Simulation of the formation of Ushant thermal front

Abstract

The formation of the Ushant thermal front off northwestern France is simulated numerically with a one-dimensional model of the vertical thermal structure of the sea. This simulation is derived from a Niiler and Kraus (1977 , Modelling and prediction of the upper layers of the ocean, pp. 143–172) mixed layer model in which bottom friction is introduced. The model inputs are the meteorological parameters measured at Ushant and the barotropic tidal currents computed with a numerical model ( Mariette et al., 1982 , Oceanologica Acta, 5, 149–159). Both very fine time steps ( Δt = 1h) and spatial steps (horizontal mesh size: Δx = Δy = 2nmi; vertical step: Δz = 1m) are used. This study is intended to be mainly concerned with the processes acting during the formation of the Ushant front. Comparison of the model results with satellite pictures and in situ measurements shows good agreement. We point out that biological blooms could occur according to a spring-neap cycle, during the thermal front formation. The prevailing effect of local processes such as tidally induced bottom friction and energy budget at the sea surface on the thermal evolution of the Mer d'Iroise is then emphasized. During the formation of the front, these effects seem to be more important than the advective processes which are not considered in this work.