Journal Article

Internal wave mixing in the Baltic Sea: Near-inertial waves in the absence of tides

Umlauf, L., van der Lee, E. M.

2011

Overview

The dynamics of near‐inertial motions, and their relation to mixing, is investigated here with an extensive data set, including turbulence and high‐resolution velocity observations from two cruises conducted in 2008 (summer) and 2010 (winter) in the Bornholm Basin of the Baltic Sea. In the absence of tides, it is found that the basin‐scale energetics are governed by inertial oscillations and low‐mode near‐inertial wave motions that are generated near the lateral slopes of the basin. These motions are shown to be associated with persistent narrow shear‐bands, strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin. In spite of different stratification, near‐inertial wave structure, and atmospheric forcing during summer and winter conditions, respectively, the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way. This scaling was different from the Gregg‐Henyey‐type models used for the open ocean, but largely consistent with the MacKinnon‐Gregg scaling developed for the continental shelf. Key Points First detailed observations are made of internal wave mixing during different seasons High dissipation in narrow shear‐bands is major source of mixing in halocline Internal wave mixing in the Baltic Sea follows scaling by MacKinnon and Gregg

• Publisher: Blackwell Publishing Ltd,American Geophysical Union
• Subject: Atmospheric forcing / Baltic Sea / Continental shelves / Earth sciences / Earth, ocean, space / Exact sciences and technology / Geophysics / Internal waves / Marine / mixing / near-inertial waves / Oceanography / Oceans / Physical oceanography / Summer / Tides / Winter