Density currents induced by freezing in a shallow polar ocean : some heuristic models

An ice cover which progrades over a shallow ocean in the form of an ice front may induce a baroclinic flow in the underlying water, provided that the brine excluded from the freezing surface layer is mixed rapidly through out the underlying water column. A series of models are developed to gain insight into the significance of this process in an initially motionless ocean and of its interaction with a pre-established circulation. For a simplified case, the near-surface flow rate is of the order of 2 cm sec⁻¹ and directed to the east in the northern Hemisphere. The volume transport under the ice front and assuming a typical depth of 100 m, is about 0.1 Sverdrup. However, for the case of an interaction with pre-existing circulation, and in particular with a circular motion, higher speeds of the induced currents may be expected. It might be concluded that the process may play a significant role in winter circulation in shallow marginal polar seas, where a winter pycnocline is absent

M2 Currents And Residual Flow In The Gulf Of California

The full nonlinear, vertically-averaged equations of motion are time integrated through a finite difference scheme on a regular grid that reproduces the topography of the Gulf of Califor nia with a mesh size of 14 km. The model is forced through a remote, open-ocean boundary to the south by a linear superposition of the incoming and outgoing Kelvin waves of the M2 tide. Fric tion, parameterized by a quadratic depth dependent law, and a viscosity close to 103.m2.s -I, suffice to stabilize the model. The resulting maps of tidal elevations and phase agree adequately with those drawn by other authors from coastal observations. The computed tidal flows are dominated by the effects of the large bathymetric gradients; the currents acceler ate markedly in the narrows and straits around the Gulf's large islands, and effects of resonance and bottom friction are evident in the shallow north ern Gulf. An average of the computed flow over an integral number of tidal cycles yields a residual circulation with typical values two orders of mag nitude smaller than those of the instantaneous flow, organized in basin-wide closed cells with ad jacent gyres rotating in opposite senses. The lar gest cell is a counterclockwise gyre found in the northern Gulf, which is probably driven by a trans fer of tidal vorticity to the mean field through a bottom frictional torque. Residual flows are also important in the region of the large islands where the rectification is induced by the large topo graphic gradients in the channels and sills. The M2 residual circulation should remove a passive tracer from the upper Gulf in several months, or close to one year. No appreciable residual effects of the M2 tide are observed in the southern part of the Gulf.Las ecuaciones de movimiento completas no lineales promediadas verticalmente se integran en el tiempo a través de un esquema de diferencias finitas en una cuadrícula regular que reproduce la topografía del Golfo de California con un tamaño de malla de 14 km. El modelo es forzado a través de un límite remoto de mar abierto hacia el sur por una superposición lineal de las ondas Kelvin entrantes y salientes de la marea M2. La fricción, parametrizada por una ley dependiente de la profundidad cuadrática, y una viscosidad cercana a 103.m2.s -I, son suficientes para estabilizar el modelo. Los mapas resultantes de elevaciones y fases de las mareas concuerdan adecuadamente con los elaborados por otros autores a partir de observaciones costeras. Los flujos de marea calculados están dominados por los efectos de los grandes gradientes batimétricos; las corrientes se aceleran marcadamente en los estrechos y angostos alrededor de las islas grandes del golfo, y los efectos de la resonancia y la fricción del fondo son evidentes en el norte poco profundo del golfo. Un promedio del caudal calculado durante un número entero de ciclos de marea produce una circulación residual con valores típicos dos órdenes de magnitud más pequeños que los del caudal instantáneo, organizados en celdas cerradas a lo ancho de la cuenca con giros adyacentes que giran en sentidos opuestos. La celda más grande es un giro en sentido contrario a las agujas del reloj que se encuentra en el norte del Golfo, que probablemente es impulsado por una transferencia de la vorticidad de las mareas al campo medio a través de un par de fricción en el fondo. Los flujos residuales también son importantes en la región de las islas grandes donde la rectificación es inducida por los grandes gradientes topográficos en los canales y sills. La circulación residual de M2 ​​debería eliminar un marcador pasivo de la parte superior del Golfo en varios meses, o cerca de un año. No se observan efectos residuales apreciables de la marea M2 en la parte sur del Golfo.Universidad Nacional, Costa Rica.Departamento de Físic

LAGRANGIAN OBSERVATIONS OF NEAR-SURFACE CURRENTS IN CANALDEBALLENAS

ABSTRACT Radio buoys tracked at depths of 0 and 50 m in the northern portion of Canal de Ballenas, Gulf of California, in June 1982, show a current dominated by alongchannel oscillations, on which are superimposed smaller-scale turbulent eddies. Maximum displacements and velocities of the motions recorded alongchannel are O(15 km; 3 m sec-'), and crosschannel they are O(5 km; 0.5 m sec-I). The mean velocity was generally towards the southeast along the channel and close to two orders of magnitude smaller than the maximum instantaneous velocities observed. The eddy-like motions, which had no preferred sense of rotation, had typical scales similar to those of the crosschannel excursions and appeared to trap the buoys after these had traveled southeastward for a few hours. This suggests a spatial variation of the flow, which might be associated with the rough configuration of the channel's coastline and bottom topography. Spectral analysis of the drogue excursions about their mean positions indicated that the motions were probably related to the semidiurnal tide and, at the surface, to the strong winds that blew from the mountain passes in Baja California, and also showed a large spatial variability. Logistics and weather conditions made longer and more precise measurements impossible, and it is difficult to conclude here on the dynamics of the flow in Canal de Ballenas. These measurements, to our knowledge, represent the first direct measurements of circulation reported for this area. aproximadamente dos 6rdenes de magnitud menor que las velocidades instantheas maximas observadas. Los giros turbulentos se observaron carentes de sentido preferente de rotaci6n y con escalas tipicas comparables a la de 10s desplazamientos transversales a1 canal. En apariencia, dichos giros atraparon a las boyas despuCs de que Cstas se desplazaron hacia el sureste por algunas horas. Esto sugiere una variaci6n espacial del flujo, asociada posiblemente con la configuraci6n irregular de la costa y del fondo del canal. El analisis espectral de la excursi6n de las boyas con respecto a su posici6n media indica una relaci6n probable con la marea semidiurna y, en superficie, con 10s vientos fuertes provenientes de 10s pasos de las montaiias de Baja California, 10s cuales presentaron asimismo una marcada estructura espacial. Las condiciones atmosfkricas locales y logisticas no permitieron efectuar observaciones mas prolongadas y precisas, por lo que es dificil concluir aqui sobre la dinarnica de la corriente en Canal de Ballenas. El principal inter& de nuestr6s resultados reside en que representan las primeras observaciones directas de la circulaci6n dentro del canal. RESUME