A long-distance connection: the polar climate affects the strength of the trade winds in the tropics

The impact of changes in sea surface temperature in the tropical Pacific on the global climate has long been recognized. For example, episodic warming of the tropical Pacific during El Niño events causes sea ice to melt in large parts of the Southern Ocean through its effect on global atmospheric circulation. A new study, published this week in the journal Advances in science by an international team, proves that the opposite path also exists.

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

“Can climate signals propagate from the polar regions to the tropics through 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 of Hawaii? I in Mānoa. “Our simulations of climate models 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 authors found that in more complex model simulations, which include realistic representations of the ocean, atmosphere, land, and sea ice, abnormal cooling in both hemispheres leads to a strengthening of tropical trade winds.

Lead author Sarah Kang of the Ulsan National Institute of Science and Technology in South Korea explained the reasoning behind these experiments: “One of the biggest sources of uncertainty in the current generation of climate models is bias in the representation of clouds over the cold Southern Ocean. We wanted to explore what effect the excessive reflection of solar radiation from these clouds in space could have on the global climate. Also, large aerosol emissions at the end of the 20th century due to industrial activity in the Northern Hemisphere from North America, Europe, and Asia has caused a slight, temporary reduction in the rate of global warming due to increased greenhouse gas emissions. ”

According to the authors’ findings, both of these effects could potentially explain why the Pacific trade winds have been exceptionally strong in recent decades.

“If communication between the poles and the tropics were only through the atmosphere, we would see a rather distinct response in the tropics depending on whether anomalous cooling is from the Arctic or Antarctic,” added Stuecker. “This is because the intertropical convergence zone – the largest rainfall belt on Earth – is north of the equator. It effectively blocks communication from the Arctic to the equator via the atmosphere.”

Contrasting experiments with and without a realistic depiction of the ocean, the authors show that greater upwelling of cold subsurface water in the eastern tropical Pacific is able to communicate arctic cooling to the tropics and thereby strengthen the trade winds.

An important implication of the findings is that reducing uncertainty in the simulated extra-tropical climate can also lead to better simulation of the climate in the tropics. The model hierarchy developed by the authors can be used to further explore the two-way interactions between the tropics and polar regions for both future climate projections and to interpret reconstructions of climatic states in the geological past.

###