Drought-enhanced dust as a driver of decadal changes in Tasman Sea phytoplankton

Ocean colour observations over the Tasman Sea show that summer chlorophyll gradually increased by a factor of 3 from 2003 to 2009 and then returned to the pre-2003 values by 2011. Associated with these changes in summer chlorophyll was a corresponding increase and cessation of extreme dust events over the Australian continent, related to one of the most severe droughts ever recorded. A number of studies have attempted to link individual dust events with surface chlorophyll responses in the Tasman Sea, but generally they do not find a clear correspondence between transport of dust over the region and its impact on chlorophyll. In our work we used a biogeochemical model to show that dust storms, rather than creating instantaneous phytoplankton responses, replenish the upper mixed layer of the ocean with iron and boost the summer phytoplankton concentrations. When interpreting the observations based on this mechanism, we find a strong correlation between the dust that crossed the region during the pre-bloom period and the magnitude of the surface chlorophyll seasonal maximum. Interestingly, the correlation increases when we take into account black carbon particles in addition to dust. Our study presents the first observational link between decadal climate variability, atmospheric aerosols and fertilization of iron-limited waters.