PICCOLO
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A glider emerges from under an iceberg. Credit: Karen Heywood.7 March 2024
Prof Karen Heywood
We must have discovered a new force of nature – the attraction between ocean gliders and ice! When we are piloting our gliders, we typically rely on satellite imagery to tell us how much ice is in the region. However, what the ice looks like on the satellite images is sometimes quite different to what it looks like on the ground – or on the water I should say.
Ice – whether sea ice (frozen seawater) or icebergs (bits of glacier from the land) – is hard. Rock hard. It moves with the waves and the currents. It doesn’t take any prisoners. Gliders are small (about as big as a person), and they have delicate antennas, sensors, wings and rudders. Every few hours, when the glider comes to the surface to communicate with the pilots back in the UK, sending back data and receiving instructions, the glider is at the mercy of the ice. Our gliders are quite smart – if they come up underneath ice, they try to call home a few times, and if they don’t get a signal (just like your mobile phone when you’re out in the wilds), they go back down and carry on with their mission. We don’t panic if we don’t hear from the glider for a few dives – we know it’s probably under the ice and will come back eventually. We cross our fingers and hope…
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When we want to recover the gliders back to the ship, however, we have to get the glider to the surface. That’s the most risky part of the glider’s mission. Our glider pilots liaised closely with us, aiming to have the glider at the surface as we arrived on the ship to pick it up. Arrive too late, and the ship wastes valuable time waiting for the glider to surface. Arrive too soon, and the glider is in a vulnerable spot for longer.
Two of our gliders communicated regularly every few hours, every time they surfaced – until the very moment when they were told to come to the surface for recovery. The ship arrives to collect them, but there are no calls from the gliders. I’m on the bridge coordinating the recovery, reporting on the gliders’ locations. There are large sea ice floes around the ship. The chance of the glider coming up underneath one is low, as there is mostly open water around us, but we know what gliders are like. “The gliders should be calling any minute,” I say confidently to the Captain, “they’re due at the surface now”. A few more long minutes go by. No calls on my computer. “Probably, the gliders came up under one of those ice floes and have gone down for another dive, they’ll be another 20 minutes or so,” I tell Will, the ship’s Captain. We go quiet. Minutes tick by. We make a cup of tea. Then, suddenly, both gliders start calling at once – they’re both at the surface! “Just like buses,” comments one of the pilots back at UEA in our group chat. The glider GPS locations appear on my computer and I relay these to the Captain – we set off to lift the gliders out of the water and onto the ship. Another glider encounter with ice has ended well!
One of our gliders surfaced beautifully, on time, and was easily spotted not far from the ship ready for us to collect. But then it began to drift towards a large nearby iceberg, as if drawn by an invisible force of attraction. We held our breath. The ship’s turbulent wake pushed the glider further towards the iceberg as we inched nearer to it. Then the glider went beneath the overhanging side of the iceberg. It looked very vulnerable and small in comparison with the huge iceberg. “Don’t worry,” Captain Will said calmly, “we’ll just wait until it pops out at the other end.” And so we watched for a few minutes as it bobbed along past the iceberg and made its way into the open water. There we safely recovered it, and breathed a big sigh of relief.
It was heart-breaking to have been so close to getting it back safely on board after its multi-month mission, only for it to get crushed on its last day.
Prof Karen Heywood (UEA)
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Sometimes things don’t end so positively. Another glider had been profiling away in relatively open water. We approached during the night, located the glider, and waited for daybreak. On approaching the glider, we found that it had drifted into a patch of broken up iceberg – technically called bergy bits. What is the chance that a glider will find its way into the only bit of ice for miles around? Well you know what those pesky gliders are like…. The bits of ice were moving up and down with the swell, and the poor little glider was being bashed between big lumps of ice. It would have been too dangerous to deploy a small boat to try to extricate the glider. As we tried to manoeuvre the ship in to get it, the glider became more difficult to see, disappearing for a few seconds, and then it was gone for good. We kept hoping it would reappear outside the patch of ice, but it didn’t. It was heart-breaking to have been so close to getting it back safely on board after its multi-month mission, only for it to get crushed on its last day. Thankfully, gliders send their data back throughout their missions, so their scientific work is done.
In ocean science, we risk losing our instruments every time we put them into the ocean – perhaps from bad weather, waves or strong currents. With gliders, however, we have another hazard to consider – the glider’s unerring affinity with ice in its many forms!