Cooling of the West Spitsbergen Current: Isopycnal diffusion by topographic vorticity waves

Abstract

We investigate the heat budget in the West Spitsbergen Current on the basis of theoretical considerations and current meter data across the slope at 78.83°N between September 1997 and September 1998. The dispersion relation for topographically trapped waves along the West Spitsbergen Slope with a barotropic along‐slope velocity is calculated by use of an idealized numerical model. Signal processing and coherence calculations of the current meter data show that barotropic vorticity waves with enhanced energy at the diurnal period exist along the slope. A wave length of 32 km is given by the dispersion relation for the 24‐hour period and a corresponding second mode with a 35‐hour period. Coherence calculations and complex demodulations of the current meter time series confirm the spatial and temporal variations given by the dispersion relation. The eddy characteristics of the first two modes are used to estimate the heat loss of the warm core of the West Spitsbergen Current by isopycnal eddy diffusion. We present the first isopycnal heat loss estimates obtained through dynamical considerations of the West Spitsbergen Current and compare them with earlier diagnostic estimates for the mean heat loss in the subsurface layers. Within the standard errors we estimate the heat flux to be O(1000) W/m2 throughout the year except for the summer months June–July. If the 9798 heat flux time series are representative for the subsurface layer between 100 and 500 m depth, the heat loss is on the same order of magnitude as the mean winter value of 1050 W/m2 from earlier diagnostic estimates.