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We resumed oceanographic sampling at about 1600 local time yesterday and have completed a further three stations. We have not seen much of the aurora on this trip, but last night we had the right combination of conditions (clear skies, far enough south for the auroras to occur, far enough north to get some darkness at night) to enjoy a midnight display. As samples are processed in the labs and calibrated, some of the early results of the voyage are starting to come in. The samples we were able to collect near the Mertz Glacier while waiting for the weather to improve show that the region continues to feel the impact of the calving of the Mertz Glacier Tongue in February 2010. The region is significant because it is one of the few locations where dense Antarctic Bottom Water is formed and sinks to the deep ocean. Calving of the glacier tongue changed the behaviour of the polynya in this region. Polynyas are regions within the pack that remain free of ice even in winter: as rapidly as the sea ice forms, it is swept away by strong winds. As a result, polynyas produce lots of ice and lots of dense (cold + salty) water. We expected that the calving of the glacier tongue would lead to a smaller and less active polynya, with less dense water formed. Measurements we made last summer confirmed this, showing that the salinity decreased by an amount equivalent to 50 years of the long-term freshening trend in this region. What we have discovered on this voyage is that the region continues to change rapidly. The salinity of the dense water produced in the polynya is now so low that the water is not dense enough to sink to the sea floor. In effect, the calving of the Mertz Glacier Tongue has turned off the tap and the region no longer produces true Antarctic Bottom Water. These observations emphasise how sensitive the region is to changes in the “icescape,” whether caused by natural events (like the calving of the glacier tongue) or to human activities (like global warming or the ozone hole).
Regards, Steve and Barbara
Map
A map showing Australia and Antarctica. The map shows the journey of one voyage that has occured in the season, with each route highlighted in a distinct colour.