Coastally trapped internal waves of tidal periodicity are investigated in an Arctic fjord, namely the Van
Mijen fjord (VMfjorden hereinafter) in Svalbard, see
Fig 1.
Figure 1: Map of VMfjorden. Crosses indicate time series stations TS1-TS6.
A series of measurements in VMfjorden were conducted over the period 9th-14th of August 2010.
RCMs were deployed (to be recovered later) in addition to CTD/LADCP sections and time series.
The time series were 13 hour long, with sample interval of one hour. Placements were near the coast around the fjord in order to observe the horizontal distribution of tidal internal waves (label
TS1-TS6, see Fig 1). Fig 2 shows salinity and density for stations TS2-TS5. In TS2 we see abrupt mixing, possibly from the hydraulic jump just inside
Akselsundet. At TS3-TS4 the baroclinic signal is evident with a suggested period of 12-13 hours. At
TS5 this signal is much reduced. At station TS6 the baroclinic signal is less clear (not shown).
The 3D-model MITgcm is utilized to simulate conditions in VMfjorden, see Fig 3 for bottom topography. Barotropic tide is the only forcing, the hydrography is horizontally constant initially as indicated for salinity in Fig 5. Fig 4 show three plots of salinity at 22.75m depth after 84, 90 and 96 hours. The signal is mainly generated at the northern entrance, and propagates with the coast to the right. Fig 6 shows salinity at TS4 from 72 to 96 hours sim. time. We observe baroclinic oscillations with period 12-13 hours, and amplitude up to ~12 m.
The measurements show clear indication of internal tidal wave activity in VMfjorden, even though supercritical currents (not shown) and hydraulic jumps are observed at the fjord entrance. The difference between stations TS4 and TS5 strongly suggest that the wave motion is confined to the western and southern part of the fjord, as is expected in stratified fjords wider than the baroclinic Rossby radius.
The model reproduces an internal Kelvin wave much as can be predicted from theory. Assuming a two layer system with relative density difference
0.002 kgm -3 and upper layer thickness 22 m, we obtain a baroclinic Rossby radius of 4.6 km, and an internal phase velocity of 0.65 ms -1 . This is comparable in magnitude to the model results in
Fig 4. Comparing results such as Fig 6 to the observed Fig 2 is difficult due to the number of simplifying factors in the numerical model. A more thorough analysis of integrated quantities is work in progress.
The entrance of the fjord is blocked by an island, limiting water exchange to mainly the northern sound, Akselsundet. This constriction should favour generation of baroclinic waves when the fjord is stratified. The barotropic tide in the area is dominated by the M2 component with a surface amplitude of about 0.5 m. Further, the fjord is wide with respect to the baroclinic Rossby radius. It is thus our goal to investigate the possibility of internal
Kelvin waves of period 12-13 hours inside
VMfjorden during summer time, when stratification is largest.
The currents through Akselsundet are supercritical in parts of the tidal period which may prevent internal wave generation. This is what defines
VMfjorden as a “jet-type” fjord according to the definition of Stigebrandt and Aure (1989). Mode 1 internal waves are however observed in jet-like fjords (Inall et.al, 2004, 2005; Stashchuk et.al,
2007) encouraging this study.
Figure 2: Salinity (color) and density (black lines) for time series stations TS2-TS5.
Figure 3,4,5: Model topography, salinity contours at
22.75m depth, and initial salinity profile Figure 6: Model salinity at TS4 after 72 hours
Thanks to Prof. Ilker Fer at Univ. of Bergen for organizing the Van Mijen cruise and letting me participate
Stigebrandt, A. and J. Aure (1989). "Vertical Mixing in Basin Waters of Fjords." Journal of Physical Oceanography 19(7): 917-926
Inall, M., F. Cottier, et al. (2004). "Sill dynamics and energy transformation in a jet fjord." Ocean Dynamics 54: 307-314
Inall, M., T. Rippeth, et al. (2005). "Evolution and distribution of TKE production and dissipation within stratified flow over topography." Geophysical Research Letters 32(L08607)
Stashchuk, N., M. Inall, et al. (2007). "Analysis of Supercritical Stratified Tidal Flow in a Scottish Fjord." Journal of Physical Oceanography 37(7): 1793-1810