18 September 2018

SAR and the mushroom

Our sampling strategy on the Sikuliaq this month has been adaptive, meaning we did’t come out here with an exact plan of where and when we want to be at specific locations. We wanted to sample interesting features at the shelf break and near the ice edge. One of the most useful tools we have to find those interesting features is satellite imagery, especially the images that come from Synthetic Aperture Radar.

Synthetic Aperture Radar (SAR) relies on electromagnetic wavelengths that are much longer than optical wavelengths to map surface reflectivity. The long wavelengths are unaffected by clouds and work just as well at night. On the ocean, reflectivity is dependent on things like waves and sea surface height. Different water masses at the surface have slightly different properties that can often be distinguished by SAR. Ice is extremely reflective and easy to pick out.

We have two radar engineers on the cruise, John Hargrove and Neil Williams from the University of Miami, who have been requesting and archiving SAR imagery from the areas we’re studying and making it available to everyone on the ship. These have been extremely helpful for making decisions about where we slow down the boat to make our measurements. John and Neil have also been monitoring SAR images they’re taking from the ship’s mast which give a closer view of the features more immediately around us.

Last week, when we began sampling along the ice edge and got our first SAR image, we noticed what looked like a long filament of ice moving to the east and then north and swirling a bit as it created the southern boundary of the ice field. It looked like a good place to stop and put our profilers in the water. As the days went by, the filament took on a more distinct mushroom shape and it became clear that we were sampling a plume of very warm Pacific water. In fact, the warmest temperatures we’ve seen yet were right in its center.

The SAR images help us make sense of what we’re seeing from our limited small-scale perspective. For example, when we made a north-south slice through the mushroom plume we noticed the currents below the surface layer reversed. Without any regional context, we’d be left to guess at the reason for this reversal. From the image, we can visualize the plume circulation with a northeastward flow through the center and a return flow to the north.

The SAR images also give us ideas about new physics to look for. Zooming in on the icy parts of the mushroom, Tom Peacock from MIT, noticed bands that he guessed were the surface signature of internal waves. Yesterday, we did a short survey with the FastCTD, sampling only down to 40 m and back in quick succession for about an hour. Sure enough, Tom found internal waves with a period of about 20 minutes traveling along the pycnocline. He thinks that the ice itself is creating them by interacting with the currents and generating lee waves.

And just like that, a new scientific endeavor is born from just a single picture.