Impacts of Indo-Pacific variability on the East Asian climate

Abstract

Interaction between the Indian and Pacific Oceans through ocean-atmosphere coupling can initiate and modulate the climate variability. The El Niño-Southern Oscillation (ENSO), which originates in the tropical Pacific Ocean, affects the Indian Ocean by atmospheric bridge. While the Indian Ocean sea surface temperature (SST) can significantly feed back onto the evolution of ENSO. The Indian ocean warming produces the atmospheric Kelvin waves that propagate eastward and increase the equatorial easterlies in the western Pacific, and thus cool the eastern Pacific SST. These tropical variabilities extensively influence on the global weather and climate, in particular East Asia with great socioeconomic importance for densely populated regions. The East Asian monsoon, which is one of the most active climate system, is considerably contributed by the tropical Indian and Pacific Oceans through the tropics-extratropics teleconnection. In particular, in the winter of El Niño years, the warm SST in the central-eastern Pacific Ocean is responsible for the East Asian warming in December but insignificant relationship in January. The equatorial central Pacific and WNP precipitation anomalies associated with the El Niño play relative roles in modulating atmospheric teleconnections over the North Pacific that result the subseasonal evolution in East Asia. The dominant intraseasonal variability in the tropics, i.e. Madden-Julian oscillation (MJO), also develops the significant East Asian cooling during phase 3. Using a thermodynamic equation, contributions of MJO-related meridional advection and adiabatic cooling terms to cooling tendency in East Asia are suggested. The MJO phase 7-like dipole convective forcing in the tropical Indo-Pacific Ocean possibly leads to the cold anomalies in East Asia through the Rossby wave propagation and a local overturning circulation. Although the many attempts have been made, the impact of tropical Indo-Pacific variability on the East Asian climate is not fully understood yet. Here, the mechanism of how does the tropical Indo-Pacific Ocean affects to the East Asian climate is investigated. The negative relationship is evident between the North Indian Ocean SST in June and East Asian surface temperature anomalies in July with 1-month leading role. The sensitive relationship between SST and precipitation in the Indian Ocean exists only in June due to the strengthening of southewesterly summer monsoons. Thus, the Indian Ocean warming in June effectively drives atmospheric Kelvin waves propagating into the western Pacific, and then develops the anticyclonic anomalies in the WNP. This is source of the Pacific-Japan teleconnection that brings cold surface anomalies to the Korea-Japan region as a result of radiative and horizontal advections by cyclonic circulation. The comprehensive understanding of tropical Indo-Pacific teleconnections to East Asia is applied for the summer 2020. The East Asian countries have experienced heavy rainfalls triggering flooding and landslides. In July 2020, the Indian Ocean warming is responsible for the zonally elongated rainband observed in East Asia, suggesting the above dynamical process. The East Asian precipitation increase in August is related to the precipitation decrease in the subtropical WNP that brings the southwesterlies on the northwestern side of anticyclonic anomalies. Thus, the record-breaking rainfall events in East Asia during July and August 2020 can potentially be explained by such Indian Ocean warming and subtropical WNP precipitation decrease, respectively. It has been an outstanding challenge to predict the East Asian weather and climate, mainly because of complex climate perturbations around East Asia. Under a climate change, the uncertainties of climate in East Asia may increase including high risks of extremes. Therefore, these results for impacts of Indo-Pacific variability in East Asia will be useful for understanding the East Asian climate.