Estimates of vertical mixing during a Lagrangian experiment off the Galician coast
- a Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, PO Box 140, Yerseke 4400 AC, The Netherlands
- b Centre for Applied Oceanography, Marine Science Laboratories, University of Wales, Bangor, Menai Bridge, Anglesey, LL59 5EY, UK
- c School of Ocean Sciences, Marine Science Laboratories, University of Wales, Bangor, Menai Bridge, Anglesey, LL59 5EY, UK
- Received 22 November 2000, Accepted 22 June 2001, Available online 20 December 2001
Abstract
An adjoint 1-D model was used to determine vertical diffusivity coefficients from temperature profiles collected within a filament escaping from the Galician coast following an upwelling event. The optimisation scheme ended with relatively high diffusivity values within the thermocline (9×10−5 m2 s−1). Such high values are relevant for biogeochemical exchanges between surface and deep waters in stratified areas.
The optimised values were several orders of magnitude higher than the bulk of diffusivity measurements recorded with a free-falling device; however, the optimisation solution was consistent with the arithmetic mean of the measurements in the thermocline (7.7×10−5 m2 s−1), giving more weight to the few largest values. Below the thermocline, the data assimilation method failed because of the three-dimensional nature of the advective field of the upwelling system. Ignoring this advective forcing in the model led to estimates that were two orders of magnitude too high.
The results suggest that turbulent mixing is a random process where a few intense events determine the average mixing that drives the long-term evolution of the water column structure. This statistical property is very important when one wants to use instantaneous diffusivity measurements for modelling purposes.
Keywords
- 1-D numerical modelling;
- NE Atlantic;
- Ocean margin;
- Data assimilation;
- Vertical mixing
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