"For example, these aerosols are partly responsible for intensifying features such as larger ocean gyres, causing them to shift southward, along with maximum sea surface temperature gradients, mid-latitude storms and the westerly jet stream," researcher Tim Cowan told environmentalresearchweb. "The process intensifies atmospheric features such as the Southern Annular Mode, a system that describes variations of pressure contrasts between mid and high latitudes in the southern hemisphere. The atmospheric changes in turn act to reinforce the oceanic shift, leading to a positive feedback."
Together with Wenju Cai, Cowan compared two sets of 20th century simulations in a coupled ocean-atmosphere global climate model. They found that increasing northern hemisphere aerosols both intensified the trend of the Southern Annular Mode and modified its structure.
"Through our earlier research we know that increasing greenhouse gases and stratospheric ozone depletion both lead to a southward shift in southern hemisphere weather systems," said Cowan. "Our results imply that aerosols that act to mitigate some of the ocean warming in fact lead to what we could now call a triple edged sword: that is, the result of increased aerosols plus ozone depletion plus greenhouse gases all impact the sea surface temperature gradients, leading to a shift and strengthening of the zonal winds that influence the weather systems of the southern mid-latitude regions."
Other studies in the northern hemisphere and tropics have shown that anthropogenic aerosols affect summertime floods and droughts in China, weaken the South Asian monsoon, and increase rainfall over northwest Australia.
Next the team hopes to look at the impact of aerosols on a more regional scale, looking at factors such as rainfall over northern Australia or southern China. "We also need to understand what role aerosols play in masking global warming, and if there is a reduction of aerosols – due to tighter pollution controls and cleaner fuels – what impact this will have on rapid climate change," said Cowan.
The researchers reported their work in Geophysical Research Letters.