Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm

Abstract

We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a single two-peak storm over the Catalan inner shelf (NW Mediterranean Sea). Velocity measurements at 24gm water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with measurements at 50gm water depth. During both wind pulses, the flow accelerated at 24gm until shortly after the wind maxima, when the bottom stress was able to compensate for the wind stress. Concurrently, the sea level also responded, with the pressure-gradient force opposing the wind stress. Before, during and after the second wind pulse, there were velocity fluctuations with both super-and sub-inertial periods likely associated with transient coastal waves. Throughout the storm, the Coriolis force and wave radiation stresses were relatively unimportant in the along-shelf momentum balance. The frictional adjustment timescale was around 10gh, consistent with the folding time obtained from bottom drag parameterizations. The momentum evolution at 50gm showed a larger influence of the Coriolis force at the expense of a decreased frictional relevance, typical in the transition from the inner to the mid-shelf.