The Agulhas current is one of the strongest western boundary currents (WBCs) in the world’s oceans [Lutjeharms, 2006]. In a recent study, Le Goff et al [Le Goff al 2021] estimate this current by processing the messages sent by the commercial vessels through the Automatic Identification System (AIS). These estimations show a remarkable improvement in the spatio-temporal resolution compared to altimetric-derived geostrophic currents. One result of this study was the fact that the main strength (weakness) of the AIS-derived currents is the very high (low) density of estimates in the absence (presence) of southwestern storm-swell wave conditions. To prevent the extreme sparsity of observation during these storm events and to take advantage of all available observation sources: we use AIS derived surface current in synergy with along-track altimetric data from DUACS DT2021. The merging is done through the Multiscale Inversion for Ocean Surface Topography (MIOST) [Ubelmann et al., 2020] variational tool to retrieve both geostrophic and ageostrophic current. This tool allows the decomposition of the signal into representors representing different time and space scales (mesoscale to large-scale) and different physical signals (geostrophy, internal waves, near-inertial oscillations, Ekman current…). Once the oceanic current is estimated, we study the behavior of the ships and especially their anormal behavior during the storms.
Ocean waves Interacting with ocean currents is a frequent cause of sea-state variability [Ardhuin et al 2017, Quilfen et al 2018, Quilfen and Chapron 2019]. Such situations can lead to sea-state hazards, crucial for shipping security. The Great Agulhas current system is an area of very intensive maritime traffic, where dangerous localized sea-state amplificated by the current are regularly reported.
In absence of wind and wave-induced motions, the heading and drift of every ship along its trajectory can be estimated from oceanic surface current map. This first guess can then be compared with real ship parameters obtained from satellite-collected ship Automatic Identification System (AIS) messages. During Southwestern storm-swell wave conditions, with wind and waves aligned against the current, some ships experience pronounced navigation difficulties, slowing down up to 2 m/s, and frequently maneuvering to keep their heading perpendicular to dominant waves. Superposed multiple individual ship trajectories can then help map anomalous areas, and to relate them to localized strong wave-current effects such as large refraction of waves by the oceanic current.
[Lutjeharms, 2006] : LUTJEHARMS, J. R. E. The Agulhas Current Springer-Verlag. Berlin, Germany, 2006.
[Le Goff et al 2021] : Le Goff, C., Boussidi, B., Mironov, A., Guichoux, Y., Zhen, Y., Tandeo, P., et al. (2021). Monitoring the greater Agulhas Current with AIS data information. Journal of Geophysical Research: Oceans, 126, e2021JC017228. https://doi.org/10.1029/2021JC017228
[Ardhuin et al 2017] : Ardhuin, F., S. T. Gille, D. Menemenlis,C. B. Rocha, N. Rascle, B. Chapron, J. Gula, and J. Molemaker (2017), Small-scale open ocean
currents have large effects on wind wave heights, J. Geophys. Res. Oceans, 122, 4500–4517, doi:10.1002/2016JC012413.
[Quilfen et al 2018] :Quilfen Yves, Yurovskaya M., Chapron Bertrand, Ardhuin Fabrice (2018). Storm waves focusing and steepening in the Agulhas current: Satellite observations and modeling. Remote Sensing Of Environment, 216, 561-571. Publisher's official version :
[Quilfen and Chapron, 2019] : Quilfen, Y., & Chapron, B. (2019). Ocean surface wave-current signatures from satellite altimeter measurements.
Geophysical Research Letters, 46.
[Ubelmann et al., 2020] : Ubelmann C., G. Dibarboure, L. Gaultier, A. Ponte, F. Ardhuin, M. Ballarota, and Y. Faugère (2020): Reconstructing Ocean Surface Current Combining Altimetry and Future Spaceborne Doppler Data. Submitted to Journal of Geophysical Research.