Diane Masson

Seasonal water mass analysis for the straits of Juan de fuca and Georgia

Abstract A quantitative analysis of water masses in the coastal waters of southern British Columbia is performed with the Optimum Multiparameter (OMP) analysis method that optimizes the use of a hydrographie dataset by solving an over‐determined linear set of mixing equations. The method is applied to a seasonal dataset collected over five years in the Strait of Georgia, a large semi‐enclosed coastal basin, as well as in Juan de Fuca Strait, its main connection to the Pacific Ocean. Abundant freshwater discharge into the coastal basin forces an estuarine exchange with oceanic shelf water. Six water characteristics of five source water types are used to obtain mixing proportions over the estuary for each of the four seasons. The model results are found to corroborate known aspects of the local dynamics such as the presence of a deep shelf inflow into Juan de Fuca Strait and of the Columbia River plume in winter at the mouth of the strait. The analysis also quantifies lesser known features of the region, such as the characteristics of the mid‐depth intrusions within the Strait of Georgia and the marked effect of remineralization on nutrient distributions in the deep water of the coastal basin.

Fortnightly modulation of the estuarine circulation in Juan de Fuca Strait

Riverine discharge into the Strait of Georgia sets up a well-defined estuarine circulation within Juan de Fuca Strait, the main path for the freshwater outflow to the continental shelf. At the landward end of Juan de Fuca Strait, the water flows through narrow channels in which strong tidal currents are known to induce significant mixing of the water column, and a spring-neap modulation of the estuarine exchange. A three-dimensional prognostic numerical model has been developed to study the circulation around Vancouver Island, British Columbia. In a series of simulations, the estuarine circulation within Juan de Fuca Strait is established by the Fraser River freshwater discharge. A fortnightly modulation is imposed on the mixing over the various sills to simulate the spring-neap tidal mixing regime. The resulting variation in the estuarine circulation is found to be largely limited to the eastern section of Juan de Fuca Strait, in the vicinity of the sills. Data from current meter moorings and surface salinity data from lighthouse stations compare favorably with the model results. The effect of local wind forcing on the estuarine exchange is also examined. The model is capable of simulating those rare events during which a concurrence of river freshet, neap tide and northwest wind allows a stronger pulse of fresh surface water to escape relatively unmixed into the eastern end of Juan de Fuca Strait. The disturbance then propagates along the northern shore of the strait as a first mode internal Kelvin wave. Finally, the effect of the fortnightly modulation on the export of freshwater onto the continental shelf is examined. It is found that small amplitude coastal trapped waves are generated near the mouth of Juan de Fuca. However, this fortnightly signal is weak in comparison to the energetic wind-induced variations typically found over the shelf.

Stratification and Mean Flow Effects on Diurnal Tidal Currents off Vancouver Island

Abstract A series of numerical experiments with a three-dimensional, baroclinic model were conducted to study the influences of density stratification and wind-driven currents on the K1 tide over the continental shelf off Vancouver Island. The region is one of anomalously large diurnal tidal currents due to the generation of coastally trapped waves at the entrance to Juan de Fuca Strait. Model results are compared with data obtained from a number of moorings deployed over the shelf, including a line extending for over 300 km in the alongshore direction. The results show that inclusion of stratification significantly improves the representation of K1 currents in the model, particularly with respect to the alongshore phase propagation of the clockwise and counterclockwise rotary components. With homogeneous water, the coastally trapped waves propagate relatively slowly and are dissipated before reaching Brooks Peninsula. In contrast, the ambient stratification permits coastally trapped motions to propagate ...

Distribution and Cycling of Suspended Particles Inferred from Transmissivity in the Strait of Georgia, Haro Strait and Juan de Fuca Strait

Abstract Transmissometer profiles collected during quarterly cruises in 2000–03 indicate that most suspended particles in the Strait of Georgia and Juan de Fuca Strait are confined to the top and bottom of the water column. In Haro Strait, however, the particles are mixed throughout the water column by strong tidal currents, producing an estuarine turbidity maximum. The distribution and cycling of particles provide important aquatic controls for contaminant transport, photochemical reactions and photosynthesis. Particle‐associated contaminants that enter Haro Strait may remain in suspension and enter the food web at all water depths. In the autumn and sometimes in the summer, the rate of photochemical transformation of dissolved organic matter is probably higher in Juan de Fuca Strait than in the Strait of Georgia or Haro Strait, because the surface water in Juan de Fuca Strait is less turbid.

Numerical Simulations of a Buoyancy-Driven Coastal Countercurrent off Vancouver Island

Abstract A three-dimensional prognostic numerical model has been developed to study the ocean circulation around Vancouver Island, British Columbia. In a series of simulations, the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of Vancouver Island. The forcing is provided by the Fraser River discharge into the Strait of Georgia. An estuarine circulation establishes itself in Juan de Fuca Strait, from which a distinctive right-bounded current is formed and advances along the coast. Sensitivity studies are conducted to determine the robustness of this current to initial conditions, opposing wind, enhanced vertical mixing, and grid resolution. Finally, various characteristics of the numerically modeled coastal flow are compared with observations.

Observations and modeling of seasonal variability in the Straits of Georgia and Juan de Fuca

The Strait of Georgia is a large semi-enclosed basin on the southern coast of British Columbia. Its main connection to the Pacific is to the south, through Juan de Fuca Strait. Abundant freshwater discharge, mainly from the Fraser River, forces an estuarine exchange with oceanic shelf water. The resulting circulation is modulated by tides and winds. Both the coastal wind stress and the flux of freshwater are subject to strong seasonal modulations, producing a marked seasonal cycle in the water properties and circulation of the region. The seasonal variability of the Strait of Georgia and Juan de Fuca Strait is described using data from a series of recent cruises conducted over a five-year period, in addition to longer term historical data sets. To complement the observations, the data are compared with numerical simulations based on the Princeton Ocean Model (POM). Forced with tides, freshwater discharge and seasonal wind stress, the model is integrated over a few years until the system approaches statistical equilibrium. Results show good agreement with observations from Juan de Fuca Strait, as well as over the upper part of the water column within the Strait of Georgia. However, simulation of the seasonal cycle of the deeper waters of the Strait of Georgia is more problematic. The deep water properties apparently are determined by a delicate balance between dense intrusions from the sill area and local vertical mixing.

Water column organic carbon in a Pacific marginal sea (Strait of Georgia, Canada)

Marginal seas provide a globally important interface between land and interior ocean where organic carbon is metabolized, buried or exported. The trophic status of these seas varies seasonally, depending on river flow, primary production, the proportion of dissolved to particulate organic carbon and other factors. In the Strait of Georgia, about 80% of the organic carbon in the water column is dissolved. Organic carbon enters at the surface, with river discharge and primary production, particularly during spring and summer. The amount of organic carbon passing through the Strait (approximately 16x10(8) kg C yr(-1)) is almost twice the standing inventory (approximately 9.4x10(8) kg C). The organic carbon that is oxidized within the Strait (approximately 5.6x10(8) kg yr(-1)) presumably supports microbial food webs or participates in chemical or photochemical reactions, while that which is exported (7.2x10(8) kg yr(-1)) represents a local source of organic carbon to the open ocean.

A Model Simulation of Future Oceanic Conditions along the British Columbia Continental Shelf. Part I: Forcing Fields and Initial Conditions

Abstract Techniques for, and the results of, downscaling forcing and initial fields for an ocean-only regional climate model of the British Columbia continental shelf are presented. Fields from one regional-global model combination within the North American Regional Climate Change Assessment Program archive are shown to be representative of a larger ensemble of eight. Because model winds over the baseline period of 1970 to 1999 represent upwelling favourable conditions poorly, a strategy of computing future-minus-contemporary anomalies and adding them to the analogous values used in a recent hindcast simulation for the same region is justified and adopted. Average surface air temperatures over the future period of 2040 to 2069 are projected to be warmer for all months, with those in January generally having the highest increases and those in June the lowest. Because average precipitation is generally projected to increase in winter and decrease in summer, total freshwater discharges follow a similar pattern, increasing by about 10% in all months except June to August when they are projected to decrease by up to 10%. Though projected changes to the seasonally averaged heat flux are shown here, the consequence of applying all these forcing and initial fields to a circulation model of the British Columbia continental shelf is described in a subsequent companion manuscript.

A Model Simulation of Future Oceanic Conditions along the British Columbia Continental Shelf. Part II: Results and Analyses

Abstract An ocean circulation model for the British Columbia continental shelf is run with future initial conditions and forcing fields downscaled from the North American Regional Climate Change Assessment Program archive. Average seasonal sea surface temperatures for the period 2065 to 2078 are projected to increase by between 0.5° and 2.0°C with respect to analogous averages from 1995 to 2008. Seasonal sea surface salinities are projected to decrease by as much as 2.0 over the same period, though there are some regions where and periods when small increases are projected. Though stronger winter winds result in larger Haida Eddies, slightly stronger summer winds along the western Vancouver Island shelf do not result in appreciable changes to either the cross-shelf upwelling or to the magnitude of Juan de Fuca Eddies or the timing of their formation. However, increased flows are projected in some seasons for the Rose Spit, Middle Bank, and Goose Island Bank eddies. More precipitation over the watersheds emptying into coastal waters produces larger freshwater discharges and, in particular, a stronger estuarine flow in Juan de Fuca Strait and a stronger Vancouver Island Coastal Current. Generally increasing winds and decreasing density mean that the winter minus summer range of sea surface heights is projected to increase all along the coast.

Vertical heat flux in the ocean: Estimates from observations and from a coupled general circulation model

The net heat uptake by the ocean in a changing climate involves small imbalances between the advective and diffusive processes that transport heat vertically. Generally, it is necessary to rely on global climate models to study these processes in detail. In the present study, it is shown that a key component of the vertical heat flux, namely that associated with the large-scale mean vertical circulation, can be diagnosed over extra-tropical regions from global observational data sets. This component is estimated based on the vertical velocity obtained from the geostrophic vorticity balance, combined with estimates of absolute geostrophic flow. Results are compared with the output of a non-eddy resolving, coupled atmosphere-ocean general circulation model. Reasonable agreement is found in the latitudinal distribution of the vertical heat flux, as well as in the area-integrated flux below about 250 m depth. The correspondence with the coupled model deteriorates sharply at depths shallower than 250 m due to the omission of equatorial regions from the calculation. The vertical heat flux due to the mean circulation is found to be dominated globally by the downward contribution from the Southern Hemisphere, in particular the Southern Ocean. This is driven by the Ekman vertical velocity which induces an upward transport of seawater that is cold relative to the horizontal average at a given depth. The results indicate that the dominant characteristics of the vertical transport of heat due to the mean circulation can be inferred from simple linear vorticity dynamics over much of the ocean.