Effects Of Indian Summer Monsoon And Enso Events On Summer Precipitation In East Asia

Using the reanalysis data,sea temperature data,ocean heat content and precipitation data from 1951 to 2019,statistical methods such as statistical inspection and synthetic analysis were mainly used to study the impact of the Indian summer monsoon and ENSO events on summer precipitation in East Asia.According to the definition of the Indian summer monsoon index and ENSO events,we divided the monsoon and ENSO into three groups and six categories: resonance group [weak monsoon-El Ni?o(WM-EN),strong monsoon-La Ni?a(SM-LN)],normal monsoon-ENSO group [normal monsoon-El Ni?o(NM-EN),normal monsoon-La Ni?a(NM-LN)] and monsoon-non-ENSO group [weak monsoon-non-ENSO(WM-NE),strong monsoon—non-ENSO(SM-NE)].We found that the Indian summer monsoon and ENSO events are not always strongly correlated.Only when certain conditions are met can strong coupling occur.Therefore,we referred to the two strong coupling categories(WM-EN and SM-LN)as resonance effects.We analyzed from the two processes of atmosphere and ocean,and the analysis showed that when the signs of the sea temperature anomalies on the tropical Indian Ocean and the equatorial Middle East Pacific are the same and are opposite to the sea temperature anomalies near the ocean continent,which was an important factor for the resonance between the Indian summer monsoon and ENSO events.We synthesized the summer precipitation anomalies in East Asia under different classifications.The distribution of summer precipitation anomalies in the developing phase of ENSO is similar to that in the strong and weak monsoon,so we believed that the developing phase of ENSO is more closely related to the Indian summer monsoon.When studying the East Asian summer precipitation anomaly under the resonance effect,it was found that the East Asian summer rainfall anomaly under the resonance effect has a larger range and stronger intensity;WM-EN is mainly reflected in thesignificant negative precipitation anomalies in North China,while SM-LN produces stronger positive precipitation anomalies in the Yangtze River Basin.In addition,when there were only monsoon events and no ENSO events,the East Asian summer precipitation anomalies in strong and weak monsoon years did not show a clear reverse distribution,which indicated that the ENSO events play an important role in the Indian summer monsoon affecting the East Asian summer precipitation.In the absence of the influence of the monsoon event,only the East Asian summer precipitation anomalies under the influence of ENSO were also somewhat different from the results obtained from previous studies.Therefore,we believed that the ENSO events and the Indian summer monsoon events are complementary to each other.Only when they act together,the anomalies to the precipitation in East Asia are stronger.When only one factor is analyzed and the other factor is excluded,the conclusion consistent with previous studies cannot be drawn.The effect of resonance on the distribution of the extreme index R99 was not obvious,which may be due to the low data coverage and annual data;for the extreme index Rx5 day,the summer anomaly distributions were similar to the general precipitation anomalies.Although the variances of monsoon-non-ENSO were still the largest,the precipitation anomalies in both classifications were negative anomaly and did not have the characteristics of inverse phase,which is inconsistent with previous studies.Therefore,we believed that the resonance effect also has a certain effect on extreme precipitation,but it was not as significant as general precipitation.We believed that the resonance effect has a strong impact because the resonance group contains many strong events,a positive feedback mechanism is formed between the low-level wind field and the ocean subsurface heat content,and there is more water vapor transport.Through mechanism analysis,we found that the abnormal precipitation in East Asia in summer or extreme precipitation is not caused by a single factor.The ENSO events can drive circulation,and the Indian summer monsoon brings water vapor to East Asia.The effect is stronger only when the two produce resonance.