The spectral nudging approach is used to avoid model drift and ensure a realistic mean state and eddy variability. There are a maximum of 46 levels in the vertical with thicknesses increasing from 6 m at the surface to 200 m at a depth of 1750 m and reaching the maximum value of 250 m at the bottom. The model has nominal horizontal resolution of approximately J u in longitude with latitude increments chosen to provide roughly square grid cells everywhere. To investigate this possibility, an eddy-admitting model based on the ocean component OPA (Oc ́an Parallelis ́ ) of the NEMO (Nucleus for European Modelling of the Ocean) was used.
This introduces the possibility that there may be some advantage to the shark’s diving behaviour by reducing or benefiting from advection by ocean currents. Ocean observations and numerical models show clearly that the speed of ocean currents associated with the Gulf Stream decrease significantly with depth below the surface. Although the contour plots were based on discrete temperature- at-depth measurements made by the PAT, the time-weighted trajectories of individual blue sharks were based on binned time at depth measurements from the PAT, and thus do not represent the exact trajectory between time intervals. This approach allowed us to reflect in the plots all collected data and avoid artefacts or excessive smoothing caused by more sophisticated techniques of data gridding. No extrapolations were made outside of the bounding data points, and the interpolation was only performed over gaps not exceeding 100 m vertically and three days temporally. Then spatial gaps between vertical profiles were interpolated (linearly) with a mixture of observed and interpolated data. A linear interpolation method was used first to fill the missing data for each vertical profile or corresponding grid’s column where at least partial measurements were already present. Each depth-temperature observation was assigned to its closest grid point and if more than one observation was found within a 3-hr (time) by 4-m (depth) range of a certain grid point, those values were averaged before entering the grid.
The grids underlying these plots were of uniform time and depth steps of 6 hours and 8 metres respectively, encompassing the entire duration and vertical range of the analysed records. Depth-temperature measurements recorded by the PAT were used to construct time-depth-temperature contour plots, over which the time-weighted diving trajectories of individual sharks were overlaid. on sea surface temperature and ambient light at depth measurements recorded by the PAT, analyzed with the state-space model ukfsst described by.
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