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| Antarctic voyage: here may be monsters | Antarctic voyage: here may be monsters |
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| Two nights ago, a group of the scientists on board the Shokalskiy began a continuous 30-hour stretch of throwing bits of their equipment into the ocean. Every hour (sometimes every half-hour), they would throw a buoy or a temperature sensor overboard. At around 20 hours in, they even threw in a robot. | Two nights ago, a group of the scientists on board the Shokalskiy began a continuous 30-hour stretch of throwing bits of their equipment into the ocean. Every hour (sometimes every half-hour), they would throw a buoy or a temperature sensor overboard. At around 20 hours in, they even threw in a robot. |
| On the way to Antarctica, the oceanographers on board the expedition wanted to find the “convergence zone” in the Southern Ocean, the point at which the subtropical waters give way to the polar waters. This is the location of a branch of the Antarctic Circumpolar Current, the strongest current in the world and the flow of water that isolates the frozen continent from the tropics. | On the way to Antarctica, the oceanographers on board the expedition wanted to find the “convergence zone” in the Southern Ocean, the point at which the subtropical waters give way to the polar waters. This is the location of a branch of the Antarctic Circumpolar Current, the strongest current in the world and the flow of water that isolates the frozen continent from the tropics. |
| Mapping the deep currents that move heat, carbon, plankton (and even waste plastics), around the world is vital for a detailed understanding of the Earth's climate system. And, though the Southern Ocean connects all the other oceans together and plays a key role in global climate, it is the least studied because it is so hostile and remote. | Mapping the deep currents that move heat, carbon, plankton (and even waste plastics), around the world is vital for a detailed understanding of the Earth's climate system. And, though the Southern Ocean connects all the other oceans together and plays a key role in global climate, it is the least studied because it is so hostile and remote. |
| Locating the convergence zone in the Southern Ocean involves measuring the temperature of the water every few kilometres in the region where previous satellite data has suggested it might be. In their survey, the AAE team threw 28 eXpendable BathyThermistor (XBT) probes into the water. “It's basically a thermometer you throw into the ocean and, as it falls down from the surface, it records temperature,” says Erik van Sebille, an oceanographer at the University of New South Wales. | |
| The first diagram below (figure 1) is the temperature map of the area around the convergence zone in this part of the Southern Ocean, taken from the 30-hour experiment. It has never before been directly measured and it is the first data from the AAE. | The first diagram below (figure 1) is the temperature map of the area around the convergence zone in this part of the Southern Ocean, taken from the 30-hour experiment. It has never before been directly measured and it is the first data from the AAE. |
| Each vertical purple line represents data from one XBT probe as it descended through the water. The team started throwing in probes just before 56 degrees south (on the right of the diagram) and finished at around 59 degrees south (on the left). The wider gaps between purple lines are data taken around 20km apart, while the narrower ones are 10km apart. The total distance the ship covered in the 30-hour stretch was around 440km. | Each vertical purple line represents data from one XBT probe as it descended through the water. The team started throwing in probes just before 56 degrees south (on the right of the diagram) and finished at around 59 degrees south (on the left). The wider gaps between purple lines are data taken around 20km apart, while the narrower ones are 10km apart. The total distance the ship covered in the 30-hour stretch was around 440km. |
| The results show that the convergence zone is clearly visible at between 56.5 and 57 degrees south. The water on the sub-tropical side is noticeably warmer in its upper 300 metres than the same depth on the polar side. | The results show that the convergence zone is clearly visible at between 56.5 and 57 degrees south. The water on the sub-tropical side is noticeably warmer in its upper 300 metres than the same depth on the polar side. |
| Another interesting feature is a cold tongue of water on the left of the map, at 100 metres depth on the left and reaching all the way to 57.5 degrees south. “This is certainly water that's formed around Antarctica, otherwise it can't get so cold,” says van Sebille. “It needs sub-zero air temperatures to get so cold. This tongue made its way all the way maybe from Commonwealth Bay or somewhere else and moved, sub-surface, through the ocean, moving north around 800km.” | Another interesting feature is a cold tongue of water on the left of the map, at 100 metres depth on the left and reaching all the way to 57.5 degrees south. “This is certainly water that's formed around Antarctica, otherwise it can't get so cold,” says van Sebille. “It needs sub-zero air temperatures to get so cold. This tongue made its way all the way maybe from Commonwealth Bay or somewhere else and moved, sub-surface, through the ocean, moving north around 800km.” |
| Pinpointing exactly where this water has come from would require measurements of its saltiness. “Later on, we will also do that – we did this experiment now because we wanted to know where exactly this front was and temperature is good enough,” says van Sebille. | Pinpointing exactly where this water has come from would require measurements of its saltiness. “Later on, we will also do that – we did this experiment now because we wanted to know where exactly this front was and temperature is good enough,” says van Sebille. |
| By mapping the precise position of the convergence point, the scientists will have a baseline onto which they can then superimpose other data they collect, such as measurements of water currents or amounts of plankton or plastics in the water. | By mapping the precise position of the convergence point, the scientists will have a baseline onto which they can then superimpose other data they collect, such as measurements of water currents or amounts of plankton or plastics in the water. |
| “The XBT section was there mainly to be sure where the front was, after the fact, so that we can sub-divide the experiments we did north of the front from the experiments we did south of the front,” says van Sebille. | “The XBT section was there mainly to be sure where the front was, after the fact, so that we can sub-divide the experiments we did north of the front from the experiments we did south of the front,” says van Sebille. |
| The scientists also threw in buoys, which van Sebille describes as “free-floating beachballs” that can drift along with the currents on the surface of the ocean for up to five years. They record their position every six hours and send that information, via text message, to a central database. “Every time, we threw in two at the same time, they start out at 10 metre difference,” says van Sebille. “What we're interested in is how quickly they separate. In total, we've put out 20 buoys, 10 pairs.” | The scientists also threw in buoys, which van Sebille describes as “free-floating beachballs” that can drift along with the currents on the surface of the ocean for up to five years. They record their position every six hours and send that information, via text message, to a central database. “Every time, we threw in two at the same time, they start out at 10 metre difference,” says van Sebille. “What we're interested in is how quickly they separate. In total, we've put out 20 buoys, 10 pairs.” |
| The most sophisticated piece of equipment thrown over the side of the Shokalskiy in the 30-hour stint was an Argo float, a robotic probe that autonomously measures the temperature and salinity of the upper 2km of the water. | The most sophisticated piece of equipment thrown over the side of the Shokalskiy in the 30-hour stint was an Argo float, a robotic probe that autonomously measures the temperature and salinity of the upper 2km of the water. |
| Since 2004, Argo floats have become an important part of the ocean observation network. There are around 3,000 of them around the world, slowly drifting with the deep currents, mapping the oceans. Every so often, the probes will rise to the surface, send back their measurements, and then descend back into the depths. Their batteries last for about three to four years and, which means around a thousand new Argos need to be deployed in the world's oceans every year to keep the mapping project going. | Since 2004, Argo floats have become an important part of the ocean observation network. There are around 3,000 of them around the world, slowly drifting with the deep currents, mapping the oceans. Every so often, the probes will rise to the surface, send back their measurements, and then descend back into the depths. Their batteries last for about three to four years and, which means around a thousand new Argos need to be deployed in the world's oceans every year to keep the mapping project going. |
| Not everything goes to plan out in the field, however. In analysing their data from the XBT probes, the AAE team noticed that one of their sensors reported some strange numbers (see figure 2), towards the end of their 30-hour deployment. The second to last XBT that they threw overboard was the curious one. “That's the only one that went wrong,” says van Sebille. “It started nicely until 200m down and then suddenly you get a spike to 12C warm. Then back to its normal profile and then back to 600m or so when it goes to 37C.” | Not everything goes to plan out in the field, however. In analysing their data from the XBT probes, the AAE team noticed that one of their sensors reported some strange numbers (see figure 2), towards the end of their 30-hour deployment. The second to last XBT that they threw overboard was the curious one. “That's the only one that went wrong,” says van Sebille. “It started nicely until 200m down and then suddenly you get a spike to 12C warm. Then back to its normal profile and then back to 600m or so when it goes to 37C.” |
| It might have been a malfunctioning probe, or perhaps the AAE have stumbled upon a special part of the ocean – at just before 59 degrees south – that is inexplicably hotter than the rest. Both are unlikely, says van Sebille, who added that he had seen nothing like this before in his career. | It might have been a malfunctioning probe, or perhaps the AAE have stumbled upon a special part of the ocean – at just before 59 degrees south – that is inexplicably hotter than the rest. Both are unlikely, says van Sebille, who added that he had seen nothing like this before in his career. |
| But, perhaps recalling the tales of the old seafarers who would write “there be dragons” on the unknown regions of their maps, van Sebille didn't hesitate to put forward a tantalising possibility for the anomalous result. “It might have been eaten by something, something warm-blooded,” he says. “A sperm whale, perhaps?” | But, perhaps recalling the tales of the old seafarers who would write “there be dragons” on the unknown regions of their maps, van Sebille didn't hesitate to put forward a tantalising possibility for the anomalous result. “It might have been eaten by something, something warm-blooded,” he says. “A sperm whale, perhaps?” |
| Not quite dragons but, in these remote waters, there might well be monsters lurking in the deep. | Not quite dragons but, in these remote waters, there might well be monsters lurking in the deep. |
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