The Control of Fjord Circulation by Fortnightly Modulation of Tidal Mixing Processes

Abstract

Tidal processes that control basin?ocean water exchange across a sill, namely, the fortnightly modulation of mixing intensity associated with the spring-neap tidal cycle, are examined. The study proceeds analytically by modeling flow in a vertical two-dimensional plane where a zone of enhanced mixing of finite horizontal extent is embedded within a vertically sheared steady estuarine flow. The intensity of mixing is allowed to vary with the fortnightly cycle. As the mixing intensity increases and hence approaches the maximum at spring tides, landward propagating and seaward propagating internal waves are generated in response to the horizontal density gradients induced by the localized mixing. Furthermore, these internal waves interact with the ambient steady sheer flow to create strong upwelling over the mixing zone, which is accompanied by bottom-intensified seaward currents on the landward side of the mixing zone and surface-intensified seaward currents on the seaward side of the mixing zone, and which are superimposed on the background estuarine flow. Accordingly, a considerable fraction of the bottom water flowing landward into the mixing zone is refluxed back seaward as part of the surface water at spring tides. As the mixing intensity decreases, the internal disturbances propagate away in both directions from the mixing zone, so that the bottom inflow from outside continues landward at neap tides. This model reproduce observations of deep-water replacement in coastal basins and shows that bottom water intrusions from outside occur during periods of weak tidal currents.