Report on the representation of vertical velocities in the Iberian Biscay Irish Ocean Analysis and Forecasting system using multi-platform observations

Abstract

This report aims at the quantification of the performances of the Iberian-Biscay-Irish Ocean Analysis and Forecasting system, a hydrodynamical model based on the quite popular NEMO Ocean Engine code. We choose to focus exclusively on the vertical velocity variable, as it has a structure that is still today poorly understood. It is of great interest to understand more accurately the mixing between the top layer and the ocean’s interior, and in consequence for nutrient fluxes, phytoplankton bloom and photosynthesis. The same algorithm retrieving vertical velocities from density arrays thanks to the quasi-geostrophic omega equation is applied to both model data and in-situ measurements corresponding to the same period, to allow comparison. Independent measurements from surface drifters are used to provide another validation method. This data is extracted from the 2018 Calypso cruise dataset, carried out in theWestern Mediterranean waters during May and June. On the sampled front, North-West of the Eastern Alboran gyre, velocities can reach values of 20mday-1 in the upper layer and almost 100mday-1 at a depth of 90m. The major asset of the model turns out to be a powerful realism of the physical variables, that nonetheless encounters issues in the location and intensity of oceanic phenomena. Amongst other things, we notice an underestimation of velocities of coefficient 4 by the model on the date of the Calypso 2018 experiment.