The Impact of Internal Wave Seasonality on the Continental Shelf Energy Budget
Abstract:
Heating-stirring models are widely used to simulate the timing and strength of stratification in these environments. Such models are based on bulk potential energy (PE) budgets: the loss of PE due to thermal stratification is being balanced by wind and tidal mixing. The model often fails to accurately predict stratification, as it only considers forces acting on the surface and bottom boundary of the water column. This highlights the need for additional internal energy sources to close this budget and produce an accurate seasonal cycle of stratification.
We present exciting new results that show the importance of considering internal waves within this framework that features continuous measurements of unprecedented detail and temporal extent spanning 16 months (April 2014 - July 2015) collected on the NW European Shelf. A mooring array recorded full depth vertical structure, dynamics and meteorological data over a full seasonal cycle at a station 120 km away from the continental shelf break.
We observe a highly variable but energetic internal wave field from the onset of stratification that suggests a continuous supply of internal mixing. The heating-stirring model reproduces the bulk characteristics of the seasonal cycle of stratification. While it accurately predicts the timing of the onset in spring and peak stratification in late summer there is a persistent 20 Jm-3 positive offset between the model and observations throughout this period. A peak in observed baroclinic energy towards the end of the stratified period leads to a delay in the timing of stratification breakdown in the model, as it significantly underestimates vertical mixing. The internal wave field contains much of the additional energy (20-100 Jm-3) required to close this budget.