G. I. Shapiro

Dynamics of Dense Water Cascades at the Shelf Edge

Abstract Plumes of dense shelf water cascade down continental slopes in many parts of the world’s oceans and provide a mechanism for shelf–ocean exchange. In this paper a nonlinear process-orientated theory is developed and used to examine the dynamics of cascading. The theory is formulated in terms of a “1½-layer” model and incorporates bottom topography, earth rotation, internal and bottom friction, and entrainment as well as externally imposed pressure gradients. The theory occupies a niche between the stream tube class of model (which considers only bulk properties of a plume) and the full three-dimensional primitive equation approach. The model provides useful insights into the complex interplay between the controlling forces, and it allows one to recover the shape and trajectory of dense plumes as well as the three-dimensional flow field inside the bottom layer. Asymptotic limits are investigated and lead to several basic results. A typical thickness of a fully developed plume is found to be twice t...

Distribution and spreading of Red Sea Water and salt lens formation in the northwest Indian Ocean

Abstract A long-term hydrographic data set collected between 1935 and 1981 is used to examine the spreading and mesoscale structure of Red Sea Water (RSW) in the northwest Indian Ocean. Four main branches of RSW are identified by statistical methods. It is shown by water mass and isopycnal analyses, that the spreading of RSW occurs partially as high-salinity, isolated patches (lenses), some of which also have positive temperature anomalies. Typical thermohaline properties of the RSW lenses (“Reddies”) are summarized. All of the identified Reddies are found in the regions where the factor P (which denotes the probability of RSW being present at a hydrographic station) has high horizontal gradients. The most probable mechanism of Reddy generation is the instability of the RSW main tongue.

The Malin cascade in winter 1996

We report evidence for cascading of shelf-edge water down the continental slope northwest of Ireland at 55N in February 1996. The measured surface density contrast between shelf-edge and slope waters was 0.01 kg m-3 and the resulting dense plume was 20-50 m thick over the continental slope and penetrated to a depth of 500 m. Cascade waters were warmer but saltier than overlying slope waters and their shelf-edge origin was confirmed by relatively higher chlorophyll concentrations and a distinctive nutrient signature. Other cross-slope transects 100 km north and south of the cascade site showed dense water at the shelf edge (a situation assumed to be favorable for cascading) but no deep downslope penetration of shelf water. A one-and-a-half layer dynamical model indicates that the observed downslope penetration of the cascade can be accounted for by the measured excess density of shelf-edge waters even with a realistic level of entrainment of overlying slope waters.

Spreading pattern and mesoscale structure of Mediterranean outflow in the Iberian Basin estimated from historical data

Abstract The location, spreading pattern, mesoscale structure and the influence of bottom topography on the Mediterranean Water (MW) outflow were studied using historical hydrographie data from the Iberian Basin of the North Atlantic. It was shown on the basis of joint analysis of continuous and discrete component of the MW distribution, that after leaving the Gulf of Cadiz the MW outflow is broken into two main cores: Northwestern and Southwestern ones. The wide, intense Northwestern core spreads in a regular and continuous manner. West off the Tejo plateau it is split into three branches, the most intense of them keeps continuity up to 14 ° W. The less intense Southwestern core passes south of the Gettysburg bank and is split into two branches just after the Gulf of Cadiz. This core has a lenticular intermittent character starting with 11° W. West of 15° W all the branches of MW outflow spread in the form of isolated salty patches. In the Iberian Basin (32 ° − 44 °N, 8 ° − 22 ° W) 30 MW lenses were found out, 12 of which were not mentioned in the literature before.

Cross‐shelf exchange in the northwestern Black Sea

The transports of water, heat, and salt between the northwestern shelf and deep interior of the Black Sea are investigated using a high-resolution three-dimensional primitive equation model. From April to August 2005, both onshore and offshore cross-shelf break transports in the top 20 m were 0.24 Sv on average, which is equivalent to the replacement of 60% of the volume of surface shelf waters (0–20 m) per month. Two main exchange mechanisms are studied: Ekman transport, and transport by mesoscale eddies and associated meanders of the Rim Current. The Ekman drift causes nearly uniform onshore or offshore flow over a large section of the shelf break, but it is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of 15–22 April and 1–4 July, some 0.66 × 1012 and 0.44 × 1012 m3 of water were removed from the northwestern shelf, respectively. In comparison, the single long-lived Sevastopol Eddy generated a much larger offshore transfer of 2.84 × 1012 m3 over the period 23 April to 30 June, which is equivalent to 102% of the volume of northwestern shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m−3 and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m−3 and sharpened the shelf break front.

A young lens of Red Sea Water in the Arabian Sea

CTD data collected in March–April 1980 in the Arabian Sea during the 22nd cruise of R.V. Akademik Vernadsky were used to investigate the structure and dynamics of a young lens of Red Sea Water (Reddy) in the initial stage of its formation. The core of the young lens with maximum temperature of 11.07°C and maximum salinity of 35.56 was located at depths of 640–830 m, and had temperature and salinity anomalies of 0.75°C and 0.22 with respect to the background water. The lens core, of thickness of 190 m, was connected to the main Red Sea Water tongue by a thin layer with vertical extent of about 20 m. In contrast to Meddies frequently observed in the North Atlantic, the density anomaly of the Reddy did not exceed 0.04 kg · m-3. A trajectory of the lens movement was reconstructed. The dynamic impact of a deep-reaching cyclonic meander on the large-scale tongue of Red Sea Water resulted in the Reddy formation near 16°N, 61°E and its subsequent movement to the south over a distance of about 370 km.

On the Structure of Wave Fronts in Nonlinear Dissipative Media

Wave fronts are well known to be the regions of space where the properties of motion vary sharply in a certain direction. This means that one of the components of the gradients of density, velocity, stress, etc., at the wave fronts exceeds essentially not only other components but also corresponding gradients outside the fronts. The large gradients require that the analysis of motion within the fronts (the front structure) be based on equations more detailed than those sufficient in the surroundings. Meanwhile these more detailed equations admit simplification of the boundary-layer type because the fronts are necessarily slender in one direction. In the “external” approximation the fronts are represented by discontinuity surfaces (Figure 1, a).

Capture and release of Lagrangian floats by eddies in shear flow

A mechanism to explain the observed trapping or release of Lagrangian drifters by eddies is proposed, based on an interaction between the eddy circulation and the background flow. Eddies strong in relation to the background have a large “trap zone” within which particles are constrained to circulate in closed paths about the center, while relatively weak eddies have a smaller trap zone. Particles outside the trap zone drift freely, while those inside rotate with the eddy. As an eddy is advected by non-uniform shear flow, its ‘trap zone’ expands or contracts so particles previously free become trapped or trapped particles are freed. Numerical experiments with idealized eddies embedded in different flows illustrate the concept. It is shown that drifters may begin to loop, as in some previously reported observations, without any need for eddy generation. Similarly, they may cease looping, without eddy destruction. The particular case of a strong cyclonic eddy south of Gran Canaria is modeled, using the observed density field and two drifter trajectories. Predicted drifter trajectories are compared with the observations. It is shown that the trajectories of buoys launched near the edge of the eddy are sensitive to small variations in the background field.

Using a lowered acoustic Doppler current profiler for measuring current velocities in the Black Sea

The results obtained with the use of a lowered acoustic Doppler current profiler (LADCP) are presented. The use of the LADCP from a vessel was the first in the history of the study of the Black Sea. The measurements were carried out in the northeastern Black Sea under the auspices of the Black Sea Ecosystem Recovery Program (BSERP) in May 2004. The effect of the computation parameters on the quality and accuracy of the calculations of velocity profiles was studied. It was shown that the use of optimal parameters and reliable navigation data and setting the instrument as close to the bottom as possible could essentially enhance the accuracy of the measurements. The current velocity calculations from the LADCP data were compared with the data on the vessel drift under calm weather. The accuracy of the calculations reached 6–8 cm/s. Recommendations on the choice of the optimal parameters for processing the data on the current velocity are presented.

On the dynamics of lenslike eddies

Abstract The shallow water reduced gravity model is used to obtain some integral properties of lenslike eddies. It is shown that a nearly circular monopole eddy transporting water in its core cannot exist in a stationary motion, one exception being compact lenslike eddies for which the problem is open. A generalized near-geostrophic model is presented which allows to describe the dynamics of motion of different types. The rate of the non-frictional decay of a lenslike eddy is estimated on the basis of the integral approach using the generalized near-geostrophic model. A simple analytical formula is presented for the rate of decay of a nearly circular eddy greater than the radius of deformation.