The Yangtze River valley (YRV) often suffers from frequent and intense heatwaves in midsummer (July-August) due to the abnormal northward and westward advance of the western Pacific subtropical high. In 2022 midsummer, high-intensity and long-lasting heatwaves hit the YRV, with the number of heatwave days exceeding climatology by four standard deviations, setting the highest historical record since 1961. Even if the long-term linear trends are removed, the 2022 midsummer remains to set the hottest record during the 1961–2022 period. The heatwave-induced drought disasters include Poyang Lake is short of water, severe wildfire in Chongqing and persistent drought for crops, which have affected about 38 million people and caused direct economic losses of about five billion dollars. While we have a general understanding of the causes of the Yangtze River valley hot summer, what makes the 2022 unprecedented YRV heatwaves remains a mystery.
In this context, Shaobo Qiao from Sun Yat-sen University, Shankai Tang from Lanzhou University, Guolin Feng from China Meteorological Administration and Bin Wang from the University of Hawaii at Manoa led a study exploring the causes of the record-breaking heatwave days over the YRV in the 2022 midsummer. We present convergent empirical and modelling evidence to show that the 2022 tripe-dip La Niña, along with the record-breaking Pakistan rainfall, produces anomalous high pressure over YRV, causing intense heatwaves. The specific mechanisms are summarized in the schematic diagram in the Figure 1 below.
On the one hand, 2022 is the third year of a rare three-year La Niña. Usually, El Niño-Southern Oscillation (ENSO) decays from winter to summer. However, the SST anomalies associated with the 2022 La Niña enhanced and extended westward from the preceding winter to the next midsummer, strengthening the zonal SST gradient in the equatorial western Pacific, which ranks the second since 1979. Physically, the La Niña-induced zonal SST gradients shift the Walker Circulation westward, suppressing convection over the equatorial western Pacific and enhancing convection over the Maritime Continent. The anomalous heating associated with these convective anomalies reinforces the low-level western Pacific subtropical High by generating descending Rossby waves over the western North Pacific. However, La Niña alone is insufficient to fully explain such extreme heatwaves. The La Niña-forced western Pacific subtropical High mode was not exceptionally strong, and it covers only the middle and lower reaches of the YRV, which cannot explain the high-pressure anomalies over the upper reach of the YRV and the eastern Tibet Plateau in 2022. Besides, the La Niña-forced western Pacific subtropical High mode does not show a strong linkage with Yangtze River valley heatwaves: The correlation coefficient is merely 0.44. Therefore, other factors must be in play.
On the other hand, Pakistan suffered from unprecedented heavy rainfall and floods in the 2022 midsummer. The accumulated rainfall exceeds 500 mm and sets the highest historical record. We find that the Pakistan rainfall and Yangtze River valley SAT are well coupled in both the interannual timescale during the historical period and the intraseasonal timescale during the 2022 midsummer. Because the record-breaking Pakistan rainfall could impact YRV heatwaves by promoting downstream Rossby wave trains, we examined the upper-level circulation anomalies. As we expect, the upper-troposphere circulation shows an anomalous wave train pattern resembling the positive circumglobal teleconnection (CGT) pattern, which is closely associated with the Pakistan rainfall rather than ENSO. The robust CGT wave pattern is reinforced by Rossby wave responses to the huge diabatic heating over Pakistan and northwest India, whicih extends the upper-level South Asia High eastward and controlling the entire Yangtze River valley. Finally, the overlay of the upper-level South Asia High and low-level western Pacific subtropical High sustains sinking motion and increases solar radiation reaching the ground, causing the 2022 Yangtze River valley heatwaves.
In summary, we find that traditional thinking of the role of La Niña cannot explain its extremity, and the unprecedented Pakistan floods play a leading role in generating the 2022 Yangtze River valley heatwaves. These findings help us understand and predict East Asia’ s extreme heatwave events.