Physical Oceanography of the Strait of Georgia, British Columbia

Abstract

A descriptive and quantitative analysis of the physical oceanography of the Strait of Georgia has been made.The area has been characterized by extreme seasonal and regional variability of the surface waters. Deep water undergoes only small change. Runoff, principally from the Fraser River, is the major cause of salinity variation. Surface warming by insolation is reflected in high summer surface water temperatures in the Central and Northern Strait.Analysis of the Strait of Georgia has been based on a hypothetical model of a deep, rectangular basin connected to the sea by mixing baffles and a long channel. Fresh water inflow is concentrated near the southern end of the basin. The Strait of Georgia–Juan de Fuca Strait system has basically three water masses: (1) the brackish surface water from runoff in the Strait of Georgia; (2) the deep water of oceanic origin in Juan de Fuca Strait; and (3) a mixture of (1) and (2) which forms at the sills. This mixture contributes to the deep water in the Strait of Georgia and to the upper seaward-flowing layer in Juan de Fuca Strait. Bottom water is formed in late autumn when dense sea water from Juan de Fuca Strait intrudes into the mixing area of the southern sills. An Intermediate Water is formed during some cold winters in the Northern Strait. A slow intrusion of warm Intermediate Water occurs from the southern sills in late summer.A general counter-clockwise circulation exists in the Strait of Georgia. Tides, runoff, and winds are the principal generating forces. Topography, Coriolis force, and centrifugal force are the main directive factors. Circulation has been studied from drift bottle experiments, mass distribution, and isentropic analysis.Some of the effects of winds in mean sea level changes and surface currents have been evaluated. Wind effects are most pronounced in influencing the circulation of the upper brackish layer.Waters are most stable in the Central and Northern Strait. Intensive tidal mixing renders the waters of the Southern Strait nearly homogeneous, particularly in winter. The largest amount of mixing energy comes from the tide, but winds contribute substantially to mixing in the surface water. The potential energy change from a stratified to a mixed column of water in the Southern Strait has been computed. Keulegan's criterion of mixing is applied to the system at the Fraser River estuary.A technique for determining the fresh water budget in the Strait of Georgia has been developed. This has been evaluated on the basis of the 1950 meteorological, hydrological, and oceanographic data. At a particular time there is a volume of fresh water in the Strait of Georgia equivalent to 1⅓ years of Fraser River discharge based on an average salinity of 33.8‰ in the inflowing oceanic water. Little effect in the overall fresh water content of the system is caused by sudden increases or decreases in the runoff.The heat budget based on five stations where the necessary meteorological and oceanographic data were available has been evaluated for 1950. Considerable variation in evaporation is largely dependent on the variation in surface water temperature. Peak evaporation occurs in the Southern Strait in late autumn with negative (condensation) values in late summer. Maximum evaporation occurs in mid-summer in the Central and Northern Strait. On a yearly basis, there is a loss of heat from the system through the transport seaward of surface water.Some concepts of inshore oceanography are given with general guiding principles for the planning and conduct of surveys.