Possible influences of a La Niña event on a continuous tropical cyclone landfall event in east China

Abstract

Four tropical cyclones (TCs) formed over the western North Pacific (WNP) basin and continuously made landfall on the coastal areas of the eastern China mainland during July–August in 2018 and caused significant damage to the coastwise areas. Such a continuous TC landfall event affecting adjacent regions within a short period has never happened in previous decades, and the landfall event was attributed to anomalous environments over the WNP. During July–August in 2018, the dynamic and thermodynamic conditions were favorable for increased TC genesis, which is a prerequisite for the continuous TC landfall. However, the low sea surface temperature and high outflow temperature prohibited further intensification of the four landfall TCs. More importantly, the northwestward shift of the subtropical high and stronger monsoonal circulation resulted in anomalous easterlies that steered the four TCs toward the coastline of East China. Further investigation indicates that an earlier La Nina event that occurred from late autumn in 2017 to early spring in 2018 may have played a key role during the period. The robust monsoon circulation and enhanced precipitation over the low-latitudinal regions of the WNP basin were induced by the earlier La Nina event and provided advantageous conditions for TC genesis and movement, thus leading to the continuous TC landfall event. Significant negative correlations are found between the summer monsoon activity and earlier El Nino Southern Oscillation (ENSO) events. After an earlier La Nina event, more TCs are expected to form during the following summer such as in the scenario in 2018 due to the vigorous monsoonal circulation, which provides favorable large-scale environments. In addition, the abnormal Hadley circulation, which may be associated with the earlier La Nina event, shifted the subtropical high northward and resulted in the anomalous easterlies that steered the TCs toward East China.