Low-frequency variability of currents and temperatures on the Pacific continental shelf off northern Baja California, 1978 to 1979

Currents, coastal winds, coastal sea level, and coastal ocean temperatures were observed at a number of northern Baja California Pacific sites between October 1978 and October 1979. Coastal winds were weakly southward on average and fluctuated north-south throughout the year. Fluctuations were uncorrelated over alongshore separations of 200 km. Coastal winds differed considerably from the large-scale offshore wind estimate (Bakun's Index) both in strength and in variability. At 30°50′N the mean currents were weakly equatorward at 25 m depth and weakly poleward at 42 and 60 m in a water depth of 75 m. The seasonal mean flow was equatorward from October to April and poleward from April to October at the shallower level but poleward all year near bottom. The fluctuations on a time scale of days to weeks were about an order of magnitude stronger than the seasonal variation, were oriented predominantly alongshore, and were quasi-barotropic in nature. Empirical orthogonal analyses showed that almost 90% of the variability could be represented by a single empirical mode. The alongshore fluctuations were significantly correlated with coastal sea level from October to July but during the rest of the year correlation was insignificant, possibly as a result of offshore eddies approaching the shelf.Observations at 25- and 60-m depth at a second mooring 100 km south of the first indicated a mean alongshore divergence from April to July 1979, consistent with long-term mean geostrophic flow patterns for the area. In general, currents were correlated alongshore but a two-week event in May, which manifested itself differently at the two sites, was suggestive of the impingement of an eddy onto the continental shelf. Currents and winds were poorly correlated in general.Temperature variation at sites along 700 km of coast showed a strong seasonal variation with the winter cool period extended by spring upwelling. Days-to-weeks scale fluctuations were similar at the five northernmost sites and correlations alongshore were significant for separations of up to 400 km. No evidence of propagating events was found in coastal temperatures or in currents.     

Mesoscale dynamics and walleye pollock catches in the Navarin Canyon area of the Bering Sea

Large canyons incise the shelf break of the eastern Bering Sea to be preferred sites of the cross-shelf exchange. The mesoscale eddy activity is particularly strong near the shelf-break canyons. To study the mesoscale dynamics in the Navarin Canyon area of the Bering Sea, the time series of velocities derived from AVISO satellite altimetry between 1993 and 2015, drifters, Argo buoys, and ship-borne data are analyzed. We demonstrate that the strength of anticyclonic eddies along the shelf edge in spring and summer is determined by the wind stress in March–April. The increased southward wind stress in the central Bering Sea forced a supply of low-temperature and low-salinity outer shelf water to the deep basin and formation of the anticyclonic mesoscale circulation seaward of the Navarin Canyon. Enhanced northwestward advection of the Bering Slope Current water leads to increase in an ice-free area in March and April and increased bottom-layer temperature at the outer shelf. The strong (weak) northwestward advection of the eastern Bering Sea waters, determined by eastern winds in spring, creates favorable (unfavorable) conditions for the pollock abundance in the western Navarin Canyon area in summer.     

The mesoscale eddies and Kuroshio transport in the western North Pacific east of Taiwan from 8-year (2003–2010) model reanalysis

The relationship between the Kuroshio volume transport east of Taiwan (~24°N) and the impinging mesoscale eddies is investigated using 8-year reanalysis of a primitive equation ocean model that assimilates satellite altimetry and SST data. The mean and fluctuations of the model Kuroshio transport agree well with the available observations. Analysis of model dynamic heights and velocity fields reveals three dominant eddy modes. The first mode describes a large eddy of ~500 km in diameter, centered at ~22° N. The second mode describes a pair of the north–south counter-rotating eddies of?~?400 km in diameter each, centered at 23° and 20° N, respectively. The third mode describes a pair of the east–west counter-rotating eddies of?~?300 km in diameter each, centered at 21° N. The associated velocity fields indicate eddies extending to 600–700 m in depth with vertical shears concentrated in the upper 400 m. All three modes and the model Kuroshio transport have similar dominant timescales of 70–150 days and generally are coherent. The decreased Kuroshio volume transports typically are associated with the impinging cyclonic eddies and the increased transports with the anticyclonic eddies. Selected drifter trajectories are presented to illustrate the three eddy modes and their correspondence with the varying Kuroshio transports.     

A theoretical model for the intrusion of the Kuroshio across the continental shelf of East China Sea

Here we use a two-layer model to study the dynamics of the intrusion of the Kuroshio onto the continental shelf. Results show that the interaction of the Kuroshio water and shelf water produces a stable upwelling zone above 100 m depth northeast of Taiwan, which provides a dynamical explanation to the presence of the cold core previously observed by satellite. The affected shelf water from the interaction has an onshore portion, which turns right and becomes a northward alongshore flow when it moves closer to shore. This implies that the Kuroshio water cannot penetrate deep onto the inner part of the continental shelf, but it generates a strong northward jet that is formed mainly by the shelf water.     

Physicochemical conditions in coastal waters off Syndney, central NSW, Australia

The continental shelf off Sydney is narrow and characterized by extensive areas of rocky reef and sandy sediment. The overlying coastal waters are dynamic with a complex current structure. Important oceanographic processes include East Australian Current (EAC) activity, northward propagating coastal trapped waves, local wind driven currents and relatively high frequency internal tides and waves. These produce influences on a wide range of temporal and spatial scales. The activity of the EAC and its eddies has been associated with episodic incursions of waters which can quickly replace large parts of the shelf waters off Sydney. Thermal stratification and the episodic presence of cold, nutrient rich waters intruded from the continental slope are important features of the water column. Thermal stratification of up to 6°C generally exists for all but a few months of the year. Nutrient concentrations are generally low in surface waters but are higher and more variable at depth because of irregular intrusions of slope waters from depths greater than 150–200 m. The trace element levels in surface seawater entering the Sydney area are expected to be extremely low.     

Three-dimensional structure of mesoscale eddies in the western tropical Pacific as revealed by a high-resolution ocean simulation

The three-dimensional structure of mesoscale eddies in the western tropical Pacific(6°S–20°N, 120°E–150°E)is investigated using a high-resolution ocean model simulation. Eddy detection and eddy tracking algorithms are applied to simulated horizontal velocity vectors, and the anticyclonic and cyclonic eddies identified are composited to obtain their three-dimensional structures. The mean lifetime of all long-lived eddies is about 52 days, and their mean diameter is 147 km. Two typical characteristics of mesoscale eddies are revealed and possible dynamic explanations are analyzed. One typical characteristic is that surface eddies are generally separated from subthermocline eddies along the bifurcation latitude(~13°N) of the North Equatorial Current in the western tropical Pacific, which may be associated with different eddy energy sources and vertical eddy energy fluxes in subtropical and tropical gyres. Surface eddies have maximum swirl velocities of 8–9 cm s~(-1) and can extend to about 1500 m depth. Subthermocline eddies occur below 200 m, with their cores at about 400–600 m depth, and their maximum swirl velocities can reach 10 cm s~(-1). The other typical characteristic is that the meridional velocity component of the eddy is much larger than the zonal component. This characteristic might be due to more zonal eddy pairs(two eddies at the same latitude),which is also supported by the zonal wavelength(about 200 km) in the high-frequency meridional velocity component of the horizontal velocity.     

Detailed structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea

The detailed three-dimensional structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea is revealed by the CTD, ADCP, and satellite-tracked drifter observations. The length and width of the cold core of the Kuroshio frontal eddy are about 60 and 40 km, respectively, and its phase speed is about 30 cm s^-1. The calculated buoy tracks with the use of the observed ADCP data well reproduce the observed tracks of satellite-tracked drifters around the frontal eddy. The observed maximum horizontal velocity around this frontal eddy are 40 cm s^-1 and the center of this eddy shifts offshore in the deep layer. Nutrient is advected onshore across the shelf edge by passing of this frontal eddy while it is advected offshore without the frontal eddy at the shelf edge.     

Impacts of the Kuroshio intrusion on the two eddies in the northern South China Sea in late spring 2016

During mid-May to early June 2016, a cold eddy and a warm eddy were captured on the continental slope of the northern South China Sea by the in situ measurements. A salty lens-shaped water mass in the subsurface layer existed in these two detected eddies, which indicated they had a Kuroshio water origin. The trajectories of the observed eddies from satellite altimeter data show that the cold eddy was generated in the central part of the Luzon Strait, while the warm eddy was formed southwest of Taiwan. The genesis of the cold eddy is related to a weak Kuroshio loop current, while that of the warm eddy is associated with a strong Kuroshio loop current. The warm eddy east of the Luzon Strait may trigger the Kuroshio from a leaping path to a looping path. During the evolution of these detected eddies, they had interactions with the Kuroshio and Luzon Gyre. Energy analysis from ocean reanalysis data showed that the baroclinic conversion between the cold eddy and the Kuroshio was stronger than that between the cold eddy and Luzon Gyre. During the eddy shedding stage, the warm eddy mainly acquired energy from the Kuroshio loop current through the baroclinic conversion.     

Biweekly periodic variation of the Kuroshio axis northeast of Taiwan as revealed by ocean high-frequency radar

An analysis of surface current data obtained from 2002 to 2005 using long-range high-frequency radar provides the first evidence for the presence of biweekly (11–14 day) periodic variations of the Kuroshio axis northeast of Taiwan. This analysis clarifies the spatiotemporal characteristics of these variations and reveals that cyclonic/anticyclonic eddies propagating along the shelf slope from the vicinity of the deep channel east of Taiwan induce these variations northeast of Taiwan. The behavior of the cyclonic/anticyclonic eddies on the shelf slope is well explained by 2nd-mode interior shelf waves advected by the Kuroshio's mean flow. Remote effects from the vicinity of the deep channel east of Taiwan, or from outside the East China Sea, are believed to play an important role in the generation of these biweekly periodic variations of the Kuroshio axis northeast of Taiwan. Moreover, on the shelf slope, these variations cause an onshore current across the shelf slope, suggesting topographically controlled upwelling. Therefore, the biweekly periodic variations of the Kuroshio axis northeast of Taiwan might contribute not only to the onshore transport of Kuroshio surface water but also to transport nutrient-rich Kuroshio subsurface water onto the shelf in the East China Sea.     

中美洲鹦鹉湾气旋涡的亚中尺度结构特征

海洋中普遍存在的涡旋对全球海洋热、盐通量有重要贡献.一条于2000年6月在中美洲鹦鹉湾采集的地震剖面L115捕获到了一个海水次表层透镜状反射结构.透镜状反射的宽度约150 km,厚度约500 m,深度从100 m延伸到约600 m,核心深约200 m.结合和地震采集时间近同步的再分析数据中的流速和海水温度数据以及计算的Rossby数和Okubo-Weiss参数,将其解释为气旋涡,且可能是亚中尺度相干涡(SCV).由于地震剖面并未穿过涡旋核心水,涡旋顶底与核心水外层水团的双扩散作用使得剖面上的气旋涡中心表现为上下相邻的强反射双核结构.同航次采集的与L115大致正交的其他几条地震剖面也捕捉到了这个气旋涡.这些剖面整体的反射特征一致性较好,但和L115差异较大,涡旋内部普遍表现为近水平的中等强度反射.涡旋上边界为倾斜的强反射,这是气旋涡的等温线上凸引起的,这里也是亚中尺度锋面的发育区.这些地震剖面的涡旋反射特征的差异表明了该气旋涡空间结构的不对称性和时间演化特征.     

Wind-forced continental shelf waves from a geographical origin

The response of a barotropic coastal ocean on a step-shaped continental shelf to a traveling sinusoidal wind stress forcing is predicted theoretically using a frictional force proportional to the alongshore current velocity. This theory is compared to a small set of observations from the northeast coast of Australia where a sudden widening of the continental shelf provides a geographical origin. The comparison is accomplished by means of frequency response functions relating alongshore wind stress with alongshore velocity. Amplitudes of the response functions are predicted to increase with alongshore distance equatorward and also to decrease with frequency at any location. These predictions are verified by the measurements. Predicted phase lags are generally less than about 30°, with observations agreeing with theory to within about 20°C. In general, the measurements provide reasonable evidence to support the theory of wind-forced continental shelf waves from a geographical origin.     

Dynamical structure and wind-driven upwelling in a summertime anticyclonic eddy within Funka Bay,Hokkaido, Japan

We conducted hydrographic observations in 2002 to investigate the anticyclonic eddy that emerges every summer in Funka Bay, Hokkaido, Japan, and elucidate dynamical structure and wind-driven upwelling within the eddy. The anticyclonic eddy has a vertical scale of 32 m and is characterized by a strong baroclinic flow and a sharp pycnocline with a concave isopycnal structure. The sharp pycnocline occurs below a warm and relatively low-salinity water termed summer Funka Bay water (FS), which is formed by heating from solar radiation and dilution from river discharge in summertime Funka Bay. Flow of the anticyclonic eddy rotates as a rigid body at each layer, and the horizontal scale and rotation period of the eddy in the surface layer are about 15 km and 2.2 days, respectively. The dynamical balance of the anticyclonic eddy is well explained by the gradient flow balance. The contribution of centrifugal force to the gradient flow balance is about 27%. Therefore, the effect of the nonlinear term associated with centrifugal force cannot be neglected in considering the dynamics of the anticyclonic eddy in summertime Funka Bay. In addition, upwelling of subsurface water was observed in the surface layer of the central part of the eddy. The formation mechanism of this upwelling is consistent with interaction between horizontal uniform wind and the eddy. This upwelling is driven by upward Ekman pumping velocity related to the horizontal divergence of Ekman transport. In summertime Funka Bay, there are two wind effects that affect the anticyclonic eddy: a decay effect of the upwelling of subsurface water resulting from horizontal uniform wind (mainly northwesterly wind), and a maintenance or spin-up effect of horizontal non-uniform wind (mainly southerly–southeasterly seasonal wind) with negative wind stress curl.     

Seawater temperature changes associated with the North Brazil current dynamics

This study used hydrographic data (CTD and ADCP) collected along the north Brazilian continental shelf and the adjacent oceanic area (the north Brazilian zone) and 13 years of monthly data of sea surface temperatures (SSTs) obtained from the Tropical Rainfall Measuring Mission satellite microwave images. In July and August 2001, the core of the North Brazil Current (NBC) with a velocity exceeding 1.2 m s^?1 in the upper 150 m was observed near the break of the north Brazilian continental shelf. The satellite and in situ SSTs from the same time indicate that relatively cold waters (<27.5 °C) extended throughout the NBC region where the vertical distribution of temperature displayed a rise of the 26 °C isotherm to near the surface on transects where the NBC was more intense. These observations indicated that the NBC plays an important role in temperature changes in the water column near and/or over the north Brazilian continental shelf margin even at times when, theoretically, the NBC may be expected to be less intense (April–May) and warm waters dominate the area. The distribution of accumulated temperature change reveals that the NBC may contribute to possible nutrient upwelling of the bottom boundary layer along the continental shelf to surficial areas and offshore following the NBC deflection or merely into the North Atlantic.     

Modelling the potential transport of tropical fish larvae in the Leeuwin Current

Application of an individual-based particle tracking model to the migration of tropical fish larvae along the continental shelf between the Houtman Abrolhos Islands and Rottnest Island (Western Australia) has shown that there is potential for the southwards advection of passive particles/larvae in the Leeuwin Current system throughout the year. However, seasonal variations in the prevailing wind field result in corresponding seasonal changes in the surface current flow (both alongshore and cross-shelf) on the continental shelf, leading to a pulse of modelled particles arriving at Rottnest Island during the autumn months. This matches, within a month, the observed April/May peak in annual recruitment of 2 species of damselfish (Abudefduf sexfasciatus and A. vaigiensis), at the time when the Leeuwin Current is strengthening. It is assumed that the larvae are in the uppermost 20 m of the water column and that there are no vertical diurnal movements.The model has a 10 km grid spacing, and so can resolve some of the current gradients across the continental shelf. Comparison of the modelled near-surface currents with ADCP measurements at 2 current mooring sites as well as with a broader range of historical current measurements off south-western Australia indicates that the alongshore net transport is reasonably well reproduced by the model, whereas agreement with the cross-shelf flow is not as good (this may be partly attributed to the paucity of high-quality near-surface current measurements in the area of study). Because of limitations in our knowledge of the swimming ability, choice of swimming direction and habitat selection of larval fish, as well as the inability of the model to reproduce the small-scale circulation around Rottnest Island, the swimming capacity of the late-stage larval fish is not specifically included; they are considered as potential settlers once they have reached within 20 km of the Island.The observed inter-annual variability in recruitment is not as well matched by the model as is the seasonal pattern, but this is almost certainly due to uncertainties in biological factors such as spawning strength, food and predation en route, which are not known.The modelled results are also applied to a more general discussion of the transport of eggs and larvae of commercial fish and invertebrate species on the Western Australian continental shelf, and it is shown that the seasonality and position on the shelf of the spawning may play a large role in the movement (and hence survival and ultimate recruitment) of different species.     

Disequilibria betweenRa, Pb and Po in the Arctic Ocean and the implications for chemical modification of the Pacific water inflow

Measurements have been made of²²⁶Ra and both dissolved and particulate forms of²¹⁰Pb and²¹⁰Po in a vertical profile at 85°50′N, 108°50′W in the Arctic Ocean.In the upper water column²²⁶Ra shows a concentration maximum that is coincident with one in the nutrients, silicate, phosphate, and nitrate, while at the same depth, dissolved and particulate²¹⁰Pb and²¹⁰Po all show minimum concentrations. It is suggested that the concentration maxima are partly due to sources of the respective elements in the continental shelf sediments, the shelf waters being subsequently advected into the Arctic Ocean basins. The²¹⁰Pb and²¹⁰Po minima have similarly been explained by interaction between the shelf sediments and overlying waters. An estimate is made of the possible contributions of shelf sediments to the layer of silica-rich water which covers the Canada Basin at a depth of 100–150 m.Residence times have been calculated for dissolved²¹⁰Pb and²¹⁰Po at various depths in the water column. Surface water residence times of dissolved and particulate forms of these radionuclides are longer than in surface Atlantic waters, probably due to lower biological activity in the surface waters of the Canada Basin. An estimatee has been made of the average sinking velocity of particulate material.     

Mesoscale eddies and baroclinic instability over the eastern Sakhalin shelf of the Sea of Okhotsk: a model-based analysis

Based on an eddy-permitting numerical model, the mesoscale variability in the East-Sakhalin Current is investigated during the winter-spring period. Analysis of necessary conditions for the development of baroclinic instability showed that the nearshore component of the East-Sakhalin Current is potentially baroclinic unstable in the first half-year. The simulated circulation uncovered a generation of anticyclonic eddies on the eastern Sakhalin shelf. It was established that a spatial scale of these eddies and the first baroclinic Rossby radius of deformation are values of the same order; a lifetime of these eddies varies from 4 to 6 weeks, given the Rossby number varies from 0.05 to 0.2. Analysis of the rate of eddy energy conversion on the eastern Sakhalin shelf showed that the generation of the revealed mesoscale eddies results from, mainly, baroclinic instability, whereas barotropic instability can be both favoring and preventing to the generation of these eddies.     

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.     

Eddy resolving modelling of the Gulf of Lions and Catalan Sea

The generation process of strong long-lived eddies flowing southwestwards along the Catalan slope was revealed through numerical modelling and in situ observations. Careful analyses of a particular event in autumn 2007 demonstrated a link between a “LATEX” eddy, which remained in the southwestern corner of the Gulf of Lions and a “CATALAN” eddy, which moved along the Catalan Shelf, since the death of the former gave birth to the latter. The origin of such eddies was found to be an accumulation of potential energy in the southwestern corner of the Gulf of Lions: under the influence of the negative wind stress curl associated with the Tramontane, a warm and less dense water body can be isolated and fed by a coastal current carrying warm water from the Catalan Sea. In summer, this structure can grow and intensify to generate a strong anticyclonic eddy. After a long period of Tramontane, a burst of southeasterlies and northerlies appeared to detach the “LATEX” eddy, which flowed out of the Gulf of Lions, migrating along the Catalan continental slope and continued into the Balearic Sea as the “CATALAN” eddy.     

Physical processes around a cuspate foreland:: implications to the evolution and long-term maintenance of a cape-associated shoal

Understanding across-margin transport has long been recognized as crucial for wise management of our coastline and shelf waters. Issues related to sewage outfalls, nutrient and pollutant dispersal, carbon export, and shoreline sediment budgets all require an understanding of these processes. Across-margin transport of water and sediment at cuspate foreland headlands has been largely unrecognized, and the processes responsible for this export unappreciated. We examined physical process on Cape Lookout Shoal, a cape-associated shoal on the North Carolina continental shelf, through numerical modeling and field observations of near-bottom currents. The cuspate foreland setting of the northern South Atlantic Bight has been previously characterized as wave-dominated with a principal alongshore directed sediment transport and physical circulation forced by wave and wind-driven currents along the inner and mid-shelf. Our findings instead suggest that a seaward-directed, tidal-driven headland flow many play a significant role in the direction of net sediment transport on the shoal and ultimately its location and long-term maintenance. The shoal's location relative to the promontory-induced residual eddies and the region of active deposition differs from traditionally held ideas on sedimentary processes at headland-related sand banks. In addition, the headland flows may also serve as a first-order mechanism for rapidly exporting nearshore and estuarine waters to the outer-shelf.     

A continental shelf scale examination of the Leeuwin Current off Western Australia during the austral autumn-winter

A continental shelf scale survey from 22°S to 34°S along the Western Australia coast provides the first detailed synoptic examination of the structure, circulation and modification of the southward flowing Leeuwin Current (LC) during the late austral autumn-early winter (May-June 2007). At lower latitudes (22°S-25°S), the LC was masked within a broad expanse of warm ambient surface water, which extended across the shelf and offshore before becoming constrained at the shelf break and attaining its maximum velocity of ∼1.0 m s⁻¹ at 28°S. The temperature and salinity signature of the LC experienced substantial modification as it flowed poleward; surface temperature of the LC decreased by ∼5.25 °C while surface salinity increased by ∼0.72, consistent with climatology estimates and smaller (larger) for temperature (salinity) than those found during summer. Subsequently, LC water was denser by ∼2σ_T in the south compared to the north, and the surface mixed layer of the LC revealed only a small deepening trend along its poleward trajectory. Modification of the LC resulted from a combination of mixing due to geostrophic inflow and entrainment of cooler, more saline surrounding subtropical waters, and convective mixing driven by large heat loss to the atmosphere. Air-sea heat fluxes accounted for 50% of the heat lost from the LC in the south, whilst only accounting for 25% in the north, where large geostrophic inflow occurred and the LC displayed its maximum flow. The onshore transport was characterised by distinct jet-like structures, enhanced in the upper 200 m of the water column, and the presence of eddies in the vicinity of the shelf break generated offshore transport.