PUBLICATIONS RESEARCH ARTICLE Key Points: The variability of the intermediate water (IW) east of Taiwan is assessed North Pacific IW is the dominant water mass found in the region Layer thickness of the Kuroshio and nature of the IW east of Taiwan are related Correspondence to: V. Mensah, vmensah@hotmail.com Citation: 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, J. Geophys. Res. Oceans, 120, 5473 5489, doi:10.1002/ 2015JC010768. Received 4 FEB 2015 Accepted 25 JUN 2015 Accepted article online 30 JUN 2015 Published online 7 AUG 2015 VC 2015. American Geophysical Union. All Rights Reserved. Intraseasonal to seasonal variability of the intermediate waters along the Kuroshio path east of Taiwan Vigan Mensah 1, Sen Jan 1, Ming-Huei Chang 1, and Yiing-Jang Yang 1 1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan Abstract The variability of the intermediate water (IW) east of Luzon and Taiwan is investigated using data acquired from moored instrumented lines and shipboard hydrographic and current velocity surveys. The IW is defined as the water mass with a local salinity minimum along the Kuroshio path. An empirical formula is developed to estimate the IW salinity minimum east of Taiwan using temperature measurements around 580 m depth. Properties of the IW east of Taiwan vary greatly as a result of variable contributions from three water masses including the high-salinity South China Sea Intermediate Water (SCSIW), the lowsalinity North Pacific Intermediate Water (NPIW), and the intermediate-salinity Kuroshio Intermediate Water (KIW). Our analysis concludes that NPIW is predominantly found east of Taiwan, and the northward transport of KIW from northeast of Luzon to east of Taiwan is not a steady process. Concurrent mooring measurements at these two locations enable us to correlate the variations of the layer thickness of the Kuroshio near its origin (KLT o ) northeast of Luzon to the nature of the IW east of Taiwan. When the Kuroshio is deep, i.e., large KLT o, KIW is transported northward across the Luzon Strait, where its salinity increases presumably due to turbulence mixing with SCSIW. This modified KIW is then transported to the east of Taiwan. When the Kuroshio is shallow, i.e., small KLT o, the KIW transport east of Luzon is nil or southward. East of Taiwan, NPIW feeds in below the Kuroshio and is transported northward beyond the I-Lan Ridge. 1. Introduction The Kuroshio is the western boundary current in the North Pacific, which transports heat and salt from the low latitudes to the midlatitudes, and thus, affects the ocean-atmosphere interactions and influences the climate along its path [Kwon et al., 2010]. Therefore, a better understanding of the Kuroshio water masses changes is crucial for predicting the variability of the poleward volume, heat, and salinity transports. The Kuroshio water masses are evolved from the water masses of the North Pacific Subtropical Gyre and are often classified into tropical and intermediate waters. The Kuroshio Tropical Water (KTW) is an evolution of the North Pacific Tropical Water (NPTW), which is found in most of the western North Pacific Ocean and whose primary layer lies between 150 and 250 m [Rudnick et al., 2011], where a salinity maximum can be found. Its properties and flow have been reported by several previous studies [e.g., Nitani, 1972; Chen, 2005]. A recent study focuses on its spatial variability and the processes underlying these changes [Mensah et al., 2014]. The intermediate water associated with the Kuroshio stems from the North Pacific Intermediate Water [Gordon et al., 2014], a cold and less saline water mass formed in the mixed water region east of Japan where the Kuroshio Extension and Oyashio meet [Talley, 1993]. Yasuda [1997] proposed that the freshwater property of the North Pacific Intermediate Water (NPIW) is a remote contribution from the Okhotsk Sea Mode Water, which is modified and transported via the Oyashio along the Kuril islands and the east of Hokkaido. This fresh, low-potential vorticity water mixes with the saltier subtropical intermediate water near the Kuroshio Extension and forms the NPIW. NPIW then spreads over the northern Pacific Ocean and reaches southward to 158N 208N. In the western tropical Pacific Ocean, the NPIW is characterized by a salinity minimum (S min ) around 34.10 34.20 [Talley, 1993] at its core at the depths around 600 m. Along the Kuroshio path east of Luzon, the S min is generally higher (34.3) than within the West Philippine Sea Basin, according to the averaged Kuroshio h-s properties described in Mensah et al. [2014]. We define this saltier variation of NPIW found along the Kuroshio, east of the Luzon Island as the Kuroshio Intermediate Water (KIW). This definition of KIW is different from those given by Chern and Wang [1998], Chen [2005], and Nakamura et al. [2013]. In these three papers, KIW generally refers to the intermediate water mass found east of Taiwan, MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5473
Figure 1. Isobaths (in meters), main OKTV stations, and mooring site east of Taiwan. The analysis is based on surveys with data collected at either two of transects KTV1, KTV2, or KTV3. Data were acquired along KTV4 during Surveys 2, 4, 8, 9, and 10. which may contain both South China Sea Intermediate Water (SCSIW) and NPIW. The KIW defined in this analysis enables the IW near the origin of the Kuroshio to be distinguished from SCSIW and NPIW. Among the major progresses in the study of the intermediate water along the Kuroshio, Chern and Wang [1998] observed the presence of a subsurface front in the Luzon Strait, separating the fresher NPIW to the east of 1228E from a saltier water mostly made of SCSIW to the west. SCSIW is nutrient-rich and saline with a S min of 34.4. It has been found on the continental shelf northeast of Taiwan [Chen and Huang, 1996]. Chen [2005], from several synoptic surveys data taken in the Luzon Strait and the Okinawa Trough, also found traces of SCSIW off the southern Japan. Recently, Nakamura et al. [2013], using climatological data, provided details about the nature of the intermediate water east of Taiwan and described it as a mix of SCSIW and NPIW. However, based on hydrographic survey data acquired along several transects east of Taiwan, Chern and Wang [1998] found no characteristic signature of SCSIW north of Green Island (22.808N, Figure 1). They proposed that the shallow ridge between Taiwan and Green Island, <500 m deep, blocks the northward flow of SCSIW, whose S min south of Green Island is located typically around the depth of 500 m [Nakamura et al., 2013]. East of northern and central Taiwan, they identified NPIW with a S min smaller than 34.25. The ambivalent results from these studies suggested that the intermediate water mass along the Kuroshio path east of Taiwan is highly variable, presumably with SCSIW and NPIW being alternatively present below the core of the Kuroshio. It is essential to study the variability of the intermediate water because it impacts the heat, salt, and nutrient budgets along the Kuroshio path. The current study aims at analyzing the variability (<1 year) of the intermediate water east of Taiwan and providing an insight into the processes that modify the characteristic of the water mass located below the velocity core of a western boundary current. To define this variability, time series of the intermediate water salinity is needed. Data from a thermistor chain (A2 in Figure 1) moored between November 2012 and November 2014 east of Taiwan are used to estimate the S min associated with the intermediate water and provide an overview of the amplitude and time scale of the intermediate water variability. The estimation is based on a least squares fit using historical conductivity-temperature-depth (CTD) data and CTD casts taken MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5474
Table 1. Presentation of Surveys of the OKTV Project a Survey # Duration Transects Covered Stations (LADCP, CTD) Completed 1 7 15 Nov 2012 KTV1, KTV2, KTV3 101 108, 201 212, 303 315 2 23 25 Apr 2013 KTV2, KTV3, KTV4 201 210, 301 312, 401 403 3 22 27 Jun 2013 KTV1, KTV3 101 108, 301 309 4 2 4 Sep 2013 b KTV2, KTV3, KTV4 201 204, 301 306, 401 404 5 1 2 Oct 2013 b KTV1 101 108 6 16 Nov 2013 b KTV1 101 103 7 24 25 Dec 2013 b KTV1 101 103, 105 108 8 8 12 Mar 2014 b KTV1, KTV2, KTV3, KTV4 101 108, 201 202, 301 306, 401 403 9 4 15 Jul 2014 KTV1, KTV2, KTV3, KTV4 101 108, 201 208, 301 308, 401 403 10 10 14 Sep 2014 KTV1, KTV2, KTV3, KTV4 101 108, 201 209, 301 310, 401 403 a The results of five of these surveys (Surveys 1, 2, 3, 9, and 10) are discussed in the current study, and the seven last surveys including data at KTV1 have been used to evaluate the uncertainty of the salinity minimum time series. b Surveys for which incompletely sampled transects did not provide sufficient information for analysis. during our recent experiments. This time series of salinity minimum is supplemented with hydrographic and current velocity data, collected over the course of several surveys and over a wide area east of Taiwan. These observational data enable us to get a further understanding of the variability of intermediate water, and to infer the origin of the water masses found east of Taiwan. Section 2 provides details of the data and methodology adopted to derive a S min time series at A2. Section 3 discusses the analysis results from the time series of S min and the shipboard surveys. In section 4, data from an upstream mooring northeast of Luzon Island are analyzed and compared with observations east of Taiwan. Possible causes of the spatial and temporal variations of the intermediate water east of Taiwan are discussed. A summary is provided in section 5. 2. Data and Methodology 2.1. Hydrographic Surveys As part of the observations of the Kuroshio Transports and their Variability (OKTV) project [Jan et al., 2015], 10 hydrographic and current velocity surveys were carried out between November 2012 and September 2014 east of Taiwan using R/Vs Ocean Researcher I, III, and V. The survey area (Figure 1) consists of three zonal transects extending from the east coast of Taiwan to 123.008E. The northernmost line, KTV1, comprises eight stations located between (23.868N, 121.748E) and (23.628N, 123.008E) close to the southern entrance of the East China Sea. The line KTV2 (13 stations) extends toward the east from (22.758N, 121.208E) to (22.758N, 123.008E), passing just north of Green Island. The southernmost transect, KTV3, with 15 stations, extends from the southern tip of Taiwan at (22.008N, 121.008E) to far offshore (22.008N, 123.008E), crossing Lan-Yu Island. Besides, survey along a meridional transect, KTV4, is also carried out. The KTV4 comprises three stations located between Lan-Yu Island and Green Island from (22.218N, 121.508E) to (22.568N, 121.568E). Due to ship time constraints, measurements along these lines were not conducted during every survey, and the easternmost stations of these transects were often skipped. The geographic location of the most frequently sampled stations is presented in Figure 1, and various information regarding the timing of the surveys and the stations covered are indicated in Table 1. In order to provide a more complete picture of the intermediate water distribution and properties east of Taiwan, only surveys comprising at least two of the three zonal transects (KTV1, KTV2, and KTV3), and with sampling extending at least as far as 1228E are analyzed. Five surveys meet these requirements (Surveys 1, 2, 3, 9, and 10). Each station includes hydrographic measurements using a Seabird SBE9 11plus CTD profiler sampling at 24 Hz. The data were averaged within 1 m bins. A 300 khz lowered acoustic Doppler current profiler (LADCP) was attached to the CTD frame to measure a current profile at each station. The LADCP was set up to measure current at 8 m resolution and its data were detided using the two-dimensional model produced tidal current described in Jan et al. [2002]. While the magnitude of the barotropic tidal current is negligible along KTV1, this model s results exhibit a barotropic tidal current amplitude of O (10 22 210 21 )ms 21 at the westernmost stations of the other transects. The intermediate water volume transport was estimated at each station by integrating the current velocity for the LADCP bins located between the 1026.1 and 1027.1 kg m 23 isopycnals, which encompass the core of the intermediate water MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5475
Figure 2. (a) h-s envelope for the reference SCSIW (blue dashed curve), NPIW (green dashed curve), and KIW (red dashed curve) calculated from the historical CTD data collected in rectangle areas shown in Figure 2b. Black stars represent the potential temperature at the depth of 580 m for each of these curves. The regions of the rectangles in Figure 2b are selected according to Mensah et al. [2014]. The green, red, and blue solid curves in Figure 2a represent the averaged properties of low, mid, and high-salinity minimum (S min ) profiles computed from data collected within 15 km of A2 (between 1985 and 2013, source Taiwan Ocean Database) with colored stars representing their potential temperature at the depth of 580 m. mass (Figure 2). The depths of these two isopycnals at each station have been previously determined from the associated CTD data. The horizontal limit of integration adopted for the calculation of volume transport was the easternmost station available at each of the KTV lines, which was 1238E for KTV1, 122.58E for KTV2, and KTV3 in most cases. 2.2. Synthetic S min Time Series 2.2.1. Description To understand the temporal variation of the intermediate water east of Taiwan, we analyze the temperature time series recorded by the moored instruments at A2, which was located at station K103 (23.818N, 122.008E) within the KTV1 transect (Table 1 and Figure 1). This mooring comprised an upward looking 150 khz ADCP located at the depth of 530 m and 10 Seabird SBE39 temperature sensors, one located at 580 m, eight placed every 100 m from 630 to 1330 m depths, and one positioned at the depth of 1500 m. All these instruments, including the ADCP, measured the temperature every 2 min from November 2012 to November 2014. The moored line and its instruments were recovered and redeployed approximately every 6 months during these 2 years; there is no data gap larger than 1 day. The data were 72 h low-pass filtered to remove temperature variations related to baroclinic-tide-induced isotherm displacements. The low-pass filtered data were then resampled at an interval of 1 day. Deducing the S min from temperature data relies on the assumption that a relationship exists between the temperature near the core of the intermediate water and the value of the S min. Indeed, assuming that the core of the intermediate water remains on a narrow isopycnic range, S min and the potential temperature at the core of the intermediate water would be closely correlated, despite potential turbulence mixing. This assumption is reasonable with regard to NPIW and SCSIW, with NPIW entering the South China Sea Basin [Wyrtki, 1961], and being subsequently modified into SCSIW through turbulent mixing [Qu et al., 2000]. Figure 2a introduces the reference h-s properties of SCSIW, KIW, and NPIW, as defined in Mensah et al. [2014], with the notable difference that these water masses are not delineated by an average curve, but by an envelope corresponding to the mean curve 6 1 standard deviation. The S min of NPIW, for example, varies mostly between 34.17 and 34.26 for a mean of 34.22. The S min of KIW varies mostly between 34.26 and 34.36 with a mean of 34.31. The S min of SCSIW is 34.42 and exhibits little variation with typical values MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5476
Figure 3. (a) Temperature at 580 m versus salinity minimum for all historical profiles located within 15 km of A2; (b) scatterplot of measured (historical data) versus least squares fitted salinity minimum using (historical) temperature data at 530, 580, and 630 m. (c) Three-day low-pass filtered time series of the least squares fitted salinity minimum using observed temperature at 530, 580, and 630 m at A2 (black curve) with its 61 standard deviation envelope (gray shading); the envelope was adapted on the time series from the range varying standard deviation in Figure 3d. The green and red dashed line in Figure 3c represent the limit separating low from mid-s min profiles and mid from high S min east of Taiwan, respectively. The colored squares in Figure 3c represent the values obtained from seaglider or shipboard CTD measurements at A2 during various OKTV cruises, and the attached digits indicate the survey numbers referred in Table 1. Survey 2 and Survey 4, which did not include measurements at A2, are not indicated on the time series, because observation along KTV1 was not included in the two surveys. ranging between 34.40 and 34.43. We classify the water masses present east of Taiwan (60 profiles sampled 15 km around A2 between 1985 and 2013) into three categories: (1) a low S min (<34.26), (2) a midrange S min (between 34.26 and 34.31), and (3) a high S min (>34.31). Our choice of 34.26 for discriminating the low S min from the midrange S min is consistent with the S min value separating NPIW from KIW. Although no profiles exist outside the upper bound of KIW, the profiles with S min within the range of KIW (34.26 34.36) required to be classified into two categories. Here the value of 34.31 for separating the midrange and high S min was chosen considering that profiles on both sides of this threshold value display distinct ds=dh properties (Figure 3b). This contrast in ds=dh might indicate a difference in mixing state between the two categories. The h-s values at the depth of 580 m (where the A2 mooring comprises a temperature sensor) for the three averaged S min classes are compared with those obtained from the nominal NPIW, KIW, and SCSIW. The low- S min -averaged profile shows a strong similarity to the NPIW type. However, the reference NPIW (green shading in Figure 2a) has S min located deeper than 600 m and h 580m 5 8.58C, whereas the low-s min profile east of Taiwan (green solid curve in Figure 2a) has S min located at 550 m depth and h 580m 5 6.88C. The averaged midrange profile (red solid curve) has a S min similar to that of KIW (pink shading), but shows a very different h-s structure than this intermediate water. The S min of midrange profile is located at a deeper depth with larger potential density (26.85 kg m 23 instead of 26.5 kg m 23 ), and possesses a smaller curvature around S min, with the low-salinity layer (layer with a salinity within 6 0.01 of S min ) approximately 120 m thick. This feature of thick low-salinity layer is enhanced for the averaged high-s min profile with a S min layer thickness of about 200 m (blue solid curve). It exhibits two salinity minima at depths of 500 and 700 m. Because S min of the KIW, NPIW, and SCSIW are located at different depths, mixing of these three types of waters near their S min may result in such multiple low-salinity minima structure. That the h-s structures east of Taiwan are different from those of NPIW, SCSIW, and KIW suggests that mixing plays a significant role in modifying MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5477
the intermediate water along the Kuroshio path. While the h-s of the low-s min water mass is in reasonable agreement with that of NPIW, the moderate salinity and high-salinity waters could be the result of turbulent vertical mixing and/or horizontal mixing of KIW, NPIW, and SCSIW. The characteristic of the water masses found east of Taiwan will be further assessed in sections 3 and 4. The distinct curvature of these three h-s curves, as well as the deepening or shoaling of the S min compared to the reference curves, let a relationship appear between the temperature at 580 m and the S min, i.e., the warmer the temperature at 580 m, the saltier the S min. The relationship between h 580m and S min for the 60 aforementioned CTD casts near A2 station is illustrated in Figure 3a. This correlation also exists between S min and temperature at other depths around the depth of S min and down to around 650 m, with the linear fit between S min and temperature yielding a coefficient of determination R 2 5 0.72, 0.78, and 0.74 for the temperature at 530, 580, and 630 m, respectively. We fit a synthetic salinity minimum SS min linearly with temperature measurements at these three depths as SS min ðþ5b i 0 1 b 1 T 530 ðþ1b i 2 T 580 ðþ1b i 3 T 630 ðþ1e i ðþ: i (1) Coefficients b 0, b 1, b 2, and b 3 are solved by minimizing the sum of residual squared e(i) 2. T 530, T 580, and T 630 are in situ temperatures at 530, 580, and 630 m, respectively. The number of observations (i51 to 60) is the number of hydrographic profiles located within a 15 km radius around the position of A2. 2.2.2. Error Estimations The least squares solution generates a synthetic SS min whose comparison with the observed S min (Figure 3b) yields a linear fit with a coefficient of determination R 2 5 0.80 and a standard error S e 5 0.029. The coefficient of determination reduces to 0.66 when the radius is increased to 20 km, indicating that, in a region of large spatial variability east of Taiwan, the estimate of SS min using temperature measurements becomes inappropriate in a large spatial domain. The estimated coefficients are applied to the 72 h low-pass filtered time series of temperature at the depths of 530, 580, and 630 m at A2 mooring. The resulting SS min time series is shown in Figure 3c. During each of the mooring deployments, the depth of the temperature sensors was slightly different. The depth z of the T z ðþterms i was therefore adapted accordingly in equation (1), and for each deployment, similar values of R 2 and S e were obtained. A gap in the time series exists between 22 May and 3 June 2013, as the mooring line became extremely slanted and the depth of the reference temperature sensors far exceeded their nominal value, making the relationship invalid during this period. A 6 1 standard deviation envelope was also estimated from the measured S min data displayed in Figure 3b. From 34.15 to 34.40 at 0.01 interval, all the synthetic SS min within a span of 0.025 were selected, and the standard deviation of the corresponding measured S min was calculated. The 0.025 span was chosen as it corresponds to 10% of the variability range observed from these profiles. This range-dependent envelope (Figure 3d) was then applied to the time series (Figure 3c), revealing significant variations of the error, with a generally higher error in the range 34.23 34.28, and a lower error for both the low and large SS min. The error associated with our least squares fit determination may arise from isopycnal vertical displacement related to baroclinic tides. Such a displacement could influence the temperature at the sensor level without changing the value of S min in the vertical profile. Approximate isopycnic displacement has been estimated from the CTD upcast and downcast profiles from 10 cruises. We calculate the depth difference of the 1026.80 kg m 23 isopycnal (which roughly corresponds to the density which the S min is found at, east of Taiwan) between each downcast and upcast passage, as well as the temporal difference between these passages, and deduce the displacement rate over a semidiurnal tidal period. Results suggest that the vertical isopycnal displacement at A2 is weak, with a value of 19 m 6 10 m over a semidiurnal tidal cycle at this location. This estimated vertical displacement is similar to that produced by the baroclinic tide model described in Jan et al. [2008]. Another source of error could be related to the fact that turbulent mixing may lead to a nonunique temperature-s min relation. In particular, the curvature or depth of the S min layer could vary, leading to nonunique relations between the temperature at the various depths and the value of the S min. Supported by the high value of R 2 (0.8) between the fitted SS min and the observed S min, we assume that the impact of these error, although potentially present, is minimal to our following analysis. The error of the salinity conversion process is further estimated by calculating the root mean squared (rms) difference between the S min obtained through CTD/seaglider casts and the SS min obtained simultaneously MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5478
from the mooring data. With eight calibration casts (red and green squares in Figure 3c), a rms error of 0.034 is found, which is nearly 1 order of magnitude below the variability observed. Six out of eight observed S min taken during the cruises agree well with the SS min. In February 2013, the SS min is highly overestimated compared to the S min observed by Seaglider (green square in Figure 3c), but within the standard deviation envelope. The S min during Survey 6 (34.31) is significantly underestimated by our estimation (SS min 5 34.26), which may be explained by the large uncertainty in this synthetic salinity range (Figure 3d). Notice that the CTD measurements of Survey 1 occurred 1 day before the beginning of our time series, i.e., we do not have a direct comparison. Figure 4. Variance preserving spectrum of SS min at A2. The shaded envelope represents the 90% confidence interval; the dashed black lines underline the main energy peaks at 19 and 100 days. 3. Results 3.1. Time Series The time average of the 3 days low-pass filtered time series of the SS min at A2 station is 34.25 which is identical to the S min averaged from historical CTD and Argo float data east of Taiwan by Nakamura et al. [2013]. The standard deviation is high at 0.054, and the time series (Figure 3c) frequently reveals large variations of O(10 21 ), with a minimum of 34.12 in early January 2013 and a maximum of 34.37 in late April 2013. The temporal variability is dominated by signals of period 100 and 19 days, from the results of the ninetapers variance preserving spectrum of SS min (Figure 4). The dominance of a large intraseasonal variability suggests that the averaged properties of the intermediate water east of Taiwan do not represent a homogeneous water mass; advection or mixing may lead to modifications of the water mass. In particular, the low-ss min (<34.26) events, whose salinity minimum is within the envelope of NPIW, prevail for 376 days or 53% of the whole time series. These events are generally long and well defined, lasting from 50 to 90 days. There is little or no trace of SCSIW (S min > 34.40), suggesting that the presence of SCSIW east of Taiwan is not permanent. High-SS min (>34.31) events, on the contrary, are numerous but short-lived (rarely exceeding 15 days) and occur 14% or 98 days of the whole time series. The maximum SS min seldom exceeds 34.35, a value which is well below that of SCSIW (34.41). This implies that if SCSIW is advected to the east of Taiwan, it must have been mixed with NPIW or KIW along the Kuroshio upstream path (Figure 2). The remaining part of the time series (33% or 231 days) exhibits ass min varying between 34.26 and 34.31. These values are all within the lower envelope of KIW. The SS min in these periods may then indicate the presence of KIW or a mix of several water masses, e.g., NPIW and KIW. The percentages of low, midrange, and high SS min from the time series were compared with those obtained from the 60 historical hydrographic profiles at and in the vicinity of A2. The reference profiles show a proportion of 55% (low), 15% (midrange), and 30% (high), against 53%, 33%, and 14%, respectively, for the time series. These results confirm that NPIW is predominantly found east of Taiwan, while the number of occurrences of midrange events is overestimated in our time series, to the detriment of high-s min events. Interestingly, low-salinity events appeared mostly during the northeasterly monsoon (November-March), when highsalinity events are very scarce (Figure 3c). The high-s min events are more prevalent from April to August. Although the dominant scale of variability is intraseasonal, the processes associated to the East Asian monsoon presumably have strong effect on modifying the water mass properties along the Kuroshio path east of Taiwan. The variability of SS min described above is likely due to local or remote turbulence mixing and advection of various water masses. To understand the origin of the various water masses east of Taiwan, survey data along KTV1, KTV2, and KTV3 are analyzed, and compared with results from the S min time series. MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5479
Figure 5. (a) Map of surface and intermediate water volume transport (in Sv, 1 Sv 5 10 6 m 3 s 21 ) east of Taiwan during Survey 1. The surface water volume transport is defined as the integrated transport between the surface and the 26.1 kg m 23 isopycnal, whereas intermediate water is defined as the layer between the 26.1 and 27.1 kg m 23 isopycnals. The dashed black line represents the Kuroshio boundaries as defined by Mensah et al. [2014]. h-s diagrams in Figures 5b 5d represent the properties of the intermediate waters at various stations along KTV1, KTV2, and KTV3, respectively. Reference h-s curves for SCSIW (dash-dotted gray line), NPIW (solid gray line), and KIW (dashed gray line) are overlaid in Figures 5b 5d. The size of each h-s point in Figures 5b 5d is velocity dependent, and the filled (empty) dots represent positive (negative) velocities. 3.2. Qualitative Analysis of the Intermediate Water Volume Transport To identify the origin of the low SS min, the volume transport in the intermediate water layer is computed using data taken from Survey 1. Data were collected from 7 to 15 November 2012, starting from KTV3 and KTV2 and ending at KTV1. Figure 5a displays the surface (integrated from the surface to the 1026.1 kg m 23 isopycnal) and intermediate water volume transport (calculated between the 1026.1 and 1027.1 kg m 23 isopycnals) along each transect, with the Kuroshio western and eastern boundaries indicated in the map for informative purpose. The intermediate water volume transport along KTV3 is southward from the east of Taiwan to 122.58E. This implies that none of the intermediate water in the Luzon Strait (whether KIW or SCSIW) is accompanying the Kuroshio during our survey. The h-s diagrams in Figures 5b 5d, which also indicate the current magnitude and direction, provide additional information. At station 308 (Figure 5d), the layer just above the intermediate water, down to 350 m (1026.05 kg m 23 isopycnal), flows northward with the Kuroshio. It is considerably saltier than the southward flowing waters beneath, whose h-s characteristics are identified as NPIW. Similarly, at the stations from the coast to west of 122.008E along KTV2 (Figures 5a and 5c), the core of the intermediate water (NPIW) also flows southward while their surface salty layer goes to the North. Further east, the water is exclusively NPIW as shown by the different h-s properties at K209, and flows massively northward (Figure 5a). As a result, a divergence of intermediate water flow appears between KTV3 and KTV2. Along KTV1, most of the stations MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5480
Figure 6. Same as Figure 5, for Survey 2 with h-s diagrams in Figures 6b 6d representing the intermediate waters along KTV2, KTV3, and KTV4, respectively. display a consistently northward volume transport down to the salinity minimum, with a very strong NPIW signature (Figure 5b). The data of this survey suggest that the Kuroshio had a shallow extent when it left the Luzon Strait with very limited northward transport of intermediate water. Southeast of Taiwan, intermediate water flowed mostly southward. The large positive transport of NPIW east of 122.008E at KTV2 and along the whole of KTV1 implies that NPIW flowing from the West Philippine Sea Basin is entrained below the Kuroshio and transported northward, resulting in the absence of a significant trace of SCSIW or KIW further north. Survey 10 presents similar characteristics to Survey 1 in h-s properties, with a very weak northward transport along KTV3 instead of the southward transport seen during Survey 1. Notice that the surface and intermediate volume transports along KTV2 and KTV3 are consistently lower during these low-salinity events than during the high and midrange S min events described in the following two paragraphs. The high-salinity case is typified by Survey 2, occurring from 23 to 25 April 2013, with measurements taken along KTV2, KTV3, and KTV4. The downstream S min time series at A2 exhibits a high peak (S min > 34.31) between 22 April and 1 May 2013, presumably a result of the advection of the upstream high-salinity event. In this case, illustrated in Figure 6, the intermediate water transport on the western side of KTV3 is largely positive, with saltier waters transported northward down to 800 m (1027.1 kg m 23 ). These waters have h-s properties close to those of SCSIW and KIW, and are also found flowing northward along KTV2 until west of 122.008E, whereas a significant portion of the intermediate water flows eastward out of the Taidung Basin (Figure 1), between the Green and Lan-Yu islands (KTV4; Figures 6a and 6d). Along KTV2, the S min is MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5481
Figure 7. Same as Figure 5 but for Survey 3, with h-s diagrams in Figures 6b and 6c representing the intermediate waters along KTV1 and KTV3, respectively. high (>34.35) and has a broad vertical extent, similar to that observed in the high-salinity h-s curve of Figure 2. Further east, there is a sharp change of water properties with a strong inflow of NPIW at all depths in a largely northward transport. The SS min time series at A2 reaches a high of 34.37, implying that the intermediate waters associated with the Kuroshio have flowed further north. It is reasonable to assume that the water, flowing beneath the western part of the Kuroshio as far as A2, comes from the Luzon Strait and is either KIW, SCSIW, or a mix of them. Survey 2 represents a case of a strong and deep northward transport of KIW/SCSIW in the western flank of the Kuroshio as far north as the I-lan Ridge, and a transport of NPIW under the eastern side of the Kuroshio. A divergence of intermediate water flow is also present between KTV2 and KTV3. Survey 3 (22 27 June 2013) occurs during a period of midrange SS min (>34.26 and <34.31), with data acquired along KTV1 and KTV3 (Figure 7). The hydrographic profiles along KTV1 (Figure 7b) display particular features because the upper layer of intermediate water down to 450 m (1026.4 kg m 23 ) contains h-s characteristics between those of KIW and SCSIW. At deeper depths, a transition exists with the salinity decreasing quickly below 34.3. The h-s curve below the S min exhibits properties set between the NPIW and KIW curves, but with a generally wider S min layer. This implies that the upper layer of intermediate water comes together with the Kuroshio from the Luzon Strait or the east of Luzon island, whereas the lower layer has a different origin, most likely from the West Philippine Sea Basin (NPIW). It is likely that the limit of the higher-salinity water marks the maximum depth of the upstream Kuroshio. This hypothesis is discussed in the next section. Along KTV3, data taken from two stations (K307 and K308) east of Lan-Yu Island exhibit a similar two layer structure. Profiles west and east of these two stations present the KIW/SCSIW, and NPIW properties, respectively. The northward transport of intermediate water is particularly strong, and it is MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5482
Figure 8. (a) Thirty day low-pass filtered time series of Kuroshio Layer Thickness (KLT o ) east of Luzon Island (black curve) and SS min east of Taiwan at A2 (red curve); lagged correlation coefficient between KLT o and the SS min east of Taiwan within three 6 month segments of (b) November 2012 and May 2013, (c) January 2013 and July 2013, and (d) March 2013 and September 2013. possible that the data sampled along KTV3 represent a transition between the mid and high-salinity case, as a strong increase of SS min is observed in the time series shortly after Survey 3 (Figure 3c). Survey 9 (4 15 July 2014) also appears to represent a similar transition phase. The strong increase of salinity (Figure 3c) 15 days after this survey supports this assumption. 4. Discussion 4.1. The Relationship Between Kuroshio Thickness and the Nature of the Intermediate Water Mass A link may exist between the layer thickness of the Kuroshio and the value of the S min east of Taiwan. In the case of a low S min, the SCSIW or KIW is only found in the uppermost layer of the intermediate water (IW), whereas it appears to a large depth in the case of a high-s min event. This implies that if the Kuroshio is thick, saltier intermediate water from as far as the origin of the Kuroshio (Kuroshio origin), KIW, may be transported to the east of Taiwan and beyond. Conversely, if the Kuroshio is shallow, none or little of KIW is transported to the north, and NPIW would then be found east of Taiwan. To verify this assumption, we introduce an essential notion: the Kuroshio Layer Thickness (KLT), defined as the maximum depth of the 0.3 m s 21 isotach of northward velocity across the Kuroshio. KLT o (KLT at the Kuroshio origin) is estimated using velocity data collected northeast of Luzon Island by an array of six ADCP moorings along 18.758N, between 122.008E and 122.878E [Lien et al., 2014]. Each mooring comprises an ADCP measuring the current velocity from 450 m upward. The moorings were deployed from June 2012 to June 2013. The choice of the 0.3 m s 21 isotach as a base for the determination of the KLT was made as a trade-off between a current velocity weak enough to represent the lower limit of the Kuroshio, and high enough so that its depth does not exceed 450 m. The ADCP time series were 72 h low-pass filtered at all depths and resampled at a daily rate, and KLT o is defined as the maximum depth of the 0.3 m s 21 northward velocity across the moored array. The daily KLT o and SS min time series were 30 days low-pass filtered for capturing the intraseasonal to interseasonal variability (Figure 8a). Cross correlations between SS min and KLT o are computed in three 6 month segments, defined as 13 November 2012 to 13 May 2013, 13 January 2013 to 13 July 2013, and 13 March 2013 to 13 September 2013 (Figures 8b 8d). Segmentation was adopted to avoid any aliasing induced by changes of the intermediate water velocity or path between the northeast of Luzon and A2. The analysis of the data in these three segments exhibits a high correlation coefficient r 5 0.75 at a 62 days lag, r 5 0.74 at a 39 days lag and r 5 0.75, at a 35 days lag, for the first, second, and third segment, respectively. These results suggest that the S min east of Taiwan is indeed related to the KLT o, such that a deep (shallow) Kuroshio east of Luzon induces a MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5483
Figure 9. Zonal salinity sections along the KTV3 transect during (a) a high-klt o event (Survey 2, April 2013) and (b) a low-klt o event (Survey 1, November 2012). The white and black lines designate isopycnals and isohalines, respectively. high (low) S min east of Taiwan. The lag of 1 2 months is consistent with the time taken for a water particle traveling 600 km from the entrance of the Luzon Strait to the east of central Taiwan at a speed of 0.1 to 0.2 m s 21, a reasonable scenario. Besides the KLT o, the path of the Kuroshio in the Luzon Strait may also affect the intermediate water properties east of Taiwan. In a numerical study, Sheremet [2001] demonstrated that the Kuroshio may loop further into the northern South China Sea when its current is weak and may leap over the Luzon Strait when its current is strong. These varying states of the Kuroshio path have also been observed from the trajectories of satellite-tracked surface drifters [Centurioni et al., 2004]. Hsin et al. [2010], using sea surface height (SSH)- derived geostrophic surface current, suggested that the looping Kuroshio in the Luzon Strait may lead to an eastward shift of the Kuroshio axis and direction southeast of Taiwan in winter. The Kuroshio path in the Luzon Strait might affect the level of entrainment of SCSIW and impact the IW properties east of Taiwan. We identify the Kuroshio path in the Luzon Strait using gridded surface geostrophic current data obtained from Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) (http://www.aviso.oceanobs.com). The longitude and shooting angle of the Kuroshio main axis are defined as the longitude and the direction of the maximum surface current along 228N and are computed from June 2012 to November 2014. Cross-covariance analyses with the SS min are performed in the same three 6 months segments (not shown) as the similar analysis between KLT and SS min. Among these three parameters, correlation with KLT o yield the best results. KLT o is the dominant factor responsible for the intraseasonal to seasonal variability of IW east of Taiwan. 4.2. Variations of the KLT and Its Impact on the Intermediate Water Flow In the event of a low KLT o, the Kuroshio depth is shallower than the upper level of IW (350 m), the intermediate water below the Kuroshio is transported southward via the Luzon Undercurrent [Qu et al., 1997; Wang et al., 2015], and KIW is not expected to be found east of Taiwan. The Kuroshio east of Taiwan is nearly a baroclinic geostrophic current which satisfies the thermal wind relation @v @z 52 g @q q 0 f @x [Liang et al., 2003; Jan et al., 2015]. When KLT o is high, KIW is found under the western flank of the Kuroshio whereas NPIW is found under its eastern flank, and the zonal density gradient is negative toward the east. Figure 9a, presenting a salinity section overlaid with isopycnals along KTV3 during Survey 2, illustrates such a situation of the eastward deepening of the isopycnals generating a northward geostrophic current. In the case of a low KLT o, however, such as during Survey 1 (Figure 9b), the zonal density gradient is positive in the western half of the section, causing a negative shear in the IW layer, and hence a southward flow to occur in this region (Figure 5a). We hypothesize that when KLT o is small, the inertia in the IW layer may be low. This would cause the water mass to stagnate within the Taidung basin, between the Taiwan coast and Lan-Yu and Green islands, and possibly mix with NPIW. The reduced S min and thickening of the layer bounded by the 1026.5 and 1027.0 kg m 23 isopycnals west of 121.58E (250 m during Survey 1 against 200 m during Survey 2) supports this assumption. Water mass modifications potentially lead to a reversal of the zonal density gradient MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5484
Figure 10. Volume transport in the surface layer and intermediate water layer during (a) Survey 1 and (b) Survey 10, and corresponding SSH and geostrophic surface velocity in Figures 10c and 10d. at the IW level and, in turn, of the velocity vertical shear and current. Further investigation is needed to verify the validity of this theory. Along KTV1, the intermediate water flows consistently to the north, even in situations of low KLTo, implying that the intermediate water is entrained into the Kuroshio. The Kuroshio tends to accelerate along the east coast of Taiwan [Rudnick et al., 2011; Liang et al., 2003], due to the presence of a quasi-permanent high-pressure system [Chern et al., 2010], or Kuroshio recirculation gyre [Gordon et al., 2014] located east of the Luzon Strait. The high-pressure system may be modified by the frequent visit of westwardpropagating eddies off the east of Taiwan. We propose that this southward-to-northward reversal of current could be caused by the modulation of this high-pressure system. Figures 10a and 10c show a northward current between KTV2 and KTV1 induced by the high-pressure system during Survey 1. During Survey 10 (Figures 10b and 10d), the northward current enhancement occurs closer to KTV3, in the vicinity of the local highest SSH. This current reversal beneath the Kuroshio may depend on the position of the highest pressure east of Taiwan. Further investigation of the vertical extent of the effect of the highpressure system on the IW transport is needed. In the case of a high KLTo, after a 30 60 days time lag, a high-smin intermediate water is observed east of Taiwan, with higher Smin than KIW but a lower h-s curvature (Figure 2). We suggest that intense mixing in the Luzon Strait affects this water mass on its way north. Alford et al. [2011] observed high vertical diffusivity K of O(102321022) m2 s21 below 400 m in the Luzon Strait. The high vertical diffusivity observed in the Luzon Strait is related to the large dissipation associated with the conversion of barotropic to baroclinic MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5485
Figure 11. (a) h-s diagram of averaged profiles collected during low, mid, and high-ssha events, and (b) corresponding CTD sampling locations of the three events within the area of definition of NPIW. The 6 1 standard deviation envelopes for the low (blue shading), high (red shading), and mid SSHa (black dashed curve) are drawn in Figure 11a. energy in this region, and is reproduced by numerous models [Niwa and Hibiya, 2004; Jan et al., 2008; Simmons et al., 2011]. We infer that in the Luzon Strait, the northward flowing KIW can potentially mix with the high-s min SCSIW from the underlying water column. Advection of SCSIW, possibly entrained by the strong Kuroshio flow, may also affect the area southeast of Taiwan, evidenced by the h-s properties of SCSIW found at K301 and K303 during Survey 2 (Figure 6c). However, as such high-salinity water is not seen at A2 or along the KTV1 line (S. Jan, personal communication, 2015), SCSIW should be limited to southeast of Taiwan. While quantifying the fraction of SCSIW southeast of Taiwan is beyond the scope of this study, we suggest that a salt budget into and out of the Luzon Strait, calculated from time averaged historical data, may settle the issue. After the period of midrange KLT o (250 350 m), SS min at A2 station varies between 34.26 and 34.31. The upper layer of KIW is transported northward, contributing to the upper intermediate water east of Taiwan, whereas the lower layer primarily comprises NPIW, as seen from the h-s properties of Survey 3 (Figure 7). The separation depth between the two intermediate water layers is close to the depth of S min. A combination of several processes such as higher current shear between the two layers, interleaving, or a phenomenon related to the baroclinic tide in the region east of Taiwan, could lead to an erosion of the low NPIW S min, and explain the midrange properties of the intermediate water. This topic merits further study. Results in Figures 8b 8d demonstrate that the correlation between KLT o and SS min east of Taiwan is consistently around 0.75, indicating that the KLT o is a major, but not the only factor to impact on the nature of the intermediate water. Another potential cause of variability could be the intrinsic variability of NPIW, due to local recirculation of the Kuroshio or impinging eddies. Nonlinear mesoscale eddies propagating from the western Pacific at 100 days period [Yang et al., 1999; Zhang et al., 2001] may transport [Early et al., 2011] and modify water mass from their origin, to a depth located below the core of the main thermocline [Qiu and Chen, 2005]. Therefore, eddies impinging with the Kuroshio east of Taiwan may potentially transport a different variant of NPIW to the region. To verify this hypothesis, we use SSH anomaly (SSHa) data acquired between 1993 and 2012, and 294 historical hydrographic data sampled within the area extending from 208N to238n and from 1248E to 124.58E, i.e., the area used to define NPIW in this study. The historical hydrographic database is a composite of CTD data from the Taiwan Ocean Data Bank and Argo float profiles obtained from the National Oceanographic Data Centre (NODC). This database includes data acquired between 1992 and 2011. The hydrographic profiles were averaged into three categories depending on whether they were sampled during a high, mid, or low-ssha event. High and low-ssha events are defined as the 25% highest and lowest SSHa in the time series, while mid-ssha corresponds to the 25% data closest to zero. The results in Figure 11a reveal that differences in water properties exist, with profiles acquired during low-ssha events (42 profiles) displaying a slightly lower salinity than those collected during MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5486
Figure 12. Schematic view of the intermediate water evolution from the Luzon Strait to the east of Taiwan during (a) high and (b) low-klt conditions. Bold black lines represent 0.3 m s 21 isotach; white dashed lines represent 20.1 m s 21 isotach. Abbreviations KIW, SCSIW, and NPIW are Kuroshio Intermediate Water, South China Sea Intermediate Water, and North Pacific Intermediate Water. RC means the Ryukyu Current. high events (85 profiles). Profiles sampled during mid-ssha events (74 profiles) present properties very close to those of NPIW. SSH anomalies could then influence the NPIW properties, however, the standard deviation envelopes of each curve overlay and further study should be conducted to ascertain the validity of these results. The potential impact of mesoscale eddies on the salinity variations is also implied by the high energy at the 100 days period on the variance preserving spectrum of S min (Figure 4). Whether this eddy influence is direct (i.e., due to westward propagating eddies coming from due east) or indirect (i.e., due to the eddies influence on the Kuroshio origin and KLT o ) merits further study. Another cause leading to intermediate water variability east of Taiwan is the local mixing, with evidence of large vertical overturns of O(10) m (M.-H. Chang, personal communication, 2015), possibly related to interleaving of layers within KIW and NPIW. Shear instability as a result of the interactions of the Kuroshio with a cyclonic eddy on its eastern side or with a counter current on its western side could be the cause of turbulence mixing. Jan et al. [2015] described such counter current, as well as the complex current structure and peculiar h-s characteristics for the stations located in the vicinity of the current reversal. 5. Conclusion The time series of SS min derived from the method developed in this study exhibits large variations with a minimum of 34.12 in January 2013 and a maximum of 34.37 in May 2013 and reveals that NPIW is the dominant (53%) water mass east of Taiwan. The variability displays high energy mainly in the 100 and 20 days periods, and can be classified as low and high events. Low-S min events (associated with NPIW) are generally long, lasting for 50 90 days, with higher occurrences in winter. The characteristics of the intermediate water during the low-s min events suggest that there is no continuous northward flow of intermediate water from the Luzon Strait to the east of Taiwan. High events are short-lived but numerous, and occur in all seasons except winter. Results analyzed from ship survey data imply that the layer thickness of the Kuroshio northeast of Luzon Island (KLT o ) plays an essential role in influencing the nature of the downstream intermediate waters. The greater the penetration depth of the Kuroshio northeast of Luzon, the more KIW is transported northward to the east of Taiwan. The S min east of Taiwan is significantly correlated with the Kuroshio layer thickness south of the Luzon Strait. During high-klt o events (Figure 12a), the KIW is transported northward from the east of Luzon, and presumably mixes with SCSIW in the Luzon Strait under the effect of intense turbulent mixing. The high diffusivity in this region is due to the large energy dissipation associated with the high rate of barotropic to baroclinic energy conversion prevalent in the region [Niwa and Hibiya, 2004; Jan et al., 2008]. The resulting MENSAH ET AL. KUROSHIO INTERMEDIATE WATERS VARIABILITY 5487