Report of the Workshop on the Indices of Mesoscale Structures (WKIMS)

Long-term indices of ocean climate are generally based on large scale features (e.g. NAO). Such indices are limited in providing insight on the possible response of fish because the scale of the physical processes described is far greater than the scale at which fish behaviour is understood. Process understanding of fish response to the environment is generally more obvious at the mesoscale (10–100 km, days-weeks) where oceanographic features such as fronts, plumes, upwelling or eddies occur. An understanding of fish response to climate compatible with process understanding requires that mesoscale oceanic features be detected and tracked over long period of times. The aim of the workshop was to apply automated detection of mesoscale hydrological structures in a number of coastal and oceanic systems in order to provide long-term time series of these indices. These new time-series would complement those of existing indices (e.g., regional SST, NAO, Baltic inflow, etc.). The ultimate goal is to relate time-series mesoscale indices to times series of fish populations on the basis of process understanding gained at the mesoscale by field/process studies. During the workshop, emphasis was first put on fronts and eddies which are found in many areas and are often associated with important biological processes (such as Bakun's triad of production, concentration and retention/transport). The work focussed on the identification of (1) the principal data types available for observing front and eddies, (2) the available techniques for detecting, characterising and tracking fronts and eddies, and (3) the possible indices which can be constructed in relation to front and eddies. For each, examples were provided and when possible, the method was applied to other datasets. In addition to front and eddies, indices which specifically deal with physical-biological interactions such as larval transport where considered.  
The Workshop recommends that ICES oceanography committee (probably through WGPBI) should review progress in the development of tools for the identification and characterisation of mesoscale physical structures in the ocean and of indices of mesoscale oceanographic features; and that ICES should also ensure that the reliability of those indices is assessed for their use in explaining spatial and temporal variability in fish populations (possibly through WGRED).