Preprints
https://doi.org/10.5194/os-2021-23
https://doi.org/10.5194/os-2021-23

  01 Apr 2021

01 Apr 2021

Review status: this preprint is currently under review for the journal OS.

A tidally driven estuary close to an amphidromy

Sissal Vágsheyg Erenbjerg1,2, Jon Albretsen3, Knud Simonsen4, Erna Olsen1, Eigil Kaas2,5, and Bogi Hansen6 Sissal Vágsheyg Erenbjerg et al.
  • 1Dept. of Fjord Dynamics, Fiskaaling A/S, við Áir 11, FO-430 Hvalvík, Faroe Islands
  • 2Niels Bohr Institute, Copenhagen University, Juliane Maries Vei 30, DK-2100 Copenhagen, Denmark
  • 3Institute of Marine Research, P.O. box 1870 Nordnes, NO-5817, Bergen, Norway
  • 4Dept. of Science and Technology, University of the Faroe Islands, J. C. Svabosgøta 14, FO-100 Tórshavn, Faroe Islands
  • 5National Center for Climate Research, Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark
  • 6Faroe Marine Research Institute, P.O. 3051, FO-110 Tórshavn, Faroe Islands

Abstract. This paper describes the implementation of a high-resolution three-dimensional model of the estuary “Sundalagið Norður”. This estuary is bound by narrow sills in both ends and has a large semidiurnal tidal variation. The proximity to an amphidromic region, results in a periodically varying difference in sea level height between both ends of the estuary, which generates strong semidiurnal tidal currents across the sills. The available observational data support the model results. The model results verify the dominance of tidal forcing with semidiurnally varying volume fluxes into and out of the estuary. The amplitudes of these fluxes vary in strength with the fortnightly variation between spring and neap tides. More surprisingly, the model also indicates a strong fortnightly variation of net fluxes averaged over 25 hours reducing both diurnal and semidiurnal tidal currents. These variations are caused by fortnightly variations in sea level difference between both ends of the estuary, which are verified by comparison with observed sea level variations. This rather surprising result implies that exchanges within the estuary and with its surroundings vary systematically; typically with one week of net northward flow followed by one week of net southward flow. This variation also appears to affect the mixing processes in the estuary and should be taken into account in planning development or activities. More observational data would be beneficial to validate the model more thoroughly and we recommend that a dedicated experiment with combined observations and numerical modeling is implemented.

Sissal Vágsheyg Erenbjerg et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-23', Manuel Diez-Minguito, 16 Apr 2021
  • RC2: 'Comment on os-2021-23', Anonymous Referee #2, 11 May 2021
  • RC3: 'Comment on os-2021-23', Anonymous Referee #3, 26 May 2021

Sissal Vágsheyg Erenbjerg et al.

Sissal Vágsheyg Erenbjerg et al.

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Short summary
Here, we describe an estuary that has narrow and shallow sills in both ends and is close to an amphidromic region. This generates tidally driven flows into and out of the estuary, but with very different exchange rates across the entrances in both ends so that the estuary behaves like a mixture between a sound and a fjord. Using a numerical model, we find a rather surprising fortnightly signal in the net transport through the estuary. Our findings are verified by observations.