Researchers decode key airflow patterns impacting global climate

The Hadley circulation is a key atmospheric circulation pattern in the tropics. This helps even out the temperature between the equator and the poles by shifting energy and momentum to higher latitudes. However, recent studies indicate that Hadley’s circulation cells have expanded poleward in the 21st century, increasing by several degrees each year. This expansion can have a significant impact on global weather and climate patterns, leading to adverse events such as heat waves, droughts and tropical cyclones.

Although some studies have investigated the factors affecting the north-south variations of the Hadley cell, there is a lack of understanding of the dynamic mechanisms responsible for year-to-year changes.

Now, in a recent study published in the journal npj Climate and atmospheric science and led by Professor Kyong-Hwan Seo of Pusan ​​National University, the researchers proposed a physical model that explains the latitude changes in the latitude of the Northern Hemisphere’s Hadley cells.

According to Professor Seo, “During the expansion phase of the Hadley cell, many regions on the periphery of the subtropics experience unfavorable climatic conditions, such as prolonged periods of drought and more frequent heat waves. In our study, the physical processes behind the interannual variation in Hadley Cell Edge (HCE) latitude have been elucidated for the first time. Changes in subtropical and midlatitude eddy activity, not change in the location of the jets, control the variation of the HCE.

In this study, a total of 41 years of ERA5 climate reanalysis data (compiled by the European Center for Medium-Range Weather Forecasts) and sea surface temperature data were analyzed. The researchers found that the Hadley Cell’s latitude changes were primarily caused by changes in wind eddy activity and the breaking of “stationary” and “transient” waves. Additionally, a significant portion of the year-to-year variation was associated with El Niño, La Niña, and the Arctic Oscillation.

These phenomena contribute to a shift towards the poles of the edge of the Hadley cell by inducing a movement of Eddy activity towards the poles. Additionally, the results of 28 CMIP5 (Coupled Model Intercomparison Project Phase 5) simulation models were observed to match the variations observed in the Hadley cell, thus confirming the validity of the proposed mechanism.

The results of this study are expected to be critical in addressing the significant impacts on regional meteorology and global climate resulting from the poleward expansion of the Hadley Cell. This involves effects such as worsening aridity in the American southwest, Mediterranean Europe and southern China in the Northern Hemisphere, as well as reduced rainfall in southern Australia and the southern Amazon in the Southern Hemisphere.

“The increase in heat waves in central and western Europe could be linked to the expansion of the Hadley cell during the boreal summer. This expansion also shifts the tracks of tropical cyclones towards the pole, due to the bulge of the subtropical high pressure zone towards the pole. Our study offers crucial insights into the physical events driving this expansion,” says Professor Seo.

Overall, this research is expected to have significant implications for our understanding of regional and global weather patterns.