Summary:

We propose to instrument two independent, autonomous, deep ocean profiling platforms with shear microstructure sensors. By combining the tried and tested technology of ARGO-floats and seabed platform technology with aerofoil shear probes, powerful on-board processing and newly available wide-band satellite communications we propose to build and make available such devices.

Science Background:

Vertical mixing mediated by breaking internal waves may be the primary controlling mechanism of the Meridional Overturning Circulation (MOC) (Munk and Wunsch, 1998). Alternatively the MOC may be controlled by horizontal mixing (Marshall, 2002). Irrespective of which turns out to be nearer the truth, it is likely that, globally, vertical mixing is implicated in the strength of the MOC and, locally, turbulent vertical mixing is crucial to the downward diffusion of heat necessary to create and maintain the observed thermocline structure (Polzin et al, 1997). Echoing this debate a proposal for a Joint IAPSO/SCOR Working Group to investigate deep ocean mixing comments that, our “understanding of deep-ocean mixing processes generally can be construed to have fallen behind our grasp of large-scale ocean circulation”.

This gulf in our understanding stems in large part from the present lack of a suitable instrument capable of directly estimating vertical mixing in the deep ocean. The instrument used in the only deep ocean measurements made to date (Polzin at al, 1997) is not commercially available. From measurements of the irreversible dissipation of turbulent kinetic energy (TKE) and density stratification the vertical eddy diffusivity can be calculated and vertical mixing fluxes estimated. Causality is difficult to establish in physical oceanography: by directly observing the irreversible process of mixing strong statements about causality can be made.

Commercial instruments currently available are tethered and depth limited to about 350m. Alternative means of measuring vertical mixing are either by the use of passive tracers, or by acoustic current meters and CTD profiles. Passive tracer experiments reveal only the effects of mixing. Acoustic methods reveal either the fine scale-structure, not necessarily related to the irreversible mixing, or require long averages (10 minutes) to compute statistics of velocity fluctuations and cannot be made from a moving platform. Both methods place heavy demands on dedicated ship time.   

Autonomous platforms: The future for observational oceanography

ARGO-floats are already transforming our observational database of the world’s oceans. The technology is well established with over 300 floats already deployed, a number set to rise to 3000 by 2005. The eulerian profiling platform ‘HOMER’ could herald a second revolution in autonomous ocean profiling. We propose to develop a sensor package that would use the ARGO-float buoyancy engine and the HOMER profiling pod to measure profiles of TKE dissipation and buoyancy frequency (in addition to the standard profiles of T and S). Two shear probes and attendant circuitry will be installed; on-board processing power will be upgraded to enable the large amount of data compression and processing needed (velocity shear is typically measured at ~300Hz to adequately sample the inertial sub-range).  ARGO-floats presently use the Argos satellite communication system; however newly available wide-band satellite communications (Iridium) will be required to transmit profiles of turbulence parameters. SRSL has been resourced by the US ARGO programme office to participate in a field evaluation of Iridium modems, which will represent a valuable contribution in kind to the proposed work. Further in-kind support will come from the UK ARGO programme, who will ensure that data from our floats enters the relevant national and international data archives.

Overall objectives

The objective is to produce working instruments by Summer 2007.

References:
Marshall, J., H. Jones, H. Karsten, and R. Wardel, 2002, Can Eddies Set Ocean Stratification? Journal of Physical Oceanography, 32, 26-38.
Munk, W.H., C. Wunsch, 1998.  Abyssal recipes II: energetics of tidal and wind mixing.  Deep-Sea Research, 45, 1977-2010.
Polzin, K.L., J.M. Toole, J.R. Ledwell, and R.W. Schmitt, 1997, Spatial Variability of Turbulent Mixing in the Abyssal Ocean. Science, 276, 93-96.