Investigating frontal variability from repeated glider transects in the Ligurian Current (North West Mediterranean Sea)

Abstract

Mesoscale variability of the Ligurian Current (North Western Mediterranean Sea) is studied using glider data from a repeated cross-frontal transect. The Ligurian Current is characterized by a complex time variability, with propagation velocities that can be compared to those of the glider. As a consequence, time variability can appear as folded into space variability in the glider data through the mechanism of Doppler smearing. In this paper, we provide a general framework to illustrate the effects of spatial and temporal variability and introduce a simple methodology to at least partially unfold the uncertainty. We capitalize on the fact that the glider repeatedly covers the transect, and we build time series of isopycnal depth at fixed points using nonlinear interpolation in time and spatial smoothing. Estimates of the isopycnal space patterns are obtained at various times, and a function fitting is used when appropriate allowing to describe the frontal time variability through a few descriptive parameters. Results show a mesoscale signal with scales consistent with what is known in the literature, and characterized by a cross-shore oscillation of the front between approximately 15 and 50km offshore. The frontal oscillation is quite smooth during the first four transects with propagation speed significantly lower than the glider (≈8km/day), so that the deconvolution method is robust. Local wind also appears to play a role. During the last two transects, instead, the system undergoes dramatic changes with time scales of the order of a day, so that the deconvolution is more problematic.