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1.
We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O2>4.2 ml l?1), and lowest in silica (Si<135 μmol kg?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=106 m3 s?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.  相似文献   

2.
Full-depth conductivity-temperature-depth-oxygen profiler (CTDO2) data at low latitudes in the western North Pacific in winter 1999 were analyzed with water-mass analysis and geostrophic calculations. The result shows that the deep circulation carrying the Lower Circumpolar Water (LCPW) bifurcates into eastern and western branch currents after entering the Central Pacific Basin. LCPW colder than 0.98°C is carried by the eastern branch current, while warmer LCPW is carried mainly by the western branch current. The eastern branch current flows northward in the Central Pacific Basin, supplying water above 0.94°C through narrow gaps into an isolated deep valley in the Melanesian Basin, and then passes the Mid-Pacific Seamounts between 162°10′E and 170°10′E at 18°20′N, not only through the Wake Island Passage but also through the western passages. Except near bottom, dissolved oxygen of LCPW decreases greatly in the northern Central Pacific Basin, probably by mixing with the North Pacific Deep Water (NPDW). The western branch current flows northwestward over the lower Solomon Rise in the Melanesian Basin and proceeds westward between 10°40′N and 12°20′N at 150°E in the East Mariana Basin with volume transport of 4.1 Sv (1 Sv=106 m3 s−1). The current turns north, west of 150°E, and bifurcates around 14°N, south of the Magellan Seamounts, where dissolved oxygen decreases sharply by mixing with NPDW. Half of the current turns east, crosses 150°E at 14–15°N, and proceeds northward primarily between 152°E and 156°E at 18°20′N toward the Northwest Pacific Basin (2.1 Sv). The other half flows northward west of 150°E and passes 18°20′N just east of the Mariana Trench (2.2 Sv). It is reversed by a block of topography, proceeds southward along the Mariana Trench, then detours around the south end of the trench, and proceeds eastward along the Caroline Seamounts to the Solomon Rise, partly flowing into the West Mariana and East Caroline Basins. A deep western boundary current at 2000–3000 m depth above LCPW (10.0 Sv) closes to the coast than the deep circulation. The major part of it (8.5 Sv) turns cyclonic around the upper Solomon Rise from the Melanesian Basin and proceeds along the southern boundary of the East Caroline Basin. Nearly half of it proceeds northward in the western East Caroline Basin, joins the current from the east, then passes the northern channel, and mostly enters the West Caroline Basin (4.6 Sv), while another half enters this basin from the southern side (>3.8 Sv). The remaining western boundary current (1.5 Sv) flows over the middle and lower Solomon Rise, proceeds westward, then is divided by the Caroline Seamounts into southern (0.9 Sv) and northern (0.5 Sv) branches. The southern branch current joins that from the south in the East Caroline Basin, as noted above. The northern branch current proceeds along the Caroline Seamounts and enters the West Mariana Basin.  相似文献   

3.
Pacific ocean circulation based on observation   总被引：2，自引：1，他引：1  
A thorough understanding of the Pacific Ocean circulation is a necessity to solve global climate and environmental problems. Here we present a new picture of the circulation by integrating observational results. Lower and Upper Circumpolar Deep Waters (LCDW, UCDW) and Antarctic Intermediate Water (AAIW) of 12, 7, and 5 Sv (106 m3s−1) in the lower and upper deep layers and the surface/intermediate layer, respectively, are transported to the North Pacific from the Antarctic Circumpolar Current (ACC). The flow of LCDW separates in the Central Pacific Basin into the western (4 Sv) and eastern (8 Sv) branches, and nearly half of the latter branch is further separated to flow eastward south of the Hawaiian Ridge into the Northeast Pacific Basin (NEPB). A large portion of LCDW on this southern route (4 Sv) upwells in the southern and mid-latitude eastern regions of the NEPB. The remaining eastern branch joins nearly half of the western branch; the confluence flows northward and enters the NEPB along the Aleutian Trench. Most of the LCDW on this northern route (5 Sv) upwells to the upper deep layer in the northern (in particular northeastern) region of the NEPB and is transformed into North Pacific Deep Water (NPDW). NPDW shifts southward in the upper deep layer and is modified by mixing with UCDW around the Hawaiian Islands. The modified NPDW of 13 Sv returns to the ACC. The remaining volume in the North Pacific (11 Sv) flows out to the Indian and Arctic Oceans in the surface/intermediate layer.  相似文献   

4.
Larval transport from distant populations is essential for maintenance and renewal of populations in patchy and disturbed ecosystems such as deep-sea hydrothermal vents. We use quasi-geostrophic modeling to consider the potential for long-distance dispersal of hydrothermal vent larvae in mesoscale eddies interacting with the northern East Pacific Rise. Modeled eddy dynamics were similar to the observed propagation dynamics of Tehuantepec eddies, including their ability to cross the ridge. Simulated surface anticyclones were associated with coherent cyclones in the deep layer with relatively strong current velocities that could significantly increase the dispersal potential of passive particles. Eddy interactions with ridge topography further enhanced tracer dispersal along the ridge axis through shearing and elongation of the eddy core. Simulations suggest that the passage of an eddy would result in local loss from the vent field and aggregate transport with potential enhancement of dispersal between vent fields separated by up to 270 km. Based on the latitude at which most Tehuantepec eddies cross the ridge, eddy-induced flows would enhance connectivity between the 13°N, 11°N, and 9°N vent fields along the East Pacific Rise asymmetrically with higher transport from northern vent fields to southern vent fields.  相似文献   

5.
Rose-Bengal-stained benthic foraminifera in six pilot-core samples and one multicore sample collected from the Hess Rise and Suiko Seamount in August 1994 were studied in order to understand foraminiferal distributions between two areas divided by an oceanic front in the central North Pacific. Samples from the Hess Rise were collected in depths of 2167–3354 m under the warm, saline Kuroshio Extension, while samples from Suiko Seamount came from depths of 1811–1955 m under the cold, less-saline subarctic current. Sediment-trap results for the year prior to our sediment sampling show that organic matter fluxes were about 2.5 times greater at Suiko Seamount than at the Hess Rise. However, the hydrographic structure between 1800 and 3400 m, based on CTD observations, is almost the same at both sites. Temperature decreases from 2.2 to 1.7°C over the depth range of 1800–3400 m, salinity increases from 34.5 to 34.7, and the dissolved oxygen content gradually increases from 1.5 to 3.0 ml l−1. The faunal populations at the Hess Rise are quite different from those at Suiko Seamount. The abundant species at the Hess Rise are Epistominella exigua, Brizalina pacifica, Fursenkoina cedrosensis and Alabaminella weddellensis. These species characteristically inhabit phytodetrital aggregates deposited on an oligotrophic seafloor. The populations at Suiko Seamount are dominated by Triloculina frigida, Lagenammina cf. arenulata, Reophax subfusiformis, and Reophax scorpiurus. The reason for differences between these populations is unclear. However, the typical phytodetritus-dwelling species E. exigua is dominant at the Hess Rise, which is located in a subtropical area that has a pulsed supply of settling organic matter in the spring. On the other hand, E. exigua is rare at Suiko Seamount, a subarctic site where there are more stable and greater fluxes of organic matter in summer and autumn. Occurrences of this species may be related to the seasonally short supply of organic matter that reaches the seafloor in the oceanic North Pacific.  相似文献   

6.
Direct velocity measurements undertaken using a nine-system mooring array (M1–M9) from 2004 to 2005 and two additional moorings (M7p and M8p) from 2003 to 2004 reveal the spatial and temporal properties of the deep-circulation currents southwest of the Shatsky Rise in the western North Pacific. The western branch of the deep-circulation current flowing northwestward (270–10° T) is detected almost exclusively at M2 (26°15′N), northeast of the Ogasawara Plateau. It has a width less than the 190 km distance between M1 (25°42′N) and M3 (26°48′N). The mean current speed near the bottom at M2 is 3.6±1.3 cm s?1. The eastern branch of the deep-circulation current is located at the southwestern slope of the Shatsky Rise, flowing northwestward mainly at M8 (30°48′N) on the lower part of the slope of the Shatsky Rise with a mean near-bottom speed of 5.3±1.4 cm s?1. The eastern branch often expands to M7 (30°19′N) at the foot of the rise with a mean near-bottom speed of 2.8±0.7 cm s?1 and to M9 (31°13′N) on the middle of the slope of the rise with a speed of 2.5±0.7 cm s?1 (nearly 4000 m depth); it infrequently expands furthermore to M6 (29°33′N). The width of the eastern branch is 201±70 km on average, exceeding that of the western branch. Temporal variations of the volume transports of the western and eastern branches consist of dominant variations with periods of 3 months and 1 month, varying between almost zero and significant amount; the 3-month-period variations are significantly coherent to each other with a phase lag of about 1 month for the western branch. The almost zero volume transport occurs at intervals of 2–4 months. In the eastern branch, volume transport increases with not only cross-sectional average current velocity but also current width. Because the current meters were too widely spaced to enable accurate estimates of volume transport, mean volume transport is overestimated by a factor of nearly two, yielding values of 4.1±1.2 and 9.8±1.8 Sv (1 Sv=106 m3 s?1) for the western and eastern branches, respectively. In addition, a northwestward current near the bottom at M4 (27°55′N) shows a marked variation in speed between 0 and 20 cm s?1 with a period of 45 days. This current may be part of a clockwise eddy around a seamount located immediately east of M4.  相似文献   

7.
We conducted full-depth hydrographic observations in the southwestern region of the Northwest Pacific Basin in September 2004 and November 2005. Deep-circulation currents crossed the observation line between the East Mariana Ridge and the Shatsky Rise, carrying Lower Circumpolar Deep Water westward in the lower deep layer (θ<1.2 °C) and Upper Circumpolar Deep Water (UCDW) and North Pacific Deep Water (NPDW) eastward in the upper deep layer (1.3–2.2 °C). In the lower deep layer at depths greater than approximately 3500 m, the eastern branch current of the deep circulation was located south of the Shatsky Rise at 30°24′–30°59′N with volume transport of 3.9 Sv (1 Sv=106 m3 s−1) in 2004 and at 30°06′–31°15′N with 1.6 Sv in 2005. The western branch current of the deep circulation was located north of the Ogasawara Plateau at 26°27′–27°03′N with almost 2.1 Sv in 2004 and at 26°27′–26°45′N with 2.7 Sv in 2005. Integrating past and present results, volume transport southwest of the Shatsky Rise is concluded to be a little less than 4 Sv for the eastern branch current and a little more than 2 Sv for the western branch current. In the upper deep layer at depths of approximately 2000–3500 m, UCDW and NPDW, characterized by high and low dissolved oxygen, respectively, were carried eastward at the observation line by the return flow of the deep circulation composing meridional overturning circulation. UCDW was confined between the East Mariana Ridge and the Ogasawara Plateau (22°03′–25°33′N) in 2004, whereas it extended to 26°45′N north of the Ogasawara Plateau in 2005. NPDW existed over the foot and slope of the Shatsky Rise from 29°48′N in 2004 and 30°06′N in 2005 to at least 32°30′N at the top of the Shatsky Rise. Volume transport of UCDW was estimated to be 4.6 Sv in 2004, whereas that of NPDW was 1.4 Sv in 2004 and 2.6 Sv in 2005, although the values for NPDW may be slightly underestimated, because they do not include the component north of the top of the Shatsky Rise. Volume transport of UCDW and NPDW southwest of the Shatsky Rise is concluded to be approximately 5 and 3 Sv, respectively. The pathways of UCDW and NPDW are new findings and suggest a correction for the past view of the deep circulation in the Pacific Ocean.  相似文献   

8.
In this paper we use a temperature and salinity based mixing model to assess the dilution of Antarctic Bottom Water (AABW) as it moves away from the Weddell Sea and into the Southwest Indian Ocean. By combining these results with CFC tracer measurements we have been able to make direct estimates of the large-scale translation rates of AABW in this region. We confirm that there is a major northward flow of AABW via a gap in the Southwest Indian Ridge at 30°E, and thence across the Agulhas Basin into the Mozambique Basin, with a translation rate from the Greenwich Meridian of 0.8–1.0 cm s−1 and a volume transport between the two basins of 1.5×106 m3 s−1. A second, smaller flow cuts the Del Cano Rise through the Prince Edward Fracture Zone but is indistinguishable from the general bottom waters once on the northern side of the rise. The third flow moves eastward along the southern flank of the Del Cano Rise to pass north of the Conrad Rise. This has bottom velocities of 0.7 cm s−1 and a volume transport of 1.6×106 m3 s−1. This water is probably the source of the AABW-rich Circumpolar Deep Water that flows through the gap to the west of Crozet Island, and which is traceable again at stations on the northern flanks of the ridge. Flow between the Conrad Rise and the Del Cano Rise is complicated by the influence of a fourth flow, the AABW that passes south of the former and thence into the Crozet Basin via the Crozet-Kerguelen Gap. We suggest that a portion of this flow loops into the channel between the Del Cano Rise and the Conrad Rise, modifying the bottom waters at the easternmost stations within this channel. We will go on in Part 2 of this paper to use these results to estimate the dissolution rates of silica in the SWINDEX area.  相似文献   

9.
From August 2002 to September 2004 a high-resolution mooring array was maintained across the western Arctic boundary current in the Beaufort Sea north of Alaska. The array consisted of profiling instrumentation, providing a timeseries of vertical sections of the current. Here we present the first-year velocity measurements, with emphasis on the Pacific water component of the current. The mean flow is characterized as a bottom-intensified jet of O (15 cm s−1) directed to the east, trapped to the shelfbreak near 100 m depth. Its width scale is only 10–15 km. Seasonally the flow has distinct configurations. During summer it becomes surface-intensified as it advects buoyant Alaskan Coastal water. In fall and winter the current often reverses (flows westward) under upwelling-favorable winds. Between the storms, as the eastward flow re-establishes, the current develops a deep extension to depths exceeding 700 m. In spring the bottom-trapped flow advects winter-transformed Pacific water emanating from the Chukchi Sea. The year-long mean volume transport of Pacific water is 0.13±0.08 Sv to the east, which is less than 20% of the long-term mean Bering Strait inflow. This implies that most of the Pacific water entering the Arctic goes elsewhere, contrary to expected dynamics and previous modeling results. Possible reasons for this are discussed. The mean Atlantic water transport (to 800 m depth) is 0.047±0.026 Sv, also smaller than anticipated.  相似文献   

10.
Five moorings ML1–ML5 were deployed on the slope of the Solomon Rise in the Melanesian Basin in the western North Pacific, northeastward at increasing water depths. We measured the velocities of the western branch current of the deep western boundary current (DWBC) and the upper deep current carrying the Lower and Upper Circumpolar Waters (LCPW, UCPW), respectively. The daily mean velocity data from 1–3 February 1999 to 24–26 February 2000 were analyzed, and variability of the DWBCs was clarified. Although the current meters did not entirely cover the western branch current of the DWBC composed of two or three streams, a stream of the western branch current was observed at a depth of 4700 m at ML4 or 4260 m at ML5 for more than half of the observation period. The stream had a mean velocity of 3.7 cm s−1 and alternated between ML4 and ML5 at 20- to 40-day intervals without occupying both of ML4 and ML5 simultaneously. This shows that the width of the stream is less than 120 km (distance between ML4 and ML5), and the position changes in a similar range. In contrast to the velocity of the eastern branch current of the DWBC, that of the western branch current did not decrease with decreasing depths to 4000 m. This reflects the vertical division into the branch currents by the bifurcation of the DWBC. The western branch current of the DWBC is located at the deep side of the countercurrent which was almost always observed at depths of 3880 and 4080 m at ML3. The countercurrent was thought to be the return flow of the western branch current that is partly reversed in the East Mariana Basin. The previous estimate of geostrophic transport of LCPW at the time of the mooring deployment was corrected to 1.4 Sv (106 m3 s−1) in the western branch current, 1.7 Sv in the countercurrent, and 1.1 Sv in the inflow to the East Caroline Basin. The upper deep current was located over the slope of the Solomon Rise with water depth less than 4500 m including ML1–ML3. It flowed at depths of approximately 2000–3500 m with the highest velocity in the middle of this layer and seldom reached the near-bottom where eddy-like disturbances existed. Its volume transport at the mooring deployment was 10.4 Sv. The upper deep current during the first half of the observation period had double cores divided by the countercurrent at ML1, whereas that during the second half had a single core, as the countercurrent at ML1 disappeared in early September 1999. The vector mean velocities of the upper deep current were 5.0 (2650 m, ML2) and 3.6 cm s−1 (1880 m, ML3) during the first half of the observation period and 7.0 cm s−1 (2670 m, ML1) during the second half; they ranged from 3 to 7 cm s−1. Similarly, those of the countercurrent at ML1 during the first half were 6.4, 3.8, 4.6 cm s−1 (2170, 2670, 3570 m).  相似文献   

11.
To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.  相似文献   

12.
Echograms (3.5 kHz) and bottom photographs reveal that the northward flowing Antarctic Bottom Water (AABW) has strongly influenced the modern depositional regime on the southwest Bermuda Rise. The spatial distribution of echo character types, the orientation and nature of current-controlled structures, and limited current meter data show that AABW flows with varying intensities along three primary pathways around and over the southwest Bermuda Rise. The main core of AABW flows clockwise around the eastern and western flanks of the southern Bermuda Rise, roughly parallel to the 5400 m isobath. This current bifurcates at 28°30′N, 69°W where a portion flows northeast over the southwest Bermuda Rise and the remainder continues north along the physiographic boundary between the southwest Bermuda Rise and the Hatteras Abyssal Plain. Secondary ribbons of AABW branch off the main core of AABW during its southerly journey along the southeastern Bermuda Rise, and flow west through fracture zones. Finally, a diffuse, northward flowing AABW sweeps the entire southwest Bermuda Rise.

A progression of current-controlled bedforms occurs beneath the main path of the AABW reflecting the spatially varying current velocities and sediment supply. The main core of AABW flows west through the narrow Vema Gap creating erosional furrows along the border between the southwest Bermuda Rise and the Vema Gap. Current velocities greater than 20 cm s−1 are inferred from the bedforms in this region. Farther north along the southwestern edge of the Bermuda Rise, sediment waves become more prevalent. This transition from erosional to more depositional bedforms results from diminished current velocities (5–15 cm s−1) and increased sediment supply. Although some of these bedforms on the southwest Bermuda Rise appear to be relict, their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs.  相似文献   


13.
The effects of tropical instability waves (TIW) within the eastern equatorial Pacific during the boreal fall of 2005 were observed in multiple data sets. The TIW cause oscillations of the sea surface temperature (SST), meridional currents (V), and 20 °C isotherm (thermocline). A particularly strong 3-wave packet of ～15-day period TIW passed through the Galápagos Archipelago in Sep and Oct 2005 and their effects were recorded by moored near-surface sensors. Repeat Argo profiles in the archipelago showed that the large temperature (>5 °C) oscillations that occurred were associated with a vertical adjustment within the water column. Numerical simulations report strong oscillations and upwelling magnitudes of ～5.0 m d?1 near the Tropical Atmosphere Ocean (TAO) buoy at 0°, 95°W and in the Archipelago at 92°W and 90°W. A significant biological response to the TIW passage was observed within the archipelago. Chlorophyll a measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) increased by >30% above 1998–2007 mean concentrations within the central archipelago. The increases coincide with coldest temperatures and the much larger increases within the archipelago as compared to those of 95°W indicate that TIW induced upwelling over the island platform itself brought more iron-enriched upwelling waters into the euphotic zone.  相似文献   

14.
Recent hydrographic measurements within the eastern South Pacific (1999–2001) were combined with vertically high-resolution data from the World Ocean Circulation Experiment, high-resolution profiles and bottle casts from the World Ocean Database 2001, and the World Ocean Atlas 2001 in order to evaluate the vertical and horizontal extension of the oxygen minimum zone (<20 μmol kg−1). These new calculations estimate the total area and volume of the oxygen minimum zone to be 9.82±3.60×106 km2 and 2.18±0.66×106 km3, respectively. The oxygen minimum zone is thickest (>600 m) off Peru between 5 and 13°S and to about 1000 km offshore. Its upper boundary is shallowest (<150 m) off Peru, shoaling towards the coast and extending well into the euphotic zone in some places. Offshore, the thickness and meridional extent of the oxygen minimum zone decrease until it finally vanishes at 140°W between 2° and 8°S. Moving southward along the coast of South America, the zonal extension of the oxygen minimum zone gradually diminishes from 3000 km (15°S) to 1200 km (20°S) and then to 25 km (30°S); only a thin band is detected at ∼37°S off Concepción, Chile. Simultaneously, the oxygen minimum zone's maximum thickness decreases from 300 m (20°S) to less than 50 m (south of 30°S). The spatial distribution of Ekman suction velocity and oxygen minimum zone thickness correlate well, especially in the core. Off Chile, the eastern South Pacific Intermediate Water mass introduces increased vertical stability into the upper water column, complicating ventilation of the oxygen minimum zone from above. In addition, oxygen-enriched Antarctic Intermediate Water clashes with the oxygen minimum zone at around 30°S, causing a pronounced sub-surface oxygen front. The new estimates of vertical and horizontal oxygen minimum zone distribution in the eastern South Pacific complement the global quantification of naturally hypoxic continental margins by Helly and Levin [2004. Global distribution of naturally occurring marine hypoxia on continental margins. Deep-Sea Research I 51, 1159–1168] and provide new baseline data useful for studies on the role of oxygen in the degradation of organic matter in the water column and the related implications for biogeochemical cycles. Coastal upwelling zones along the eastern Pacific combine with general circulation to provide a mechanism that allows renewal of upper Pacific Deep Water, the most oxygen-poor and oldest water mass of the world oceans.  相似文献   

15.
Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (σ1.5=34.70 kg m-3) was observed eastward as far as 5°W. The deep CFM maximum (σ4=45.87 kg m-3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water “ages” computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.  相似文献   

16.
A hydrographic section between Tasmania and Antarctica was occupied in late winter 1991 as part of the World Ocean Circulation Experiment (WOCE). The primary purpose of the WOCE repeat section SR3 is to measure the exchange between the Indian and Pacific Oceans south of Australia. This paper describes the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50°S and 51°S and is the most striking feature of the vertical sections. Two additional fronts at 53°S and 59°S are associated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53°S. Very deep (>500 m) mixed layers are found north of the SAF, confirming that Subantarctic Mode Water (SAMW) is formed in this region by deep convection in winter. Chlorofluorocarbons (CFCs) are significantly undersaturated (≈90–92% of equilibrium values) in these deep mixed layers, indicating that gas exchange rates are not rapid enough to bring these deep mixed layers to equilibrium by the end of the winter period of deep convective mixing. Northward Ekman drift of cold, fresh water across the SAF is likely to be responsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is relatively low in oxygen and CFCs (≈60–70% and 10–20% of saturation values, respectively), high in potential vorticity, and high in nutrients. These characteristics suggest that the AAIW on this section is not renewed by direct and rapid ventilation near this location. Water mass properties suggest that water from the Tasman Sea spreads south and west across the northern portion of the SR3 section between 800 and 3000 m depth. A cold, fresh, CFC-rich variety of Antarctic Bottom Water is formed along the Wilkes-Adelie coast of Antarctica. The net transport across the section relative to the deepest common depth is 160 Sv. The band of eastward flow between 50°S and 53°S including the SAF carries 137 Sv to the east and dominates the net transport. Weaker flow south of 58°S contributes an additional 70 Sv. The eastward flow is compensated in part by 37 Sv of westward flow between Tasmania and 48.5°S and 8 Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six ALACE floats deployed at about 950 m confirm the sense of flow inferred from the choice of a deep reference level.  相似文献   

17.
On 12 November 2006, 3 kg of sulfur hexafluoride were released in a 1.2 km long streak in the axial summit trough of the East Pacific Rise at 9°30′N to study how circulation and mixing affect larval dispersion. The first half of a tracer survey performed approximately 40 days after the injection found a small percentage of the tracer on the ridge axis between 9°30′N and 10°10′N, with the main concentration near 9°50′N, a site of many active hydrothermal vents. These observations provide evidence of larval connectivity between vent sites on the ridge. The latter half of the survey detected the primary patch of tracer west of the ridge and just south of the Lamont Seamounts, as a majority of the tracer had been transported off the ridge. However, by the end of the survey, the eastern edge of this patch was transported back to within 10 km of the ridge crest at 9°50′N by a reversal in the subinertial flow, suggesting another pathway for larvae between points along the ridge. Both the horizontal and vertical distributions of the tracer were complex and were likely heavily influenced by topography and vents in the area. Elevated tracer concentrations within the axial summit trough and an adjacent depression on the upper ridge flank suggest that tracers may be detained in such depressions. Correlated tracer/turbidity profiles provide direct evidence of entrainment of the tracer into vent plumes from 9°30′N to 10°N. A comparison of the vertical tracer inventory with neutral density vent-plume observations suggests that on the order of 10% of the tracer injected was entrained into vent plumes near the injection site. The results imply that effluent from diffuse hydrothermal sources and larvae of hydrothermal vent fauna can be entrained in significant quantities into plumes from discrete sources and dispersed in the neutrally buoyant plumes.  相似文献   

18.
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19.
Very large subaqueous sand dunes were discovered on the upper continental slope of the northern South China Sea. The dunes were observed along a single 40 km long transect southeast of 21.93°N, 117.53°E on the upper continental slope in water depths of 160 m to 600 m. The sand dunes are composed of fine to medium sand, with amplitudes exceeding 16 m and crest-to-crest wavelengths exceeding 350 m. The dunes' apparent formation mechanism is the world's largest observed internal solitary waves which generate from tidal forcing on the Luzon Ridge on the east side of the South China Sea, propagate west across the deep basin with amplitudes regularly exceeding 100 m, and dissipate extremely large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. While subaqueous dunes are found in many locations throughout the world's oceans and coastal zones, these particular dunes appear to be unique for two principal reasons: their location on the upper continental slope (away from the influence of shallow-water tidal forcing, deep basin bottom currents and topographically-amplified canyon flows), and their distinctive formation mechanism (approximately 60 episodic, extremely energetic, large amplitude events each lunar cycle).  相似文献   

20.
The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically tracked, covering a period from December 1992 to October 1996. Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5°S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s-1. At the Brazil–Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.  相似文献   

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