Oceanic eddy formation and propagation southwest of Taiwan

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011jc007386, 2011 Oceanic eddy formation and propagation southwest of Taiwan Feng Nan, 1,2 Huijie Xue, 2 Peng Xiu, 2 Fei Chai, 2 Maochong Shi, 1 and Peifang Guo 1 Received 15 June 2011; revised 25 October 2011; accepted 25 October 2011; published 31 December [1] Oceanic eddies are active and energetic southwest of Taiwan. The formation and propagation of eddies in this area were investigated using 17 year satellite altimeter data. Cyclonic eddies (CEs) and anticyclonic eddies (ACEs) often coexisted, but there were more CEs than ACEs generated during the period from October 1992 to October ACEs were stronger and, in general, lived longer than CEs. ACEs occurred more often in winter than in other seasons, while CEs were more frequent in summer. Compared with the direct local wind forcing, the Kuroshio path variability appears to be a dominant factor for eddy formation in this area. A conceptual model of an eddy-kuroshio interaction is proposed. In summer, there exists an outflow northwest of Luzon Island, and the Kuroshio likely leaps across the Luzon Strait. To the north of the outflow and left of the Kuroshio axis, CEs are often formed, which in turn induce ACEs to the west of CEs. In winter, under the influence of northeasterly monsoon, the Kuroshio Current Loop (KCL) appears southwest of Taiwan more frequently than in other seasons, and ACEs are frequently shed from the KCL. Most of the ACEs propagate westward, and, as a result, CEs are often spun up to the east of the ACEs. The surface South China Sea outflow in summer and the KCL in winter are, however, likely related to the monsoons. Therefore, the indirect effects of monsoon winds are also evident in the seasonal variations of eddy occurrence. Citation: Nan, F., H. Xue, P. Xiu, F. Chai, M. Shi, and P. Guo (2011), Oceanic eddy formation and propagation southwest of Taiwan, J. Geophys. Res., 116,, doi: /2011jc Introduction [2] In the past decades, eddy activities in the South China Sea (SCS) have attracted much attention from oceanographers. Some observational work has been done on eddies in the SCS [e.g., Dale, 1956; Wang and Chern, 1987; Li et al., 1998; Chu et al., 1999; Chu and Fan, 2001; Hu et al., 2011; Nan et al., 2011b]. Their statistical characteristics are well documented based on satellite data [Hwang and Chen, 2000; Wang et al., 2003; Xiu et al., 2010; Chen et al., 2011]. The results all show that southwest of Taiwan is a region with a high probability of eddy occurrence (see also Figure 1d). Correspondingly, two strong eddy kinetic energy (EKE) centers were observed east of Vietnam and southwest of Taiwan [Chen et al., 2009; Cheng and Qi, 2010] (see also Figure 1c). [3] The Kuroshio Current Loop (KCL) southwest of Taiwan was observed [Li and Wu, 1989; Yuan et al., 2006] and reproduced in several numerical models [e.g., Farris and Wimbush, 1996; Xue et al., 2004; Wu and Chiang, 2007; Chern et al., 2010; Sheu et al., 2010]. The KCL is traditionally defined as an anticyclonic current 1 College of Physical and Environmental Oceanography, Ocean University of China, Qingdao, China. 2 School of Marine Sciences, University of Maine, Orono, Maine, USA. Copyright 2011 by the American Geophysical Union /11/2011JC loop southwest of Taiwan with the Kuroshio flowing into the SCS in the middle of the Luzon Strait and flowing out in the northern part of the Luzon Strait [e.g., Li and Wu, 1989; Yuan et al., 2006; Nan et al., 2011a]. Yuan et al. [2006] analyzed satellite altimeter data and demonstrated that the anticyclonic intrusion of the Kuroshio is a transient phenomenon rather than a persistent circulation pattern in the Luzon Strait, which was also shown by Nan et al. [2011a]. Li et al. [1998] observed a warm-core anticyclonic eddy (ACE) located at E, 21 N in September 1994, and based on water characteristics in the eddy center the ACE appeared to be detached from the KCL. Eddy shedding from the KCL in the SCS has since been studied by many authors. Metzger and Hurlburt [2001] showed the nondeterministic nature of eddy shedding from the KCL. Jia and Liu [2004] concluded that the most frequent eddy shedding intervals are 70, 80, and 90 days and the most frequent eddy shedding locations are between E and 120 E. Jia and Liu [2005] suggested that eddy shedding was intimately related to the Luzon Strait transport. Wang et al. [2008a] examined the origins of two ACEs in the northeastern SCS during winter and, on the basis of observed water mass properties, they suggested that one of the two ACEs was shed from the KCL. [4] Eddies in different areas of the SCS may be generated by different mechanisms. Wind stress curl (WSC) was thought to be an important but not the sole mechanism of cyclonic eddy (CE) and ACE forming in the SCS [Xiu et al., 1of15

2 Figure 1. (a) Topography of the northern SCS. Contours show the 200 and 1000 m isobaths. (b) Mean absolute dynamic topography (ADT) (cm) and the corresponding surface geostrophic currents (m s 1 ) derived from the 17 year satellite altimeter data. (c) Mean EKE (cm 2 s 2 ). (d) Climatological spatial pattern of eddy occurrence (%), quantified as the frequency of an eddy presence at each pixel during the entire time series from October 1992 to October Boxes in Figures 1c and 1d denote areas with high EKE and eddy occurrences southwest of Taiwan, respectively. 2010]. Wang et al. [2008b] suggested that the orographic wind jets associated with the wintertime northeasterly monsoon and the gaps in the mountainous island chain along the eastern boundary of the SCS can spin up CEs and ACEs. Eddies were generated in the regions to the southeast of Vietnam in the winter and off central Vietnam in the summer because of wind-driven coastal jet separation and strong flow variability [Gan and Qu, 2008]. Mesoscale flow instabilities that are due to Kuroshio intrusion through the Luzon Strait dominate the sea surface height (SSH) variability in the northern SCS compared with direct atmospheric forcing [Metzger and Hurlburt, 2001]. Chen et al. [2011] indicated that the eddy number in the northern SCS has a good correlation with the strength of background flows. [5] The Luzon Strait is a deep (2500 m) meridional gap of 360 km wide along the western wall of the Kuroshio (Figure 1a), which is also the main passage for water exchange between the SCS and the western Pacific Ocean [Metzger and Hurlburt, 2001; Xue et al., 2004; Liang et al., 2008]. When passing by the Luzon Strait, a branch of the Kuroshio flows northwestward into the SCS. Most of the Kuroshio water flows out of the SCS subsequently, but a portion of it intrudes into the SCS (see Figure 1b). The SSH field in the northern SCS with the Kuroshio intrusion through the Luzon Strait is quite different from that without the Kuroshio intrusion [Liu et al., 2001]. [6] It has been shown that the Kuroshio takes different intruding paths southwest of Taiwan by observation [Caruso et al. 2006], numerical models [Sheremet, 2001; Xue et al., 2004; Wu and Chiang, 2007; Sheu et al., 2010], and laboratory experiments [Sheremet and Kuehl, 2007; Kuehl and Sheremet, 2009]. Nan et al. [2011a] defined a Kuroshio SCS Index (KSI) using the integral of geostrophic vorticity (GV) southwest of Taiwan, and three typical paths (the looping path, the leaking path, and the leaping path) were quantitatively differentiated based on the KSI derived from the 16 year satellite altimeter data. The Kuroshio path in the Luzon Strait can change from one path to another in several weeks. [7] The prior work of Hwang and Chen [2000] and Wang et al. [2003] mainly focused on ACE shedding from the KCL southwest of Taiwan. However, the present study reveals that there were more CEs (41) than ACEs (27) generated in this area from 118 E to 121 E and north of 20 N (see the box in Figure 1) during the period from October 1992 to October 2009, agreeing with the eddy polarity analysis by Chen et al. [2011]. The process of ACE shedding from the KCL has been investigated by a number of authors [e.g., Metzger and Hurlburt, 2001; Jia and Liu, 2004; Yuan et al., 2006], but the conclusions vary because of the differences in models and the ways in which eddies were depicted. This paper focuses on the characteristics of eddy formation and propagation southwest of Taiwan based on an objective criterion of eddy identification (see section 2.2). Seasonal analysis of eddy occurrence shows that ACEs occurred more often in winter than in other seasons, while CEs were more frequent in summer, which was not shown in prior works. On the average, ACEs have longer mean lifetimes than those of CEs and can propagate farther. Some long-lived ACEs could propagate westward 2of15

3 to the interior of the SCS, while none of the CEs passed the Dongsha Islands. The possible reasons are discussed in this paper. The interannual variations of eddy activities in this area are also shown. As mentioned above, the paths of the Kuroshio are unstable southwest of Taiwan. How the variable paths of the Kuroshio related to eddy formation is also discussed in this paper. In addition, ACEs often occurred next to CEs and vice versa (see section 4.3), resulting in the coexistence of ACEs and CEs, which influences eddy propagation and leads to the speculation of eddy-eddy interaction. [8] The rest of the paper is organized as follows. Section 2 describes the satellite data as well as the eddy detection methodology. Section 3 presents the results of eddy activities southwest of Taiwan. Section 4 discusses the effects of local wind and Kuroshio paths on eddy formation. Using the satellite altimeter data and blended wind data, we investigate the dominant factor for eddy formation in this area. A conceptual model of eddy-kuroshio interaction is proposed to illustrate the relationship between the Kuroshio intruding paths and the formation of CEs and ACEs. Finally, section 5 summarizes the main findings. 2. Data and Methodology 2.1. Satellite Data [9] The absolute dynamic topography (ADT) data and sea level anomaly (SLA) data used in this paper are produced by the French Archiving, Validation, and Interpolation of Satellite Oceanographic (AVISO) data project. The merged data from the combination of Jason, TOPEX/POSEIDON, Envisat, GFO, ERS, and Geosat altimeters are interpolated onto a global grid of 1/4 resolution and are archived in weekly averaged frames. The data set covers the period from October 1992 to the present, while in this study the 17 year data from October 1992 to October 2009 are used. Gridded SLA products are computed with respect to a seven year mean from January 1993 to December ADT products are obtained by summing the SLA with the mean dynamic topography (MDT). MDT, on the other hand, is derived based on the 4.5 years of Gravity Recovery and Climate Experiment (GRACE) data and 15 years of altimetry and in situ data (hydrographic and drifters data) (see aviso.oceanobs.com/en/data/products/auxiliary-products/mdt/). The geostrophic currents derived from ADT have been validated with drifter data in the Kuroshio region [Rio and Hernandez, 2004]. Although both tidal and sea level pressure corrections are incorporated, the data in shelf areas are still contaminated by aliases from tides and internal waves [Yuan et al., 2006]. Following the work of Yuan et al. [2006], the data in areas where the water depth is less than 200 m are excluded. [10] Monthly blended wind [Zhang et al., 2006] data from October 1992 to October 2009 at 1/4 resolution are obtained from The wind stress (t) is calculated using the same formula as that used by Wu [1982] and Cheng and Qi [2010]: t! ¼ rc D U!! 10 U 10; C D ¼ 0:8 þ 0:065U! ; ð1þ where C D is the drag coefficient, U 10 is wind velocity at 10 m above the sea surface, and r = 1.33 kg m 3 is the density of air. The WSC ( t y x differencing. t x y ) is then computed using center 2.2. Identification of Eddies [11] Different criteria have been used to identify eddies in the SCS. Hwang and Chen [2000] defined the 5 cm contour of dynamic height marked from the center of the eddy as the eddy edge. Wang et al. [2003] used the difference in the SSH anomaly between the center and the outermost enclosed contour greater than 7.5 cm to identify eddies. A census of eddy activities in the SCS during was conducted by Xiu et al. [2010] using the Okubo-Weiss [Okubo, 1970; Weiss, 1991] method. Following Isern-Fontanet et al. [2003] and Xiu et al. [2010], we use the objective criterion of the Okubo-Weiss parameter, defined as W ¼ ðs n Þ 2 þ ðs s Þ 2 w 2 ; where s n, s s, and w are the normal component of strain (i.e., stretching deformation), the shear component of strain (i.e., shearing deformation), and the relative vorticity of the flow, respectively, given by s n ¼ u v x y ; s s ¼ v x þ u y ; w ¼ v x u y : [12] The surface horizontal geostrophic velocities anomaly (u and v ) can be derived from SLA maps as follows: ð2þ ð3þ u ¼ g ðslaþ ; v ¼ g ðslaþ ; ð4þ f y f x where g is gravitational acceleration and f is the Coriolis parameter. This criterion allows us to separate the flow into different regions: vorticity-dominant regions (W < 0.2s w ), strain-dominant regions (W > 0.2s w ), and a background field (jwj 0.2s w ), where s w is the standard deviation in the entire domain of interest. The region of W < 0.2s w is designated as the eddy core. It is an ACE (CE) when the mean SLA is positive (negative) in the eddy core. The detailed eddy identification procedure (six steps) and eddy autotracking method were described by Xiu et al. [2010], and following the method of Xiu et al. [2010], only eddies with lifetimes longer than 4 weeks and spatial radii larger than 45 km were counted. [13] Other parameters to characterize eddies include the EKE and energy density (EI). The EKE is calculated as (u 2 +v 2 )/2, and the EI, following the work of Chen et al. [2011], is defined as the mean EKE in the eddy core, that is, EI = EKE pr, where R is the eddy radius. In addition, the 2 horizontal background geostrophic velocities (u, v) are derived from ADT as follows: u ¼ g ðadtþ ; v ¼ g ðadtþ : ð5þ f y f x 3. Results 3.1. Eddy Kinematic Characteristics [14] Taking the SCS basin as a whole, the number of ACEs was larger than the number of CEs based on shorter satellite time series from the 1990s [Hwang and Chen, 2000; 3of15

4 Table 1. Statistics of Kinematic Properties of CEs and ACEs Generated Southwest of Taiwan a Property Mean Standard Deviation Minimum Maximum Radius (km) 61/64 6.8/6.6 49/52 78/81 Lifetime (weeks) 7.2/ /7.0 5/5 20/39 SLA (cm) 9.3/ / / /19.5 EKE (cm 2 s 2 ) 360/ / / /954 Distance traveled (km) 247/ /362 52/97 678/1879 Propagation speed (cm s 1 ) 5.5/ / / /11.0 EI (10 3 cm 2 s 2 km 2 ) 31.2/ / / /65.7 Relative vorticity b 8.64/ / / / 6.88 Shearing deformation b 0.25/ / / /3.43 Stretching deformation b 0.74/ / / /4.54 a See the box in Figure 1. There are a total of 41 CEs and 27 ACEs. b The units of relative vorticity (w), shearing deformation (s s ), and stretching deformation (s n ) are 10 6 rad s 1. Wang et al., 2003], whereas the number of CEs and ACEs were comparable based on longer satellite time series from the last two decades [Xiu et al., 2010; Chen et al., 2011]. According to Wang et al. [2003], most eddies generated southwest of Taiwan are ACEs. However, in this study we find from the longer (October 1992 to October 2009) satellite time series that there were more CEs (41) than ACEs (27) generated southwest of Taiwan from 118 E to 121 E and north of 20 N (see the box in Figure 1). The analysis of eddy polarity by Chen et al. [2011] also showed that there were more CEs than ACEs generated southwest of Taiwan. Li et al. [2011] used drifter data from 1979 to the present to identify eddies in the northern SCS. Because of its sparse coverage compared with satellite altimeter data, drifter data might miss many eddies. Nevertheless, for eddies with radii larger than 60 km, there were more CEs (22) than ACEs (10). The reason why there are more CEs than ACEs is discussed in section 4. [15] Table 1 shows the kinematic properties of eddies generated southwest of Taiwan. ACEs are a little bigger (radius) and stronger (SLA and relative vorticity) than CEs and have a longer lifetime. According to the results of Xiu et al. [2010], the number of eddies with lifetimes longer than 150 days becomes considerably small. All 41 CEs generated southwest of Taiwan lasted fewer than 150 days, while several long-lived ACEs lasted more than 150 days. The averaged relative vorticities of CEs ( rad s 1 ) and ACEs ( rad s 1 ) southwest of Taiwan appear to be much stronger than those of CEs ( rad s 1 ) and ACEs ( rad s 1 ) for the entire SCS basin [Hwang and Chen, 2000]. The difference may be caused by different eddy definitions, since the Okubo-Weiss method used in this study depicts the eddy edge as the position where W = 0.2s w while Hwang and Chen [2000] defined the 5 cm contour of dynamic height as the eddy edge. The shearing and stretching deformations of ACEs are a little larger than those of CEs, indicating that ACEs are more deformed and noncircular. Statistically, 19 of 27 ACEs (about 70%) were generated from the KCL. The mean EKE and EI of ACEs were relatively larger than their CE counterparts, suggesting that ACEs likely obtained more kinetic energy from the Kuroshio. This may contribute to the intensity and sustenance of ACEs as on the average ACEs have longer mean lifetimes than those of CEs and can propagate farther, which agrees with the findings of Chen et al. [2011] that ACEs in the SCS basin as a whole are more stable than CEs. [16] Most mesoscale eddies propagate westward in the SCS [Xiu et al., 2010] and have propagation speeds similar to the first-mode baroclinic Rossby wave (10 cm s 1 )in the northern SCS [Wu and Chiang, 2007; Wang et al., 2008a; Bayler and Liu, 2008]. Almost all eddies that propagated westward to the Luzon Strait in the North Pacific are blocked by the island chain in the Luzon Strait and the Kuroshio axis. Figure 2 shows the propagation pathways of eddies southwest of Taiwan. There were 31 CEs and 22 ACEs propagating westward, which means that eddies disappeared west of the formation position, disregarding possible eastward and westward wobbling in between. However, Figure 2. Pathways of (a) CEs and (b) ACEs generated southwest of Taiwan. Blue and red lines represent trajectories of eastward and westward propagating eddies, respectively. Black dots and triangle represent positions of eddy formation and Dongsha Islands, respectively. 4of15

5 Figure 3. Seasonal patterns of (left) CE and (right) ACE occurrences (%), quantified as the frequency of an eddy presence at each pixel in separate seasons: (a, b) spring, (c, d) summer, (e, f) autumn, and (g, h) winter. 5of15

6 Figure 4. Monthly (shaded) and yearly (dotted solid line) variations of (a) CE occupied area (CEOA), (b) ACE occupied area (ACEOA), and (c) total eddy occupied area (TEOA) in the box (see Figure 1d) from October 1992 to October Units are 10 3 km CEs and 5 ACEs propagated eastward. None of the CEs passed the Dongsha Islands, whereas five long-lived ACEs propagated westward more than 500 km, bypassed the Dongsha Islands, and reached the interior of the SCS. The reason why ACEs likely propagate farther westward will be discussed in section 4.3. Following the Kuroshio, two CEs generated in the northern Luzon Strait propagated northeastward and moved out of the SCS east of the southern tip of Taiwan (Figure 2a). The mean propagation speed of all 27 ACEs (6.4 cm s 1 ) is a little faster than that of the 41 CEs (5.5 cm s 1 ). The propagation speed of eddies varies from 2 to 11 cm s 1 during their lifetime. The radii, SLAs, and relative vorticities of eddies also vary during their lifetimes (not shown), suggesting complex interactions between eddies and the ambient currents in the northern SCS, which also is discussed in section Seasonal Characteristics of Eddy Occurrence [17] In this section, we focus on the seasonal variability of eddy characteristics southwest of Taiwan. From October 1992 to October 2009, there were 12, 8, 4, and 17 CEs generated in spring, summer, autumn, and winter, respectively. As well, there were 5, 5, 4, and 13 ACEs generated in spring, summer, autumn, and winter, respectively. Winter is the favorite season for eddy generation, whereas autumn is the opposite. Eddy shedding from the KCL dominated the ACEs. A careful examination of the weekly background geostrophic current maps indicated that there were 2, 1, 3, and 13 ACEs shed from the KCL in spring, summer, autumn, and winter, respectively. In other words, all of the 13 ACEs generated in winter were shed from the KCL. Among the 17 CEs generated in winter, 7 were generated around the KCL, and another 7 were generated to the east of the ACEs shed from the KCL. [18] The seasonal eddy occurrence, calculated as the frequency of an eddy presence at each pixel during each season of the entire time series from October 1992 to October 2009, is shown in Figure 3. Among the centers of high eddy occurrence, two to the southwest of Taiwan stand out: One is the high ACE occurrence in winter (Figure 3h) and another is the high CE occurrence in summer (Figure 3c). The center of high ACE occurrence in winter with a maximum frequency of presence at 23.3% was mainly caused by ACE shedding from the KCL, because all 13 ACEs generated in winter were shed from the KCL. The locations of high ACE occurrence are basically consistent with the result of Jia and Liu [2004] that the most frequent eddy shedding location is between E and 120 E. The probability of ACE occurrence decreased gradually from this center to the Dongsha Islands, which also illustrates the prevailing propagation path of these eddies. There were CEs that occurred around this center at 120 E, 20 N), E, 22 N), and E, 22 N (Figure 3g) in winter, but the probability of CE occurrence with the maximum at 15.5% was lower than that of ACEs. [19] In summer, there was a center southwest of Taiwan having a high probability of CE occurrence with the maximum at 22.0%, which was not shown in prior works to the best of our knowledge. The mean WSC was weakly negative in summer southwest of Taiwan, which could not produce CEs through linear dynamics. The reason why there was a center with high CE occurrences in summer is shown in section 4.2. The CE occurrence center was confined to the southwest of Taiwan, which again illustrates that the CEs generated in summer did not propagate far in accordance with Figure 2a. In contrast, there was almost no ACE occurring in this region during the summer. Instead, the maximum ACE occurrence of 12.1% appeared to the west of the center of high CE occurrence. In spring, there were a few more CEs than ACEs. Autumn was the season with the fewest eddies generated and lowest eddy occurrence Year-to-Year Variations of Eddy Activity [20] Figure 4 shows the monthly time series of eddy occupied area in the box (see Figure 1) from October 1992 to October Tick marks along the time axis indicate the 6of15

7 Figure 5. Seasonal patterns of WS (N m 2) and WSC (10 8 N m 3) averaged over the 17 years: (a) spring, (b) summer, (c) autumn, and (d) winter. beginning of a year. The periods with no eddy presence spans 18% of the time over the entire 17 years, most of which appeared after CEs and ACEs often co-occurred in this area (Figures 4a and 4b), which can also be deduced from Figure 3. The combined appearance of CEs and ACEs accounts for 48% of the entire time series. The periods of CE only and ACE only are comparable, each possessing 17% of the entire time series. [21] The total area of the box as shown in Figure 1 is 94,400 km2. Figure 4c shows the variations of the total eddy occupied area (TEOA). The mean and maximum yearly TEOAs are 10,540 and 16,200 km2, respectively, accounting for 11% and 17% of the total area of the box, respectively. Seasonal and interannual variations are clear for both the CE occupied area (CEOA) and the ACE occupied area (ACEOA) (Figures 4a and 4b). Spectrum analysis of CEOA, Figure 6. Monthly averaged (a) WSC (10 8 N m 3) and (b) GV (10 6 rad s 1) of the ambient current in the box (see Figure 1d) from October 1992 to October The red line and blue line in Figures 6a and 6b represent positive and negative values, respectively. 7 of 15

8 Table 2. Correlation Coefficient (R) and Corresponding Significance (P) Among Monthly Variations of WSC, GV, and Eddy Occupied Area a PWSC NWSC PGV NGV CEOA ACEOA PWSC 1 NWSC 0.29/ PGV 0.29/ / NGV 0.17/ / / CEOA 0.03/ / / / ACEOA 0.09/ / / / / a If P is less than 0.05, the correlation R is significant. The significant R values are bolded in the table. PWSC, positive WSC; NWSC, negative WSC; PGV, positive GV of the ambient current; NGV, negative GV of the ambient current; CEOA, CE occupied area; ACEOA, ACE occupied area. ACEOA, and TEOA time series suggests that the annual and semiannual signals dominate the variations (figures not shown). They also have several secondary peaks with periods of 90, 120, 150, and 240 days. Interannual variations of ACEOA (Figure 4b) are similar to the occurrence of the Kuroshio looping path variations [Nan et al., 2011a, Figure 5B], indicating that most ACEs are likely generated from the KCL. A noticeable trend is a smaller ACEOA between 2005 and During the strong El Niño year of , the CEOA is larger, while the ACEOA is smaller, which is caused by the weakened KCL during In contrast, the ACEOA is larger in 1996 and 2000, a result of a stronger KCL before and after the El Niño [Nan et al., 2011a, Figure 2]. The TEOA has a decreasing trend after 2000, which is perhaps related to the decreasing trend of the number of eddies in the entire SCS [Chen et al., 2011] and a less-frequent of KCL after 2000 [Yuan et al., 2006]. 4. Discussion 4.1. Wind Variations and Eddy Formation [22] This section focuses on the effect of local wind on the eddy formation southwest of Taiwan. The seasonally reversing monsoon winds in the SCS typically blow from the northeast during boreal winter and from the southwest during boreal summer, and the monsoon is stronger in winter but weaker in summer. Figure 5 shows the seasonal patterns of the WS and WSC in the vicinity of the Luzon Strait. The WS and WSC are larger in winter and autumn, but smaller in spring and summer. Because of the orographic effect, negative WSC (NWSC) and positive WSC Figure 7. Seasonal patterns of geostrophic current (m s 1 ) derived from ADT (cm) averaged over the 17 years: (a) spring, (b) summer, (c) autumn, and (d) winter. 8of15

9 Figure 8. Monthly Ekman transport (blue; 10 6 m 3 s 1 ) and surface transport (red; 10 4 m 3 s 1 ) through the Luzon Strait from October 1992 to October (PWSC) coexist in winter southwest of Taiwan. Conceptually, the PWSC (NWSC) produces divergence (convergence) in the surface water, which induces upwelling (downwelling) and hence generates CEs (ACEs) [Hwang and Chen, 2000]. By comparing ACE generation location and NWSC distributions, Wang et al. [2008b] supported the concept that the NWSC may be an important mechanism for ACEs formed southwest of Taiwan in winter. Similarly, it is found in the present study that the region with higher ACE occurrence (Figure 3h) corresponds to a strong NWSC (Figure 5d) in winter, but this is also where the consistency stops. The WSC pattern in autumn is similar to that in winter except for being a little weaker. Yet the eddy occurrence pattern in autumn (Figures 3e and 3f) is quite different from that in winter. On the other hand, WSC southwest of Taiwan is the weakest in summer (Figure 5b), while the CE occurrence is the highest (Figure 3c). WSC west of Luzon Island is large and positive in spring, autumn, and winter. However, the CE occurrence is not high. These inconsistencies between the seasonal WSC patterns and seasonal eddy occurrence patterns lead us to conclude that although it may contribute to eddy formation by changing SLA through Ekman pumping, the WSC cannot be the sole control factor for eddy formation in this area. [23] To quantify the effect of local WSC on eddy formation, the time series of monthly mean WSC and GV averaged in the boxed area shown in Figure 1 are shown in Figure 6. Because PWSC and NWSC often coexist, positive (red lines) and negative (blue lines) values were calculated separately. A correlation analysis among monthly WSC, GV, CEOA, and ACEOA was conducted (Table 2). The correlation coefficient between PWSC and NWSC is 0.29 (above 95% significance level). A peak-to-peak comparison of PWSC and NWSC also shows that the PWSC and NWSC often coexist, especially in autumn and winter (Figures 5 and 6a). PWSC and NWSC have a positive correlation with positive GV (PGV) and negative GV (NGV), respectively. However, the correlation coefficients are less than 0.30, indicating that the local WSC has limited effects on the GV southwest of Taiwan. The correlation coefficients between the WSC and the eddy occupied areas are not significant. On the other hand, the correlation coefficient between NGV and ACEOA is 0.63, suggesting that ambient current variations play an important role in the ACE formation southwest of Taiwan, which is consistent with the results shown in section 3.2, i.e., that most ACEs are shed from the KCL. However, the correlation coefficient between PGV and CEOA is 0.34, indicating that in addition to ambient current variations, there might be other factors (such as interactions with ACEs) affecting the CE formation. Figure 9. Geostrophic current (m s 1 ) derived from ADT (cm) averaged from the times (a) when CEs are generated in summer and (b) when ACEs are generated in winter. 9of15

10 Figure 10. Schematic showing the Kuroshio paths near the Luzon Strait and formation of CEs and ACEs southwest of Taiwan (a) in summer and (b) in winter Kuroshio Path Variations and Eddy Formation [24] The large-scale circulation in the SCS is driven mainly by the East Asian monsoon, with significant influence from the Kuroshio intrusion on the northern SCS [e.g., Qu, 2000; Su, 2004; Xue et al., 2004]. The Kuroshio intrusion through the Luzon Strait is important to not only the general circulation but also eddy formation in the northeastern SCS [e.g., Li et al., 1998; Wu and Chiang, 2007; Xiu et al., 2010]. Metzger and Hurlburt [2001] showed that the flow instability that is due to the Kuroshio intrusion dominates in the area southwest of Taiwan compared with the wind forcing. Figure 7 shows the seasonal pattern of geostrophic current in the vicinity of the Luzon Strait. Figure 8 shows the variation of the monthly, wind-induced Ekman transport (calculated using an Ekman depth of 65 m near the Luzon Strait) and the surface transport through the Luzon Strait. The surface transport, calculated using the surface geostrophic current across the E meridian multiplied by the width of the Luzon Strait and 1 m in depth, is negative in winter and positive in summer, indicating that at the surface the Pacific water mainly flows into the SCS in winter and the SCS water flows out in summer. The Kuroshio forms a loop in winter southwest of Taiwan (Figure 7d). In summer, there often exists an outflow northwest of the Luzon Island, and the Kuroshio likely leaps across the Luzon Strait (Figure 7b), which is the reason why the surface transport is positive in summer. This seasonal intrusion feature has been shown in previous studies [e.g., Shaw, 1991; Chern and Wang, 1998]. The correlation coefficient between the Ekman transport and the surface transport is 0.77 (above 95% significance level), suggesting that monsoon winds play an important role for the seasonal variation of the Kuroshio intrusion into the SCS. According to Nan et al. [2011a], the vertical depth of the KCL can reach 500 m in depth. The SCS summer outflow above 300 m was observed by current profile deployed north of Luzon Island [Liang et al., 2008]. In winter, the northeasterly monsoon plays an important role in the formation of the winter KCL, as suggested by Farris and Wimbush [1996], whereas in summer the southwesterly monsoon drives the outflow north of the Luzon Island, as suggested by Liang et al. [2008]. [25] Compared with spring and autumn, summer and winter have a higher probability of eddy occurrence. We thus turn our attention to relationships between the Kuroshio path variations and eddy formation in summer and winter, respectively. Figure 9 shows the mean geostrophic current averaged from the times when CEs are generated in summer and when ACEs are generated in winter. In summer, ambient flows are anticyclonic northwest of the Luzon Island. The SCS water flows out of the SCS in the southern part of the Luzon Strait. The SCS western boundary current flows eastward at 18 N toward the deep basin in summer [Nan et al., 2011b]. This branch can meander to the Luzon Strait, which may be the main source of the SCS outflow water in summer. When CEs are formed in summer, the SCS water outflows in the southern part of the Luzon Strait, while the averaged Kuroshio axis leaps across the Luzon Strait because of the SCS outflow (Figure 9a). CEs are often induced north of the SCS outflows Figure 11. The (a) first and (b) second EOF spatial mode of the ambient current GV. 10 of 15

11 Figure 12. The amplitude time series associated with the two spatial modes in Figure 11: (a) the first mode and (b) the second mode. Blue and red dots in Figure 12a denote CE and ACE formation dates in summer and winter, respectively. and left of the Kuroshio leaping axis. An ACE is often present to the northwest of the CE. [26] In winter, the circulation in the SCS is mainly cyclonic, driven by the northeasterly monsoon. The KCL often appears southwest of Taiwan in winter (Figure 7d) because of the winter monsoon. When ACEs form in winter (Figure 3h), the averaged Kuroshio axis makes a loop southwest of Taiwan and forms the KCL (Figure 9b). As mentioned in section 3.2, 13 ACEs generated in winter were all shed from the KCL and moved westward after being shed. As the ACEs moved westward, CEs were sometimes induced east of the ACEs (see Figure 3g). Moreover, among those 13 ACEs generated in winter, 5 lasted into spring, which may be the reason why in spring the CE occurrence remained relatively high (Figure 3a). In conclusion, the eddy occurrence patterns seen in Figures 3c and 3h are basically in accordance with the Kuroshio path patterns in summer and winter, suggesting that the Kuroshio path variations play a dominant role in eddy formation southwest of Taiwan. [27] As a summary of the analyses above, we propose a conceptual model of eddy-kuroshio interaction (Figure 10) to show the coevolution of the Kuroshio path and eddy formation in summer and winter, respectively. In summer (Figure 10a), there exists an outflow northwest of the Luzon Island, and the Kuroshio likely leaps across the Luzon Strait. Such a flow regime favors the generation of ACEs northwest of Luzon Island. To the north of the outflow and left of the Kuroshio axis, CEs are often formed, which in turn induce ACEs to the west of CEs. In winter (Figure 10b), under the influence of the northeasterly monsoon, the KCL appears southwest of Taiwan more frequently than during other seasons, and consequently more ACEs are shed from the KCL. Surrounding the KCL, CEs often appear. Most of the ACEs propagate westward. To the east of the ACEs, it may induce CEs. In this model, we emphasize that variation of the Kuroshio path is the control factor for the eddy formation southwest of Taiwan, whereas the direct effect of local wind is less important (Table 2). However, the SCS outflow in summer and the KCL in winter are intimately related to the monsoons. Therefore, the eddy occurrence has apparent seasonal variations (Figure 3), indicative of indirect monsoon effects. [28] An empirical orthogonal function (EOF) analysis of GV depicts the dominant patterns of the ambient current. Figures 11 and 12 show the first two spatial modes and their corresponding amplitude time series, respectively. Spectral Figure 13. Spectral analysis of the two amplitude time series in Figure 12: (a) the first mode and (b) the second mode. The red dotted line denotes a 95% significance curve. 11 of 15

12 Figure 14. Geostrophic current (m s 1 ) derived from ADT (cm) of (left) one CE and (right) one ACE generated in July 1996 and December 2007, respectively. Evolutions (formation time, 2 weeks, 4 weeks, and 6 weeks) of (a d) the CE and (e h) the ACE are shown. 12 of 15

13 analysis of the amplitude time series is shown in Figure 13. The annual signal dominates in mode 1, which accounts for 24% of the total variance. The spatial distribution of mode 1 (Figure 11a) shows a positive anomaly next to the southern tip of Taiwan. The corresponding amplitude shows that the center has negative vorticity with the maximum occurring in winter, indicating that KCL dominates the ambient current field in winter. The KCL sometimes leads to ACE shedding in winter (red dots in Figure 12a), but not every winter. There are three positive vorticity centers with smaller magnitudes around the anticyclonic center, which is consistent with the CE occurrence pattern in winter (Figure 3g) and supports the eddy-kuroshio interaction conceptual model given in Figure 10b. Because of the dominant annual signal, the spatial distribution of mode 1 flips in summer. In summer, the anticyclonic center with negative vorticity changes to a positive center. This positive vorticity center sometimes leads to CE formation in summer (blue dots in Figure 12a). The amplitude of the summer positive center is smaller than that in winter, and the number of CEs in summer is fewer than the ACEs in winter. This positive vorticity center and the negative vorticity center to its southwest coexist, which also supports the eddy-kuroshio interaction conceptual model given in Figure 10a. The other two negative vorticity centers to the south and east of the positive vorticity center may correspond to anticyclonic gyres to the right of the SCS outflow in summer Eddy Propagation [29] One typical CE generated in July 1996 (Figure 14, left) and one typical ACE generated in December 2007 (Figure 14, right) are selected to show the propagation after the CE and ACE generated in summer and winter, respectively. ADT and the corresponding geostrophic currents at the time of eddy formation 2, 4, and 6 weeks later are shown. The CE in summer was generated north of the outflow present off the northwest Luzon Island and left of the Kuroshio axis. To the west of the CE, there existed an ACE as the CE evolved. Because of the summer outflow and the ACE to the west, the CE did not propagate much during 6 weeks, which is also the reason why the CE occurrence center is confined to the southwest of Taiwan in Figure 3c. [30] The ACE in winter was shed from the KCL. After being shed, it propagated southwest. It is more likely for ACEs to propagate westward into the interior of the SCS because of southwestward background flows in the northern SCS. As mentioned in section 3.1, five long-lived ACEs can propagate to the interior of the SCS. Among the five longlived ACEs, three were shed from the KCL between 120 E and 121 E in December 1992, December 2001, and January After being shed from the KCL, they all propagated westward and reached the Dongsha Islands (118 E) in February, then moved southwest along the northern shelf of the SCS. The other two of the five long-lived ACEs were generated to the southeast of the Dongsha Islands (118.5 E, 20.5 N) in April 1993 and September All together, four of the five long-lived ACEs propagated to the Dongsha Islands (118 E) in winter and one in spring. It is easier for four eddies to propagate westward into the interior of the SCS because of southwestward prevailing flows in the northern SCS in winter. In spring, the southwestward prevailing flows become weaker due to weaker northeast monsoon (Figure 7b). The ACE generated in April 1993 also propagated to the interior of the SCS, but had the shortest lifetime among the five long-lived ACEs. To the east of the ACE, a CE was spun up as the ACE moved away (Figure 14h). [31] There exist eddy-eddy interactions as eddies evolve (Figure 14). Mode 2 accounts for 19% of the total variance. Its spatial distribution (Figure 11b) shows a positive vorticity center east of the Dongsha Islands. Around this positive vorticity center there are negative vorticity centers. Some GV patterns of mode 2 (Figure 12b) with maximum or minimum amplitude were examined (figures not shown). It can be seen that CEs and ACEs often coexist. Around strong and big CEs or ACEs, eddies often exist that are weaker and smaller ACEs or CEs. The shearing and stretching deformations of eddies vary considerably as eddies evolve (Table 1). Eddies are often deformed and noncircular, which may be caused by eddy-current and eddy-eddy interactions. Mode 2 has five main periods shorter than one year and they are 40, 60, 90, 130, and 180 days (Figure 13b), which may also indicate the complexity of eddy-eddy interactions. On the other hand, tidal aliasing may cause some of the significant peaks in altimetry data, e.g., the 62 day and 173 day aliasing periods that are due to M 2 and K 1, respectively, in TOPEX/POSEIDON [Chelton et al., 2001]. However, the AVISO products (both ADT and SLA) are composites from multiple altimeters with different orbital altitudes, inclinations, and repeat periods. The significance of tidal aliasing and the tidal aliasing periods are not readily available. 5. Summary and Conclusions [32] The weekly satellite data of 17 years are used to study the eddy formation and propagation southwest of Taiwan. Eddies with lifetimes longer than 4 weeks and spatial radii larger than 45 km are identified using the objective criterion of the Okubo-Weiss parameter. Unlike the predominant ACEs southwest of Taiwan suggested by Hwang and Chen [2000] and Wang et al. [2003], because they mainly focused on ACE shedding from the KCL, we found that there were more CEs (41) than ACEs (27) generated in this area during the period from October 1992 to October Nevertheless, ACEs were indeed stronger and lived longer than CEs. Our analyses also showed that CEs and ACEs often co-occurred. CEs were confined to east of Dongsha Islands, while some long-lived ACEs could propagate westward from the Luzon Strait to the interior of the SCS. Eddies deformed during their propagation processes because of complex interactions between eddies and the ambient currents in the northern SCS. [33] Eddy occurrence in this area had a notable seasonal pattern. Summer and winter were the favorite seasons for eddy formation, while autumn was the season with fewest eddies generated and the lowest eddy occurrence. ACEs appeared more often in winter than in other seasons, while CEs were more frequent in summer. Monthly blended wind data from October 1992 to October 2009 were used to investigate the effects of wind on eddy formation. A correlation analysis of the local WSC and eddy activities suggested that the effect of local WSC on eddy occurrence was 13 of 15

14 limited. A comparison of ambient geostrophic currents and eddy occurrence patterns shows that the Kuroshio path variation is likely the control factor for eddies forming in this area. Based on the statistics of eddy occurrence and the Kuroshio path variation, a conceptual model of eddy- Kuroshio interaction (Figure 10) is proposed for the summer and winter, respectively. [34] 1. In summer, there exists an outflow northwest of Luzon Island. The Kuroshio likely leaps across the Luzon Strait. To the north of the outflow and left of the Kuroshio axis, CEs are often formed, which often induce ACEs to the west of the CEs. [35] 2. In winter, the KCL appears southwest of Taiwan more frequently than during other seasons, and ACEs are frequently shed from the KCL. As most of the ACEs propagate westward, CEs are often induced east of the ACEs. [36] In this model, variability of the Kuroshio path is emphasized as the control factor for eddies forming southwest of Taiwan, while the direct effect of local wind is less important. However, monsoons are important in driving the SCS outflow in summer and the KCL in winter, both of which have important effects on the seasonal patterns of the eddy occurrence. EOF analysis of the ambient GV also supports the proposed eddy-kuroshio interaction model. [37] The results of this paper are based exclusively on weekly satellite observations with 1/4 resolution. The data in areas where the water depth is less than 200 m are excluded in this study. Although only a few eddies may propagate from deep water to the northern slope, the trajectories less than 200 m of those eddies are not shown. Those eddies smaller than 45 km or eddies lasting for shorter than one month are also ignored. In addition, three-dimensional eddy dynamics may be important for eddy properties. 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