Riders on the Storm

By Matthew Wood 28/02/2010

Turns out there are no reservations about hitchhiking in Antarctica.

On the Victoria Land coast, microscopic particulate matter, from volcanic ash to sea salt, powdered rock to helicopter exhausts, make a habit of snatching free rides on the winds of Ross Sea cyclonic systems or on the gravity-driven katabatic gales that flow off the ice sheets most ferociously during the long dark of the Antarctic winter.

Microscopic View Snow Samples

The Evans Piedmont Glacier is a small ice dome that sits on the coast, snuggled between the McMurdo Dry Valleys and the lofty Transantarctic Mountains to the west, and the Ross Sea to the east. With the McMurdo Volcanic Group to the south and a handful of research stations nearby, it’s the perfect repository for stray aerosols whose chemistries recount a tale of changing local wind patterns. In the summer of 2007, Julia Bull of the Antarctic Research Centre travelled to Evans to read that story for herself.

The study is the first of its kind in the area to analyse the snowpack for a broad suite of major and trace elements. Four to five metre-deep snow pits, burrowed into the glacier surface, reveal a continuous profile of snow accumulation clearly marked out by low-density summer hoar horizons. To descend into such a snow pit is to step back in time well over a decade. The annual snow layers — while outwardly unassuming — are actually a detailed chronicle of climatic change as shown by fluctuations in a range of proxies, including: particulate matter elemental concentrations; stable isotopes of hydrogen and oxygen, which respond to seasonal temperature change; and methanesulphonate, an atmospheric acid that forms in response to primary productivity in the surface layer of the ocean and is indicative of summer accumulation when the Ross Sea is relatively free of sea ice.

Many terrestrial mineral dust element concentrations correlate well with annual maximum wind speed, but with marine aerosol the relationship isn’t as clear. Inter-annual changes are revealed when plotting the ratio between certain terrestrial- and marine-sourced elements, mirroring equivalent shifts in mean summer wind strength at a sub-decadal frequency. Weather station temperature measurements document a similar pattern, which included a cooling of the Victoria Land coast during the 1990s.

Wind Sculpted Snow

Climate change deniers predictably pounce on any climate records that show a cooling trend, but often their ‘interpretations’ will ignore how localised the signal is or fail to consider the effects of natural climate variability superimposed on a human-induced global warming trend. Julia’s findings support the claim that the Ross Sea climate experiences regular forcing by the El Niño-Southern Oscillation, which periodically enhances the inflow of cold continental air masses from the south.

Studies like Julia’s, set within the timescale of instrumental climate records, are extremely important in calibrating the paleo-climate proxy toolbox that can then be applied to the longer-term climate histories provided by ice cores.

Figure (c) Julia Bull 2009. Photo (c) Matthew Wood 2003

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