Abstract
Abstract. Internal hydraulic control, which occurs when stratified water masses are forced through an abrupt constriction, plays an enormous role in nature on both large and regional scales with respect to dynamics, circulation, and water mass formation. Despite a growing literature on this subject surprisingly few direct observations have been made that conclusively show the existence of and the circumstances related to internal hydraulic control in nature. In this study we present observations from the Little Belt, Denmark, one of three narrow straits connecting the Baltic Sea and the North Sea. The observations (comprised primarily of along-strait, detailed transects of salinity and temperature; continuous observations of flow velocity, salinity, and temperature at a permanent station; and numerous vertical profiles of salinity, temperature, fluorescence, and flow velocity in various locations) show that internal hydraulic control is a frequently occurring phenomenon in the Little Belt. The observations, which are limited to south-going flows of approximately two-layered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one. This is connected to the depth of the pycnocline on the upstream side and the topography, which is both deepening and contracting toward the narrow part of the Little Belt. The existence of two possible flow configurations is known from theoretical and laboratory studies, but we believe that this has never been observed in nature and reported before. The water masses formed by the intense mixing, which is tightly connected with the presence of control, may be found far downstream of the point of control. The observations show that these particular water masses are associated with chlorophyll concentrations that are considerably higher than in adjacent water masses, showing that control has a considerable influence on the primary production and hence the ecosystem in the area.
Highlights
In recent years there has been a growing interest in the concept of internal hydraulic control, which occurs when a flowing, stratified fluid is subject to an abrupt constriction
The observations, which are limited to south-going flows of approximately twolayered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one
In the present situation the well-mixed water masses found in the narrow part of the Little Belt are the result of mixing between the two water masses found to the north, the mixing ratio being roughly 80 % upper layer water and 20 % lower layer water
Summary
In recent years there has been a growing interest in the concept of internal hydraulic control, which occurs when a flowing, stratified fluid is subject to an abrupt constriction. Internal hydraulic control has attracted attention, for example in connection with estuarine circulation and bottom water renewal in semi-enclosed shelf seas and biological and environmental effects thereof The purpose of this study is to present and discuss observations of internal hydraulic control (in the following and where ambiguities can be ruled out, it is just referred to as hydraulic control) made in the Little Belt, Denmark (see Fig. 1). We find this pertinent for mainly two reasons.
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