Future research

It would be interesting, in the future, to repeat this aspect of the DIMES project (the simultaneous deployment of XCTD, CTD and microstructure profilers) in areas with higher stratification and/or mixing. One possible location would be the Brazil Basin (site of a number of the early observational studies measuring diapycnal diffusivity - see chapter 1 and Polzin et al., 1997; Ledwell et al., 2000), which incorporates both a smooth abyssal plain and the rough topography of the mid-Atlantic ridge, and has generally higher stratification than in the Southern Ocean. One advantage to carrying out such a study in the Brazil Basin would be to determine if there has been any change since the previous studies, but conversely, choosing another location with a suitable range in topographic roughness could extend the global coverage of diffusivity estimates.

Density profiles from any nearby Iridium Argo floats could also be considered, as these provide high-resolution (2 m) profiles that are able to resolve fine-scale (tens to hundreds of meters) strain (Wu et al., 2011). Data from Seagliders could

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also be incorporated. Beaird et al. (2012) discuss a method to infer the rate of dissipation of turbulent kinetic energy (ǫ, from which κ is easily calculated) using data from Seagliders. Their Seaglider Large-Eddy Method utilises finescale vertical velocity measurements inferred from the vertical flight model of the Seaglider, as well as density measurements. They show that this method performs well (by comparison with a microstructure survey) in the energetic turbulence of the Faroe Bank Channel. Comparisons between this method and purely density profile based methods would also be interesting. Seagliders and Iridium Argo floats both provide profiles at much less expense than microstructure profilers, and the data coverage provided by these types of instruments is increasing rapidly.

As mentioned in section 3.6, the mooring in Shag Rocks Passage used for this study was not designed to measure κ_z, and in particular lacked temperature and salinity instrumentation in the water depths ensonified by the ADCP. Figure 6.2 outlines a possible improved design for a mooring intended forκz measurements.

The basic design is similar to the mooring in Shag Rocks Passage, with the addition of a string of instruments giving reasonably accurate temperature and salinity readings (such as Seabird Electronics MicroCATs, as indicated here), in the depths ensonified. The spacing of these instruments is such that they fall at the centre of 100 m bins upwards from the ADCP, so that, if the quality of the ADCP velocity measurements is sufficient, the diffusivity estimates can be made every 100 m rather than averaged over the entire depth ensonified. If the mooring were to be deployed in an area where particularly complex stratification is expected, additional MicroCATs might be necessary, and similarly, in regions with particularly simple stratification fewer MicroCATs might be sufficient. 100 m spacing seems a reasonable baseline. RCMs are included above and below the depths ensonified, to provide verification that the velocities recorded by the ADCP (and, in particular, unexpected features/variability) are not instrument artefacts.

The near-bottom RCM and MicroCAT are included so the bottom boundary layer can be compared with waters above. Pressure loggers are included so that mooring motion can be analysed. The portion of the mooring marked as ’Repeat Section’

could be repeated upwards in the water column to give a more complete vertical picture. Increased mixing in the upper part of the water column has been ascribed to the influence of the wind in other studies (e.g. Jing and Wu, 2010; Wu et al., 2011;

Waterman et al., 2012a), but a long time series of diffusivity estimates showing correlation with local winds would provide additional verification.

If such moorings were to be deployed in Shag Rocks Passage, it would be interesting to place three of them such that they encircled the seamount. If we

length depends on depths of interest

50 m 100 m 100 m 100 m 100 m 100 m 100 m

50 m

length depends on size of bottom boundary layer

Anchor RCM

RCM, with MicroCAT and pressure logger

RCM, with MicroCAT and pressure logger

MicroCAT MicroCAT MicroCAT MicroCAT MicroCAT MicroCAT

Upward-looking LongRanger ADCP, with MicroCAT and pressure logger

Repeat section

Figure 6.2: Schematic of a mooring designed forκ measurements. This is based on the moorings in Shag Rocks Passage, but includes MicroCATs in the water depths ensonified by the ADCP, so that a more accurate profile of buoyancy frequency is available.

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are correct in our supposition of topographically trapped waves propagating around the seamount, variability due to these waves should be apparent in all the moorings.

However, it would be of more general interest to place such moorings in locations covering a wide range in stratification and mixing regimes, current strengths and variability, eddy activity, and atmospheric conditions. With repeating chains of ADCPs, RCMs and MicroCATs, one could hypothetically achieve time series of diffusivity estimates showing the influence of stratification, currents, winds and eddies, and the time-and-depth variations in those effects, as well as increasing the dataset of diffusivity estimates around the global ocean. Time series of a year or more in length would give a first look at the natural climate variability in diapycnal diffusivity in the chosen locations.

Deploying moorings for the specific purpose of measuring diffusivity at a wide range of locations would amount to a very ambitious (and expensive!) program.

More realistically, moorings could be modified on an opportunistic basis to allow for diffusivity estimates. If a mooring for another project already included a LongRanger ADCP, it would be relatively inexpensive to add RCMs above and below (if not already part of the mooring design), and a chain of MicroCATs in the water depths ensonified to provide the necessary temperature and salinity measurements.

It would also be sensible to test diffusivity estimates calculated from the data produced by such moorings against estimates from a microstructure profiler deployed in the same location. If possible, one would take concurrent CTD/LADCP and microstructure profiles in the vicinity of such a test mooring in a couple of different locations with smooth/rough topography, and at times with some different wind strengths. This should highlight regimes where the reduced vertical resolution (compared with normal CTD/LADCP profiles) is significantly degrading the diffusivity estimates. These tests could be combined with the hypothetical deployment, discussed above, of concurrent and co-located XCTDs, CTD/LADCP, microstructure profilers, Iridium Argo floats and Seagliders. Thus the microstructure profiles could be used efficiently to test a wide variety of finescale instruments and methods of calculating diapycnal diffusivity, which could also be inter-compared. These might provide an opportunity for some improvement in the finescale parameterization methods, which were originally developed by comparison with microstructure instruments (e.g. Gregg and Kunze, 1991; Polzin et al., 1995, 2002), but are still being modified and improved (e.g. Thurnherr, 2012).