Robin D. Pingree

Slope turbulence, internal waves and phytoplankton growth at the Celtic Sea shelf-break

The edge of the Celtic Sea shelf is characterized during the summer by a band of cold water (ca. 100 km broad), which is generally conspicuous in high resolution infrared images from satellites, particularly under high pressure atmospheric conditions with clear skies. Preliminary studies of mixing in this region were made in 1972, 1973 and 1974 and were followed by more detailed interdisciplinary studies in 1976, 1979 and 1980 relating phytoplankton growth to the ways in which turbulence in the environment controls the availability of nutrients and light energy. The results have shown the cooler water to be about 1-2 °C colder than the adjacent surface waters of the Celtic Sea and Atlantic Ocean. This cold band also exhibits higher than background surface values of inorganic nitrate and chlorophyll a. Although these values are generally low compared with the values that have been observed near the neighbouring shelf tidal fronts, the increased surface values along the shelf break in summer appear to be significant. The observed increases of chlorophyll a are thought to be related to physical processes associated with the slopes, ridges and canyons where enhanced mixing, particularly due to internal waves or upwelling, results in nutrient renewal and subsequent phytoplankton growth along the shelf-break region of the Celtic Sea.

Structure, seasonal development and sunglint spatial coherence of the internal tide on the Celtic and Armorican shelves and in the Bay of Biscay

Abstract The generation and propagation of internal tides from the shelf break in the Bay of Biscay is now a well-documented process, but a description of the spatial coherence of the internal tides has so far been impossible with conventional in situ observations. This paper first analyses the shelf measurements of internal tides and shows that by studying available remotely sensed images over a number of years, particularly in the visible band (which we term “sunglint” images), it is possible to gain significant insight into the spatial coverage, long-crestedness, and seasonal development of these features. The sunglint images provide a synoptic description of the internal tides, and show that they may occur up to 250 km onshelf from the shelf break with coherent crests extending over 400 km in the along shelf direction. The oceanic signal was observed to extend from the shelf break right across the Bay of Biscay (∼300 km). The images allow the tidal wavelengths to be reliably estimated both onshelf and offshelf from the shelf break, without complications arising from advection by the barotropic tide which occur when in situ measurements are made. A strong seasonal signal was found which results from the development of the stratification in the upper water column. By approximating observed temperature profiles, a simple two-layer model is developed for the onshelf waves which provides a relationship between the tidal wavelengths, and the thickness of and the temperature difference across, the upper layer. It is possible to use this relationship in combination with remote sensed images, providing sea-surface temperature and tidal wavelength, to infer the depth of the upper layer, which generally increases as the stratification develops, so allowing a simple method for estimating the stratification in the upper water column. The effect of nonlinearity is important in determining wave structure but has only a small effect on phase speed or wavelength in the presence of Earths rotation.

A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well-defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature > ∼ 13.25°C, salinity > ∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reached the Horseshoe Abyssal Plain.

Position and structure of the Subtropical/Azores Front region from combined Lagrangian and remote sensing (IR/altimeter/SeaWiFS) measurements

The position and structure of the North Atlantic Subtropical Front is studied using Lagrangian flow tracks and remote sensing (AVHRR imagery: TOPEX/POSEIDON altimetry: SeaWiFS) in a broad region ( similar to 31 degree to similar to 36 degree N) of marked gradient of dynamic height (Azores Current) that extends from the Mid-Atlantic Ridge (MAR), near similar to 40 degree W, to the Eastern Boundary ( similar to 10 degree W). Drogued Argos buoy and ALACE tracks are superposed on infrared satellite images in the Subtropical Front region. Cold (cyclonic) structures, called storms, and warm (anticyclonic) structures of 100-300 km in size can be found on the south side of the Subtropical Front outcrop, which has a temperature contrast of about 1 degree C that can be followed for similar to 2500 km near 35 degree N. Warmer water adjacent to the outcrop is flowing eastward (Azores Current) but some warm water is returned westward about 300 km to the south (southern Counterflow). Estimates of horizontal diffusion in a Storm (D=2.2t10 super(2) m super(2) s super(-1)) and in the Subtropical Front region near 200 m depth (D sub(x)=1.3t10 super(4) m super(2) s super(-1), D sub(y)=2.6t10 super(3) m super(2) s super(-1)) are made from the Lagrangian tracks. Altimeter and in situ measurements show that Storms track westwards. Storms are separated by about 510 km and move westward at 2.7 km d super(-1). Remote sensing reveals that some initial structures start evolving as far east as 23 degree W but are more organized near 29 degree W and therefore Storms are about 1 year old when they reach the MAR (having travelled a distance of 1000 km). Structure and seasonality in SeaWiFS data in the region is examined.

Coupling between physical and biological fields in the North Atlantic subtropical front southeast of the Azores

Abstract The physical and biological properties of the North Atlantic subtropical region southeast of the Azores were studied during an oceanographic cruise carried out in March 1992. The main patterns of physical variability were defined by the presence of (a) a thermohaline front (subtropical front; STF) located at 34°N–35°N and extending from 15°W to 28°W, which separates warmer more saline Western Atlantic Water from colder and fresher Eastern Atlantic Water, and (b) a strong eastward flow running along the subtropical front (Azores Current; AC) extending down to at least 250 m and showing velocities of 50–70 cm s −1 at the core of the current. Overall a close linkage was observed between the STF-AC physical feature and high levels of chlorophyll a . A high resolution survey showed chlorophyll a fluorescence associated with the southern frontal boundary (consisting of chain forming diatoms and flagellates) and with the AC (made up of cells in the less than 2 μm size-class), respectively. Primary production rates measured in the frontal high-chlorophyll region (> 1 mg C m −3 h −1 ) were much higher than previous measurements carred out in the same area in late spring and summer and about 2 times higher than modelling estimates for the region. The large spatial extension of the biological signature associated with the STF-AC system suggests that carbon fixation within the frontal structure could be significant for regional carbon budgets of the subtropical northeast Atlantic.

The Eastern Subtropical Gyre (North Atlantic): Flow Rings Recirculations Structure and Subduction

Thirteen carefully prepared drogued buoy assemblies have been deployed in the eastern subtropical North Atlantic giving ~20 buoy years of Lagrangian data at a depth of 200 m. The buoy results together with hydrography have revealed the structure of the eastward flowing Azores Current (AC). The main jet had a transport of 26 Sv (near 28°W) with compensating counterflows in March 1992. Jet and counterflows were readily seen in the ADCP current structure and evident in the upper layer temperature (salinity) structure on an isopycnal surface. Buoys and hydrography showed that the adjacent westward flowing counterflows resulted in recirculation both north (anticlockwise circulation) and south (clockwise circulation) of the AC - the Subtropical Recirculations. South-south-west flow (3 cm s −1 ) occurred in the central region of study west of the Canary Islands. The long-term movement measured in the south or northern North Equatorial Current region was 3·5 cm s −1 west-south-west. The mean south displacement per year of the buoys was 2·2° of latitude to the south and most of this displacement occurred in the first half of the year (February-August). The maximum westward displacement rate occurred six months later in October and November. The Subtropical Front/Azores Current region was identified as a zone of increased levels of kinetic energy (>100 cm 2 s −2 ) at a latitude near 34°N at the 200 m level, stretching for >2000 km, and as a marked horizontal winter sea surface temperature contrast at a latitude near 36°N. Winter mixing created an outcropping region of water with density in the range 26–4 0 −3 along the Subtropical Front near 34°N with a maximum vertical extent of 200 m.

Structure of a meddy (Bobby 92) southeast of the Azores

Abstract Meddy Bobby was found near 34.75°N, 23.5°W in March 1992, some 1500 km from the Strait of Gibraltar. The meddy core had a maximum salinity of 36.531 psu at a depth of 1320 m. A detailed hydrographic survey defined the temperature, salinity, oxygen, nitrate and silicate structure of the meddy. The velocity field was derived from the gradient equation, drogued Argos buoys and ADCP measurements. The property distributions suggest that about 1 km of the central water column (∼650–1600 m) was travelling with the meddy, though the vertical influence of the meddy extended over a much greater interval, >3km. Current structure reached the sea surface and isopycnals were depressed (by∼100 m) with respect to far field values even at a depth of 3 km. There was little evidence to suggest that significant mixing of the central core had yet taken place. Profiles in the meddy core showed the characteristic temperature maximum (at ∼ 775 m), the stability minimum (at ∼ 1250 m) and the stability maximum (at ∼ 1500 m) associated with Mediterranean Water outflow near the southwest corner of the Iberian Peninsula. Dynamic structures gave a core depth of ∼1100–1200 m, and at this level a number of well-defined structures were found that characterized the horizontal influence of the meddy. Surrounding an inner one (temperature >12.2°C, salinity >36.4 psu) of diameter about 25 km, in near solid body rotation (with centre period ∼3.7 days and relative normalized vorticity −0.5) there was a dynamically compatible broad potential vorticity front at a radius of ∼10–20 km. Maximum azimuthal currents (of ∼30 cm s −1 ) occurred at a radius of ∼22 km and at a radius of ∼27 km there was a water mass front where horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually travelling with the meddy, giving it an oblate discuss shape with aspect ration of 1.8%. Finally, at a radius of ∼40 km, where the potential vorticity gradient was no longer apparent, there was a perturbation front, indicative of mixing and exchange of water where the meddy penetrates the surrounding field.

The Droguing of Meddy Pinball and Seeding with Alace Floats

This paper reports the results of a hydrographic survey and the successful deliberate deep droguing of a meddy (Pinball) and the seeding of its core with two ALACE floats. The drogued buoy results give important kinematic properties of the eddy core in real timesemicolon the ALACE have allowed the position of the meddy to be tracked for seven months. Pinball was found against the continental slope near Lisbon canyon. The maximum core salinity was 36·564 psu, at a depth of 1260 m, but the maximum rotation rate with period ~2·5 d was in the upper core near 700 m, where temperatures reached 13·2°C. The azimuthal transport to a radius of 50 km was ~13 Sv. Pinball moved from the continental slope near Lisbon to the central Tagus Abyssal Plain, returned towards the continental slope and then moved westwards crossing the central Tagus Abyssal Plain a second time. At times it had a marked remote sensing infra-red sea-surface signature. It moved ~550 km over 204 d and the near real-time data meant that, in principle, this eddy could have been re-surveyed, redrogued or reseeded with floats during this period.

Physical, chemical and biological features of a cyclonic eddy in the region of 61°10'N 19°50'W in the North Atlantic

Abstract The second leg (CD61) of a two cruise investigation of coccolithophore biogeochemistry in the NE subarctic Atlantic provided the opportunity to make a detailed study of a cyclonic eddy in the vicinity of 61°N 20°W. The eddy field in the NE Atlantic is thought to be particularly important with regard to the physics of this region, and may influence the resulting chemical and biological properties of subarctic Atlantic waters. This eddy was ca. 50 km in diameter, moved at ca. 1.5 km d −1 to the north of east, with a geostrophic circulation around the feature of ca. 25 cm s −1 and probably extended as far as the ocean floor, where it may have interacted with the bottom topography. The horizontal salinity, nitrate and biological gradients between adjacent waters and the eddy were less marked in the present study than in a previous investigation of a cyclonic eddy in the vicinity of 48N 22°W (Mittelstaedt, 1987), possibly due to the surface waters of the eddy mixing with surrounding waters. Satellite image sequences clearly link this feature with those studied in a mesoscale coccolithophore bloom studied in the same region on a previous cruise (CD60). Rates of primary production within the eddy were almost twice the mean values reported for Ocean Weather Station India (OWSI) at this time of year, but were similar to those noted during studies at the MLML site to the SE of the eddy location. Other biological rate measurements also indicated that the NE sub-polar Atlantic in mid-summer is more active than previously thought. Despite the extensive coccolithophore bloom studied immediately previously by CD60, there was no measurable coccolithophore calcification in the waters within the eddy in the present study. This is consistent with phytoplankton taxonomic data, which demonstrates that coccolithophore abundance was almost one hundred fold lower at this location on CD61 relative to CD60 and that lith and coccolithophore abundances were grestest in the water column beneath the mixed layer, suggesting sinking. These observations suggest that the decline of the bloom had occurred in the period between the two cruises.

Drifting buoy in the field of flow of two eddies on East Thulean Rise (northeast Atlantic)

The track of an Argos buoy, drogued at a depth of 70 m, over a years deployment (from April 18, 1989 to April 18, 1990) in the NE Atlantic is described. The mean speed of the buoy over a years period was 1.4 cm s−1 to the NE. For 320 days there was little net movement, and during this time the buoy spent 66 days in a cyclonic eddy and 130 days in an anticyclonic eddy. The anticyclonic eddy diameter was about 200 km, and the buoy completed 21 complete revolutions around the eddy center. At a radius of 55 km, the rotation rate was 10 days. In the eye of the anticyclonic eddy the rotation rate was 36 hours. The inertial period for the latitude (∼51.5°N) was 15.3 hours, and inertial motions were occasionally marked in the anticyclonic eddy; the observed inertial period was about half an hour longer than the inertial period corresponding to the latitude. Both eddies appeared to be partially trapped to East Thulean Rise (water depth, 3000 m) and moved at a mean speed of about 1 cm s−1 to the NW along the rise, following water depth contours, though the anticyclonic eddy was stationary for about 50 days over the more central region of the rise. The track of an imperfectly drogued buoy in a circular eddy field is then examined analytically. A simple balance of stress forces on the drogued assembly is assumed, and commonly adopted assumptions for the stress forces are made. With assumptions, the idealized study shows that under conditions of a steady surface stress, a surface wind or current for example, a buoy can remain trapped within an eddy field and show no tendency for steady translation in the direction of the surface stress. For a clockwise eddy, the displacement of the buoys track is to the right of the surface stress direction. For larger surface stresses or time varying winds the buoy may be “blown out” of an eddy of limited extent.