Water exchange through the Kerama Gap estimated with a 25-year Pacific HYbrid Coordinate Ocean Model*

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1 Chinese Journal of Oceanology and Limnology Vol. 3 o. 6, P , Water exchange through the Kerama Gap estimated with a 2-year Pacific HYbrid Coordinate Ocean Model* ZHOU Wenzheng ( 周文正 ) 1, 2, YU Fei ( 于非 ) 1, **, A Feng ( 南峰 ) 1 1 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 26671, China 2 University of Chinese Academy of Sciences, Beijing 149, China Received May 8, 216; accepted in principle Sep. 26, 216; accepted for publication ov. 9, 216 Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag GmbH Germany 217 Abstract Variations in water exchange through the Kerama Gap (between Okinawa Island and Miyakojima Island) from 1979 to 23 were estimated with the.8 Pacific HYbrid Coordinate Ocean Model (HYCOM). The model results show that the mean transport through the Kerama Gap (KGT) from the Pacific Ocean to the East China Sea (ECS) was 2.1 Sv, which agrees well with the observed mean KGT (2. Sv) for Over the time period examined, the monthly KGT varied from -1.9 Sv to 1.8 Sv and had a standard deviation of. Sv. The water mainly enters the ECS via the subsurface layer (3 m) along the northeastern slope of the Kerama Gap and mainly flows out of the ECS into the southwest of the Kerama Gap. The seasonal and interannual variations of the KGT and the Kuroshio upstream transport were negatively correlated. The Kuroshio upstream transport was largest in summer and smallest in autumn while the KGT was smallest in summer (1.2 Sv) and largest in spring (2.94 Sv) and autumn (2.44 Sv). The seasonal and interannual variations in the Kuroshio downstream (across the P-line) transport differed significantly from the Kuroshio upstream transport but corresponded well with the KGT and the sum of the transport through the Kerama Gap and the Kuroshio upstream, which indicates that information about variation in the KGT is important for determining variation in the Kuroshio transport along the P-line. Keyword : Kerama Gap; Kuroshio; Ryukyu Current; HYbrid Coordinate Ocean Model (HYCOM); water exchange 1 ITRODUCTIO Water exchange often occurs between two basins that are interconnected by a shallow or a deep sill (Song, 26). The Ryukyu Island chain acts as a barrier to deep water exchange between the orthwestern Pacific (P) and the East China Sea (ECS) (Jin et al., 21). The Kerama Gap, which extends over about 2 km in the center of the Ryukyu Island chain between Miyakojima Island and Okinawa Island (Fig.1) (akamura et al., 213) and has a sill depth of more than 1 m, is the only deep channel in the chain (Sibuet et al., 199). In recent decades, water exchange through the Kerama Gap has attracted much attention from oceanographers (Andres et al., 28b; akamura et al., 213; a et al., 214; Yu et al., 21). The upper circulation around the Kerama Gap is dominated by strong and persistent subtropical current systems, namely the Kuroshio and the Ryukyu Currents (Zhu et al., 23; Centurioni et al., 24). The Kuroshio enters the ECS mainly through the East Taiwan Channel (ETC) and flows out through the Tokara Strait, while the Ryukyu Currents flow northeastward along the eastern slope of the Ryukyu Islands (akamura et al., 213). The exchange of upper-layer water between the ECS and the P is mainly through the Kerama Gap, except at the entrance from the ETC where the sill depth is 77 m and at the exit to the Tokara Strait where the sill depth is 69 m (Yu et al., 21); deep water exchange only occurs through the Kerama Gap. Soeyanto et al. * Supported by the ational atural Science Foundation of China (o ), the ational Ocean Subject (o. XDA11261), the SFC Shandong Joint Found for Marine Science Research Centers (o. U14641), and the SFC Innovative Group Grant (o ) ** Corresponding author: yuf@qdio.ac.cn

2 1288 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol Kyushu Tokara Strait East China Sea Okinawa -4 Pacific Ocean - -1 Miyakojima -1 Taiwan East Taiwan Channel E 132 Fig.1 Maps of the East China Sea (ECS) and the orthwestern Pacific (P) with 1 m interval bathymetry The thin black line between the Miyakojima and Okinawa represents the Kerama Gap. Red (or blue) lines represent the trajectory of the drifters through the Kerama Gap. The solid circles represent the start position of the drifters. -7 (214) pointed out that the volumes transported by the Kuroshio upstream and downstream transects were spatially consistent, but there was a significant discrepancy in the volume transported between the Kuroshio upstream and downstream. Andres et al. (28a) pointed out that the Kerama Gap is a bottleneck for interactions between the Ryukyu Currents and the ECS-Kuroshio, and, even though the bulk of the Kuroshio enters the ECS through the ETC, the Kerama Gap may still be an important communication conduit with the ECS. They also suggested that the effect of mesoscale eddies arriving at Okinawa from the interior of the Pacific Ocean may drive changes in the Kuroshio transport across the P-line (KT) through the Kerama Gap (Fig.2). Zheng et al. (28) found that the flow from the Ryukyu Currents into the ECS through the Kerama Gap enhanced the KT and that there was a significant discrepancy between the volumes transported by the Kuroshio upstream and the Kuroshio downstream. Jin et al. (21) found that a shift in the Kuroshio axis determined the water exchange through the Kerama Gap, and that the water flowed from the ECS (P) to the P (ECS) when the Kuroshio axis shifted to (away from) the Kerama Gap. Part of the KT is supplemented by the volume transported through the Kerama Gap (KGT), and the different water masses from the open ocean will modify the properties of the Kuroshio water mass (Oka and Kawabe, 1998). To date, there have been few studies of the KGT (Andres et al., 28b; a et al., 214). Yuan (199) used current meter moorings and estimated that a volume of 2.4 Sv entered the ECS from the KGT between ovember 1991 and September Morinaga et al. (1998) also used current meter measurements and estimated that the KGT transport from the P into the ECS was 7.2 Sv during the summer of a et al. (214) measured the annual mean KGT with an array of current and pressurerecording inverted echo sounders (CPIES) and current

3 o.6 ZHOU et al.: Water exchange through the Kerama Gap Kyushu 3 28 M P PM Kerama Gap Okinawa Section1 Section2 Miyakojima ETC.3 m/s E 132 Fig.2 Mean surface currents from 1992 to 22 as shown by the Pacific HYCOM model The five red sections represent the Kerama Gap, Kuroshio upstream section (PM-line), Kuroshio downstream section (P-line), continental shelf section (M-line) and ETC. Sections 1 and 2 represent the section through the Kerama Gap monitored by the World Ocean Database (WOD). meter moorings from June 29 to June 211, and found that the 2-year mean KGT from the P into the ECS was about 2. Sv. The many numerical models that have been used to estimate the KGT have all shown that the KGT transport is from the P to the ECS. Using a 1/6 (~18 km) model, You and Yoon (24) reported that the KGT was.6 Sv. Guo et al. (26), by running a 1/18 (~6 km) nested ocean model from September 1991 to December 1998, found that the inflow through the Kerama Gap was.49 Sv; they also found that the Kuroshio onshore fluxes across the shelf break of the ECS were largest in autumn and smallest in summer. Soeyanto et al. (214), with a data-assimilative ocean model (JCOPE2), estimated that the KGT from 1993 to 212 was.18 Sv. Their model comprised two submodels connected by a one-way nesting method. The inner model had a horizontal resolution of 1/12 (~9 km) with 46 vertical layers and the outer model had a horizontal resolution of 1/4 (~27. km) with 21 vertical layers. Yu et al. (21) studied the seasonal variations in the KGT from 1993 to 212 using the 1/12. global HYCOM reanalysis and estimated that the KGT was 2.3 Sv. They also pointed out that the HYCOM reanalysis showed bottom-trapped inflow and that the maximum velocity occurred near the depth of the sill, which is not consistent with the observations. Understanding the variations in water exchange through the Kerama Gap will contribute new insights into variability in the ECS-Kuroshio (a et al., 214). In this study, we investigated seasonal and interannual variations in the KGT and discussed the relationship between the KGT and the ECS-Kuroshio transport

4 129 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol. 3 using the Pacific HYCOM model. We found that, in some ways, the Pacific HYCOM model had more advantages than the global HYCOM reanalysis, as discussed further in Section 2.3. In this paper, we describe the data, the numerical model, and model verification in Section 2; present the results in Section 3; discuss seasonal and interannual variability of the KGT in Section 4, and report our main findings in Section. 2 MATERIAL AD METHOD 2.1 Satellite-tracked drifting buoys Satellite-tracked drifting buoys (drifters) collect direct velocity measurements worldwide from the sea surface layer at a nominal depth of 1 m as part of the Global Drifter Program. The raw data of each drifter is quality controlled and interpolated to 6-h intervals (Centurioni et al., 24). Most drifters enter the ECS- Kuroshio through the ETC and drift northeastward along the Kuroshio principal axis, but some drift out of the Kuroshio through the Kerama Gap. Thirtythree drifters passed through the Kerama Gap from 1979 to 21 (Fig.1). Sixteen of them (red line) drifted from the ECS to the P mainly through the southern part of the Kerama Gap and the other 17 (blue line) moved from the P to the ECS, mainly through the northern part of the Kerama Gap. Out of the 16 drifters that crossed the Kerama Gap into the P, 14 drifted from the Kuroshio upstream and the other two drifted from the Kuroshio downstream (across the P-line). Almost all the drifters that moved from the P to the ECS through the Kerama Gap travelled northeastward along the mean Kuroshio path, with most transport in autumn. The drifters tracks suggest that water exchange through the Kerama Gap may lead to variations in the volumes transported between the Kuroshio upstream and the Kuroshio downstream. 2.2 umerical model In this study we used the Pacific HYbrid Coordinate Ocean Model (HYCOM PACa.8) with an equatorial horizontal resolution of.8 ( grid points, spaced at an average of 6. km) that covered the orth and Equatorial Pacific regions from 98 E to 78 W and from 2 S to 66. The model has 33 hybrid vertical coordinate layers and the bathymetry is derived from a quality controlled ETOPO 2. dataset. Surface forcing of the model was done with 6-hourly gridded data from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis product (ERA-1 forcing), including wind stress, wind speed, heat flux and precipitation (Kelly et al., 27). The model outputs included monthly sea surface height (SSH), temperature, salinity, and currents for the period from 1979 to 23. There is no data assimilation in this model. A more detailed description of the Pacific HYCOM model is provided by Kelly et al. (27). 2.3 Model verification Kelly et al. (27) pointed out that the high resolution HYCOM model was much better than lowresolution models for reflecting the characteristics of the Kuroshio region and gave results that were closer to the observations. They also reported some bias between high resolution models without assimilation and the assimilated observations but considered they were still useful for studying ocean dynamics and for predictions of seasonal and interannual variabilities. To verify the reliability of the Pacific HYCOM, we compared the current velocity, temperature, and salinity distributions with the observed data Current velocity in the Kerama Gap The Kuroshio entered the ECS mainly through the ETC and flowed out of the ECS through the Tokara Strait, as shown in Fig.2. Although the Kuroshio is a persistently strong western boundary current, there was significant discrepancy in the seasonal variability between the upstream and downstream. The KT comprises the Kuroshio transport of the PM-line (KMT), the KGT, and the transport of the continental shelf (M-line) (MT). There was a strong correlation between seasonal variability in the KMT and transport of the ETC (correlation coefficient of.96), and a weak correlation between the KMT and the KT (correlation coefficient of.33) (Table 2). The Kerama Gap, between the PM-line and the P-line, plays an important role in water exchange between the ECS and the P, as shown by the drifter tracks in Fig.1. The mean MT was only about 1. Sv and varied from -.8 to 2.8 Sv, while the mean KGT was 2.1 Sv and varied from -1.9 Sv to 1.8 Sv (Table 1), which indicates that the KGT may have more influence on the temporal variability of the KT than the MT. This is discussed further in Section 4. The vertical structure of the velocity along the Kerama Gap transect from the global HYCOM reanalysis (Fig.3a), Pacific HYCOM (Fig.3b), and observations of a et al. (214) (Fig.3c) are compared

5 o.6 ZHOU et al.: Water exchange through the Kerama Gap Depth (m) -1-1 a b c Velocity (m/s) Fig.3 Vertical distribution of velocity (m/s) along the Kerama Gap transect a. global HYCOM reanalysis; b. Pacific HYCOM; c. linear interpolation of the observations (a et al., 214) (The figure is from Yu et al., 21). Table 1 Statistics of transport volumes (Sv) through all the sections Section Mean Std Max Min Spring Summer Autumn Winter Kerama (total) Kerama (deep) P PM M ETC Kerama (total)+pm Std is the standard deviation. Kerama Gap (total) is from Miyakojima to Okinawa (~2 km). Kerama Gap (Deep) represents the deep gap (~ km). Table 2 Correlation coefficients (month/season/year) among all the sections P PM Kerama M ETC P PM.27/.33/.34 Kerama.4/.44/ /-.63/-.64 M.43/.8/ / * / *.28/.29/.23 ETC.28/.3/.31.93/.96/ /-.9/-.63 */.11 /* PM + Kerama.87/.9/.89.31/.34 /.43.1/.2 /.42.21/.39/.22.28/.33/.38 PM + M.33/.4/.38.99/.99/ / -.8 /-.63 */.18 /*.93/.96/.9 The correlation coefficients are all significant to the 9% confidence level except those in bold font (9% 9%). Asterisks represent the correlation coefficients that are significant at levels below 9%. in Fig.3. This figure shows that there is better agreement between the current structure and the observations with the Pacific HYCOM than with the global HYCOM reanalysis. The global HYCOM seems to have a bottom-trapped inflow, with a maximum velocity close to the sill depth (Yu et al., 21). There is a discrepancy between the Pacific HYCOM and the global HYCOM because the Modular Ocean Data Assimilation System (MODAS) database is used to project surface information downward to the water column in the global HYCOM. Cummings and Smedstad (213) noted that MODAS was only marginally useful in areas where the historical profile data seemed inadequate to statistically represent the Ryukyu Current in the northeast of the Kerama Gap. The Pacific HYCOM

6 1292 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol. 3 Depth (m) a b Depth (m) E E Temp. ( C) c E -8-1 d E Salinity Fig.4 Vertical distributions of the temperature (units: C) and salinity along section 1 from the Pacific HYCOM (a, c) and WOD (b, d) The HYCOM data is for February 1988 and the observed WOD is from 22 February 1988 to 23 February adopted an improved methodology, the Improved Synthetic Ocean Profile (ISOP), that adjusts the model forecast density field so that it agrees with the observations. The Pacific HYCOM has more advantages than the global HYCOM reanalysis. For example, the Pacific HYCOM agrees that there is a thin vertical layer near the bottom with intensified inflow across the Kerama Gap, as suggested by akamura et al. (213). The Pacific HYCOM indicates that 6% of the KGT is in the upper m, which is closer to the observations (6%) (a et al., 214), whereas the global HYCOM reanalysis indicates that 61% of the KGT is in the upper 7 m (Yu et al., 21). Although the current structures of the two models are significantly different, Yu et al. (21) argued that the mean inflow into the ECS from both the Pacific HYCOM (2.1 Sv) and the global HYCOM (2.3 Sv) agreed well with the observations (2. Sv) and did not seem to be impacted by the difference in the flow structure in the deep layer. However, the seasonal variability of the KGT derived from the two models is different Temperature and salinity We chose two observed sections (section 1 and section 2 in Fig.2) from the World Ocean Database (WOD) (Boyer et al., 213) to verify the model results. Figures 4 and show that the temperature and salinity distributions from the Pacific HYCOM agreed well with the observations. Figure 4a and 4b show that the temperature of section 1 decreased vertically from the surface (23 C) to the bottom ( C). Figure 4c and 4d show that the salinity of section 1 was more than 34.9 at depths of between 1 and 2 m and less than at depths of between 6 and 7 m. Figure a and b show that the temperature of section

7 o.6 ZHOU et al.: Water exchange through the Kerama Gap 1293 Depth (m) a E b E Temp. ( C) 3 Depth (m) c E 128 d E Salinity Fig. Vertical distributions of the temperature (units: C) and salinity along section 2 from the Pacific HYCOM (a, c) and WOD (b, d) The HYCOM data is from January 1999 and the observed WOD is from January to January decreased vertically from the surface (23 C) to the bottom (3 C). Figure c and d show that the salinity was more than between 1 and 2 m deep and less than 34.2 between 6 and 7 m deep. Figure also shows that the properties of the Okinawa Trough water mass and the Pacific water were similar, which indicates that the Pacific water can influence the water mass properties of the Okinawa Trough through the Kerama Gap. 3 RESULT 3.1 Seasonal variability in the vertical distributions of velocity There were significant seasonal variations in the vertical distributions of the velocity in the upper and deep layers of Kerama Gap. The drifter tracks (Fig.1) illustrate that surface water of the Kerama Gap flows into and out of the ECS throughout the year and reaches a maximum in the autumn, when it drifts from the P into the ECS. Figure 6 further verifies that the maximum and minimum surface water volumes flowed into the ECS from the Kerama Gap in autumn and summer, respectively. Variation in the Kuroshio is important for determining the water exchange through the Kerama Gap (Jin et al., 21). The distance between the Kuroshio Current axis and the Kerama Gap can determine the effect of the Kuroshio on the Kerama Gap. When the transport volume of the Kuroshio reaches a maximum, the flow amplitude of the Kuroshio will be at its widest and the current axis will be closest to the Kerama Gap, which will cause water to flow out of the ECS. In contrast, when the volume transport of the Kuroshio is at a minimum, the

8 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol Depth (m) Depth (m) a E b E c E E Fig.6 Average vertical distributions of the velocity (m/s) of the Kerama Gap transect for the (a) spring, (b) summer, (c) autumn, and (d) winter between 1979 and 23 Positive values indicate the inflow into the ECS from the P. d flow amplitude of the Kuroshio is narrowest and the current axis is farthest away from the Kerama Gap; when this happens, the water will flow into the ECS and increase the transport volume of the Kuroshio. Zheng et al. (28) suggested that inflow from the Ryukyu Currents through the Kerama Gap could enhance the volume transported downstream by the Kuroshio. Compared with the surface velocity, the subsurface velocity (3 m) is much stronger from the P to the ECS and persists throughout almost the whole year with a minimum in summer and a maximum in spring. The Kuroshio upstream was strongest (Table 1), and the Kuroshio Current axis was closest to the Kerama Gap, in summer, which may cause the water to flow out of the ECS. The maximum and minimum deep water exchange (from the P to the ECS) through the Kerama Gap were in summer and winter, respectively. The distributions of the surface, subsurface, and deep-layer horizontal velocities will be discussed in next section. 3.2 Horizontal velocity fields around the Kerama Gap Although the mean current vectors measured with current meters in the upper (4 m), middle (6 7 m), and deep levels (9 1 m) in the Kerama Gap have been studied by a et al. (214) and Yu et al. (21), they were only able to study two or three observation points because of a lack of observed data. To date there have been no studies of the seasonal horizontal velocity distributions in the upper and deep layers in the Kerama Gap and the surrounding region, even though they are needed to provide an understanding of the dynamics of the Kerama Gap overflow and the seasonal variations in

9 o.6 ZHOU et al.: Water exchange through the Kerama Gap cm/s 2 cm/s 2 2 a E cm/s b E cm/s 2 2 c E 128 d E 128 Fig.7 Average (a) spring, (b) summer, (c) autumn, and (d) winter horizontal velocity fields around the Kerama Gap over the period from 1979 to 23 for the surface layer (1 m) from Pacific HYCOM the water exchange through the Kerama Gap. In particular, while the control of the Kuroshio on surface flow, and mesoscale eddies have both been studied (Andres et al., 28b; Jin et al., 21; akamura et al., 213), there is little information about subsurface and deep-layer flows surrounding the Kerama Gap. In this study therefore, we examined the seasonal horizontal velocity distributions at the surface (1 m), subsurface (3 m), and in the deep layers (1 m) in the Kerama Gap and in the surrounding region using the Pacific HYCOM. Figure 7 shows the seasonal variability of the surface horizontal velocity fields surrounding the Kerama Gap for the period from 1979 to 23 modelled by Pacific HYCOM. In spring, the Kerama Gap overflow (from the ECS to the P) was mainly in the southwest of the gap and the inflow (from the P to the ECS) was mainly in the northeast of the gap. The overflow was intensified along Miyakojima. The inflow grew weaker in summer, intensified in autumn throughout the entire Kerama Gap, and then became weaker again in winter. Surface horizontal velocity fields show that surface water mainly flowed out of the ECS into the southern part of the Kerama Gap in summer and mainly flowed into the ECS into the northern part of the Kerama Gap in autumn, which is consistent with the drifter tracks (Fig.1). Figure 8 shows the seasonal variability of the

10 1296 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol. 3 2 cm/s 2 cm/s 2 2 a E 128 b E cm/s 2 cm/s 2 2 c d E E 128 Fig.8 Same as Fig.7 but for the subsurface layer (3 m) horizontal velocity fields around the Kerama Gap for the period from 1979 to 23 at a depth of 3 m. We chose this depth because the velocity at 3 m provides a good representation of the core subsurface inflow from the P to the ECS through the Kerama Gap. agano et al. (27) indicated that the maximum velocity of the Ryukyu Currents was quite variable between 3 and 8 dbar deep. Thoppil et al. (216) thought that the subsurface velocity of the Ryukyu Currents was at a maximum between 8 and 1 m and that there was a transient shallow velocity core at about 3 m followed by anticyclonic eddies. akamura et al. (213) showed that the intermediate water of the P intensified along the northeast slope of the Kerama Gap into the ECS and could be regarded as a persistent flow throughout the year. The results of this study show that the subsurface water of the P bifurcated from the Ryukyu Current System and flowed into the ECS along the northeastern slope of the Kerama Gap throughout the year, with a maximum in spring and a minimum in summer. The Kerama Gap plays an important role in deep water exchange between the P and the ECS but there is little information about the deep flow pattern in the Kerama Gap and even the direction of the mean flow has not yet been firmly established (Andres et al., 28a). Thoppil et al. (216) found that there were significant variations in the Ryukyu Currents with

11 o.6 ZHOU et al.: Water exchange through the Kerama Gap cm/s 2 cm/s 2 2 a E 128 b E cm/s 2 cm/s 2 2 c d E E 128 Fig.9 Same as Fig.7 but for the deep layer (1 m) maximum currents in winter and spring and minimum currents in summer. akamura et al. (28) pointed out that the deep countercurrent beneath the Kuroshio in the northern Okinawa Trough was approximately bounded by the 1 m isobaths and was much stronger during the winter-spring period than in the summer-autumn period. The horizontal velocity of the deep layer (1 m) surrounding the Kerama Gap (Fig.9) suggests that the Kuroshio had vanished and that the deep countercurrents beneath the Kuroshio were more stable along the continental shelf of the northern Okinawa Trough during the winter-spring period than during the summer-autumn period. The Kerama Gap deep outflow was also significant during the winter-spring period and the inflow was noticeable during the summer-autumn period, which indicates that deep water exchange through the Kerama Gap may be dominated by the deep countercurrent beneath the Kuroshio. We can therefore conclude that the Kuroshio and the mesoscale eddies result in upper water exchange through the Kerama Gap (Andres et al., 28b; Jin et al., 21; a et al., 214), but the deep countercurrent beneath the Kuroshio determines the deep water exchange of the Kerama Gap. Besides, the upper water entered the ECS with a maximum velocity of 1 cm/s in spring while the deep water flowed into the ECS with a maximum velocity of cm/s in summer.

12 1298 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol. 3 4 DISCUSSIO 4.1 Seasonal variation in the KGT Although the mean KGT was only 2.1 Sv and only contributed about 6% of the mean Kuroshio transport, the fact that it varied widely (from -1.9 Sv to 1.8 Sv with a standard deviation of. Sv) means that it may have had a significant impact on the seasonal variability of the ECS-Kuroshio transport. The large variation in the KGT may be caused by the bifurcation of the Kuroshio and Ryukyu Currents and the high frequency of mesoscale eddies. Yu et al. (21) calculated the standard deviation of the KGT and the KT and concluded that the yearly transport standard deviation of the KT was highly correlated with the standard deviation of the KGT (correlation coefficient of.64). Although they reported that there was a relationship between the KT and the KGT, they did not discuss the relationship between the KGT and the KMT and the continental shelf transport (MT). In this study, we calculated the ETC transport, KMT, KT, KGT, and MT (Table 1) for each month from 1979 to 23 and discussed the relationships between them. The most significant feature of the monthly variability is that the KT was positively correlated with the sum of KGT and the KMT, with a correlation coefficient of.87 (Fig.1a); the KGT was negatively correlated with the KMT with a correlation coefficient of -.66 (Fig.1b). akamura et al. (28) reported that a small portion of the Kuroshio flow separated from the western slope in the southern Okinawa Trough and moved toward the Kerama Gap and that its greatest portion flowed along the eastern slope of the northern Okinawa Trough. When the Kuroshio goes toward (away from) the Kerama Gap, the water will flow from the ECS (P) to the P (ECS), thereby causing a decrease (increase) in the KGT, which is agreement with the findings of Jin et al. (21). The PM-line is between the ETC and the P-line, and they are all part of the Kuroshio. It is interesting to find that the monthly variability of the KMT was strongly correlated with the transport of the ETC (correlation coefficient of.93), but not so strongly correlated with the KT (correlation coefficient of.27) (Table 2). This shows that there was little variability in the Kuroshio upstream transport (between the ETC and the PM-line) but that there was significant variability in the Kuroshio midstream transport (between the PMline and the P-line), which is consistent with the results reported in other studies (Lin et al., 24; Zheng et al., 28; Soeyanto et al., 214). The location of the Kerama Gap, between PM-line and the P-line, may be an important control on the variation between the KMT and the KT. The monthly variability in the KT was positively correlated with the KGT and MT, with correlation coefficients of.4 and.43, respectively, but was weakly correlated with the KMT, with a correlation coefficient of.27. Besides, the monthly variability of KT is more positively correlated with sum of KMT and KGT (correlation coefficients of.87) than the sum of KMT and MT (correlation coefficients of.33) (Table 2). We can conclude that the variation in the KT was determined by the variability in the KGT, KMT, and MT, but that the KGT was perhaps the most important factor. The most significant feature of the seasonal variability is that it was consistent with the monthly variability. The KT was positively correlated with the sum of KGT and KMT (correlation coefficient of.9) and the KGT was negatively correlated with the KMT (correlation coefficient of -.63) (Fig.11). The positive correlation between the seasonal variability in the KT and the MT was stronger than the correlations with either KGT or KMT (Table 2); the standard deviation and the range of the KGT (. Sv, 26.7 Sv) were bigger than those of the KMT (4. Sv, 23.8 Sv) and the MT (.6 Sv, 3.6 Sv) (Table 1), which also demonstrates that KGT may have more control on the seasonal variation in the KT than either the KMT or MT. The 2-year mean KGT from the P into the ECS derived from the Pacific HYCOM was about 2.1 Sv, and had a standard deviation of ±. Sv. The temporal variability of the total transport (~2 km) was strongly correlated (correlation coefficient of.97) with the deep gap (~ km) transport (1.93 Sv). Figure 12 shows that the 2-year monthly mean KGT was largest in April (3.83 Sv) and smallest in July (.31 Sv), which is not consistent with the findings of Yu et al. (21), who reported that the KGT was largest in October (3.4 Sv) and smallest in ovember (.4 Sv). The discrepancy between our results and those of Yu et al. (21) may reflect the bottom velocity of the model. As they pointed out, in the global HYCOM reanalysis the inflow is bottomtrapped with a maximum velocity occurring near the sill depth, but the current structure in the Pacific HYCOM model shows better agreement with the observations. Although the Pacific HYCOM model has no data assimilation, it is important for predicting seasonal and interannual variability (Kelly et al., 27). Seasonal variability of the KGT was greatest

13 o.6 ZHOU et al.: Water exchange through the Kerama Gap 1299 P 4 R=.87 Kerama+PM a 2 21 Kerama Gap 2 1 R=-.66 PM b Year Fig.1 Comparisons of the monthly volume transports from 1979 to 23 from the Pacific HYCOM a. P-line (blue) and the sum of transports through the Kerama Gap and the PM-line (green); b. Kerama Gap (blue) and the PM-line (green). Positive volume transport through the Kerama Gap is in the direction from the P to the ECS R(T2:T1)=.9 R(T2:T3)=.33 R(T2:T4)=.44 R(T3:T4)= T1: Kerama+PM T2: P T3: PM T4: Kerama Gap Year Fig.11 Comparisons of the seasonal cycle of volume transports through the Kerama Gap (green), P-line (red), PM-line (black) and total transports through the Kerama Gap and the PM-line (blue) from 1979 to 23 from the Pacific HYCOM model

14 13 CHI. J. OCEAOL. LIMOL., 3(6), 217 Vol T1: PM T2: P T3: Kerama+PM T4: Kerama Gap Month Fig.12 Monthly mean transports of the Kerama Gap, P-line, and PM-line, and the sum of the transports of the Kerama Gap and the PM-line from1979 to 23 from the Pacific HYCOM model in spring (2.94 Sv) and autumn (2.44 Sv) and was least in summer (1.2 Sv). The KMT, with a maximum in summer ( Sv) and a minimum in autumn (32. Sv) (Table 1), was negatively correlated with the KGT but was consistent with the MT. The KT, with a maximum in spring (37. Sv) and a minimum in winter (34.7 Sv), was positively correlated with the sum of KGT and KMT. This also illustrates that the discrepancy in the seasonal variations between the KT and the KMT is related to variations in the KGT. 4.2 Interannual variability The yearly transport and its standard deviations through the Kerama Gap, PM-line, P-line, and the sum of the transports through the Kerama Gap and the PM-line from 1979 to 23 estimated by the Pacific HYCOM model are shown in Fig.13. The interannual variation of the KGT was positively correlated with the KT (correlation coefficient of.4) but was negatively correlated with the KMT (correlation coefficient of -.64). The correlation coefficient between the KT and the KMT was.34, confirming that the interannual variability of the KT also corresponded more closely with variability of the KGT than of the KMT. Yu et al. (21) estimated that the two time series of the yearly standard deviation between the KGT and the KT were highly correlated, with a correlation coefficient of.64 between 1992 and 212, which is less significant than the correlation reported in this study; the correlation coefficient between the KT and the total of KGT and the KMT was high (.89). The yearly mean MT ((1.±.6) Sv) was not useful for estimating the contribution to the KT. The interannual variability of the KT was mainly determined by the sum of the KGT and KMT (correlation coefficient of.86). COCLUSIO The Kerama Gap plays an important role in water exchange between the ECS and the P and makes a significant contribution to the variability between the KMT and the KT. We investigated variations in water exchange through the Kerama Gap from 1979 to 23 with a.8 Pacific HYCOM model. The 2- year mean KGT from the P into the ECS was (2.1±.) Sv. The monthly KGT varied widely and reached a maximum of 1.8 Sv (March 1999) and a minimum of -1.9 Sv (June 199). The mean transport of the deep gap (~ km) was (1.93±4.3) Sv and varied from -9.9 Sv to 13.9 Sv. The monthly mean KGT was greatest in April (3.83 Sv) and smallest in July (.31 Sv). There was significant seasonal variation in the KGT, with maximum values in spring (2.94 Sv) and autumn (2.44 Sv) and a minimum in summer (1.2 Sv). The KMT reached a maximum in summer ( Sv) and a minimum in autumn (32. Sv). Water flows into the ECS mainly from the northern part of the Kerama Gap and flows out of the ECS from the southern part of the Kerama Gap in the surface layer. The subsurface currents are the most important

15 o.6 ZHOU et al.: Water exchange through the Kerama Gap P Kerama+PM PM Kerama Gap a b Year Fig.13 Yearly mean transport (a) and its standard deviation (b) in the Kerama Gap (green), P-line (blue), PM-line (black), and total transports through the Kerama Gap and the PM-line (red) from 1979 and 23 derived from the Pacific HYCOM model 1 factor in water exchange through the Kerama Gap. They persistently flow into the ECS along the northeastern slope of the Kerama Gap throughout the year, with a maximum in spring and a minimum in summer. The deep water exchange through the Kerama Gap is determined by the deep countercurrents beneath the Kuroshio and is dominated by strong inflows during the summer-autumn period and an outflow during the winter-spring period. The seasonal variability of the KT and the sum of KGT and KMT were positively correlated, while the KGT and the KT were negatively correlated. The KMT was almost consistent with the Kuroshio transport (across the ETC), but differed significantly from the KT, indicating that the KGT plays an important role in the variation of the volume transported by the PM-line and the P-line. The correlation between the KT and the KGT was larger than the correlation between the KT and the KMT, which also demonstrates that the variations in the KT are determined by the variability of the KGT rather than the KMT. The interannual variability was almost consistent with the seasonal variations. We found that both the Kerama Gap and the Kuroshio transports varied on seasonal and interannual time scales, which will be useful for future observations (Qu and Song, 29). The water mass properties from the P to the ECS-Kuroshio through the Kerama Gap may be modified. Future studies will focus on (1) tracing the intermediate and bottom water from the Pacific Ocean to the Okinawa Trough through the Kerama Gap, with reference to Chen. (2), and (2) the effect of the Kuroshio and mesoscale eddies on water exchange via the Kerama Gap. 6 ACKOWLEDGEMET The satellite-tracked drifting buoy data are available from the Global Drifter Program (GDP), with support from their website (ftp://ftp.aoml.noaa. gov/pub/phod/buoydata/). The World Ocean Database (WOD) is available from the website ( nodc.noaa.gov/oc/select/dbsearch/dbsearch. html). The HYCOM data are available for download from the Global Ocean Data Assimilation Experiment (GODAE) from the website (ftp://ftp.hycom.org/ datasets/paca.8/).

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