If you haven’t gleaned this already, studying hidden processes miles below the ocean’s surface is challenging. Even worse, we’re cruising in an unpredictable region in the Tasman Sea, a spot notoriously dubbed the “Roaring Forties” due to the strong winds ripping from the west through the Southern Hemisphere between 40 and 50 degrees latitude. We’re limited to the tools we can carry on the ship, so we come prepared. If an instrument breaks, we shift, rethink and make do with what we have. And the last few days have been a reminder of this.
Sunday we recovered a mooring (to re-deploy elsewhere) that had one of the McLane Moored Profilers on it (shown here in a happy pre-deployment status). It robotically climbs up and down the mooring line at a speedy 33 cm/s, measuring temperature, salinity, and velocity as it goes. When it works, it provides an extremely valuable high-resolution view of the ocean by taking smooth, continuous vertical profiles of temperature versus depth, as opposed to the chunky measurements other sensors take from individual spots on the line.
But a moored profiler is a finicky beast. For starters, it must be “neutrally buoyant,” meaning that it should be “floating” up and down the line in the middle of the ocean like a hot-air balloon in the sky. To do this, it must be equally as dense as the surrounding water. Imagine a dog-chewed tennis ball floating halfway to the bottom of a swimming pool. If it’s either too heavy or too light, it either sinks or floats. Similarly, if the profiler is too heavy, its motor must work extra hard to crawl up and down the line. Therefore we carefully adjust its weight by compensating the heavy stuff inside of it—like the instruments measuring things—with glass balls filled with air to make it light (glass is heavy but air is very light compared to water). The net effect is the fabled Goldilocks juuuuuust right.
When Matthew Alford pulled the data from the recovered profiler on Sunday, he looked at the diagnostics on his computer and then frowned. The poor little motor had been working very hard and the instrument had not been crawling well (“I think I can, I think I can…”). We initially fretted that it had been ballasted poorly, meaning that our formula for adding up the weight was wrong. If this were the case, the formula could’ve been wrong for the rest of the moorings as well. But upon closer inspection, we realized that the profiler was actually heavy (very heavy!) because its internal glass ball had cracked and was not filled with air, but filled with seawater. Doh!
Manufacturing a glass sphere to withstand the intense pressure of the deep ocean with zero imperfections is a huge engineering feat (it has to endure 10,000 psi, or five tons of weight per square inch).The engineering is usually perfect, but sometimes deficiencies bleed through. Luckily this sphere only cracked and flooded itself. Sometimes a crack can cause the ball to literally explode, taking out everything else in its path. We walked away from this one feeling like we had gotten off pretty light.
Today we take the “sphere is half full” perspective. We were very lucky to have pulled this particular mooring up and notice the problem, instead of letting it be in the ocean half-working for the next two months. Now we have the opportunity to remove the busted profiler from the mooring, re-design a new mooring and throw it back into the ocean. If we were on land, we would spend many weeks carefully designing each mooring, weighing all the components and considering the stress and pressure they each would be subjected to. But sometimes the ocean throws a curve ball and the team must re-design a brand new mooring with only a few hours to spare. Check inventory! Laptops out! Time for more coffee!
—Julia Calderone and Jennifer MacKinnon, The Revelle
The Tasman Tidal Dissipation Experiment//Supported by the National Science Foundation