Undulating CTD Research Articles

Quantification of dissolved oxygen dynamics in a semi-enclosed sea – A comparison of observational platforms

In this study, we present dissolved oxygen (DO) concentration data collected during an observational program conducted in the central Baltic Sea, six months after a Major Baltic Inflow (MBI) event from December 2014 has ended a long period of stagnation. The main working area was the Eastern Gotland Basin (EGB). The spatial distribution of DO was obtained by two ship based CTD-systems (classical CTD probe, towed undulating CTD probe) and by using a glider (Slocum), while the temporal variability of DO concentrations was revealed by a profiling mooring (GODESS - Gotland Deep Environmental Sampling Station). In this paper, we compare the performance of all platforms used and show that integrating modern oceanographic tools into an existing observation network can help to enhance the capability of the network to capture the spatiotemporal variability of DO in the Baltic Sea on both a large (seasonal; 10–500 km) and a small (diurnal; 1–10 km) scale.It was found that in comparison to a classical standard CTD system, a towed undulating CTD system (ScanFish) is better suited to resolve the meso- and submeso-scale distribution patterns of DO concentrations. In particular, in the sub-halocline layers where the variability of DO concentrations was consistently high (1–70 µM). The highly dynamic post-inflow situation was also observed by the glider and the profiling mooring. For comparison purposes four different sub-halocline layers in a depth range from 80 m to 180 m depth were defined and the mean oxygen content of these layers was calculated for both platforms. It turned out, that the mean DO concentrations for the different layers, which were determined from glider and mooring measurements, differ only slightly from each other. Differences were greatest (up to 7 µM) below the halocline (80–120m) and could be explained by the spatial and temporal variability of DO concentrations in this area and by the large response time of the gliders oxygen optode. However, the temporal development of inflow events could be determined and investigated in more detail by using autonomous platforms. The frequency and size of smaller inflows, for instance, that move along the halocline or in deeper layers thus could be evaluated.

Processes controlling mid‐water column oxygen minima over the Texas‐Louisiana shelf

AbstractWe investigate distributions of dissolved oxygen over the Texas‐Louisiana shelf using spatially highly resolved observations in combination with a regional circulation model with simple oxygen dynamics. The observations were collected using a towed, undulating CTD during the Mechanisms Controlling Hypoxia (MCH) program. Mid‐water oxygen minimum layers (dissolved oxygen lower than 3.2 mL L−1) were detected in many transects. These oxygen minimum layers are connected with the bottom boundary layer and follow the pycnocline seaward as a tongue of low oxygen into the mid‐water column. T‐S diagrams highlighting the low oxygen minima in both observations and simulations imply direct connections between low‐oxygen bottom water and the oxygen minimum layer. The dynamics of these oxygen minimum layers in the mid‐water column are examined using a three‐dimensional hydrodynamic model, based on the Regional Ocean Modeling System (ROMS). Convergence within the bottom boundary layer relative to density surfaces is calculated, results show that there is a convergence in the bottom boundary layer at the location where the pycnocline intersects the bottom. Buoyancy advection forced by bottom Ekman transport creates this convergent flow, and the corresponding low‐oxygen intrusion. Similar intrusions of near‐bottom water into the pycnocline are observed in other regions. The presence of hypoxia within the bottom boundary layer in the northern Gulf of Mexico creates a unique situation in which these intrusions are also associated with low dissolved oxygen.

Spatial variability of internal waves in an open bay with a narrow steep shelf in the Pacific off NW Mexico

Small scale spatial patterns (<10km) in nearshore internal wave fields are rarely reported on, yet can have a large impact on nearshore mixing and productivity. In this study, the spatial pattern of internal wave characteristics were explored in Todos Santos Bay, Baja California (Mexico), using time series of temperature and currents from moored and towed thermistor chains and acoustic profiling current meters, as well as cross-shore transects with a towed undulating CTD system. Spectra of temperature and currents showed significant spatial variability within the bay, with the northern sector dominated by the internal tidally-forced semidiurnal signal, and the southern sector dominated by wind-forced, subinertial, baroclinic, diurnal fluctuations, which decreased with distance from shore. Semidiurnal internal tidal waves were generated by the barotropic tide at various sites on the continental slope to the west of the bay. They traveled toward the NE and reached the observation site in the northern part of the bay, after bouncing once or twice off the surface and the bottom. Despite the narrowness of the shelf, the semidiurnal internal tides at this site presented a first-mode structure, although not completely formed at times. On average, the semidiurnal internal waves had a ~9km wavelength, traveled in the form of an arc, and propagated with a phase velocity of ~20cm/s. When they reached shallow waters near the coast, they disintegrated rapidly into groups of short, nonlinear internal waves, with 15–20m amplitudes, 5–20min periods, and 50–200m wavelengths. The spatial patterns found in this study are most likely due to variability in distance from generation sites, complex bottom topography, and small scale (<10km) spatial variability in meteorological conditions such as winds.

Tidal and wind-event variability and the distribution of two groups of Pseudo-nitzschia species in an upwelling-influenced Ría

High-resolution physical and biological measurements were carried out in May–June 2007 during the ‘HABIT-Pontevedra 2007’ survey in Ría de Pontevedra (Galician Rías Baixas, NW Spain) to study small-scale physical–biological interactions in the distribution of microphytoplankton, with special emphasis on harmful species. Longitudinal transects from the Ría to the adjacent shelf were sampled to describe the spring–neap tidal and circadian variability. An in situ particle profiler, a moored ADCP, and a towed undulating CTD (Scanfish) were used during the survey, which took place after an upwelling pulse at neap tides during a downwelling–upwelling cycle and coincided with the annual maximum of Pseudo-nitzschia spp. Cell maxima of Pseudo-nitzschia seriata (2×106cellsL−1) and Pseudo-nitzschia delicatissima (6×105cellsL−1) groups were observed during the first half of the cruise during downwelling, and a significant decrease in cell numbers occurred during subsequent relaxation-upwelling conditions. Thin layers were eroded during downwelling and formed again in the subsequent upwelling pulse. Cells of the P. seriata group were always dominant in terms of biomass but the contribution of the P. delicatissima group increased with stratification. Water exchange between the Ría and the adjacent shelf was mainly controlled by the upwelling/downwelling cycle, and tidal (both semidiurnal and spring–neap) variability appeared as a modulation of the response of the Ría circulation to wind variability. The circadian variability was regulated by tidal forcing, and Pseudo-nitzschia spp. maxima were associated with high stratification during low tide. The magnitude of spring–neap tidal and circadian variability has to be considered when designing and implementing water quality and harmful algae monitoring programmes. Blooms of Pseudo-nitzschia ssp. were not associated with the occurrence of domoic acid in shellfish even when Pseudo-nitzschia australis was dominant. These results confirm that just cell densities of the potential toxin producer are not enough for early warning in monitoring of Pseudo-nitzschia events.

The circulation dynamics associated with a northern Benguela upwelling filament during October 2010

Upwelling filaments, a common feature in all the major upwelling systems, are also regularly observed in the Benguela upwelling system and are thought to provide an effective mechanism for the exchange of matter between the shelf and the open ocean. The mesoscale dynamics of a northern Benguela upwelling filament located at approximately 18.5°S were examined and the associated transport was quantified. The development of the filament was tracked using optimal interpolated SST satellite data and two transects were consequently sampled across the feature using a towed undulating CTD (ScanFish). Additional hydrographic, nutrient and biological parameters were investigated at several stations along each transect. Following 7 days of strong upwelling favorable winds, sampling coincided with a period of relative wind relaxation and the filament was presumably in a decaying state. The basic mesoscale structure of the investigated filament corresponded well to what had previously been described for filaments from other eastern boundary current systems. The cross-shore transport associated with the filament was found to be significantly greater than the integrated Ekman transport in the region. With the combination of the high resolution dataset and a MOM-4 ecosystem model the complex mesoscale flow field associated with the feature could be observed and the counterbalancing onshore transport, associated with subsurface dipole eddies, was revealed within the filament. The results further suggest that an interaction between the offshore bending of flow at the Angola-Benguela Front (ABF), the detachment of the strong poleward flow from the coast as the thermal front meanders and the observed dipole eddies may be driving filament occurrence in the region off Cape Frio.

The contribution of the deep chlorophyll maximum to primary production in a seasonally stratified shelf sea, the North Sea

Results from extensive cruises in the years 2000 and 2001 throughout distinct ecohydrodynamic regions of the central and southern North Sea are presented and used to generate estimates of gross primary production and new production. An undulating CTD fitted with a fluorometer was towed over a distance of 12,000 kms. Fluorescence data were used to determine the chlorophyll distribution and derive estimates of phytoplankton biomass. These results were combined with estimates of primary production (new and regenerated) from experiments from one cruise in order to estimate gross production for a greater geographical extent. Results from repeat inter-annual transects showed that the strength of the thermocline and the associated deep chlorophyll maximum were variable. However, when the primary production was integrated over the 15–40 m depth, the variability between years was low. While the depth and strength of the deep chlorophyll maximum varied across the region, a deep chlorophyll maximum (DCM) is a consistent and widespread feature of this region at around 30 m depth. In 2001 the calculated average primary production rate in summer for the whole area surveyed was 0.91 g C m−2 day−1. This daily production equates to ~130 g C m−2 for the summer stratified period. In the offshore stratified regions around the Dogger Bank and Eastern Central North Sea primary production of 64 g C m−2 associated with the deep chlorophyll maximum (15–40 m) accounted for 60 % of total primary production during the summer stratified period (after the spring bloom). Approximately 66 % of new production in these areas occurred in the DCM. This study shows the extent of the DCM in the North Sea and demonstrates its importance in sustaining primary production after the spring bloom.

Variation in the Hatteras Front density and velocity structure Part 2: Historical setting

On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic Bight and warm salty South Atlantic Bight shelf waters converge alongshelf 90% of the time, causing strong alongshelf gradients in temperature and salinity known as the ‘Hatteras Front’. The resulting density gradient supports strong shoreward velocities in the cross-shelf oriented ‘nose’, of the Front in wintertime. To investigate further, the Frontal Interactions near Cape Hatteras (FINCH) project used shipboard ADCP and a towed undulating CTD to examine Hatteras Front property, density and velocity fields in August 2004, January 2005, and July 2005. Strong property gradients were encountered across the nose of the Hatteras Front in all cases, but the density gradient, dynamic height gradient, and observed along-front cross-shelf velocities evolved in time. FINCH along-Front velocities were strong and shoreward in fall and winter, and weakly mixed shoreward and seaward in July. Several archived data sets were examined, and demonstrate that the density evolution and wind forcing seen in FINCH are characteristic of other years. Evidence suggests the width of the Hatteras Front does not vary dramatically in time, so that consistently large fall and winter density contrast across the Front implies consistently large shoreward velocities along it in winter. Weak density contrasts across the Hatteras Front in spring suggest the magnitude and sign of springtime density gradients and along-Front velocities could vary interannually. Recruitment success of commercially important stocks on the shelf that depend on cross-shelf transport may thus be affected year to year.

Variation in the Hatteras Front density and velocity structure Part 1: High resolution transects from three seasons in 2004–2005

On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic Bight and warm salty South Atlantic Bight shelf waters converge alongshelf 90% of the time, causing strong alongshelf gradients in temperature, salinity, and density known as the ‘Hatteras Front’. Mechanisms of shoreward transport in this region have long been a topic of interest, since many commercially important species spawn on the outer shelf, but utilize the adjacent Albemarle and Pamlico Sounds for nurseries, requiring some physical transport mechanism to move the eggs and larvae from the outer shelf to these nursery areas. One mechanism providing such shoreward transport is strong shoreward velocity along the cross-shelf oriented ‘nose’ of the Hatteras Front. The Frontal Interactions near Cape Hatteras (FINCH) project used shipboard ADCP and a towed undulating CTD to examine Hatteras Front property, density and velocity fields in August 2004, January 2005, and July 2005. Strong property gradients were encountered across the nose of the Hatteras Front in all cases, but the density gradient evolved in time, and along with it the dynamic height gradient driving the observed along-front cross-shelf velocities in the nose of the Front. In August and January FINCH data, MAB shelf waters on the north side of the Hatteras Front are less dense than SAB shelf waters, driving shoreward velocities along the Hatteras Front. By July, MAB shelf waters are slightly more dense than SAB shelf waters, with areas of weak seaward and shoreward velocities within the Hatteras Front. As Part 1 of a pair of contributions, this article focuses on FINCH data to illustrate the range of density gradients encountered and resulting cross-shelf velocities. Whether these observations are typical of variability in the Hatteras Front is explored in a second article, Part 2.

Scales characterising a high density thin layer of Dinophysis acuta Ehrenberg and its transport within a coastal jet

An investigation into the distribution of Dinophysis spp. in coastal waters off the south coast of Ireland was carried out in July 2007. Dinophysis acuta was present as a sub surface layer containing up to 55,000cellsL−1. The population had a high percentage of viable cells (mean: 89%; median: 94%; n=24) with a high specific division rate (∼0.55d−1). The layer, of approximately 5m thickness, did not coincide with the fluorescence maximum and was present as a patch of horizontal dimension less than 10km×7km. Both conventional and towed undulating CTD used in conjunction with high vertical resolution sampling methods showed the patch of Dinophysis to move with a similar speed and direction as the coastal flow, which ran parallel to the coast in the form of a coastal jet with speed of the order of 6.5–7kmday−1. The implications of the alongshore transport of populations of harmful species in coastal jets for monitoring programmes and predictive models are discussed.

The Hatteras Front: August 2004 velocity and density structure

The Hatteras Front is a persistent mesoscale cross‐shelf oriented front off Cape Hatteras, North Carolina. It is the boundary between relatively cool, fresh Mid‐Atlantic Bight shelf waters and warmer, saltier shelf waters of the South Atlantic Bight, which both converge along‐shelf upon Cape Hatteras year round. The Frontal Interaction Near Cape Hatteras (FINCH) project was conducted in 2004–2005 to intensively sample the Hatteras Front with shipboard ADCP and undulating towed CTD. This paper documents velocity and density structures associated with the cross‐shelf oriented zone of Hatteras Front during the August 2004 field season. Property gradients across the Hatteras Front are large, with temperature (T) and salinity (S) differences of ∼4–6°C, 2–5 psu, respectively over distances of 1–2 km. The T and S are not completely compensating, and a strong density (ρ) gradient also exists, with Δρ of ∼2 kg/m3 across a gentler 10 km wide front. The density gradient results in a steric sea‐level height gradient of ∼1–2 cm across the Front, which is in approximate geostrophic balance with a surface intensified jet, directed shoreward along the cross‐shelf oriented Front. The velocity is sheared with depth at 3.0 × 10−2 to 5.0 × 10−2 s−1 in the upper 5 m of the jet; a rate consistent with the density gradient according to the thermal wind relationship. Shoreward transport of ∼4.8 × 104 m3/s results from the surface intensified jet. The structure of the velocity field associated with the Hatteras Front resembles that of a slope‐controlled buoyant plume, as described by Lentz and Helfrich (2002). Velocity and density structures are similar during both advancing (southwestward) and retreating (northeastward) motion of the Front.

The Irish coastal current: A seasonal jet-like circulation

During 25 July–3 August 2001 a comprehensive cruise was undertaken along the western coast of the Republic of Ireland to resolve the three-dimensional density field and circulation. Towed undulating CTD revealed the strong thermocline beyond the 60 m contour with more diffuse stratification inshore. Argos drifters deployed in the late summer of 1999, 2000 and 2001 demonstrated strongly northward flow, their motion was uncorrelated with wind forcing but was consistent with the predicted geostrophic currents generated by the bottom fronts intersecting the seabed. These flows were evident in the data from the vessel mounted 153.6 kHz acoustic Doppler current profiler (ADCP) although the jets are relatively narrow (15–20 km). This region is heavily influenced by storm events and Atlantic influence with high salinity contributing to strong vertical and horizontal density gradients particularly in the south of the region. However, the seasonal heating cycle coupled with tidal mixing is the dominant force of buoyancy production that ensures that the frontal features occur every year. In combination with similar features found in the Celtic Sea [Brown, J., et al., 2003. Observations of the physical structure and seasonal jet-like circulation of the Celtic Sea and St. George's Channel of the Irish Sea. Continental Shelf Research 23, 533–561] these observations demonstrate the existence of a continuous pathway in summer from the north Cornish coast to Malin Head with average residual velocities >7.5 cm s −1. The data set described here provides the necessary basis for environmental management and knowledge of the pathway for contaminants, the dynamics of nutrients and an understanding of the movement of larvae and juvenile fish, in a geographic location that had previously been little studied.

Vertical location of seasonal nutriclines in the western Gulf of Finland

About 350 profiles of phosphate, nitrate and silicate concentration were collected on board the R/V Aranda in the western Gulf of Finland in summers 1992–1999. The study sites were located close to the coastal slope where the effect of wind-forced physical processes is well expressed. To determine the mesoscale background, hydrographical fields were mapped with a towed undulating CTD. The seasonal nutricline is a layer where the nutrient concentration increases just below the nutrient-depleted upper layer and is characterised by its depth and gradient. A clear ordering of seasonal nutriclines in the pycnocline/thermocline was observed with increasing depth: silicocline, phosphacline and nitracline. The silicocline and phosphacline were found close to each other. The spacing between the phosphacline and the nitracline was usually 3–5 m and about 14 m in the cases of a deep nitracline. The latter can be explained by the utilisation of nitrate reserves by a vertically migrating dinoflagellate Heterocapsa triquetra in cold summers when the surface layer temperature remained below 16 °C. Almost all nutricline depths (except the deep nitraclines) were well correlated with the pycnocline depth. Cold nutrient-rich intrusions in the nutricline layer, mostly 4–9 m thick, were frequently found in the vicinity of internal thermohaline/nutrient fronts in the Finnish coastal sea area. It is shown that the vertical transport of phosphate and nitrate by upwelling and turbulent mixing essentially depends on the separation of the phosphacline and nitracline.

The front on the Northern Flank of Georges Bank in spring: 1. Tidal and subtidal variability

During March–April 1999, 2 weeks of undulating CTD and shipboard acoustic Doppler current profiler surveys revealed the variability of the intense internal tide on the northern edge of Georges Bank. The nature of the internal tide was modulated by episodic surface intrusions of cool, fresh Scotian Shelf Water (SSW), stratifying the otherwise vertically well‐mixed outer bank. The introduction of SSW created a system analogous to that in summer, when the outer bank is thermally stratified and separated from well‐mixed regions by a tidal mixing front. During SSW intrusions, internal tidal behavior is characterized by tidal advection that is significantly faster than internal wave propagation speeds on the bank (supercritical flow) and slower than the lowest internal modes in deeper water to the north (subcritical flow). A large‐amplitude internal lee wave develops over the slope during off‐bank tidal flow. This stalled energy is released to propagate on‐bank as a high‐frequency internal wave when the tide reverses. It is suggested, by analogy with the summer case, that a portion of this energy is used by mixing at the on‐bank limit of SSW. The presence of SSW thus modifies the internal tidal response, enabling a mechanism that contributes to its assimilation with ambient bank water. The off‐bank‐propagating internal tide is dominated by a mode 1 internal wave at the M2 frequency, with cross‐bank velocities comparable to the barotropic tide.

Observations of the physical structure and seasonal jet-like circulation of the Celtic Sea and St. George's Channel of the Irish Sea

During July–September 1998 a comprehensive survey was undertaken of the Celtic Sea and St. George's Channel of the Irish Sea in order to resolve the three-dimensional density field and circulation. Towed undulating CTD data revealed that isolated below the seasonal thermocline was a dense (cold and saline) pool flanked by strong near-bed density gradients. The trajectories of 23 satellite-tracked Argos drifters demonstrated the existence of a cyclonic (anti-clockwise) circulation pattern. This was in accord with persistent and narrow (ca. 20 km) cores of cyclonic flow (dense water to the left) as revealed by geostrophic current estimates and moored acoustic Doppler current profiler data, with velocities >0.1 m s −1 and exceeding 0.3 m s −1 in St. George's Channel. The results are consistent with a seasonal baroclinic circulation that advects saline Atlantic water through the south and west of the region to St. George's Channel. Westward flow across St. George's Channel is directed south into the Celtic Sea and west along the Irish coast. The southward flow, an artefact of the baroclinic circulation, often appears in infrared satellite imagery as a meander in the Celtic Sea front. At the southward extension a secondary clockwise circulation is formed (diameter∼40 km) before water joins westward flow. Fresher water of coastal origin, present in the northern Celtic Sea in late spring, is advected westward by the summer circulation. The advent of summer thermal stratification and associated flow fields reduces exchange between the Celtic and Irish Seas. The understanding provided by such datasets such as those described here provides the necessary basis for environmental management and knowledge of the pathways for contaminants, the dynamics of nutrients and an understanding of the movement of larvae and juvenile fish. It also provides a template against which to test the prognostic capabilities of present and emerging baroclinic numerical models.

Physical and biological patchiness of an upper ocean transect from South Africa to the ice edge near the Greenwich Meridian

An undulating CTD (SeaSoar) was towed from the Subtropical Front at 42°S, 12°E to the ice edge at 57.5°S, 2°W. The instrument measured temperature, salinity, pressure, and light in the top 400 m of the water column. Fluorescence and particle counts also were recorded. Fluorescence was calibrated to chlorophylla, and particle counts were converted to biovolume to provide estimates of phyto- and zooplankton, respectively. The size of particles counted covered the range 0.25–8.0 mm. Three major fronts, Subtropical, Subantarctic, and Polar were intersected and identified by their physical characteristics. The Polar Front showed surface and subsurface signatures separated by nearly 300 km. Considerable small-scale structure was observed between the fronts, in particular a large (150-km diameter), warm-core eddy was found in the northern part of the Subantarctic Zone. Both fronts and eddy were associated with significant biological signals. Phyto- and zooplankton correlated well in the frontal regions and south of the Polar Front. The Polar Frontal Zone was associated with elevated concentrations of biomass, whereas concentrations were low at the Subantarctic Front and south of 55°S. In contrast, the warm-core eddy in the Subantarctic Zone contained the highest zooplankton biovolume of the section while chlorophyll concentration was only moderate. We hypothesise that such a distribution results from grazing. The overall distribution of phyto- and zooplankton changed across the Subantarctic Front. The vertical extent of biomass deepened from north to south across the front. Zooplankton tended to be concentrated in relatively narrow, deep-extending bands to the north but were more widely spread to the south. The small-scale hydrographic structure correlated significantly with the distribution of plankton, suggesting strong physical controls on both small zooplankton and phytoplankton.

Mesoscale subduction at the Almeria–Oran front: Part 1: Ageostrophic flow

This paper presents a detailed diagnostic analysis of hydrographic and current meter data from three, rapidly repeated, fine-scale surveys of the Almeria–Oran front. Instability of the frontal boundary, between surface waters of Atlantic and Mediterranean origin, is shown to provide a mechanism for significant heat transfer from the surface layers to the deep ocean in winter. The data were collected during the second observational phase of the EU funded OMEGA project on RRS Discovery cruise 224 during December 1996. High resolution hydrographic measurements using the towed undulating CTD vehicle, SeaSoar, traced the subduction of Mediterranean Surface Water across the Almeria–Oran front. This subduction is shown to result from a significant baroclinic component to the instability of the frontal jet. The Q-vector formulation of the omega equation is combined with a scale analysis to quantitatively diagnose vertical transport resulting from mesoscale ageostrophic circulation. The analyses are presented and discussed in the presence of satellite and airborne remotely sensed data; which provide the basis for a thorough and novel approach to the determination of observational error.

Seasonal variability of the temperature and salinity fields in the coastal zone of the states of Jalisco and Colima, Mexico

The results of the analysis of data obtained from a series of oceanographic surveys conducted in an ecological monitoring polygon on the continental shelf off Jalisco and Colima (Mexico) are presented. The surveys were conducted monthly between 1995 and 1997 with an undulating CTD. In one day, up to 100 soundings were made at stations with a maximum depth of 150 m, in a spatial grid of points with an area of 50 · 15 km. The vertical profiles of temperature and salinity were obtained by averaging the data from the oceanographic surveys. It is shown that the vertical distributions and hydrophysical characteristics analyzed present a significant seasonal variability related to the annual variations of solar energy input and variability of evaporation, precipitation and coastal discharges in the study area. The seasonal oscillations cover a water layer greater than 100 m, but the subsurface layers of the coastal waters are observed with greater intensity. The mean profiles present a high statistical reliability, since the deformations caused by the internal waves were completely suppressed.

Spatial structure of the temperature and salinity fields in the presence of internal waves on the continental shelf of the states of Jalisco and Colima (Mexico)

A method to filter fluctuations in the hydrophysical properties of the continental shelf, caused by intense internal tides, is proposed. The method is based on a smoothing of the field of the properties analyzed with a filter, the parameters of which are determined by the shape of the spatial correlation function of the field pulsations. The filtering method was tested on data from a fast oceanographic survey, conducted in an area of the monitoring polygon of the waters around Barra de Navidad. The survey was conducted on 25 and 26 November 1995, with an undulating CTD. In one day, 86 vertical profiles of temperature and salinity were obtained from nine transects perpendicular to the coast (survey area of 100 · 25 km) to a depth of 100 m. The filtering algorithm of the internal waves is studied in detail. The field measurements present an analytical shape obtained from an equation in which the elements describe the components: low frequency, pulsating and related to daily behavior. The results of the analysis of the initial temperature and salinity fields with respect to homogeneity and isotropy are discussed, as well as the structural peculiarities of the estimates of the spatial correlation functions. It is observed that the temperature and salinity fields along the shelf, within the limits of the polygon scale, are neither homogenous nor isotropic. The correlation functions of the field pulsations analyzed are more clearly observed in the thermocline. The characteristic scales perpendicular to the coast are smaller than along it. Examples of smoothing the temperature and salinity fields in the thermocline are given.

Density and flow structure in the Clyde Sea front

A well-defined front in temperature and salinity separates the stratified Clyde Sea water from the vertically well mixed water of the North Channel. The detailed structure of the front was observed in autumn 1990 by a combination of, repeated crossings of the front using a ship-borne ADCP and a towed undulating CTD system, and the deployment of a fixed mooring system with temperature, salinity and velocity sensors for a period of 12 days. The results show that the front was situated on the Great Plateau near a contour of log 10( H/ U 3 2)=2.7∼3.7 where H is the water depth and U 2 the amplitude of M 2 tidal velocity. The temperature structure in the Clyde Sea was inverted and the Clyde Sea surface temperature was lower than that of the vertically well mixed water in the North Channel. Since the salinity gradient was stronger than the temperature gradient with fresher water on the surface, the density structure was predominantly controlled by salinity. There were indications of warm and saline bottom water upwelling on the mixed side of the front during spring tides. This upwelling disappeared and the salinity and temperature structure at the front was more diffuse during the neap tide period. A jet-like along-front residual current was observed flowing to the northwest in the surface layer with a counter flow to the southeast in the bottom layer. The vertical difference in velocity was about 9 cm s −1 and was approximately consistent with the shear determined from the thermal wind relation. Both cross- and along-front components of the current observed at the mooring station varied in response to the advection of the front, although both components had large variations with periods of less than one day and several days. The front was advected past the mooring system by a mean flow from the North Channel to the inner basin, while oscillating 3–5 km back and forth with the tidal currents. From the velocity at a current meter mooring and CTD data, the front was estimated to have moved up to 20 km during the observational period and the cross frontal velocity was inferred to be 3–4 cm s −1.

The changing structure of the upper ocean in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment

As part of the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) the French vessel Le Noroit completed a series of 18 sections from 5°S to 5°N on 156.5°E, using a towed undulating CTD (SeaSoar) and a hull‐mounted acoustic doppler current profiler (ADCP). Here we present results from the cruise. We describe both the mean structures and the response of the upper ocean to changing meteorological conditions in terms of changes in the dynamic and thermodynamic structure over a 10° latitude band and over a period of approximately 70 days. The mean thermal and haline structures were consistent with the ongoing El Niño conditions of 1992–1993. Only between 1°N and 3°N did salinity play a greater role than that of temperature in the mean stratification of the upper 80 m. The zonal currents were weak in the mean but highly variable, correlating significantly in the 0–60 m depth range with the zonal winds. We show that a cyclonic, cold core structure, identified at 2°S around January 9, will heavily influence any heat budget performed there. Meteorological conditions varied dramatically during the experiment, with strong westerlies, strong easterlies, and periods of little wind observed. Throughout these conditions and at all latitudes a diurnal thermal cycle was seen. The maximum depth of the diurnal thermocline varied from 30 to 35 m, being shallower at more southerly latitudes. The near‐surface temperature showed a mean diurnal signal of approximately 0.4°C. The halocline depth also exhibited a diurnal signal of around 17 m, suggesting that salinity stratification plays a role in limiting vertical mixing at all latitudes, and more so in northern latitudes. Thicker barrier layers were found north of the equator; we show that barrier layers thicker than approximately 25 m correspond to significantly reduced entrainment cooling of the mixed layer (0(0.1°C)). The observed spatial and temporal variability is discussed in the context of the analyses made by others within the intensive flux array near 2°S.