Mean Sea Level Change Research Articles

Radiation stress and mass transport in gravity waves, with application to ‘surf beats’

This paper studies the second-order currents and changes in mean surface level which are caused by gravity waves of non-uniform amplitude. The effects are interpreted in terms of the radiation stresses in the waves.The first example is of wave groups propagated in water of uniform mean depth. The problem is solved first by a perturbation analysis. In two special cases the second-order currents are found to be proportional simply to the square of the local wave amplitude: (a) when the lengths of the groups are large compared to the mean depth, and (b) when the groups are all of equal length. Then the surface is found to be depressed under a high group of waves and the mass-transport is relatively negative there. In case (a) the result can be simply related to the radiation stresses, which tend to expel fluid from beneath the higher waves.The second example considered is the propagation of waves of steady amplitude in water of gradually varying depth. Assuming no loss of energy by friction or reflexion, the wave amplitude must vary horizontally, to maintain the flux of energy constant; it is shown that this produces a negative tilt in the mean surface level as the depth diminishes. However, if the wave height is limited by breaking, the tilt is positive. The results are in agreement with some observations by Fairchild.Lastly, the propagation of groups of waves from deep to shallow water is studied. As the mean depth decreases, so the response of the fluid to the radiation stresses tends to increase. The long waves thus generated may be reflected as free waves, and so account for the 'surf beats’ observed by Munk and Tucker.Generalle speaking, the changes in mean sea level produced by ocean waves are comparable with those due to horizontal wind stress. It may be necessary to allow for the wave stresses in calculating wind stress coefficients.

Physical Oceanography of the Strait of Georgia, British Columbia

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.

Some Factors Affecting the Extent of Ice in the Barents Sea Area

Variations in flow of the Gulf Stream system and in transport of warm Atlantic water across the Wyville-Thomson Ridge northeastwards (via the North Cape Current) affect the areal extent of ice in the Barents Sea. Theoretical considerations and data are presented relating variations in the Florida Current (southern Gulf Stream system), as indicated by mean sea level changes at Charleston, S.C., and Miami, Fla., to ice conditions in the Barents Sea three years later. Low sea level at Charleston and Miami means strong flow of the Florida Current, contraction of the North Atlantic eddy, little warm Atlantic surface water discharged into the Barents Sea and thus more ice. Conversely, high sea level at Charleston and Miami results in more Atlantic water in Barents Sea and less ice. Relationship of ice to winds in Barents Sea area is also briefly discussed. In the period 1925-1938, good agreement between sea level, ice, and wind curves occurs in 12 of the 14 years.

Geodetic Investigations in the Canadian Arctic

Arctic work of the Canadian Geodetic Survey started in 1935. Between 1942 and 1950, 300 astronomical stations were established on the mainland north of the tree-line and in the southern Canadian Arctic Islands. The shoran electronic method of position fixation has superseded the astronomical method in most areas. Methods are described, as is the extension of the Canadian shoran net since 1949. Future extensions, gravity observations, and needed investigations of changes in mean sea level are briefly discussed.