On residual currents in the central strait of Georgia,B.C.: Research note

Abstract

Numerical modelling effort to understand low‐frequency circulation in the Strait of Georgia has been found to underestimate the strength of the circulation by roughly an order of magnitude. At least in part, this model defect may be due to the absence of statistical‐dynamical tendencies that result from eddy interactions (in reality). This defect is generic to ocean numerical models ranging from estuarine to global‐scale applications. A simple change to the formulation of eddy viscosity may help, making models somewhat “less wrong” if not yet “right”.     

Wind‐driven depth‐averaged circulation in Queen Charlotte Sound and Hecate Strait

Abstract

We develop a wind‐driven depth‐averaged model of the circulation on the continental shelf around the Queen Charlotte Islands. The model captures a major feature of the winter current‐meter observations: a flow in Moresby Trough against the direction of the prevailing winds. Moresby Trough is a steep submarine canyon cutting across the shelf from the Pacific Ocean to the mainland. The flow patterns revealed by simulated drifters lead to four generalizations about the depth‐averaged, wind‐driven flow: (1) the flow is subject to strong topographic steering, (2) the exchange between Queen Charlotte Sound and the Pacific Ocean is limited to small regions near Cape St James and Cape Scott, (3) the exchange between Queen Charlotte Sound and Hecate Strait is controlled by Moresby Trough, and (4) the observed outflows past Cape St James are not explained by the dynamics of this model.     

Low‐frequency subsurface current and density fluctuations in the strait of Georgia

Abstract

The depth‐time structure of low‐frequency subsurface current and density fluctuations in central Strait of Georgia is described. The measurements were made between February and May, 1981 with profiling Cyclesondes. The depth range of approximately 50 to 290 m was sampled in water depths of about 325 m. The mean current was cross‐strait and decreased with depth. The low‐frequency current fluctuations occurred predominantly at periods exceeding a week and also decreased with depth. Below about 200 m, there was an increasing tendency for the fluctuations to be aligned with the local topography. The maximum vertical separation for components of current to be significantly coherent was 100 m and was in the lowest frequency band. Currents separated by 4.1 km in the horizontal were correlated in the more energetic north‐south component.     

Residual currents in juan de Fuca strait

Abstract

We examine the residual (non‐tidal) flow in Juan de Fuca Strait on the west coast of Canada using current and bottom pressure data collected on cross‐channel sections in the summers of 1975 and 1984. A positive estuarine circulation was evident in both sections but was better defined at the mid‐strait section than at the outer section near the mouth of the strait. Magnitudes of the volume transports in both the upper and lower layers of the channel ranged from ～ 0.25 X 10 m s at the mid‐strait section to ～ 0.15 X 10 m s～ at the outer section. The method of geostrophic levelling (Garrett and Pétrie, 1981; Pétrie et al, 1988) is used to determine the relationship between the daily averaged long‐strait velocity component and the cross‐strait pressure difference. A statistically significant relationship, consistent with a cross‐strait geostrophic balance, is obtained between the time series of shallow currents and shallow bottom pressures at the mid‐strait section. The deep currents and cross‐strait pressure differences were correlated at both sections but, because of the placement of the pressure gauges and/or ageostrophic effects such as bottom friction, were not related through a simple geostrophic balance. Cross‐spectral analysis and the calculation of the current amplitude ratios (square root of the energy ratio between the deep and shallow currents) are used to examine the baroclinicity of the flow as a function of frequency. Results suggest that flow variability in Juan de Fuca Strait is strongly baroclinic and has marked cross‐channel structure throughout the low‐frequency band.     

Deep and intermediate water replacement in the Strait of Georgia

Abstract

We have studied deep‐water replacement processes in the Strait of Georgia using data from two different observational programs. From the monthly hydrographic data of Crean and Ages (1971) we have recognized the propagation of cold, brackish and well oxygenated waters northwards from Boundary Passage at depths between 75 and 200 m. We found a significant correlation over the years 1967–78 between surface cooling and temperature drops at those depths some months later. Measurements at an array of moorings in the central Strait of Georgia (Stacey et al., 1987) revealed the presence during summer months of currents concentrated near the bottom and varying with fortnightly and monthly periods. We have interpreted this phenomenon in terms of gravity currents originating from Boundary Passage during periods of neap tides and introducing at depth salty waters from the Strait of Juan de Fuca. Our analysis confirms in part the validity of the deep‐water replacement mechanisms advanced by Waldichuk; however, we find that wintertime replacement does not usually reach bottom while summertime penetration of waters from the Strait of Juan de Fuca clearly does. Because of the important role played by tidal mixing, monthly sampling is inadequate to resolve and understand the deep‐water replacement processes.     

Spatial and Temporal Variability of Ocean Temperature over the Labrador Shelf

Abstract

The mid‐to‐bottom waters of the Labrador Shelf are shown to exhibit an anomalous along‐shelf temperature gradient, with warmer waters found in the north. This feature is present in summer and autumn but appears to reverse in December. Inadequate data are available during winter and spring to draw firm conclusions regarding this feature. A time averaged heat loss of the shelf waters to the atmosphere would result on average, in colder waters in the south (because of north‐south advection); however, it is shown that there is a net annual‐mean input of heat to the shelf waters. An examination of the seasonal temperature cycle at standard depths reveals that its phase is almost uniform below 30‐m depth on the northern banks of the Labrador Shelf. The limited phase variation suggests the influence of a plume of well mixed water originating near Hudson Strait. It thus appears that mixing at the entrance to Hudson Strait imparts a phase anomaly to the seasonal cycle in the north that contributes to the observed inversion of the expected latitudinal temperature gradient.     

Stochastic and deterministic multicloud parameterizations for tropical convection

Despite recent advances in supercomputing, current general circulation models poorly represent the variability associated with organized tropical convection. In a recent study, the authors have shown, in the context of a paradigm two baroclinic mode system, that a stochastic multicloud convective parameterization based on three cloud types (congestus, deep and stratiform) can be used to improve the variability and the dynamical structure of tropical convection. Here, the stochastic multicloud model is modified with a lag type stratiform closure and augmented with an explicit mechanism for congestus detrainment moistening. These modifications improve the representation of intermittent coherent structures such as synoptic and mesoscale convective systems. Moreover, the new stratiform-lag closure allows for increased robustness of the coherent features of the model with respect to the amount of stochastic noise and leading to a multi-scale organization of slowly moving waves envelopes in which short-lived and chaotic convective events persist. Congestus cloud decks dominate the suppressed-dry phase of the wave envelopes. The simulations with the new closure have a higher amount of stochastic noise and result in a Walker type circulation with realistic mean and coherent variability which surpasses results of previous deterministic and stochastic multicloud models in the same parameter regime. Further, deterministic mean field limit equations (DMFLE) for the stochastic multicloud model are considered. Aside from providing a link to the deterministic multicloud parameterization, the DMFLE allow a judicious way of determining the amount of deterministic and stochastic “chaos” in the system. It is shown that with the old stratiform heating closure, the stochastic process accounts for most of the chaotic behavior. The simulations with the new stratiform heating closure exhibit a mixture of stochastic and deterministic chaos. The highly chaotic dynamics in the simulations with congestus detrainment mechanism is due to the strongly nonlinear and numerically stiff deterministic dynamics. In the latter two cases, the DMFLE can be viewed as a “standalone” parameterization, which is capable of capturing some dynamical features of the stochastic parameterization. Furthermore, it is shown that, in spatially extended simulations, the stochastic multicloud model can capture qualitatively two local statistical features of the observations: long and short auto-correlation times of moisture and precipitation, respectively and the approximate power-law in the probability density of precipitation event size for large precipitation events. The latter feature is not reproduced in the column simulations. This fact underscores the importance of gravity waves and large scale moisture convergence.     

On the mean and tidal currents in Hudson strait

Abstract

The vertical structures of the mean and tidal flows in Hudson Strait are described from moored current‐meter data collected during an 8‐week period in August to October of 1982. The residual flow in the strongly stratified waters off Quebec is directed along the Strait to the southeast, is highly baroclinic and is concentrated near shore (within an offshore length scale of approximately an internal Rossby radius). Maximum mean speeds of 0.3 m s^?1 were observed near‐surface (30 m). In the weakly stratified waters on the northern side of the Strait along Baffin Island the mean flow is northwestward. The maximum speeds are 0.1 m s^?1 near‐surface (30 m) and the current amplitudes decrease to 0.05 m s^?1 at 100 m. The mean southeastward transport is estimated to be 0.93 ±0.23 × 10⁶ m³ s^?1 with a northwestward transport of 0.82 ± 0.24 × 10⁶ m³ s^?1. Over most of the Strait the across‐channel residual currents are directed towards the Quebec shore with velocities ranging from 0.02 to 0.1 ms^?1. Current variability is dominated by the tides, the M₂ being the major tidal constituent. In the vicinity of the mooring the M₂ tide is primarily barotropic, progressive in nature, and has along‐channel current amplitudes varying across the Strait from 0.20 to 0.45 m s^?1. Observed differences in tidal sea‐level elevations across the Strait can be accounted for by the cross‐channel variations characteristic of Kelvin waves.     

Oceanographic features of Hecate Strait and Queen Charlotte Sound in Summer

Abstract

We present evidence of previously unresolved oceanographic features in Queen Charlotte Sound and Hecate Strait using data collected in the summer of 1990 and interpreted using a three‐dimensional, finite‐element diagnostic numerical model for two separate simulations: baroclinic flow without wind‐forcing and barotropic flow with wind‐forcing. Features include a strong, prevailing southward flow along the east coast of Moresby and Kunghit Islands, clockwise circulation around the edge of Middle Bank and a cold‐water plume flowing from the shallows at the north end of Aristazabal Island toward the south and through the trough between Middle Bank and Goose Island Bank A persistent (near‐surface) outflow into the Pacific Ocean is found near the surface within 20 km of Cape St. James at the southern tip of the Queen Charlotte Islands and intermittent surface outflows are observed across the mouth of Queen Charlotte Sound. In central Hecate Strait, to the north of Middle Bank, prevailing along‐strait currents are weak and there is an east‐west interleaving of two water masses: warm water from the west side of the strait and cold water from the east side.     

The Rose Spit Eddy in Dixon Entrance: Evidence for its existence and underlying dynamics

Abstract

The existence and dynamics of the so‐called “Rose Spit Eddy” in Dixon Entrance, British Columbia, are investigated by (i) analysing published observations of low‐frequency Eulerian and Lagrangian currents in the region; (ii) interpreting tidal residuals produced by the Hecate Model (a non‐rotating hydraulic model of Hecate Strait and Dixon Entrance); and (iii) running a barotropic, non‐linear numerical tidal model over simplified topography to investigate residuals produced over the Rose Spit sill.

Observations have consistently revealed persistent basin‐wide, surface‐intensified cyclonic shears within central and eastern Dixon Entrance. The Hecate hydraulic tidal model also produced a tidal residual cyclonic gyre in central Dixon Entrance, but with velocities considerably larger than those observed. Barotropic numerical simulations of tidal streams flowing over a representation of the Rose Spit sill produced residual flows along the sill in reasonable agreement with observations and theory. A southward‐directed jet flow was produced off Cape Chacon. Elsewhere, tidal rectification was weak. Run without the Coriolis force, organized flow along the sill broke down, although the headland jet off Cape Chacon persisted.

We submit that the observed Rose Spit eddy results from interactions between buoyancy‐driven coastal currents and tidally rectified flows generated over the Rose Spit sill, and near Cape Chacon, and perhaps indirectly, over the western flank of Learmonth Bank (which although west of the Rose Spit eddy, contributes to the cross‐channel flow across the Entrance). These regions of localized tidal stress will each favour recirculation of a portion of the coastal current within the Entrance, helping to form the eddy.

We believe that the Hecate hydraulic model eddy was generated to a significant degree by phase errors introduced at the northern open boundary, where a rocking barrier was used to force currents. A second rocking barrier also produced a large cyclonic gyre, not supported by observations, near the model's southern boundary.     

The baroclinic circulation in Hudson strait

Abstract

The baroclinic circulation in the mouth of Hudson Strait is modelled using general results for nearly geostrophic flow along an indented coastline. A simple T‐junction model is first discussed, followed by a somewhat more faithful idealization that includes the sharp northern tip of Labrador, the southwest tip of Baffin Island and part of Ungava Bay. The results show that the mouth of Hudson Strait does not present a significant obstacle to baroclinic flow in and out of it. We thus conclude that the observed recirculation must be due to other effects.     

Winter‐time near‐surface currents in the strait of Juan de Fuca

Abstract

During November 1976 to February 1977 near‐surface wind, current and temperature measurements were made at three sites along the Strait of Juan de Fuca. Strong tidal currents and major intrusions of warmer, fresher offshore coastal water were superimposed upon the estuarine circulation of near‐surface seaward flow. The r.m.s. amplitudes of the diurnal and semidiurnal tidal currents were ～30 cms^‐1 and 30–47 cm s^‐1, respectively. The vector‐mean flow at 4 m‐depth was seaward and decreased in speed from 28 cm s^‐1 at 74 km from the entrance to 9 cm s^‐1 at 11 km from the entrance. On five occasions intrusions of 1–3 C warmer northeast Pacific coastal water occurred for durations of 1–10 days. The 25 cm s^‐1 up‐strait speed of the intrusive lens agreed to within 20% of the gravity current speed computed from Benjamin's (1968) hydraulic model. The near‐surface currents associated with the intrusions and the southerly coastal winds were significantly correlated, indicating that the intrusions were initiated when shoreward Ekman currents advected Pacific coastal water into the Strait. The reversals were not significantly coherent with the along‐strait sea surface slope measured along the north side of the Strait nor were they strongly related to local wind forcing.     

A diagnostic study of the southern hemisphere summer circulation of the CCC general circulation model

Abstract

The medium‐scale wave regime, consisting largely of zonal wavenumbers 5–7, frequently dominates the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hövmöller diagrams, space‐time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium‐scale wave regime.

The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with the observations. After comparison with the observed December‐January‐February 1979 distributions, some quantitative differences remain: the heat transport is too weak aloft and too large near the surface, whereas the momentum transport tends to be too weak. The baroclinic and barotropic conversions show a maximum in the medium‐scale waves. The time evolution of the Richardson number of the mean flow suggests that the medium‐scale wave is due to a baroclinic instability.     

Northern hudson bay and foxe basin: Water masses,circulation and productivity

Abstract

Total alkalinity and total carbonate determinations together with salinity and temperature are used to characterize water masses in Foxe Basin, Hudson Bay and Hudson Strait. From these measurements, we are able to infer the amounts of fresh water from river runoff and from sea‐ice meltwater. The average ice cover is estimated to be 1.9 m, and the residence time of river runoff in Hudson Bay is 3—4 years. Estimates of biological productivity were made by “correcting” total carbonate measurements for effects of biological processes, giving a value of 24 gC m a for new production.     

THE INTERACTION BETWEEN EDDY FLUX AND ZONAL AVERAGED FLOW IN“JUNE SUDDEN CHANGE”

In this paper,Eliassen-Palm theory and associated diagnostic method are used to discuss the dynamicalmechanism of the interaction between eddy flux and zonal averaged flow in the sudden change of the generalcirculation in the Northern Hemisphere in early summer of 1982,which indicates that the northward jump ofsubtropic jet stream is closely related to the enhanced transport of mid-latitude eddy energy to the subtropic tro-popause.However,the direction of the transport is conditioned by the structure of zonal averaged flow.It isevident that the adjustment of zonal averaged flow responses quickly to the eddy transport during this episode.As regard to the suddenness of the change of circulation,the critical role is played by the eddy forcing,     

Numerical prediction of an intense convective system associated with the July 1987 montreal flood. part I: Gravity waves and the squall line

Abstract

In this study, a 24‐h high‐resolution numerical prediction of a prefrontal squall line associated with the 14 July 1987 Montreal flood is employed to investigate the origin and role of mesoscale gravity waves in the development of the squall system. The 24‐h integration using an improved mesoscale version of the Canadian regional finite‐element model is first validated against available observations; then non‐observable features are diagnosed to reveal the relationship between deep convection and gravity wave events. It is shown that the model reproduces well many aspects of the squall line, such as the propagation and organization of the convective system, as well as its associated precipitation. It is found that gravity waves are first excited near Lake Erie, following the initiation of early convective activity. Then, these waves propagate eastward and northeastward at speeds of 20 and 35 m s^‐1, respectively. As the waves propagate downstream, deep convection radiates rapidly behind the wave trough axis, forming a long line of squall convection. Because the squall line moves with the gravity waves in a “phase‐locked” manner, deep convection has a significant influence on the structure and amplitude of the gravity waves. The sensitivity of the wave‐squall prediction to various parameters in convective parameterization is also examined.     

The circulation and residence time of the strait of Georgia using a simple mixing‐box approach

Abstract

New observations in the Strait of Georgia, British Columbia, Canada show that temperature and dissolved oxygen have a pronounced seasonal cycle, with a spatially varying phase. Phase lags in oscillating systems arise due to internal time scales which can be interpreted in fluid systems as residence times. Exploiting phase we construct a quantitative and internally consistent circulation scheme for this body of water after dividing it into four regions: the Fraser River plume, the surface waters down to 50 m, the intermediate waters down to 200 m, and the deep water. In this scheme the intermediate water, the largest region by volume, is continually renewed, and its characteristics change in response to continuous changes in the characteristics of source waters. The dependence of the estuarine circulation on variations in fresh inflow is weak. The deep water is volumetrically less important, but seasonal changes in the density of oceanic source waters can produce a variation in the overall circulation by driving an additional inflow which leads to both deep renewal and increased upwelling. In turn, this increased upwelling results in lower surface temperatures than might otherwise be expected. Intermediate water residence times are about 160 days. Deep water is renewed once per year in summer and is affected only by vertical diffusion during the rest of the year. Surface water residence times for the entire Strait are a few months at most, but the Fraser River plume has a freshwater residence time of approximately 1 day. In addition, we find that the residence time of oceanic source waters in the Strait is 1.7 years due to a substantial recirculation in Haro Strait. Other consequences of this scheme are consistent with independent estimates of horizontal transports, air‐sea heat fluxes, subsurface oxygen (O₂) utilization, and primary production. Finally, analysis of the spatial phase variations suggests that the intermediate inflow enters the Strait as a boundary current along the slopes of the Fraser delta.     

On the interannual variability of arctic sea‐level pressure and sea ice

Abstract

Monthly mean sea‐level pressure (SLP) data from the Northern Hemisphere for the period January 1952‐December 1987 are analysed. Fluctuations in this field over the Arctic on interannual time‐scales and their statistical association with fluctuations farther south are determined. The standard deviation of the interannual variability is largest compared with that of the annual cycle along the seaboards of the major land masses. The SLP anomalies are generally in phase over the entire Arctic Basin and extend south over the northern Russia and Canada, but tend to be out of phase with fluctuations at mid‐latitudes. The anomalies are most closely associated with fluctuations over the North Atlantic and Europe except near the Chukchi Sea to the north of Bering Strait. The associations with the North Pacific fluctuations become increasingly more prominent at most Arctic sites (e.g. the Canadian Arctic Archipelago) as the time‐scale increases.

Associations between the SLP fluctuations and atmospheric indices that represent processes affecting sea‐ice drift (wind stress and wind stress curl) are determined. In every case local associations dominate, but some remote ones are also evident. For example, changes in the magnitude of the wind stress curl over the Beaufort Sea are increased if the atmospheric circulation over the North Pacific is intensified; wind stress over the region where sea ice is exchanged between the Beaufort Gyre and the Transpolar Drift Stream is modulated by both the Southern and North Atlantic Oscillations.

Severe sea‐ice conditions in the Greenland Sea (as measured by the Koch Ice Index) coincide with a weakened atmospheric circulation over the North Atlantic.     

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate. Correspondence to: CJC Reason     

Effects of variable wind shear on the mesoscale circulation forced by slab‐symmetric diabatic heating

Abstract

We examine the response of stably stratified airflow to a slab‐symmetric diabatic forcing associated with condensation in long‐lasting precipitation bands. The steady‐state linearized Boussinesq equations are used to model the diagnostic relationship between the vertical motion field, the heating source and the ambient flow. The basic‐state flow is assumed to be horizontally uniform and non‐rotating, but the static stability and wind vary in the vertical. Linear theory shows that the speed of the along‐band wind component is unimportant for slab‐symmetric heating since it cannot contribute towards the advection of buoyancy or vertical motion.

For typical atmospheric stratification and a moving heating source associated with a cloud band, the Taylor‐Goldstein equation is solved numerically. The numerical results show that the cross‐band wind shear tilts the updraft core and broadens it. While the magnitude of the shear is increased, the circulation becomes stronger. The details of the wind profile are also important in determining the intensity and structure of the circulation. When the wind profile indicates a convex bulge (i.e. the low‐level shear is weaker than the upper‐level shear), the circulation becomes slightly weaker in comparison with the linear wind profile. Conversely, the circulation becomes stronger when the wind profile has a concave shape. Increasing the concave bulge tends to enhance the circulation but not in a monotonic fashion. This non‐monotonic relation between the vertical motion and the parabolic wind profile is interpreted in terms of kinetic energy changes of parcels that interchange their altitudes.