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The main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5-6 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) through the Samoan Passage. A recent field campaign has shown that this fl...
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The main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5-6 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3-2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the "island rule." Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4-2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.
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The deep western boundary current (DWBC) was studied based on a full-depth mooring east of Luzon Island in the Northern Philippine Sea deep basin during the period from January 2018 to May 2020. On average, the DWBC in the Philipp...
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The deep western boundary current (DWBC) was studied based on a full-depth mooring east of Luzon Island in the Northern Philippine Sea deep basin during the period from January 2018 to May 2020. On average, the DWBC in the Philippine Sea flows southward with a velocity of approximately 1.18 cm s~(-1) ata depth of 3050 m. Significant intraseasonal and seasonal variations of the DWBC are identified. The intraseasonal variations have multiple spectral peaks in the range of 30-200 days, with the most obvious peak at approximately 120 days. On the seasonal time scale, the DWBC intensifies in summer/autumn and weakens in winter/spring, corresponding well with the seasonal variation of the ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment. Both intraseasonal and seasonal variations have no significant correlation with the temporal variations in the upper and middle layers but have a certain correlation with transport through the Yap-Mariana Junction (YMJ). A set of experiments based on an inverted-reduced-gravity model and the OBP data reveal that the temporal variations originating from the YMJ could propagate counterclockwise along the boundary of the deep basin to the western boundary of the deep Philippine Sea, dominating the temporal variations of DWBC.
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We report helium isotope data collected in the central Indian Ocean, from the Arabian Sea to the Southern Ocean, during a Japanese GEOTRACES cruise in 2009 - 2010. We found hydrothermal helium-3 plumes and confirmed that 3(He)/(4)...
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We report helium isotope data collected in the central Indian Ocean, from the Arabian Sea to the Southern Ocean, during a Japanese GEOTRACES cruise in 2009 - 2010. We found hydrothermal helium-3 plumes and confirmed that 3(He)/(4)(He) ratio anomalies were almost the same as those observed in WOCE cruises conducted in 1990s, which indicates the hydrothermal activity and abyssal currents have not changed largely for the last few decades. Maximum delta He-3 value over 14% was observed at mid-depth (2000 - 3000 m) in the northern part (north of 30 degrees S) in the central Indian Ocean, whereas lower delta He-3 was found in the southern part at the same depth, where delta He-3 is defined as the percent deviation of the helium isotopic ratio relative to the atmospheric standard. The vertical distribution of delta He-3 shows a similar trend with dissolved iron and manganese distributions in the hydrothermal plume. Lateral delta He-3 distribution at mid-depth using our GEOTRACES data together with WOCE data suggest that the helium-3 plume in the central Indian Ocean derived from the Central Indian Ridge around 20 degrees S. It does not flow northward along the ridge but flows eastward as previously reported. The source of the helium-3 plume observed in the region adjacent to the Indian subcontinent might be in the Gulf of Aden as inferred from water properties. The delta He-3 distribution could reveal clockwise deepwater circulation in the Arabian Sea.
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We report helium isotope data collected in the central Indian Ocean, from the Arabian Sea to the Southern Ocean, during a Japanese GEOTRACES cruise in 2009 - 2010. We found hydrothermal helium-3 plumes and confirmed that 3(He)/(4)...
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We report helium isotope data collected in the central Indian Ocean, from the Arabian Sea to the Southern Ocean, during a Japanese GEOTRACES cruise in 2009 - 2010. We found hydrothermal helium-3 plumes and confirmed that 3(He)/(4)(He) ratio anomalies were almost the same as those observed in WOCE cruises conducted in 1990s, which indicates the hydrothermal activity and abyssal currents have not changed largely for the last few decades. Maximum delta He-3 value over 14% was observed at mid-depth (2000 - 3000 m) in the northern part (north of 30 degrees S) in the central Indian Ocean, whereas lower delta He-3 was found in the southern part at the same depth, where delta He-3 is defined as the percent deviation of the helium isotopic ratio relative to the atmospheric standard. The vertical distribution of delta He-3 shows a similar trend with dissolved iron and manganese distributions in the hydrothermal plume. Lateral delta He-3 distribution at mid-depth using our GEOTRACES data together with WOCE data suggest that the helium-3 plume in the central Indian Ocean derived from the Central Indian Ridge around 20 degrees S. It does not flow northward along the ridge but flows eastward as previously reported. The source of the helium-3 plume observed in the region adjacent to the Indian subcontinent might be in the Gulf of Aden as inferred from water properties. The delta He-3 distribution could reveal clockwise deepwater circulation in the Arabian Sea.
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The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circu...
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The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circulation north and cyclonic circulation south of the Antarctic Circumpolar Current. This image mostly follows from geostrophic estimates from hydrography using a bottom level of no motion for the eastward flow regime which typically yield transports near 170 Sv. Net eastward transport of about 145 Sv for this region results from subtracting those westward flows. This estimate is compatible with the canonical 134 Sv through Drake Passage with augmentation from Indonesian Throughflow (around 10 Sv). A new image is developed from World Ocean Circulation Hydrographic Program sections I8S and I9S. These provide two quasi-meridional crossings of the South Australian Basin and the Australian-Antarctic Basin, with full hydrography and two independent direct-velocity measurements (shipboard and lowered acoustic Doppler current profilers). These velocity measurements indicate that the belt of eastward flow is much stronger, 271 ± 49 Sv, than previously estimated because of the presence of eastward barotropic flow. Substantial recirculations exist adjacent to the Antarctic Circumpolar Current: to the north a 38 ± 30 Sv anticyclonic gyre and to the south a 76 ± 26 Sv cyclonic gyre. The net flow between Australia and Antarctica is estimated as 157 ± 58 Sv, which falls within the expected net transport of 145 Sv. The 38 Sv anticyclonic gyre in the South Australian Basin involves the westward Flinders Current along southern Australia and a substantial 33 Sv Subantarctic Zone recirculation to its south. The cyclonic gyre in the Australian-Antarctic Basin has a substantial 76 Sv westward flow over the continental slope of Antarctica, and 48 ± 6 Sv northward-flowing western boundary current along the Kerguelen Plateau near 57°S. The cyclonic gyre only partially closes within the Australian-Antarctic Basin. It is estimated that 45 Sv bridges westward to the Weddell Gyre through the southern Princess Elizabeth Trough and returns through the northern Princess Elizabeth Trough and the Fawn Trough - where a substantial eastward 38 Sv current is hypothesized. There is evidence that the cyclonic gyre also projects eastward past the Balleny Islands to the Ross Gyre in the South Pacific. The western boundary current along Kerguelen Plateau collides with the Antarctic Circumpolar Current that enters the Australian-Antarctic Basin through the Kerguelen-St. Paul Island Passage, forming an energetic Crozet-Kerguelen Confluence. Strongest filaments in the meandering Crozet-Kerguelen Confluence reach 100 Sv. Dense water in the western boundary current intrudes beneath the densest water of the Antarctic Circumpolar Current; they intensely mix diapycnally to produce a high potential vorticity signal that extends eastward along the southern flank of the Southeast Indian Ridge. Dense water penetrates through the Ridge into the South Australian Basin. Two escape pathways are indicated, the Australian-Antarctic Discordance Zone near 125°E and the Geelvinck Fracture Zone near 85°E. Ultimately, the bottom water delivered to the South Australian Basin passes north to the Perth Basin west of Australia and east to the Tasman Basin.
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The cross-equatorial flow of grounded abyssal ocean currents in a differentially rotating meridional channel with parabolic bottom topography is examined. In particular, the dependence is determined of the cross-equatorial volume ...
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The cross-equatorial flow of grounded abyssal ocean currents in a differentially rotating meridional channel with parabolic bottom topography is examined. In particular, the dependence is determined of the cross-equatorial volume flux on the underlying flow parameters including the slope of the channel's walls s, the half-width of the channel l, the half-width and height of the abyssal current a and H, respectively, the magnitude of the rotation vector ?, the Earth's radius R, and the reduced gravity g'. In addition, it is shown that the ratio between the width of the channel and the zonal wavelength of a narrow wave structure that is formed by the current in the equatorial region plays a crucial role in determining into which hemisphere the current flows after its interaction with the equator. It is found that some parameters (e.g. a and H) do not have any significant effect on the zonal wavelength, while variations in other parameters (e.g. l, s,Ω, R and g') change the zonal wavelength and, consequently, can dramatically alter the qualitative trans-equatorial behavior of the abyssal current.After examining an auxiliarymodel of a particle in a rotating equatorial channel, it is shown that the zonal wavelength of the equatorial wave is linearly proportional to the equatorial length scale defined as L_(eq) = (g's/l)~(1/2)/β,where β = 2Ω/R is the equatorial value of the beta-parameter.
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This is Part II of a two-part theoretical study into the midlatitude-cross-equatorial dynamics of a deep western boundary current (DWBC) in an idealized meridionally aligned, differentially rotating ocean basin with zonally varyin...
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This is Part II of a two-part theoretical study into the midlatitude-cross-equatorial dynamics of a deep western boundary current (DWBC) in an idealized meridionally aligned, differentially rotating ocean basin with zonally varying parabolic bottom topography. Part I determined the midlatitude flow across the planetary vorticity gradient and the dynamics of the DWBC as it begins to enter the equatorial region in the intermediate equatorial region. Part II determines the nonlinear dynamics of the DWBC as it flows across the basin along the equator in the inner equatorial region. The large-scale structure of the flow within the inner equatorial region corresponds to a zonally aligned nonlinear stationary planetary wave pattern that meanders about the equator in which the flow exits the equatorial region on the eastern side of the basin. In addition to numerically determining the pathlines for the large-scale equatorial flow, an approximate nonlinear model is introduced for which an analytical solution can be obtained for the nonlinear planetary wave along the equator. If the DWBC exits the equatorial region into the opposite hemisphere from its source hemisphere, the characteristic curves associated with the flow must necessarily intersect within the inner equatorial region. It is in the regions of intersecting characteristics that dissipation makes a leading-order contribution to the dynamics and induces the requisite potential vorticity adjustment permitting the cross-equatorial flow of a DWBC that is in planetary geostrophic dynamical balance in midlatitudes.
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Current observations at several depths between 250 and 3750 m are reported from a 30°S, deep sea site 150 km off the Chile coast for the period July 1993-June 2001. These results are used with current observations from a nearby s...
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Current observations at several depths between 250 and 3750 m are reported from a 30°S, deep sea site 150 km off the Chile coast for the period July 1993-June 2001. These results are used with current observations from a nearby slope site, satellite altimeter data, and hydrographic and Acoustic Doppler Current Profiler data from the WOCE P6E section along 32.5°S to address mean and variable flow in the Chile Basin. Strong current variability in the upper ocean for periods more than 100 day's is explained in terms of remotely forced Rossby waves, local baroclinic instabilities of coastal currents and variable wind forcing. Intraseasonal to seasonal current variability at the deep ocean site was greatest during La Nina events while intraseasonal variability at the slope site was greatest during the 1997-1998 El Nino event. Mean westward and poleward flow was observed at all depths at the deep sea site but upper ocean means were not significantly different from zero. There was a well-defined, mean poleward flow of 0.6 ±0.3 cm s~(-1) at 2450m depth there. Geostrophic current calculations are presented for the P6E section with levels of no motion based on our current observations and other constraints. These reference choices yield reasonable and consistent results for the steady-state heat balance of the deep Chile and Peru Basins. Results show a deep equatorward flow of 3-4 Sv on the eastern flank of the East Pacific Rise and a deep poleward boundary flow of about 10Sv within 1500km of the Chile coast. Up to half of the total mid-depth outflow of the South Pacific may take place east of the Rise. Thus the deep poleward boundary current off Chile is a major component of the deep circulation of the global ocean but the dynamics of this current remain a puzzle.
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Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes i...
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Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current's denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC's flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.
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Deep-sea currents in the Northeast Pacific Ocean are examined using observations at Station M, located on the abyssal plain 220 km offshore of central California. Collected near the seabed at similar to 4000 m depth from October 2...
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Deep-sea currents in the Northeast Pacific Ocean are examined using observations at Station M, located on the abyssal plain 220 km offshore of central California. Collected near the seabed at similar to 4000 m depth from October 2014-October 2018, the current meter observations supplement long-term investigations of deep-sea benthic ecology at this site. To investigate mechanisms for variability at different time scales, the observed currents are separated into low-frequency fluctuations with periods of 1.4-60 days, fluctuations that are coherent with astronomical tidal forcing, and other high-frequency fluctuations. These motions contribute similarly to the overall variability at this location. Low-frequency currents exhibit elevated energy at periods of similar to 30 days, consistent with previous observations elsewhere in the abyssal North Pacific. However, unlike some other observations in the deep ocean, there is no consistent seasonal cycle in the variability of low-frequency motion. The theoretical prediction of Sverdrup balance between transport and local wind stress curl does not explain the timing or the magnitude of low frequency currents, suggesting that they are driven by remote winds or eddy dynamics. The non-tidal high-frequency fluctuations show evidence of a near-inertial peak and clockwise rotary motion, consistent with theoretical expectations for the internal gravity wave spectrum. Like the low-frequency currents, interannual variability of internal wave energy appears to be stronger than seasonal variability. Observations of the vertical structure 2.5-17.5 m above bottom are used to characterize the interactions between currents and the seabed. Observational estimates of diurnal and semidiurnal tidal ellipses are consistent with results from an analytical model that incorporates the effects of the Earth's rotation and tidal acceleration. A numerical model of the time dependent boundary layer provides estimates of bottom roughness and friction velocity that are consistent with previous observations at this site. This study shows that the strength, time scale and polarization of the near-bottom currents at Station M vary significantly between years, demonstrating the value of multi-year records in studies of the lateral transport of organic matter, sediment and organisms at this abyssal site.
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