Synoptic

The visualization platform developed in the ESA funded GlobCurrent Project (http://www.globcurrent.org) and the Ocean Virtual Laboratory Project (http://ovl.oceandatalab.com) are systematically explored to maximize the benefit of sensor synergy for advances in studies and understanding of upper ocean mesoscale processes and dynamics. The highly complex and dynamic greater Agulhas Current regime is chosen. Both radar altimetry, microwave and infrared radiometry, imaging spectrometer and synthetic aperture radar data are explored in combination with in-situ observations from Argo profiling floats and surface drifter data. Individually the multi-sensor satellite data have inherent capabilities and limitations with regards to spatial resolution, coverage and repeat sampling. The sensor synergy is a highly attractive approach to shortcut some of the limitations. As such, it advances the interpretation and application as previously clearly demonstrated by Kudryavtsev et al. (2012).

In this presentation, the focus is on detection and interpretation of ocean frontal boundaries. Under ideal imaging conditions (no clouds, weak to moderate winds, etc) essentially all optical and microwave remote sensing sensors manifest presence of ocean fronts such as related to gradients and changes in: (i) infrared- and microwave radiometer-based sea surface temperature; (ii) imaging spectrometer-based chlorophyll distribution and concentration; (iii) altimeter-based sea level anomaly; (iv) SAR-based surface roughness and (vi) SAR-based range Doppler shift. A common driver for the expression of these frontal boundaries is the surface currents and their spatial changes and meandering pattern including instabilities and formation of eddies. In particular, surface current changes and gradients related to shear, vorticity and strain can lead to rotation, divergence and convergence with corresponding upwelling and downwelling in the upper ocean. These distinct surface current changes therefore establish a spatial correlation in gradients of sea surface temperature, sea level anomaly and sea surface roughness. Capitalizing on this correlation it is demonstrated that satellite sensor synergy clearly advances the routine monitoring of the two-dimensional surface expression of mesoscale upper ocean processes and dynamics. This approach, here demonstrated for meanders and eddies in the Agulhas Current core and in the Agulhas Return Current, largely shortcut the limitation with individual sensor imaging capabilities. Jointly with coincident in-situ observations, it evidently creates an important step toward advances in near real-time monitoring and quantitative interpretation of mesoscale upper ocean processes and dynamics. As such, it is also a promising approach in terms of assessment as well as systematic use of the detection capabilities provided by the approved NASA and CNES Surface Water and Ocean Topography (SWOT) mission expected to be launched in 2021. Arguably, the approach will also strengthen high resolution model validation and assimilation. 

The latest European Space Agency’s (ESA) Sentinel missions support emergency response such as oil spill monitoring by providing timely and cost-free data at high temporal, spectral and spatial resolution.

With the launch of Sentinel-3A early 2016, and the previously launched Sentinel-1A/B and Sentinel-2A/B satellite series, the three different missions now offer the potential for the synergistic use of optical, thermal and SAR measurements for the detection and mapping of oil spills anywhere in the world’s ocean. Scanning the Earth’s surface with two satellites significantly increases revisit capability, estimated to be approximately 5 days for Sentinel-1 and Sentinel-2, and 1 day for Sentinel-3 at the equator. This significant improvement in data acquisition frequency, in comparison to the previous ESA Envisat 2002-2012 mission, allows for the synergetic use of three Sentinel satellites for near-real time oil spill monitoring.

The recent incident that occurred 400 km offshore in the China Sea with the Sanchi Iranian tanker on 14th January 2018 is of a sufficiently large size (i.e., several oil slicks of light crude oil covering a surface area > 300 km2) that it allows for this synergistic mapping to be tested.

We investigate the combined use of Sentinel-1A/B SAR, Sentinel-2A/B Multi-Spectral Instrument (MSI) and Sentinel-3A Ocean and Land Colour Instrument (OLCI) imagery acquired from the 17th January 2018 onwards for improved discrimination of this oil slick from the various sensors. Interferometric Wide swath (IW; 250 km) and Extra Wide swath (EW; 400 km wide) modes acquisitions from Sentinel-1, specifically used for wide area coastal monitoring including ship traffic and oil spill monitoring, and full resolution (300 m) scenes from Sentinel-3 at 865 nm in particular, proved very useful for the oil slick mapping and the evaluation of its environmental impact by tracking its dispersion over time and space.

We will show that the synergetic use of ESA Sentinel satellites offers an enormous potential for innovation in the context of oil spill monitoring, and how this combined approach can be successfully transferred to our new exciting project on oil spills detection along the Great Barrier Reef in Australia.

In studies of upwelling usually data from infrared and optical sensors are used which provide information on the sea surface temperature (SST) and the chlorophyll-a (Chl-a) concentration. In this paper it is shown that also synthetic aperture radars (SAR) images can also give valuable contribution to such studies. Upwelling  regions become detectable by SAR because they are associated with a reduction of the radar backscatter due to 1) a change of the stability of the air-sea interface or/and 2) the presence of biogenic slicks. While the change of the stability of the air-sea interface due to the presence of cold surface water in the upwelling region causes only a small reduction of the radar backscatter, biogenic cause a very strong reduction, usually of more than 10 dB. In areas of strong upwelling, the biological productivity is high due to increased nutrient supply from lower water levels. The biota living in this area secrete surface active material that ascends to the sea surface and forms there biogenic slicks, which often cover large areas in the form as monomolecular layers. The biogenic slicks damp the short-scale surface waves, which are responsible for the radar backscattering, as strongly as mineral oil films, and thus areas covered with the surface films become visible on SAR images as areas of strongly reduced radar backscatter (dark areas). Biogenic slicks often get entrained in the surface current field of eddies and thus render the structure of eddies visible on SAR images.

This paper focusses on upwelling caused by cyclonic eddies which are generated by different mechanisms: 1) detachment from a filament of an upwelling front (in the Benguela upwelling system), 2) interaction of a strong current with a headland (Cap-Vert at the east coast of Senegal), 3) intensification of a meandering current (the Agulhas Return Current off the coast of South Africa and the Atlantic Ionian Stream in the Mediterranean Sea off the south coast of Sicily). These upwelling events are studied by using Sentinel-1 SAR images, Modis SST and Chl-a maps and model data of geostrophic surface currents. It is shown that this synergism yields new insights into upwelling mechanisms, e. g., into upwelling associated with cyclonic meanders of the Agulhas Return Current. A Sentinel-1 SAR image is presented that shows that on the rim of a cyclonic meander strong surface convergence occurs, which causes increase in the short-scale sea surface roughness and thus of the radar backscatter. At the same time, the Chl-a map shows on the rim increased the Chl-a concentration indicating increased of biogenic productivity, while the SST map shows a double-sign structure of the rim (a warm outer ring and a cool inner ring). A physical explanation of this phenomenon is presented.

Making use of the fine resolution and repeat capabilities of two Sentinel-1 SARs, optical imagery from Sentinel-2, and optical/thermal IR imagery Landsat8, multiple acquisitions over a few days period that provide fascinating and enticing repeat observations of rapidly evolving small eddies.  Often these sequences require coincident SST and ocean color imagery to help clarify the signatures.  The study will show recent observations over Southern California and Lake Superior that illustrate the value of rapid sequences using multiple sensors. Also shown will be sequences of small eddies in the marginal ice zone obtained from multiple SAR sensors.  From such sequences, useful derivations of eddy rotational velocity may be obtained.