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Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan
Author’s Accepted Manuscript Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan Po-Chun Hsu, Quanan Zheng, Ching-Yuan Lu, Kai-Ho Cheng, Hung-Jen Lee, Chung-Ru Ho www.elsevier.com/locate/csr
PII: DOI: Reference:
S0278-4343(17)30507-1 https://doi.org/10.1016/j.csr.2018.10.012 CSR3831
To appear in: Continental Shelf Research Received date: 4 October 2017 Revised date: 11 September 2018 Accepted date: 22 October 2018 Cite this article as: Po-Chun Hsu, Quanan Zheng, Ching-Yuan Lu, Kai-Ho Cheng, Hung-Jen Lee and Chung-Ru Ho, Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan, Continental Shelf Research, https://doi.org/10.1016/j.csr.2018.10.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan Po-Chun Hsua,b, Quanan Zhengc, Ching-Yuan Lua, Kai-Ho Chenga, Hung-Jen Leea, Chung-Ru Hoa* a
Department of Marine Environmental Informatics, National Taiwan Ocean University, Keelung, Taiwan
b
Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan
c
Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA
*Corresponding author: [email protected] (C.-R. Ho)
Abstract The interaction of coastal water in I-Lan Bay, a bay near northeast Taiwan, and the Kuroshio Current is studied using the data from hydrologic survey and satellite remote sensing. An index for water mass distinguished is used for clarifying the origin of water mass in I-Lan Bay. The velocity profile from acoustic Doppler current profile data indicates a countercurrent that flows southward along the northeast coast of Taiwan toward I-Lan Bay with a speed around 0.1-0.2 m/s. The index of water mass indicates that the water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water, thus forming a clear ocean front between these two areas. The near surface water temperature in the I-Lan Bay is about 2 degrees Celsius lower than that in the Kuroshio region. The seasonal average near surface salinity of I-Lan Bay can exist 0.4 psu fresher than the Kuroshio area. From the analysis of dynamic process, the coastal countercurrent in I-Lan Bay is primarily affected by 1) the occurrence of cold dome in northeast of Taiwan mainly occurs in summer, and 2) the wind-driven current from the Taiwan Strait along the north coast and the northeast coast of Taiwan caused by the southwesterly monsoon makes the countercurrents strongly in summer. The dynamics of the countercurrent occurrence can be explained by the Ekman transport theory. Keywords: Kuroshio; coastal countercurrent; cold dome; water masses; northeast of Taiwan; interaction. 1. Introduction Previous investigators have explored the effects of dynamic processes produced by mesoscale eddies and typhoons on the Kuroshio Current (KC) adjacent to Taiwan Island (Zheng et al., 2014; Andres et al., 2015, 2017) and conducted comprehensive field observations of the KC structure related to multi-scale processes from the Luzon Strait to northeast of Taiwan. For example, evolution of KC Tropical Water and variability of KC intermediate waters was one of focuses
(Mensah et al., 2014, 2015; Jan et al., 2015; Yang et al., 2015). The World Ocean Circulation Experiment (WOCE) PCM-1 line was the first project aimed to systematic measurement of the KC volume transport using a mooring array deployed in the south of I-Lan Bay from 1994 to 1996 (Johns et al., 2001; Zhang et al., 2001). Recently, three bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed across the KC along 23.75°N east of Taiwan Island, named KTV1 line, providing measurements of the KC velocity, which were used to analyze interaction of the KC with mesoscale eddies (Jan et al., 2015). Both experiments focused on variation of the KC velocity and transport. However, flow field structure in I-Lan Bay located off the northeastern coast of Taiwan Island and on the KC nearshore side, as well as impacts of its variation on the hydrodynamic environment remain unclear (Fig. 1). Previous investigators have addressed the KC branch invading the East China Sea shelf and a cold dome phenomenon on the northern shelf of Taiwan Island. Cheng et al. (2009) found the cold dome occurred every month and its centroid was located between 25.25°N and 26°N. Takahashi et al. (2009) used ocean high-frequency radar to capture the flow pattern and found that a countercurrent occurs off northeastern Taiwan Island. Jan et al. (2011) indicated that the summer Taiwan Strait outflow and winter monsoon-driven China Coastal Current may affect a counterclockwise flow around the cold dome and intensify upwelling. The phenomenon of countercurrent has been also observed (Tang et al., 2000; Wu et al., 2008). When an upwelling area (counterclockwise flow of the cold dome) develops, it will produce a countercurrent along the northeast of Taiwan Island. However, the questions that where does the countercurrent eventually flow, whether does it affect the coastal shelf area, and what is its relation to the KC axis shift remain unclear. This study aims to understand the flow field structure and hydrological characteristics on the KC nearshore side from the continental shelf area northeast of Taiwan to the I-Lan Bay area. The interaction between low-temperature and low-salt coastal water with high-temperature and high-salt
KC water around the I-Lan Bay area will be clarified. The baseline includes shipboard acoustic Doppler current profiler (Sb-ADCP) data, conductive, temperature, and depth (CTD) profiles, and satellite remote sensing data.
Figure 1. Bathymetry of the study area around I-Lan Bay. The solid blue line represents the location of KC main axis.
2. Data and methods 2.1 In-situ data In-situ current data are derived from drifters and Sb-ADCP measurements. The drifter data were collected and processed by the Atlantic Oceanographic and Meteorological Laboratory under the Global Drifter Program (formerly the World Ocean Circulation Experiment-Surface Velocity Programme) and obtained from Fisheries and Oceans Canada (http://www.dfo-mpo.gc.ca). Historical CTD profiles from 1998 to 2010 and Sb-ADCP data from 1991 to 2017 acquired from the vicinity of Taiwan by R/V Ocean Researcher I, II, and III (www.odb.ntu.edu.tw). CTD profiles from 1986 to 2016 obtained from the World Ocean Database of the National Oceanographic Data Center (https://www.nodc.noaa.gov) are interpolated to standard depth levels. The historical Sb-ADCP data used in this study have been gridded to a 0.25° x 0.25° grid in latitude and longitude.
The depth ranges from 0 m to 300 m. In addition to reference water masses (northern shelf water, southwest water of Taiwan, and the KC), the data were divided into four areas along I-Lan Bay toward the mainstream area of the KC (Fig. 2).
Figure 2. Observation areas 1 to 4 of the CTD data. The red line represents the location of KC main axis.
2.2 Satellite data The monthly absolute geostrophic current data with 0.25 degree spatial resolution from January 1993 to December 2016 are calculated from the absolute dynamic topography (ADT) data which are derived from satellite altimeter data. This dataset was provided by the Copernicus Marine and Environment Monitoring Service (CMEMS). The monthly wind data with 0.25 degree spatial resolution are derived from Quick Scatterometer (QuikScat) from January 2000 to December 2008 and C-2015 Advanced Scatterometer (ASCAT) from January 2009 to December 2016. QuikScat and ASCAT data are produced by Remote Sensing Systems which is sponsored by the National Aeronautics and Space Administration (NASA) Ocean Vector Winds Science Team. Data are
available at www.remss.com.
2.3 Index for water mass analysis The water mass analysis conducted in this study is based on a simplified index defined as (Li et al., 2006) β=
(𝑇−𝑇̅ ) ∆𝑇
+
(𝑆−𝑆̅) ∆𝑆
𝛼,
where β is a non-dimensional index, representing the contribution of temperature and salinity to water mass stability. The index is used to quantify the tendency of a water mass to change from warm and salty water to cold and fresh water. 𝑇 and 𝑆 are the temperature and the salinity in each CTD profile. 𝑇̅ and 𝑆̅ are the mean temperature (22.7℃) and salinity (34.45 psu) of the study area (19°N−27°N, 119°E−124°E), respectively. ∆𝑇(4.0) and ∆𝑆 (0.5) are the average annual mean differences of the temperature and the salinity between the northern shelf water and the KC water upstream of the I-Lan Ridge, respectively. 𝛼 is the contribution ratio and set as 5 (Li et al., 2006). The parameters in the index are determined based on historical in situ data from the World Ocean Atlas 2013, Version 2 (www.nodc.noaa.gov/OC5/woa13/) and Hybrid Coordinate Ocean Model data (HYCOM) (https://hycom.org/). HYCOM data have been used by previous research into smallto medium-scale flow fields and hydrological vertical structures adjacent to Taiwan Island (Zheng and Zheng, 2014; Hsu et al., 2016; Yin et al., 2017).
3. Results 3.1 Characteristics of the flow field off the northeast coast of Taiwan The countercurrent, flowing an opposite direction to the KC, occurs off the north coast of Taiwan Island, flows clockwise into the Keelung Sea Valley and I-Lan Bay, and continues to flow southward across the I-Lan shelf or turn northeastward and interflow with the KC (Fig. 3). Clear indications of annual countercurrents of geostrophic flow have also been found from the results of
the first mode of the empirical orthogonal function (EOF) analysis (Fig. 4). However, when each month was examined separately, the frequency of the countercurrent in June and July was more than 80% but only 20% in January and February. As shown in Figures 4d and 4e, the flow field of the northeastern coast of Taiwan was significantly different in summer and winter. In contrast, the current speed and pattern of the KC axis outside of I-Lan Bay had a significant difference in summer and winter, but the average position of the flow axis was only ~10 km different (Hsu et al., 2016). The historical Sb-ADCP data are used to obtain the in situ flow fields of the northeastern continental shelf of Taiwan and the I-Lan Bay. Figure 5 presents the seasonally mean velocity profiles from the coast along 25.25°N, 25°N, 24.75°N, and 24.5°N. Figure 5a shows the results along the Keelung Sea Valley and the Mian-Hwa Canyon from 121.75°E to 122.25°E. The V-component velocity reaches −0.27 m/s in summer and the average speed at the depth of 150–250 m reaches −0.17 m/s. Figure 5b shows that the countercurrent at 122°E is clearly visible in summer and autumn, and the average surface V-component velocity is close to zero in winter and spring, which may be due to an average flow in the opposite directions. However, at 100–200 m, the countercurrent is observed in all seasons. Figure 5c shows the flow field in the I-Lan Bay, and the average V-component velocity is between −0.1 and −0.2 m/s southward for all seasons. Figure 5d shows the flow field at the southern end of I-Lan Bay, which is characterized by that the coastal countercurrent continually exists in the subsurface layer. From the data of drifters, satellite altimeters, and historical Sb-ADCP, we found that countercurrent flow occurs from the continental shelf northeast of Taiwan and southward along the coast to the I-Lan Bay and possibly further southward. The variations of the KC did not seem to affect the waters of I-Lan Bay. The influence of the KC axis shift may be limited outside of 122°E. Whether the hydrological environment of I-Lan Bay was affected by the variations of the KC path near the shore side or not could be further analyzed by hydrological data.
Figure 3. Trajectories of drifters adjacent to northeast of Taiwan Island. Red and blue lines represent the trajectories of the countercurrent and the KC, respectively.
Figure 4. The first mode of the empirical orthogonal function (EOF) of monthly geostrophic currents northeast of Taiwan (a) U-component, (b) V-component, (c) current vectors, and comparison of mean current fields in (d) summer and (e) winter. Red boxes show the current fields off the northeast coast of Taiwan Island.
Figure 5. Seasonally mean velocity (U positive eastward and V positive northward) profiles from the coast of northeast Taiwan to open ocean along (a) 25.25°N, (b) 25°N, (c) 24.75°N and (d) 24.5°N from 1991 to 2017. The blank indicates no data.
3.2 Water mass distribution from I-Lan Bay to the KC region A cross section of the vertical structure of climatological mean shows significant temperature and salinity differences between the coastal and the KC waters. Because of few field measurements in winter, the seasonal analysis is only available in the other three seasons. In the near-surface layer (0−20 m), the water temperature in the I-Lan Bay is about 2℃ lower than the water temperature in the east of 122.2°E (KC region) in all three seasons (Fig. 6). Salinity differences in the near-surface water layer in the I-Lan Bay are only 0.1 psu fresher than that in the KC region in spring and
summer, but the water is 0.4 psu fresher in the I-Lan Bay (33.95 psu) than that in the KC region (34.32 psu) in autumn (Fig. 6). In the 50–200 m deep layer, the salinity in the I-Lan Bay is about 0.2 psu fresher than that in the KC region in spring and autumn, and 0.1 psu fresher in summer. The water temperature in the I-Lan Bay is about 1℃ lower than that in the KC region in all three seasons.
Figure 6. Cross sections of salinity and temperature from the coastal area (I-Lan Bay) to the KC region in (a) spring (b) summer, and (c) autumn derived from the CTD data.
To further confirm the salinity difference between the coastal water and the KC, a histogram of salinity maximums from the four observed areas is analyzed (Fig. 7). To ensure the quality of the calculation, the CTD data only at depths deeper than 150 m are used. Figures 7a – d represent areas
1 to 4, respectively. The first to third rows represent the seasonal states of spring, summer, and autumn. Figure 7d exhibits a peak of salinity greater than 34.8 psu and an average salinity maximum of about 34.76 psu in Area 4. Areas 1 and 2 show a peak around 34.55 to 34.6 psu and an average salinity maximum of about 34.57 psu. The water mass in the area of 121.9° to 122.1°E does not seem to be affected by the variation of the KC path. In Area 3, an interesting salinity maximum multiple-peak phenomenon is observed during three seasons. We think that this phenomenon results from intensive water mixing taking place in this area, i.e., a portion of the water is similar to the coastal (inshore) water, while other portions may come from the KC (east of 122.2°E). The salinity difference between its maximum and the value at the upper layer could be used to separate different type of water mass. For example, the lower salinity difference value indicates that the upper salinity varies slightly with depth and is close to the characteristics of the inshore water (Liu et al., 2015); and the higher difference value represents the upper salinity changed significantly with depth, consistent with KC-type water (Mensah et al., 2014). But this rule still needs to consider the occurrence of rainfall, the surface and upper layer of water could be freshened during heavy rainfall events, especially in summer (Hsu and Ho, 2018). The results in Figure 8 clearly show that the salinity value near surface is significantly fresher lower than that in other seasons. Therefore, it is good to use the maximum salinity and salinity at 10 m (𝑆10𝑚 ) to distinguish the characteristics of water mass. Figure 8 presents a histogram of salinity at 10 m from four observed areas, the 𝑆10𝑚 in the inshore water is fresher than that in the KC region. Together with Figures 7 and 8, the water mass of the I-Lan Bay, which is affected by coastal countercurrent, has different water characteristics with the KC water where exists in the region east of 122.2°E.
Figure 7. Maximum salinity in the study areas in spring (upper), summer (middle), and autumn (lower). (a) to (d) are Areas 1, 2, 3, and 4 in Fig. 2.
Figure 8. Salinity at 10 m in the study areas in spring (upper), summer (middle), and autumn (lower). (a) to (d) are the Areas 1, 2, 3, and 4 in Fig. 2.
Three theta-salinity curves of water mass reference are selected for further comparison with the water type in the study area. The first curve is the water of the KC upstream of the I-Lan Ridge (121.8°E−122.5°E, 23°N−24°N) from Mensah et al. (2014). This area is located on the KC axis,
which is a reference for the KC. The second curve is the southwest water of Taiwan Island (119°E−120°E, 22°N−23°N), which is located in the mixing area of the KC branch and the northern South China Sea water, where the water continues to flow through the Taiwan Strait to the north of Taiwan Island. The third curve is the water of the north of Taiwan Island (121.5°E−122°E, 25.2°N−25.5°N), which reflects the characteristics of the coastal and shelf waters. The KC water shows a higher salinity maximum and a higher temperature than the north shelf water, and some portion of the north shelf water may come from the mixed southwest water of Taiwan and fresh water from the Taiwan Strait. Figure 9 shows the comparison of water masses in different areas with contoured isopycnals. The solid and dashed black dot lines represent the water of Areas 1 and 4. The theta-salinity curves illustrate a difference of <0.1 psu along the 23–25 kg m−3 isopycnal between the coastal water (red) and the I-Lan Bay water (solid black) in the spring, summer, and autumn. The coastal water in the upper layer is fresher and colder than that in Area 1. However, the water in Area 4 is almost consistent with that of the upstream KC water. This study finds that the water mass in the I-Lan Bay has the lower salinity and temperature than that in the KC region in all seasons, especially in Area 1. Meanwhile, the water mass in Area 3 is slightly closer to that of the KC water, but retains some of the coastal water properties. This implies that Area 3 may serve as a boundary between the coastal water and the KC water.
Figure 9. Reference theta-salinity curves representing the coastal and shelf waters north of Taiwan Island (red), southwest of Taiwan (green), and the KC upstream of I-Lan ridge (blue). The solid and dashed lines represent the waters
in Areas 1 and 4 in Fig. 2. Asterisk line in the right column represents the water in Area 2. The columns from the left to the right represent the spring, summer, and autumn, respectively.
3.3 Composition and source of the water in I-Lan Bay The simplified index for the water mass analysis (see section 2.2 for details) defined by Li et al. (2006) is used in this study. This method helps to clarify the composition and source of the water in the I-Lan Bay as shown in Figure 10. The red and blue lines represent the KC water upstream of I-Lan ridge (warm and salty) and the water on the north shelf of Taiwan Island (cold and fresh), respectively. These two indicators serve as reference water masses for the water in I-Lan Bay. The square points represent the water mass in Area 1 (I-Lan Bay). The surface water mass (0–10 m) is similar to the shelf water from March to October, and more biased toward the KC water in November and December. However, the water mass at 100 m in the I-Lan Bay is close to the shelf water in all months. The water masses in Area 3 behave like transitional waters, which possibly mix the coastal fresher water and the warm KC water influenced by the swing of the KC path. The water masses of Area 4 located in the KC main path show characteristics close to the KC water, and only slight difference is found in the surface water between June and August, because the KC surface water tends to have a high temperature and is fresher in summer.
Figure 10. Comparison of monthly mean water mass index (a) at 0–10 m and (b) at 100 m. The red and blue lines represent warm and salty water masses and cold and fresh water masses, respectively. The square, triangle, diamond, and cross points represent the water masses in Areas 1, 2, 3, and 4, respectively. Some values are so close that the patterns overlap, the value of square pattern in November and December is 0.57 and 1.20.
4. Discussion 4.1 Mechanisms The dynamic processes of the occurrence of countercurrents are discussed according to the results of EOF analysis. It is divided into two types based on the first and second mode of EOF from monthly geostrophic current data between January 1993 and December 2016 (Fig. 11). The first type (Type 1) is according to the first mode of EOF (EOF1) result, which the countercurrent flows along the northeast coast of Taiwan. The second type (Type 2) is according to the second mode of EOF (EOF2) result, which is the countercurrent occurrence due to the formation of a cold dome. Combining with the geostrophic current data and sea surface temperature (SST) images showed as examples of both countercurrent types. The daily SST imagery with 2 km spatial resolution is produced from Himawari-8 and provided by the P-Tree System, Japan Aerospace Exploration Agency (JAXA). The dynamic process of Type 1 countercurrent in the I-Lan Bay is caused by the wind-driven current, from the Taiwan Strait flows along the north coast and the northeast coast of Taiwan. The temporal variation of EOF1 (Fig. 11a) presents that the countercurrents in the I-Lan Bay may occur in every season but the intensity is stronger in summer. The average amplitude of temporal variation of EOF1 in summer is 5.9 and is the highest amplitude of a year. This result is consistent with historical hydrological data (Fig. 5). The mean countercurrent speed in summer is the fastest. Figure 12 shows the EOF1 results of winds from monthly wind data between January 2000 and December 2016. It contains 84% of the total variance. The southwesterly winds mainly occur in June, July, and August (boreal summer). Compare the temporal variations of EOF1 between currents and winds from January 2000 to December 2016, the correlation coefficient is about 0.57 (p <0.05). This implies that the southwesterly winds could
cause the countercurrents strongly in summer. According to Jan et al. (2002), the northward Taiwan Warm Current from the South China Sea is driven by the southwesterly winds in summer. When it passes the Taiwan Strait, the surface Ekman transport (U) forces the current deflecting to its right along the north coast and then northeast coast of Taiwan as (Chang et al., 2010): 𝑈 = 𝜏0 ⁄𝑓0 (1 + 𝑅𝑜𝑠 ),
(1)
where 𝜏0 is the wind stress, 𝑓0 is the Coriolis parameter, and 𝑅𝑜𝑠 is the surface non-dimensional vorticity. This suggested that the flow set in motion by the wind stress was deflected eastward (right of the wind direction) along the northern coast of Taiwan into the I-Lan Bay during the southerly or southwesterly wind to form the countercurrent. During the northerly monsoon season, the flow north of Taiwan is mainly the China Coastal Current (Jan et al., 2010). The currents are affected by wind stresses and flows along the continental shelf into the I-Lan Bay. The composition and source of the Taiwan Strait Current can be referred to Jan et al. (2002). Figure 13 depicts representative images with SST when countercurrent occurred in I-Lan Bay in each season. The cold water flows from the Taiwan Strait side and enters the I-Lan Bay with countercurrents, which caused a sharp ocean thermal front between the I-Lan Bay and the KC.
Figure 11. EOF results of spatial pattern and temporal variation of monthly geostrophic currents from January 1993 to December 2016. (a) The first mode, and (b) the second mode. The percentage of variance explained by the first mode is 79 %, and the second mode is 6%.
Figure 12. The first mode of EOF results of spatial pattern and temporal variation of monthly winds from January 2000 to December 2016. The percentage of variance explained by the first mode is 84 %.
Figure 13. Examples of Type 1 countercurrent with the SST images of Himawari-8 and geostrophic currents derived from satellite altimetry on (a) April 5-6, 2016, (b) June 26, 2016, (c) September 17-19, 2015, and (d) December 1-2, 2015, respectively. White arrows denote geostrophic current.
The dynamic process of Type 2 countercurrent in the I-Lan Bay is caused by the occurrence of a cold dome near northeast of Taiwan. The formation of the cold dome is related to the process of Kuroshio deflection caused by the wind stress and the topography effects (Hsueh et al., 1992; 1993, Chen et al., 1996). The size and shape of the cold dome can be affected by the KC in the East China Sea, the current and coastal flow from the Taiwan Strait, and the impact of monsoon (Jan et al., 2011; Shen et al., 2011; Gopalakrishnan et al., 2013). They suggested the position variability and interannual variability are caused by wind stress curl and the on-shelf water transport of the KC through the North Men-Hua Canyon. A schematic diagram of the outer current of the cold dome to flow to the I-Lan Bay as a countercurrent is shown in Figure 14. The second mode of EOF (EOF2) results (Fig. 11b) presented the cold dome could exist in each month and mainly occurred in summer season. Compare EOF2 results of currents and EOF results of winds between January 2000 and December 2016, the correlation coefficient of temporal variations is 0.75 (p <0.05). These results consistent with the pervious study (Cheng et al., 2009), which used SST and sea surface height anomaly data and found that the cold dome exists in a whole year but the most often occurs in summer. The southwesterly monsoon could cause the cold dome to be more likely to occur in summer. Based on the basin-scale, eddy-resolving dual-domain Pacific Ocean Model results from Shen et al. (2011), the formation of cold dome could be separated into four possible mechanisms. Two major mechanisms cause the cold dome to occur most in summer: 1) dynamics isotherm uplift resulting from the geostrophic component of Kuroshio, which is expected to be large in summer due to the fast Kuroshio speed and the Kuroshio axis position shift; and 2) Ekman transport causing divergence and upwelling during the cyclonic summer monsoon. As mentioned above, the wind is auxiliary for the cold dome formation. Figure 15 presents examples of the cold dome and the occurring countercurrents in the I-Lan Bay. One can see that the cold water generated by the cold dome is accompanied by an anti-clockwise flow field and flows into the I-Lan Bay, which contrasts sharply with the high-temperature characteristics of the KC.
Figure 14. A schematic diagram of I-Lan Bay countercurrent (Type 2).
Figure 15. Same as Figure 13, but for Type 2 countercurrent. (a) April 16, 2016, (b) August 24-25, 2016, and (c) October 25-26, 2015, respectively.
4.2 Chlorophyll-a concentration and attenuation coefficient Coastal and shelf waters flow into I-Lan Bay, causing significant ocean fronts with the KC (Fig. 16). The water quality of the KC water is clear relative to the Taiwan Strait water and the East China Sea water. To discuss the interaction of these different water masses, monthly moderate-resolution imaging spectroradiometer (MODIS) chlorophyll-a (Chl-a) concentrations and attenuation coefficient at 490 nm (Kd_490) data (https://modis.gsfc.nasa.gov/) are used as additional indicators for identifying the water masses and the fronts of different water masses (Kim et al., 2017; Ciancia et al., 2018). Figure 17 shows the mean geostrophic currents field with the
Chl-a and Kd_490 in summer and others season. In general, the clear KC water has low Chl-a concentration and low Kd_490. Lower Kd_490 value means less attenuation and higher clarity of ocean water. If the current flows from the Taiwan Strait side to the Kuroshio side of eastern Taiwan (countercurrents occur), the water mass is expected to have high Chl-a concentration and low transparency characteristics. The Chl-a concentration (mg/m3) is 0.53 (winter), 0.56 (spring), 0.43 (summer) and 0.62 (autumn) in Area 1 (see Fig. 2). In contrast, the Chl-a concentration in Area 4 (see Fig. 2) is only 0.22, 0.17, 0.16, and 0.16 mg/m3 for winter, spring, summer, and autumn, respectively from July 2002 to December 2016. The Chl-a concentration in the I-Lan Bay always maintains a high value, and there is no obviously seasonal variability. The value of Kd_490 (m−1) also indicates the same trend. The seasonal mean in Area 1 is 0.07 (winter), 0.07 (spring), 0.07 (summer), and 0.08 (autumn), higher than the mean in Area 4 (0.05 in winter, 0.04 in spring, 0.04 in summer, and 0.04 in autumn). Unlike the more transparent KC water, the water in the I-Lan Bay trends toward the turbid coastal waters.
Figure 16. Average geostrophic currents field (white arrows), Chl-a (a and b) and Kd_490 (c and d). (a) and (c) in summer season; (b) and (d) in winter, spring and autumn. The black dash line indicates the Kuroshio main axis with maximum current velocity.
5. Summary In this study, CTD, Sb-ADCP, drifting buoys, satellite altimeter and MODIS are used to analyze the hydrological properties of the coastal countercurrent in the I-Lan Bay and the northeast of Taiwan Island, as well as variations of the KC nearshore side. From the results of the flow field analysis, drifting buoys recorded the presence of a coastal countercurrent in the I-Lan Bay, which may originate from the cold dome and the northeastern shelf of Taiwan Island. The EOF analysis of satellite altimeter data also shows the same results. The velocity profile from the Sb-ADCP data indicate that the remarkable surface countercurrent occurs in the north of the I-Lan Bay in summer and autumn. The surface countercurrent still occurs in winter, but has a slower velocity. However, at the lower depth of 100–200 m, the evident countercurrent flows along the northeast coast of Taiwan Island to the I-Lan Bay. The velocity of the V-component is between −0.1 to −0.2 m/s in the I-Lan Bay. From the perspective of hydrological analysis, the cross sectioned vertical structure of seasonal means shows significant salinity difference between the water in the I-Lan Bay and the KC water, especially the gap between the 23 to 25 kg m−3 isopycnal and the theta-salinity curve. The I-Lan Bay water has a lower 𝑆𝑚𝑎𝑥 than that of the KC water. The simplified index for the water mass analysis is used to classify composition of the water in the I-Lan Bay. The results show that it is close to fresh shelf water. This study indicates that the water in the I-Lan Bay is mainly affected by the northern shelf waters of Taiwan Island and mixed with the KC nearshore water. Thus, we conclude that the I-Lan Bay water has unique characteristics. This study will help further analysis of biochemical systems around this area. In addition, there is a basic understanding, i.e., the future study on interaction between the KC and the I-Lan Bay should have a comprehensive analysis on land-based radar, buoy observations and glider observations.
Acknowledgements Research product of sea surface temperature used in this paper was supplied by the P-Tree System, Japan Aerospace Exploration Agency (JAXA). This work is supported by the Ministry of Science and Technology of Taiwan through grants MOST 106-2611-M-019-015 and MOST 106-2917-I-019-001.
References Andres, M., Jan, S., Sanford, T. B., Mensah, V., Centurioni, L. R., & Book, J. W. (2015). Mean structure and variability of
the
Kuroshio
from
northeastern
Taiwan
to
southwestern
Japan.
Oceanography,
28(4),
84-95,
doi:10.5670/oceanog.2015.84. Andres, M., V. Mensah, S. Jan, M.-H. Chang, Y.-J. Yang, C. M. Lee, B. Ma, and T. B. Sanford (2017), Downstream evolution of the Kuroshio's time-varying transport and velocity structure, Journal of Geophysical Research: Oceans, 122, 3519-3542, doi:10.1002/2016JC012519. Ciancia, E., Coviello, I., Di Polito, C., Lacava, T., Pergola, N., Satriano, V., & Tramutoli, V. (2018). Investigating the chlorophyll-a variability in the Gulf of Taranto (North-western Ionian Sea) by a multi-temporal analysis of MODIS-Aqua Level 3/Level 2 data. Continental Shelf Research, 155, 34-44, doi:10.1016/j.csr.2018.01.011. Chen, H. T., Yan, X. H., Shaw, P. T., & Zheng, Q. (1996). A numerical simulation of wind stress and topographic effects on the Kuroshio current path near Taiwan. Journal of physical oceanography, 26(9), 1769-1802, doi: 10.1175/1520-0485(1996)026<1769:ANSOWS>2.0.CO;2. Chang, Y. L., Oey, L. Y., Wu, C. R., & Lu, H. F. (2010). Why are there upwellings on the northern shelf of Taiwan under northeasterly winds?. Journal of Physical Oceanography, 40(6), 1405-1417, doi:10.1175/2010JPO4348.1. Cheng, Y. H., Ho, C. R., Zheng, Z. W., Lee, Y. H., & Kuo, N. J. (2009). An algorithm for cold patch detection in the Sea off northeast Taiwan using multi-sensor data. Sensors, 9(7), 5521-5533, doi:10.3390/s90705521. Cheng, Y. H., Hu, J., Zheng, Q., & Su, F. C. (2017). Interannual variability of cold domes northeast of Taiwan. International Journal of Remote Sensing, 1-11, doi:10.1080/01431161.2017.1395972 Gopalakrishnan, G., Cornuelle, B. D., Gawarkiewicz, G., & McClean, J. L. (2013). Structure and evolution of the cold dome off northeastern Taiwan: A numerical study. Oceanography, 26(1), 66-79, doi:10.5670/oceanog.2013.06 Hsueh, Y., J. Wang, and C.‐ S. Chern (1992), The intrusion of the Kuroshio across the continental shelf northeast of Taiwan, Journal of Geophysical Research: Oceans, 97(C9), 14323–14330, doi:10.1029/92JC01401. Hsueh, Y., Chern, C. S., & Wang, J. (1993). Blocking of the Kuroshio by the continental shelf northeast of Taiwan. Journal of Geophysical Research: Oceans, 98(C7), 12351-12359, doi: 10.1029/93JC01075. Hsu, P. C., Lin, C. C., Huang, S. J., & Ho, C. R. (2016). Effects of cold eddy on Kuroshio meander and its surface properties, east of Taiwan. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(11), 5055-5063, doi:10.1109/JSTARS.2016.2524698. Hsu, P. C., & Ho, C. R. (2018). Typhoon-induced ocean subsurface variations from glider data in the Kuroshio region adjacent to Taiwan. Journal of Oceanography, doi:10.1007/s10872-018-0480-2 Jan, S., Wang, J., Chern, C. S., & Chao, S. Y. (2002). Seasonal variation of the circulation in the Taiwan Strait. Journal of Marine Systems, 35(3-4), 249-268, doi:10.1016/S0924-7963(02)00130-6
Jan, S., Tseng, Y. H., & Dietrich, D. E. (2010). Sources of water in the Taiwan Strait. Journal of Oceanography, 66(2), 211-221, doi:10.1007/s10872-010-0019-7 Jan, S., Chen, C. C., Tsai, Y. L., Yang, Y. J., Wang, J., Chern, C. S., ... & Kuo, J. Y. (2011). Mean structure and variability of the cold dome northeast of Taiwan. Oceanography, 24(4), 100-109, doi:10.5670/oceanog.2011.98. Jan, S., Y. J. Yang, J. Wang, V. Mensah, T.-H. Kuo, M.-D. Chiou, C.-S. Chern, M.-H. Chang, and H. Chien (2015), Large variability of the Kuroshio at 23.75°N east of Taiwan, Journal of Geophysical Research: Oceans, 120, 1825– 1840, doi:10.1002/2014JC010614. Johns, W. E., Lee, T. N., Zhang, D., Zantopp, R., Liu, C. T., & Yang, Y. (2001). The Kuroshio east of Taiwan: Moored transport observations from the WOCE PCM-1 array. Journal of Physical Oceanography, 31(4), 1031-1053, doi:10.1175/1520-0485(2001)031<1031:TKEOTM>2.0.CO;2. Kim, H. C., Son, S., Kim, Y. H., Khim, J. S., Nam, J., Chang, W. K., ... & Ryu, J. (2017). Remote sensing and water quality indicators in the Korean West coast: Spatio-temporal structures of MODIS-derived chlorophyll-a and total suspended solids. Marine pollution bulletin, 121(1-2), 425-434, doi:10.1016/j.marpolbul.2017.05.026. Li, G., Han, X., Yue, S., Wen, G., Rongmin, Y., & Kusky, T. M. (2006). Monthly variations of water masses in the East China Seas. Continental Shelf Research, 26(16), 1954-1970, doi:10.1016/j.csr.2006.06.008. Liu, S., Qiao, L., Li, G., Li, J., Wang, N., & Yang, J. (2015). Distribution and cross-front transport of suspended particulate matter over the inner shelf of the East China Sea. Continental Shelf Research, 107, 92-102, doi:10.1016/j.csr.2015.07.013. Mensah, V., Jan, S., Chiou, M. D., Kuo, T. H., & Lien, R. C. (2014). Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan. Deep Sea Research Part I: Oceanographic Research Papers, 86, 68-81, doi:10.1016/j.dsr.2014.01.005. Mensah, V., S. Jan, M.-H. Chang, and Y.-J. Yang (2015), Intraseasonal to seasonal variability of the intermediate waters along the Kuroshio path east of Taiwan, Journal of Geophysical Research: Oceans, 120, 5473–5489, doi:10.1002/2015JC010768. Shen, M. L., Tseng, Y. H., & Jan, S. (2011). The formation and dynamics of the cold-dome off northeastern Taiwan. Journal of Marine Systems, 86(1), 10-27, doi:10.1016/j.jmarsys.2011.01.002. Tang, T. Y., Tai, J. H., & Yang, Y. J. (2000). The flow pattern north of Taiwan and the migration of the Kuroshio. Continental Shelf Research, 20(4), 349-371, doi:10.1016/S0278-4343(99)00076-X. Takahashi, D., Guo, X., Morimoto, A., & Kojima, S. (2009). Biweekly periodic variation of the Kuroshio axis northeast of Taiwan as revealed by ocean high-frequency radar. Continental Shelf Research, 29(15), 1896-1907, doi:10.1016/j.csr.2009.07.007. Wu, C.-R., H.-F. Lu, and S.-Y. Chao (2008), A numerical study on the formation of upwelling off northeast Taiwan, Journal of Geophysical Research, 113, C08025, doi:10.1029/2007JC004697. Yang, Y. J., Jan, S., Chang, M. H., Wang, J., Mensah, V., Kuo, T. H., ... & Lai, J. W. (2015). Mean structure and fluctuations of the Kuroshio East of Taiwan from in situ and remote observations. Oceanography, 28(4), 74-83, doi:10.5670/oceanog.2015.83. Yin, Y., X. Lin, R. He, and Y. Hou (2017), Impact of mesoscale eddies on Kuroshio intrusion variability northeast of Taiwan, Journal of Geophysical Research: Oceans, 122, 3021–3040, doi:10.1002/2016JC012263.
Zhang, D., Lee, T. N., Johns, W. E., Liu, C. T., & Zantopp, R. (2001). The Kuroshio east of Taiwan: Modes of variability and relationship to interior ocean mesoscale eddies. Journal of Physical Oceanography, 31(4), 1054-1074, doi:10.1175/1520-0485(2001)031<1054:TKEOTM>2.0.CO;2. Zheng, Z. W., & Zheng, Q. (2014). Variability of island-induced ocean vortex trains in the Kuroshio region southeast of Taiwan Island. Continental Shelf Research, 81, 1-6, doi:10.1016/j.csr.2014.02.010. Zheng, Z.-W., Q. Zheng, C.-Y. Lee, and G. Gopalakrishnan (2014), Transient modulation of Kuroshio upper layer flow by directly impinging typhoon Morakot in east of Taiwan in 2009, Journal of Geophysical Research: Oceans, 119, 4462–4473, doi:10.1002/2014JC010090.
Highlights 1. Ship-board acoustic Doppler current profiles are used to observeflow structure from I-Lan Bay to Kuroshio region. 2. Simplified index of water mass based on CTD data isused to analyze
compositionsand source of
water in I-Lan Bay. 3. The water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water. 4. Dynamic processes of the occurrence of countercurrent in I-Lan Bay are explained.
PII: DOI: Reference:
S0278-4343(17)30507-1 https://doi.org/10.1016/j.csr.2018.10.012 CSR3831
To appear in: Continental Shelf Research Received date: 4 October 2017 Revised date: 11 September 2018 Accepted date: 22 October 2018 Cite this article as: Po-Chun Hsu, Quanan Zheng, Ching-Yuan Lu, Kai-Ho Cheng, Hung-Jen Lee and Chung-Ru Ho, Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan, Continental Shelf Research, https://doi.org/10.1016/j.csr.2018.10.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan Po-Chun Hsua,b, Quanan Zhengc, Ching-Yuan Lua, Kai-Ho Chenga, Hung-Jen Leea, Chung-Ru Hoa* a
Department of Marine Environmental Informatics, National Taiwan Ocean University, Keelung, Taiwan
b
Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan
c
Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA
*Corresponding author: [email protected] (C.-R. Ho)
Abstract The interaction of coastal water in I-Lan Bay, a bay near northeast Taiwan, and the Kuroshio Current is studied using the data from hydrologic survey and satellite remote sensing. An index for water mass distinguished is used for clarifying the origin of water mass in I-Lan Bay. The velocity profile from acoustic Doppler current profile data indicates a countercurrent that flows southward along the northeast coast of Taiwan toward I-Lan Bay with a speed around 0.1-0.2 m/s. The index of water mass indicates that the water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water, thus forming a clear ocean front between these two areas. The near surface water temperature in the I-Lan Bay is about 2 degrees Celsius lower than that in the Kuroshio region. The seasonal average near surface salinity of I-Lan Bay can exist 0.4 psu fresher than the Kuroshio area. From the analysis of dynamic process, the coastal countercurrent in I-Lan Bay is primarily affected by 1) the occurrence of cold dome in northeast of Taiwan mainly occurs in summer, and 2) the wind-driven current from the Taiwan Strait along the north coast and the northeast coast of Taiwan caused by the southwesterly monsoon makes the countercurrents strongly in summer. The dynamics of the countercurrent occurrence can be explained by the Ekman transport theory. Keywords: Kuroshio; coastal countercurrent; cold dome; water masses; northeast of Taiwan; interaction. 1. Introduction Previous investigators have explored the effects of dynamic processes produced by mesoscale eddies and typhoons on the Kuroshio Current (KC) adjacent to Taiwan Island (Zheng et al., 2014; Andres et al., 2015, 2017) and conducted comprehensive field observations of the KC structure related to multi-scale processes from the Luzon Strait to northeast of Taiwan. For example, evolution of KC Tropical Water and variability of KC intermediate waters was one of focuses
(Mensah et al., 2014, 2015; Jan et al., 2015; Yang et al., 2015). The World Ocean Circulation Experiment (WOCE) PCM-1 line was the first project aimed to systematic measurement of the KC volume transport using a mooring array deployed in the south of I-Lan Bay from 1994 to 1996 (Johns et al., 2001; Zhang et al., 2001). Recently, three bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed across the KC along 23.75°N east of Taiwan Island, named KTV1 line, providing measurements of the KC velocity, which were used to analyze interaction of the KC with mesoscale eddies (Jan et al., 2015). Both experiments focused on variation of the KC velocity and transport. However, flow field structure in I-Lan Bay located off the northeastern coast of Taiwan Island and on the KC nearshore side, as well as impacts of its variation on the hydrodynamic environment remain unclear (Fig. 1). Previous investigators have addressed the KC branch invading the East China Sea shelf and a cold dome phenomenon on the northern shelf of Taiwan Island. Cheng et al. (2009) found the cold dome occurred every month and its centroid was located between 25.25°N and 26°N. Takahashi et al. (2009) used ocean high-frequency radar to capture the flow pattern and found that a countercurrent occurs off northeastern Taiwan Island. Jan et al. (2011) indicated that the summer Taiwan Strait outflow and winter monsoon-driven China Coastal Current may affect a counterclockwise flow around the cold dome and intensify upwelling. The phenomenon of countercurrent has been also observed (Tang et al., 2000; Wu et al., 2008). When an upwelling area (counterclockwise flow of the cold dome) develops, it will produce a countercurrent along the northeast of Taiwan Island. However, the questions that where does the countercurrent eventually flow, whether does it affect the coastal shelf area, and what is its relation to the KC axis shift remain unclear. This study aims to understand the flow field structure and hydrological characteristics on the KC nearshore side from the continental shelf area northeast of Taiwan to the I-Lan Bay area. The interaction between low-temperature and low-salt coastal water with high-temperature and high-salt
KC water around the I-Lan Bay area will be clarified. The baseline includes shipboard acoustic Doppler current profiler (Sb-ADCP) data, conductive, temperature, and depth (CTD) profiles, and satellite remote sensing data.
Figure 1. Bathymetry of the study area around I-Lan Bay. The solid blue line represents the location of KC main axis.
2. Data and methods 2.1 In-situ data In-situ current data are derived from drifters and Sb-ADCP measurements. The drifter data were collected and processed by the Atlantic Oceanographic and Meteorological Laboratory under the Global Drifter Program (formerly the World Ocean Circulation Experiment-Surface Velocity Programme) and obtained from Fisheries and Oceans Canada (http://www.dfo-mpo.gc.ca). Historical CTD profiles from 1998 to 2010 and Sb-ADCP data from 1991 to 2017 acquired from the vicinity of Taiwan by R/V Ocean Researcher I, II, and III (www.odb.ntu.edu.tw). CTD profiles from 1986 to 2016 obtained from the World Ocean Database of the National Oceanographic Data Center (https://www.nodc.noaa.gov) are interpolated to standard depth levels. The historical Sb-ADCP data used in this study have been gridded to a 0.25° x 0.25° grid in latitude and longitude.
The depth ranges from 0 m to 300 m. In addition to reference water masses (northern shelf water, southwest water of Taiwan, and the KC), the data were divided into four areas along I-Lan Bay toward the mainstream area of the KC (Fig. 2).
Figure 2. Observation areas 1 to 4 of the CTD data. The red line represents the location of KC main axis.
2.2 Satellite data The monthly absolute geostrophic current data with 0.25 degree spatial resolution from January 1993 to December 2016 are calculated from the absolute dynamic topography (ADT) data which are derived from satellite altimeter data. This dataset was provided by the Copernicus Marine and Environment Monitoring Service (CMEMS). The monthly wind data with 0.25 degree spatial resolution are derived from Quick Scatterometer (QuikScat) from January 2000 to December 2008 and C-2015 Advanced Scatterometer (ASCAT) from January 2009 to December 2016. QuikScat and ASCAT data are produced by Remote Sensing Systems which is sponsored by the National Aeronautics and Space Administration (NASA) Ocean Vector Winds Science Team. Data are
available at www.remss.com.
2.3 Index for water mass analysis The water mass analysis conducted in this study is based on a simplified index defined as (Li et al., 2006) β=
(𝑇−𝑇̅ ) ∆𝑇
+
(𝑆−𝑆̅) ∆𝑆
𝛼,
where β is a non-dimensional index, representing the contribution of temperature and salinity to water mass stability. The index is used to quantify the tendency of a water mass to change from warm and salty water to cold and fresh water. 𝑇 and 𝑆 are the temperature and the salinity in each CTD profile. 𝑇̅ and 𝑆̅ are the mean temperature (22.7℃) and salinity (34.45 psu) of the study area (19°N−27°N, 119°E−124°E), respectively. ∆𝑇(4.0) and ∆𝑆 (0.5) are the average annual mean differences of the temperature and the salinity between the northern shelf water and the KC water upstream of the I-Lan Ridge, respectively. 𝛼 is the contribution ratio and set as 5 (Li et al., 2006). The parameters in the index are determined based on historical in situ data from the World Ocean Atlas 2013, Version 2 (www.nodc.noaa.gov/OC5/woa13/) and Hybrid Coordinate Ocean Model data (HYCOM) (https://hycom.org/). HYCOM data have been used by previous research into smallto medium-scale flow fields and hydrological vertical structures adjacent to Taiwan Island (Zheng and Zheng, 2014; Hsu et al., 2016; Yin et al., 2017).
3. Results 3.1 Characteristics of the flow field off the northeast coast of Taiwan The countercurrent, flowing an opposite direction to the KC, occurs off the north coast of Taiwan Island, flows clockwise into the Keelung Sea Valley and I-Lan Bay, and continues to flow southward across the I-Lan shelf or turn northeastward and interflow with the KC (Fig. 3). Clear indications of annual countercurrents of geostrophic flow have also been found from the results of
the first mode of the empirical orthogonal function (EOF) analysis (Fig. 4). However, when each month was examined separately, the frequency of the countercurrent in June and July was more than 80% but only 20% in January and February. As shown in Figures 4d and 4e, the flow field of the northeastern coast of Taiwan was significantly different in summer and winter. In contrast, the current speed and pattern of the KC axis outside of I-Lan Bay had a significant difference in summer and winter, but the average position of the flow axis was only ~10 km different (Hsu et al., 2016). The historical Sb-ADCP data are used to obtain the in situ flow fields of the northeastern continental shelf of Taiwan and the I-Lan Bay. Figure 5 presents the seasonally mean velocity profiles from the coast along 25.25°N, 25°N, 24.75°N, and 24.5°N. Figure 5a shows the results along the Keelung Sea Valley and the Mian-Hwa Canyon from 121.75°E to 122.25°E. The V-component velocity reaches −0.27 m/s in summer and the average speed at the depth of 150–250 m reaches −0.17 m/s. Figure 5b shows that the countercurrent at 122°E is clearly visible in summer and autumn, and the average surface V-component velocity is close to zero in winter and spring, which may be due to an average flow in the opposite directions. However, at 100–200 m, the countercurrent is observed in all seasons. Figure 5c shows the flow field in the I-Lan Bay, and the average V-component velocity is between −0.1 and −0.2 m/s southward for all seasons. Figure 5d shows the flow field at the southern end of I-Lan Bay, which is characterized by that the coastal countercurrent continually exists in the subsurface layer. From the data of drifters, satellite altimeters, and historical Sb-ADCP, we found that countercurrent flow occurs from the continental shelf northeast of Taiwan and southward along the coast to the I-Lan Bay and possibly further southward. The variations of the KC did not seem to affect the waters of I-Lan Bay. The influence of the KC axis shift may be limited outside of 122°E. Whether the hydrological environment of I-Lan Bay was affected by the variations of the KC path near the shore side or not could be further analyzed by hydrological data.
Figure 3. Trajectories of drifters adjacent to northeast of Taiwan Island. Red and blue lines represent the trajectories of the countercurrent and the KC, respectively.
Figure 4. The first mode of the empirical orthogonal function (EOF) of monthly geostrophic currents northeast of Taiwan (a) U-component, (b) V-component, (c) current vectors, and comparison of mean current fields in (d) summer and (e) winter. Red boxes show the current fields off the northeast coast of Taiwan Island.
Figure 5. Seasonally mean velocity (U positive eastward and V positive northward) profiles from the coast of northeast Taiwan to open ocean along (a) 25.25°N, (b) 25°N, (c) 24.75°N and (d) 24.5°N from 1991 to 2017. The blank indicates no data.
3.2 Water mass distribution from I-Lan Bay to the KC region A cross section of the vertical structure of climatological mean shows significant temperature and salinity differences between the coastal and the KC waters. Because of few field measurements in winter, the seasonal analysis is only available in the other three seasons. In the near-surface layer (0−20 m), the water temperature in the I-Lan Bay is about 2℃ lower than the water temperature in the east of 122.2°E (KC region) in all three seasons (Fig. 6). Salinity differences in the near-surface water layer in the I-Lan Bay are only 0.1 psu fresher than that in the KC region in spring and
summer, but the water is 0.4 psu fresher in the I-Lan Bay (33.95 psu) than that in the KC region (34.32 psu) in autumn (Fig. 6). In the 50–200 m deep layer, the salinity in the I-Lan Bay is about 0.2 psu fresher than that in the KC region in spring and autumn, and 0.1 psu fresher in summer. The water temperature in the I-Lan Bay is about 1℃ lower than that in the KC region in all three seasons.
Figure 6. Cross sections of salinity and temperature from the coastal area (I-Lan Bay) to the KC region in (a) spring (b) summer, and (c) autumn derived from the CTD data.
To further confirm the salinity difference between the coastal water and the KC, a histogram of salinity maximums from the four observed areas is analyzed (Fig. 7). To ensure the quality of the calculation, the CTD data only at depths deeper than 150 m are used. Figures 7a – d represent areas
1 to 4, respectively. The first to third rows represent the seasonal states of spring, summer, and autumn. Figure 7d exhibits a peak of salinity greater than 34.8 psu and an average salinity maximum of about 34.76 psu in Area 4. Areas 1 and 2 show a peak around 34.55 to 34.6 psu and an average salinity maximum of about 34.57 psu. The water mass in the area of 121.9° to 122.1°E does not seem to be affected by the variation of the KC path. In Area 3, an interesting salinity maximum multiple-peak phenomenon is observed during three seasons. We think that this phenomenon results from intensive water mixing taking place in this area, i.e., a portion of the water is similar to the coastal (inshore) water, while other portions may come from the KC (east of 122.2°E). The salinity difference between its maximum and the value at the upper layer could be used to separate different type of water mass. For example, the lower salinity difference value indicates that the upper salinity varies slightly with depth and is close to the characteristics of the inshore water (Liu et al., 2015); and the higher difference value represents the upper salinity changed significantly with depth, consistent with KC-type water (Mensah et al., 2014). But this rule still needs to consider the occurrence of rainfall, the surface and upper layer of water could be freshened during heavy rainfall events, especially in summer (Hsu and Ho, 2018). The results in Figure 8 clearly show that the salinity value near surface is significantly fresher lower than that in other seasons. Therefore, it is good to use the maximum salinity and salinity at 10 m (𝑆10𝑚 ) to distinguish the characteristics of water mass. Figure 8 presents a histogram of salinity at 10 m from four observed areas, the 𝑆10𝑚 in the inshore water is fresher than that in the KC region. Together with Figures 7 and 8, the water mass of the I-Lan Bay, which is affected by coastal countercurrent, has different water characteristics with the KC water where exists in the region east of 122.2°E.
Figure 7. Maximum salinity in the study areas in spring (upper), summer (middle), and autumn (lower). (a) to (d) are Areas 1, 2, 3, and 4 in Fig. 2.
Figure 8. Salinity at 10 m in the study areas in spring (upper), summer (middle), and autumn (lower). (a) to (d) are the Areas 1, 2, 3, and 4 in Fig. 2.
Three theta-salinity curves of water mass reference are selected for further comparison with the water type in the study area. The first curve is the water of the KC upstream of the I-Lan Ridge (121.8°E−122.5°E, 23°N−24°N) from Mensah et al. (2014). This area is located on the KC axis,
which is a reference for the KC. The second curve is the southwest water of Taiwan Island (119°E−120°E, 22°N−23°N), which is located in the mixing area of the KC branch and the northern South China Sea water, where the water continues to flow through the Taiwan Strait to the north of Taiwan Island. The third curve is the water of the north of Taiwan Island (121.5°E−122°E, 25.2°N−25.5°N), which reflects the characteristics of the coastal and shelf waters. The KC water shows a higher salinity maximum and a higher temperature than the north shelf water, and some portion of the north shelf water may come from the mixed southwest water of Taiwan and fresh water from the Taiwan Strait. Figure 9 shows the comparison of water masses in different areas with contoured isopycnals. The solid and dashed black dot lines represent the water of Areas 1 and 4. The theta-salinity curves illustrate a difference of <0.1 psu along the 23–25 kg m−3 isopycnal between the coastal water (red) and the I-Lan Bay water (solid black) in the spring, summer, and autumn. The coastal water in the upper layer is fresher and colder than that in Area 1. However, the water in Area 4 is almost consistent with that of the upstream KC water. This study finds that the water mass in the I-Lan Bay has the lower salinity and temperature than that in the KC region in all seasons, especially in Area 1. Meanwhile, the water mass in Area 3 is slightly closer to that of the KC water, but retains some of the coastal water properties. This implies that Area 3 may serve as a boundary between the coastal water and the KC water.
Figure 9. Reference theta-salinity curves representing the coastal and shelf waters north of Taiwan Island (red), southwest of Taiwan (green), and the KC upstream of I-Lan ridge (blue). The solid and dashed lines represent the waters
in Areas 1 and 4 in Fig. 2. Asterisk line in the right column represents the water in Area 2. The columns from the left to the right represent the spring, summer, and autumn, respectively.
3.3 Composition and source of the water in I-Lan Bay The simplified index for the water mass analysis (see section 2.2 for details) defined by Li et al. (2006) is used in this study. This method helps to clarify the composition and source of the water in the I-Lan Bay as shown in Figure 10. The red and blue lines represent the KC water upstream of I-Lan ridge (warm and salty) and the water on the north shelf of Taiwan Island (cold and fresh), respectively. These two indicators serve as reference water masses for the water in I-Lan Bay. The square points represent the water mass in Area 1 (I-Lan Bay). The surface water mass (0–10 m) is similar to the shelf water from March to October, and more biased toward the KC water in November and December. However, the water mass at 100 m in the I-Lan Bay is close to the shelf water in all months. The water masses in Area 3 behave like transitional waters, which possibly mix the coastal fresher water and the warm KC water influenced by the swing of the KC path. The water masses of Area 4 located in the KC main path show characteristics close to the KC water, and only slight difference is found in the surface water between June and August, because the KC surface water tends to have a high temperature and is fresher in summer.
Figure 10. Comparison of monthly mean water mass index (a) at 0–10 m and (b) at 100 m. The red and blue lines represent warm and salty water masses and cold and fresh water masses, respectively. The square, triangle, diamond, and cross points represent the water masses in Areas 1, 2, 3, and 4, respectively. Some values are so close that the patterns overlap, the value of square pattern in November and December is 0.57 and 1.20.
4. Discussion 4.1 Mechanisms The dynamic processes of the occurrence of countercurrents are discussed according to the results of EOF analysis. It is divided into two types based on the first and second mode of EOF from monthly geostrophic current data between January 1993 and December 2016 (Fig. 11). The first type (Type 1) is according to the first mode of EOF (EOF1) result, which the countercurrent flows along the northeast coast of Taiwan. The second type (Type 2) is according to the second mode of EOF (EOF2) result, which is the countercurrent occurrence due to the formation of a cold dome. Combining with the geostrophic current data and sea surface temperature (SST) images showed as examples of both countercurrent types. The daily SST imagery with 2 km spatial resolution is produced from Himawari-8 and provided by the P-Tree System, Japan Aerospace Exploration Agency (JAXA). The dynamic process of Type 1 countercurrent in the I-Lan Bay is caused by the wind-driven current, from the Taiwan Strait flows along the north coast and the northeast coast of Taiwan. The temporal variation of EOF1 (Fig. 11a) presents that the countercurrents in the I-Lan Bay may occur in every season but the intensity is stronger in summer. The average amplitude of temporal variation of EOF1 in summer is 5.9 and is the highest amplitude of a year. This result is consistent with historical hydrological data (Fig. 5). The mean countercurrent speed in summer is the fastest. Figure 12 shows the EOF1 results of winds from monthly wind data between January 2000 and December 2016. It contains 84% of the total variance. The southwesterly winds mainly occur in June, July, and August (boreal summer). Compare the temporal variations of EOF1 between currents and winds from January 2000 to December 2016, the correlation coefficient is about 0.57 (p <0.05). This implies that the southwesterly winds could
cause the countercurrents strongly in summer. According to Jan et al. (2002), the northward Taiwan Warm Current from the South China Sea is driven by the southwesterly winds in summer. When it passes the Taiwan Strait, the surface Ekman transport (U) forces the current deflecting to its right along the north coast and then northeast coast of Taiwan as (Chang et al., 2010): 𝑈 = 𝜏0 ⁄𝑓0 (1 + 𝑅𝑜𝑠 ),
(1)
where 𝜏0 is the wind stress, 𝑓0 is the Coriolis parameter, and 𝑅𝑜𝑠 is the surface non-dimensional vorticity. This suggested that the flow set in motion by the wind stress was deflected eastward (right of the wind direction) along the northern coast of Taiwan into the I-Lan Bay during the southerly or southwesterly wind to form the countercurrent. During the northerly monsoon season, the flow north of Taiwan is mainly the China Coastal Current (Jan et al., 2010). The currents are affected by wind stresses and flows along the continental shelf into the I-Lan Bay. The composition and source of the Taiwan Strait Current can be referred to Jan et al. (2002). Figure 13 depicts representative images with SST when countercurrent occurred in I-Lan Bay in each season. The cold water flows from the Taiwan Strait side and enters the I-Lan Bay with countercurrents, which caused a sharp ocean thermal front between the I-Lan Bay and the KC.
Figure 11. EOF results of spatial pattern and temporal variation of monthly geostrophic currents from January 1993 to December 2016. (a) The first mode, and (b) the second mode. The percentage of variance explained by the first mode is 79 %, and the second mode is 6%.
Figure 12. The first mode of EOF results of spatial pattern and temporal variation of monthly winds from January 2000 to December 2016. The percentage of variance explained by the first mode is 84 %.
Figure 13. Examples of Type 1 countercurrent with the SST images of Himawari-8 and geostrophic currents derived from satellite altimetry on (a) April 5-6, 2016, (b) June 26, 2016, (c) September 17-19, 2015, and (d) December 1-2, 2015, respectively. White arrows denote geostrophic current.
The dynamic process of Type 2 countercurrent in the I-Lan Bay is caused by the occurrence of a cold dome near northeast of Taiwan. The formation of the cold dome is related to the process of Kuroshio deflection caused by the wind stress and the topography effects (Hsueh et al., 1992; 1993, Chen et al., 1996). The size and shape of the cold dome can be affected by the KC in the East China Sea, the current and coastal flow from the Taiwan Strait, and the impact of monsoon (Jan et al., 2011; Shen et al., 2011; Gopalakrishnan et al., 2013). They suggested the position variability and interannual variability are caused by wind stress curl and the on-shelf water transport of the KC through the North Men-Hua Canyon. A schematic diagram of the outer current of the cold dome to flow to the I-Lan Bay as a countercurrent is shown in Figure 14. The second mode of EOF (EOF2) results (Fig. 11b) presented the cold dome could exist in each month and mainly occurred in summer season. Compare EOF2 results of currents and EOF results of winds between January 2000 and December 2016, the correlation coefficient of temporal variations is 0.75 (p <0.05). These results consistent with the pervious study (Cheng et al., 2009), which used SST and sea surface height anomaly data and found that the cold dome exists in a whole year but the most often occurs in summer. The southwesterly monsoon could cause the cold dome to be more likely to occur in summer. Based on the basin-scale, eddy-resolving dual-domain Pacific Ocean Model results from Shen et al. (2011), the formation of cold dome could be separated into four possible mechanisms. Two major mechanisms cause the cold dome to occur most in summer: 1) dynamics isotherm uplift resulting from the geostrophic component of Kuroshio, which is expected to be large in summer due to the fast Kuroshio speed and the Kuroshio axis position shift; and 2) Ekman transport causing divergence and upwelling during the cyclonic summer monsoon. As mentioned above, the wind is auxiliary for the cold dome formation. Figure 15 presents examples of the cold dome and the occurring countercurrents in the I-Lan Bay. One can see that the cold water generated by the cold dome is accompanied by an anti-clockwise flow field and flows into the I-Lan Bay, which contrasts sharply with the high-temperature characteristics of the KC.
Figure 14. A schematic diagram of I-Lan Bay countercurrent (Type 2).
Figure 15. Same as Figure 13, but for Type 2 countercurrent. (a) April 16, 2016, (b) August 24-25, 2016, and (c) October 25-26, 2015, respectively.
4.2 Chlorophyll-a concentration and attenuation coefficient Coastal and shelf waters flow into I-Lan Bay, causing significant ocean fronts with the KC (Fig. 16). The water quality of the KC water is clear relative to the Taiwan Strait water and the East China Sea water. To discuss the interaction of these different water masses, monthly moderate-resolution imaging spectroradiometer (MODIS) chlorophyll-a (Chl-a) concentrations and attenuation coefficient at 490 nm (Kd_490) data (https://modis.gsfc.nasa.gov/) are used as additional indicators for identifying the water masses and the fronts of different water masses (Kim et al., 2017; Ciancia et al., 2018). Figure 17 shows the mean geostrophic currents field with the
Chl-a and Kd_490 in summer and others season. In general, the clear KC water has low Chl-a concentration and low Kd_490. Lower Kd_490 value means less attenuation and higher clarity of ocean water. If the current flows from the Taiwan Strait side to the Kuroshio side of eastern Taiwan (countercurrents occur), the water mass is expected to have high Chl-a concentration and low transparency characteristics. The Chl-a concentration (mg/m3) is 0.53 (winter), 0.56 (spring), 0.43 (summer) and 0.62 (autumn) in Area 1 (see Fig. 2). In contrast, the Chl-a concentration in Area 4 (see Fig. 2) is only 0.22, 0.17, 0.16, and 0.16 mg/m3 for winter, spring, summer, and autumn, respectively from July 2002 to December 2016. The Chl-a concentration in the I-Lan Bay always maintains a high value, and there is no obviously seasonal variability. The value of Kd_490 (m−1) also indicates the same trend. The seasonal mean in Area 1 is 0.07 (winter), 0.07 (spring), 0.07 (summer), and 0.08 (autumn), higher than the mean in Area 4 (0.05 in winter, 0.04 in spring, 0.04 in summer, and 0.04 in autumn). Unlike the more transparent KC water, the water in the I-Lan Bay trends toward the turbid coastal waters.
Figure 16. Average geostrophic currents field (white arrows), Chl-a (a and b) and Kd_490 (c and d). (a) and (c) in summer season; (b) and (d) in winter, spring and autumn. The black dash line indicates the Kuroshio main axis with maximum current velocity.
5. Summary In this study, CTD, Sb-ADCP, drifting buoys, satellite altimeter and MODIS are used to analyze the hydrological properties of the coastal countercurrent in the I-Lan Bay and the northeast of Taiwan Island, as well as variations of the KC nearshore side. From the results of the flow field analysis, drifting buoys recorded the presence of a coastal countercurrent in the I-Lan Bay, which may originate from the cold dome and the northeastern shelf of Taiwan Island. The EOF analysis of satellite altimeter data also shows the same results. The velocity profile from the Sb-ADCP data indicate that the remarkable surface countercurrent occurs in the north of the I-Lan Bay in summer and autumn. The surface countercurrent still occurs in winter, but has a slower velocity. However, at the lower depth of 100–200 m, the evident countercurrent flows along the northeast coast of Taiwan Island to the I-Lan Bay. The velocity of the V-component is between −0.1 to −0.2 m/s in the I-Lan Bay. From the perspective of hydrological analysis, the cross sectioned vertical structure of seasonal means shows significant salinity difference between the water in the I-Lan Bay and the KC water, especially the gap between the 23 to 25 kg m−3 isopycnal and the theta-salinity curve. The I-Lan Bay water has a lower 𝑆𝑚𝑎𝑥 than that of the KC water. The simplified index for the water mass analysis is used to classify composition of the water in the I-Lan Bay. The results show that it is close to fresh shelf water. This study indicates that the water in the I-Lan Bay is mainly affected by the northern shelf waters of Taiwan Island and mixed with the KC nearshore water. Thus, we conclude that the I-Lan Bay water has unique characteristics. This study will help further analysis of biochemical systems around this area. In addition, there is a basic understanding, i.e., the future study on interaction between the KC and the I-Lan Bay should have a comprehensive analysis on land-based radar, buoy observations and glider observations.
Acknowledgements Research product of sea surface temperature used in this paper was supplied by the P-Tree System, Japan Aerospace Exploration Agency (JAXA). This work is supported by the Ministry of Science and Technology of Taiwan through grants MOST 106-2611-M-019-015 and MOST 106-2917-I-019-001.
References Andres, M., Jan, S., Sanford, T. B., Mensah, V., Centurioni, L. R., & Book, J. W. (2015). Mean structure and variability of
the
Kuroshio
from
northeastern
Taiwan
to
southwestern
Japan.
Oceanography,
28(4),
84-95,
doi:10.5670/oceanog.2015.84. Andres, M., V. Mensah, S. Jan, M.-H. Chang, Y.-J. Yang, C. M. Lee, B. Ma, and T. B. Sanford (2017), Downstream evolution of the Kuroshio's time-varying transport and velocity structure, Journal of Geophysical Research: Oceans, 122, 3519-3542, doi:10.1002/2016JC012519. Ciancia, E., Coviello, I., Di Polito, C., Lacava, T., Pergola, N., Satriano, V., & Tramutoli, V. (2018). Investigating the chlorophyll-a variability in the Gulf of Taranto (North-western Ionian Sea) by a multi-temporal analysis of MODIS-Aqua Level 3/Level 2 data. Continental Shelf Research, 155, 34-44, doi:10.1016/j.csr.2018.01.011. Chen, H. T., Yan, X. H., Shaw, P. T., & Zheng, Q. (1996). A numerical simulation of wind stress and topographic effects on the Kuroshio current path near Taiwan. Journal of physical oceanography, 26(9), 1769-1802, doi: 10.1175/1520-0485(1996)026<1769:ANSOWS>2.0.CO;2. Chang, Y. L., Oey, L. Y., Wu, C. R., & Lu, H. F. (2010). Why are there upwellings on the northern shelf of Taiwan under northeasterly winds?. Journal of Physical Oceanography, 40(6), 1405-1417, doi:10.1175/2010JPO4348.1. Cheng, Y. H., Ho, C. R., Zheng, Z. W., Lee, Y. H., & Kuo, N. J. (2009). An algorithm for cold patch detection in the Sea off northeast Taiwan using multi-sensor data. Sensors, 9(7), 5521-5533, doi:10.3390/s90705521. Cheng, Y. H., Hu, J., Zheng, Q., & Su, F. C. (2017). Interannual variability of cold domes northeast of Taiwan. International Journal of Remote Sensing, 1-11, doi:10.1080/01431161.2017.1395972 Gopalakrishnan, G., Cornuelle, B. D., Gawarkiewicz, G., & McClean, J. L. (2013). Structure and evolution of the cold dome off northeastern Taiwan: A numerical study. Oceanography, 26(1), 66-79, doi:10.5670/oceanog.2013.06 Hsueh, Y., J. Wang, and C.‐ S. Chern (1992), The intrusion of the Kuroshio across the continental shelf northeast of Taiwan, Journal of Geophysical Research: Oceans, 97(C9), 14323–14330, doi:10.1029/92JC01401. Hsueh, Y., Chern, C. S., & Wang, J. (1993). Blocking of the Kuroshio by the continental shelf northeast of Taiwan. Journal of Geophysical Research: Oceans, 98(C7), 12351-12359, doi: 10.1029/93JC01075. Hsu, P. C., Lin, C. C., Huang, S. J., & Ho, C. R. (2016). Effects of cold eddy on Kuroshio meander and its surface properties, east of Taiwan. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(11), 5055-5063, doi:10.1109/JSTARS.2016.2524698. Hsu, P. C., & Ho, C. R. (2018). Typhoon-induced ocean subsurface variations from glider data in the Kuroshio region adjacent to Taiwan. Journal of Oceanography, doi:10.1007/s10872-018-0480-2 Jan, S., Wang, J., Chern, C. S., & Chao, S. Y. (2002). Seasonal variation of the circulation in the Taiwan Strait. Journal of Marine Systems, 35(3-4), 249-268, doi:10.1016/S0924-7963(02)00130-6
Jan, S., Tseng, Y. H., & Dietrich, D. E. (2010). Sources of water in the Taiwan Strait. Journal of Oceanography, 66(2), 211-221, doi:10.1007/s10872-010-0019-7 Jan, S., Chen, C. C., Tsai, Y. L., Yang, Y. J., Wang, J., Chern, C. S., ... & Kuo, J. Y. (2011). Mean structure and variability of the cold dome northeast of Taiwan. Oceanography, 24(4), 100-109, doi:10.5670/oceanog.2011.98. Jan, S., Y. J. Yang, J. Wang, V. Mensah, T.-H. Kuo, M.-D. Chiou, C.-S. Chern, M.-H. Chang, and H. Chien (2015), Large variability of the Kuroshio at 23.75°N east of Taiwan, Journal of Geophysical Research: Oceans, 120, 1825– 1840, doi:10.1002/2014JC010614. Johns, W. E., Lee, T. N., Zhang, D., Zantopp, R., Liu, C. T., & Yang, Y. (2001). The Kuroshio east of Taiwan: Moored transport observations from the WOCE PCM-1 array. Journal of Physical Oceanography, 31(4), 1031-1053, doi:10.1175/1520-0485(2001)031<1031:TKEOTM>2.0.CO;2. Kim, H. C., Son, S., Kim, Y. H., Khim, J. S., Nam, J., Chang, W. K., ... & Ryu, J. (2017). Remote sensing and water quality indicators in the Korean West coast: Spatio-temporal structures of MODIS-derived chlorophyll-a and total suspended solids. Marine pollution bulletin, 121(1-2), 425-434, doi:10.1016/j.marpolbul.2017.05.026. Li, G., Han, X., Yue, S., Wen, G., Rongmin, Y., & Kusky, T. M. (2006). Monthly variations of water masses in the East China Seas. Continental Shelf Research, 26(16), 1954-1970, doi:10.1016/j.csr.2006.06.008. Liu, S., Qiao, L., Li, G., Li, J., Wang, N., & Yang, J. (2015). Distribution and cross-front transport of suspended particulate matter over the inner shelf of the East China Sea. Continental Shelf Research, 107, 92-102, doi:10.1016/j.csr.2015.07.013. Mensah, V., Jan, S., Chiou, M. D., Kuo, T. H., & Lien, R. C. (2014). Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan. Deep Sea Research Part I: Oceanographic Research Papers, 86, 68-81, doi:10.1016/j.dsr.2014.01.005. Mensah, V., S. Jan, M.-H. Chang, and Y.-J. Yang (2015), Intraseasonal to seasonal variability of the intermediate waters along the Kuroshio path east of Taiwan, Journal of Geophysical Research: Oceans, 120, 5473–5489, doi:10.1002/2015JC010768. Shen, M. L., Tseng, Y. H., & Jan, S. (2011). The formation and dynamics of the cold-dome off northeastern Taiwan. Journal of Marine Systems, 86(1), 10-27, doi:10.1016/j.jmarsys.2011.01.002. Tang, T. Y., Tai, J. H., & Yang, Y. J. (2000). The flow pattern north of Taiwan and the migration of the Kuroshio. Continental Shelf Research, 20(4), 349-371, doi:10.1016/S0278-4343(99)00076-X. Takahashi, D., Guo, X., Morimoto, A., & Kojima, S. (2009). Biweekly periodic variation of the Kuroshio axis northeast of Taiwan as revealed by ocean high-frequency radar. Continental Shelf Research, 29(15), 1896-1907, doi:10.1016/j.csr.2009.07.007. Wu, C.-R., H.-F. Lu, and S.-Y. Chao (2008), A numerical study on the formation of upwelling off northeast Taiwan, Journal of Geophysical Research, 113, C08025, doi:10.1029/2007JC004697. Yang, Y. J., Jan, S., Chang, M. H., Wang, J., Mensah, V., Kuo, T. H., ... & Lai, J. W. (2015). Mean structure and fluctuations of the Kuroshio East of Taiwan from in situ and remote observations. Oceanography, 28(4), 74-83, doi:10.5670/oceanog.2015.83. Yin, Y., X. Lin, R. He, and Y. Hou (2017), Impact of mesoscale eddies on Kuroshio intrusion variability northeast of Taiwan, Journal of Geophysical Research: Oceans, 122, 3021–3040, doi:10.1002/2016JC012263.
Zhang, D., Lee, T. N., Johns, W. E., Liu, C. T., & Zantopp, R. (2001). The Kuroshio east of Taiwan: Modes of variability and relationship to interior ocean mesoscale eddies. Journal of Physical Oceanography, 31(4), 1054-1074, doi:10.1175/1520-0485(2001)031<1054:TKEOTM>2.0.CO;2. Zheng, Z. W., & Zheng, Q. (2014). Variability of island-induced ocean vortex trains in the Kuroshio region southeast of Taiwan Island. Continental Shelf Research, 81, 1-6, doi:10.1016/j.csr.2014.02.010. Zheng, Z.-W., Q. Zheng, C.-Y. Lee, and G. Gopalakrishnan (2014), Transient modulation of Kuroshio upper layer flow by directly impinging typhoon Morakot in east of Taiwan in 2009, Journal of Geophysical Research: Oceans, 119, 4462–4473, doi:10.1002/2014JC010090.
Highlights 1. Ship-board acoustic Doppler current profiles are used to observeflow structure from I-Lan Bay to Kuroshio region. 2. Simplified index of water mass based on CTD data isused to analyze
compositionsand source of
water in I-Lan Bay. 3. The water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water. 4. Dynamic processes of the occurrence of countercurrent in I-Lan Bay are explained.