Variability of the Kuroshio east of Taiwan was observed at a cross-stream transect ~ 50km south of the PCM-1 line with an array of three moored ADCPs measuring for ~ 23 months, supplemented with ...eleven repeated shipboard surveys. Observations of the Kuroshio's velocity structure reveal the absence of an obvious regular seasonal signal, but significant variability at 70–200 day period for both maximum velocity axis migration and transport due to interactions with mesoscale eddies. Empirical orthogonal function (EOF) analysis shows the migration and transport modes explain 46% and 29% of the total variance, respectively, which is in contrast to the findings at the PCM-1 line where the transport mode explained more variance than did the migration mode. The Kuroshio transport in the upper 500m across a 150km section is 17.2Sv with a standard deviation of 5Sv. The estimated Kuroshio transport is 4.3Sv lower than that reported for the PCM-1 line, likely due to the interannual variations related to abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions. When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increases and decreases the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.
•Kuroshio strongly varies in migration and transport modes associated with eddies.•Local barotropic/baroclinic responses and the upstream steering affect the transport.•Flow divergence/convergence adjacent to the Kuroshio edge affects its axis.
Eddy‐Kuroshio Interactions: Local and Remote Effects Jan, Sen; Mensah, Vigan; Andres, Magdalena ...
Journal of geophysical research. Oceans,
December 2017, 2017-12-00, 20171201, Letnik:
122, Številka:
12
Journal Article
Recenzirano
Odprti dostop
Quasi‐geostrophic mesoscale eddies regularly impinge on the Kuroshio in the western North Pacific, but the processes underlying the evolution of these eddy‐Kuroshio interactions have not yet been ...thoroughly investigated in the literature. Here this interaction is examined with results from a semi‐idealized three‐dimensional numerical model and observations from four pressure‐sensor equipped inverted echo sounders (PIESs) in a zonal section east of Taiwan and satellite altimeters. Both the observations and numerical simulations suggest that, during the interaction of a cyclonic eddy with the Kuroshio, the circular eddy is deformed into an elliptic shape with the major axis in the northwest‐southeast direction, before being dissipated; the poleward velocity and associated Kuroshio transport decrease and the sea level and pycnocline slopes across the Kuroshio weaken. In contrast, for an anticyclonic eddy during the eddy‐Kuroshio interaction, variations in the velocity, sea level, and isopycnal depth are reversed; the circular eddy is also deformed to an ellipse but with the major axis parallel to the Kuroshio. The model results also demonstrate that the velocity field is modified first and consequently the SSH and isopycnal depth evolve during the interaction. Furthermore, due to the combined effect of impingement latitude and realistic topography, some eddy‐Kuroshio interactions east of Taiwan are found to have remote effects, both in the Luzon Strait and on the East China Sea shelf northeast of Taiwan.
Plain Language Summary
Mesoscale eddies are everywhere in the ocean. These ocean swirls of either clockwise or counterclockwise spinning with diameter of about 100‐300 km and rounding current speed of about 0.5 m/s, carrying energy and certain type of water mass, move westward and eventually reach the western boundary of each ocean. The evolution of these eddies and the interaction which occurs when they encounter the western boundary current, e.g. the Kuroshio in the western North Pacific, is important in redistributing ocean energy and, in turn, shaping the large scale ocean circulation. This study focuses on the processes underlying the interaction of nonlinear mesoscale eddies with the Kuroshio, which have not yet been thoroughly investigated in the literature. Using pressure‐sensor equipped echo sounder and satellite observations interpreted in the context of semi‐idealized numerical simulations, this study find (1) locally, eddy arrivals modify velocity structure in the Kuroshio first, followed by changes in sea level and isopycnal depths leading to seesaw‐like variations of the sea level and density slopes across the Kuroshio, and (2) modeled remote effects, i.e., Kuroshio intrusions, manifest in the Luzon Strait and on the East China Sea shelf and depend on the eddies' impingement latitude, strength, and polarity.
Key Points
Observations of eddy‐Kuroshio interactions are interpreted through numerical simulations with varying eddy strength and impinging latitude
Locally, eddy arrivals change sea level and isopycnal depths leading to seesaw‐like patterns across the Kuroshio
Modeled remote effects manifest at the Luzon Strait and East China Sea shelf and depend on the eddies' impingement latitude and polarity
Tropical cyclones (TCs) cause severe natural hazards and drive intense upper ocean cooling through a series of oceanic and atmospheric physical processes, including vertical mixing and upwelling. ...Among these processes, TC-induced warming of near-surface waters in the open ocean has rarely been noted. This study provides a detailed analysis of upper ocean responses to 30 TC events observed by two buoys in the western North Pacific between 2016 and 2021. Supplemented with numerical experiments, we suggest that downwelling frequently occurs at the periphery of upwelling regions (around the radius of the 34 knot wind speed) following the passage of a TC. Downwelling is identified via pronounced warm anomalies under a shallow mixed layer depth, and its dynamics are attributed to negative wind stress curl and current-induced convergence. These findings highlight the important role played by TC-induced downwelling and offer insights for reconsidering the influence of TCs on biogeochemical processes.
Variability of water mass and throughflow transport in the Taiwan Strait are analyzed using strait‐wide conductivity‐temperature‐depth (1985–2003) and sectional acoustic Doppler current profiler ...(1999–2001) data. Results from a cluster analysis, temperature‐salinity diagrams, and direct transport calculations indicate that the strong northeast monsoon drives the brackish Mixed China Coastal Water into the northern strait and hinders the northward intruded saline Kuroshio Branch Water (KBW) in the southeastern strait from December to January. The mean throughflow transport across the central strait is about 0.1 Sv southward during this period, thus supporting previous observations that there is no persistent northward flowing current throughout the strait in winter. The weakening of the northeast monsoon in February–March, however, emancipates the KBW to intrude northward into the East China Sea (ECS). In June, the increase in this northward transport accompanied by the decrease in the westward intrusion of Kuroshio through the Luzon Strait leads to the replacement of the KBW by the less saline South China Sea Water (SCSW). The northward transports ranging from 1.16 to 2.34 Sv between March and August yield 0.131–0.238 × 1015 W and 53.33–81.74 × 106 kg/s of temperature and salt transports, respectively, toward the ECS. The inception of the northeast monsoon in October marks both the change of water mass from fall to winter patterns and the decrease of the northward throughflow transport. Our results also reveal that the subsurface KBW and SCSW remain consistent throughout the observation periods between 100 and 200 m and below 200 m depths, respectively, in the southeastern strait.
Submesoscale interleaving layers are caused by lateral intrusions of dissimilar water masses in frontal zones, which are significant processes in shaping physical, biogeochemical, and ecological ...parameters in the ocean. Possible interleaving layers were sometimes observed by ship-based conductivity-temperature-depth (CTD) surveys with coarse spacing between adjacent stations in the Kuroshio region east of Taiwan but have never been examined dynamically. Here we show the characteristics of interleaving layers observed by a Seaglider with two repeated hydrographic surveys along a triangle track east of Taiwan from December 2016 to March 2017. Salinity profiles indicate that prominent interleaving layers appeared in the intermediate layer (approximately 500-800 m) with vertical and horizontal length scales of O(50) m and O(10-100) km, respectively, during our observations. A dipole eddy pair and a relatively large anticyclonic eddy impinged on the Kuroshio during the first and second surveys, respectively, which brought certain impacts on the interleaving motion as the eddy potentially altered the density slope across the Kuroshio. The associated instability analysis and the Turner angle suggest that the double diffusive instability is the primary driving mechanism for the development of interleaving layers.
Abstract
Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is ...strong (1.5 m s
−1
) and unsteady (±0.5 m s
−1
) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity
K
ρ
is concentrated from
O
(10
−4
) to
O
(10
−3
) m
2
s
−1
and has a maximum value of 10 m
2
s
−1
. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Biological organic carbon production and consumption play a fundamental role in the understanding of organic carbon cycling in oceans. However, studies on them in the Kuroshio, the western boundary ...current in the North Pacific Ocean, are scarce. To better understand the variations of plankton community respiration (CR) and particulate organic carbon (POC), eight cruises. which covered four seasons over a 2-year period, were surveyed across the Kuroshio at the KTV1 transect east of Taiwan. Spatially, a coastal uplift of isotherms (i.e., onshore lifting and offshore deepening) was observed along the KTV1 transect. During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth, resulting in higher values of phytoplankton, POC, and plankton CR on the onshore side. In this study, phytoplankton was dominated by picophytoplankton including Prochlorococcus, Synechococcus, and picoeukaryotes. Plankton CR was low, and its mean depth-normalized integrated rate (the upper 100 m water depth) ranged from 7.07 to 22.27 mg C m−3 d−1, to which the picophytoplankton and heterotrophic bacteria contributed the most. The mean depth-normalized integrated value of POC ranged from 12.7 to 21.6 μg C L−1. POC is mainly associated with phytoplankton biomass with a mean carbon ratio of chlorophyll a/POC ≈ 1.03. All results suggest that plankton CR and POC variations may be associated with picoplankton dynamics in the Kuroshio.
Abstract
This study presents amended procedures to process and map data collected by pressure-sensor-equipped inverted echo sounders (PIESs) in western boundary current regions. The modifications to ...the existing methodology, applied to observations of the Kuroshio from a PIES array deployed northeast of Luzon, Philippines, consist of substituting a hydrography-based mean travel time field for the PIES-based mean field and using two distinct gravest empirical mode (GEM) lookup tables across the front that separate water masses of South China Sea and North Pacific origin. In addition, this study presents a method to use time-mean velocities from acoustic Doppler current profilers (ADCPs) to reference (or “level”) the PIES-recorded pressures in order to obtain time series of absolute geostrophic velocity. Results derived from the PIES observations processed with the hydrography-based mean field and two GEMs are compared with hydrographic profiles sampled by Seagliders during the PIES observation period and with current velocity measured concurrently by a collocated ADCP array. The updated processing scheme leads to a 41% error decrease in the determination of the thermocline depth across the current, a 22% error decrease in baroclinic current velocity shear, and a 61% error decrease in baroclinic volume transports. The absolute volume transport time series derived from the leveled PIES array compares well with that obtained directly from the ADCPs with a root-mean-square difference of 3.0 Sv (1 Sv ≡ 10
6
m
3
s
–1
), which is mainly attributed to the influence of ageostrophic processes on the ADCP-measured velocities that cannot be calculated from the PIES observations.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor‐equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of ...Luzon. Zero lag correlations between PIES‐measured acoustic travel times and satellite‐measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy‐influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw‐like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6 month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21 days later in the downstream Kuroshio east of Taiwan.
Key Points
Eddy‐Kuroshio interaction is observed with 11 pressure sensor‐equipped inverted echo sounders
Eddy‐influence weakens across the Kuroshio east of Taiwan and northeast of Luzon
Eddy‐influence causes seesaw‐like SSHa changes across the Kuroshio most obviously east of Taiwan
The spatial and temporal variations of baroclinic tides in the Luzon Strait (LS) are investigated using a three-dimensional tide model driven by four principal constituents, O
1
, K
1
, M
2
and S
2
, ...individually or together with seasonal mean summer or winter stratifications as the initial field. Barotropic tides propagate predominantly westward from the Pacific Ocean, impinge on two prominent north-south running submarine ridges in LS, and generate strong baroclinic tides propagating into both the South China Sea (SCS) and the Pacific Ocean. Strong baroclinic tides, ∼19 GW for diurnal tides and ∼11 GW for semidiurnal tides, are excited on both the east ridge (70%) and the west ridge (30%). The barotropic to baroclinic energy conversion rate reaches 30% for diurnal tides and ∼20% for semidiurnal tides. Diurnal (O
1
and K
1
) and semidiurnal (M
2
) baroclinic tides have a comparable depth-integrated energy flux 10–20 kW m
−1
emanating from the LS into the SCS and the Pacific basin. The spring-neap averaged, meridionally integrated baroclinic tidal energy flux is ∼7 GW into the SCS and ∼6 GW into the Pacific Ocean, representing one of the strongest baroclinic tidal energy flux regimes in the World Ocean. About 18 GW of baroclinic tidal energy, ∼50% of that generated in the LS, is lost locally, which is more than five times that estimated in the vicinity of the Hawaiian ridge. The strong westward-propagating semidiurnal baroclinic tidal energy flux is likely the energy source for the large-amplitude nonlinear internal waves found in the SCS. The baroclinic tidal energy generation, energy fluxes, and energy dissipation rates in the spring tide are about five times those in the neap tide; while there is no significant seasonal variation of energetics, but the propagation speed of baroclinic tide is about 10% faster in summer than in winter. Within the LS, the average turbulence kinetic energy dissipation rate is O(10
−7
) W kg
− 1
and the turbulence diffusivity is O(10
−3
) m
2
s
−1
, a factor of 100 greater than those in the typical open ocean. This strong turbulence mixing induced by the baroclinic tidal energy dissipation exists in the main path of the Kuroshio and is important in mixing the Pacific Ocean, Kuroshio, and the SCS waters.
PDF
