Observed seasonality in internal wave energy and associated mixing in a temperate shelf sea

Abstract

<p>Long-term observations (March’14 − July’15) of ocean density and velocity from the North West European shelf reveal a seasonality in internal wave energy linked to the seasonal cycle of stratification. Further, this seasonality extends to internal mixing associated with internal waves that can be effectively described by the buoyancy frequency (N<sup>2</sup>), with the strongest mixing associated with strongly stratified summer conditions. To better understand these results a model was used that employed three different, commonly used parameterisations of internal mixing. Each parameterisation produced some degree of seasonality in internal mixing. Contrary to observed results however, all three model scenarios produced a minimum in internal mixing during summer, with enhanced mixing observed during spring and autumn. This failure in each model was attributed to the lack of realistic levels of enhanced baroclinic energy and shear (S<sup>2</sup>) that is identified in observations to be attributable to internal waves. These observations reveal a close relationship between N<sup>2</sup>and S<sup>2</sup>, resulting in a near continuous state of marginal stability; where the gradient Richardson number is maintained at a near critical level. Due to the observed strong dependence of internal wave energy and internal mixing on stratification, a modified version of the MacKinnon and Gregg (2003a) turbulence scaling was employed. This modified parameterisation successfully replicated the observed seasonality in internal mixing. This important result implies that future parameterisations should aim to scale internal mixing on enhanced levels of S<sup>2</sup> from internal waves, which are shown here to be suitably predicted by the seasonal cycle of stratification (N<sup>2</sup>).</p>

The deep water oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

“…The time series of temperature and salinity were collected by 23 CTDs fixed to the same mooring deployment. Temperature and salinity data were collected with a temporal resolution of 5 minutes and a spatial resolution which varied with depth: 5m (10m − 35m and 54m − 79m below the surface), 2m (37m − 49m below the surface), and 10m (89m − 129m below the surface) [47].…”