The quasi-biweekly oscillation (QBWO) characteristics of Meiyu precipitation in the Yangtze-Huaihe River Basin (YHRB) in 2016 and 2020 are compared by filtering and composite methods using Sea Surface Temperature (SST), atmospheric circulation reanalysis data and daily precipitation data in China. The results show that precipitation exhibited significant QBWO characteristics in both years. However, the QBWO and the low-frequency precipitation in 2016 are stronger than that in 2020. The low-frequency anticyclone moving northwestward in the lower troposphere over the South China Sea (SCS) and the western Pacific and the low-frequency cyclone migrating southeastward under the influence of the westerly jet in the mid-high latitudes are the main reasons for the QBWO of Meiyu precipitation in the YHRB in 2016. In the Meiyu period of 2020, the QBWO precipitation was mainly influenced by the low-frequency cyclones moving southwestward from southern Japan, the low-frequency anticyclones moving northwestward from the Philippine Sea and the low-frequency cyclones moving westward associated with the polar vortex westward rotating in the mid-high latitudes. The continuous SST warming over the SCS, the tropical Western Pacific and the tropical Indian Ocean triggered and maintained the QBWO of convective activities and atmospheric circulations over the SCS and the Western Pacific in 2016 through air-sea interaction. In 2020, positive SST anomalies in the Northwest Pacific and the Kuroshio and its extension region triggered alternative generation and migration of low-frequency cyclones and anticyclones in the region via air-sea interaction, which regulated the advance and retreat of the Western Pacific Subtropical High and thereby affected the QBWO precipitation in the YHRB. In 2016 and 2020, on the quasi-biweekly scale, the anomalous anticyclone in the lower troposphere was favorable for the SST increase, and positive SST anomalies further triggered an anomalous cyclonic in the lower atmosphere, which led to negative SST anomalies and subsequently promoted the development of anomalous anticyclone. Through the air-sea interaction, the QBWO signals in the lower troposphere continuously propagates to the Jianghuai region, affecting the QBWO of rainfall there during the Meiyu period.

Owing to limited observations, it remains unknown whether the impact of El Niño–Southern Oscillation (ENSO) on the Indian Ocean–Northwest Pacific (IO–NWP) climate showed decadal changes in the early 20th century. Using multi-source reanalysis and hindcast datasets from the ECMWF and NOAA extending back to 1901, this study investigates interdecadal variations of the impact of ENSO on the IO–NWP climate from 1901 to 2009. It is found that the influence of ENSO on the IO–NWP climate shows “strong–weak–strong” interdecadal change during 1901–2009. This is characterized by much weaker Indian Ocean sea surface temperature (SST) warming and a weaker NWP subtropical anticyclone (NWPSA) in the following summer of El Niño during 1946–1967, compared with those in the other two periods (1901–1945 and 1968–2009). Analyses of the datasets indicate that the interdecadal variation is mainly associated with the change in ENSO amplitude. In contrast to the period of 1946–1967, a greater SST variance occurred in the central–eastern equatorial Pacific during 1901–1945 and 1968–2009. A stronger El Niño tends to generate more significant anticyclonic anomalies over the southeast Indian Ocean through teleconnection. The northwesterly anomalies to the south of the anticyclone weaken the southeast trade winds and warm the south Indian Ocean SST via wind–evaporation–SST feedback, and the positive south Indian Ocean SST anomalies trigger westward-propagating oceanic Rossby waves to induce stronger warming of the southwest Indian Ocean, leading to a significant asymmetric wind pattern across the equator in spring. The profound northeastward winds on the north side weaken the southwest monsoon, leading to a “second warming” over the north Indian Ocean in summer, which anchors the eastward-propagating warm Kelvin waves and results in a stronger NWPSA by inducing surface divergence and suppressing deep convection.