Strongly stratified easterly flows across Spitsbergen

Abstract

Strongly stratified easterly flow across the Svalbard archipelago is investigated by means of anumerical model with a grid distance of 10 km. Idealized simulations of flow over idealized andreal orography with Brunt–Väisälä frequency fixed at N=0.019 s^-1 and with a uniform backgroundvelocity varying between 8.7 and 26 ms^-1 are performed. With a maximum height of the orography at 916 m, the non-dimensional mountain height ĥ=Nh/U varies between 0.67and 2.0. The Rossby number Ro=U/fL is of order unity. Robust features of these simulationsare an upstream velocity minimum south of Hinlopenstretet and a downstream wake to thenorth of Isfjorden. Over the downslopes, upstream of the wake, the velocities are high in thesimulations at the lower end of ĥ due to vigorous gravity wave activity. As ĥ is increasedtowards 2.0, the gravity wave activity in this area is greatly reduced, and the associated velocitymaximum disappears. Downstream jets are radiating from the three resolved fjords Isfjorden, van Mijenfjorden and Hornsund, and the two former jets merge into one further downstream. The absolute velocity maximas are found in the south, where the velocities are enhanced relativeto the upstream undisturbed velocity by factors of 1.7 to 2.2. The highest velocities are recognizedto be associated with the gravity wave activity. However, velocities are generally high in thesouth due to the left–right asymmetry induced by planetary rotation. The simulation of a realcase largely confirms these findings. In this case wind speeds reaching between 35 and 40 ms^-1 are simulated near the southern tip of Spitsbergen. An investigation of the zonal wind speedsat Spitsbergen is also presented and reveals that from 1990 to 1997, 13 situations with upstreamwind speed of the same strength as in the case study have occurred.