Along or across front ocean survey strategy? an operational example at an unstable front and the impact on the estimation of quasi-geostrophic vertical velocities and temperature fluxes

We present results of the optimization of near-real time on-board sampling strategy in the Iceland-Faroes oceanic frontal area, based on the outputs of a mesoscale 3D operational data assimilation forecasting experiment. By minimizing a root mean square error cost function, we show that in this example an along-front sampling strategy, i.e. with transects parallel to the front, produces smaller errors in temperature, salinity, nitrate, phytoplankton, and zooplankton fields, as a result of a combination of the direction of the sampling of the front and errors associated with the asynopticy of observations (Doppler effect). This is contrary to the classic across-front sampling strategies that are used in most field experiments reported in the literature, i.e. where transects are perpendicular to the front. A control model shows that at these spatio-temporal scales, the along front sampling strategy is optimal when the frontal instability has sufficiently developed. We further examine the impact of optimised sampling strategies on the accuray of derived vertical motion and temperature fluxes. It is shown that the unusual along front sampling strategy may also provide better estimations of vertical velocities and temperature fluxes compared to the classic across front sampling strategy, especially when the front is sufficiently developed.