Polar climate affects the strength of the trade winds in the tropics

The impact of sea surface temperature variations in the tropical Pacific on the global climate has long been recognized. For example, the episodic warming of the tropical Pacific during El Niño events causes sea ice to melt in deep parts of the Southern Ocean via its effect on global atmospheric circulation. A new study, published this week in the journal Scientific advances by an international team, shows that the opposite way also exists.

Using a hierarchy of climate model simulations, the authors demonstrate the physical pathways through which polar climatic variations can affect the trade winds in the tropics.

“Climate signals can travel from the polar regions to the tropics via the atmosphere or the ocean,” explained Malte Stuecker, co-author and assistant professor in the Department of Oceanography and International Pacific Research Center at the University. from Hawaii to Mānoa. “Our climate model simulations were designed to study the relative role of these pathways and whether their importance differs for disturbances from the North Pole or the South Pole.”

The authors found that in the most complex model simulations, which include realistic representations of the ocean, atmosphere, land, and sea ice, abnormal cooling in either hemisphere leads to a strengthening of the tropical trade winds.

Lead author Sarah Kang from the National Institute of Science and Technology in Ulsan, South Korea explained the rationale behind these experiments: “One of the biggest sources of uncertainty in the current generation of climate models are the biases in the representation of clouds over the cold Southern Ocean. We wanted to explore the effect that too much reflection of solar radiation from these clouds on outer space could have on the climate global., and Asia has resulted in a slight temporary reduction in the rate of global warming which is due to increased greenhouse gas emissions. “

According to the authors’ results, these two effects could explain why the Pacific trade winds have been unusually strong in recent decades.

“If the communication between the poles and the tropics only occurred via the atmosphere, we would see a fairly distinct response in the tropics depending on whether abnormal cooling is coming from the Arctic or from the Antarctic,” Stuecker added. “This is because the Intertropical Convergence Zone – the largest band of rain on Earth – is located north of the equator. It effectively blocks communication from the Arctic to the equator via the atmosphere.”

Contrasting experiments with and without a realistic representation of the ocean, the authors show that an increased upwelling of cold groundwater in the eastern tropical Pacific is able to communicate cooling from the Arctic to the tropics and thus reinforce the trade winds.

An important implication of the results is that reducing the uncertainty in the simulated extratropical climate can also lead to better simulation of the climate in the tropics. The hierarchy of models developed by the authors can be used to further explore bidirectional interactions between the tropics and polar regions both for future climate projections as well as to interpret reconstructions of climatic states from the geological past.