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The Beijing–Tianjin–Hebei (BTH) region has encountered increasingly severe and frequent haze pollution during recent decades. This study reveals that El Niño–Southern Oscillation (ENSO) has distinctive impacts on interannual variations of haze pollution over BTH in early and late winters. The impact of ENSO on the haze pollution over the BTH is strong in early winter, but weak in late winter. In early winter, ENSO-related sea surface temperature anomalies generate double-cell Walker circulation anomalies, with upward motion anomalies over the tropical central-eastern Pacific and tropical Indian Ocean, and downward motion anomalies over the tropical western Pacific. The ascending motion and enhanced atmospheric heating anomalies over the tropical Indian Ocean trigger atmospheric teleconnection propagating from the north Indian Ocean to East Asia, and result in the generation of an anticyclonic anomaly over Northeast Asia. The associated southerly anomalies to the west side lead to more serious haze pollution via reducing surface wind speed and increasing low-level humidity and the thermal inversion. The strong contribution of the Indian Ocean heating anomalies to the formation of the anticyclonic anomaly over Northeast Asia in early winter can be confirmed by atmospheric model numerical experiments. In late winter, vertical motion and precipitation anomalies are weak over the tropical Indian Ocean related to ENSO. As such, ENSO cannot induce a clear anticyclonic anomaly over Northeast Asia via atmospheric teleconnection, and thus has a weak impact on the haze pollution over BTH. Further analysis shows that stronger ENSO-induced atmospheric heating anomalies over the tropical Indian Ocean in early winter are partially due to higher mean SST and precipitation there. Significance Statement There exist large discrepancies regarding the contribution of El Niño–Southern Oscillation (ENSO) events to the wintertime haze pollution over North China. Several studies have indicated that ENSO has a weak impact on the haze pollution over North China. However, some studies have argued that ENSO events can exert impacts on the occurrence of haze pollution over North China. In this study, we present evidence to demonstrate that ENSO has distinctive impacts on interannual variations of the haze pollution over the Beijing–Tianjin–Hebei (BTH) region in North China in early and late winters. Specifically, ENSO has a strong impact on the haze pollution over BTH in early winter, whereas the impact of ENSO on the haze pollution over BTH is fairly weak in late winter. Results of this study could reconcile the discrepancy of previous studies about the impact of ENSO on the haze pollution over North China.

El Niño–Southern Oscillation (ENSO) and the Tibetan Plateau snow cover are important factors in interannual climate variability. The relationship between ENSO and the Tibetan Plateau snow variation is still undetermined. While some studies suggested that ENSO is a key factor of changes in snow cover over the Tibetan Plateau, other studies noted independence between the two. The present study revealed a prominent interdecadal change in the relationship between ENSO and the spring Tibetan Plateau snow-cover variation in the early 2000s. There is a significant positive correlation between ENSO and the spring Tibetan Plateau snow-cover variation in the period 1988–2003, but an obvious negative relationship is detected in the period 2004–19. The interdecadal change in the ENSO–snow relationship is related to the distinct pathway of ENSO influence on the spring Tibetan Plateau snow-cover variation during the two periods. In the period 1988–2003, ENSO induces anomalous convection over the tropical western North Pacific that in turn causes atmospheric circulation and moisture anomalies over the Tibetan Plateau. The resultant winter snow anomalies over the central-eastern Tibetan Plateau persist to the following spring. In the period 2004–19, ENSO induces North Atlantic sea surface temperature (SST) anomalies in winter that are maintained to the following spring. The North Atlantic SST anomalies then stimulate the atmospheric circulation anomalies extending to the Tibetan Plateau that induce snow-cover anomalies there in spring. The different processes of ENSO influence lead to opposite anomalies of spring snow cover over the Tibetan Plateau in the two periods.

This study conducts correlation and regression analyses of the JRA-55 reanalysis data and observational rainfall datasets from China’s National Climate Center. The analyses reveal that interdecadal enhancement in the relationship between the East Asian summer monsoon (EASM) and the Indian Ocean Basin mode (IOBM) after the early 1990s, and the diminished correlation between the EASM and the Niño-3 index. The analyses also reveal that the relationship between EASM-related rainfall/circulation with IOBM also experienced an interdecadal shift at the same time. During the first epoch (1977–1989), EASM-related rainfall was correlated significantly with the Niño-3 index, and accompanied by a Pacific–Japan-like anomaly pattern of horizontal winds. In a subsequent epoch (1994–2014), EASM-related rainfall was correlated significantly with IOBM, and accompanied by a meridional dipole pattern in the horizontal winds. After the 1990s, IOBM exerted influence on EASM through land–sea thermal contrast, and the critical land area was the region 33°–47°N, 110°–140°E. The interdecadal strengthening in the EASM–IOBM linkage around the early 1990s may be attributable to a faster rate of decay of El Niño after the 1990s.

Previous studies revealed the influence of the autumn–winter Tibetan Plateau snow cover on atmospheric circulation and climate in the North American region. The present study documents the relationship between the eastern Tibetan Plateau snow cover and the North American air temperature in spring and the associated physical processes using satellite-based snow cover, reanalysis atmospheric and surface variables, observation-based surface air temperature (SAT), and sea surface temperature (SST). A stable relationship is identified between the eastern Tibetan Plateau snow cover and the North American SAT in spring before the mid-2000s. Positive snow-cover anomalies over the eastern Tibetan Plateau induce cooling in the local atmospheric column. The atmospheric cooling stimulates a large-scale atmospheric wave pattern at the upper level that extends northeastward from the eastern Tibetan Plateau via northeast Asia and the North Pacific to North America. An anomalous high forms over North America, accompanied by anomalous descent. In the northwestern part, the horizontal advection by anomalous southerly winds along the west flank of anomalous anticyclone induces SAT increase. In the central part, the enhanced surface sensible heat flux following anomalous descent-induced downward shortwave radiation increase leads to SAT increase. The relationship between the eastern Tibetan Plateau snow cover and the North American SAT is weakened after the mid-2000s. The weakened relationship is attributed to an intensified impact of tropical central Pacific SST anomalies on the North American SAT variations through a Pacific–North America-like atmospheric circulation pattern, which overcomes the influence of the Tibetan Plateau snow-cover anomalies.

The present study investigates the impacts of the North Atlantic sea surface temperature (SST) anomalies on the East Asian winter monsoon (EAWM) variability. It is found that the northern component of the EAWM variability is associated with a dipole pattern of preceding summer North Atlantic SST anomalies during 1979–2016. The processes linking preceding summer North Atlantic SST to EAWM include the North Atlantic air–sea interactions and atmospheric wave train triggered by the North Atlantic SST anomalies. Atmospheric wind anomalies in the preceding spring–summer result in the formation of a dipole SST anomaly pattern through surface heat flux changes. In turn, the induced SST anomalies provide a feedback on the atmosphere, modifying the location and intensity of anomalous winds over the North Atlantic. The associated surface heat flux anomalies switch the North Atlantic SST anomaly distribution from a dipole pattern in summer to a tripole pattern in the following winter. The North Atlantic tripole SST anomalies excite an atmospheric wave train extending from the North Atlantic through Eurasia to East Asia in winter, resulting in anomalous EAWM. However, the relationship of the northern component of EAWM to preceding summer North Atlantic SST anomalies is weak before the late 1970s. During 1956–76, due to weak air–sea interaction over the North Atlantic, no obvious tripole SST anomaly pattern is established in winter. The atmospheric wave train in winter is located at higher latitudes, leading to a weak connection between the northern component of EAWM and the preceding summer North Atlantic dipole SST anomaly pattern.