Considerable progress has been made towards understanding the extratropical atmospheric response to the tropical forcing associated with El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomalies (Yeh et al. 2018). The Rossby wave dynamics lead to changes in the extratropical atmospheric circulation, as a direct response to the ENSO-related tropical diabatic heating (Horel and Wallace 1981; Hoskins and Karoly 1981). Indeed, many previous studies suggested that the ENSO can influence the East Asian monsoon system in boreal summer (Nitta 1987; Huang and Wu 1989; Zhang et al. 1996; Kosaka and Nakamura 2006; Xie et al. 2009; Xie et al. 2016). The impacts of ENSO on the East Asian climate are also significant during the El Niño mature phase (i.e. winter), through the tropics-extratropics teleconnection (Rasmusson and Carpenter 1982; Zhang et al. 1999; Wang et al. 2000; Son et al. 2014; Gong et al. 2015; Kim et al. 2017; Kim and Kug 2018; Kim et al. 2018).
An observational study (Nitta 1987) hypothesized that during post-El Niño summers an anomalous anticyclone in the western North Pacific (WNP) can affect the summer climate in East Asia through atmospheric meridional dipoles, referred to as the Pacific-Japan (PJ) pattern. This lower-tropospheric teleconnection has been extensively considered as one of the common summer teleconnection patterns in East Asia (Nitta 1987; Huang 1992; Huang and Sun 1992). Zhang et al. (1996) found that the suppressed convective activity over the equatorial western Pacific, as a result of warming in the eastern Pacific, intensifies the East Asian summer monsoon. Strong positive anomalies of outgoing longwave radiation over the Maritime Continent lead to the atmospheric cooling that accompanies the anomalous anticyclone. In a subsequent study, the intensified and westward shift of the anticyclonic anomalies in the WNP was suggested to be responsible for the negative precipitation anomalies over both southern and northern parts of China, and bounding positive anomalies over central parts (Zhang et al. 1999).
Evidence presented in Wang et al. (2000) also shows that the recurrent anomalous anticyclone after an El Niño event is a result of an atmospheric Rossby wave response to the SST cooling over the WNP. The WNP anticyclonic anomalies persist until the following summer through positive thermocline feedback between the anticyclone and negative SST anomalies. In accordance with summer, the WNP anticyclonic anomalies act as a key system that bridges the ENSO and East Asian winter monsoon, transporting warm and moist air towards East Asia (Wang et al. 2000). Previous studies on the ENSO impacts have mainly focused on the East Asian summer and winter monsoons, but the dynamical influences of ENSO are somewhat independent on the season (Zhang et al. 1999; Wang et al. 2000; Wu et al. 2003).
In addition, Wu et al. (2003) documented the spatial-temporal evolution of ENSO-related rainfall anomalies in East Asia. Because the rainband in East Asia exhibits large meridional migration over the seasons (Tao and Chen 1987), it is necessary to understand how ENSO affects anomalous rainfall changes differently with the season. According to Wu et al. (2003), the most robust influence of ENSO is seen as a positive correlation that migrates from southern China in the fall of the ENSO developing phase to eastern-central China and southern Japan through the following spring. Another is a negative correlation over northern China in summer and fall during the ENSO developing years. These two main rainfall anomalies in East Asia are induced by different anomalous atmospheric circulation systems (Wu et al. 2003). The evolution of positive rainfall in southern China is closely related to the lower-level anticyclone anomalies over the WNP. The anticyclonic anomalies are established over the South China Sea in fall of ENSO developing years, move eastward to the Philippine Sea during the ENSO mature phase, and shift northward in spring and summer of ENSO decaying years. In comparison, the negative rainfall in northern China is associated with the northerly anomalies on the western side of the cyclonic anomalies that displace southwestward along the East Asian coast during the developing phase of ENSO.
More recently, Son et al. (2016) suggested that a significant negative correlation exists in September, the ENSO developing phase, between Niño3 SST and precipitation over the Korean Peninsula. Interestingly, the positive precipitation anomalies that exist over the subtropical North Pacific associated with ENSO can be considered a unique pattern only in this month. This subtropical diabatic heating in September plays a critical role in generating the cyclonic flow over the subtropical North Pacific as a Rossby wave response. The resultant northerly anomalies on the left side of cyclonic circulation act to decrease the precipitation over the Korean Peninsula in September. Therefore, Son et al. (2016) emphasized that the subtropical cyclone is a key component in linking ENSO signals to East Asia accompanying the northerly anomalies in September.
Despite the above studies on the seasonally-varying ENSO teleconnections in East Asia, detailed studies focusing on the ENSO-related seasonal transition have not been conducted sufficiently. In particular, the ENSO possibly contributes to the seasonal transition from summer to winter; owing to the East Asian perturbations associated with ENSO being considerably different in summer and winter, respectively. Although limited studies have investigated this season, there still remains a need to understand the ENSO impacts on the East Asian fall climate in terms of its spatial/temporal evolution, intensity, and dynamical processes. Therefore, in this study, we investigate the ENSO impacts on seasonal transition, in particular from summer to winter (i.e., fall) in East Asia.
The fall, namely autumn, can be defined diversely but simply refers to the season between summer and winter. Astronomically, fall is regarded in the Northern Hemisphere as from the fall equinox to the winter solstice. In meteorology, fall is generally considered to include the months of September, October, and November in the Northern Hemisphere; March, April, and May in the Southern Hemisphere. We compare the atmospheric seasonal transition and its regional impacts on East Asia, from summer to winter, related to the ENSO forcings using observational data. In addition, we assess how well the Coupled Model Intercomparison Project Phase 5 (CMIP5) models simulate the ENSO impacts on seasonal transition during fall in East Asia.
Section 2 describes the observed and simulated datasets. Section 3 examines the ENSO-related seasonal transition in East Asia with the observed results. In Sect. 4, the long-term climate simulations of CMIP5 models are investigated to support the observational findings, and the fidelity of seasonal transition in current models is also evaluated. A summary and discussion follow in Sect. 5.