For years, climate models warned that warm deep water around Antarctica could edge closer to the continent\u2019s icy fringe. Now the ocean itself appears to be confirming it.<\/p>\n
A decades-long analysis of ship records and robotic float data found that a mass of relatively warm water called circumpolar deep water has expanded and shifted toward Antarctica\u2019s continental shelf over the past 20 years. That matters because this water can slip beneath floating ice shelves and melt them from below.<\/p>\n
\u201cIt\u2019s concerning, because this warm water can flow beneath Antarctic ice shelves, melting them from below and destabilizing them,\u201d said Joshua Lanham, lead author of the study at Cambridge Earth Sciences<\/a>.<\/p>\n Ice shelves already sit in a precarious role. They act like braces along the edge of the continent, helping hold back inland glaciers and ice sheets. Those ice reserves together contain enough freshwater to raise global sea level by about 58 metres.<\/p>\n A ship deploying a set of sensors as part of long-running ocean monitoring campaign. Ship-based measurements capture a detailed picture of the entire water column, but data is collected infrequently. (CREDIT: Laura Cimoli, University of Cambridge \/ Scripps Institution of Oceanography, UC San Diego) <\/p>\n The new work, led by the University of Cambridge with collaborators from UCLA<\/a> and UCSD<\/a>, draws on long-running ocean observations to show this change across the Southern Ocean, the vast ring of water that surrounds Antarctica.<\/p>\n A clearer picture from ships and floats<\/p>\n Scientists had hints before, but not enough evidence to track the shift directly. Ship-based measurements offered detailed slices of the Southern Ocean, including temperature, salinity, and nutrients from top to bottom. However, those transects were often repeated only about once a decade.<\/p>\n To get around that problem, the researchers combined ship observations with data from Argo floats, autonomous instruments that drift through the upper ocean and provide ongoing measurements. The float network does not go back as far as the ship record. But it fills in the missing months and years that older surveys could not capture on their own.<\/p>\n Using machine learning, the team merged those two streams into monthly snapshots spanning the last four decades. That let them detect a broad shift in where ocean heat<\/a> is sitting.<\/p>\n \u201cIt\u2019s the first time that scientists have observed the shift in deep-ocean heat throughout the Southern Ocean,\u201d Lanham said. \u201cIt\u2019s something that had been predicted by climate models due to global warming, but we hadn\u2019t seen it in data.\u201d<\/p>\n The study found that the change is strongest in the upper 2000 metres of the ocean. Near Antarctica, the warm-water layer thickened. Farther north, it thinned. Taken together, the pattern points to a poleward movement of the warm water mass.<\/p>\n Argo floats are robotic devices that drift through the upper water column gathering continuous snapshots of ocean data, but the programme hasn\u2019t been running as long as ships have been collecting hydrographic sections. (CREDIT: Laura Cimoli, University of Cambridge \/ Scripps Institution of Oceanography, UC San Diego) The cold barrier is weakening<\/p>\n One reason this shift matters so much is that Antarctica\u2019s ice shelves were not always as exposed.<\/p>\n \u201cIn the past, the ice sheets were protected by a bath of cold water, preventing them from melting. Now it looks like the ocean\u2019s circulation has changed, and it\u2019s almost like someone turned on the hot tap and now the bath is getting warmer!\u201d said Prof. Sarah Purkey, one of the senior authors of the study from Scripps Institution of Oceanography.<\/p>\n That \u201ccold bath\u201d was linked to dense, very cold water forming near Antarctica and sinking into the deep ocean. This dense water, including dense shelf water and Antarctic Bottom Water, helped create a barrier that limited how easily warmer circumpolar deep water could reach the ice.<\/p>\n The new study suggests that barrier is shrinking in some places.<\/p>\n In the Weddell Sea and parts of East Antarctica, the expansion of circumpolar deep water near the continent was matched by a contraction in Antarctic Bottom Water and dense shelf water. In West Antarctica<\/a>, where those colder waters are much less prominent in the upper 2000 metres, the expanding warm layer was instead paired with a reduction in Antarctic Intermediate Water.<\/p>\n The researchers also found that north of the Antarctic Circumpolar Current, the warm deep water shrank while Subantarctic Mode Water expanded.<\/p>\n That may sound technical, but the broader point is simple: as warm water moves south, neighboring water masses shift around it.<\/p>\n Schematic overview of the analysis workflow used in this study, illustrating the main processing steps. (CREDIT: Communications Earth & Environment) A signal large enough to measure<\/p>\n The study did more than identify the pattern. It also measured its pace.<\/p>\n Using the Argo-based record, the researchers estimated a circumpolar mean poleward migration of the warm-water boundary of 1.26 kilometres per year, with a 95 percent confidence interval of 0.53 to 1.98 kilometres per year. The movement was strongest in the Weddell Sea at 2.39 kilometres per year. It was intermediate in East Antarctica at 1.31 kilometres per year, and weaker in West Antarctica at 0.80 kilometres per year.<\/p>\n Within the 60 to 65 degrees south latitude band, the ocean heat content within the circumpolar deep water layer increased at a rate of 2.81 terawatts, with a 95 percent confidence interval of 2.0 to 3.6 terawatts.<\/p>\n The researchers say that warming along the Antarctic margin fits what models have suggested for years. Climate simulations have indicated that warming air and added freshwater from melting ice<\/a> would reduce the formation of the cold, dense water that normally forms near Antarctica. As that dense water retreats, warmer deep water can move in.<\/p>\n \u201cWe can now see this scenario is already emerging in the observations,\u201d said Lanham. \u201cThis isn\u2019t just a possible future scenario suggested by models; it\u2019s something that is happening now, bringing wider implications for how carbon, nutrients and heat are cycled through the global ocean.\u201d<\/p>\n One striking result from the analysis was that this poleward shift emerged as the leading mode of non-seasonal variability in the warm-water layer over the last two decades.<\/p>\n Circumpolar distribution of Southern Ocean water mass fractions. (CREDIT: Communications Earth & Environment) What may be driving it<\/p>\n The study does not pin the shift on a single cause.<\/p>\n The authors say one possible driver is the observed contraction of Antarctic Bottom Water and dense shelf water, which would remove part of the cold barrier near the continent. Another is changing wind patterns over the Southern Ocean<\/a>. The westerly winds have strengthened in recent decades. Climate models project they may keep shifting poleward under continued warming. Either process, or both together, could help push warm deep water closer to Antarctica.<\/p>\n The researchers were careful to note limits in their analysis. They assumed that certain biogeochemical end members stayed unchanged during the Argo period, and they acknowledged that selecting those end members has an unavoidable subjective element. They also noted uncertainty in how best to represent source waters through time.<\/p>\n Still, they say the main finding held up across sensitivity tests, including tests involving stronger warming of Antarctic Bottom Water and different model setups.<\/p>\n Practical implications of the research<\/p>\n The immediate concern is ice.<\/p>\n