Variations of tidally driven three-layer residual circulation in fjords
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Residual, or tidally averaged, circulation in fjords is generally assumed to be density driven and two layered. This circulation consists of a thin surface layer of outflow and a thick bottom layer of sluggish inflow. However, development of different vertical structures in residual circulation in fjords can arise from wind, remote, and tidal forcing that may modify the two-layer circulation. Particularly, theoretical results of tidal residual flows in homogeneous semienclosed basins indicate that their vertical structure is determined by the dynamical depth of the system. This dynamical depth can be considered as the ratio between the water column depth and the depth of frictional influence in an oscillatory flow (inverse of Stokes number). When the frictional depth occupies the entire water column, the tidal residual flow is one layered as in shallow basins. But when the frictional depth is only a small portion of the water column (>6 times smaller), the tidal residual is three layered. In relatively deep fjords (say deeper than 100 m), where frictional depths typically occupy a small portion of the water column, the tidal residual flow is expected to be three layered. Ample observational evidence presented here shows a three-layered exchange flow structure in fjords. On the basis of observational and theoretical evidence, it is proposed that the water exchange structure in deep fjords (more than six frictional layers deep, or inverse Stokes number >6) is tidally driven and is three layered. The tidally driven three-layer structure of residual flows could be regarded in some cases as the fundamental structure. However, this structure will only be observed sporadically as it will be masked by wind forcing, remote forcing from the ocean, and freshwater pulses.
KeywordsFjord circulation Three-layer flows Tidal residual flow
Current data from moorings deployed in 2006 at Reloncavi Fjord were obtained under project CONA-C12F 06–02 supported by National Oceanographic Committee of Chile. A.VL acknowledges support from NSF project 0825876 and a Fellowship under Program of Short Stays 801100008 supported by CONICYT (Chile), to analyze some of the data at Universidad de Valparaiso. A.VL also acknowledges support from the Fulbright Commission and CSIC, Spain, which allowed completion of this document. Comments from two anonymous reviewers helped clarify several aspects of this presentation.
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