Warm-core Ring Research Articles

Vertical distribution, abundance and community structure of oncaeid copepods in the Oyashio region, western subarctic Pacific

The vertical distribution, abundance and community structure of oncaeid copepods were investi- gated in the Oyashio region, western subarctic Pacific. Samples were collected with a 0.10 mm mesh closing type net from five discrete layers down to a maximum depth of 2,000 m in September and December 1996 and in April and October 1997. The copepods were widely distributed from epipelagic to bathypelagic zones, and showed prominent peaks of abundance above the ther- mocline and/or between 250 and 1,000 m depth. Standing stocks of total oncaeid copepods ranged from 1.5 to 2.5·10 5 inds m 2 at 0-2,000 m in the water col- umn, which are the same order of magnitudes as those reported in tropical, subtropical and polar regions by previous workers. A total of 38 species and two forms belonging to the genera Oncaea, Triconia, Spinoncaea, Conaea and Epicalymma, and two provisionally classi- fied species of the family Oncaeidae were identified in this study. Of these, 14 species have already been re- corded from the eastern subarctic Pacific. Several warm- water species were also found in December 1996 and/or October 1997, when the effect of warm-core rings orig- inating from the Kuroshio Current was evident in the epipelagic zone. Dominant species throughout the whole water column were T. borealis, T. canadensis, O. grossa, O. parila, O. rimula, O. lacinia, Epicalymma spp. and Oncaea sp. A, and these eight species together always made up about half of the total oncaeid numbers. Community analysis revealed that species composition in epipelagic and upper mesopelagic zones varied tem- porally due to the changes of hydrographic conditions, whereas those in lower mesopelagic and bathypelagic zones were relatively stable, reflecting the constancy of deep environments.

Numerical Study on the Oyashio Water Pathways in the Kuroshio–Oyashio Confluence*

Abstract In this paper, results of a high-resolution regional model of the Kuroshio–Oyashio confluence, where the mixed water region (MWR) forms off the northeastern coast of Japan, are discussed. The model simulates major characteristics of the Kuroshio and the Oyashio system well, such as the separation of the Kuroshio Extension from the Japanese coast and southward intrusion of the Oyashio. Further, potential temperature and salinity structures in the intermediate layer σθ = 27.0 resemble those obtained from historical data. Upon the success of this simulation, the authors focus on the diagnosis of the Oyashio water pathways intruding into the subtropics. It is found that the pathways of the Oyashio water form in the vicinity of the Japanese coast, where warm core rings and the Oyashio intrusion are active. These pathways are shown to be primarily eddy driven. Of particular interest is the water that originates in the Sea of Okhotsk, characterized by low potential vorticity (PV). Impacts of the Okhotsk...

Scotian Slope circulation and eddy variability from TOPEX/Poseidon and frontal analysis data

Sea level observations from TOPEX/Poseidon (T/P) altimetry, in conjunction with concurrent frontal analysis data from infrared imagery, are used to study temporal and spatial variability of sea‐surface currents over the Scotian Slope. Geostrophic surface current anomalies normal to ground tracks are derived from the sea level anomalies relative to local means for the study period. The altimetric results reveal prominent spatial variability of cross‐track currents with an overall intensification toward the west and south, typically 20–30 cm/s over the upper and lower slope, 20–50 cm over the continental rise, and up to 70–75 cm/s near the Gulf Stream northern wall. A rapid westward intensification on the western slope contrasts with nearly uniform distribution on the eastern slope. The intensification seems to be associated with the high occurrence of Gulf Stream warm core rings (WCR) and with the close proximity to the Gulf Stream. Crossover analyses of altimetric current anomalies indicate that the total current variability can be estimated by a factor of 1.5 from the cross‐track current variability and the variance is isotropic to within 14%. The rotational speed of the Gulf Stream rings can reach 1–2 m/s over the Scotian Slope, with the root‐mean‐square variability of ∼60 cm/s. The relative vorticity of the interior core is estimated to be ∼1.5 ×10−5 1/s. When combined with CTD data, T/P observations can produce the vertical profile of the total geostrophic current that is in approximate agreement with ADCP measurements.

Gulf Stream cross-frontal exchange: possible mechanisms to explain interannual variations in phytoplankton chlorophyll in the Slope Sea during the SeaWiFS years

During the first 4 years of the SeaWiFS mission (September 1997 through August 2001), the spring blooms in the Slope Sea increased in magnitude. During the same time, the mean path of the Gulf Stream shifted northward. The northward trend of the Gulf Stream is evidenced in satellite sea-surface temperature imagery as well as in situ temperature, salinity, and current vector information collected by the merchant vessel Oleander on its weekly trip between New Jersey and Bermuda. Surface temperature and salinity increased in the Slope Sea over the 4 years. It surprised us to find a collective increase in phytoplankton chlorophyll, temperature, and salinity, contrary to the commonly observed inverse relationship between temperature and chlorophyll in surface waters of this region. While the Gulf Stream surface waters are depleted of nutrients and low in biomass content, waters at depth are rich in nutrients. Although the Gulf Stream usually serves as a barrier between Sargasso waters to the south and Slope waters to the north, cross-stream exchange occurs when there is upward flow along isopycnals toward the surface waters of the Slope Sea. Under certain conditions, warm-core rings and associated streamers shed by the Gulf Stream may also bring nutrient-rich water up into the euphotic zone. The outflow of surface water from the Labrador Sea appears to influence both the size and horizontal transport of the Slope Sea. When the Slope waters are warm and saline there is less Labrador water present, resulting in less dilution of the Gulf Stream waters leaking into the Slope Sea and less horizontal advection within its cyclonic gyre. While the intensity of the spring blooms during this period has dramatic interannual variability, we found the total surface chlorophyll concentration integrated over the Slope Sea remains nearly unchanged. This suggests that the Labrador is not the major supplier of nutrients, but rather that the Slope Sea receives a steady nutrient supply from the sub-surface Gulf Stream waters.

Distribution patterns of larval myctophid fishes in the transition region of the western North Pacific

We examined the species composition and horizontal distribution of myctophid fish larvae in the transition region of the western North Pacific during the early summer. In total, 4,760 myctophid larvae were collected at 44 stations; 18 species of myctophids from 15 genera were collected, and the 8 most abundant species accounted for >95% of larvae. The distribution patterns of these larvae were well defined by the hydrographic structures of the study area including the Oyashio and Kuroshio fronts, the Subarctic Boundary, and a warm core ring. The horizontal distribution patterns of the eight dominant species were categorized into three groups: northern transition water (Stenobrachius nannochir, Tarletonbeania taylori, and Lampanyctus jordani), southern transition water (Symbolophorus californiensis, Diaphus theta, and Nannobrachium regale), and Kuroshio (Myctophum asperum and Diaphus garmani). The Subarctic Boundary defined the distributions of the northern and southern transition-water groups. The importance of areas of western North Pacific transition water as spawning and nursery grounds for subarctic, transitional, and subtropical myctophid fishes was indicated by the relationship between the horizontal distribution patterns of larvae, juveniles, and adults and the physical oceanographic structures.

Species composition and depth distribution of chaetognaths in a Kuroshio warm-core ring and Oyashio water

Chaetognath species composition, diversity and depth distribution were investigated inside and outside a warm-core ring oV the north-east coast of Japan. Time series samples were collected at a station inside a Kuroshio warm-core ring (KWCR) and at a station in the surrounding Oyashio water. Greater mean abundance of 2.51± 0.116 (mean ± SE) chaetognaths m –3 was found outside the ring compared with 1.75± 0.321 chaetognaths m –3 collected within the ring. However, species diversity values were higher within the KWCR (Shannon–Weaver index). Eukrohnia hamata was the dominant species outside the ring, comprising 79.8–87.9% of the total chaetognaths per haul. Inside, Sagitta minima was dominant, comprising 35.1–44.3%. Most E. hamata were collected in deeper layers within the KWCR and their abundance was on average only 9% of that found in the Oyashio region. Only E. hamata and Sagitta scrippsae had diVerent depth distributions in the KWCR. The vertical distribution of E. hamata by body size appeared altered by the KWCR. Although the mean length of E. hamata was not signiWcantly diVerent between regions, sexual development appeared inhibited in the ring. Sagitta elegans collected in the KWCR were mostly small in size (<10 mm), signiWcantly smaller than in the Oyashio water.

Properties of flow and pressure over Georges Bank as observed with near‐surface drifters

During the 1995–1999 Northwest Atlantic GLOBEC project, 203 drifters drogued at nominally 10 m were deployed on or near Georges Bank. The objectives of the deployments were to characterize flow patterns and residence time properties. During all seasons, there was a tendency for clockwise flow around the bank, and this flow, along with the residence time, increased during the summer. Residence times ranged from typically 40 days during the winter to 90 days during the summer. The seasonal change in mean flow implies a role for processes beyond simple barotropic tidal rectification to be important in the dynamics. Regional subtidal frequency pressure patterns (estimated using the drifter data) dominate the sea level variability, but they play less of a role in the subtidal current variability. Instead, current variations seem to be more dominated by shorter‐scale patterns and (during winter) by flow associated with the surface Ekman layer. At 10 m depth, net current variability does not change much on a seasonal basis. A few events with particularly good drifter coverage are used to illustrate the effects of a Warm Core Ring south of the bank, and of strong winter and spring storms.

Stability of Multilayer Ocean Vortices: A Parameter Study Including Realistic Gulf Stream and Agulhas Rings

Abstract Ocean rings, when isolated from major ocean currents, can have life spans on the order of years. This study focuses on the stability of such isolated ocean rings. Assuming axisymmetric basic-state profiles, the linear stability of a wide variety of rings is analyzed by examining the properties of the modes to which they become unstable and the associated energy conversions. Earlier studies have indicated that corotating rings, with a large barotropic component, are far less unstable than counterrotating ones. This sharp contrast between co- and counterrotating rings appears to be a consequence of the choice for a radial profile of the azimuthal velocity that decays only gradually on the ring's outer flank. For more realistic velocity profiles, co- and counterrotating rings have similar growth rates. Nearly compensated rings, that is, those with a weak flow in the deepest layer, are found to be the least unstable ones. In this paper, the problem for warm-core rings with a Gaussian profile is first...

Distributions of carbohydrate species in the Gulf of Mexico

In order to study the role of polysaccharides in the cycling of marine organic matter and transparent exopolymeric particles (TEP), the concentrations of total carbohydrates (p-TCHO), total uronic acids (URA) and total acid polysaccharides (APS) in suspended and sinking particles, as well as carbohydrates in the filter-passing “dissolved” phase (d-TCHO), were measured in vertical profiles along a N–S transect in the Gulf of Mexico, across a cold core (CCR) and a warm core (WCR) ring (eddy) during both July 2000 and May 2001. The concentrations of d-TCHO in 2000 ranged from 4 to 22 μM C, with a subsurface maximum, which was located slightly above the depth of chl a maximum, amounting to, on average, 34% of DOC in the CCR, and 13% in the WCR. The concentration of particulate carbohydrates (p-TCHO) in different size fractions (0.7–10, 10–53, and >53 μm) ranged from 0.04 to 1.1, 0.005 to 0.40, and 0.006 to 0.26 μM C, respectively, indicating that carbohydrates are mostly concentrated in small particles (0.7–10 μm). URA and APS were similarly concentrated in small particles, in which, on average, URA accounted for 87% and 57% of total URA, and APS for 92% and 88% of total APS in 2000 and 2001, respectively. URA accounted for 3–9% of carbohydrates in suspended particles, suggesting that URA are a minor component of the p-TCHO pool. Due to its surface-reactive nature, URA could play a major role in the coagulation of particles and macromolecules despite its relatively low abundance. While, on average, p-TCHO and total APS were more enriched in suspended particles than in sinking particles in both 2000 and 2001, the opposite was true for URA in both years. The greater contents of URA that are present in settling particles compared to suspended particles could indicate a mass flow in the direction of sinking particles, either caused by coagulation, by bacterial reworking of particulate and colloidal organic matter, or by their more refractory nature.

Control of acid polysaccharide production and 234Th and POC export fluxes by marine organisms

Ratios of particulate organic carbon (POC) to particulate 234Th activities (POC/234Th) in the ocean are used to determine the POC export flux, and thus, the power of the biological pump. In order to understand the main reasons why this ratio frequently varies as a function of depth, size, and kind (suspended vs. sinking particles), we measured vertical profiles in a cold core ring and warm core ring in the Gulf of Mexico in May 2001. Here we show that particulate 234Th/POC ratios in the Gulf of Mexico are positively correlated to the content of different Th(IV)‐binding polysaccharide fractions (uronic acids, total acid polysaccharides, total polysaccharides) in both suspended and sinking particles as well as to prymnesiophyte abundance, but negatively correlated to bacterial production. Variations in acid polysaccharide compounds, produced by both algae and bacteria, but degraded only by bacteria, can account for observed variations in POC/234Th ratios seen in the ocean.

ADCP-Referenced Kuroshio and Oyashio Water Transports for North Pacific Intermediate Water Formation*

Abstract The ADCP-referenced geostrophic circulation of the Oyashio and Kuroshio waters in the density range 26.6–27.2 σθ in the Kuroshio–Oyashio interfrontal zone is examined by synoptic observations with a CTD and shipboard ADCP in May 1992 and May 1994 in order to investigate the absolute transports of the source waters for North Pacific Intermediate Water (NPIW) formation. The Kuroshio and Oyashio components are calculated by the ADCP-referenced transport combined with the mixing ratio assuming isopycnal mixing between pure Kuroshio and Oyashio waters. As a result, the Kuroshio transport involved in the NPIW formation is estimated to be 7–8 Sv (Sv ≡ 106 m3 s−1), which is the net component along the Kuroshio Extension (KE) from the separation point near the Cape Nojima to the east side of the first crest of the KE. The Oyashio transport is estimated to be 5–6 Sv, which consists of coastal 1–2 Sv and offshore 3–4 Sv separated by warm core rings, and which subsequently merges with the KE at the east side...

Stability‐induced modification of sea surface winds over Gulf Stream rings

Satellite‐borne scatterometer and infrared data collected over Gulf Stream warm and cold core rings are used to study the effect of the sea‐air temperature difference on the wind speed over rings. The observed acceleration of the wind over warm core rings and deceleration over cold core rings is found to be consistent with that predicted by the planetary boundary layer model of Brown and Foster [1994]. In addition it is shown that the distance over which the winds respond to an ocean surface temperature step is short (≤25km) while the distance over which the marine boundary layer responds to a surface temperature step is long (≥175km).

The South China Sea warm-core ring 94S and its influence on the distribution of chemical tracers

In this paper, the distributions of currents and chemical tracers were studied along two hydrographic sections across ring 94S, a warm-core ring found in the South China Sea. Results suggest that while currents on its offshore side maintained quasigeostrophic balance, such a balance was not reached on its onshore side. Therefore, it is suggested that interactions of ring 94S with the slope may play an important role: it may break down the quasigeostrophic balance and cause a deformation of the current field. The observed distribution of δ18O supports the hypothesis that water masses inside the ring originate from the Kuroshio. Distributions of chemical tracers reveal a strong vertical disturbance of the isolines at the edge of ring 94S, where it approaches the shelf. This phenomenon may play an important role in the vertical exchange of biochemical elements across the thermocline. Possible reasons, i.e., enhanced vertical mixing and upwelling, are discussed.

Particulate and dissolved 210Pb activities in the shelf and slope regions of the Gulf of Mexico waters

Concentration profiles of dissolved and particulate 210Pb were measured on horizontal transects of the continental shelf and slope regions of the Gulf of Mexico. Vertical profiles in the slope station in the presence and absence of warm-core ring enable us to evaluate the differences in the scavenging intensity of 210Pb. A comparison of the rate of production of 210Pb in the overlying water column to that of its mean annual flux calculated from the sediment inventory allows us to get a better insight in to the lateral transport and deposition of 210Pb to the slope sediments over a <100 year time scale. Our results indicate that there is no general increase of dissolved or particulate 210Pb concentration with depth. The scavenging residence time of 210Pb for the entire 1500-m water column varied between 0.06 and 4.7 years. There was a strong correlation between the inventory of total (dissolved+particulate) 210Pb and depth of the water column. The ratio of measured sediment inventory (F) of 210Pb to its production (P) in the overlying water column varied between 0.69 and 1.63, suggesting that, at time, lateral inputs from boundary scavenging in a few sites are dominant, while in other places, vertical scavenging is the primary mechanism of transport for 210Pb.

Ecological–physical modelling for warm core ring 93A of Kuroshio extension focused on a blooming event inside WCR

An ecological–physical coupled model was applied to simulate warm core ring (WCR) 93A off the Sanriku area of Japan for ADEOS campaign period, April–June 1997. Southeastward movement of WCR as observed by satellite imaging was reproduced. The increase in phytoplankton concentration with time and its subsequent decay inside the WCR, accompanied by Kuroshio water intrusion, were also well simulated by the model. These processes can be explained by the horizontal advection of surrounding high chlorophyll water into the WCR grazing of phytoplankton by zooplankton.

Mechanism of Nutrient Supply to Warm-Core Ring off Sanriku, Japan

The mechanism by which nutrient is supplied to a warm-core ring (WCR) was investigated in order to understand the greater productivity of WCR than that of the Kuroshio, where the WCR originattes. A single WCR was observed in January and May, 1997. The thermostad (a layer of isothermal and isohaline water) of the WCR had different properties from January to May, the differences: Δwater temperature: −0.698°C, Δsalinity: −0.048, Δsigma θ: +0.072, Δnitrite+nitrate-N: +1.83 μM, Δphosphate: +0.011 μM and Δsilicate: +3.2 μM. We examined three possible mechanisms for nutrient supply to WCR in winter, namely: 1) inflow of the Oyashio surface water into WCR; 2) isopycnal mixing with Oyashio water; 3) entrainment of the water below the WCR into the WCR. The results were as follows: 1) When the decrease of salinity was due to the inflow of the Oyashio surface water, the increase of nutrients (nitrite+nitrate-N, phosphate-P and silicate-Si) was estimated to be only 17–27% of the observed increase. 2) When the decrease of salinity was due to isopycnal mixing, the increase of nutrients was estimated to be 30–42% of the observed increase. 3) When the decrease of salinity in the WCR in May was due to entrainment of the water below the WCR in winter by convection, the mixing depth was calculated be 620 m according to the salt budget. The increase of nutrients in this case was calculated to be 82–95% of the observed increase. The main mechanism of nutrient supply to WCR was concluded to be due to the entrainment of the water below the WCR by winter mixing.

Present state and future about application of satellite remote sensing for fisheries around Japan

Satellite images delineate several meso-scale features, such as warm-core ring, warm and cold streamer. Warm-core ring is very important factor for finding fishing ground. Warm streamer is selectively utilized by fish schools as their migration routes from the Kuroshio Extension to the warm-core rings in the north and from the warm-core rings to the coastal waters. These migration routes were investigated using research vessels, satellite images, aerial observations, and commercial catch data. Fish schools were distributed in the warm streamer, especially at their heads, where phytoplankton and zooplankton were most heavily concentrated. Sometimes these warm streamers grazed coastal area with pelagic fish schools and supplied many fishes into the set-net. Tip of warm streamer always have some slick-phenomena, which were found in the coastal area from the land. These slick-phenomena become to put mark on approach of warm streamer with many fish schools. These observing methods had better be utilized at the same time for research of the warm streamer in detail. It is very important for total observing system, which was spatial and simultaneous system, to make clear about fisheries oceanographic phenomena.

Behavior of Warm-Core Rings in a Double-Gyre Wind-Driven Ocean Circulation Model

Numerical experiments have been performed to understand the movement of warm-core rings in the Kuroshio-Oyashio Transition region, considering the double-gyre ocean circulation represented by a 2.5-layer primitive equation model. Rings, eddies, and frontal disturbances are simulated, and analytical descriptions of warm-core ring behavior are examined. Warm-core rings are usually generated at the south of the zero wind stress curl line (ZWCL), either off-shore or near-shore, and a near-shore ring gradually moves northward along the coast, supplied with energy from the sub-tropical front and other rings. When they approach the ZWCL, two different types of movements occur, which is affected by the intensity of the rings and controlled by the wind stresses. In case the ring intensity is strong, it moves northward along the coast against the subarctic western boundary current system and cold-core rings, whereas a moderate ring is advected by them to the northeastward. The advection process of moderate rings seems to be important for northeastward movement of the Kuroshio warm-core rings in the middle of the Oyashio current system. The mechanism of large-scale forcing dynamics influences the cross-shore location of rings and implications for the fishing ground formation of the Pacific saury are described.

Evidence for the trapping and amplification of near-inertial motions in a large anticyclonic ring in the Oyashio

Three ARGOS drift buoys were deployed in the Oyashio Current off the Kuril Islands near 45°N in fall, 1990, during a joint Russia/Canada study of western boundary current dynamics in the Subarctic Pacific Ocean. We here report on one buoy deployed within an anticyclonic warm core ring (WCR86B) which shows evidence of large amplitude inertial motions of near-diurnal frequency. During its first week within the ring the buoy drifted with a mean azimuthal current speed of 0.40–0.45 m s−1 and a radius of rotation of 15–20 km. However, superimposed on the mean rotation of the ring at this time were “loops” of near-diurnal period, radius 7–8 km and speeds exceeding 1 m s−1. During successive rotations the buoy spiraled outward, its mean period of rotation increased and the amplitude of the near-diurnal motions decreased. The large motions are explained by inertial wave trapping and amplification within the extremely large and weakly stratified eddy, wherein the negative vorticity of the eddy reduces the local inertial frequency to near-diurnal frequency. We here suggest that either tidal or wind forcing may generate these high-amplitude “loop” motions.

Skewed Water Temperature Occurrence Frequency in the Sea off Sanriku, Japan, and Intrusion of the Pure Kuroshio Water

Temperature and salinity data obtained by the Iwate Fisheries Technology Center were analyzed for the period 1971–1995. It was found that occurrence frequency distributions of temperature and salinity are very skewed at depths deeper than 200 m and that temperature sometimes exceeds m + 5σ (m: mean and σ: standard deviation. If such abnormally high temperatures are real the 3σ criterion cannot be adopted. Oceanic conditions were surveyed in 1972 and 1979, when temperatures exceeding m + 5σ were observed. It was found that the abnormally high temperature (and high salinity) water was the pure Kuroshio Water introduced into the region due to some special conditions such as abnormal approach of large warm-core ring to the Sanriku Coast or abnormal northward extension of the Kuroshio along the coast. These events are very rare, occurring only twice in the 25-year period analyzed, but the abnormally high temperature observed is real.