan
Feb
Cyclone number
75
83
76
Explosive cyclone
number
12
20
The proportion of
explosive cyclone
16
24.1
2005~2006
total
2006~2007
Total
Dec Jan Feb total
Dec Jan Feb
total
234
76
95
75
247
92
82
79
253
734
15
47
13
13
12
38
17
19
14
50
135
19.7
20
17.1 13.7 16
15.4
18.5 23.1 17.7 19.8
18.4
Table. 2 Monthly numbers of various types of cyclones
Month
Number of
type A(<970
hPa)
2004-12
2005-01
2005-02
367
337
481
Number of
type
B(970-980
hPa)
383
417
373
2005-12
2006-01
2006-02
338
352
370
382
352
420
2006-12
427
2007-01
Number of
type
C(980-99
0 hPa)
418
300
200
Number of
type D(>990
hPa)
Total
128
149
48
1296
1203
1102
346
250
191
166
174
95
1232
1128
1076
386
241
149
1203
462
345
182
105
1094
2007-02
342
346
231
90
1009
Total
3476
3404
2359
1104
10343
Table 3. Monthly number of explosive cyclones over the southern oceans
Month
Strong
intensification
cyclone
（>1.8Bergeron）
Moderate
intensification
cyclone(1.3-1.8
Bergeron）
Weak
intensification
cyclone（<1.3
Bergeron）
Total
Average
cyclone
deepening
rate
2004-12
2005-01
2005-02
0
1
0
3
5
3
9
14
12
12
20
15
1.23
1.24
1.20
2005-12
2006-01
2006-02
2
1
3
6
5
2
5
7
7
13
13
12
1.39
1.34
1.43
2006-12
1
6
10
17
1.29
2007-01
2
11
6
19
1.46
2007-02
2
3
9
14
1.31
Total
12
44
79
135
1.32