Researchers at the University of Hawaii at Manoa are describing how the climate in polar regions can influence the strength of tropical trade winds. It was previously established that warming in the tropical Pacific affects the climate on a global scale, such as causing sea ice to melt in parts of the Southern Ocean. The new study shows that this long distance connection between the poles and the tropics works both ways.
“Climate signals can propagate from the polar regions to the tropics either via the atmosphere or the ocean,” explained study co-author Professor Malte Stuecker. “Our climate model simulations were designed to investigate the relative role of these pathways and whether their importance differs for perturbations originating from the North pole or the South pole.”
The researchers used complex computer models that included realistic representations of the ocean, atmosphere, land, and sea ice. The simulations revealed that an anomalous cooling event in either hemisphere leads to a strengthening of the tropical trade winds.
“One of the largest sources of uncertainty in the current generation of climate models are biases in the representation of clouds over the cold Southern Ocean,” explained study lead author Sarah Kang. “We wanted to explore what effect too much reflection of solar radiation by these clouds to outer space might have on global climate.”
“In addition, large emissions of aerosols in the late 20th century due to industrial activity in the Northern Hemisphere from North America, Europe, and Asia resulted in a slight, temporary reduction of the global warming rate that is due to increasing greenhouse gas emissions.”
The study suggests that both of these effects could potentially explain why the Pacific trade winds were anomalously strong in recent decades.
“If the communication between the poles and the tropics would only occur via the atmosphere, we would see quite a distinct response in the tropics depending on whether an anomalous cooling arises from the Arctic or the Antarctic,” said Professor Stuecker. “This is because the Intertropical Convergence Zone – the largest rainband on Earth – is located to the north of the equator. It effectively blocks a communication from the Arctic to the equator via the atmosphere.”
The team conducted contrasting experiments with and without a realistic ocean representation. The findings revealed that enhanced upwelling of cold subsurface water in the eastern tropical Pacific is able to transfer the Arctic cooling toward the tropics, which strengthens the trade winds.
The model developed for this research can be used to further explore two-way interactions between the tropics and polar regions. This will not only improve future climate projections, but could also help experts to interpret reconstructions of past climate states.
The study is published in the journal Science Advances.