Sydney’s Red Dawn
The dust storm season of spring 2009 was particularly active. On September 22 Sydney awoke to a “Red Dawn”. The storm continued into the following day. A new paper in the Journal of Marine & Freshwater Research was published in this fall (2015).
While some might think of blue skies replaced with red dusty skies to be a disaster for ocean pastures it is a blessing of biblical proportions. At the very least it reminds one of a great musical ballad.
Red skies smiling at me
Nothing but red skies do I see
Ocean deserts turning to green
Nothing but green seas do I see…
The dust from this wonderous storm came from the Lake Eyre Basin in central Australia. It was the largest dust storm to hit Sydney since records began in 1940. This “Red Dawn” event, carried winds of 80-100km/h and sustained elevated dust at numerous Dustwatch stations across New South Wales on September 22 and 23. The storm affected visibility in Brisbane and as far north as Cairns. An estimated 2.5 million tonnes of sediment was blown out to sea off the Australian coast in a 3,000-kilometre-long dust plume, making it the largest loss of soil from the Australian continent ever reported.
Out to sea
This dust blew out over the ocean, where it was deposited on the Tasman Sea, aided by rainfall. There was a second event in mid-October 2009.
Research published shows the two dust storms that traversed the south-east coast of Australia caused a widespread spike in phytoplankton biomass in the Tasman Sea. This was especially so south of the Tasman Front (at around 32 degrees south), where the East Australian Current veers east from the Australian coastline.
Interestingly, although the “Red Dawn” event of late September was one of the largest on record in terms of total soil loss, the biological response was almost as strong following the less intense mid-October event. This suggests the timing and mode of delivery of dust-derived nutrients are critical.
A previous 20-year study of dust storms suggested that it is unusual for dust storms to happen at the right time to cause plankton blooms.
Measuring ocean plants
Ocean phytoplankton contain chlorophyll that empowers them to convert CO2 into energy via photosynthesis, thus enabling them to grow. It’s a green substance and thus a good indicator of overall plant health: robust forests, lush lawns and vibrant phytoplankton blooms appear green.
Chlorophyll concentration in the surface ocean is a means to measure phytoplankton biomass and thus ocean pasture productivity and can be estimated using satellite-based ocean colour sensors (such as NASA’s MODIS Aqua).
During the spring of 2009 chlorophyll was well above the long-term climatological mean for the southern spring (0.3-0.4 milligrams per cubic metre). Values were as high as 0.9 milligrams per cubic metre or about 100% above baseline concentrations.
Sadly the Australians were unable to get out to sea to collect water samples to measure the amount of iron concentrations in the water. However results from other studies in the Tasman suggest that iron can limit phytoplankton growth south of the Tasman Front, which is indeed where the greatest phytoplankton growth was seen.
As a result of global warming, droughts will likely become more frequent and the number of severe dust storms in Australia may increase in the future. There is some evidence that the amount of dust in the atmosphere over the continent is already changing due to enhanced dust storm activity.
Although the researchers data describe a very active dust storm season, they raise the possibility that more dust storms in future may mean more phytoplankton, in turn absorbing more carbon into the sea.