Distinct impact of the Pacific multi-decadal oscillation on precipitation in Northeast China during April in different Pacific multi-decadal oscillation phases

The impact of the Pacific multi-decadal oscillation (PMO) on precipitation in April during the sowing season in Northeast China (NEC) was analysed. According to the results, there was a significant inter-decadal change in the relationship between the PMO in December of the previous year and the precipitation in NEC in April of the following year during different PMO phases. Before 1980, when the PMO was in a negative phase, there was no significant correlation between the PMO and NEC April precipitation. After 1980, when the PMO changed to a positive phase, the relationship between the two variables became significant. Based on further analysis, the main reason for this relationship is that as the PMO changes from a negative phase to a positive phase, the northern North Pacific warms, and the north-south temperature gradient in the mid- to high latitudes of the North Pacific decreases, which weakens the mid- to high-level westerlies, thus increasing the impact of North Pacific water vapour transport on the April precipitation in NEC. When the PMO changes from a negative phase to a positive phase, the increased sea surface temperature (SST) significantly strengthens the Aleutian low (AL), which can enhance the Alaska warm current and further increase the sea temperature in the PMO region. This air-sea interaction forms a positive feedback that maintains the SST signal from December of the previous year to April of the following year, which affects the circulation and water vapour conditions of NEC and ultimately affects NEC April precipitation. This positive feedback mechanism was further verified in the ECHAM5 model. When the positive anomalous SST appeared in the PMO region, as in the observations after 1980, the AL was significantly enhanced, which in turn affected the SST enhancement.