Aerosols could be weakening summertime circulation in the Northern Hemisphere

Over the past several decades, summer jet streams (or west to east wind flow) and weather systems in the Northern Hemisphere have weakened. Projections suggest the trend will continue, which could make extreme heat events more likely and affect air quality.

Some studies have hypothesized that the weakening is related to Arctic amplification, or the way the Arctic is warming more quickly than the rest of the planet, because this phenomenon reduces the temperature difference between the equator and the North Pole. But others have suggested that anthropogenic emissions of aerosols, which lead to a similarly weakened gradient, may be more directly to blame.

Using Detection and Attribution Model Intercomparison Project (DAMIP) data, Joonsuk Kang and colleagues studied how anthropogenic factors may have influenced summertime circulation patterns between 1980 and 2020.

They found that aerosols play just as big a role as greenhouse gases in the slowdown of wind patterns and atmospheric flow during the summer months. Changes in aerosol emissions can influence the strength of the weather systems by altering the flow of energy between land and ocean. The study is published in the journal AGU Advances.

A reduction in aerosol emissions in North America and Europe during this period meant more sunlight reaching the surface, causing a greater energy contrast between these land surfaces and the ocean. This caused energy export to the air over the ocean.

As a result, the energy converged over the higher-latitude ocean (40°N–70°N), weakening the gradient between the poles and the equator, as well as the weather systems. This effect is about twice as pronounced over the Pacific because aerosol emissions were reduced more in Eurasia than in North America.

Increased aerosol pollution from South and East Asia had the same weather-weakening effect, but through the opposite process: The increased pollution decreased the amount of solar energy that reached the surface and reduced the energy transport between land and the lower-latitude (25°N–40°N) Pacific Ocean. Ultimately, less energy converged over the lower-latitude Pacific, further weakening the energy gradient and the weather systems.

Because aerosols have shaped summertime circulation patterns over the past 40 years, it will be important to continue research on how they may shape future summer climate trends, the researchers write.

This story is republished courtesy of Eos, hosted by the American Geophysical Union. Read the original story here.