The 'global thermohaline circulation' Image:


Preliminary results are already available from the sampling that occurred around the Mertz Glacier several weeks ago. The calving of the glacier tongue after collision with the passing B9B is of significant interest as it one of the few locations where dense Antarctic Bottom Water is formed. Such formation occurs at polynyas.

A polynya is a region within the pack ice that remains ice free all year round due to strong winds sweeping the ice away as soon as it forms. As a result the water is very salty (salts being left behind when ice forms) and very cold (goes without saying if it’s cold enough to produce ice). Due to the increased density of very cold, very salty water, it sinks in to the deep ocean forming part of the global circulation I keep harping on about.

According to Dr Rintoul it was expected that the calving of the glacier tongue in February 2010 would lead to a smaller, less active polynya and less dense water formation. In today’s “sitrep” he explains that the measurements from last summer demonstrated that the decrease in salinity was equivalent to 50 years of the long-term freshening trend in this region; where freshening refers to the dilution of salt water with freshwater i.e. from the continent. This voyage’s salinity measurements indicate that this rapid change has continued and that the salinity is too low to continue forming true Antarctic Bottom Water. He concludes his report by saying that these observations emphasize how sensitive the region is to changes in the ‘icescape’, whether they be natural or due to human activity.

This is a very important point and something that climate scientist must grapple with constantly; what is natural variability within a system and what it the overprint of human activity? In this instance, the calving of a glacier that reduces regional salinity is a natural event that exacerbates the long term freshening trend. However, accelerated global warming or a hole in the ozone layer, these are not natural events however they have the same impact by accelerating glacial melt and increasing freshening. In either case, deep water formation slows down with implications for the global ‘conveyor belt’ circulation. There I go again, it’s all about circulation.

All this information and we haven’t even crossed the Subantarctic Front.