Turbulent Diurnal Jets as Observed by an Ocean Microstructure Glider.

It is notoriously difficult to measure turbulence in very upper (<10m) ocean. However, the recent advent of turbulence enabled ocean gliders has allowed for long - of the order of weeks – continuous observations to be made very close (<5m) to the ocean surface. Sparsely available more traditional profiler-based observations of very near surface ocean turbulence have hinted at the presence of processes known as diurnal jets. These phenomena result in the diurnal intensification of near surface turbulence during times of peak incoming solar radiation. The periodic intensification of near surface ocean turbulence has clear implications for the transfer of heat, momentum and gases between the ocean and atmosphere.

It is proposed that diurnal intensification of near-surface turbulent kinetic energy is caused by surface-trapping of quasi constant wind energy within the shoaling near surface layer caused by daily solar re-stratification. The result is a relative intensification of turbulent kinetic energy within a shallow day-time ocean surface layer.

Work presented here forms part of the NERC-funded ‘ALTERECO’ project, which aims to gain a better understand how changing physical and chemical conditions in UK Shelf Seas affects the marine ecosystem and ocean health.

We present an analysis of 9 weeks, consisting of more than 2800 profiles, of continuous velocity microstructure derived observations of turbulence kinetic energy dissipation rates , temperature and salinity. Contextualised within atmospheric forcing provided by ERA- interim reanalysis products, our analysis provides repeated evidence, in the form of a two order of magnitude in increase of e above the background near surface level, of these diurnal turbulent processes.

We further test our hypothesis that these processes may well be a ubiquitous feature of the upper global ocean, by initialising a one-dimensional turbulence closure model (GOTM) with wind, radiative forcing and stratification terms derived from our data set, as well those from wider parameter space. The numerical model is successfully shown to also reproduce these diurnal patterns of intensified near surface turbulence within realistic regions of parameter space.

The observational and numerical simulation results are then synthesised in order to form a parametrisation representing the potential intensified near surface turbulence, based on the macro scale forcing. This parametrisation is then applied to readily available global maps of the relevant forcing in order to ascertain if diurnal jets may indeed be ubiquitous but sparely observed features driving intensified near surface turbulence.