A Large-Scale Seasonal Modeling Study of the California Current System

A high-resolution, multi-level, primitive equation ocean model has been used to investigate the combined role of seasonal wind forcing, seasonal thermohaline gradients, and coastline irregularities on the formation of currents, meanders, eddies, and filaments in the entire California Current System (CCS) region, from Baja to the Washington-Canada border. Additional objectives are to further characterize the meandering jet south of Cape Blanco and the seasonal variability off Baja. Model results show the following: All of the major currents of the CCS (i.e., the California Current, the California Undercurrent, the Davidson Current, the Southern California Countercurrent, and the Southern California Eddy) as well as filaments, meanders and eddies are generated. The results are consistent with the generation of eddies from instabilities of the southward current and northward undercurrent via barotropic and baroclinic instability processes. The meandering southward jet, which divides coastally-influenced water from water of offshore origin, is a continuous feature in the CCS, and covers an alongshore distance of over 2000 km from south of Cape Blanco to Baja. Off Baja, the southward jet strengthens (weakens) during spring and summer (fall and winter). The area off southern Baja is a highly dynamic environment for meanders, filaments, and eddies, while the region off Point Eugenia, which represents the largest coastline perturbation along the Baja peninsula, is shown to be a persistent cyclonic eddy generation region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mean flow and variability in the southwestern East Sea

The Ulleung Basin is one of three deep basins that are contained within the East/Japan Sea. Current meter moorings have been maintained in this basin beginning in 1996. The data from these moorings are used to investigate the mean circulation pattern, variability of deep flows, and volume transports of major water masses in the Ulleung Basin with supporting hydrographic data and help from a high-resolution numerical model. The bottom water within the Ulleung Basin, which must enter through a constricted passage from the north, is found to circulate cyclonically—a pattern that seems prevalent throughout the East Sea. A strong current of about 6 cms⁻¹ on average flows southward over the continental slope off the Korean coast underlying the northward East Korean Warm Current as part of the mean abyssal cyclonic circulation. Volume transports of the northward East Korean Warm Current, and southward flowing East Sea Intermediate Water and East Sea Proper Water are estimated to be 1.4 Sv (1 Sv=10⁻⁶ m³ s⁻¹), 0.8 Sv, and 3.0–4.0 Sv, respectively. Deep flow variability involves a wide range of time scales with no apparent seasonal variations, whereas the deep currents in the northern East Sea are known to be strongly seasonal.     

加利福尼亚流系流场的季节性演变

本文利用三维数值模型(ROMS-Co Si NE)分析了整个加利福尼亚流系水平流场的季节性演变过程,研究了美国加州中部海域流场垂直结构的季节性变化特征,并探讨了其动力学机制。研究发现:(1)数值模型能够较为准确的模拟流场的季节性变化,与浮标观测数据以及前人的研究结果符合良好;(2)从表层到200m,加利福尼亚潜流向高纬度扩张,近岸上升流急流则向高纬度撤退,加州南部海域的中尺度涡更显著;(3)在加州中部海域,近岸急流的最大值(约15cm/s)发生在夏季,位于近岸的表层海域;加利福尼亚潜流最大值(约4cm/s)发生于冬季,出现在离岸100km的125m处;加利福尼亚流在春季达到全年最大值(约5cm/s),流轴位于离岸(400—600km)的表层海水。加利福尼亚流系的流场具有显著的季节性变化,研究进一步表明这主要受地转关系调控。     

Seasonal Variability of the California Undercurrent: Statistical Analysis Based on the Trajectories of Floats with Neutral Buoyancy

The seasonal variation of the kinetic energy of the California Undercurrent and the eddy field in the region of this current are reconstructed by analyzing the trajectories of floats with neutral buoyancy (RAFOS) launched by the researchers of the Naval Postgraduate School (Monterey, California, USA) for 10 yr (since 1992). These data are processed by using a mathematical approach based on the combination of a procedure of averaging over cells and a special filtration approach with an aim to improve the statistical reliability of numerical calculations. The analysis of the accumulated data shows that the kinetic energy of the California Undercurrent has a well pronounced seasonal variation with two extrema in late spring and early autumn. The intensification of the California Undercurrent in autumn is accompanied by a decrease in its extension in the direction transverse to the coast. The eddy field of the California Undercurrent is especially intense in autumn. At depths of 300–600 m, the annual average kinetic energy of the eddy field increases with the distance from the coast up to about 127°W. Moreover, in this case, the zonal anisotropy of the eddy field increases.     

Summertime hydrographic features in the southeastern Hwanghae

CTD casts in the southeastern Hwanghae (Yellow Sea) were made in August 1983 and 1984 to describe the spatial structure of the summertime hydrographic features. Cold coastal water appeared around the southwestern coast of Korea, which was formed by strong tidal stirring. Tidal mixing in the study area seems to have been enhanced by the presence of many small islands. In the deeper region beyond the tidal front, stratification became much stronger and the bottom layer below seasonal thermocline was occupied mostly by the Hwanghae Cold Water characterized by a temperature lower than 10°C and salinity of 32.5–33.0%.The northeastward extension of the Changjiang Diluted Water was shown by a tongue-like plume of relatively warm fresh water, confined to the thin surface layer 10 m thick. There was no evidence for the Hwanghae Warm Current carrying high salinity water into the eastern Hwanghae along the Korean coast. The warm current was found to flow in a narrow band close to the west and north coast of Chejudo (Cheju Island) and then to pass eastward through the Cheju Strait. Thus the eastern part of the cyclonic circulation in the surface layer cannot be considered to be a northward continuation of the Hwanghae Warm Current. The local salinity maximum in the lower layer off Kunsan and the higher salinity on the west side of the central trough than on the east side would imply a northward flow on the west flank of the trough to compensate for the southward intrusion of the Hwanghae Cold Water, from which an anticyclonic circulation could be expected in the lower layer.     

The vertical structure and variability of the western boundary currents east of the Philippines: case study from in situ observations from December 2010 to August 2014

In this work, the vertical structure and variability along the western boundary of the Philippines are investigated using direct observations from acoustic Doppler current profiler (ADCP), Doppler volume sampler (DVS) and Aanderaa Seaguard instruments, which are mounted on a subsurface mooring deployed at 8°N, 127°3′E. In climatology, the southward Mindanao Current (MC) and northward Mindanao Undercurrent (MUC) play a dominant role in the upper layer. The mean currents at 1200 and 3500 m flow northward, whereas those at 2500 and 5600 m flow equatorward. The power spectral density (PSD) shows that an intraseasonal signal of 60–80 days is common from the sea surface to the bottom. The semiannual signals are strongest in the MUC layer, and the amplitude then decreases with depth to 3500 m. The seasonal variability at 2500 and 5600 m is similar between the two depths, suggesting a southward current in winter and northward flow in autumn. The current at 3500 m exhibits a northward direction in spring and southward flow in winter. In addition, the linear correlations between mooring data and altimetry products indicate that the variations in surface meridional currents along the western boundary of the Pacific Ocean can reach the bottom via low-frequency processes. The vertical-mode decomposition for observations indicates that the first four modes can effectively capture the original data. The relative contributions of different modes exhibit seasonal variability. The first baroclinic mode plays a dominant role in spring and autumn. In winter and summer, its contribution decreases and becomes comparable to that of the other modes.     

On the total geostrophic circulation of the Indian Ocean: flow patterns, tracers, and transports

The large-scale circulation of the Indian Ocean has several major components. There is a cyclonic gyre in the far southwest with its axis along about 60°S. It extends to the bottom. North of this the Circumpolar Current flows eastward south of 40°S to more than 3000 m. The axis of the great anticyclonic gyre lies along 35°S to 40°S down to about 2000 m. Below there the western end shifts northward and the axis lies along the central and southeast Indian ridges, with southward flow west of the ridges and northward flow on the east side.There is a westward flow along 10°S to 15°S, which includes water from the Pacific, through the Banda Sea. The flow near the equator is eastward down to the depth of the ridge near 73°E. Flow within both the Arabian Sea and Bay of Bengal is cyclonic down to great depth.There is a southward flow along the coast of Africa in the upper 2000 m joining the Circumpolar Current, and a southward flow along the coast of Australia that does not reach the Circumpolar Current.Below 2500 m there is a northward flow from the Circumpolar Current along the east coast of Madagascar and on into the Somali and Arabian basins.     

The termination of the Equatorial Undercurrent in the eastern Pacific

The termination of the Equatorial Undercurrent (EUC) in the eastern tropical Pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5°N, 10°S, 80°W and 100°W. Three distinctive hydrographic features associated with the EUC are used, along with dynamic topography, to trace the mean path of the EUC and to investigate aspects of its seasonal variation. These features are (1) the 13°C thermostad, (2) the high-salinity core, and (3) the high dissolved oxygen concentration tongue.The thermostad is thickest just south of the equator to the west of the Galapagos Islands, with a decreasing maximum extending southeast to the coast of South America near 7°S. The thermostad is poorly developed in the mean near the equator east of the Galapagos Islands. This pattern appears to be produced by convergence of eastward flow in the EUC and in the Southern Subsurface Countercurrent, as the EUC flows southeastward from the Galapagos. This interpretation is supported by the geostrophic flow calculated from the mean dynamic topography. The isolated high-salinity core is found to the south of the equator and is traced all the way to the coast of South America. The high oxygen tongue associated with the EUC splits at the Galapagos, with the southward projecting branch disappearing rapidly as an identifiable feature. The other branch of the tongue continues around the islands to the north and along the equator to the coast of Ecuador. Some evidence of recirculation of EUC water in the South Equatorial Current is seen to the northwest of the Galapagos.A strong seasonal pulsation of the EUC affects these features in a consistent way, with the pulse occurring first west of the Galapagos, and later along the coast. The thermostad first reaches maximum development, and then a pronounced downwelling of the thermocline occurs rapidly, leading to minimum thermostad extent. Associated wtih this downwelling is an eastward protruding high-salinity tongue near the equator which is later seen as an isolated core along the coast at 4°S and 6°S. Pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the EUC, and ultimately the Peru-Chile Undercurrent. The pressure gradient reversal along the equator occurs prior to the annual wind reversal, suggesting that remote forcing plays a role in the seasonal pulse of the EUC in the eastern Pacific.     

南海的季节环流─TOPEX/POSEIDON卫星测高应用研究

应用1992～1996年的TOPEX/POSEIDON卫星高度计遥感资料,研究了冬、夏季风强盛期多年平均的南海上层环流结构。研究结果表明,南海上层流结构呈明显的季节变化,在很大程度上受该海区冬、夏交替的季风支配。冬季总环流呈气旋型,并发育有两个次海盆尺度气旋型环流;夏季总环流大致呈反气旋型、但在南海东部18°N以南海域未见明显流系发育。研究还表明,南海环流的西向强化趋势明显,无论冬、夏在中南半岛沿岸和巽他陆架外缘均存在急流,其流向冬、夏相反,是南海上层环流中最强劲的一支。鉴于该海流的动力特征与海洋动力学中定义的漂流不同,有相当大的地转成分,建议称为“南海季风急流(South China Sea MonsoonJet)”.冬季南下的季风急流在南海南部受巽他陆架阻挡折向东北,沿加里曼丹岛和巴拉望岛外海有较强东北向流发育。夏季北上的季风急流在海南岛东南分为两支:北支沿陆架北上,似为传统意义上的南海暖流;南支沿18°N向东横穿南海后折向东北;二者之间(陆架坡折附近)为弱流区。两分支在汕头外海汇合后,南海暖流流速增强。就多年平均而言,黑潮只在冬季侵入南海东北部,并在南海北部诱生一个次海盆尺度的气旋型环流,这时南海暖流只出现在汕头以东海域.夏季南海北部完全受东北向流控制,未见黑潮入侵迹象.用卫星跟踪海面漂流浮标观测进行的对比验证表明,以上遥感分析结果与海上观测一致。     

Coastal upwelling in the California current system

Coastal upwelling in the California Current system has been the subject of large scale studies off California and Baja California, and of small scale studies off Oregon. Recent studies of the winds along the entire coast from 25°N to 50°N indicate that there are significant along-shore variations in the strength of coastal upwelling, which are reflected in the observed temperature distribution. Active upwelling appears to be restricted to a narrow coastal band (about 10–25 km wide) along the entire coast, but the region influenced by coastal upwelling may be much wider. Intensive observations of the upwelling zone during summer off Oregon show the presence of a southward coastal jet at the surface, a mean vertical shear, a poleward undercurrent along the bottom, and persistently sloping isopycnals over the continental shelf; most of the upwelling there occurs during relatively short periods (several days long) of upwelling-favorable winds. During the upwelling season off Oregon, the offshore Ekman transport is carried by the surface Ekman layer, and the onshore return flow occurs through a quasi-geostrophic interior. It is not known whether the structure and dynamics observed off Oregon are typical of the upwelling zone along the entire coast, though some of the same features have been observed off Baja California. Current and future research will eventually show whether the Oregon results are also applicable in the region of persistently strong upwelling-favorable winds off northern California, and in the region of complex bathymetry off central and southern California.     

On Structure and Temporal Variation of the Bifurcation Current off the Kii Peninsula

The Kii Bifurcation Current is often found along the southwest coast of the Kii Peninsula, and its frequency of occurrence reaches about 70% in the period from 1988 to 1996 (Takeuchi et al., 1998a). In order to clarify the structure and short-period variability of the Kii Bifurcation Current, detailed observations were made four times on board the R/V Seisui-maru of Mie University on October 29–31, 1996, on June 24–26, 1997, October 14–16, 1997, and December 3–4, 1997. The measured horizontal structure of the Kii Bifurcation Current indicates that the eastern portion of the Current (eastward flow near Cape Shionomisaki) consists of a part of the current zone of the Kuroshio. It is shown that the current structure, including the Kii Bifurcation Current in the vicinity of Cape Shionomisaki, is stable when the Kuroshio is flowing in a stationary straight path, but that the current structure is considerably changed when small-scale eddies pass by the cape. Such short-period variation can be monitored by using the daily variation of the sea level difference between Kushimoto and Uragami. In particular, in the case of October 29–31, 1996, when an eminent small-scale eddy passed by Cape Shionomisaki, and when the Kuroshio axis tentatively moved southwards about 50 km apart from the coast, the Kii Bifurcation Current seems to have disappeared.     

The mid-depth circulation of the northwestern tropical Atlantic observed by floats

A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16°N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.     

The β-induced drift of separated boundary currents

Western boundary currents flow poleward from low latitudes until they ultimately separate from the coast and turn eastward into the ocean interior. The separation is mainly due to either: (i) the variation of the Coriolis parameter with latitude (β) which causes vanishing of the near-wall depth; (ii) vanishing wind stress curl over the ocean interior which forces zero meridional transport; or (iii) opposing currents that flow toward the equator and force the northward flowing currents to turn offshore (Agra and Nof, Deep Sea Research I, 40, 2259–2282). Here, we focus on the third kind of separated currents and show that, due to β, such separated currents migrate along the wall. A nonlinear “reduced gravity” one-and-a-half layer model is used to compute the desired migration speed. Solutions of the primitive equations are constructed analytically assuming that the translation rate is steady. It is found that the migration rate along the wall is given by βR_d² cosα/2 sinγ, where R_d is the Rossby radius, α an angle that measures the inclination of the joint offshore currents relative to the north, and γ is the angle between the axis of the joint offshore currents and the wall. The migration meridional component can be either northward or southward (depending on the inclination of the wall) but the zonal component is always westward. When the separated joint offshore flow is in the east-west direction (i.e. α = π/2 or 3π/2 so that the separated flow is zonal) no migration is taking place. It turns out that the above migration formula is so robust that it is also describes the migration rate in a two-and-a-half layer model where one current is allowed to, at least partially, dive under the other. For most separated currents the computed migration rate is a few centimeters per second.Possible application of this theory to the Confluence zone in the South Atlantic (where significant seasonal movement of the separation latitude has been observed) is discussed.     

Why are rings regularly shed in the western equatorial Atlantic but not in the western Pacific?

The western equatorial Atlantic is characterized by the formation and shedding of 3–4 large anticyclonic rings per year. These rings originate from the North Brazil Current which, in response to the vanishing wind stress curl (over the ocean interior), retroflects and turns eastward at around 4°N. After their formation and shedding the rings propagate toward the northwest along the South American coast carrying an annual average of about 4Sv. As such, the rings constitute an important part of the meridional heat flux in the Atlantic.The same cannot be said, however, of the western equatorial Pacific. Here, the situation is entirely different even though the South Equatorial Current retroflects at roughly the same latitude as its Atlantic counterpart, the North Brazil Current. Although the South Equatorial Current retroflection is flanked by two quasi-permanent eddies (the so-called Halmahera and the Mindanao eddies), these eddies are an integral part of the current itself and are not shed. Consequently, they are not associated with any meridional heat flux. An important question is, then, why the two oceans behave in such a fundamentally different way even though the source of the rings, the retroflected currents, are very similar in the two oceans.To answer this question, the two oceans are compared using recently developed analytical and numerical models for the western equatorial oceans. It is first pointed out that, according to recent developments in the modelling of the western equatorial Atlantic, the North Brazil Current retroflection rings are formed, shed and drift to the west because, in the Atlantic, this is the only way by which the momentum flux of the approaching and retroflecting current can be balanced. In this scenario, the northwestward flow force exerted by the approaching and retroflecting North Brazil Current (analogous to the force created by a rocket) is balanced by the southwestward force exerted by the rings as they are formed (analogous in some sense to the kickback associated with a firing gun).On the other hand, in the western equatorial Pacific, the formation and shedding of rings is unnecessary because the southward flowing Mindanao Current provides an alternative mechanism for balancing the northward momentum flux of the South Equatorial Current. This implies that it is the absence of a counter current (such as the Mindanao) in the western Atlantic that causes the formation and shedding of North Brazil Current rings. A remaining difficulty with the above scenario is that most colliding and retroflecting currents (i.e. the Mindanao and South Equatorial currents) are not “balanced” in the sense that they cannot be stationary but rather must drift along the coast. It is shown that, in the case of the western Pacific, the long-shore migration is arrested by the Indonesian Throughflow which allows the “unbalanced” fraction of the approaching currents to leak out into the Indian Ocean. This resolves the above difficulty and allows the retroflection to be approximately steady.     

Seasonal variations of water system distribution and flow patterns in the southern sea area of Hokkaido,Japan

Hydrographic data and composite current velocity data (ADCP and GEK) were used to examine the seasonal variations of upper-ocean flow in the southern sea area of Hokkaido, which includes the “off-Doto” and “Hidaka Bay” areas separated by Cape Erimo. During the heating season (April–September), the outflow of the Tsugaru Warm Current (TWC) from the Tsugaru Strait first extends north-eastward, and then one branch of TWC turns to the west along the shelf slope after it approaches the Hidaka Shelf. The main flow of TWC evolves continuously, extending eastward as far as the area off Cape Erimo. In the late cooling season (January–March), part of the Oyashio enters Hidaka Bay along the shallower part of the shelf slope through the area off Cape Erimo, replacing almost all of the TWC water, and hence the TWC devolves. It is suggested that the bottom-controlled barotropic flow of the Oyashio, which may be caused by the small density difference between the Oyashio and the TWC waters and the southward migration of main front of TWC, permits the Oyashio water to intrude along the Hidaka shelf slope.     

赤道中印度洋上层环流结构与季节变化特征分析

本文通过分析RAMA印度洋观测浮标系统锚系ADCP实测资料,对赤道中印度洋上层海流季节变化进行了研究。研究结果表明,0°,80.5°E纬向流垂向剖面呈现上150m层一致的东向流,而经向流在100m以浅呈现表层向北次表层向南的翻转流结构。赤道中印度洋上层纬向流季节信号被半年周期的东向射流Wyrtki Jets(WJs)所控制。WJs发生于季风方向转换的季节,4—5月份较弱,10—11月份较强。赤道中印度洋上层经向流年周期信号显著。北半球夏季与冬季分别出现风应力旋度驱动的Sverdrup南向流与北向流。本文结论为赤道中印度洋上层环流季节变化特征的研究提供了观测角度的支持。     

Altimeter-derived surface circulation in the large–scale NE Pacific Gyres. : Part 1. seasonal variability

The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are:

1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region.

2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System.

3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries.

4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current.

5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer.

6. The maximum values of EKE occur when each of the boundary currents are maximum.

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a⁴⁰Ar/³⁹ Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.