The northern extratropics—including regions in northern Europe, northeast Asia, and North America—experienced extremely prolonged heat waves during May–August 2018. Record-breaking surface temperatures, which caused numerous deaths, were observed in several cities. The 2018 heat waves exhibited a circumglobal characteristic owing to a circumpolar perturbation (CCP) in the middle–upper troposphere of the Northern Hemisphere (NH). The CCP had two parts: a wave-like perturbation and a hemispheric perturbation that was almost zonally symmetric. Singular value decomposition analysis revealed that the zonally symmetric perturbation was coupled to the SST warming trend, whereas the wave-like perturbation was primarily coupled to the interannually-varying SST anomaly (SSTA), particularly in the tropical North Pacific, which reached an extreme in 2018. Numerical experiments confirmed that the zonally symmetric component was primarily resulted from the SSTA associated with the warming trend, whereas the interannually-varying SSTAs in the NH contributed mostly to the wave-like perturbation. The warming trend component of SSTA, especially that in the tropics, compounded by the unusually large SSTAs in 2018, was hypothesized to have contributed to inducing the circumpolar circulation anomaly that caused the record-breaking heat waves in the extratropical NH in 2018.
In July and August (JA) 2018, the monsoon trough in the western North Pacific (WNP) was unusually strong, the anticyclonic ridge was anomalously northwardshifted, and enhanced and northward-shifted tropical cyclone activity was observed. Studies have examined the effects of sea surface temperature (SST) anomalies on the North Atlantic (NA), the Indian Ocean (IO), and the tropical North Pacific. However, a synthetic view of SST forcings has yet to be identified. Based on a series of numerical experiments, this study demonstrated that the SST anomaly in the tropical WNP was the key forcing that formed the structure of the observed anomalous phenomena in the monsoon trough. Moreover, the combined effect of the SST anomaly in both the tropical and extratropical WNP resulted in enhanced circulation anomalies in the WNP. The NA SST anomaly also enhanced the monsoon trough in the presence of WNP SST anomaly. By contrast, the individual SST anomaly in the NA, IO, the extratropical WNP, and the subtropical eastern North Pacific could not force the enhanced monsoon trough. We proposed that the local effect of both the tropical and extratropical WNP SST anomaly as the major driver and the remote effect of NA SST anomaly as a minor contributor jointly induced the anomalous circulation and climate extremes in the WNP during JA 2018.
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