The Northern Kvark strait


The northern Kvark is one of the many straits in the Baltic Sea, Figure 1. This strait connects the very fresh Bothnian Bay and the slightly saltier Bothnian Sea and influences the exchange properties considerable. The understanding of the dynamics of the strait is therefore important and it is surprising that the area is almost an observational desert.

 

(Click image to enlarge)

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Figure 1. Map of the northern Baltic Sea with the topography of the Northern Kvark shown in detail to the right.

 

Our work started with theoretical studies. We first worked on the parameterizations of the effects of straits flow in models (Omstedt and Axell, 1998). Later it was suggested that the long-term average flow through the Northern Kvark is hydraulic controlled (Stigebrandt, 2001). Based on modeling on decadal scale Omstedt and Axell (2003) confirmed this important suggestion.

 

In 2002 we started to perform oceanographic measurements in the region. Intensive field experiments were conducted in December 2002, May 2003 and October-December 2004. The main aim was to obtain a description of the physical oceanography in the Northern Kvark. We conclude from these observations and from theoretical considerations that the strait is often blocked by barotropic water exchange and hence are filled with water from either the Bothnian Bay or the Bothnian Sea. Apparently, this occurs about 45% of the time. The rest of the time there is vertical stratification in the strait, which often can be approximated by two homogeneous layers. The observation also indicates that the strait may experience a linearly increasing density profile. The flows in the stratified regimes were shown to commonly be hydraulically controlled, which has a strong and important impact on the flow dynamics. The results are presented in Green et al., (2006).

 

(Click image to enlarge)

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Figure 2. Time-depth plots of the along-strait flows velocities (cm/s) in the western channel. The time scale is in days starting from October 19, 2004. The figure illustrates strong barotropic currents that are oscillating back and forth. In fact 90% of the variance can be explained by this oscillation.

 

References:

  • Green, M.,J.,A., Liljebladh, B., and A., Omstedt (2006). Physical oceanography and water exchange in the Northern Kvark Strait. Continental Shelf Research 26, 721-732.
  • Omstedt, A. and L., Axell (1998). Modeling the seasonal, interannual and long-term variations of salinity and temperature in the Baltic proper. Tellus, 50A, 637-652.
  • Omstedt, A. and L., Axell (2003). Modeling the variations of salinity and temperature in the large Gulfs of the Baltic Sea. Continental Shelf Research, 23, 265-294.
  • Stigebrandt, A. (2001). Physical Oceanography of the Baltic Sea. Chapter 2 in A Systems Analysis of the Baltic Sea (F. Wulff, L. Rahm and P. Larsson, eds.). Ecological studies 148. Springer Verlag, pp. 19-74.


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