1. Observation of sub-mesoscale eddies over Baltic Sea using TerraSAR-X and
Oceanographic data
Aikaterini Tavri (1), Suman Singha (2), Susanne Lehner (3), Konstantinos Topouzelis (4)
(1) Technische Universität München, Munich, Germany, (2), (3) German Aerospace Center, DLR – IMF, (4) University of the Aegean, Lesvos, Greece
Summary
Oceanographic data
The present study is focused on detection and mapping of sub-mesocale features like spiral eddies over the Baltic Sea region using Synthetic Aperture Radar images of medium resolution from TerraSAR-X satellite. Main objective is to provide an extensive analysis on the occurrence and statistics of small scale oceanic eddies in the region, over a 4 year period, in combination with oceanographic data retrieved either from in-situ measurements or model analysis. Exploiting information from sea surface temperature (SST) and chlorophyll distribution (Chl-a) maps as well as wind and surface currents circulation conditions, a categorization of the formations is made and a possible triggering physical mechanism is assigned.
An example of the additional collected data and generated maps is shown for where several black eddies were located in an algal bloom in the Gulf of Finland.
Categories of sub-mesoscale eddies
Sub-mesoscale eddies are spiral dynamical formations detected on sea surface. Due to their size and interaction within the upper boundary layer and capillary waves damping, these formations are mostly observed using SAR data [1]. Sub-mesoscale eddies similar to mesoscale eddies are highly connected with ocean near surface dynamics and transportation of biogenic material, surfactants or pollutants [2].
Distribution Map and Seasonality
Black Eddies
White eddies
Ice eddies
Black eddies are characterized by low backscatter signatures and were related to algal blooms and low current velocities. White eddies were detected during strong western winds, characterized from higher backscatter values, closely related to surface circulation and current velocities. During winter season on the northern part of the basin, ice eddies with characteristic moderate backscatter signature values and smoother texture characteristics, were detected. A color transformation for the images was applied to separate high and low frequency variations.
Based on the resulting distribution formation three main hot spots could be retrieved near the coastline and Gulf regions. The detected sub-mesoscale eddies were observed exclusively during summer and autumn, with the exception of ice eddies which were observed during winter (February and March).
REMOTE SENSING TECHNOLOGY INSTITUTE [IMF]
Eddies categories
Average size
Predominant Rotation
Average estimated wind field
Black eddies
1 to 3 km
Cyclonic
Low (4 to 5.2 m/s)
White eddies
4 to 5.5 km
Anticyclonic
Moderate (7.4 to 8.6 m/s)
Ice eddies
4 to 6 km
High (11 to 12.2 m/s)
Conclusions
Given the large examined period very few sub-mesoscale eddies were located over the Baltic Sea, compared to previous years studies. Black eddies seem to formulate due to surface mixing, during low wind conditions over regions with high SST and Chl-a concentrations and low current velocities, as it is supported from the present research results. White eddies are subjected to strong shear changes and current velocities . Finally Ice eddies are the most particular ones related to water density variations and intermediate current velocities.
References
[1] Johannesen, J.A., Kudryvtsev, V., Akimov, D., Eldevik, T., Winther, N., and Chapron, B., On radar i´maging of current fatures: 2. Mesoscale eddy and current front detection”, Journal of Geophysical Resources, 110, C07017, (2005),
[2] Alpers, W., Brandt, P., Lazar, A., Dagorne, D., Soe, B., Faye, S. Hansen, M., Ruhbino, A., Poulain, P.M., and Brehmer, P.,”A small-scale oceanic eddy off the coast of West Africa studies by multi-sensor satellite and surface drifter data, Journal of Remote sensing of Environment, 129, p , (2012).
*CCAR, Colorado center for Astrodynamics Research, Global Modis viewers for SST and ocean color data.
DLR – German Aerospace Center
Technische Universität München
Technische Universität München
*I received an EC Student Grant to attend the LPS 2016