Effects of super typhoons on cyclonic ocean eddies in the western

PUBLICATIONS
Journal of Geophysical Research: Oceans
10.1002/2013JC009575
Effects of super typhoons on cyclonic ocean eddies in the
western North Pacific: A satellite data-based evaluation
between 2000 and 2008
Special Section:
Liang Sun1,2, Ying-Xin Li1, Yuan-Jian Yang1,3, Qiaoyan Wu2, Xue-Tao Chen1, Qiu-Yang Li1, Yu-Bin Li4,
and Tao Xian1
AUTHOR’S PREFACE
TO A SPECIAL
COLLECTION
Western Pacific Ocean
Circulation and Climate
Correspondence to:
L. Sun,
[email protected]
Citation:
Sun, L., Y.-X. Li, Y.-J. Yang, Q. Wu, X.-T.
Chen, Q.-Y. Li, Y.-B. Li, and T. Xian
(2014), Effects of super typhoons on
cyclonic ocean eddies in the western
North Pacific: A satellite data-based
evaluation between 2000 and 2008, J.
Geophys. Res. Oceans, 119, 5585–5598,
doi:10.1002/2013JC009575.
Received 7 NOV 2013
Accepted 6 AUG 2014
Accepted article online 11 AUG 2014
Published online 2 SEP 2014
1
Key Laboratory of the Atmospheric Composition and Optical Radiation, CAS, School of Earth and Space Sciences,
University of Science and Technology of China, Hefei, People’s Republic of China, 2State Key Laboratory of Satellite Ocean
Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, People’s Republic of
China, 3Key Laboratory of Atmospheric Sciences and Satellite Remote Sensing of Anhui Province, Anhui Institute of
Meteorological Sciences, Hefei, People’s Republic of China, 4School of Applied Meteorology, Nanjing University of
Information Science and Technology, Nanjing, People’s Republic of China
Abstract A composite time series of the merged satellite altimeters sea surface height anomaly (SSHA)
data and satellite-observed sea surface temperature (SST) data were used to identify eddies in the Western
North Pacific Ocean (WNPO), where there were numbers of intense typhoons. This study systematically
investigated 15 super typhoons during the period of 2000-2008 in the WNPO to study their impacts on the
pre-typhoon ocean features, e.g., the cyclonic ocean eddy (COE) feature (closed contours of SSHA < 26 cm)
and neutral condition (SSHA between 26 and 6 cm). Two new COEs are generated by two super typhoons,
and 18 pre-existing COEs are intensified by 13 super typhoons. 5 of the 13 super typhoons each influenced
two pre-exisiting COEs. Although the typhoon-induced maximum cooling centers had a right bias along the
tracks due to wind conditions, pre-existing COEs also play a significant role in determining the strength and
location of large SST cooling. Three possible factors (maximum wind speed, typhoon translation speed and
the typhoon forcing time, Tf) are employed to explain the interactions. Above all, the changes of the COE geometric and physical parameters (e.g., effective radius, area, SST, SSHA, and eddy kinetic energy) were mostly
related to the typhoon forcing time, Tf. This is because Tf is a parameter that is a combination of the typhoon’s
translation speed, intensity and size. Although the typhoons may significantly impact COEs, such samples
were not commonly observed. Thus, the impact of typhoon on the strength of COEs is generally inefficient.
1. Introduction
When typhoons pass over the ocean, their strong winds stir the upper ocean and generate divergent outward
flows through local (entrainment, vertical mixing, and upwelling) and nonlocal (horizontal advection, horizontal mixing, and pressure gradients) processes [Price, 1981; Lin et al., 2003; Davis and Yan 2004; Han et al.,
2012]. The wind stress vector of a typhoon turns clockwise with time on the right side of the track, which is
roughly resonant with the current in the mixed layer. This, together with the asymmetry of wind stress, makes
the upper ocean exhibit stronger sea surface temperature (SST) cooling on the right side of typhoon track
[Price, 1981; Dickey and Simpson, 1983; Stramma et al., 1986; Vincent et al., 2013]. In addition to wind stress, it
has been suggested that preexisting negative sea surface height anomaly (SSHA) features or cyclonic ocean
eddies (COEs) play important roles in the response of the upper ocean to typhoons [Nan et al., 2005; Walker
et al., 2005; Zheng et al., 2008, 2010; Liu et al., 2009]. The COEs provide a relatively unstable thermodynamic
structure that easily elevates cold and nutrient-rich water. On the other hand, COEs might be influenced by
typhoons in various ways. The preexisting cyclonic ocean circulation might be intensified under appropriate
conditions after the passage of a typhoon [Shang et al., 2008; Sun et al., 2009, 2012; Yang et al., 2010, 2012b].
New cyclonic eddies were generated by looping trajectory typhoons in the South China Sea [Chu et al., 2000;
Hu and Kawamura, 2004]. A cyclonic ocean eddy was observed in the preexisting positive SSHA area, which
was result from three sequential typhoons in September 2008 [Yang et al., 2012a]. Recently, two COEs result
from long forcing time of strong wind stress curl were found in two certain locations along the trails of binary
typhoons [Yang et al., 2012b]. Above studies implied that certain typhoons, such as looping trajectory
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Figure 1. (a) Tracks of 15 super typhoons. (b) The forcing time for typhoon Chanchu (2006).
typhoons or sequential typhoons, passing over the same region, may play a notable role in intensifying or
generating COEs. However, the underlining physical processes of typhoon’s impact on COEs are still not clear.
Due to the harsh conditions bought out by typhoons, little in situ data were available to study the
typhoon-ocean interactions. Fortunately, microwaves can penetrate clouds with little attenuation, providing an uninterrupted view of the ocean surface accompanying a typhoon [Wentz et al., 2000]. The
responses of COEs to typhoons were mostly studied by using microwave satellite data. However, the
features of COEs observed by satellite are very limited. COEs are mostly represented by SST and SSHA.
In this paper, we explore the response of COEs to typhoon in a more complete way. COEs are generally represented by geometric parameters and physical parameters. Geometric parameters include the
effective radius, the area, and the distance from the COE center to the typhoon center. Physical parameters include SST, SSHA, and eddy kinetic energy (EKE) of the COE. Then, the responses of COEs to
typhoons are studied by analyzing the correlation of these COE parameters and the properties of
typhoons (i.e., intensity, translation speed, and forcing time).
The western North Pacific Ocean (WNPO) is a region with high frequency of typhoons and abundant COEs
observed all year round. This paper primarily investigated the activity of 20 COEs influenced by 15 super
typhoons during the period of 2000–2008 in the WNPO (Figure 1a). Supper typhoons are defined as storms
whose maximum 1 min sustained winds speed exceeding 59 m/s, see section 2 for details. The rest of the
paper is organized as follows. In section 2, data and data processing method are presented. Two typical
activities of COEs induced by typhoons Dujuan (2003) , Sudal (2004), and Chanchu (2006) are described in
section 3. In section 4, the influences of typhoons on COEs are fully explored. Finally, a summary and discussion are presented in section 6.
2. Data and Method
2.1. Satellite Data and Parameters of COE
Merged altimeter data were obtained from multiple satellite sensors, including those of the systems
of Jason-1, TOPEX/Poseidon, Geosat Follow-On (GFO), European Remote Sensing 2 (ERS-2), and Environmental Satellite (ENVISAT). Data were produced and distributed by the Archiving, Validation and
Interpretation of Satellite Oceanographic (AVISO) organization. Near real-time merged sea surface
height anomaly (SSHA) data (TOPEX/POSEIDON or Jason-11 ERS-1/2 or ENVISAT) with Mercator
grids are available at www.aviso.oceanobs.com. Currently, the products are available on a daily
scale with 0.25 3 0.25 resolution in the global ocean. The daily SST data were derived from the
Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), with a spatial resolution
of 0.25 3 0.25 .
The oceanic dynamic features are defined as positive-SSHA (anticyclonic eddy) feature (closed contours,
SSHA > 6 cm), negative-SSHA (cyclonic eddy) feature (closed contours, SSHA < 26 cm), and neutral
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condition (SSHA between 26 and 6 cm) [Lin et al., 2008] with a mononuclear eddy constrain [Li et al., 2014].
The eddy is usually in the form of an irregular circle with the eddy’s area of ACOE.
The meridional component, ug, and the zonal component, vg, of the geostrophic velocity of the ocean current were calculated as follows:
ug 5
g @g
g @g
; vg 5
f @y
f @x
(1)
where g is the acceleration of gravity, f is the Coriolis parameter, and g is the SSHA. If the eddy covers
a region with n grids of 0.25 3 0.25 , the EKE is computed [e.g., Xu et al., 2011]:
EKE5
n
X
1
i51
2
ðu2g 1vg2 ÞqAi Hi
(2)
where q 5 1020 kg/m3 is the density of seawater, Ai is the area of the ith grid, Hi is the depth of the ocean
at ith grid according to the ‘‘Smith & Sandwell’’ database (the Gridded Global Relief Data (ETOPO2v2)) from
National Geophysical Data Center. This database is a worldwide set of 2 min gridded ocean bathymetry
derived from 1978 satellite radar altimetry of the sea surface.
The changes of COE parameters induced by typhoon are defined as the differences between before
COE under the influence of typhoon and COE after typhoon passage. In general, changes of COE
parameters at a point might include two parts: the changes due to COE variations and the changes due
to COE propagation. We use both the extremes and averages within a region as measurement to
exclude the influences of COE propagation. Consequently, the directly calculated max SST cooling [e.g.,
Walker et al., 2005] might be larger than both the change of extreme and averaged SST of a COE. To
eliminate the influence from typhoon, we take 5 day averages as the reference state before typhoon.
Then the differences of COE parameters are calculated with daily data within a week. dSSHAmax
(dSSTmax) represents the maximum difference of SSHA (SST) extreme within a COE region, while dSSHAmean (dSSTmean) and dEKE represent the difference of mean SSHA (SST) and mean EKE, respectively. In
this study, we use a criterion of dSSHAmax < 26 cm as significance of COE change. If dSSHAmax does not
meet the criterion, the case is not included for further investigation. And dACOE represents COE area
change before and after typhoon passage. To reveal the relationship between the location of the COE
and the extreme SST cooling induced by the typhoon, two more parameters are also defined. LCOE is
defined as the center location of the COE away from the typhoon track, and LdSSTmax as the center location of the largest SST cooling away from typhoon track. The positive and negative values of location
mean location at the right and the left of the typhoon track, respectively.
2.2. Typhoon Data and Forcing Time
The ‘‘best-track data sets’’ of the WNPO were obtained from the Joint Typhoon Warning Center (JTWC). Each
best-track file contains the tropical cyclone center locations, the central pressure, and the maximum sustained wind (MSW) speeds (i.e., the 1 min mean maximum sustained wind speed at a 10 m height, Vmax in
Table 1), the wind radius, and the radii of the specified winds (17, 25, 33, or 51 m/s) for four quadrants, at 6
h intervals.
The super typhoons, which MSW speeds surpass 115 knots (59 m/s, Category 4 storm in the
Saffir-Simpson scale), were obtained from the Japan Aerospace Exploration Agency (JAXA).
According to JAXA/EORC Tropical Cyclone Database (http://sharaku.eorc.jaxa.jp/TYP_DB/index_e.
shtml), there are 49 super typhoons observed in the WNPO during 2000–2008. But only 15 of 49
super typhoons (JAXA/EORA) have significant influence on COEs, which have dSSHAmax <26 cm
after typhoon.
Although the translation speed (Ut) was widely used to represent the air-sea interaction time scale [e.g.,
Price, 1981; Price et al., 1994; Schade and Emanuel, 1999], it may be more appropriate to use the forcing time
(Tf) [e.g., Sun et al., 2010] as interaction time scale. The reason is that the air-sea interaction time scale is relatively longer at the right side of a clockwise curved track than at the left side of the curved track [Sun et al.,
2010]. Both translation speed (Ut) and the forcing time (Tf) of typhoon are calculated in this study. The forcing time Tf is calculated following the same steps used by Sun et al. [2010]. The JTWC data were used to
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Table 1. Parameters of 15 Typhoonsa
Typhoon (Year)
Vmax (m/s)
Lifetime
Vmax (m/s)
Ut (m/s)
Tf (h)
Study Region
(SR)
Is COE
Existed?
How COE
Changed?
71.4
71.4
66.3
66.3
63.8
66.3
63.8
63.8
74
66.3
66.3
63.8
58.7
63.8
63.8
63.8
71.4
71.4
71.4
68.9
71.4
71.4
63.8
66.3
61.2
66.3
63.8
53.6
74
63.8
66.3
63.8
56.1
63.8
61.2
63.8
71.4
71.4
71.4
58.7
4.2
3.01
3.19
5.38
7.6
6.43
1.52
2.55
3.23
1.79
3.7
3.68
1.27
6.54
6.54
1.95
8.23
4.31
3.18
7.47
43.2
57.9
30.6
43
23.2
26.1
91.2
71.7
63
61.2
42.9
49.2
92.4
41.4
22.2
48.3
16.2
30.6
41.1
22.8
153 E–159 E, 19 N–23 N
153 E–158 E, 15 N–19 N
144 E–150 E, 25 N–28 N
150 E–155 E, 8 N–12 N
122 E–127 E, 19 N–22.5 N
125 E–131 E, 11 N–15 N
126 E–132 E, 13 N–19 N
130 E–136 E, 19 N–23 N
130 E–139 E, 11 N–15 N
128 E–132 E, 14 N–18 N
131 E–141 E, 6 N–12 N
128 E–137 E, 16 N–22 N
129 E–133 E, 19.5 N–24 N
125 E–130 E, 21 N–25 N
130 E–134 E, 19 N–24 N
114 E–117 E, 13 N–16 N
143 E–148 E, 19 N–24 N
122 E–126 E, 19 N–23 N
125 E–130 E, 14 N–18 N
130 E–135 E, 21 N–26 N
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Intensified
Intensified
Intensified
Intensified
Intensified
Intensified
Intensified
Intensified
Intensified
Generated
Intensified
Intensified
Intensified
Intensified
Intensified
Generated
Intensified
Intensified
Intensified
Intensified
Podul (2001)
Podul (2001)
Wutip (2001)
Pongsona (2002)
Dujuan (2003)
Imbudo (2003)
Ketsana (2003)
Ketsana (2003)
Lupit (2003)
Sudal (2004)
Sudal (2004)
Tokage (2004)
Kirogi (2005)
Longwang (2005)
Longwang (2005)
Chanchu (2006)
Yagi (2006)
Sepat (2007)
Sepat (2007)
Rammasun (2008)
a
The number after name of the typhoon refers to the different eddies along the track of the typhoon.
calculate the forcing time. At first, the region under the influence of typhoon is defined as wind speed over a
critical value Uc (e.g., Uc 5 17 m/s) along its track. Then, the duration of wind speed over Uc within the
typhoon-influenced area is calculated. Maximum duration within the area defines as the forcing time of the
typhoon. The 6 h track is interpolated to shorter time (e.g., half hour). Figure 1b shows the forcing time calculated for Typhoon Chanchu (2006) as an example. The forcing time is relatively large at typhoon slow moving
and sharp turning region. The longest forcing time in this region is about 50 h with an error less than 1 h.
The Argo floats profiles were extracted from the real-time quality-controlled Argo database of China Argo
Real-time Data center. The Argo data were collected and made freely available by the International Argo
Project and the national programs that contribute to it.
A linear regression was applied to calculate the relationships between the differences in the parameters of
the COE and the properties of the typhoon. The correlation coefficients (r) were tested using a t distribution.
Statistic significance was evaluated by p values.
3. Typical Cases
3.1. New COEs Generated by Typhoon Chanchu (2006) and Sudal (2004)
Typhoon Chanchu was formed in the tropical Pacific Ocean (135.3 E, 8.7 N) on 8 May 2006. The typhoon
moved into our study region (12.6 N–16.3 N, 113.7 E–117.3 E) at 06:00 on 14 May and left this region 36 h
later (Figure 2a). When Chanchu influenced this region, its maximum wind speed reached 64 m/s, its
translation speed was 1.5 m/s, and its forcing time was 48.3 h.
Figure 2 shows the daily SST on four different days and weekly mean SSHA change in four different
weeks induced by Typhoon Chanchu’s passage in the study region. The typhoon’s forcing time is
plotted in Figure 2a before typhoon. TRMM TMI-derived SST serial images revealed the evolution of
sea surface cooling induced by Chanchu (Figures 2a–2d). After the passage of Chanchu, a distinctly
extreme cold patch was observed in this region, with a maximum temperature cooling of 25.7 C (Figure 2c). The cold path located at large forcing time region (Tf > 45 h). This cold patch lasted for about
1 week, which is generally consistent with previous studies [e.g., Hart et al., 2007; Price et al., 2008;
Dare and McBride, 2011; Mei and Pasquero, 2013]. The ocean features also changed from neutral condition before Chanchu’s passage (Figure 2e) to a COE with a negative-SSHA feature category after
Chanchu’s passage (Figure 2f). This new eddy was well developed and lasted for more than several
weeks (figure not shown). The values of Ac of the COE reached about 68.9 3 103 km2 after 2 weeks
(Figure 2h).
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Figure 2. The (a–d) SST (colored) and (e–h) SSHA (contours) changed on four different stages. Typhoon Chanchu moved into this study region (12.6 N–16.25 N, 113.7 E–
117.25 E) on 14 May and left this region 36 h later. The time interval of the typhoon track is 6 h. Figures 2a and 2e represent SST and SSHA before Typhoon Chanchu passage, respectively. The typhoon’s forcing time was contoured (hours) in Figure 2a. The SSHA smaller than 26 cm was filled with color. The track of the typhoon Chanchu is
represented by bold line in Figures 2a and 2f.
Typhoon Sudal (2004) had similar but more significant impacts on the ocean (Figure 3). It had a similar
sharp turning track and a relatively slowly transport speed (1.8 m/s). The forcing time was large (>60
h) in this region (Figure 3a), where a new COE generated later (Figure 3f). The SST cooling of 22.8 C,
however, was not so pronounced. This might be due to the different pretyphoon condition. Before
Sudal’ passage, there was a weak warm core eddy (Figure 3e).The stronger stratification of upper
ocean might make it hard to entrain the deeper cold water to the surface.
3.2. Preexisting COE Intensified by Typhoon Dujuan (2003)
Typhoon Dujuan was formed over the subtropical Pacific Ocean (138.1 E, 17.8 N) on 28 August 2003. The
typhoon moved into our study region (19 N–22.5 N, 122 E–127 E) at 18:00 on 31 August and left this
region 18 h later (Figure 4a). When Dujuan influenced this region, its maximum wind speed reached
62 m/s, its translation speed was 7.6 m/s, and its forcing time was 23 h.
Figure 4a shows the SST before Typhoon Dujuan and the typhoon’s forcing time in this region. After
the passage of Dujuan, a distinctly extreme cold patch was observed in the COE region (Figures 4b
and 4c), with a maximum cooling of 22.3 C (Figure 4c). This cold patch lasted for more than 1 week
(Figures 4b and 4c). Although the SST is relatively homogenous, there is an evident COE existing (Figure
4e), with Ac of 46.4 3 103 km2. In comparison with the pretyphoon condition, the maximum drop of the
SSHA was 8 cm, and the Ac of the COE reached about 50.5 3 103 km2 after 2 weeks (Figure 4h).
Dujuan significantly strengthened the north part of the preexisting COE, which propagated westward.
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Figure 3. Same as Figure 2, except for typhoon Sudal. Typhoon Sudal moved into the study region (11 N–17 N, 129 E–133 E) on 10 April 2004 and left this region on 12 April 2004.
These cases are two type examples showing typhoon’s impact on COEs: generating and intensifying. In general, Typhoons Chanchu and Sudal had stronger impacts on the upper ocean (new COEs generated), while
typhoon Dujuan’s influence on upper ocean was relatively small (strengthening preexisting COE). Stronger
wind (intensity) and longer forcing time of Chanchu/Sudal than Dujuan may be the main reasons lead to
the difference of typhoon’s impact on upper ocean [Sun et al., 2010].
4. Influence of Super Typhoons on COEs
Similar analysis as Typhoons Chanchu, Sudal, and Dujuan was applied to other super typhoons. The results
are summarized in Tables 1 and 2.
4.1. The 15 Super Typhoons
The 15 typhoons, including Chanchu, Sudal, and Dujuan, passing over a COE or generating a new
COE in the selected areas were listed in Table 1. Vmax, Ut, and Tf. values of these 15 super typhoons
were chosen for study when they influenced the COE regions (Table 1). The values of Vmax, Ut, and Tf
of these typhoons were in the range of 59–74 m/s, 1.3–8.2 m/s, and 16.2–92.4 h, respectively. Among
these 15 typhoons, 13 of them passed over 18 corresponding preexisting COEs and only two typhoons
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Figure 4. Same as Figure 2, except for Typhoon Dujuan. Typhoon Dujuan moved into the study region (19 N–22.5 N, 122 E–127 E) at 18:00 on 31 August 2003 and left this region 18 h later.
(Sudal and Chanchu) passed over neutral condition region. After the passage of these typhoons, 18
preexisting COEs were more or less strengthened and two new COEs were generated. There were also
cases that two preexisting COEs located along or near the track were generated by the same typhoon.
These typhoons are Podul (2001), Ketsana (2003), Sudal (2004), Longwang (2005), and Sepat (2007).
4.2. Variations of COEs Induced by the Typhoons
To quantitatively analyze the eddy activities influenced by the typhoons, changes in six parameters of the
COEs (dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE) are listed in Table 2. Under the influence of
typhoons, the values of dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE are primarily in the range
Table 2. Differences of COE Parameters Between the Post and Prepassage of a Typhoon
COE Induced by
Podul
Podul
Wutip
Pongsona
Dujuan
Lmbudo
Ketsana.
Ketsana
Lupit
Sudal
Sudal
Tokage
Kirogi.
Longwang
Longwang
Chanchu
Yagi
Sepat
Sepat
Rammasun
SUN ET AL.
dSSHAmax (cm)
dSSHAmean (cm)
dSSTmax ( C)
dSSTmean ( C)
dACOE (103 km2)
dEKE (1015 J)
26
217
29
210
28
29
242
221
228
225
220
215
216
216
217
227
27
211
215
27
21.8
26.4
22.8
28
22.9
21.3
210.3
26.8
26.4
210.3
24.1
27.8
24.6
22.7
24.3
214.2
21.2
25.2
26.2
21.9
23.4
22.8
23.2
22.8
22.3
22.5
27.1
24.6
24.1
22.8
24.8
23.8
24.1
21.5
21.2
25.7
23.9
23.4
23.2
21.8
22.1
21.2
21.7
21.7
21.3
21.5
23.2
23
22.4
21.8
21.5
22.1
22.4
21
20.6
23.8
21.8
22.2
22.5
21.5
3.4
52.8
56
49.6
4.1
45.9
165.5
39.8
222.3
89.2
132
18
17.5
53.4
36.3
68.9
24.3
18
29.1
12.2
6.5
35.7
8
3.7
7.9
17.4
173.7
60.7
70.5
72.9
113.7
35.1
20
22.9
18.2
21.3
2.1
6.6
8.1
1.0
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0
37
May 08
May 18
May 22
62-1
34
SSHA (cm)
-10
62-3
62-2
-20
-30
(a) -150
-100
-50
0
d (km)
50
100
Depth
surface descending
deep upwelling
(b)
distance
0
0
150
10.1002/2013JC009575
of 242 to 26 cm, 214.2 to
21.3 cm, 27.1 to 21.2 C, 23.8
to 20.6 C, 3.4 3 103 – 222.3 3
103 km2, 1 3 1015 – 173.7 3
1015 J, respectively. Comparing
Tables 1 and 2, after the super
typhoons’ passage, 12 COEs
(60%) that exhibited strong
cooling (dSSTmax 23 C), and
12 COEs (60%) exhibited a
notable SSHA deepening
(dSSHAmaxx 212 cm). Generally, dramatic SST drop among
these COEs occurred under
two related conditions, slow Ut
(4 m/s) or long Tf (>30 h).
There is only one case with
exception, which was one of
the strongest Typhoon Yagi
(2006). If both conditions were
satisfied at the same time, the
typhoon would cause more
cooling in the regions of COEs
(e.g., Typhoons Lupit and Ketsana in 2003, Typhoon Sudal in
2004, Typhoon Kirogi in 2005).
4.3. Vertical Response Within
the COE
The typhoon-eddy interaction
will also lead to subsurface
change. Typhoons may influence subsurface of COEs in
400
400
different ways: heat extraction
and redistribution [e.g., Bourassa et al., 2010; Yang et al.,
2010], momentum input, and
associated surface water diver600
600
2900662-1
gence [e.g., Zedler, 2009;
2900662-1
2900662-2
2900662-2
Jaimes et al., 2011; Sun et al.,
2900662-3
2900662-3
2012]. Since the subsurface
2900634
2900634
observations associated with
2900637
2900637
typhoons are very limited, we
800
800
34.2
34.4
34.6
34.8
5
10
15
20
25
o
demonstrate the vertical temSalinity (psu)
Temperature ( )
(d)
(c)
perature and salinity profile
change within a COE during
Figure 5. (a) The SSHA profiles of COE and the locations of the Argo profiles. The numbers
34, 37, and 62 represent Argo float 2900634, 2900637, and 2900662, respectively. (b) The
Typhoon Rammasun (2008) as
scheme of the COE responses, the arrows indicating the surface descending and deep
an example (Figure 5). The
upwelling after typhoon. The pretyphoon (c) temperature and (d) salinity profile (black) and
SSHA profiles of COE and the
posttyphoon temperature profiles (color curves) in the upper ocean. The arrows in Figures
5c and 5d indicate the direction of upwelling in COE.
locations of the Argo profiles
(2900662/2900634/2900637)
are shown in Figure 5a. The mixed layer deepened about 40 m after the typhoon’s passage, which is consistent with the previous studies [Price, 1981; Jaimes et al., 2011; Sun et al., 2010]. In the upper ocean (0–
100 dbar) above the thermocline, the temperature decreased (Figure 5c) about 1–3 C after the typhoon.
Although the immediate vertical entrainment associated with heat extraction and redistribution could
SUN ET AL.
200
Depth (dbar)
Depth (dbar)
200
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Journal of Geophysical Research: Oceans
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Figure 6. Relationship between the COE center location and the largest SST cooling.
make surface cooling, it is more likely
due to the surface
water divergence
and the persistent
upwelling below
100 dbar in the
COE for Typhoon
Rammasun. The
upwelling also
brought saltier
water from the
deep ocean to the
upper ocean. As a
consequence, the
salinity within 50–
100 dbar increased
(Figure 5d).
The upwelling
within the COE occurred not only in the upper ocean, but also below the thermocline. Figures 5c and
5d show that both the temperature and salinity profiles within 400–800 dbar upward shifted after the
typhoon, as indicated by the arrows. So the deepening of SSHA is associated with the upwelling of
water below the thermocline. Besides, the generation of ocean fronts may also provide potential
energy in turn for the eddy growth via baroclinic instability [e.g., Mei and Pasquero, 2012]. However, it
can hardly be observed in this study due to the special-temporal resolution.
These responses can be illustrated by a conceptual model (Figure 5b). In the upper ocean, the strong wind
stirred the water and deepened the mixed layer by vertical mixing. Consequently, upwelling was well established at the top of thermocline and the surface was descending. The deep water then upwelled within the
COE. It is worth to point out that the upwelling lasted (the low SSHA in Figure 5a) for more than 2
weeks. The vertical shear instability of near-inertial currents causes turbulent vertical mixing between
mixed layer and thermocline waters and produces 75%–90% of the TC-induced mixed layer cooling
Table 3. Statistics for Areas Where SST Cooling 23 C
Typhoons (Year)
Podul (2001)
Podul (2001)
Wutip (2001)
Pongsona (2002)
Dujuan (2003)
Imbudo (2003)
Ketsana (2003)
Ketsana (2003)
Lupit (2003)
Sudal (2004)
Sudal (2004)
Tokage (2004)
Kirogi (2005)
Longwang (2005)
Longwang (2005)
Chanchu (2006)
Yagi (2006)
Sepat (2007)
Sepat (2007)
Rammasun (2008)
SUN ET AL.
SST Cooling
(23 C)
Area (A1) in Study
Region in Table 1 (103 km2)
SST Cooling
(23 C)
Area (A2) Over COE
Region (103 km2)
15.8
2.9
10.4
3.8
0.0
0.7
106.3
63.0
54.0
20.1
59.3
95.7
40.7
0.0
0.0
87.5
12.8
15.2
21.5
7.2
15.8
2.2
7.6
3.8
0.0
0.7
95.3
63.0
52.5
17.8
59.3
76.3
30.0
0.0
0.0
30.0
12.8
15.2
3.7
6.4
C 2014. American Geophysical Union. All Rights Reserved.
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Proportion
(A2/A1) (%)
100.0
75.0
73.7
100.0
100.0
89.7
100.0
97.2
88.9
100.0
79.7
73.8
34.2
100.0
100.0
17.3
90.0
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Figure 7. Correlation between the maximum wind speed (Vmax) of the typhoons and the eddy parameters.
[e.g., Price, 1981; Greatbatch, 1984; Shay et al., 2000]. The process is similar to the COE response to
Typhoon Namtheun (2004) [Sun et al., 2012].
4.4. The Locations of COEs and Maximum SST Cooling
The relationship between LCOE and LdSSTmax is shown in Figure 6. Generally, the maximum cooling centers
induced by typhoons were mainly located either close to typhoon tracks (horizontal dashed) or to the COEs
(vertical dashed). The maximum cooling centers located either at the right of the typhoon track (first and second quadrants in Figure 6) or at the same side with COEs (first and third quadrants in Figure 6), which is consistent with previous studies [Price, 1981; Shi and Wang, 2011]. Figure 6 also shows that about 30% (6/20)
maximum cooling centers do not locate in the same side of COEs, but fall in second quadrant. These cases
suggested that other factors relate to typhoon property may also play a role on the large surface cooling.
Large SST cooling can be used as a proxy for upwelling or enhanced eddy pumping after typhoon passage
[Shi and Wang, 2007, 2011; Gierach and Subrahmanyam, 2008; Wang and Zhao, 2008]. In Table 3, the areas
of the SST cooling more than 3 C were calculated following the method of Walker et al. [2005] over 20 study
regions (in Table 1) and over 20 COEs regions after the typhoons’ passage. As mentioned in section 2.1,
such cooling might be greater than the dSSTmax. As shown in Table 3, SST cooling more than 3 C appeared
in many study regions (17/20, except for the Dujuan (2003) case, and the two cases of Longwang (2005)),
and in the 17 COEs regions (same cases as above). The proportions of areas of the COE regions with SST
cooling larger than 3 C to those of study regions were found to be greater than 70% and even reached
SUN ET AL.
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Figure 8. Correlation between the translation speed (Ut) of the typhoons and the eddy parameters.
100%, except for Chanchu (2006) and Sepat (2007). This is because that the SST cooling was dominated by
the wind forcing not by the COE, as in Figure 2. The coexistence of preexisting COEs and large SST cooling
region suggests that the pretyphoon oceanic environment plays a significant role in determining the
strength and location of the large SST cooling induced by typhoons. This was found in previous studies
[Zheng et al., 2008; Yang et al., 2010].
4.5. Factors Influence COE Activity
According to previous studies [Zhao et al., 2009; Lin et al., 2009; Sun et al., 2010; Yang et al., 2012b], the activities of the COEs induced by typhoons is primarily dependent on four factors. The first factor is the
typhoon’s intensity, since it the wind forcing produces most ocean response dynamics, e.g., Ekman pumping, entrainment, and mixing. The second and third factors are the typhoon’s translation speed and size in
vortex diameter, respectively. These two factors determine the forcing time. Another important factor may
be the Coriolis parameter, which has different effect that the amplitude of the vertical displacement of isopycnals is larger at lower latitudes [Schade and Emanuel, 1999; Sun et al., 2010]. It determines the frequency
of the inertial oscillations that are dominated in the oceanic wake. It also affects the potential amplitude of
the vertical displacement of isopycnals. If any of the above factors are unfavorable, the response of the COE
to a typhoon is weakened. To investigate the relative importance of these factors for a typhoon to influence
the COE activity, the linear regression relationships between the properties of the typhoons and the differences in the parameters of the COEs are explored as below.
SUN ET AL.
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Figure 9. Correlation between the wind forcing time (Tf) of the typhoons and the eddy parameters.
Figure 7 shows dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE from all 20 COEs changes as Vmax
changes. The correlations between the parameters of COE and Vmax do not show statistical significance
according to p values. This is either because the Vmax is not important to induce COEs change or the data
set is not long enough (only 20 COEs). This might be understood as the following reasons. According to the
theory [Stern, 1965], observation [Jaimes and Shay, 2009] and simulations [Jaimes et al., 2011] on typhooninduced upwelling in eddy, the ocean current more than the wind stress is dominated. In addition, the drag
coefficient of wind stress drops with wind speed when the wind speed is very high [Powell et al., 2003]. Simulations also reported that the SST cooling is correlated to wind power index more than wind speed itself
[Vincent et al., 2013]. Thus, the wind speed alone might be not so significant. The influence of Vmax on COEs
deserves further study with more data.
Figure 8 shows dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE from all 20 COEs changes as Ut
changes. In general, dSSHAmax, dSSHAmean, dSSTmax, dSSTmean increase as Ut increases and dACOE, and dEKE
decrease as Ut increases. The correlations between COE parameter and Ut are statistical significant. Overall,
the regression results indicate that a typhoon with a slower transit speed causes larger changes in a majority of the parameters of the COEs.
Figure 9 shows dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE from all 20 COEs changes as Tf
changes. In general, dSSHAmax, dSSHAmean, dSSTmax, dSSTmean increase as Tf increases and dACOE, and dEKE
decrease as Tf increases. The correlations between COE parameter and Tf are all statistical significant. Tf shows
similar correlation to COE change as Ut. Tf is a better choice to represent typhoon, because Tf is related to not
only the translation speed of a typhoon but also intensity and the size (vortex diameter) of a typhoon.
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Journal of Geophysical Research: Oceans
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Since the typhoon-induced SSHA descending can last for more than several days [Vincent et al., 2012; Yang
et al., 2012b; Knaff et al., 2013], the parameter Tf, which can be calculated right after typhoon’s passage, might
be severed as a useful precursor for SSHA descending, and other ocean environmental changes.
5. Summary and Discussion
In this study, the interactions of 15 super typhoons and COEs were studied in WNPO during the period of
2000–2008, which resulted in the intensification of 20 preexisting COEs and the formation of two new COEs
after the passages of the typhoons. Using multiple satellite observations, six COE parameters were used to
systematically explore the activities of 20 COEs influenced by typhoons. Under the influences of typhoons,
the values of dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE, and dEKE were mainly in the ranges of 242 to
26 cm, 214.2 to 21.3 cm, 27.1 to 21.2 C, 23.8 to 20.6 C, 3.4 3 103 – 222.3 3 103 km2, 1 3 1015 – 173.7
3 1015 J, respectively. After the passages of the typhoons, the largest SST cooling centers induced by
typhoons were mainly located within 100 km along the right side of the typhoon tracks. Sixteen COEs (80%)
exhibited large cooling (dSSTmax 23 C), and 12 COEs (60%) exhibited notable SSHA deepening
(dSSHAmaxx 212 cm). Preexisting COEs were demonstrated to play a notable role in determining the
strength and location of large SST cooling. The dramatic SST drop among most of these COEs occur under
two certain conditions: slow Ut (4 m/s) and long Tf (>30 h). The importance of Tf is because that Tf involves
not only the translation speed of a typhoon but also with the intensity and size of a typhoon. Although the
typhoons may significantly impact COEs, such samples were not very common. During the study period,
there are 49 super typhoons passing over about 192 COEs. However, only about 10% of COEs were significantly influenced by these super typhoons as shown in our study. During 2003 in WNPO region, it was
found that only 2 (Lupit and Ketsana) of 11 typhoons (18%) had notable impacts on phytoplankton blooms
and SST cooling [Lin, 2012]. The reason of inefficiency of typhoon on COE may be that the energy input by
typhoon was absorbed by the surface waves. According to a hurricane-ocean coupled model simulation
[Liu et al., 2008], the surface currents (0.07 TW) take about 4% of total of 1.72 TW input by tropical cyclones.
In what circumstance that makes typhoon effectively influence COEs need to be further studied in the
future works.
Acknowledgments
We thank the anonymous reviewers
for their constructive suggestions. This
work is supported by the National
Basic Research Program of China
(2012CB417402, 2013CB430301,
2013CB430302, and 2013CB430303),
the National Foundation of Natural
Science (41376017 and 41205126), the
Open Fund of State Key Laboratory of
Satellite Ocean Environment Dynamics
(SOED1209), and the Open Research
Fund of Key Laboratory of
Atmospheric Composition and Optical
Radiation of Chinese Academy of
Sciences (Grant.JJ1102). We thank the
Joint Typhoon Warning Center (JTWC)
for providing typhoon track data,
Remote Sensing Systems for TMI SST,
AVISO for SSHA data, and National
Geophysical Data Center for ocean
depth data, China Argo Real-Time Data
Center for float profiles.
SUN ET AL.
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