Potential Temperature Salinity Research Articles

K nearest neighbors classification of water masses in the western Alboran Sea using the sigma-pi diagram

Different classification techniques of water masses have been developped using the potential temperature-salinity (θ-S) diagram and its volumetric analysis. In this study, we propose a new method to automatically classify water masses via a supervised machine learning algorithm based on the K nearest neighbors (Knn), in the potential density and potential spicity (σ-π) coordinates. This method is applied to temperature and salinity data collected in the western side of the Alboran Sea during a glider mission, dedicated to sample the Western Alboran Gyre (WAG) in late winter 2021. The water masses in the studied region were classified into five different categories following a supervised learning process, based on ocean profile databases available on the region of interest. The results corroborate previous studies of the spatial distribution of water masses in the Alboran Sea, inferred from traditional method based on the expert analysis of the (θ-S) diagram, and suggest that this methodology is efficient and reliable for water masses classification. Compared to a classical clustering computation (herein k-means), this method is more appropriate in a region where the characteristics of the water masses change considerably in both space and time.

Water masses constrain the distribution of deep-sea sponges in the North Atlantic Ocean and Nordic Seas

Water masses are bodies of water with distinctive physical and biogeochemical properties. They impart vertical structure to the deep ocean, participate in circulation, and can be traced over great distances, potentially influencing the distributions of deep-sea fauna. The classic potential temperature-salinity (θ-s) diagram was used to investigate deep-sea sponge (demosponge genus Geodia) association with water masses over the North Atlantic Ocean and Nordic Seas. A novel analysis was conducted, based on sampling the curvature of climatological mean θ-s curves at sponge locations. Sponges were particularly associated with turning points in the θ-s curves, indicative of intermediate and deep water masses. Arctic geodiid species (G. hentscheli and G. parva) were associated with Arctic intermediate and deep waters in the Nordic Seas, and with dense overflows into the northern North Atlantic. Boreal species (G. atlantica, G. barretti, G. macandrewii, and G. phlegraei) were associated with upper and intermediate water masses in the Northeast Atlantic and with upper, Atlantic-derived waters in the Nordic Seas. Taken together with distributional patterns, a link with thermohaline currents was also inferred. We conclude that water masses and major current pathways structure the distribution of a key deep-sea benthic faunal group on an ocean basin scale. This is most likely because of a combination of the physical constraints they place on the dispersal of early life-history stages, ecophysiological adaptation (evolved tolerances) to specific water masses, and the benefits to filter-feeders of certain phenomena linked to water column structure (e.g. nepheloid layers, internal waves/tides, density-driven currents).

Origin of the springtime South China Sea Warm Current in the southwestern Taiwan Strait: Evidence from seawater oxygen isotope

Oxygen isotope (δ18O) of seawater is an excellent proxy for tracing the origins of water masses and their mixing processes. Combining with hydrographic observation, hybrid coordinate ocean model (HYCOM) analysis data, and seawater oxygen isotope, we investigated the source of the South China Sea Warm Current (SCSWC) in the southwestern Taiwan Strait and its underlying mechanism. Results show that the Kuroshio subsurface water (KSSW) can intrude the continental slope in the southwestern Taiwan Strait, and thereby climb up the continental slope coupled with upwelling. The δ18O-salinity relationship further indicates that in spring, the SCSWC in the southwestern Taiwan Strait originates from the upslope deflection of the slope current formed by the KSSW intrusion into the South China Sea, rather than from the west segment of the SCSWC formed to the east of Hainan Island. In addition, the southward flowing Zhe-Min Coastal Current (ZMCC) can reach as far as the Taiwan Bank (TB) and deflects offshore over the western TB at approximately 23.5°N, to some extent affecting the SCSWC. Moreover, this study reveals that seawater δ18O is exquisitely sensitive to the determination of the origin and transport of water masses as compared with traditional potential temperature-salinity plot (θ-S) and HYCOM analysis data. In addition, their coupling can more reliably interpret the mixing processes of shelf water masses.

Cyclonic eddies and upper thermocline fine-scale structures in the Antarctic Circumpolar Current

Mesoscale eddies in the open ocean are mostly formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is converted into the kinetic energy of the flow around the eddy. As a permissible form of motion within a rapidly rotating and stratified fluid eddies driven by baroclinic instability are important for the poleward and vertical transport, not only of physical properties, but also biogeochemical ones. In this paper, we present observations from four cyclonic eddies in the Antarctic Circumpolar Current. We have sorted them by apparent age, based on altimeter data and consideration of the degree of homogenisation of the potential temperature-salinity(\U0001d703S) relationship, and then looked at the spatial distribution of measures of fine-scale variability in the upper thermocline. The youngest eddy shows isopycnals which are domed upwards and it contains a variety of waters with differing temperature-salinity characteristics. The fine-scale variability is higher in the core of the eddy. The older eddies show a core which is more homogeneous in potential temperature and salinity. The isopycnals are flatter in the centre of the eddy, and in cross-section, they can be M-shaped, so that the steepest gradients are concentrated around the edge. The fine-scale variability is more concentrated around the edges where the density gradients are stronger. We hypothesise that lateral stirring and mixing processes within the eddy homogenise the water so that the temperature-salinity relationship becomes tighter. When the eddy eventually collapses, this modified water can be released back into the flow. Thus, we see how the interplay of mesoscale and small-scale processes are modifying water mass properties and, potentially, regulate biogeochemical processes.

Assessment of the representation of Antarctic Bottom Water properties in the ECCO2 reanalysis

Abstract. We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of Antarctic Bottom Water (AABW) in the Southern Ocean. We used a 20-year (1992–2011) observational database to perform comparisons of hydrographic properties and reanalysis output for the same time period. Four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature were also evaluated. The opening and maintenance of an oceanic polynya in the Weddell Sea sector is observed after 2004 in the reanalysis product. Moreover, intense deep water production due to deep convection occurs, which leads to a scenario in which the Weddell Sea is flooded with AABW. For this reason, our analyses focused on the period that was identified as more reliable (1992–2004). The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–S) diagrams, are coincident with observations. However, the reanalysis output produces surface waters that are generally denser than observations due to the reproduction of waters that are generally saltier than expected, which probably resulted from the strong seasonality of sea ice concentrations. Bottom waters are warmer and less dense, while intermediate waters are statistically closest to the observations. The differences in bottom water properties are partially due to the inability of the reanalysis to properly reproduce the formation and export of dense waters from the shelf and the consequent absence of the densest AABW variety for most of the analyzed period. Despite differences in the absolute values, the upper AABW limit (γn ≥ 28.27 kg m−3) and AABW occupied area estimates are coincident with the observations in the World Ocean Circulation Experiment (WOCE) repeat sections SR2 and SR4. Moreover, the AABW volume export and current velocity variability are correlated with the observed time series in the most important region of dense water export (i.e., the Weddell Sea). Despite the consistency in terms of variability, the absolute volume transport and velocity estimates are underrepresented in all cases.

Wyville Thomson Ridge Overflow Water: Spatial and temporal distribution in the Rockall Trough

Wyville Thomson Ridge Overflow Water (WTOW), which is the only part of the outflow from the Norwegian Sea not to directly enter the Iceland Basin, is shown to be a significant water mass in the northern Rockall Trough. It is found primarily at intermediate depths (600–1200 m) beneath the northward flowing warm Atlantic waters, and above recirculating Mediterranean influenced waters and Labrador Sea Water (LSW). The bottom of the WTOW layer can be identified by a mid-depth inflexion point in potential temperature–salinity plots. An analysis of historical data reveals that WTOW has been present in all but eight of the last 31 years at 57.5°N in the Rockall Trough. A denser component of WTOW below 1500 m has also been present, although it appears to be less persistent (12 out of the 31 years) and limited to the west of the section. The signature of intermediate WTOW was absent in two periods, the mid-1980s and early 1990s, both of which coincided with a freshening, and probable increase in volume, of LSW in the trough. Potential temperature–salinity diagrams from historical observations indicate that WTOW persists at least as far south as 55°N (and as far west as 20°W in the Iceland Basin) although its signature is quickly lost on leaving the Rockall Trough. We suggest that a transport of WTOW down the western side of the trough exists, with WTOW at intermediate depths entering the eastern trough either via a cyclonic recirculation, or as a result of eddy activity. Further, WTOW is seen on the Rockall–Hatton Plateau and in the deep channels connecting with the Iceland Basin, suggesting additional possible WTOW transport pathways. These suggested transport routes remain to be confirmed by further observational or modelling studies.

Warming and Freshening in the Abyssal Southeastern Indian Ocean*

Abstract Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

In Situ Calibration of Moored CTDs Used for Monitoring Abyssal Water

Abstract To monitor changes in heat content and geostrophic volume transport of abyssal water accurately, 50 moored conductivity–temperature–depth (CTD) recorders used for density measurements were calibrated in situ by simultaneous observations with accurate shipboard CTDs. Comparisons of the data from the moored and shipboard CTDs showed pressure sensitivities of 0–3 mK at 6000 dbar for the temperature sensors of the moored CTDs. From the in situ calibrations, the uncertainties of the moored CTD data for the deep ocean (≥3000 dbar) were estimated to be 0.6 dbar, 0.6 mK, and 0.0026 for pressure, temperature, and salinity, respectively, relative to the shipboard CTD reference. Time drifts of the moored CTD data, estimated from the in situ calibrations before and after 17- or 14-month mooring deployments in the deep ocean, were considerably smaller than typical stabilities as specified by the manufacturer. However, time drifts of the pressure sensors tended to be negative and the result suggests that pressure data from most present Argo floats, which use the same type of pressure sensor, may have a systematic negative bias. Time series salinity data calculated from the in situ–calibrated CTDs were slightly biased (mean of +0.0014) with respect to the shipboard CTD salinity data, based on potential temperature–salinity relationships, possibly due to a disequilibrium of the moored CTD conductivity sensors during the in situ calibrations.

Water mass properties and chemical characteristics in the Saros Gulf, Northeast Aegean Sea (Eastern Mediterranean)

This work is a description of the water masses and chemical properties in the Northeast (NE) Aegean Sea, a region of recently renewed interest with respect to the hydrology of the Eastern Mediterranean Basin. Analysis of hydrological data from the study area reveals the presence of three water masses: The 20–40 m thick surface layer is occupied mainly by Black Sea Water, BSW, modified on its way through the Bosphorus, the Marmara Sea and the Dardanelles. Below the surface layer there is warm and highly saline water originating from the South Aegean and the Levantine basins, Levantine Intermediate Water (LIW), extending down to 350–400 m depth. Below this layer, the deeper than 400 m basins of the North Aegean contain locally formed, very dense water with different Potential temperature–Salinity characteristics at each sub-basin and called North Aegean Deep Water (NADW). Evidence of temperature-controlled dense water formation of the Gulf of Saros, in the NE Aegean was reported in January 2002. The nutrient concentrations varied between nitrate + nitrite = 0.19–0.37 µM, phosphate = 0.01–0.02 µM and silicate = 0.7–1.1 µM for the surface layer (BSW) during sampling periods. Dissolved nutrient concentrations in the NADW were: 1.7–3.7 µM for NO 3 + NO 2, 0.08–0.17 µM for PO 4 and 2.7–4.2 µM for silicate. During winter, relatively high nutrient levels are observed in the surface layers of the NE Aegean Sea, whilst phosphate and nitrates decrease in summer.

Frontal structure and transport in the northwestern Weddell Sea

Hydrographic data from the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project are analyzed to determine the frontal structure and transport along a section across the continental shelf and slope in the northwestern Weddell Sea. The flow is dominated by three barotropic northward flowing currents: the Antarctic Coastal Current, the Antarctic Slope Front and the Weddell Front. The strongest baroclinic flows are confined to the region between the Slope Front and the Weddell Front over the steepest part of the continental slope. The Antarctic Coastal Current flows over the continental shelf near a local steepening in the bathymetry and has a transport of ∼ 1.3 Sv . The Antarctic Slope Front is found approximately 25 km offshore of the shelf break in 800 m of water. The Slope Front, which is associated with a transport of ∼ 4 Sv , exhibits peak velocities above the bottom that reach 35 cm s - 1 as detected by lowered acoustic Doppler profiler (LADCP) measurements. A third northward current is found between the 2500 and 3000 m isobaths, corresponding to a local break in the topography. Potential temperature–salinity diagrams show that the change in water mass properties across the deep front is similar to the change found across the Weddell Front in the northern Weddell Sea. This suggests that the deep front is a crossing of the Weddell Front further upstream in the Weddell Gyre. The Weddell Front accounts for ∼ 17 Sv of northward transport across the section. A deep outflow is observed all along the continental slope between the Slope Front and the Weddell Front. Transport within the deep outflow is localized in two to three distinct cores that are tied to topographical features. The total transport across the ADELIE section is 46 ± 8 Sv . This value exceeds previous estimates because the full-depth and de-tided LADCP measurements allowed the narrow ( ∼ 20 km ) frontal currents to be resolved, leading to more accurate estimates of the barotropic component of the flow. We discuss the physical processes that may lead to the formation and maintenance of these fronts.

Ventilation of the North Pacific subtropical pycnocline and mode water formation

The annual subduction rate of the North Pacific was calculated based on isopycnally averaged hydrographic climatology (HydroBase), high-resolution winter mixed-layer climatology (NWMLC), and various wind stress climatologies from ship reports, numerical weather prediction products, and satellite products. The calculation was performed using Lagrangian coordinates in the same manner as in previous works, except a less smoothed oceanic climatology (HydroBase and NWMLC) was used instead of a World Ocean Atlas. Differences in the wind stress climatologies have very little effect on subduction rate estimates. The subduction rate census for density classes showed peaks corresponding to subtropical mode water (STMW), central mode water (CMW), and eastern subtropical mode water (ESTMW). The deeper mixed layer and the associated sharper mixed-layer fronts in the present climatology resulted in a larger lateral induction, which boosted the subduction rate, especially for the potential density anomaly ( σ θ ) range of the lighter STMW (25.0 < σ θ < 25.2 kg m −3) and lighter CMW (26.0 < σ θ < 26.2 kg m −3), compared to previous estimates. The renewal time of permanent pycnocline water was estimated as the volume of water divided by the subduction rate for each σ θ class: 2–4 years for ESTMW (24.5 < σ θ < 25.2 kg m −3), 2 years for the lighter STMW (25.0 < σ θ < 25.3 kg m −3), 5–9 years for the denser STMW (25.3 < σ θ < 25.6 kg m −3), 10–20 years for the lighter CMW (26.0 < σ θ < 26.2 kg m −3), 20–30 years for the middle CMW (26.2 < σ θ < 26.3 kg m −3), and 60 years or longer for the denser CMW (26.3 < σ θ < 26.6 kg m −3). A comparison of the water volume and subduction rate in potential temperature–salinity ( θ– S) space indicated that the upper permanent pycnocline water (25.0 < σ θ < 26.2 kg m −3) was directly maintained by nondiffusive subduction of winter surface water, including STMW and lighter CMW. The lower permanent pycnocline water (26.2 < σ θ < 26.6 kg m −3) may be maintained through the subduction of fresher and colder water from the subarctic–subtropical transition region and subsequent mixing with saltier and warmer water. Diagnosis of the potential vorticity (PV) of the subducted water demonstrated that the low PV of STMW was mainly due to the large subduction rate, whereas that of both ESTMW and CMW was due mainly to the small density advection rate (cross-isopycnal flow). Additionally, a relatively large subduction rate probably contributes to the low PV of part of the lighter CMW (ESTMW) formed in the region around 38°N and 170°W (28°N and 145°W), which is characterized by a relatively thick winter mixed layer and an associated mixed-layer front, causing a large lateral induction rate.

Observations of small-scale disturbances of the Subantarctic Front south of Australia

High-resolution XCTD and XBT observations were conducted to study eddy variability in the Subantarctic Front (SAF) and its possible impact on the properties of the adjacent Subantarctic Mode Water (SAMW) in the region south of Australia. The stations were occupied along cross-frontal transects in February 2004 and in February and March 2002. Coarse-resolution data from previous WOCE observations were also examined to reveal the water mass properties, including dissolved oxygen (DO). Small-scale (10–20 km) features were detected in the SAF for all high-resolution sections. Fluctuation of the 7 ∘ C isotherm was clearly revealed at a depth of 300–600 m for the σ θ = 26.8 – 26.9 kg m - 3 density range. Temperature and salinity inversions of a similar spatial scale were also found on the same density surface. The spatial scale and depth range of the small-scale anomalies could be consistent with those of the baroclinic instability in the top several hundred-meter layer. In the potential temperature–salinity ( θ – S) diagram, the small-scale fluctuations constitute cold/fresh intrusions from the warm/saline SAMW curve. The intrusions were found on the θ – S line connecting the deeper SAMW and the shallower Antarctic Surface Water (AASW) of the higher latitudes. From the WOCE observations, the presence of SAMW with high DO was indicated north of the SAF, as previously reported. Within the SAF, intrusions in the θ – S diagram with relatively high DO were found at around the 7 ∘ C isotherm for 26.8 – 26.9 kg m - 3 , the same densities for the small-scale anomalies and the high-DO SAMW. Analysis of heat and salt deficit suggests that the small-scale disturbances have a comparable or larger role than those of meso-scale anomalies. These imply that the small-scale anomalies contribute to the cross-frontal water exchange and to the formation of high-DO SAMW.

Generation and Initial Evolution of a Mode Water θ–S Anomaly*

Abstract Generation and evolution of an isopycnal potential temperature–salinity (θ–S), or spiciness, anomaly is studied around 20°–23°S, 110°W in the austral winter of 2004. Two profiling CTD floats deployed in the region in January 2004 provide the observations. The anomaly (defined as relative to water properties of the preceding summer) is very large (initially about 0.35 in S and about 0.9°C in θ). It is associated with the winter ventilation of a thick, low-potential-vorticity layer known as South Pacific Eastern Subtropical Mode Water. Regional lateral θ and S distributions at the surface predispose the ocean to formation of this water mass and allow significant anomalies to be generated there with relative ease. The water mass is potentially important for climate in that, after northwestward advection in the South Equatorial Current, it contributes to the Equatorial Undercurrent and eventually resurfaces in the cold tongue of the eastern equatorial Pacific Ocean. The anomaly studied is strong enough to predispose a portion of the water column to salt fingering, increasing vertical mixing. Although lateral processes are no doubt important in evolution of the anomaly, the vertical mixing appears to be sufficiently vigorous to reduce it significantly within 6 months after its formation by spreading it to denser horizons through diapycnal fluxes. By that time the anomaly is most likely sufficiently diffuse so that subsequent evolution from diapycnal fluxes is significantly reduced as it makes its way toward the equator.

Deep and Bottom Waters in the Eastern Scotia Sea: Rapid Changes in Properties and Circulation

Abstract Two meridional hydrographic transects (in 1995 and 1999) across the eastern Scotia Sea are used to investigate variability in the deep and bottom waters between the South Scotia Ridge and South Georgia. There is a significant warming of the warm deep water (WDW) south of the southern boundary of the Antarctic Circumpolar Current (ACC); waters are approximately 0.1°–0.2°C warmer in 1999 than 1995. This is due mainly to raised WDW potential temperatures in the Weddell Sea being fed through to the Scotia Sea as the WDW flows northeastward in the Weddell Gyre. There is a warming of the Weddell Sea Deep Water (WSDW) of approximately 0.05°C across the whole extent of the section, and an accompanying change in salinity that maintains the potential temperature–salinity relationship. This is caused by variability in the properties of the water overflowing the South Scotia Ridge, rather than enhanced outflow of the bottom layer of the Scotia Sea or movements of the ACC fronts, and may be related to changes...

The Role of Cabbeling and Double Diffusion in Setting the Density of the North Pacific Intermediate Water Salinity Minimum

The top of the North Pacific Intermediate Water (NPIW) in the subtropical North Pacific is identified with the main salinity minimum in the density range σθ = 26.7–26.8. The most likely source of low salinity for the NPIW salinity minimum is the Oyashio winter mixed layer, of density σθ = 26.5–26.65. The Oyashio waters mix with Kuroshio waters in the broad region known as the Mixed Water Region (MWR), between the separated Kuroshio and Oyashio Fronts just east of Japan. It is shown that cabbeling during mixing of the cold, fresh Oyashio winter mixed layer water with the warm, saline Kuroshio water increases the density of the mixture by up to σθ = 0.07 at densities around σθ = 26.6–26.65, regardless of the mixing mechanism. Thus cabbeling accounts for about half of the observed density difference between the Oyashio winter mixed layer water and the top of the NPIW. Double diffusion during mixing of the interleaving layers of Oyashio and Kuroshio waters in the MWR can also change the density of the mixing intrusions. Density ratios favorable to double diffusion are shown to be especially prominent in Oyashio intrusions into a Kuroshio warm core ring in the 1989 data examined here. The average potential temperature–salinity profile of the new subtropical NPIW just east of the MWR, with its nearly uniform salinity, suggests the dominance of salt fingering over diffusive layering. Using the observed salinity and density differences between Oyashio surface water and the NPIW salinity minimum, after subtracting the density difference ascribed to cabbeling, an effective flux ratio of about 0.8 is estimated for possible double diffusive processes in the MWR.

Formation and circulation of the water masses between the southern Indian Ocean and Antarctica: Results from δ&lt;SUP&gt;18&lt;/SUP&gt;O

We present measurements of the stable isotopes of oxygen ( 18 O and 16 O) from seawater samples collected during the Antarctic Deep Outflow Experiment (ADOX) cruises in the Southern Ocean and southern Indian Ocean, February to March, 1993 and 1994. The data are used in conjunction with hydrographic data to infer characteristics of the formation and circulation of water masses found in the region. The waters on the continental shelf of Antarctica (adjacent to the Princess Elizabeth Trough; PET) are isotopically the lightest found due to the injection of about 1% of glacial meltwater, and are probably advected to the region from farther east by the current associated with the Antarctic Slope Front. They appear to be locally disassociated from the Antarctic Surface Water and Winter Water (WW) farther north in the PET. The WW of the Enderby Basin is isotopically lighter than the PET WW and also fresher, indicating the presence of an additional component of glacial meltwater or high-latitude precipitation. North of the Antarctic Circumpolar Current, the surface δ 18 O values correlate strongly with salinity, but extrapolate to an apparent freshwater endmember which is isotopically too light to be reasonable; advection and mixing of the water masses dominate over the local water balance at this location. The Subantarctic Mode Water of the southern Indian Ocean lies on the line of the surface waters in salinity-δ 18 O space due to its local formation by deep convection. The Antarctic Intermediate Water also lies on this line, but it is known that the major sources for this water mass are outside the region. Consequently this observation does not necessarily imply formation by local deep convection. The δ 18 O measurements have shown that the major sources for PET Antarctic Bottom Water (AABW) lie outside the Weddell and Ross seas. Based on the potential temperature-salinity of the PET AABW we further deduce that the PET itself is not the formation location of a significant amount of AABW. Its major component is advected to the region from the Australian-Antarctic Basin farther east.

Eddy stirring and phytoplankton patchiness in the subarctic North Atlantic in late summer

The spatial structure of phytoplankton distributions in the mesoscale eddy field of the North Atlantic (vicinity of 20°W, 59°N) was examined with a series of high resolution, towed transects. Data were obtained after the fall transition in August 1991 as part of the Marine Light-Mixed Layers (MLML) program. Measurements of chlorophyll fluorescence (CF) and the beam attenuation coefficient anomaly ( Δc 490) show that chlorophyll and particles are confined to the surface mixed layer and seasonal pycnocline. Two water masses account for the potential temperature–salinity ( θ– S) structure in the region: North Atlantic Central Water from the sub-tropical gyre and Sub-Arctic Intermediate Water from the sub-polar gyre. These water masses carry distinct particle distributions based on measurements of CF and Δc 490. Spatial distributions of CF and S in the mixed layer are spatially coherent for horizontal wavenumbers k less than a transition wavenumber k t≈0.15 cycles km -1 (transition wavelength λ t= k -1 t≈7 km). The horizontal distribution of θ in the mixed layer is uncorrelated with S. Autospectra of θ, S, CF, and Δc 490 vary approximately as k -2 for k less than k t. For k⩾ k t, CF and Δc 490 decrease approximately as k -3, while S spectra continue as k -2. These observations are consistent with physical control of phytoplankton distributions for k< k t by eddy advection. They are consistent with control by non-conservative biological processes for k⩾ k t. This contradicts some theoretical models of chlorophyll spectra, which predict physical control of phytoplankton distributions above k t and biological control below. The contradiction may arise because the scales of variance sources assumed in the models are not consistent with observed scales in the MLML region.

A Deep Eddy in the Antarctic Intermediate Water North of the Chatham Rise

In mid-1996, a hydrographic survey was made over the Chatham Rise, near New Zealand. A deep isolated eddy of Antarctic Intermediate Water was found north of the rise between 1200 and 1800 dbar. It was widest on a potential density surface (s1) of 32.1. Potential-temperature salinity and potential-temperature oxygen characteristics identify the eddy as having a source south of the Chatham Rise. This eddy was too deep to have passed over the Chatham Rise, and the only way it could have reached this position was by flowing around the eastern end of the Chatham Rise. The authors hypothesize that the intrusion may have been generated by a similar mechanism to that proposed for Mediterranean eddies, so that eddies are formed by separation of a frictional boundary layer at the end of the Chatham Rise. Once the intrusion is formed, it was most likely advected to its observed position, 500 km from the generation region, by recirculation within a much largerscale permanent eddy found off the south east of the North Island (the Wairarapa Eddy).

Abyssal currents generated by diffusion and geothermal heating over rises

A continuously stratified (in both salinity and temperature) diffusive time-dependent one-dimensional f-plane model over a sloping bottom is constructed. The model is used to investigate the role of mixing of density near the bottom on large-scale abyssal flow near mid-ocean rises. For realistic abyssal values, both geothermal heating from the bottom and diffusion can be important to the dynamics of flow over mid-ocean rises. When diffusion dominates, buoyancy is transported toward the bottom and the θ- S (potential temperature-salinity) relation remains nearly linear. When geothermal heating dominates, the θ- S relation hooks near the bottom and a convectively driven mixed layer forms. Both effects reduce the density and stratification near the bottom. In contrast, bottom-intensified diffusion has the same effect near the bottom but results in an increase of density and stratification some distance above the bottom. If the bottom slopes, a horizontal density gradient results, setting up a geostrophic, bottom-intensified, along-slope flow that can effect mass transport. Evidence of the importance of these processes is found in the abyssal Pacific. Just over the western flank of the East Pacific Rise, a 700–900 m thick layer of low N 2 (buoyancy frequency) is warmer, saltier, and lighter than interior water at the same depth. This layer is described with CTD data from recent hydrographic sections at nominal latitudes 15°S and 10°N. If the interior is motionless, this low N 2 layer transports 4 and 8 × 10 6 m 3 s −1 equatorward above the western flank of the rise at 15°S and 10°N, respectively. This equatorward current, a direct result of diffusion and heating over a sloping sea-floor, has a volume transport comparable to those of the deep western boundary current at these latitudes.

South Atlantic thermocline ventilation

South Atlantic Central Water (SACW) is carried poleward within a narrow longitude interval off the Argentine coast by the Brazil Current after confluence with the Malvinas (Falkland) Current. Hydrographic data obtained from the R.V. Atlantis II in December 1979 and January 1980 indicate that within the SACW poleward extension, convective processes are important in ventilating the main thermocline to the north. A number of relatively warm, salty intrusions 100 m thick are found within the 12 to 17°C layer of the thermocline along 38°S. These features are about 0.2 × 10 −3 more saline than the regional potential temperature-salinity (θ–S) curve of the SACW thermocline and represent local oxygen maxima and nutrient minima. The characteristics of the intrusion are similar to thick remnant winter-mixed layers within various pockets (warm-core eddies) of SACW found further south. The observations support McCartney's 1977 ( A Voyage of Discovery, M. Angel, editor, pp. 103–119, Pergamon) suggestion that the Brazil-Malvinas confluence region is the source of a thermostad in the South Atlantic thermocline. The present data suggest that a family of winter altered, relatively salty mixed layers and intrusions are formed in the poleward extension of the SACW thermocline waters, which then spread northward into the main thermocline. The number of family members and their range of θ–S characteristics depend on the distribution of warm eddies and meanders during the winter period. The lower boundary of the intrusions have density ratios ( R = αΔT/ βΔS) of 1.2 to 1.3, making them unstable to salt-finger activity. Salt fingers are likely to be the primary process that integrates the excess salt of the intrusions into a broader, but less extreme positive salinity anomaly (relative to the South Atlantic thermocline T-S curve), which is associated with the 12 to 17°C weak thermostad of the southwest Atlantic. It is suggested that deep winter convective processes within the SACW extension not only transfer salt and oxygen to mid-thermocline depths, but it also induces significant downward salt flux by salt-finger activity at mid-thermocline depths. Similar processes may be important in all poleward edges of the world thermocline, particularly along the western sectors.