Properties of In-Situ Lagrangian Surface Drifters and the Relationship with Stokes Drift in the Baltic Sea

Delpeche-Ellmann, N.; and Giudici, A., 2024. Properties of in-situ Lagrangian surface drifters and the relationship with Stokes drift in the Baltic Sea. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 417-421. Charlotte (North Carolina), ISSN 0749-0208. Properties of Lagrangian surface drift are important for understanding the pathways of substances (such as debris, pollutants, marine life, etc.). The surface drift however does not act alone, and often is a combination of driving forces such as wind, waves, tides etc. The degree of influence however of each driving force is not homogenous and may vary depending on the atmospheric conditions, underlying ocean state etc. The impact of winds (Ekman velocities) are known to be highly correlated with surface drift, however the influence of waves (Stokes drift) have been less explored. As a result, this study examines: (i) the pathways of Lagrangian surface drift in the Baltic Sea and (ii) connections of Stokes drift to the overall surface drift. This was performed by incorporating a synergy of 13 in-situ surface drifters, meteorological stations (from which wind and wave data was obtained) along with a hindcast wave model from which Stokes drift was derived. The analysis was performed for two years (2013 and 2018) to capture the seasonal patterns of surface drift. Results show that surface drift varied from 0.12–0.2 m/s and the ratio of influence of drifter speed to wind speed varied from 1.0–2.4 %, thus emphasizing the strong influence of winds. The impact of Stokes drift on the overall velocity was most prominent during high wave conditions and was estimated to be > 60% of the overall drift at these conditions. These results demonstrate the importance of considering Stokes drift when predicting the pathways of substances in marine areas.