Heat waves have gotten hotter in the Northern Hemisphere in recent decades. Home to about 90% of the world’s population, with the largest fraction living in the mid-latitudes, more frequent and more severe heat waves and droughts have occurred in the Northern Hemisphere—in Europe in 2003, 2010 and 2019, in North America during 2018 and 2021, and in eastern China in 2013 and 2022.

Given the large causalities and economic losses, it’s been frustrating that predicting these events with global computer models is more difficult outside the Earth’s tropical zone than within it.

Now, scientists from China have shown that a key global climate connection has shifted since the late 1970s, with the most severe heat wave and drought impacts over eastern Europe, eastern Asia and southwestern North America. Their work has been published in Nature Communications.

Both empirical observations and modeling evidence show an essential role of the El Niño-Southern Oscillation (ENSO) behind this change.

Before the late 1970s, ENSO indirectly affected the boreal summer circumglobal teleconnection in a way that modulated the Indian summer monsoon. That link weakened starting in the 1980s, with the ENSO forcing displaced westward, causing a shift of the previous centers of the GCT.

ENSO, the El Niño-Southern Oscillation, is the nonperiodic change in winds and water temperature over the western tropical Pacific that manifests as El Niños (warmer surface water) and La Niñas (cooler surface water) in the central and eastern equatorial Pacific Ocean. These changes have global consequences, with the former often increasing average global surface temperature and the latter decreasing the average global surface temperature, affecting rainfall in certain regions too.

In the era of anthropogenic global warming, the El Niños frequently lead to record temperatures, as happened in 2023 and 2024. The “Southern Oscillation” occurs in the atmosphere and is tied to the shifting sea surface temperatures.

ENSO patterns, which have been shown to be unpredictable, have a significant effect on climate in the tropics, rainfall, and crop production. Through “teleconnections” they can influence distant parts of the globe as well, often thousands of kilometers away.

One is the boreal summer (June to August) circumglobal teleconnection (CGT) discovered in 2005, an upper-tropospheric atmospheric circulation anomaly that occurs during the Northern Hemisphere summer.

Using reanalysis data—datasets of past weather and climate observations—fed into climate models, it was found the CGT’s structure and variability changed after the late 1970s, along with another global scale, the western North Pacific–North American teleconnection. But back then it could only be roughly predicted about a month in advance.

The CGT consists of Rossby waves—inertial waves present in a rotating fluid—in the upper atmosphere, with 5 wavenumbers in total associated with the Northern Hemisphere jet stream flowing west to east.

The research team from China, with lead author Shankai Tang from Sun Yat-Sen University and Guangdong Laboratory in Zhuhai, China, seven co-authors in China and one at the University of Hawaii at Manoa, has discovered that the northern hemisphere Rossby waves have shifted one-half wavelength since the late 1970s.

Utilizing rainfall data compiled from the National Oceanic and Atmospheric Administration’s Precipitation Reconstruction, and global wind, rainfall, surface temperature and global geopotential height provided by the European Center for Medium-Range Weather Forecasts Reanalysis 5 (ERA5), and sea surface temperatures and a precipitation-evapotranspiration from still other databases, they used the Community Earth System Model version 2 from the National Center for Atmospheric Research (NCAR) to do numerical experiments to examine the CGT for statistical evidence of any changes.

Why the half-wavelength change? One obvious possibility is manmade climate change, but they found that the change discovered could not be simulated using 24 state-of-the-art climate models from the Coupled Model Intercomparison Project Phase 6 with the known anthropogenic forcings, “suggesting that they are driven by natural variability” they write.

Looking at anomalous summertime ENSO years, they found the structure of the summertime ENSO had experienced a significant change over multiple decades, with the sea surface temperatures shifting westward from the equatorial eastern Pacific Ocean and the Chilean coast to the equatorial central Pacific. (The change was found to be the same for both El Niños and La Niñas.)

The descending motion of the Walker Circulation led to suppressed convection and rainfall over the tropical western North Pacific. Through a chain of atmospheric interactions, the result is the half-wavelength western shift of the North Pacific center of the CGT, “and other centers downstream.”

The group say their work can be helpful for future model improvements for making projections and predictions. “A deeper understanding of the recent CGT structure change,” they wrote, “should help improve the seasonal predictions of the mid-latitude Northern Hemisphere heat waves and droughts associated with the ENSO.”

A better understanding of the ENSO-CGT relationship should also improve projections of future climate change over the same mid-latitude region.

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