Tag Archives: Eddy

Ocean Eddies & Slicks in SAR Imagery

In a recent post, I talked about observing an eddy in the Arabian Sea in L-band ALOS PALSAR SAR imagery. In this post, I want to talk briefly about the physical interaction between SAR signals and eddies.

Spiral eddies are often convergence zones and act as accumulators of surface slicks. These surface slicks (could be biogenic / natural oil seeps / mineral oil etc.) make a surface layer over the ocean and actually dampen the surface waves of the ocean through a phenomenon called Marangoni Damping (see this seminal paper by Alpers and Hühnerfuss).

However, sometimes it is also possible that an eddy may appear brighter in SAR imagery than the surrounding ocean, due to wave-current and shear interactions.

In my paper on ocean currents from sequential SAR imagery, I talk about this phenomenon in the introduction, and you can also find some good references therein.

For further interest, here are a few other seminal papers on the science of ocean wave damping by surface slicks:

Discovering Submesoscale Eddies in the Arabian Sea through SAR Images

I am working on a research study to analyze physical oceanography features in the Arabian Sea using Synthetic Aperture Radar (SAR) remote sensing imagery. For this study, we are using L-band ALOS PALSAR imagery. In the first phase, we have been looking at summer monsoon upwelling and related biogenic slicks. After processing some SAR imagery, and just going through some of the images visually, we discovered a really nice sub-mesoscale eddy in one image.

For the uninitiated, submesoscale eddies are fleeting and shy creatures of physical oceanography, and have not proven easy to find, due to their short temporal and spatial scales. Submesoscale eddies and dynamics are subjects of current research in physical oceanography, both in terms of modeling and observations (see,, Few ships with wakes are also clearly visible in the SAR image.

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Submesoscale eddy in the Arabian Sea. Image from JAXA. Data processed and analysed by Waqas Qazi and Aaqib Javad.

The image is a processed SAR image from ALOS PALSAR. Processing steps include calibration, speckle filtering, geocoding, and resampling through automated processing workflows. I am working with a graduate student to analyze ALOS PALSAR-1 and PALSAR-2 images spread over 4 years to analyze physical oceanography features in the Arabian Sea. This research is supported by the International Foundation for ScienceJAXA Research Announcement 4 (RA-4), and the Institute of Space Technology. We have also published some basic work on identifying a temperature front in SAR imagery in the Arabian Sea (more on that in an upcoming blog post).

I had previously found a submesoscale eddy in the California Current System when deriving ocean currents from sequential SAR imagery, see the paper here. Also, Marmorino et al. (2010) found submesoscale eddies in SAR imagery in the Southern California Bight.

To learn more about the state-of-the-art in submesoscale ocean dynamics, see: