Westward Currents Research Articles

Conjugate high-intensity energetic electron precipitation at high latitude

Abstract. On 6 August 1998 an intense precipitation event occurring at high latitude in the evening sector was observed by X-ray and far-ultraviolet imagers on board the Polar satellite and by several ground-based instruments. The precipitation region was centred at approximately 19:00 MLT at 74° MLAT (at an L-shell of about 13). The event started at 22:59 UT and lasted about 10 minutes. It happened during the late expansion phase of a substorm after two hours of strongly southward IMF. Imaging riometers at geomagnetically conjugate sites recorded strong absorption levels which exceeded 7 dB at 38 MHz in a transient and localized intensification occurring within a poleward moving arc-like feature. The temporal and spatial similarities between the recordings from the two conjugate regions are remarkable. The arc-like precipitation region progressed poleward with a velocity of 1.5 km/s. Ground magnetometers co-located with the imaging riometers observed disturbances consistent with poleward moving westward currents. In X-ray and riometer images which are sensitive only to energetic electrons (above 5–10 keV) the event seems isolated, but in UV images the event is seen to occur on the poleward edge of the rapidly poleward expanding evening side aurora. The energy spectrum of precipitating electrons was subject to a temporary hardening which peaked at a mean energy of about 20 keV when the event was at its most intense at 23:02 UT. The event is likely to have been caused by an accelerating mechanism at some height above the ionosphere or by an earthward flow burst in the magnetotail, or possibly both.Key words. Magnetospheric physics (energetic particles, precipitating; storms and substorms; magnetosphere-ionosphere interactions)

Impacts of salinity on the eastern Indian Ocean during the termination of the fall Wyrtki Jet

The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean‐atmosphere coupling.

On the surface currents of the Caribbean Sea

Velocity observations from near surface drifters made between 1996 and 2001 were used to construct surface circulation maps of the Caribbean Sea. In the Venezuela Basin the strongest westward currents (0.3 ms−1) are found south of 14°N and at 17°N. Two jets occur over the deep gaps of the Jamaica Ridge and move northwestward across the Cayman Basin. The two jets feed a current that impinges on Yucatan, slows down while turning north and accelerates again to become the Yucatan Current. A cyclonic gyre occurs in the southern Colombia Basin, with mean currents along the south coast in excess of 0.7 m s−1. To the east, the gyre is highly variable. Eddy motions dominate most of the Eastern Caribbean as well as the eastern Yucatan Basin, where the average surface circulation is not well defined.

Observation of field-aligned and ionospheric currents during space weather month, September 1999

September 1999 was a magnetically active month, with several geomagnetic storms occurring in sequence which influenced the current pattern of the high-latitude ionosphere. The DMI Greenland magnetometers recorded variations of the ionospheric Hall currents while Ørsted, a Danish low-altitude Earth-orbiting satellite, measured the magnetic effect of field-aligned currents (FAC). We discuss a particular dayside event which occurred a few hours after the peak of a moderate storm on Sept. 15, 1999. Ørsted proceeded polebound on a trajectory closely aligned with the Greenland west coast magnetometer chain, and the Sondrestrom Incoherent Scatter Radar was in operation scanning the ionosphere almost parallel to the Østed trajectory. Coincident solar wind (SW) and interplanetary magnetic field (IMF) data were obtained from the ACE spacecraft and ballistically propagated to a nominal subsolar bow shock location at 12 Re. A synoptic view of the data suggests that the dayside field-aligned and ionospheric current system responded quickly (within 5–10 min) to variations of the northward component of the IMF (IMF- B z ). Specifically, the latitude of the boundary between westward and eastward high-latitude Hall currents as well as the latitudes of the maxima of the eastward and westward currents followed closely IMF- B z variations.

SOFAR Floats Reveal Midlatitude Intermediate North Atlantic General Circulation. Part II: An Eulerian Statistical View

Abstract Part I of this paper has given a descriptive view of the trajectories of 26 SOFAR floats drifting near 700-m depth in the central North Atlantic during the mid-1980s, as part of the TOPOGULF experiment. Here an Eulerian analysis of the 53.4 collected float years is performed by grouping data in 2° latitude × 4° longitude boxes. The mean circulation lacks a significant southward Sverdrupian flow and shows instead zonal bands of alternating westward and eastward currents (except in the Canary Basin). West of the ridge and north of 38°N, the general northeastward flow of the Gulf Stream system is recovered while, south of 38°N the westward recirculation observed from historical float data between 70° and 55°W is shown to extend as far as 40°W. A mean Azores Current is observed both west of the ridge near 33°N and east of 30°W near 34°N. In between, east of the ridge, a tongue of high eddy energy indicates stronger eddy activity and local instabilities of the Azores Current. Eddy kinetic energy and e...

Seismic evidence for bottom current activity at the Agulhas Ridge

In the South Atlantic water masses from the Atlantic and Indian Oceans meet Antarctic water masses. The Agulhas Ridge, a pronounced elevation of the ocean bottom in the eastern South Atlantic, has acted as a barrier for deep oceanic currents since the Cretaceous, such as the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), or water masses derived from AABW such as Circumpolar Deep Water (CDW). The history of these currents is recorded in the sedimentary sequence in the adjacent Cape and Agulhas Basins. Seismic profiles over the Agulhas Ridge show sediment packages in the Cape Basin which are interpreted as contourite sheets. These consist of thick sequences interrupted by widespread hiatuses, with a predominantly low seismic reflectivity. The seismic data shows prominent reflectors inside contourite drift bodies which, at the location of the drill-sites of Ocean Drilling Program (ODP) Leg 177, can be attributed to hiatuses in the Early Oligocene, the Middle Miocene, around the Miocene/Pliocene boundary and in the early Pleistocene. In this way, ODP Leg 177 cores were used to date an elongate contourite drift in the Cape Basin. This drift shows sediments deposited by a westward current, implying that the bottom current in the Oligocene followed the same trajectory as present-day CDW does.

The overflows across the Ninetyeast Ridge

Abstract Water deeper than, say, 3800 m in the Central Indian Basin comes from the West Australian Basin across deep saddles on the Ninetyeast Ridge near Lats. 5°S, 10°S, and 28°S. The WOCE Hydrographic Program in the Indian Ocean (1994–1996) provided opportunities for more detailed measurements of water properties in these overflows than any made earlier. With zero-velocity surfaces inferred from contrasts in oxygen and silicic-acid concentrations and from distributions of specific-volume anomaly, geostrophic estimates of these overflow rates are, respectively, 1.0, 1.1, and 0.1×10 6 m 3 s −1 (westward). These constructions also imply strong zonal flows of mid-depth water (between 2000 db and the zero-velocity surfaces) through the passages: 2.2 and 6.7×10 6 m 3 s −1 westward at Lats. 5°S and 10°S, respectively, and 1.8×10 6 m 3 s −1 eastward at Lat. 28°S. As in the southward flow through Denmark Strait, density stratification explains the splitting of westward currents at 5°S and 10°S into mid-depth and deep flows as they pass over their saddles (but not why such vertically extended flows should exist in the first place). The cause of opposed eastward flow at 28°S is less certain, but might be related to the particular geometry of the West Australian Basin.

Os‐Sr‐Nd results from sediments in the Bay of Bengal: Implications for sediment transport and the marine Os record

Sr, Nd, and Os isotopic data are presented for sediments from diverse locations in the Bay of Bengal. These data allow the samples to be divided into three groups, related to their sedimentary contexts. The first group, mainly composed of sediments from the shelf off Bangladesh and the currently active fan, has Sr and Nd characteristics consistent with a dominantly Himalayan source. Their 187Os/188Os ratios (∼1.2‐1.5) show that the average detrital material delivered by the Ganga‐Brahmaputra (G‐B) river system is not unusually radiogenic. A large difference in 187Os/188Os ratio exists between these Bengal Fan sediments and Ganga bedloads (187Os/188Os ∼2.5, Pierson‐Wickmann et al. [2000]). This difference mainly reflects addition of a less radiogenic Brahmaputra component, though mineralogical sorting and loss of radiogenic Os during transport may also play some role. The second sample group contains sediments from elsewhere in the Bay, particularly those located on the continental slope. They display Os isotopic compositions (0.99–1.11) similar to that of present seawater and higher Os and Re concentrations. These characteristics suggest the presence of a large hydrogenous contribution, consistent with the lower sedimentation rate of these samples. Sr and Nd ratios indicate that a significant fraction of these sediments is derived from erosion of non‐Himalayan sources, such as the Indo‐Burman range. These observations could be explained by the deflection of sediments from the G‐B river system by westward currents in the head of the Bay. The third group contains only one sample, but shows that in addition to a Himalayan source, sediment discharge from Sri Lanka may influence the detrital component in the distal part of the fan. The similarity between the isotopic compositions of the group I R/V Sonne samples and those of Ocean Drilling Program Leg 116 [France‐Lanord et al., 1993; Reisberg et al., 1997] suggests that the material eroding in the Himalayas has been roughly constant since the Miocene. The high Os isotopic ratios of leachates of both Sonne group I and Miocene Leg 116 sediments imply that much of the leachable highly radiogenic Os component was conserved during transport through the estuary or interaction with seawater. In constrast, samples with lower, but still relatively high, sedimentation rates (Sonne groups II and III and Pliocene Leg 116) seem to have significantly adsorbed or exchanged Os and Re with seawater. This suggests that in some cases the Os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient marine Os record, or conversely, to date unfossiliferous sediments.

Westward moving waves or eddies ( Storms) on the Subtropical/Azores Front near 32.5°N? Interpretation of the Eulerian currents and temperature records at moorings 155 (35.5°W) and 156 (34.4°W)

Three Argos buoy-years of Lagrangian data in westward-moving cyclonic eddies, or Storms, near 32.5°N, together with hydrographic measurements, have shown that Storms move westward at nearly 3 km day −1. Water in eddies can be trapped and moved westward by advection within the eddy or by phase propagation of the eddy pattern, so we cannot say that the flow field (or Eulerian mean) is 3 km day −1 westward. Two moorings (155 and 156) deployed in the Storm Corridor have provided a further 8 instrument-years of Eulerian data. The temperature and current records confirmed that two Storms a year passed each mooring over the 2-year measurement period. As expected, there is a lag of ∼1.3 month at mooring 155 (which is 102 km to the west of mooring 156) with respect to conditions at mooring 156. The progressive vector diagrams (PVDs) derived from the current meter records exhibit fairly regular X (east or zonal) and Y (north or meridional) displacement scales that repeat with semi-annual periodicity (SAP). The dominant current signal is the north component of the SAP, which reaches an amplitude of 18 cm s −1 for the upper layer or near surface current record (∼242-m depth). The geostrophic north component values derived from altimetry were in good agreement with the upper layer current meter measurements. The large north component amplitude was not interpreted as evidence for Rossby Waves but rather due to the passage of nine eddies (eight complete) of alternate sign (cyclonic, anticyclonic) passing the mooring rigs during the 2-year deployment period. The Y scale shows that the near surface characteristic or mean maximum azimuthal speed is about 35 cm s −1 for cyclonic eddies or Storms, and that this value is reduced to 4 cm s −1 at 1400-m depth. The residual or mean Eulerian currents range from 8 cm s −1 for the upper layer currents to 1 cm s −1 for the deeper currents at ∼1400-m depth and are predominantly westward. Simple theoretical considerations and idealised numerical simulations show that the mean westward Eulerian current depends markedly on whether the eddy centres pass to the north or south of the rigs. This raises the question as to what do we mean by Eulerian residual currents, even for relatively long records (∼2 years). It is shown that the strong near surface westward current (∼6 km day −1) measured at mooring 155 is largely due to a westward-moving eddy field with variable centre offsets. The magnitude of the near surface east–west component of flow was estimated as eastward at 2 cm s −1. The north–south component of mean flow was southward at 2 cm s −1. The deeper records gave a weak westward flow of ∼1 cm s −1 but did not show a significant southward component for the mean Eulerian flow field. 7.4 float-years of Lagrangian ALACE data in the Subtropical Front region near 740 dbar gave mean east–west flows that were <0.5 cm s −1. Overall, it is shown that the eddy structures propagate westward mainly by phase propagation (i.e. a westward-moving pattern with no westward advection for the current meter to measure), though plane Rossby Wave dynamics appeared inappropriate. Theoretical and modeling considerations show that a speed of 3-km day −1 westward is too large a value for the self-advection of eddies due to the beta effect.

Coastal Boundary Layer Characteristics during Summer Stratification in Lake Ontario

Abstract Simultaneous measurements of Eulerian and Lagrangian currents along the north shore of Lake Ontario are analyzed to provide the mean flow properties and horizontal turbulent exchange characteristics in the coastal boundary layer (CBL). The summer coastal boundary layer is characterized by a frictional boundary layer (FBL) of a width of ∼3 km, in which shore and bottom friction affects the flow. In this regime the currents are predominantly shore parallel and persistent. The outer boundary layer also called an inertial boundary layer (IBL), typically of the order of 5–6 km wide, is a consequence of the adjustment of inertial oscillations to the lateral boundary. During the summer season within the CBL, the current motions are associated with thermocline displacements. The eastward (westward) wind stress causes thermocline elevation (depression) causing upwelling (downwelling). The mean subsurface westward currents associated with downwelling events are typically stronger in comparison to weak east...

Fronts, transport, and Weddell Gyre at 30°E between Africa and Antarctica

A detailed description of the frontal structure of major currents and estimates of transport between Africa and Antarctica at 30°E were made on the basis of a finely resolved hydrographic section made during the 1996 Civa‐2 cruise. Particular emphasis was put on a refinement of the eastern boundary of the Weddell Gyre by analyzing also supplementary hydrographic data from the 1993 Civa‐1 cruise and the best available historical hydrographic data in the Weddell and Enderby Basins. Contrary to the general belief, our results show that the Weddell Gyre extends far beyond 30°E, reaching at least as far east as the Enderby Land promontory (53°E). From this point, westward currents along the Antarctic continental margin constitute the gyre's southern boundary, thus closing the cyclonic circulation of the gyre at its easternmost end. As the most convincing evidence for this refined circulation scheme, significant westward transport south of 65°S has been identified from the Civa‐1 acoustic Doppler current profiler data. We have shown also, using an inertial jet model, that the blocked feature of the Weddell cold regime (∼57°S) to the west of 25°E is due to the topographic control of flow by local bathymetry. During the Civa‐2 cruise, ∼70% of the Antarctic Circumpolar Current (ACC) was concentrated near the Polar Front (51°S), with surface speeds reaching up to 70 cm s−1. The baroclinic current field in the subpolar area south of the Weddell Front (58°S for Civa‐2 and 57°S for Civa‐1) is featureless (&lt;3 cm s−1), but yielding a nonnegligible eastward transport of ∼15 Sv (1 Sv = 106 m3 s−1). This bottom‐referenced baroclinic transport is entirely compensated by a substantial westward barotropic transport south of 65°S. The ACC transport across the 30°E section during the Civa‐2 cruise is estimated to be ∼160 Sv.

A statistical study of evening sector arcs and electrojets

We present results of a statistical study of evening sector auroral arcs associated with electrojets. The study (including ∼ 1000 all-sky camera (ASC) frames) is based on data of the MIRACLE instrument network. An automatic search engine is used to define the arc periods from ASC data and the recordings of the IMAGE magnetometer network are used to estimate the strength and location of the equivalent electrojet currents. The maximum current densities and intensities of the equivalent electrojets vary in the ranges 200–600 A/km and 200–400 kA, respectively. In 85 % of cases with single arcs the current system is of a convection reversal type (an eastward electrojet equatorward of a westward electrojet). Usually the arc is within 1° distance from the latitude of maximum eastward current density which is ≤4° south from the transition region between the westward and eastward electrojets. Most of such events were observed in the dusk sector, not in the premidnight-sector, which is generally considered as the typical Harang discontinuity (HD) region of convection reversal type currents. Furthermore, the poleward boundary of the westward current is very often poleward of the northernmost IMAGE station (CGM lat. ∼76), which may mean that the westward currents are not real horizontal currents but equivalent currents due to field aligned R1 currents. This implies that a stable arc often resides a few degrees equatorward to the evening shell potential minimum. We discuss the implications of these findings from the viewpoint of previous HD-studies.

Bottom water circulation in the western equatorial Pacific as inferred from carbon tetrachloride observations

Hydrographic and tracer data from the western equatorial Pacific, collected during cruise 113 of R/V Sonne in October–November 1996, are discussed in the context of spreading of lower Circumpolar Deep Water (lCDW). The horizontal distribution of CCl4 and other properties indicate a deep western boundary current north of the Caroline Seamounts. Elevated concentrations of CCl4 at the bottom show that a small fraction of lCDW in this region is younger than about 70 years. This age is used to infer a minimum spreading rate of about 0.8 cm/s for the young components of this water mass. The current appears to bifurcate at about 143°E into topographically guided northward and westward currents.

Variability of Currents off the Northern Coast of New Guinea

The variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were examined from one year time series of current data from ADCP moorings at 2°S, 142°E and 2.5°S, 142°E. Change in the hydrographic structure induced by monsoonal wind forcing was also examined from hydrographic data along the 142°E covering consecutively two winter seasons and two summer seasons. The westward NGCUC was observed to persist year around. The annual mean depth of the current core was 220 m, the mean speed of the zonal component was 54 cm/s with a standard deviation of 15 cm/s at the 2.5°S site. Velocity fluctuations at 20–30 day period were observed year around. Seasonal reversal of the surface intensified NGCC was clearly observed. In the boreal summer characterized by the southeasterly monsoon, westward currents of over 60 cm/s were dominant in the surface layer. The warm, low-salinity layer thickened at this time and sloped down toward the New Guinea coast from the equator. This surface water accumulation may be caused by onshore Ekman drift at the New Guinea coast, combined with weak Ekman upwelling at the equator. In the boreal winter, an eastward surface current developed to 100 cm/s extending down to 100 m depth in response to the northwesterly monsoonal winds. Coastal upwelling was indicated in this season and the surface water accumulated at the equator due to Ekman convergence. Shipboard ADCP data indicated that the NGCUC intensified in boreal summer as the width and depth of the NGCUC increased.

CHARA CTERISTICS OF COASTAL CURRENTS IN TOMAKOMAI COAST

To understand the environments of water quality, sediments and ecology in a coastal sea area, the characteristics of coastal currents must be clarified, In this study, we analyzed observation data of flow velocity and wind velocity in Tomakomai Coast including the area of Mukawa River mouth.Along Tomakomai Coast, a strong south-west or south-east wind often cased eastward or westward currents parallel to the shoreline in winter. Flow velocity in offshore of Siraoi Coast was strong and more than 30cm/s at a depth of 50m. In both summer and winter wind occurred currents parallel to the shoreline.In summer, fiow fluctuations with the period of about one day were occured by sea and land wind.

A Numerical Investigation of the Local Ocean Response to Westerly Wind Burst Forcing in the Western Equatorial Pacific

Numerical simulations of the local equatorial ocean response to idealized westerly wind burst (WWB) forcing are described. In particular, the authors examine the development and evolution of the subsurface westward jet (SSWJ) that has been observed to accompany these wind events. This westward current is interpreted as the signature of equatorial waves that accompany the downwelling and upwelling that occurs along the edges of the wind forcing region. Some important features of the SSWJ include maximum intensity toward the eastern edge of the forcing region, a time lag between the wind forcing and peak SSWJ development, and an eastward spreading of the SSWJ with time. The effect of wind burst zonal profile, magnitude, duration, and fetch on the SSWJ are explored. The response of an initially resting ocean to WWB forcing is compared with that for model oceans that are spun up with annual-mean surface fluxes and monthly varying fluxes. It is demonstrated that the gross features of the response for the spun up simulations can be well approximated by adding the background zonal current structure prior to the introduction of the wind burst to the initially resting ocean current response to the WWB. This result suggests that the zonal current structure that is present prior to the commencement of WWB forcing plays a key role in determining whether or not a SSWJ will develop.

Morphological aspects of a new type of counter electrojet event

Abstract. The study describes the time and space morphologies of a rather new type of counter electrojet event on the basis of data from the excellent chain of magnetic and ionospheric observatories along the Indo-Russian longitude sector. Abnormally large westward currents are observed during almost the whole of the daytime hours on a series of days. These events do not form any vortices in the current system and do not apparently seem to be associated with tidal effects or any solar magnetosphere events or geomagnetic disturbances. The existence of a westward electric field over the equatorial ionosphere has been confirmed by the absence of an equatorial type of sporadic E in the ionograms at Thumba precisely during the periods when ∆H at Trivandrum minus ∆H at Alibag is negative. The equatorial F region anomaly was also absent on the counter electrojet day. Such counter electrojet events during the northern winter months of low solar activity years are suggested to be the result of the modified wind system in the ionosphere associated with stratospheric warming events.Key words. Geomagnetism and paleomagnetism (time variations · diurnal to secular) · Ionosphere (electric fields and currents; equatorial ionosphere)

On Seasonal and Year to Year Variation in Flow of the Alaskan Stream in the Central North Pacific

The Alaskan Stream is the westward boundary current of the North Pacific subarctic gyre. In the central region of the North Pacific, the Alaskan Stream serves as a connection between the Alaskan gyre, Western subarctic gyre and Bering Sea gyre. Its volume transport is very important in estimating the magnitude of the subarctic circulation in the North Pacific. In order to clarify its seasonal and interannual variation, we conducted observations along a north-south section at 180° during June from 1990 to 1997. Moorings were deployed from 1995 to 1997. Hydrographic casts were made at intervals of ∼37 km to a depth of 3000 m. Moorings were set between CTD stations, with Moor1 (Moor2) at the center (southern edge) of the Alaskan Stream. Geostrophic volume transport (referred to 3000 m) revealed large interannual variability in the Alaskan Stream. Average volume transport over the 8 years was 27.5 × 106 m3s-1 with a standard deviation of 6.5 × 106 m3s-1. Maximum transport was 41.0 × 106 m3s-1 (1997) and minimum was 21.7 × 106 m3s-1 (1995). Stable westward flows were observed at Moor1 1500 m (259°, 11.7 cm s-1) and 3000 m (240°, 3.7 cm s-1, 1996–1997 year average). The ratio of eddy to mean kinetic energy (KE′/\(\overline {KE} \)) was very small (<0.6) throughout the year. A relatively weak and unstable westward flow was observed at Moor2 at 3000 m depth. Conversely, the average flow direction at Moor2 5000 m was eastward.

Morphological aspects of a new type of counter electrojet event

The study describes the time and space morphologies of a rather new type of counter electrojet event on the basis of data from the excellent chain of magnetic and ionospheric observatories along the Indo-Russian longitude sector. Abnormally large westward currents are observed during almost the whole of the daytime hours on a series of days. These events do not form any vortices in the current system and do not apparently seem to be associated with tidal effects or any solar magnetosphere events or geomagnetic disturbances. The existence of a westward electric field over the equatorial ionosphere has been confirmed by the absence of an equatorial type of sporadic E in the ionograms at Thumba precisely during the periods when ∆H at Trivandrum minus ∆H at Alibag is negative. The equatorial F region anomaly was also absent on the counter electrojet day. Such counter electrojet events during the northern winter months of low solar activity years are suggested to be the result of the modified wind system in the ionosphere associated with stratospheric warming events.Key words. Geomagnetism and paleomagnetism (time variations · diurnal to secular) · Ionosphere (electric fields and currents; equatorial ionosphere)

Water masses and properties at 165°E in the western Pacific

Full‐depth conductivity‐temperature‐depth‐oxygen profiler (CTDO2) data at 165°E in 5°S–40°N in summer 1991 (World Ocean Circulation Experiment (WOCE) Hydrographic Program P13C) and 27°–48°N in spring 1993 (P13J) are analyzed. Distributions of salinity and temperature are characterized by the fronts associated with ocean currents, such as the Kuroshio Extension front at 34°–35°N, the Kuroshio Bifurcation front at 38°–40°N, the subarctic front at 40°–42°N, other salinity fronts in the tropical waters, and some oxygen fronts. Dissolved oxygen increases upward sharply, called oxycline, just above the intermediate oxygen minimum layer. The processes of water‐mass extension are clarified mainly from oxygen and potential vorticity (PV). The North Pacific Tropical Water (NPTW) forms at the sea surface in 18°–31°N from winter to summer, and the winter‐formed NPTW of 23.0–25.0σθ (where σθ is potential density) extends to 12°N by the subtropical ocean gyre, so that it occupies a layer of 23.0–25.0σθ in 12°–31°N. The Subtropical Mode Water (STMW) forms at the surface in 29°–34°N in winter and extends to 25°N by the ocean circulation; it occupies a layer around 25.4σθ between the main and seasonal pycnoclines in 25°–34°N. The North Pacific Intermediate Water (NPIW) extends to 34°–38°N on 26.83σθ by the Kuroshio Extension and its northern branch and extends to 27°–33°N on 26.77σθ and to 15°–22°N on 26.61σθ by westward currents; it occupies 15°–38°N in the intermediate subtropical gyre. The NPIW is located in the oxycline with similar PV to the STMW. The boundary of the subtropical and tropical gyres is concluded to be around 12°N (14°–16°N) in a surface (intermediate) layer &lt;25.0σθ (&gt;26.0σθ). Bottom waters are also examined.