OceThe Officiala Maganzineog of the Oceanographyra Spocietyhy

CITATION Rudnick, D.L., S. Jan, L. Centurioni, C.M. Lee, R.-C. Lien, J. Wang, D.-K. Lee, R.-S. Tseng, Y.Y. Kim, and C.-S. Chern. 2011. Seasonal and mesoscale variability of the Kuroshio near its origin. Oceanography 24(4):52–63, http://dx.doi.org/10.5670/oceanog.2011.94.

DOI http://dx.doi.org/10.5670/oceanog.2011.94

COPYRIGHT This article has been published inOceanography , Volume 24, Number 4, a quarterly journal of The Oceanography Society. Copyright 2011 by The Oceanography Society. All rights reserved.

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downloaded from http://www.tos.org/oceanography Special Issue on the Oceanography of Taiwan Seasonal and Mesoscale Variability of the Kuroshio Near Its Origin

By Daniel L. Rudnick, Sen Jan, Luca Centurioni, Craig M. Lee, Ren-Chieh Lien, Joe Wang, Dong-Kyu Lee, Ruo-Shan Tseng, Yoo Yin Kim, and Ching-Sheng Chern

Underwater photo of a glider taken off Palau just before recovery. Note the barnacle growth on the glider, fish underneath, and the twin hulls of a catamaran used for recovery in the distance. Photo credit: Robert Todd

52 Oceanography | Vol.24, No.4 Abstract. The Kuroshio is the most important current in the North Pacific. Here, we present historical data and recent observations of the Kuroshio off the coasts of Taiwan and the Philippine Archipelago, with a focus on its origins. Seasonal climatologies from shipboard hydrographic and velocity measurements, and from surface drifters, demonstrate changes in the Kuroshio caused by the monsoon. In particular, seasonal monsoon forcing affects the degree of penetration of the Kuroshio through Luzon Strait. Data from surface drifters and underwater gliders describe its mesoscale variability. Velocities derived from drifters make clear the mesoscale variability associated with the Subtropical Countercurrent east of the Kuroshio. Underwater gliders document mesoscale structure prominent in salinity extrema associated with water masses. The evolution of these water masses as they progress northward near the Kuroshio indicates strong mixing in the region.

Introduction Kuroshio passes between Taiwan and the Three major currents dominate the Ryukyu Islands, it is a strong, coherent general circulation in the western Pacific current with a mean transport of 21 Sv offshore of Taiwan and the Philippines (1 Sv = 106 m3 s–1) (Johns et al., 2001). (Figure 1): the North Equatorial Current The NEC supplies the western (NEC) flows westward, runs into the boundary with two distinct water Philippine coast, and bifurcates into the masses marked by salinity extrema. northward-flowing Kuroshio and the North Pacific Tropical Water (NPTW) southward-flowing Mindanao Current. is marked by high salinity near 200 m As the terminus of the NEC, and the depth, while North Pacific Intermediate origin of the Kuroshio and Mindanao Water (NPIW) is well known as a Current, this bifurcation region is glob- salinity minimum near 500 m depth. ally important. Of these three currents, On the basis of chlorofluorocarbon the Kuroshio has probably received the observations, Fine et al. (1994) conclude most study through observation, theory, that NPTW was in contact with the and modeling. Here, we review some of atmosphere less than three years before this work and present new findings, with reaching the western boundary, while a focus on variability. NPIW is less than 20 years old. As The Kuroshio starts as a relatively the water proceeds northward in the poorly defined current flowing from Kuroshio, the modification of these the bifurcation region (Figure 1). The water masses is evident. current flows northward to the northern The Kuroshio’s origin is marked by tip of Luzon Island, where it veers strong mesoscale variability, so strong, westward through Luzon Strait into the in fact, that it can overwhelm the mean South China Sea (SCS). The Kuroshio flow offshore of Luzon. The mesoscale generally does not penetrate very far variability is due to at least two sources: into the SCS, but penetrating incursions (1) the instability of the Kuroshio itself, happen occasionally. The Kuroshio and (2) westward-propagating eddies becomes well established as it flows arriving at the western boundary. northeastward out of Luzon Strait along The eastward-flowing Subtropical the east coast of Taiwan. By the time the Countercurrent (STCC) is a region of

Oceanography | December 2011 53 0 observations of variability in the 24°N

Taiwan Kuroshio and environs from recent 1000 22°N observational campaigns. Shipboard acoustic Doppler current profiler (ADCP) Kuroshio 20°N 2000 and surface drifter data reveal variability in currents. Autonomous underwater 18°N 3000 Luzon gliders are used to quantify variations in 16°N hydrographic properties. A special focus North Equatorial Current 4000 is the passage of the Kuroshio across the

14°N Depth (m) mouth of Luzon Strait. Mindanao Curr 5000 12°N Bottom Topography 10°N 6000 The seafloor surrounding Taiwan has complicated topography (Figure 2). East 8°N en t Palau 7000 of Taiwan, tectonic processes related indanao M to the flipping of subduction direction 6°N 8000 lead to great seafloor roughness. The 120°E 125°E 130°E 135°E Philippine Sea Plate subducts beneath Figure 1. Schematic diagram of currents in the western Pacific off Taiwan and the Philippines. The North Equatorial Current (NEC) flows westward where the Eurasia Plate along the Ryukyu it runs into the Philippine coast. The end of the NEC is the beginning of the Trench. However, the Luzon Island Arc northward Kuroshio and the southward Mindanao Current. The Kuroshio starts that belongs to the Philippine Sea Plate as a weak, unorganized current, and strengthens as it flows northward. thrusts over the Eurasia continental margin (Liu et al., 1998). As a result, the topography east of Taiwan can be sepa- enhanced eddy activity centered at about source, the region offshore of Taiwan has rated into three parts. The northern part 22°N (Qiu and Chen, 2010), presumably the second strongest mesoscale eddies in is comprised of the northwest-southeast because of instability in the STCC. Qiu the entire North Pacific. The only region trending Okinawa Trough adjacent to and Chen (2010) also find interannual of stronger mesoscale variability in the the east-west trending Ryukyu Island changes in the strength of mesoscale North Pacific is the Kuroshio Extension Arc to the south. The middle part, variability related to vertical shear in offshore of Japan. roughly between 23°N and 24.5°N, is both STCC and NEC. Regardless of In the following, we summarize a 4,000–6,000 m deep basin called the Huatung Basin. Located south of the Daniel L. Rudnick ([email protected]) is Professor, Scripps Institution of Oceanography, east-west trending Yaeyama Ridge, the La Jolla, CA, USA. Sen Jan is Associate Professor, Institute of Oceanography, National Ryukyu Trench, with its maximal depth Taiwan University, Taipei, Taiwan. Luca Centurioni is Associate Project Scientist, Scripps over 6,200 m, becomes shallow toward Institution of Oceanography, La Jolla, CA, USA. Craig M. Lee is Principal Oceanographer, the west and loses its topographic char- Applied Physics Laboratory, University of Washington, Seattle, WA, USA. Ren-Chieh Lien is acter west of 122.5°E. In the southern Principal Oceanographer and Affiliate Professor, Applied Physics Laboratory, University of part, the north-south trending Luzon Washington, Seattle, WA, USA. Joe Wang is Professor, Institute of Oceanography, National Island Arc, which is the southern bound Taiwan University, Taipei, Taiwan. Dong-Kyu Lee is Visiting Researcher, Scripps Institution of the Huatung Basin, connects to the of Oceanography, La Jolla, CA, USA. Ruo-Shan Tseng is Professor, Department of Marine Coastal Mountain Range in south- Resources, National Sun Yat-sen University, Kaohsiung, Taiwan. Yoo Yin Kim is Senior eastern Taiwan. Green Island and Lanyu Statistician, Scripps Institution of Oceanography, La Jolla, CA, USA. Ching-Sheng Chern is Island lie on the Luzon Island Arc off Professor, Institute of Oceanography, National Taiwan University, Taipei, Taiwan. southeastern Taiwan.

54 Oceanography | Vol.24, No.4 Winds search/rescue, and general education. both hydrographic and velocity data Winds measured by satellite remote To update the climatology of the in each grid are interpolated to the sensing as well as at island weather Kuroshio east of Taiwan after Liang center of the grid and then vertically stations off eastern Taiwan suggest that et al. (2003), we analyze hydrography interpolated at 10 m depth intervals. The the East Asia monsoon dominates the and current velocity data from the corresponding climatological mean of change of wind, which is southwesterly Ocean Data Bank collected before 2009. hydrography and velocity are calculated to southerly in summer and northeast- The shipboard ADCP ocean current at each grid when the data number erly in winter. The winter northeasterly velocity is blended with wind-driven is greater than three for hydrography monsoon commences in mid-September, and tidal components fluctuating at and 30 for velocity data in each grid. prevails from October to January, periods from a half-day to seasons. The It is natural to separate data collected and weakens continuously thereafter. barotropic tidal currents computed in summer and winter because of the The summer southwesterly monsoon, by a two-dimensional numerical tidal distinct monsoonal wind forcing. prevailing from June to August, is much model described in Hu et al. (2010) are Accordingly, summer is defined as from weaker than the winter monsoon, in subtracted from the ADCP velocity data. May to September and winter from particular, off southeastern Taiwan The ocean is divided into 15' × 15' grids; October to April. (Jan et al., 2006).

27°N Historical Hydrographic and Velocity Data EastEaEastst CChinahihinana SSeaeaea ShelfShehellff Since 1985, hydrography (temperature 26°N and salinity) and current velocity in ghgh the seas surrounding Taiwan have it TTrouooughu aia awawa 25°N trtrrait OkinOOkkiinn been continuously measured onboard S lanand n I-LanI-I-LaLan unini Issl ana YYuunanaguniaggu Island R/Vs Ocean Researcher 1, 2, and 3 iwiw RidgeRRiidge aaiwan St TTa l d AArc under various scientific programs spon- ele u Isllanan 24°N n ukkyu nnn RyukyuRyRy Island Ar sored by the National Science Council a Taiwan hha YaYaeyamaeyeyamma RidgRiRiddgge PPeeenghunngghhuu C (NSC) and the government agencies IIsIslandsllaandnd u HuatunHuHuatatunung RyukyuRyRy huh ukukyuyu TTrreeenchncnch 23°N g BasinBBaasisin of Taiwan. More than 40,000 historical nng enghu Channel GaguaGGa Ridge PPe GGreee n a conductivity-temperature-depth (CTD) ggu

IIsIslandslalandnd u

a hydrographic casts from 1985 to 2010, 22°N RRi LuL zon Island Arc

dg LaLanyn u u and 3,500,000 current velocity profiles g zoz

IIsIslandslaandd o e from 16 to 300 m depths measured by n IsI BBaattatanan s lal shipborne ADCP from 1991 to 2010 are 21°N in a assi IslandIIsslalandnd ndn BaB d a PhilippinePPhhiilipppipinene stored in NSC’s Ocean Data Bank oper- LuLuzozon A SeSe rcr SeaSSeea BasinBaBasisin a StStraittraaitit c ated by the Institute of Oceanography, inin 20°N Chh h BalintangBaBalliintntanang National Taiwan University. The raw uutt ChannelChChaannnenel SouthSoSo China Sea Basin data have undergone quality control BabuyanBaBabbuyayan IslandIIsslalandnd procedures following the standard 19°N recommended by the US National Philippines Oceanographic Data Center. In accor- (Luzon Island) 18°N dance with NSC policy, the archived 118°E 119°E 120°E 121°E 122°E 123°E 124°E 125°E data have been released to the public for –8000 –6000 –4000 –2000 0 2000 4000 6000 8000 purposes of scientific research, public m infrastructure engineering, marine Figure 2. Bathymetric chart and topography in the seas surrounding Taiwan.

Oceanography | December 2011 55 Climatology of the Luzon Strait, hugs the east coast of the southern tip of Taiwan, the main Kuroshio East of Taiwan Taiwan, and turns toward the north- stream passes through the gap between Currents averaged over the upper 30 m east off northeastern Taiwan. Within Taiwan and Green Island. A core of in summer and winter show changes Luzon Strait, the Kuroshio loops across strong flow likely from the Northwest caused by monsoonal wind forcing the strait in summer (Figure 3a) but Pacific subtropical gyre joins to the (Figure 3). Beginning east of Luzon partly leaks into the northern South east flank of the Kuroshio east of Green Island, the Kuroshio, with core velocity China Sea on its western flank in winter Island. As the main Kuroshio and of about 1 m s–1, flows northward across (Figure 3b). As the Kuroshio sweeps off this eastern branch join, the Kuroshio narrows and strengthens in the short distance before it arrives at I-Lan (a) Summer (b) Winter Ridge. This section of Kuroshio is also called the “East Taiwan Current.” After passing over the Ryukyu Island Arc, the Kuroshio bumps into the East China Sea shelf break and turns northeastward, leaving the coast of Taiwan at roughly 25°N. According to the roughly 20-year statistics, the Kuroshio east of Taiwan is 100 km wide and 800–1,000 m deep, with the velocity core ranging from 1 to 1.5 m s–1 (Figure 3). The mean volume transport in the upper 300 m is 9.1 Sv at 22°N between Taiwan and m Lanyu Island and is 15.7 Sv at 24°N -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 between the coast and 123°E.

0 A recent survey using shipboard U V Taiwan ADCP reveals the vertical structure (c) 100 200 of the Kuroshio as it enters Luzon Strait. In June 2010, during a neap tide, 1.0 300 Depth (m) R/V Revelle collected several sections 400 0.5 (d) across the Kuroshio (Figure 3c). The

) 500

–1 –1 01 Kuroshi –1 0.0 o ms V (m s –0.5 Figure 3. Climatological mean of shipboard 22°N acoustic Doppler current profiler (ADCP) velocity –1.0 observations at 30 m depth in (a) summer and (b) winter, and sections of meridional velocity 21°N measurements from R/V Revelle shipboard ADCP 0 in Luzon Strait in June 2010 (c). The depth scale of

N) ADCP velocity contours in (c) is exaggerated by a 0 o ( 2000 U V 20°N factor of five in order to reveal the vertical struc- 100

) ture of the Kuroshio (red shadings). The inserts

Depth (m) Lat 200 (d) and (e) show the vertical profile of zonal and 4000 meridional velocity in the middle of Luzon Strait 300 19°N

Depth (m (green vertical line in panel c) and in Balintang 119.0°E 400 119.5°E 120.0°E Channel (black vertical line in panel c), respec- 120.5°E 121.0°E21.00°°E (e) 500 18°N tively. Two yellow horizontal lines in (a) mark –1 01120.5°E120120.55°°E –1 123.0°E1 the zonal sections discussed in Figure 4. Lon (oE) ms

56 Oceanography | Vol.24, No.4 section along 20.5°N in the middle water is the relatively fresh northern seen from 500 to 650 m depth without of Luzon Strait shows that a strong South China Sea water coming from the distinct seasonal variation. northward Kuroshio exists between western Luzon Strait and attaching to the The core of the Kuroshio bends to the 120.75°E and the Luzon Island Arc, left side of the Kuroshio off southern- northeast following topography north with a width of ~ 100 km. Along the most Taiwan. A salinity maximum layer of Taiwan. The surface expression of the axis of the Kuroshio (green vertical (NPTW) is seen between 150 to 250 m Kuroshio is shifted landward in winter line in Figure 3c), the northward depths where the salinity is greater relative to summer (Figure 3). The strong current is about 0.8 m s–1 in the upper than 34.8 in both seasons. A salinity winter northeast monsoon likely plays 300 m, decreasing nearly linearly to minimum (~ 34.2) layer (NPIW) is a role in this shift in Kuroshio position. zero around 500 m depth (Figure 3d). The average zonal current in the upper Summer Winter 500 m is –0.13 m s–1, much smaller 22°N Section 24°N Section 22°N Section 24°N Section than the average meridional current of 0 –1 0.5 m s . Within the Balintang Channel –100 (black vertical line in Figure 3c), a strong –200 Depth (m) westward current of 1–1.2 m s–1 exists –300 121°E 122°E 122°E 123°E 121°E 122°E 122°E 123°E in the upper 100 m, and the northward Velocity (m s –1) current is similar to that in the middle –0.4 –0.2 0.0 0.2 0.4 0.6 0.8 1.0 of Luzon Strait (Figure 3e). The flow pattern is consistent with the climato- 22°N Section 24°N Section 22°N Section 24°N Section 0 logical mean (Figure 3a,b). –100 Figure 4 shows sections of northward –200 velocity, temperature, and salinity across –300 the Kuroshio at 22°N and 24°N (two –400 –500 horizontal yellow lines in Figure 3a). Depth (m) –600 The major velocity core is narrower –700 but deeper at 22°N, and a weaker but –800 121°E 122°E 122°E 123°E 121°E 122°E 122°E 123°E visible velocity core is seen in the upper Temperature (°C) 100 m at 122.25°E. However, at 24°N, 5 10 15 20 25 30 there is only one wider but shallower velocity core. A coastal countercurrent 22°N Section 24°N Section 22°N Section 24°N Section 0 exists below 150–200 m depths on –100 both sections. The isotherm tilt on the –200 west flank of the Kuroshio is consistent –300 with the geostrophic balance (Figure 4, –400 –500 middle panels). Isotherms level off Depth (m) –600 below 700 m, indicating the vertical –700 extent of the Kuroshio. The average –800 121°E 122°E 122°E 123°E 121°E 122°E 122°E 123°E temperature in the upper 50–100 m Salinity (psu) reaches 30°C in summer and decreases to 26°C in winter. The salinity west of 33.8 34.0 34.2 34.4 34.6 34.8 35.0 the Kuroshio in the upper 50 m layer Figure 4. Zonal sections of climatological mean meridional velocity (top panels), temperature (middle panels), and salinity (bottom panels) variations along 22°N and 24°N, (two horizontal is lower (34–34.2) in summer than in yellow lines in Figure 3a), in summer and winter. Contour interval is 0.1 m s–1 for velocity, winter (Figure 4, lower panels). This 1°C for temperature, and 0.1 psu for salinity.

Oceanography | December 2011 57 Other mechanisms that may cause (Niiler, 2001) in Luzon Strait, in the often enters the SCS through Luzon seasonal variations in the Kuroshio are northern SCS, and east of Taiwan permit Strait, and is believed to provide an changes in the NEC, fluctuations in construction of a time series. At the upper-ocean net westward mass trans- volume transport through Taiwan Strait, time of this writing, a line of drifters is port from the Philippine Sea into the and mesoscale eddies. being released monthly in the Philippine SCS (Centurioni et al., 2004, 2009). The Sea (Table 1). During the last decade, a cyclonic circulation of the northern Observations From total of more than 600 SVP drifters have SCS is intensified along the rim of the Surface Drifters been deployed in the Northwest Pacific basin and appears to form a continuous In the past decade, the US Office of in support of ONR activities. Drifters current system that connects the swift Naval Research (ONR) has sponsored were released mainly by Voluntary Luzon Strait throughflow and the several Lagrangian experiments in the Observing Ships (VOS), with the excep- fast southward current off Vietnam seas around Taiwan: South China Sea tion of few deployments from joint (Centurioni et al., 2009). The currents (SCS), East China Sea, Philippine Sea, US-Taiwan cruises in 2005, 2007, 2008, in Taiwan Strait become progressively Japan East Sea, Taiwan Strait, and Luzon and 2009. VOS operated by the Hanjin more southward as winter advances Strait. The main common goal of these Shipping Company, Hyundai, and the because the southward-moving cold and Lagrangian programs was to improve Taiwanese Coast Guard were central fresh East China Sea water modifies the understanding of the dynamical bases to the success of the programs. The summer mass distribution that main- of the interactions of the Kuroshio with US National Oceanic and Atmospheric tains the northeastward flow from the the seas around Taiwan by obtaining Administration (NOAA)-funded Global SCS into the East China Sea (Jan et al., new observations of ocean currents at Drifter Program also contributed signifi- 2002). Interestingly, while a northward 15 m depth. Of particular interest was cantly to the Northwest Pacific drifter current is still present in Taiwan Strait in the seasonal variability of the Kuroshio array described here. fall and early winter, none of the drifters in Luzon Strait, the seasonal circulation The near-surface currents measured that enter the SCS with the intruding of the northern SCS and Taiwan Strait, by the Northwest Pacific SVP drifter Kuroshio reach Taiwan Strait through the mesoscale variability east of Taiwan, array are markedly seasonal (Figure 5) the Penghu Channel. and the intrusion of the Kuroshio onto and seemingly in phase with the From April through September, a the continental shelf of the southern East monsoonal winds. From October period that includes the southwest China Sea. Multiyear deployments of through March, a period that includes monsoon, the Kuroshio intrusion Surface Velocity Program (SVP) drifters the northeast monsoon, the Kuroshio into the SCS disappears (Figure 5b),

Table 1. Summary of drifters deployed and anticipated deployments in the seas around Taiwan for the period from October 2003 through July 2012.

Number Region Time span and months Periodicity of drifters released Northern South China Sea, 2003–2006, October Weekly, five drifters per week drogued at 15 m depth 255 Luzon Strait, and Taiwan Strait through January and 75 drifters drogued at 150 m depth in 2006 2005 and 2007, One deployment per year, 25 drifters in 2007, Northern South China Sea 35 April and May 10 drifters in 2007 North Pacific, east of southern April 2007 to September Two drifters per week up to January 2009, then one 234 Taiwan and northeast of Taiwan 2009, continuous drifter per week; 122 drifters in July and August 2009 August 2010 to July 2012, Philippine Sea Nine drifters every 35 days 288 continuous

58 Oceanography | Vol.24, No.4 and a weaker, eddy-dominated flow the southern coast of Lamon Bay, to circulation at the center of the northern is established in the northern SCS ~ 16.5°N during the winter monsoon. SCS and the Pacific subtropical gyre. (Figure 5d). Tracks of drifters deployed Concurrently, the northward expression From April to September, the entire in the SCS in spring through summer of the NEC slows down. northern SCS circulation becomes eddy have shown how parcels tend to exit the The square root of the ratio of an eddy dominated, and the eddylike flow of the SCS through Luzon Strait and Taiwan to the mean kinetic energy is computed Pacific subtropical gyre extends more Strait (Centurioni et al., 2007). The flow from the drifter velocity data (Figure 5) southward (~ 13.5°N) compared to the through Taiwan Strait reverses, and and used to illustrate the seasonality October to March period, thus justifying a swift northeastward coastal current of the mesoscale variability around the reduced mean speed of the northern flows along the western coast of Taiwan. Taiwan. During the northeast monsoon NEC observed in the drifter data and West of Luzon, the southernmost season, the Kuroshio, the strong Vietnam the disappearance of the westward latitude at which the surface expres- current, and the current along the rim surface current between 18°N and 20°N sion of the Kuroshio appears as an of the northern SCS are steady relative east of Luzon Strait. organized flow shifts northward from to the flow associated with the meso- Observational evidence supports the the summer latitude of ~ 15°N, near scale eddies, while eddies dominate the concept that the westward-propagating

27°N > 4.0 (a) October–March ms–1 (c) October–March > 0.80 3.5 –1 24°N 0.3 ms 0.75 0.70 3.0 21°N 0.65 0.60 2.5

18°N 0.55 2.0 0.50 0.45 15°N 1.5 0.40

0.35 1.0 12°N < 0.30 < 0.5 27°N 4.0 ms–1 (b) April–September > 0.80 (d) April–September 3.5 24°N 0.3 ms–1 0.75 0.70 3.0 21°N 0.65 0.60 2.5 0.55 18°N 0.50 2.0 0.45 15°N 1.5 0.40 0.35 1.0 12°N < 0.30 < 0.5 110°E 115°E 120°E 125°E 130°E 110°E 115°E 120°E 125°E 130°E Figure 5. Locations of drifters color-coded in accordance with their six-hourly instantaneous speed overlaid with bin-averaged drifter velocity data for: (a) October through March and (b) April through September. The square root of the ratio of the mean eddy kinetic energy to the mean kinetic energy is computed from six-hourly drifter velocity observations for (c) October through March and (d) April through September. Velocities and ratios were averaged in 0.5° × 0.5° wide bins and are shown only for bins with more than 15 six-hourly observations.

Oceanography | December 2011 59 eddies that reach Taiwan and the Luzon over 5,000 dives were completed, the salinity extrema. Variations in spice Strait region within the Pacific subtrop- covering a total of more than 20,000 km on a wide range of scales are a consistent ical gyre, which are therefore respon- in over 1,000 glider days. feature of the observations. sible for sea level fluctuations east of The operational plan was for gliders We examine changes in thermohaline Taiwan, are positively correlated with the to be deployed off the coast of Luzon, structure along the glider trajectory indi- transport of the Kuroshio through the then to follow a zigzag route northward cated by the solid black line in Figure 6a. East Taiwan Channel (between Taiwan toward Taiwan (Figure 6a). Currents Vertical profiles of potential temperature and Iriomote, one of the westernmost tended to be weak in the deployment plotted against salinity (Figure 6c) clearly Ryukyu Islands) and cause, during times region, so gliders followed the planned show the salinity extrema of NPTW of low Kuroshio transport, pronounced track relatively precisely. Mesoscale and NPIW. These extrema are largest in meandering (intrusion) of the Kuroshio currents increased to the north, so the south and weaken moving north- onto the East China Sea continental gliders could not always follow the same ward following the path of the mean shelf (see Gawarkiewicz et al., 2011, path as 1,000 m-depth-average currents flow. Mixing with surrounding water, in this issue), with a periodicity of approached or exceeded 0.25 m s–1. either along or across isopycnals, causes approximately 100 days. Three gliders traveled down the axis this reduction in the salinity extrema, of the Kuroshio as it entered the SCS under the traditional assumption that Observations From through southern Luzon Strait, finally water properties are derived from end Underwater Gliders crossing the current at 20°N. members. The mean northward 1,000 m A program of glider observations was A crossing of the Kuroshio into the depth average flow along the glider undertaken to quantify mesoscale SCS shows the hydrographic structure of trajectory was 0.10 m s–1, so the water structure within and offshore of the the region (Figure 6b). The high-salinity column would take about three months Kuroshio (Figure 6). In April 2007, the NPTW near 200 m depth is apparent, to traverse the region. The changes in the first two gliders were deployed, and as is the low-salinity NPIW near 500 m. water masses are thus remarkably rapid. the last gliders were recovered in June The inshore edge of the Kuroshio (near We may gain some insight into the 2008. Deployments lasted three to four 450 km distance) is marked by a front processes causing stirring of NPTW and months, and the fleet included as many in salinity, indicating penetration of NPIW through examination of potential as four gliders at any one time. The NPTW into the SCS. The magnitude temperature on isopycnals (Figure 6d). two types of autonomous underwater of the NPIW salinity minimum also This rather simple expression of spice gliders (Rudnick et al., 2004) used, Spray decreases across the Kuroshio. Surface demonstrates one of the most robust (Sherman et al., 2001) and Seaglider low-salinity water originates in the SCS. features in the glider observations. (Eriksen et al., 2001), profiled as deep Fluctuations in the depth of isopycnals Layers of strong spice variability coin- as 1,000 m, repeating a cycle from the are reflections of mesoscale processes cide with the dominant water masses. surface to depth about every six hours and internal waves. Apparently, stirring is most obvious and covering 6 km. The resulting speed The dominant water masses, NPTW along salinity extrema, because these of 0.25 m s–1 was sometimes slower and NPIW, are regions of thermohaline waters are the newest in the sense that than the depth-average currents in the fluctuations on isopycnals. This ther- they were most recently ventilated. region, presenting some challenges in mohaline variability (Veronis, 1972; That is, there is a large-scale gradient piloting. Even with these challenges, the Jackett and McDougall, 1985), which available to be stirred by the prevalent gliders were able to survey the offshore by definition does not affect density, mesoscale eddies. It is often the case that eddy-rich region and cross the Kuroshio has been called “spice” (Munk, 1981). spice gradients in the two layers align in in Luzon Strait. Variables measured In the section across the Kuroshio the vertical because the eddies are thick included pressure, temperature, salinity, (Figure 6b), spice variability can be seen enough to stir both layers. Between the and depth-average velocity. Throughout as the changes in salinity (color) along extrema is a layer of very weak spice the 15 months of glider observations, isopycnals (black contours), especially in variability near 26.5 kg m–3. The same

60 Oceanography | Vol.24, No.4 eddies affect the quiet layer, but the diapycnal mixing cause the reduction in diffusivity proceeds as follows. Take the effects are not obvious because there is salinity extrema (Figure 6c). An open reduction in salinity to be 0.1 psu, and no large-scale gradient to be stirred. This question is the contribution of isopycnal the time for this change to occur to be lack of a large-scale gradient near 15°C is and diapycnal processes to the observed 100 days. The mean vertical curvature seen in the tightness of the temperature- thermohaline modification. Assuming of salinity on isopycnals in the NPTW salinity relationship (Figure 6c). only diapycnal mixing causes the is –1 × 10–4 psu m–2 (this can be esti- Isopycnal processes, as manifest reduction in salinity in the NPTW, an mated from Figure 6b,c), so the inferred in spice variability (Figure 6d), and order-of-magnitude calculation of eddy diffusivity is 1 × 10–4 m2 s–1. This value

0 500 24°N 1000

23°N 2000 16 Mar 2008 09:44:34 – 01 Apr 2008 08:31:40, Dives 24–90 0 35.0 22°N 3000 100

21°N 4000 200 ) 300 20°N 5000 34.5

Depth (m 400 ) 19°N 500 Depth (m

18°N 7000 600 Salinity (psu) 34.0 700 17°N 800 16°N 900

15°N 10000 1000 33.5 119°E 120°E 121°E 122°E 123°E 124°E 125°E 500 400 300 200 100 Distance (km)

15 Apr 2007 05:03:00 – 25 Jul 2007 04:16:55 15 Apr 2007 05:03:00 – 25 Jul 2007 04:16:55, Dives 1–421 35 35 23°N 30 30 22°N 25 25 21°N C) C) 20 20 20°N

15 15 emperature (° emperature (°

T 19°N T

10 10 tential 18°N tential Po Po 5 5 17°N

0 0 34.0 34.2 34.4 34.6 34.8 35.0 35.2 0 500 1000 1500 2000 Salinity (psu) Distance (km) Figure 6. (a) Glider tracks in the region of the Kuroshio from April 2007 through July 2008. Thirteen glider deployments produced over 5,000 dives to as deep as 1,000 m, covering more than 20,000 km in over 1,000 glider-days. (b) A section along the red track in panel (a). Salinity is contoured in color, and potential density is contoured with black lines and an interval of 0.25 kg m–3. Tick marks along the top indicate the locations of profiles. The section follows the Kuroshio as it goes through Luzon Strait from the Pacific, and finally crosses the current into the South China Sea. (c) Profiles of potential temperature against salinity, colored by latitude along the black track in panel (a). Note the spread in salinity in the regions of the salinity extrema. (d) Potential temperature on isopycnals (with a spacing of 0.25 kg m–3) along the black track in panel (a). Tick marks along the top show the locations of profiles. Note the two layers of strong variability corresponding to the salinity extrema in panel (c). Also note the region of weak variability near 15°C.

Oceanography | December 2011 61 is an order of magnitude larger than the leaving the northwest coast of Taiwan. southward flow. Analyses of large-scale 1 × 10–5 m2 s–1 canonical value in the Along the way, the Kuroshio loops observations, for example, satellite sea upper thermocline from microstructure into, and back out of, Luzon Strait. A surface height, suggest interannual vari- measurements. Presumably, the hori- second core of strong flow joins the ability in the area of the bifurcation, zontal stirring apparent in spice helps main Kuroshio from the east offshore of with a particularly southerly location in to increase the eddy diffusivity averaged Taiwan, increasing the transport. recent years. The bifurcation location over large temporal and spatial scales. Seasonal variability, driven by the is predictable from basin-scale models The calculation of eddy diffusivity is very monsoon, is a prominent feature of the driven by wind stress. In situ observa- rough, with the time required for the region. Wind forcing during the strong tions gathered using surface drifters and NPTW salinity maximum to be eroded northeast winter monsoon pushes the profiling floats deployed in the NEC are particularly worth scrutiny. However, core of the Kuroshio closer to the Taiwan being used to quantify how the west- this erosion is certainly occurring, coast. Also in winter, the Kuroshio sends ward flow splits near the bifurcation. suggesting strong mixing. a net flow into the SCS, and a strong Gliders are being deployed off Palau to The finding of layers of enhanced southward boundary current along the do repeated sections across the NEC and spice variability is not unique to this east coast of Vietnam develops. The the Mindanao Current. Gliders launched region. These layers have also been summer surface flow pattern suggests from Taiwan will cross the Kuroshio off identified through sustained glider that near-surface water parcels exit the northern Luzon and southern Taiwan. observations in the California Current South China Sea through Luzon Strait to A final glider section in the SCS will System (Todd et al., 2011) and in the join the Kuroshio. document the net transport from the subtropical gyre north of Hawaii (Cole Mesoscale variability, at timescales Pacific. Moorings will be placed at the and Rudnick, 2011). In each case, the shorter than seasonal and longer than a southern end of Luzon Strait to monitor high variance spice layers coincide with day, dominates flow in the region, espe- the Kuroshio flow at this critical point. recently ventilated waters, or with named cially away from the Kuroshio core. The Forward and assimilating models, and water masses. This sort of spice variance source of the mesoscale variability is the objective analyses, are being used in was previously identified in ship surveys object of continuing study, but instability combination with the observations to using towed vehicles (i.e., Ferrari and in the STCC to the east is certainly a establish predictability. If these efforts Rudnick, 2000; Hodges and Rudnick, factor. Mesoscale structure is especially are fully successful, they promise funda- 2006); however, the persistence of apparent in hydrographic sections. mental advances in the understanding these layers was unknown as they were The mesoscale stirring manifests itself of the NEC, the Kuroshio, and the observed though sections occupied only as interleaving in the dominant water Mindanao Current. once. With sustained glider observa- masses, that is, the shallow salty NPTW tions, it is now becoming clear that and the deeper fresh NPIW. The stirring Acknowledgements layers of persistent spice variance occur participates in the turbulent cascade, We are grateful to the Office of Naval throughout the ocean, often coinciding and a back-of-the-envelope calculation Research, whose support has been with the intrusion of major water masses. suggests enhanced mixing in this region. essential to many of the observational An active ONR-sponsored observa- programs reported here. We acknowl- Conclusion tional and modeling program, “Origins edge funding through ONR grants A combination of the historical data of the Kuroshio and Mindanao Current,” N00014-06-1-0776 (DLR), N00014- record and recent focused observations aims to quantify patterns of flow 10-1-0273 (DLR and LC), N00014-03- is establishing the mean state of the and fluxes with a goal of establishing 1-0474 (LC), N00014-08-1-0557 (LC), Kuroshio near its origins. From humble predictability. At issue is how the NEC N00014-10-1-0397 (RCL), N00014-06- beginnings at the termination of the feeds the Kuroshio and the Mindanao 1-0751, and N00014-10-1-0311 (CML), NEC off the coast of the Philippines, the Current, and what dynamics determine and NOAA Global Drifter Program Kuroshio grows to a powerful current the partitioning into northward and grant NA17RJ1231 (LC). The Ocean

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