Abstract
In this study, a reduced-gravity, primitive equation OGCM is used to investigate the seasonal variability of the bifurcation of the South Equatorial Current (SEC) into the Brazil Current ...(BC) to the south and the North Brazil Undercurrent/Current (NBUC/NBC) system to the north. Annual mean meridional velocity averaged within a 2° longitude band off the South American coast shows that the SEC bifurcation occurs at about 10°–14°S near the surface, shifting poleward with increasing depth, reaching 27°S at 1000 m, in both observations and model. The bifurcation latitude reaches its southernmost position in July (∼17°S in the top 200 m) and its northernmost position in November (∼13°S in the top 200 m). The model results show that most of the seasonal variability of the bifurcation latitude in the upper thermocline is associated with changes in the local wind stress curl due to the annual north–south excursion of the marine ITCZ complex. As the SEC bifurcation latitude moves south (north) the NBUC transport increases (decreases) and the BC transport decreases (increases). The remote forcing (i.e., westward propagation of anomalies) appears to have a smaller impact on the seasonal variability of the bifurcation in the upper thermocline.
Pacific Decadal Oscillation (PDO) index is strongly correlated with vertically integrated transport carried by the Kuroshio through the East China Sea (ECS). Transport was determined from satellite ...altimetry calibrated with in situ data and its correlation with PDO index (0.76) is highest at zero lag. Total PDO‐correlated transport variation carried by the ECS‐Kuroshio and Ryukyu Current is about 4 Sv. In addition, PDO index is strongly negatively correlated, at zero lag, with NCEP wind‐stress‐curl over the central North Pacific at ECS latitudes. Sverdrup transport, calculated from wind‐stress‐curl anomalies, is consistent with the observed transport variations. Finally, PDO index and ECS‐Kuroshio transport are each negatively correlated with Kuroshio Position Index in the Tokara Strait; this can be explained by a model in which Kuroshio path is steered by topography when transport is low and is inertially controlled when transport is high.
Data from satellite altimeters and from a 13-month deployment of
in situ
instruments are used to determine an empirical relationship between sea-level anomaly difference (SLA) across the Kuroshio in ...the East China Sea (ECS-Kuroshio) and net transport near 28°N. Applying this relationship to the altimeter data, we obtain a 12-year time series of ECS-Kuroshio transport crossing the C-line (KT). The resulting mean transport is 18.7 ± 0.2 Sv with 1.8 Sv standard deviation. This KT is compared with a similarly-determined time series of net Ryukyu Current transport crossing the O-line near 26°N southeast of Okinawa (RT). Their mean sum (24 Sv) is less than the mean predicted Sverdrup transport. These KT and RT mean-flow estimates form a consistent pattern with historical estimates of other mean flows in the East China Sea/Philippine Basin region. While mean KT is larger than mean RT by a factor of 3.5, the amplitude of the KT annual cycle is only half that of RT. At the 95% confidence level the transports are coherent at periods of about 2 years and 100–200 days, with RT leading KT by about 60 days in each case. At the annual period, the transports are coherent at the 90% confidence level with KT leading RT by 4–5 months. While the bulk of the Kuroshio enters the ECS through the channel between Taiwan and Yonaguni-jima, analysis of satellite altimetry maps, together with the transport time series, indicates that the effect of mesoscale eddies is transmitted to the ECS via the Kerama Gap southwest of Okinawa. Once the effect of these eddies is felt by the ECS-Kuroshio at 28°N, it is advected rapidly to the Tokara Strait.
Water mass formation in the intermediate and deep layers of the Okinawa Trough is investigated using two distinct data sets: a quasi‐climatological data set of the water properties of the minimum ...salinity surface produced from Argo float profiles and historical CTD data, and a velocity data set in the Kerama Gap measured by moored current meters during June 2009 to June 2011. The formation process of Okinawa Trough Intermediate Water is explained on the basis of horizontal advection and mixing of North Pacific Intermediate Water (NPIW) and South China Sea Intermediate Water (SCSIW). The salinity‐minimum water intruding into the Okinawa Trough through the channel east of Taiwan is approximately composed of 45% NPIW and 55% SCSIW, while that through the Kerama Gap is 75% NPIW and 25% SCSIW. Salinities of these water masses increase in the Okinawa Trough due to strong diapycnal diffusion; its coefficient is estimated as 6.8–21.5 × 10−4 m2 s−1 based on a simple advection‐diffusion equation. On the other hand, deep water in the Okinawa Trough, below the sill depth of the Kerama Gap (∼1100 m), is ventilated by overflow in the bottom layer of the Kerama Gap down to the deepest layer (∼2000 m) in the southern Okinawa Trough. A simple box model predicts that this bottom overflow (0.18–0.35 Sv) causes strong upwelling (3.8–7.6 × 10−6 m s−1) in the southern Okinawa Trough, which must be maintained by buoyancy gain of the deep water due to strong diapycnal diffusion (4.8–9.5 × 10−4 m2 s−1).
Key Points
The Kerama Gap bottom overflow causes the deep ventilation in Okinawa Trough
Salinity minima are attenuated by strong diapycnal mixing in Okinawa Trough
Okinawa Trough is an upwelling area of North Pacific Intermediate Water
The Kerama Gap, near the middle of the Ryukyu Island chain, is the deepest channel with a sill depth of 1050 m connecting the East China Sea (ECS) to the Northwestern Pacific. We measured the flow ...through the Kerama Gap from June 2009 to June 2011. The 2 year mean transport, 2.0 ± 0.7 Sv, is into the ECS from the Northwestern Pacific; it contributes about 11% of the mean Kuroshio transport in the ECS at the PN line. Subtidal standard deviation of the transport through the Kerama Gap is 3.2 Sv, comparable to that of the PN‐line Kuroshio transport (4.0 Sv), suggesting a significant effect of Kerama Gap transport on temporal variability of the Kuroshio transport in the ECS. Comparison with time series of satellite‐measured sea surface height maps reveals that temporal variability of the Kerama Gap transport is related to the arrival of mesoscale eddies from the east: high (low) transport into the ECS is associated with the presence of a cyclonic (anticyclonic) eddy south of the Kerama Gap.
Key Points
Mean flow through the Kerama Gap is 2 Sv into the East China Sea
Variability of subtidal Kerama Gap flow is larger than the mean
This variability is related to the arrival of mesoscale eddies from the east
Abstract
Although tides are believed to be the most important source for diapycnal mixing in the ocean, few studies have directly simulated open-ocean circulation including tides. Because the ...East/Japan Sea (EJS) has been considered to be a “miniature ocean,” tidal effects on the intermediate water of the EJS are investigated by using an eddy-resolving ocean general circulation model that can take account of M2 and K1 tides as well as oceanic flows. The simulated temperature and salinity in the intermediate layer are significantly improved by including tides. The improvement results from the combined effect of two internal tides. The M2 internal tide, propagating into the interior of the EJS, enhances vertical mixing and brings watermass characteristics closer to those observed. The K1 internal tide, trapped along the coast, induces southwestward flow along the Russian coast in the upper layer and thereby enhances the so-called Liman Current, which transports fresh waters with density conducive to the ventilation of intermediate waters in the EJS.
We present observational evidence from coastal tide station and bottom pressure data that basin‐mode oscillations are frequently excited in the Japan/East Sea (JES). The fundamental basin‐mode is a ...Kelvin‐wave‐like oscillation consisting of a single amphidromic system around which the high water propagates counter‐clockwise. Its period is about 6.7 hours and its coastal wavelength is equivalent to the circumference of the JES. The relative amplitudes of the observed oscillations agree with Rikiishi's 1986 model results except for stations near the Korea Strait where the closed boundary in the model produces unrealistically high amplitudes. The basin oscillation amplitude varies on synoptic time scales (2–17 days) and exhibits seasonal variations. The optimal wind direction to generate basin‐mode oscillations is along 60°/240° T.
The Affiliated Surveys of the Kuroshio off Ashizuri‐Misaki (ASUKA) project is a collaboration between scientists in Japan and the United States to improve our understanding of the structure and ...dynamics of the Kuroshio off the coast of southwest Japan. In this project an array of nine inverted echo sounders and nine current meter moorings were deployed for 2 years (1993–1995) along a line extending 1000 km from Shikoku. Numerous hydrographic profiles along this section were used to determine the regional gravest empirical modes (GEMs), which relate vertical profiles of temperature and specific volume anomaly to vertical acoustic echo time. These GEMs were used in conjunction with the inverted echo sounder data to calculate 2‐year time series of entire sections of relative geostrophic velocity and temperature. Current meter data provided absolute velocity references for the geostrophic velocity sections. In addition, a time series of Kuroshio position was determined from the GEM temperature sections. It shows three big (>80 km displacement) and three little offshore meanders of the Kuroshio occurring in February, May/June, and October of both years. This suggests a 4‐month periodicity, possibly phase‐locked to the annual cycle. The meanders may be caused by mesoscale eddies arriving from the east. The observed meanders were preceded by low Kuroshio volume transport and were followed by high Kuroshio volume transport. The Kuroshio volume and temperature transports were highly variable, with respective means of 65 ± 4 Sv and 3.7 ± 0.2 PW and standard deviations of 20 Sv and 0.9 PW.
From historical hydrocasts in the Okinawa Trough region of the East China Sea (ECS), acoustic echo time from 700 dbar to the surface shows a tight relationship with temperature (T) except near the ...100–200‐dbar layer. This is caused by 2nd‐ or higher‐mode baroclinic variations. Significant out‐of‐phase correlation between upper and lower layer T from the hydrocasts confirms that 2nd‐mode variations are strong. Furthermore, the 2nd‐mode variations are dominant during the ebb tide period, which suggests they are caused by 2nd‐mode internal tides (ITs) generated at the continental shelf break. For comparison, we investigate historical hydrocasts southeast of the Ryukyu Islands, where no significant 2nd‐mode variations are found. A model simulating the M2 IT agrees qualitatively with these observations: it predicts relatively strong 2nd‐mode IT energy in the ECS, but weak energy in the region southeast of the Ryukyu Islands.
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