Effects of Super Typhoons on Cyclonic Ocean Eddies in the Western North

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Journal of Geophysical Research: Oceans

AUTHOR’S PREFACE Effects of super typhoons on cyclonic ocean eddies in the TO A SPECIAL western North Pacific: A satellite data-based evaluation COLLECTION 10.1002/2013JC009575 between 2000 and 2008 Liang Sun1,2, Ying-Xin Li1, Yuan-Jian Yang1,3, Qiaoyan Wu2, Xue-Tao Chen1, Qiu-Yang Li1, Yu-Bin Li4, Special Section: and Tao Xian1 Western Pacific Ocean Circulation and Climate 1Key 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 Correspondence to: Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, People’s Republic of L. Sun, 3 [email protected] China, Key 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 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 Abstract A composite time series of the merged satellite altimeters sea surface height anomaly (SSHA) cyclonic ocean eddies in the western data and satellite-observed sea surface temperature (SST) data were used to identify eddies in the Western North Pacific: A satellite data-based North Pacific Ocean (WNPO), where there were numbers of intense typhoons. This study systematically evaluation between 2000 and 2008, J. Geophys. Res. Oceans, 119, doi:10.1002/ investigated 15 super typhoons during the period of 2000-2008 in the WNPO to study their impacts on the 2013JC009575. 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, Received 7 NOV 2013 and 18 pre-existing COEs are intensified by 13 super typhoons. 5 of the 13 super typhoons each influenced Accepted 6 AUG 2014 two pre-exisiting COEs. Although the typhoon-induced maximum cooling centers had a right bias along the Accepted article online 11 AUG 2014 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

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 Envi- ronmental 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

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: g @g g @g u 5 ; v 5 (1) g f @y g f @x

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]:

Xn 1 2 2 EKE5 ðug1vgÞqAiHi (2) i51 2

3 where q 5 1020 kg/m 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 dSSHA-

mean (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 sus-

tained 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 <26cm 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

Table 1. Parameters of 15 Typhoonsa

Vmax (m/s) Study Region Is COE How COE Typhoon (Year) Lifetime Vmax (m/s) Ut (m/s) Tf (h) (SR) Existed? Changed? Podul (2001) 71.4 71.4 4.2 43.2 153E–159E, 19N–23N Yes Intensified Podul (2001) 71.4 71.4 3.01 57.9 153E–158E, 15N–19N Yes Intensified Wutip (2001) 66.3 63.8 3.19 30.6 144E–150E, 25N–28N Yes Intensified Pongsona (2002) 66.3 66.3 5.38 43 150E–155E, 8N–12N Yes Intensified Dujuan (2003) 63.8 61.2 7.6 23.2 122E–127E, 19N–22.5N Yes Intensified Imbudo (2003) 66.3 66.3 6.43 26.1 125E–131E, 11N–15N Yes Intensified Ketsana (2003) 63.8 63.8 1.52 91.2 126E–132E, 13N–19N Yes Intensified Ketsana (2003) 63.8 53.6 2.55 71.7 130E–136E, 19N–23N Yes Intensified Lupit (2003) 74 74 3.23 63 130E–139E, 11N–15N Yes Intensified Sudal (2004) 66.3 63.8 1.79 61.2 128E–132E, 14N–18N No Generated Sudal (2004) 66.3 66.3 3.7 42.9 131E–141E, 6N–12N Yes Intensified Tokage (2004) 63.8 63.8 3.68 49.2 128E–137E, 16N–22N Yes Intensified Kirogi (2005) 58.7 56.1 1.27 92.4 129E–133E, 19.5N–24N Yes Intensified Longwang (2005) 63.8 63.8 6.54 41.4 125E–130E, 21N–25N Yes Intensified Longwang (2005) 63.8 61.2 6.54 22.2 130E–134E, 19N–24N Yes Intensified Chanchu (2006) 63.8 63.8 1.95 48.3 114E–117E, 13N–16N No Generated Yagi (2006) 71.4 71.4 8.23 16.2 143E–148E, 19N–24N Yes Intensified Sepat (2007) 71.4 71.4 4.31 30.6 122E–126E, 19N–23N Yes Intensified Sepat (2007) 71.4 71.4 3.18 41.1 125E–130E, 14N–18N Yes Intensified Rammasun (2008) 68.9 58.7 7.47 22.8 130E–135E, 21N–26N Yes Intensified

aThe 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 (135.3E, 8.7N) on 8 May 2006. The typhoon moved into our study region (12.6N–16.3N, 113.7E–117.3E) 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 25.7C (Fig-

ure 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 3 2 weeks (ﬁgure not shown). The values of Ac of the COE reached about 68.9 3 10 km after 2 weeks (Figure 2h).

Figure 2. The (a–d) SST (colored) and (e–h) SSHA (contours) changed on four different stages. Typhoon Chanchu moved into this study region (12.6N–16.25N, 113.7E– 117.25E) 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 22.8C, 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 (138.1E, 17.8N) on 28 August 2003. The typhoon moved into our study region (19N–22.5N, 122E–127E) 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 22.3C (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 3 2 4e), with Ac of 46.4 3 10 km . In comparison with the pretyphoon condition, the maximum drop of the 3 2 SSHA was 8cm,andtheAc of the COE reached about 50.5 3 10 km after 2 weeks (Figure 4h). Dujuan significantly strengthened the north part of the preexisting COE, which propagated westward.

Figure 3. Same as Figure 2, except for typhoon Sudal. Typhoon Sudal moved into the study region (11N–17N, 129E–133E) 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,andTf 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

Figure 4. Same as Figure 2, except for Typhoon Dujuan. Typhoon Dujuan moved into the study region (19N–22.5N, 122E–127E) 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 3 2 15 COE Induced by dSSHAmax (cm) dSSHAmean (cm) dSSTmax ( C) dSSTmean ( C) dACOE (10 km ) dEKE (10 J) Podul 26 21.8 23.4 22.1 3.4 6.5 Podul 217 26.4 22.8 21.2 52.8 35.7 Wutip 29 22.8 23.2 21.7 56 8 Pongsona 210 28 22.8 21.7 49.6 3.7 Dujuan 28 22.9 22.3 21.3 4.1 7.9 Lmbudo 29 21.3 22.5 21.5 45.9 17.4 Ketsana. 242 210.3 27.1 23.2 165.5 173.7 Ketsana 221 26.8 24.6 23 39.8 60.7 Lupit 228 26.4 24.1 22.4 222.3 70.5 Sudal 225 210.3 22.8 21.8 89.2 72.9 Sudal 220 24.1 24.8 21.5 132 113.7 Tokage 215 27.8 23.8 22.1 18 35.1 Kirogi. 216 24.6 24.1 22.4 17.5 20 Longwang 216 22.7 21.5 21 53.4 22.9 Longwang 217 24.3 21.2 20.6 36.3 18.2 Chanchu 227 214.2 25.7 23.8 68.9 21.3 Yagi 27 21.2 23.9 21.8 24.3 2.1 Sepat 211 25.2 23.4 22.2 18 6.6 Sepat 215 26.2 23.2 22.5 29.1 8.1 Rammasun 27 21.9 21.8 21.5 12.2 1.0

of 242 to 26 cm, 214.2 to 0 37 May 08 21.3 cm, 27.1 to 21.2C, 23.8 62-1 May 18 3 May 22 to 20.6 C, 3.4 3 10 – 222.3 3 34 3 2 15 -10 10 km ,13 10 – 173.7 3 62-3 15 62-2 10 J, respectively. Comparing Tables 1 and 2, after the super SSHA-20 (cm) typhoons’ passage, 12 COEs (60%) that exhibited strong -30 cooling (dSSTmax 23 C), and 12 COEs (60%) exhibited a -150 -100 -50 0 50 100 150 (a) d (km) notable SSHA deepening surface descending (dSSHAmaxx 212 cm). Gener- ally, dramatic SST drop among these COEs occurred under

two related conditions, slow Ut (4 m/s) or long Tf (>30 h).

Depth There is only one case with exception, which was one of the strongest Typhoon Yagi deep upwelling (2006). If both conditions were satisﬁed at the same time, the (b) distance typhoon would cause more cooling in the regions of COEs 0 0 (e.g., Typhoons Lupit and Ket- sana in 2003, Typhoon Sudal in 2004, Typhoon Kirogi in 2005).

4.3. Vertical Response Within 200 200 the COE The typhoon-eddy interaction will also lead to subsurface change. Typhoons may influ- Depth (dbar) Depth (dbar) ence subsurface of COEs in 400 400 different ways: heat extraction and redistribution [e.g., Bour- assa et al., 2010; Yang et al., 2010], momentum input, and associated surface water diver- 600 600 2900662-1 2900662-1 gence [e.g., Zedler,2009; 2900662-2 2900662-2 Jaimes et al., 2011;Sunetal., 2900662-3 2900662-3 2900634 2900634 2012]. Since the subsurface 2900637 2900637 observations associated with 800 800 typhoons are very limited, we 5 10152025 34.2 34.4 34.6 34.8 (c) Temperature (o) (d) Salinity (psu) demonstrate the vertical temperature and salinity profile Figure 5. (a) The SSHA profiles of COE and the locations of the Argo profiles. The numbers change within a COE during 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 upwelling after typhoon. The pretyphoon (c) temperature and (d) salinity profile (black) and an example (Figure 5). The posttyphoon temperature profiles (color curves) in the upper ocean. The arrows in Figures SSHA profiles of COE and the 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 1–3C after the typhoon. Although the immediate vertical entrainment associated with heat extraction and redistribution could

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 6. Relationship between the COE center location and the largest SST cooling. (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 estab- lished 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 23C SST Cooling SST Cooling (23C) (23C) Area (A1) in Study Area (A2) Over COE Proportion Typhoons (Year) Region in Table 1 (103 km2) Region (103 km2) (A2/A1) (%) Podul (2001) 15.8 15.8 100.0 Podul (2001) 2.9 2.2 75.0 Wutip (2001) 10.4 7.6 73.7 Pongsona (2002) 3.8 3.8 100.0 Dujuan (2003) 0.0 0.0 Imbudo (2003) 0.7 0.7 100.0 Ketsana (2003) 106.3 95.3 89.7 Ketsana (2003) 63.0 63.0 100.0 Lupit (2003) 54.0 52.5 97.2 Sudal (2004) 20.1 17.8 88.9 Sudal (2004) 59.3 59.3 100.0 Tokage (2004) 95.7 76.3 79.7 Kirogi (2005) 40.7 30.0 73.8 Longwang (2005) 0.0 0.0 Longwang (2005) 0.0 0.0 Chanchu (2006) 87.5 30.0 34.2 Yagi (2006) 12.8 12.8 100.0 Sepat (2007) 15.2 15.2 100.0 Sepat (2007) 21.5 3.7 17.3 Rammasun (2008) 7.2 6.4 90.0

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 3C 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 3C to those of study regions were found to be greater than 70% and even reached

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.

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 typhoon- induced 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]. Sim- ulations 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 major- ity of the parameters of the COEs.

Figure 9 shows dSSHAmax, dSSHAmean, dSSTmax, dSSTmean, dACOE,anddEKE from all 20 COEs changes as Tf changes. In general, dSSHAmax, dSSHAmean, dSSTmax, dSSTmean increase as Tf increases and dACOE,anddEKE

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.

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 21.2C, 23.8 to 20.6C, 3.4 3 103 – 222.3 3 103 km2,13 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.

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