Eddy Shedding from the Kuroshio Bend at Luzon Strait

Transcription

1 Journal of Oceanography, Vol. 60, pp to 1069, 2004 Short Contribution Eddy Shedding from the Kuroshio Bend at Luzon Strait YINGLAI JIA* and QINYU LIU Physical Oceanography Laboratory and Ocean-Atmosphere Interaction and Climate Laboratory, Ocean University of China, Qingdao , P.R. China (Received 27 June 2003; in revised form 3 January 2004; accepted 9 January 2004) TOPEX/POSEDIENT-ERS satellite altimeter data along with the mean state from the Parallel Ocean Climate Model result have been used to investigate the variation of Kuroshio intrusion and eddy shedding at Luzon Strait during The Kuroshio penetrates into the South China Sea and forms a bend. The Kuroshio bend varies with time, periodically shedding anticyclonic eddies. Criteria of eddy shedding are identified: 1) When the shedding event occurs, there are usually two centers of high Sea Surface Height (SSH) together with negative geostrophic vorticity in the Kuroshio Bend (KB) area. 2) Between the two centers of high SSH there usually exists positive geostrophic vorticity. These criteria have been used to determine the eddy shedding times and locations. The most frequent eddy shedding intervals are 70, 80 and s. In both the winter and summer monsoon period, the most frequent locations are E and 120 E, which means that the seasonal variation of eddy shedding location is unclear. Keywords: Luzon Strait, Kuroshio bend, eddy shedding. 1. Introduction The Luzon Strait (about 3 degrees in width), the main connection between the South China Sea (SCS) and the North Pacific Ocean, is an obvious gap on the western boundary of the North Pacific (Fig. 1). The western boundary current (Kuroshio) penetrates into the SCS, crossing the gap (Luzon Strait). Many researchers have studied on the characteristics of the Kuroshio s penetration in the Luzon Strait. Based on analyses of historical data, Li and Wu (1989) speculated that there was a Kuroshio Current Loop in the SCS. They suggested that a branch of the Kuroshio would turn west at the southern Luzon Strait, curve to the east and then flow out of the SCS at the northern Luzon Strait. Li, L. et al. (1998) studied the Kuroshio penetration using data from two CTD surveys conducted in March 1992 and September They found that the westward intrusion of the Kuroshio could only reach 119 E. An anticyclonic ring of probable Kuroshio origin was then observed at 21 N, E by Li, L. et al. (1998) from a CTD survey conducted in September, They further speculated that there were intermittent intrusions of the Kuroshio into the SCS and intermittent eddy shedding from the Kuroshio (Fig. 1). Metzger and Hurlburt * Corresponding author. jiayingl@ouc.edu.cn Copyright The Oceanographic Society of Japan. (2001) studied eddy shedding from the Kuroshio using a high-resolution model. They found that eddy sheddings were rare events when the model was forced by the reanalysis wind from the European Center for Medium- Range Weather Forecasts (ECMWF), but were persistent events when forced by the operational wind. Using TOPEX/POSEIDON altimeter data, Li et al. (2002) found anticyclonic eddies which were shed from the Kuroshio in the north SCS. However, because the mean background circulation (the Kuroshio) cannot be reflected by altimeter data, the eddy shedding process from the Kuroshio remains unclear. The spatio-temporal variation of the eddy shedding is unknown. In this paper, the mean state of Kuroshio bend from two models is discussed first. Secondly, the 9-year Sea Surface Height (SSH) field is obtained from the sum of the simulated mean SSH and the anomaly from T/P-ERS altimeter data. Thirdly, an eddy shedding process is discussed and criteria for eddy shedding are obtained. Using the criteria, all the eddy shedding times and locations are obtained during the 9-year period studied. Finally, the spatio-temporal variations of eddy shedding are discussed. 2. Data The Maps of Sea Level Anomaly (MSLA) altimeter products are produced by the CLS (Collecte, Localisa- 1063

2 China Anticyclonic eddy South China Sea Taiwan Philippines North Pacific Luzon Strait Kuroshio Fig. 1. Diagram showing the penetration of Kuroshio and an anticylonic eddy separated from the Kuroshio based on the speculation on eddy shedding by Li, L. et al. (1998). tion, Satellites) Space Oceanography Division as part of the European Union s Environment and Climate project. CD ROMs are produced by the AVISO (Archiving, Validation and Interpretation of Satellites Oceanographic data)/altimetry operations center. The ERS products are generated as a part of the proposal Joint analysis of ERS- 1, ERS-2 and TOPEX/POSEIDON altimeter data for oceanic circulation studies. The MSLA products merged with ERS (35-day repeat orbit periods) and TOPEX/ POSEIDON (10-day repeat orbit periods) data sets provide better space-time sampling of ocean features. The data sets is corrected for instrumental errors, environmental perturbations, ocean wave influence, tide influence and inverse barometer effect. Orbit errors are then estimated. The MSLA data is obtained using an improved space/time objective analysis method which takes long wavelength errors into account. The estimated noise is about 2 cm rms. MSLA products are available at 10 day intervals, cm (a) (c) Fig. 2. (a) Mean pattern of SSH covering from POCM. Rectangle marks the KB area (18.75 N 22 N, E 123 E). SSH contour interval is 5 cm. Mean SSH and upper-layer currents from the 1/8, 6-layer finite depth thermodynamic model of the Pacific Ocean north of 20 S with realistic bottom topography. SSH contour interval is 5 cm and reference vector at the top is 1 ms 1 (c) Geostrophic vorticity of mean SSH from POCM result. Geostrophic vorticity contour interval is s 1. ( is taken from Metzger and Hurlburt (2001).) 1064 Y. Jia and Q. Liu

3 (a) Fig. 3. (a) Surface geopotential anomaly in dynamic meters relative to 1000 db in the period from August 28 to September 10, 1994, SSH (contour) and corresponding geostrophic vorticity (shaded area) in the period from August 28 to September 7, The SSH contour interval is 5 cm. Dark shading is geostrophic vorticity less than s 1. Light shading is geostrophic vorticity greater than s 1. ((a) is taken from Li, L. et al. (1998)). with a resolution from October 1992 to August The Parallel Ocean Climate Model (POCM) output was produced during a simulation run performed by Robin Tokmakian of the Department of Oceanography at the Navy Postgraduate School at NCAR, in collaboration with Bert Semtner. The fields we use are from the 20 year (79 98) POCM_4C run. The POCM_4c is forced by ECMWF reanalysis fields, heat and freshwater flux. The POCM_4c has a horizontal resolution of about 0.25, 20 layers in vertical, and a free surface (Semtner and Chervin, 1992). The POCM result used in this paper is from 1992 to 1996, with a 10 day interval. 3. Mean State of Kuroshio Bend The mean SSH and geostrophic vorticity from POCM is shown in Figs. 2(a) and (c). A branch of the Kuroshio enters the South China Sea in the northwest direction at 20 N, 121 E south of Luzon Strait, deflects at about (21 N, 118 E) then flows out of the Luzon Strait. The deflected branch of the Kuroshio is called the Kuroshio bend. There is anticyclonic geostrophical vorticity at the northern Kuroshio bend and cyclonic vorticity at the southern end (Fig. 2(c)). The area occupied by the Kuroshio bend is called the KB area ( N, E) in this paper. The mean SSH field of the 1/8, 6-layer Naval Research Laboratory Layered Ocean Model (NLOM) result (Metzger and Hurlburt, 2001) shows that the Kuroshio penetrates to about the same longitude as that in the POCM (Fig. 2). Comparing the result from the two models (Figs. 2(a) and ), it is found that both the mean SSH and the shape of the Kuroshio bend are similar. Moreover, the simulated SSH from the POCM agrees well with the T/P data (Stammer et al., 1996; Li, W. et al., 1998), so the mean SSH from the POCM is used as a substitute for the mean observation relative to the anomaly from altimeter data (Liu et al., 2001). The SSH used in this paper is the sum of the mean from the POCM and the anomaly from T/P-ERS altimeter data. 4. Spatio-Temporal Variation of Eddy Shedding 4.1 Eddy shedding process Li, L. et al. (1998) captured an anticyclonic, separated ring from the CTD data of a section survey from August 28 to September 10, 1994 (Fig. 3(a)). The surface geopotential anomaly field (Fig. 3(a)) reveals an anticyclonic ring in the area (20 22 N, E) with a scale of about 150 km. According to Li, L. et al. (1998), this ring is detached from the Kuroshio. The SSH field during the same period also shows an anticyclonic eddy with similar scale at (20 22 N, E) (Fig. 3). The location of the anticyclonic ring from the altimetry data accords well with that from the in situ data, except for a difference of about 0.5 degree in longitude. This difference may be caused by the low zonal resolution of the altimeter data. Figure 3(a) shows two cyclonic eddies between the anticyclonic ring and the Kuroshio, so the anticyclonic ring seems to have separated from the Kuroshio. In Fig. 3, there is only one cyclonic eddy at ( N, E) and the anticyclonic ring is still connected with the Kuroshio (Fig. 3). There is no eddy, but cyclonic geostrophic vorticity in Fig. 3 at the location ( N, E). At the same location there is a cyclonic eddy with a scale of about 100 km, Fig. 3(a). The cyclonic eddy may be too small to be recognized in the altimeter data, which means that the coarse resolution of the altimetry data allows only obvious anticyclonic eddy separation to be recognized. From the SSH data with an interval of 10 days extending over 9 years, we find that both the SSH and the Eddy Shedding from Kuroshio Bend 1065

4 (a) Fig. 4. (a) rms of SSH from T/P-ERS data. Contour interval is 1 cm. rms of SSH greater than 8 cm is shaded. rms of geostrophic vorticity calculated from SSH from T/P-ERS data. Contour interval is s 1, rms of geostrophic vorticity greater than s 1 is shaded. Rectangle marks the KB area (18.75 N 22 N, E 123 E). (a) (c) (d) Fig. 5. SSH from T/P-ERS satellite altimeter data (contour) and corresponding geostrophic vorticity (shaded area). Contour interval of SSH is 5 cm. Dark shading is geostrophic vorticity less than s 1. Light shading is geostrophic vorticity greater than s 1. Rectangle marks the KB area (18.75 N 22 N, E 123 E). geostrophic vorticity varies greatly in the KB area, which means that eddy is more active in the KB area than in other areas (Figs. 4(a) and ). Forthermore, the rms of the SSH anomaly is about cm, which is much larger than the estimated noise of 2 cm rms for the T/P-ERS altimeter data. This means that the results from the SSH are not much influenced by the noise. Furthermore, the variation of the geostrophic vorticity (Fig. 4) is comparable to the mean geostrophic vorticity (Fig. 2(c)), while the variation of the SSH (Fig. 4(a)) is only about 10% of the mean SSH (Fig. 2(a)). This means that mesoscale eddies are more easily to identify from the geostrophic vorticity field. In this paper, therefore, the geostrophic vorticity field as well as the SSH are used to identify eddies. Anticyclonic eddies separated intermittently from the Kuroshio from 1992 to An example of the process of the variation of the Kuroshio bend and eddy shedding is shown in Fig. 5, where the darker shaded area denotes anticyclonic eddies and the lighter shaded area denotes cyclonic eddies. The eddy shedding process began on January 29, 1999 (Fig. 5(a)). To the east of 122 E there was an area with large a SSH gradient and negative geostrophic vorticity, which indicates the Kuroshio bend, 1066 Y. Jia and Q. Liu

5 Number Number summer winter day E Eddy Shedding Interval Longitude Fig. 6. Frequency of eddy shedding intervals in the 9 years from 1992 to x-coordinate shows the eddy shedding interval. y-coordinate shows the frequency. while to the west of the Kuroshio bend there was existed an anticyclonic eddy (centered at about 21 N, 120 E). After January 29, the Kuroshio bend began to extend westward (figure not shown) and the anticyclonic eddy west of the Kuroshio bend began to decay. On 28 February, 30 days later, the west end of the Kuroshio bend was merged with the anticyclonic eddy (Fig. 5) at about 119 E and then extended westwards to about 118 E (Fig. 5(c)) after another 30-day interval. In the next 20 days (on 19 April), the bend of the Kuroshio was again divided into two parts by a band of positive vorticity (Fig. 5(d)). The western part of the Kuroshio bend became an anticyclonic eddy with radius about 100 km centered at (21.5 N, 120 E) and the eastern part of the bend (the main flow of the Kuroshio) retreated back to the east of the Luzon Strait. The moment that the Kuroshio bend is divided into two parts by positive vorticity is called the eddy shedding time (as at April 19, 1999 and January 29, 1999) and the eddy shedding interval is defined as the time between two consecutive eddy sheddings, which is a period of 80 days in the former example (starting on January 29 and ending on April 19, 1999). 4.2 Eddy shedding criterion The SSH data revealed many eddy shedding events from October 1992 to August 2001, all of them having similar processes to the example discussed in Subsection 4.1, which means that eddy shedding criteria can be identified. The characteristics of the eddy shedding processes reveal that the distribution of the SSH and geostrophic vorticity during eddy shedding differs greatly from those before such an event. When an anticyclonic eddy separated from the Kuroshio bend, there are usually two centers of high SSH with negative geostrophic vorticity in the KB area. One center is located inside the Kuroshio bend east of the Luzon Strait and the other inside the anticyclonic eddy west of the Luzon Strait. Between the two Fig. 7. Frequency of eddy shedding locations in the 9 years from 1992 to x-coordinate shows the location of eddy shedding (longitude). y-coordinate shows the frequency. White bar shows locations of eddies shed in summer monsoon period. Gray bar shows locations of eddies shed in winter monsoon period. centers there is usually a positive geostrophic vorticity. However, during the westward extendsion of the Kuroshio bend, the SSH inside the Kuroshio bend increases monotonously eastward usually with one high SSH center in the KB area. The criteria of the eddy shedding can therefore be listed as follows: 1) In the KB area, there must be two centers of high SSH with negative geostrophic vorticity, one located to the west of the Luzon Strait, which is called the eddy shedding location (indicating the anticyclonic eddy), while the other, located to the east of the Luzon Strait, indicates the Kuroshio. 2) There must be positive geostrophic vorticity between the two centers indicating the separation of the anticyclonic eddy and the Kuroshio. The time when the SSH and geostrophic vorticity field meet these criteria is called the eddy shedding time. The interval between two consecutive eddy sheddings is called the eddy shedding interval. Using these two criteria, the eddy shedding time and location of the eddy shedding events from 1992 to 2001 have been determined and are listed in Table Spatio-temporal variation of eddy shedding From October 1992 to August 2001, a total of 33 anticyclonic eddies were shed from the Kuroshio (Table 1) with about four anticyclonic eddies per year on average. Among these anticyclonic eddies, 17 are shed during the winter monsoon period and 16 during the summer monsoon period. The intervals of eddy shedding vary from 40 to 230 days, while the most frequently occurring intervals are s (5 times), s (4 times) and 90 days (6 times) (Fig. 6). The eddy shedding locations vary from 118 E to E. In the winter monsoon period, the most frequently occurring locations are E (6 times) and 120 E (4 times), while in the summer monsoon Eddy Shedding from Kuroshio Bend 1067

6 Table 1. Compilation of eddy shedding times and locations. Summer monsoon period (May to Oct.) Eddy shedding time Period Location of shed eddy Jun. 29, 93 Sep. 07, 93 May 10, 95 Aug. 18, 95 Jul. 03, 96 Sep. 21, 96 Jun. 28, 97 May 14, 98 Jul. 03, 98 Oct. 31, 98 Jul. 18, 99 May 13, 00 Jul. 22, 00 Sep. 30, 00 May 18, 01 Aug. 06, day 110 day 120 day 50 day 50 day 120 day 100 day 50 day 140 day Eddy shedding occurs 16 times 118 E, 21 N E, 22 N 119 E, 20.5 N E, 20.5 N 120 E, 21 N E, 20.5 N E, 18.5 N E, 20.5 N 120 E, 19.5 N 120 E, 21 N 119 E, 21.5 N E, 21 N Winter monsoon period (Nov. to Apr.) Eddy shedding time Period Location of shed eddy Feb. 19, 93 Mar. 31, 93 Apr. 25, 94 Nov. 01, 94 Feb. 09, 95 Nov. 06, 95 Mar Dec. 20, 96 Feb. 28, 97 Nov. 05, 97 Jan. 04, 98 Mar. 25, 98 Jan. 29, 99 Apr. 19, 99 Jan. 24, 00 Mar. 24, 00 Dec. 29, day 230 day day 130 day 130 day 60 day 1 60 day Eddy shedding occurs 17 times E, 21 N E, 20.5 N E, 20.5 N E, 20.5 N 119 E, 21 N E, 22 N E, 21 N 118 E, 21 N E, 22 N 120 E, 21 N 120 E, 21.5 N 119 E, 21.5 N 119 E, 21 N period the most frequent locations are also E (5 times) and 120 E (5 times) (Fig. 7). Put briefly, the seasonal variation of the eddy shedding location is unclear. 5. Conclusions and Discussions In general, the Kuroshio bend varies with time and periodically sheds anticyclonic eddies. Criteria of eddy shedding have been established: 1) When the shedding event occurs, there are usually two centers of high SSH together with negative geostrophic vorticity in the KB area. 2) Between the two centers of high SSH there is usually a positive geostrophic vorticity. These criteria have allowed all of the eddy shedding times and locations to be determined. The intervals of eddy shedding vary from 40 to 230 days, the most frequently occurring intervals being 70, 80 and s. The eddy shedding locations vary from 118 E to E. In both winter and summer monsoon period, the most frequently occurring locations are E and 120 E, which means that the seasonal variation of eddy shedding location is unclear. Error caused by tidal correction in the altimetry data is relatively high in shallow coastal areas (Li et al., 1999). Although the 60-day aliased tidal signal is not dominant in the SSH variation in the area of our concern, the effect of the aliased tidal signal, especially in shallow coastal areas, should be estimated in further studies. More in situ data should also be used to evaluate the result from the altimeter data. Although the 0.25 POCM result compared well with the 1/8 NLOM result (Metzger and Hurlburt, 2001), the resolution may be not high enough for simulating eddy shedding. Higher model resolution may help to produce a more accurate mean SSH field for altimetry data, which may improve the result of this study. There is a 100-day oscillation of Kuroshio transport east of Taiwan (Zhang et al., 2001). Is there any effect of the 100-day Kuroshio oscillation on the eddy shedding? Farris and Wimbush (1996) found a relationship between the loop current stages (the degree of the Kuroshio bend) and the strength of northeast monsoon. Sheremet s work (2001) suggested that the Kuroshio penetration would increase when the Kuroshio is weakened by the northeasterly. It is worth studying the influence of the Kuroshio oscillation, local wind, and other factors on eddy shedding. Acknowledgements This study benefit from the Ministry of Science and Technology of China (G and 2001DIA50041), and the Ministry of Education of China ( ). Thanks are to CLS and AVISO/Altimetry, France, for providing the T/P-ERS satellite altimeter data disk. We would like to acknowledge receipt of the CD- ROM of POCM data from Dr. Robin Tokmakian and Peter Braccio. We are also very grateful to Prof. Ruixin Huang, Woods Hole Institute of Oceanography and Prof Y. Jia and Q. Liu

7 Zhengyu Liu, University of Wisconsin for valuable suggestions. We thank our colleagues Wei Liu and Aijun Pan for improving written English. References Farris, A. and M. Wimbush (1996): Wind-induced Kuroshio intrusion into the South China Sea. J. Oceanogr., 52, Li, L. and B. Wu (1989): A Kuroshio loop in South China Sea? On circulations of the north-eastern South China Sea. J. Oceanogr. in Taiwan Strait, 8, (in Chinese with English abstract). Li, L., W. Nowlin, Jr. and J. Su (1998): Anticyclonic rings from the Kuroshio in the South China Sea. Deep-Sea Res. I, 45, Li, L., R. Wu, Y. Li and Z. Gan (1999): A Preliminary analysis of shallow water tidal aliasing in TOPEX/POISEDON altimetric data. Acta Oceanologica Sinica, 21, 7 14 (in Chinese with English abstract). Li, W., L. Li and Q. Liu (1998): Water mass analysis in Luzon Strait and northern South China Sea. J. Oceanogr. in Taiwan Strait, 57, (in Chinese with English abstract). Li, Y., L. Li, M. Lin and W. Cai (2002): Observation of mesoscale eddy fields in the sea south west of Taiwan by TOPEX/POISEDON altimeter data. Acta Oceanologica Sinica, 24, (in Chinese with English abstract). Liu, Q., Y. Jia, P. Liu, Q. Wang and P. Chu (2001): Seasonal and intraseasonal thermocline variability in the central South China Sea. Geophys. Res. Lett., 28, Metzger, J. and H. Hurlburt (2001): The nondeternimistic nature of Kuroshio penetration and eddy shedding in the South China Sea. J. Phys. Oceanogr., 31, Semtner, J., Jr. and R. Chervin (1992): Ocean circulation from a global eddy-resolving model. J. Geophys. Res., 97, Sheremet, V. (2001): Hysteresis of a western boundary current leaping across a gap. J. Phys. Oceanogr., 31, Stammer, D., R. Tokmakian, A. Semtner and C. Wunsch (1996): How well does a 1/4 global circulation model simulate large-scale oceanic observations? J. Geophys. Res., 101, 25,779 25,811. Zhang, D., T. Lee, W. Johns, C. Liu and R. Zantopp (2001): The Kuroshio east of Taiwan: modes of variability and relationship to interior ocean mesoscale eddies. J. Phys. Oceanogr., 31, Eddy Shedding from Kuroshio Bend 1069