Shallow Sea Fronts Research Articles

Coastal oceanographic regimes of the Northern Argentine Continental Shelf (34–43°S)

The oceanographic regimes of the Northern Argentine Continental Shelf (NACS, 34°–43°S) are derived from advected waters of subantarctic origin, local sources of continental run-off, and a locally generated salinity maximum. Based on 3690 CTD profiles, monthly mean wind fields at coastal stations, and river discharge data, we define the oceanographic regimes over the shelf by analyzing salinity characteristics and spatial distribution: (1) a maximum in salinity (33.7–34.2) originating from the Gulf of San Matías; (2) a relative salinity minimum (30.0–33.3) of the El Rincón estuarine system; (3) a salinity minimum (0–33.0) originating in the Río de la Plata; and (4) waters of the continental shelf (33.5 and 33.7). Temperature over the shelf is controlled by sea–air heat exchange coupled with bathymetry. An analysis of the Simpson parameter of stability ( ϕ) provided an objective definition of a vertically homogenous coastal zone separated from seasonally stratified shelf waters south of 37°S. Bottom temperature gradients and synoptic sections in the winter and spring indicate the presence of a shallow sea front at the 40–50-m isobaths south of 37°S, persistent throughout the year. We define two seasonal periods, autumn–winter and spring–summer, based on seasonality in monthly mean winds fields, continental run-off, fresh water balance and the spatial distribution of salinity signals. Maximum seasonal variation in the extent and location of the oceanographic regimes occurs within the coastal zone. In the autumn–winter period, we observe a northward extension of the Río de la Plata and Gulf of San Matías waters, as well as a reduction of the El Rincón and Continental Shelf waters near the coast. The spring–summer period is characterized by Río de la Plata waters flowing to the south and east, a reduction of Gulf of San Matías waters and an invasion of El Rincón and Continental Shelf waters into the coastal areas. In a general sense, waters across the NACS undergo a seasonal oscillation in distribution and extension that implies a spring–summer reversal of the characteristic shelf-wide north–northeastward direction of flow within the coastal zone.

A laboratory simulation of mixing across tidal fronts

A laboratory study which simulates the dynamics of shallow sea fronts and the mixing across a tidal front is described. The experiments show, for the first time, that it is possible to simulate a stationary tidal front with the inclusion of buoyancy effects, Coriolis effects and turbulence in the laboratory. Experimental data obtained for the cross-front mixing rate are presented. The data analysis shows that the mixing rate increases with stratification and decreases with rotation. A theoretical model of the flow which collapses the experimental data is developed which shows that the cross-front mixing is controlled by baroclinic processes. The model enables an extrapolation of the laboratory results to oceanographic conditions. Estimates of the cross-front mixing velocity for oceanographic conditions give values consistent with estimates obtained from North Sea data.

The Oceanographic and Biological Tidal Cycle Succession in Shallow Sea Fronts in the North Sea and the English Channel

Two major discrepancies between existing theories concerning the positioning and development of shallow shelf sea fronts and available data are that these models (1) do not adequately describe the neap-spring adjustment of frontal position and (2) these theories fail to explain the relatively small changes in the location of the front despite the rapid seasonal decay of the stabilizing surface heat flux.In the model presented here, these weaknesses in earlier attempts to describe frontal position are overcome. This is achieved partly by incorporating wind stirring in the heat and mixing balance for the frontal area and partly by allowing for a cross-frontal circulation. This new model defines two types of shallow sea fronts: tidal 'surface' and tidal 'bottom' fronts. Surface fronts are dominated by tidal and bottom fronts by wind stirring. The front type determines the circulation pattern on the stratified side of the front. The circulation pattern, in turn, governs the biological activity. The model describes the persistent but temporally varying redistribution of water from the bottom and surface layers to the thermocline layer which supplies nutrients from the deep (light limited) to the shallow regions where phytoplankton activity is stimulated. This new interpretation of the spring-neap adjustment also implies that, during the heating season, an increasing part of the potential energy to be overcome during a tidal cycle stems from the mixing of nutrient-rich dense bottom water and light surface water imported from offshore. The existence of this mechanism is supported by field observations.

A study on generation mechanism of a shallow sea front and its variabilities—lecture by the member awarded the Okada Prize of the Oceanographical Society of Japan for 1991—

The author reviews his study on generation mechanism of a shallow sea front and its variabilities awarded the Okada Prize of the Oceanographical Society of Japan for 1991.

Coupling between mixing and advection in a shallow sea front

Tidal fronts in the Celtic Sea were numerically simulated for a 43 day period under realistic conditions of atmospheric and tidal forcing. In order to properly examine the boundary mixing we embedded a one-dimensional mixed-layer model into a three-dimensional general circulation model. Simulation from the embedded model shows generation of a tidal front that separates the well-mixed Irish Sea water from the stratified Celtic Sea water. The surface front does not respond to the spring-neap tidal variation, rather it is controlled mainly by the atmospheric forcing. During upwelling favorable winds the surface front moves with a southward Ekman transport and upwelling occurs at the edge of the tidally mixed water. Also, associated with the advancing surface front there is a prominent double-celled circulation. On the other hand, the surface front remains almost stationary during downwelling events. Model studies suggest that the atmospheric forcing plays an essential part of the thermocline evolution in shallow sea fronts.

A Numerical Study of a Shallow Sea Front Generated by Buoyancy Flux: Generation Mechanism

We studied numerically the frontogenesis of shallow sea fronts such as are observed in the Kii Channel, Japan, during winter, under conditions of sea surface cooling and buoyancy influx from coast and open ocean. Numerical experiments were carried out in a vertically two-dimensional basin with a new model (NH-model), without using the hydrostatic approximation and the convective adjustment. Considering the vertical acceleration term in the momentum equation, intermittent gravitational convections with a large aspect ratio were produced in the frontal region to intensify the horizontal convergence and to strengthen the horizontal density gradient. Consequently, a front in the tracer distribution had a sharpness comparable to the observed front in the Kii Channel and 3.6 times the sharpness in an H-Model, using the hydrostatic approximation and the convective adjustment. In the present model situation, this effect of gravitational convections on sharpening a front in the NH-model is equivalent to that of the 10-time cooling rate in the H-model on the time average. Moreover, the convections intermittently intensified the sharpness of the front up to 2.4 times the time-averaged value: In the H-model, such a sharp front could not be formed even if the cooling rate was increased 10 times. Further, we discussed the effect of eddy viscosity and diffusivity on frontogenesis.

A numerical study of a shallow sea front generated by the buoyancy flux

We numerically investigated the physical process of water exchange caused by fluctuations of the front. This front is formed in a vertically two-dimensional NH-model (non-hydrostatic model) under steady forcing and simulates well the front observed during winter in the Kii Channel, Japan. The velocity field in the model has two kinds of oscillations. The first has a period of 6∼12 hr and is caused by intermittent gravitational convection in the frontal zone. The period and the intensity of intermittent convection are determined by buoyancy flux through the side boundaries as well as surface cooling. The other is associated with large scale circulation driven at the side boundaries and is controlled by the Coriolis force and the bottom stress. Its period of 3∼4 days is determined by the sum of the inertial period and the spin down time for the baroclinic mode of the along-front velocity component. These oscillations make the position of the front fluctuate with the same periods.

Relations between environmental structure of front and the change of concentration of fish larvae and eggs. 1. Water temperature patterns and distributions of fish eggs and larvae in the vicinity of shallow sea front.

Water temperature patterns and distributions of fish eggs and larvae in the vicinity of shallow sea front were observed at the mouth of Wakasa Bay, the Japan Sea. The front was formed between nearshore and offshore waters. The concentration per 100m3 of eggs and larvae of Japanese anchovy Engraulis japonicus in the surface layer sometimes corresponded with the direc-tion and scale of the front. The effect of the swimming activity of a larva on distribution pattern was little during the first two days after hatching out. The front shifted horizontally with tidal cycle and its pattern was affected by the longer period wave. The wave longer than tide period was caused by typhoons and low pressure.

Behavior and Distribution of Larval and Early Jevenile Homarus americanus

Field behavioral observations and plankton tows show that American lobster (Homarus americanus) larvae are concentrated in downwellings characteristic of shallow sea fronts. Stage IV lobsters swimming at the surface avoided floating weeds and sticks. Seventy percent of laboratory-reared fourth stage lobsters swam near the surface for more than a minute when released but only 25% of fifth stage lobsters did so. When early fourth stage lobsters encountered bottom, they did not remain long in one place, but late fourth and early fifth stage lobsters began to burrow. Late fourth and early fifth stage lobsters were less likely to return to the surface when they reached the bottom. If the bottom was unsuitable (featureless sand), the lobsters resumed swimming. These behaviors appear to be an appropriate mechanism for substrate selection during settlement.

Fronts and Seabird Aggregations in the Southeastern Bering Sea

Fronts and Seabird Aggregations in the Southeastern Bering Sea

Microbiological and zooplankton activity at a front in Liverpool Bay

Shallow sea fronts, zones of transition between mixed and stratified waters, are generally characterized by accumulation of biomass1,2. Various mechanisms can be suggested for this accumulation. Pingree et al.1 considered that phytoplankton are limited on the stratified side by nutrient supply and on the mixed side by light intensity, whereas at the front itself, recently stabilized mixed water provides conditions for rapid growth. Adjacent water masses may also complement each other's nutrient deficiencies. At the western Irish Sea front for example, silica has been limiting on the mixed side and combined nitrogen on the stratified side, so that the front supported more algal growth than either water separately2. Another possibility, as fronts are regions of convergence, is advection of suspended matter, which may include detritus, phytoplankton and zooplankton3. This is unlikely to end with the passive accumulation of material at the front: bacterial mineralization of organic detritus will stimulate phytoplankton growth, bacterial and phytoplankton growth will support increased zooplankton activity and this in turn will increase the rate of recycling. Although little investigated, this mechanism seems to be important in a front existing in Liverpool Bay, as we report here.

A note on the circulation induced by a shallow-sea front

The density current induced by a front which is a transition between well-mixed and stratified water is examined by means of a two-dimensional numerical model. It is shown that this may cause a convergence and up-welling near the front, which could have important biological consequences, together with a flow along the line of the front.