Submarine Estimates of Arctic Turbulence Spectra (SEATS)

The arctic ocean and it’s peripheral seas comprise a seasonally variable mix of ice covered and ice-free open ocean and shelf waters that are considered to be one of the most sensitive regions to global change processes. Current estimated rates of multi-annual sea-ice loss appear to be exceeding model predictions. Despite the insulation from direct atmospheric forcing, mesoscale baroclinically unstable eddies (the oceanic’s internal ‘storm scale’, 10-100 km) have been clearly observed through repeated salinity and temperature against depth profiles deployed through ice holes and more recently by the deployment of ice-tethered profiling instruments.

Measurements of a turbulent well mixed boundary layer under sea-ice have been examined, establishing a mixed layer thickness closely coincident with an ice speed related frictional drag. For the same reason that different processes lead to the establishment of a turbulent mixed layer under ice compared to open waters, we expect that different mechanisms will lead to mixed layer re-stratification and therefore that the sub-mesoscale (1-10’s km) interaction with mesoscale eddies and frontal boundaries will vary between ice-free and ice-covered regions. The aim of this study is an improved understanding of the characteristics of mixed layer variability in marginal or seasonally variable ice-covered regions, through the analysis of a large pre-existing data resource. Royal Navy (RN) submarines have collected temperature and salinity data in the Arctic, and elsewhere for a long time. For over 15 years RN submarines have been equipped with a sensor suite that also includes a number of biogeochemical instruments particularly for pigments such as chlorophyll(a) and ‘yellow substance’. These data are normally highly classified and unavailable for research purposes outside and even largely within the MOD itself. This is because traditional research requirements include acquisition time/date and position. However, only vessel velocity, elapsed time (instrument sampling interval) and depth, within well known published ranges, are required to accompany the data-streams for us to examine the spatial cross correlation coefficients in the physical and biological parameters. This is a mechanism for looking at data collected spatially and temporally in terms of the time and space scales of the inherent variability contained within it. Specifically, discussions with our MOD collaborators indicate that significant amounts of data will be available both in the mixed layer and deeper. The wavenumber spectrum is simply proportional to the Fourier transform of these cross-correlation coefficients, and quantifies the contribution to variability from features at a range of length scales, including eddies and filaments. For general underway vessel velocities, the ‘speed’ of the observer is such that the time taken to traverse a structure is very much less than the time taken for its evolution for all but the smallest structures. The disturbance scale of the vessel O(100 m) will limit our study to sub-mesoscale filaments and above (~0.2-100 km). A key issue we can address is how closely related the spectra of other properties are to the physical spectra.

Grant reference
Natural Environment Research Council
Total awarded
£212,887 GBP
Start date
11 Dec 2011
3 years 6 months 18 days
End date
29 Jun 2015