Distinctive Rainfall Evolutions in East Asia between Super and Regular El Niño Events during Their Decaying Summers

Xiaohui Wang aKey Laboratory of Meteorological Disaster, Ministry of Education (KLME), Joint International Research Laboratory of Climate and Environmental Change (ILCEC), Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology, Nanjing, China

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Tim Li aKey Laboratory of Meteorological Disaster, Ministry of Education (KLME), Joint International Research Laboratory of Climate and Environmental Change (ILCEC), Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology, Nanjing, China
bInternational Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii

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Suxiang Yao aKey Laboratory of Meteorological Disaster, Ministry of Education (KLME), Joint International Research Laboratory of Climate and Environmental Change (ILCEC), Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology, Nanjing, China

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Abstract

While enhanced rainbands progressed northward in East Asia from June to August during the regular El Niño decaying summer, strengthened rainbands were only observed in the earlier summer and disappeared in August in the super El Niño composite. The cause of this distinctive feature is investigated through a combined observational and modeling study. The relative roles of the mean state and anomalous heating in causing the northward progression in the regular El Niño group are assessed through idealized numerical experiments. The result shows that the monthly evolving mean state is more important, while the anomalous forcing also plays a role. The distinctive rainfall feature in the super El Niño composite was primarily contributed by the 1982/83 and 2015/16 events, whereas the rainband evolution in 1998 resembled the regular El Niño composite. The cause of the different rainfall pattern in August among the super El Niño events is further investigated. A marked difference exists in the tropical sea surface temperature anomaly (SSTA) and associated anomalous precipitation patterns. A low-level cyclonic (anticyclonic) anomaly appeared south of Japan in August 1983 and 2016 (1998), inducing northerly (southerly) anomalies and thus suppressed (enhanced) rainfall in eastern China. Whereas an anomalous anticyclone in the western North Pacific (WNP) is a typical response to an El Niño during its mature and decaying phases, the formation of a cyclonic anomaly in the WNP resulted from anomalous enthalpy advection associated with the eastward retreat of an anomalous anticyclone triggered by a local cold SSTA belt in August 1983 and from a Pacific meridional mode (PMM)-like positive SSTA pattern in August 2016.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Tim Li, timli@hawaii.edu

Abstract

While enhanced rainbands progressed northward in East Asia from June to August during the regular El Niño decaying summer, strengthened rainbands were only observed in the earlier summer and disappeared in August in the super El Niño composite. The cause of this distinctive feature is investigated through a combined observational and modeling study. The relative roles of the mean state and anomalous heating in causing the northward progression in the regular El Niño group are assessed through idealized numerical experiments. The result shows that the monthly evolving mean state is more important, while the anomalous forcing also plays a role. The distinctive rainfall feature in the super El Niño composite was primarily contributed by the 1982/83 and 2015/16 events, whereas the rainband evolution in 1998 resembled the regular El Niño composite. The cause of the different rainfall pattern in August among the super El Niño events is further investigated. A marked difference exists in the tropical sea surface temperature anomaly (SSTA) and associated anomalous precipitation patterns. A low-level cyclonic (anticyclonic) anomaly appeared south of Japan in August 1983 and 2016 (1998), inducing northerly (southerly) anomalies and thus suppressed (enhanced) rainfall in eastern China. Whereas an anomalous anticyclone in the western North Pacific (WNP) is a typical response to an El Niño during its mature and decaying phases, the formation of a cyclonic anomaly in the WNP resulted from anomalous enthalpy advection associated with the eastward retreat of an anomalous anticyclone triggered by a local cold SSTA belt in August 1983 and from a Pacific meridional mode (PMM)-like positive SSTA pattern in August 2016.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Tim Li, timli@hawaii.edu

1. Introduction

As a dominant signal on the interannual time scale, El Niño–Southern Oscillation (ENSO) exerts great impacts on the global climate (Horel and Wallace 1981; Rasmusson and Carpenter 1982; Philander 1983; Webster et al. 1998; Li and Hsu 2017). Through a so-called Pacific–East Asia teleconnection pattern (Chang et al. 2000a,b; Wang et al. 2000, 2003; T. Li et al. 2017), ENSO largely modulates the climate variability over the northwestern Pacific peripheral regions, leading to natural disasters such as floods (Huang et al. 2000), drought (Yin et al. 2009), and haze (S. Li et al. 2017), and profoundly impacts the development of the agriculture and economy in East Asia, where billions of people live. Therefore, understanding the diversity of ENSO-induced climate impacts is of urgent need.

It has been clearly demonstrated that a typical El Niño conveys its impact to East Asia through an anomalous low-level anticyclone in the western North Pacific (WNP), referred to as the WNP anomalous anticyclone or WNPAC. This anomalous anticyclone is a key atmospheric bridge connecting the East Asian climate to ENSO (Zhang et al. 1996; Chang et al. 2000a,b; Wang et al. 2000, 2003). In the past two decades, great effort has been devoted to investigating the formation and maintenance mechanisms for the WNPAC [see T. Li et al. (2017) for a detailed review on this topic]. While both an internal atmospheric process (i.e., moist enthalpy advection associated with the seasonal transition of the mean state; Wu et al. 2017) and an air–sea feedback process (i.e., local wind–evaporation–sea surface temperature feedback; Wang et al. 2000) primarily contribute to its establishment, the WNPAC is maintained into the following summer after El Niño peak by the Indian Ocean (IO) capacitor effect (Xie et al. 2009; Wu et al. 2009), local sea surface temperature anomaly (SSTA) forcing (Wu et al. 2010), and a cold SSTA in central equatorial Pacific due to fast El Niño transition (B. Wang et al. 2013; Xiang et al. 2013). The southerly anomalies along the western edge of the WNPAC cause the westward extension of the western Pacific subtropical high (WPSH), resulting in a positive rainfall anomaly over the Yangtze River Valley (YRV) through northward moisture transport (Chang et al. 2000a,b; Zhou and Yu 2005; Sui et al. 2007; Wu and Zhou 2008).

Recently, El Niño diversity has become a hot topic. In addition to pattern diversity (i.e., central Pacific versus eastern Pacific El Niño) (Larkin and Harrison 2005; Ashok et al. 2007; Weng et al. 2007; Yuan et al. 2012; Feng et al. 2016a,b, 2017; Xu et al. 2017a,b, 2019), there is also El Niño intensity diversity, such as super El Niño versus regular El Niño (e.g., L. Chen et al. 2016, 2017; Hoell et al. 2016; B. Wang et al. 2017, 2020; Ke et al. 2019; Zhao et al. 2019). In contrast to regular El Niño, super El Niño exhibits an extremely warm SSTA over the equatorial central-eastern Pacific and is more likely to cause severe disasters worldwide (Smith et al. 1999). For example, a devastating flood occurred over the YRV in summer 1998, after the peak of a super El Niño in December 1997. It resulted in approximately 3000 deaths and huge economic losses (Huang et al. 1998; Jiang et al. 2008). Therefore, it is critical to distinguish the climate impact of a regular El Niño group from a super El Niño group (B. Wang et al. 2017, 2019).

The objective of the current study is to reveal the similarities of and differences between the regular and super El Niño groups and to understand the mechanisms responsible for the difference. We intend to address the following scientific questions: 1) What are the significant differences of precipitation anomalies in East Asia between the super and regular El Niño groups during ENSO decaying summer? 2) What is the mechanism responsible for distinctive precipitation evolution patterns between the two groups?

The rest of this paper is organized as follows. Section 2 introduces the datasets, method, and model used in this paper. The observed precipitation evolution patterns in the regular and super El Niño composites during ENSO decaying summers are compared in section 3. Section 4 discusses the cause of northward propagation of anomalous rainband in East Asia from June to August in the regular El Niño group. In section 5, we investigate the cause of the distinctive precipitation evolution patterns for each of the super El Niño events. Finally, a summary and discussion are in section 6.

2. Data, method, and model

a. Data

The following observational and reanalysis datasets are used in the current study: 1) the Met Office Hadley Centre’s sea ice and SST dataset (HadISST; Rayner et al. 2003) and National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperature (ERSST; Huang et al. 2017); 2) monthly and daily precipitation from the Global Precipitation Climatology Project (GPCP; Adler et al. 2003) and the interim European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-I; Dee et al. 2011); 3) monthly three-dimensional atmospheric fields including horizontal wind and geopotential height fields from ERA-I datasets. The analysis period is from 1979 to 2016 for all the datasets above except for ERA-I precipitation (1980–2016). The ensemble average of the aforementioned two SST datasets is used in the current analysis to reduce the data uncertainty. The two datasets were interpolated into the same resolution (2° × 2°) prior to the ensemble average. Precipitation data from the Climate Prediction Center Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997) and reanalysis data from National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP–NCAR; Kalnay et al. 1996) are also used for comparison, to ensure a robust result.

b. Definitions of regular and super El Niño events

An El Niño event is selected when the averaged SSTA over the Niño-3.4 region (5°S–5°N, 170°–120°W) is above 0.5°C during boreal winter [December–February (DJF)]. A super El Niño event is defined when the Niño-3.4 index exceeds 2 standard deviations. Based on the criterion above, three super El Niño events (i.e., 1982/83, 1997/98, and 2015/16) and six regular El Niño events (i.e., 1991/92, 1994/95, 2002/03, 2004/05, 2006/07, and 2009/10) since 1979 are identified.

c. Model

An atmospheric general circulation model (AGCM), ECHAM4.6, developed at the Max Planck Institute for Meteorology (Roeckner et al. 1996), is adopted for conducting idealized numerical experiments. The model has a T42 horizontal resolution (2.8° latitude × 2.8° longitude), with 19 vertical levels in a hybrid sigma–pressure coordinate from the surface to the top of the troposphere (near 10 hPa). This atmospheric general circulation model has been used to investigate the circulation responses to the IO SST and heating anomalies during El Niño mature winter (M. C. Chen et al. 2016), northward propagation of the boreal summer intraseasonal oscillation (BSISO; Jiang et al. 2004), the convection initiation and atmospheric teleconnection of the Madden–Julian oscillation (Zhao et al. 2013; L. Wang et al. 2013, 2017), and the variability of the Asian summer monsoon (Fu et al. 2002). In the current setting, a control experiment (CTL) is run with a specified climatological monthly SST field for 30 years. The sensitivity experiments (SEN) are forced by either a specified SSTA field that is superposed on the climatological mean SST field or an anomalous heating field, and they are integrated for 30 years each. The anomaly response is derived by subtracting the CTL from the SEN, averaged during the last 15-yr integration period.

3. Distinctive precipitation patterns between super and regular El Niño groups

The top panel of Fig. 1 shows the time–latitude sections of the climatological rainband in June–August (JJA) and rainfall anomalies in the regular and super El Niño composites during ENSO decaying summers averaged over 110°–130°E. Climatologically, there is a steady northward movement of rainband from south of 30°N in June to 40°N in August (Fig. 1a). Accompanying the rainband movement is the northward progression of the climatological WPSH as shown in Fig. 1d. The rainband and the anticyclone are tightly coupled, as southerly winds to the western flank of the WPSH transport high mean moisture northward from tropical oceans, strengthening the rainband.

Fig. 1.
Fig. 1.

(top) Time–latitude sections of (a) climatological mean precipitation (mm day−1) and of precipitation anomalies in (b) regular and (c) super El Niño composites during their decaying summers averaged over 110°–130°E. A 14-day running mean has been applied in the panels. In (b) and (c), the values reaching a 10% significance level are stippled. (bottom) The 5883-gpm contour lines of the WPSH in June (solid), July (black dashed) and August (red) for (d) long-term climatology, (e) the regular El Niño composite, and (f) the super El Niño composite.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

In the regular El Niño composite, positive precipitation anomalies appear in East Asia, and show a significant northward migration (Fig. 1b), similar to the climatology. The evolution of the WPSH during the regular El Niño composite resembles that in climatology (Fig. 1e). This implies that during the decaying summer of a regular El Niño, total precipitation illustrates an evolution feature similar to the climatology, while the rainfall intensity increases. This is in a great contrast to anomalous rainfall evolution pattern in the super El Niño composite (Fig. 1c) in which a stronger positive precipitation anomaly persists over the YRV (near 30°N) from June to July, and there is no obvious precipitation anomaly in August. As a result, there is no obvious northward propagation of precipitation anomalies throughout the summer in the super El Niño composite. Correspondingly, the WPSH center extends westward to the coast of East Asia in June and July compared to the climatological mean position, and retreats eastward markedly in August (Fig. 1f).

The observational analysis above reveals a distinctive evolution feature between the regular and super El Niño groups, that is, the anomalous rainband progresses slowly northward in the former whereas the enhanced rainfall stays only over the YRV in June–July but disappears in August in the latter. What causes the distinctive evolution characteristics between the two types of El Niño? In the following sections we intend to address this question. In section 4, we first explore the cause of northward movement of the positive precipitation anomaly from June to August in the regular El Niño composite.

4. Mechanism for northward movement of the precipitation anomaly in the regular El Niño group

To understand the cause of the northward movement of the positive precipitation anomaly in the regular El Niño composite, we first examine the spatial distributions of anomalous low-level atmospheric circulation, precipitation, and SST from June to August. As seen in Fig. 2, an anomalous anticyclone appears over the WNP. This anticyclone can be viewed as a Rossby wave response to the local negative heating anomaly, which is ultimately caused by both the cold SSTA in the equatorial Pacific and a warm SSTA in the Indian Ocean, as discussed previously by B. Wang et al. (2013) and Xie et al. (2009).

Fig. 2.
Fig. 2.

(left) Precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1) anomaly fields for the regular El Niño composite during its decaying (a) June, (c) July and (e) August. (right) As in the left column, but for SST (shading; °C) and 850-hPa streamfunction anomalies [contours; solid (dashed) lines denote positive (negative) values; interval: 0.8 × 106 m2 s−1]. For the precipitation and SST anomalies, the values reaching 10% significance level are stippled; the blue vectors represent the values reaching 10% significance level for wind anomalies. Letter A denotes the center of an anticyclone.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

The anomalous anticyclone center is located south of 20°N in June (Fig. 2a). The southerly anomalies on the western flank of the anticyclone bring water vapor to southeastern China, resulting in an enhanced precipitation anomaly. A local positive SSTA over the WNP, as seen in Fig. 2b, is likely a result of atmospheric impact to the ocean, as suppressed convection leads to the increase of downward shortwave radiation and thus a warm SSTA in situ. In July, the negative precipitation anomaly in the WNP moves northward to 20°N, while strengthened precipitation occurs over the YRV (Fig. 2c). The negative precipitation anomaly in the WNP shifts to about 35°N in August (Fig. 2e).

Two possible factors may cause the northward movement of the anomalous anticyclone in the WNP. The first is attributed to the northward propagation of the diabatic heating anomaly. But even the anomalous heating is fixed with time, the seasonally evolving background mean state may also cause the northward propagation movement of the anticyclone. To isolate the two factors, we design the following three sets of AGCM experiments.

In the first set of experiments (SEN_R; here R denotes the regular El Niño group), the observed anomalous negative heating from June to August in the WNP is specified. The horizontal pattern of the anomalous heating is the same as the observed precipitation anomaly shown in Figs. 2a, 2c, and 2e. An idealized vertical profile with a maximum heating in middle troposphere is specified. This set of experiments is to examine the role of both the seasonally evolving mean state and the time-dependent perturbation heating. In the second set of experiments (SEN_R_M), the seasonal mean heating anomaly is specified, which does not change with time. This experiment is designed to understand the sole effect of the seasonally evolving background mean state on northward movement of the anomalous anticyclone. The third set of experiments (SEN_R_A) is designed to reveal the pure effect of the time-dependent perturbation heating, while the mean state is fixed in July (i.e., by keeping July climatological mean SST and solar radiation at top of the atmosphere). Table 1 describes all numerical experiments.

Table 1

List of all numerical experiments using ECHAM4.

Table 1

Figures 3a–c show the simulated 850-hPa streamfunction field in June, July, and August in SEN_R when both the mean state and perturbation heating effects are considered. The pattern and temporal evolution of the WNP anomalous anticyclone are well simulated. To quantitatively measure the relative importance of the mean state and the perturbation heating, we examine the latitudinal locations of maximum southerly anomalies averaged over 100°–125°E west of the WNPAC (Fig. 3j), as the southerly anomalies associated with the WNPAC directly link to the movement of anomalous rainband. Note that the center locations in SEN_R are very close to those in the observation, suggesting that the model successfully captures the northward movement of the anomalous anticyclone in the presence of both the mean-state and perturbation heating effects. By comparing the simulation results in SEN_R_M and SEN_R_A, we note that the mean-state effect is greater. In the presence of the time-dependent mean state and perturbation heating, the maximum southerly anomalies shift northward by 11° and 9° latitude, respectively, from June to August. Thus, the mean-state effect accounts for 55% of the northward displacement (Fig. 3k).

Fig. 3.
Fig. 3.

The pattern of 850-hPa streamfunction anomaly field (contours; interval: 0.8 × 106 m2 s−1) in response to specified negative heating anomalies in the WNP (shading; °C day−1) during regular El Niño decaying (a) June, (b) July, and (c) August. (d)–(f) As in (a)–(c), but for SEN_R_M in which the JJA-mean heating anomaly is specified. (g)–(i) As in (a)–(c), but for SEN_R_A in which the mean state is fixed in July. (j) Latitudes of the maximum southerly anomalies averaged over 100°–125°E (unit: degrees) in the observation (black), SEN_R (orange), SEN_R_M (green), and SEN_R_A (blue). (k) As in (j), but for the latitude difference between August and June (unit: degrees). The numbers above the blue and green bars denote the relative contributions of the anomalous heating and the mean state, respectively.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

An interesting question is given a fixed anomalous heating in the WNP in SEN_R_M, how does the mean state cause the northward movement of the WNPAC? As shown in Fig. 1a, the climatological rainband in East Asia moves northward from June to August, so are background mean ascending motion and mean specific humidity fields. The background mean specific humidity follows closely to the mean ascending motion because of role of vertical moisture transport. While a negative heating in the WNP induces a low-level anomalous anticyclone, southwesterly anomalies to the west of the anticyclone induce a low-level convergence to the north of the anticyclone, forming a pair of meridional convergence–divergence dipole. The dipole interacts with the northward moving background mean moisture, leading to the northward propagation of a pair of anomalous moisture convergence and divergence belts. As a result, a pair of anomalous wet and dry belts move northward as the climatological mean rainband advances poleward.

It is worth mentioning that the actual effect of the mean state exceeds 55%, because the northward movement of the perturbation heating is partially attributed to the mean-state effect. This can be further demonstrated through an additional set of experiments in which SST anomalies in the tropical Pacific and Indian Ocean, rather than the anomalous heating fields in the WNP, are specified. The experiment design is the same as SEN_R, SEN_R_M, and SEN_R_A except that the monthly SSTA is specified. In the presence of the monthly evolving mean-state-only experiment, the latitudinal location of the maximum southerly anomalies associated with the WNPAC shifts by 11° latitude from June to August. In the presence of the monthly evolving SSTA only experiment, the location of the maximum southerly anomalies shifts by 7° latitude. Therefore, the mean-state effect accounts for 60%, while the SSTA effect accounts for 40%.

5. Mechanisms for distinctive precipitation evolutions in super El Niño events

As described in section 2, the super El Niño group in the current composite analysis includes three events. By carefully examining each super El Niño case, we found that the composite evolution result shown in Figs. 1c and 1f is mainly attributed to the 1982/83 and 2015/16 events, whereas the rainfall evolution for the 1997/98 event resembles the regular El Niño composite. Therefore, it is necessary to reveal physical mechanisms for each of the three events.

a. The 1997/98 event

Figure 4 shows the horizontal patterns of low-level atmospheric circulation, precipitation, and SST anomaly fields from June to August. The tropical SSTA pattern over the Pacific and Indian Ocean in general resembles that of the regular El Niño composite, except that the SSTA amplitude is greater. This led to a stronger anomalous anticyclone in the WNP. The center of the anomalous anticyclone moved slightly northward from June to August, so that anomalous rainbands were primarily located over south and central China in June–July (Figs. 4a,c), and shifted farther northward in August (Fig. 4e). The persistent and stronger tropical SSTA including a cold SSTA in the equatorial Pacific and a warm SSTA in the IO from June to August (Figs. 4b,d,f) maintained the WNP anomalous anticyclone, through the IO capacity and central Pacific SSTA forcing mechanisms (e.g., Xie et al. 2009; B. Wang et al. 2013).

Fig. 4.
Fig. 4.

As in Fig. 2, but for summer 1998.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

While the tropical SSTA in the tropical Pacific and IO is relatively stable throughout the summer, the anticyclonic center in the WNP moves northward. It is likely that the northward shift arises from the mean-state impact. To unveil the relative contributions of the mean state and the tropical SSTA in summer 1998, we also conducted three sets of numerical experiments.

In the first experiment, SEN_98, we consider both the time-dependent mean-state and SSTA effects. Observed positive SSTA over the IO and negative SSTA over the Pacific (as shown in Figs. 5a–c) as well as realistic seasonally evolving mean state are specified. In the second experiment (SEN_98_M), the JJA-mean SSTA in the tropical Pacific and IO is specified, while the mean state varies with time as in SEN_98. This sensitivity experiment is designed to examine the role of the mean-state change in the northward movement of the anticyclone. In the third experiment (SEN_98_A), the time-dependent SSTA (same as in SEN_98) is specified, while the mean state is kept in July.

Fig. 5.
Fig. 5.

The pattern of 850-hPa streamfunction anomaly field (contours; interval: 0.8 × 106 m2 s−1) in response to a specified tropical SSTA forcing (shading; °C) in (a) June, (b) July, and (c) August 1998. (d) Latitudes of the maximum southerly anomalies averaged over 100°–125°E derived from the observation (black), SEN_98 (orange), SEN_98_M (green), and SEN_98_A (blue).

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

Figure 5 shows the simulation results. The anomalous anticyclone over the WNP is successfully simulated in SEN_98 (Figs. 5a–c). The latitudinal locations of the maximum southerly anomalies associated with the WNPAC are plotted in Fig. 5d. The northward movement of the maximum southerly anomalies in SEN_98 is quite close to that in the observation. Such a northward shift is to a large extent reproduced in SEN_98_M, indicating the important role of the mean state in promoting the northward movement. The latitudinal shift in SEN_98_A is undetected, implying that the temporal change of the anomalous SST in the tropics plays a minor role.

The results above indicate that the evolutions of both the anomalous anticyclone and the anomalous rainfall in East Asia in summer 1998 resemble those of the regular El Niño composite. Such a resemblance is attributed to the fact that the tropical SSTA patterns and associated precipitation anomalies in June–August 1998 were similar to those of the regular El Niño composite. The northward progression of the anomalous rainband in East Asia in summer 1998 resulted from the combined effect of the tropical SSTA and seasonally evolving background mean state.

b. The 1982/83 event

Tropical SSTA and precipitation patterns in summer 1983 differed markedly from the regular El Niño composite in the following aspects. First, a warm rather than a cold SSTA appeared in the equatorial eastern Pacific (Figs. 6b,d,f). As a consequence, a positive precipitation anomaly appeared over the equatorial central Pacific (Figs. 6a,c,e). Second, although a warm SSTA occurred in the tropical IO, a negative rainfall anomaly occupied most of the region in June and July. This implies that the IO SSTA during that period did not play an active role in affecting the atmospheric wind in the WNP. A basinwide positive rainfall anomaly appeared in the IO in August 1983, suggesting a switch of the IO role from a passive to an active one.

Fig. 6.
Fig. 6.

As in Fig. 2, but for summer 1983. Letter C represents an anomalous cyclone in southern Japan.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

A marked feature in the SSTA pattern in summer 1983 was a strong cooling in North Pacific. Extended from this midlatitude cooling was a northeast–southwest-oriented cold SSTA belt extending from subtropical northeastern Pacific to tropical WNP. Physically, we argue that this cold SSTA belt plays a critical role in inducing an anomalous anticyclone in the WNP in summer 1983.

Note that in June–July, an anomalous anticyclone was located to the west of the cold SSTA belt in the WNP (Figs. 6b,d). Neither a warm SSTA in the equatorial eastern Pacific nor a warm SSTA in the IO could generate this anticyclone. A local positive SSTA off the coast of East Asia (Figs. 6b,d) was likely a result of the anticyclonic forcing. Southerly anomalies to the west of the anticyclone cause a positive precipitation anomaly over the YRV (Figs. 6a,c) through anomalous northward moisture transport. In August, the cold SSTA belt was strengthened a little over the subtropics and tropical WNP (Fig. 6f). Meanwhile, a basinwide positive precipitation anomaly appeared in the IO (Fig. 6e), implying that the warm SSTA in the IO started to play an active role. The combined impacts of the aforementioned SSTA forcing and the full establishment of the WNP monsoon trough (Fig. 7b) led to the strengthening and eastward shift of the anticyclone in August (Figs. 6e,f). As a result, the center of the anomalous anticyclone shifted from 130°E in June–July to 150°E in August. This shift left space for the development of an anomalous cyclone over southern Japan (Fig. 6e). Anomalous northerlies associated with the cyclone offset climatological southerlies, leading to a negative precipitation anomaly in most of East Asia. This causes the interruption of northward progression of the precipitation anomaly in August in the super El Niño composite (Fig. 1c).

Fig. 7.
Fig. 7.

Climatological mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1) in (a) June and (b) August. A red line represents the full development of the WNP monsoon trough in August, with pronounced westerlies to its south and easterlies to its north. Both the moisture and precipitation increase rapidly along the monsoon trough.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

An interesting question is what caused the development of an anomalous cyclone over southern Japan. We argue that it is caused by positive moist enthalpy advection anomalies (Wu et al. 2017) associated with southerly anomalies to the west of the WNP anticyclone. Due to the full development of the monsoon trough in late summer, the atmospheric convective response to the tropical IO and subtropical Pacific SSTA forcing shifted eastward. This caused the eastward retreat of the anomalous anticyclonic center, which left space for the development of the anomalous cyclone in southern Japan.

To test the hypothesis above, we carried out two sets of AGCM experiments. In the first experiment (SEN_83), the observed time-dependent negative SSTA belt in the subtropical and tropical Pacific is specified. Meanwhile, the observed positive SSTA over the IO is specified in August. (The IO SSTA in June–July is not considered, simply because it did not play an active role during the time period.) The model was integrated with the seasonally evolving mean SST, to allow the full development of the WNP monsoon trough. In the second experiment (SEN_83_A), the same SSTA pattern is specified as SEN_83, but under a fixed July background mean state. By doing so, we retain the same SSTA forcing but keep the mean state unchanged.

Figure 8 shows the simulation results. In SEN_83, an anomalous anticyclone forms in June and July over the WNP (Figs. 8a,c) similar to the observed. This suggests that the subtropical cold belt in the Pacific is a primary factor that sets up the anticyclone. The SEN_83 experiment successfully reproduces the eastward shift of the anomalous anticyclone in August, as well as the occurrence of an anomalous cyclone near southern Japan (Fig. 8e). The sensitivity experiment SEN_83_A confirms the role of the mean-state effect. Under a fixed July mean state, an almost identical anomalous anticyclone appears at each month in the WNP, even though the SSTA forcing differs (Figs. 8b,d,f). Therefore, this sensitivity experiment confirms the hypothesis that the seasonally evolving mean-state change, in particular the full development of the monsoon trough, is critical in resulting in the eastward shift of the anomalous anticyclone and the establishment of an anomalous cyclone in southern Japan.

Fig. 8.
Fig. 8.

(left) Simulated 850-hPa streamfunction anomaly patterns (contour; interval: 0.8 × 106 m2 s−1) in SEN_83 in response to a specified tropical SSTA forcing in (a) June, (c) July, and (e) August 1983. (right) As in the left column, but for SEN_83_A in which the mean state is fixed in July condition.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

c. The 2015/16 event

The tropical anomalous SST and precipitation patterns in summer 2016 also exhibited distinctive features. Positive SST and precipitation anomalies occupied most of the northern IO in June–July, which favored the establishment of the anomalous anticyclone in the WNP through a Kelvin wave response (Wu et al. 2009) (Figs. 9b,d). A local positive SSTA in the WNP was obviously a passive response to the anticyclonic forcing. Although a cold SSTA appears in the equatorial Pacific, this SSTA impact on the WNP circulation was hindered by a strong positive Pacific meridional mode (PMM)-like SSTA belt extending from the subtropical northeastern Pacific to the equatorial western Pacific (Figs. 9b,d). The persistence of the anomalous anticyclone from June to July favored an anomalous rain belt over the YRV (Figs. 9a,c).

Fig. 9.
Fig. 9.

As in Fig. 2, but for summer 2016.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

An interesting feature in August in the northern IO is a negative precipitation anomaly overlying a positive SSTA. This implies a shift of the SSTA role from an active one in June–July to a passive one in August. This is possible because an enhanced convection in the WNP may promote a suppressed convection over the northern IO–Indian monsoon sector through an anomalous Walker circulation or a negative moist enthalpy advection anomaly associated with large-scale cyclonic flow generated by the positive heating anomaly in the WNP (Gu et al. 2010). There might be two-way feedbacks between the enhanced convection in the WNP and the suppressed convection in the IO. On one hand, an enhanced WNP convection promoted anomalous descent in the IO. On the other hand, a suppressed heating in the IO could induce a Kelvin wave response with pronounced low-level westerly and cyclonic flow anomalies in the WNP.

What caused the initial development of the anomalous cyclone in the WNP in August 2016? We hypothesize that it is attributed to both the remote forcing from the IO and the PMM-like SSTA pattern in the Pacific. To test the hypothesis, in particular to investigate the relative roles of the IO heating anomaly versus the PMM-like SSTA pattern, we conducted three sets of experiments. In the first experiment (SEN_16), both the heating anomaly in the IO and a PMM-like SSTA pattern in the Pacific (as shown in Figs. 10a–c) are specified. In the second experiment (SEN_16_I), only the heating anomaly over the IO is specified, while there is no SSTA in the Pacific. In the third experiment (SEN_16_P), only the Pacific SSTA pattern is specified, while there is no heating anomaly in the IO.

Fig. 10.
Fig. 10.

(left) The simulated 850-hPa streamfunction anomaly patterns (contours; interval: 0.65 × 106 m2 s−1) in (a) June, (b) July, and (c) August 2016 derived from SEN_16 in which a heating anomaly over the IO (shading; °C day−1) and a PMM-like SSTA in the Pacific (shading; °C) are specified. The green box is used to estimate the strength of the WNP anomalous circulation in June, July, and August, respectively. (center) As in the left column, but for SEN_16_I in which only the heating anomaly over the IO is specified. (right) As in the left column, but for SEN_16_P in which only the Pacific SSTA pattern is specified.

Citation: Journal of Climate 36, 1; 10.1175/JCLI-D-22-0143.1

Figure 10 shows the simulation result in the above three sets of experiments. An anomalous anticyclone is simulated over the WNP in June and July (Figs. 10a,b), and an anomalous cyclone is also reproduced in the WNP in August (Fig. 10c). The simulation is in general in good agreement with the observation, except that the simulated anticyclonic center in July shifts slightly eastward and northward. The latitudinal location of the maximum southerly anomalies in the East Asian longitudes associated with the WNPAC in July, however, is quite close to the observed. The northeastward shift of the simulated anticyclonic center in July might result from an overestimated cyclonic anomaly to its southeast, which is a direct response to the warm SSTA belt in the tropical central Pacific.

To quantitatively measure the relative contributions of the IO and Pacific forcing, box-averaged streamfunction anomalies are calculated for all the three experiments, and the result is shown in Table 2. Because the simulation results in June and July are almost identical, in Table 2 we only show the June–July average result. It is found that the formation of the anomalous anticyclone in June–July was caused by the positive heating anomalies over the IO (Figs. 10d,e and Table 2). This is reasonable, because the PMM-like positive SSTA in the Pacific cannot excite an anomalous anticyclone. The formation of the anomalous cyclone in the WNP in August, however, is primarily caused by the PMM-like positive SSTA belt in the Pacific (Fig. 10i and Table 2). Our calculation shows that it contributes about 56%. The IO negative heating anomaly also played a role, and it contributes about 44%. Given that the IO heating may partially result from the WNP forcing, the major triggering mechanism for the anomalous cyclone in August 2016 is the PMM-like SSTA forcing.

Table 2

Box-averaged streamfunction anomalies (unit: 106 m2 s−1) in June–July and August 2016. The box for each month is shown in Fig. 10.

Table 2

To sum up, the observational and modeling study above reveals that the distinctive precipitation evolution pattern associated with the super El Niño group shown in Fig. 1c arose from the formation of an anomalous cyclone in the WNP in August 1983 and 2016. The formation mechanism for the anomalous cyclone in 1983 differs from that of 2016. The former was attributed to the eastward shift of the WNP anomalous anticyclone, due to full development of the WNP monsoon trough in late summer. The eastward retreat of the anticyclone promoted a positive moist enthalpy advection anomaly, leading to the development of an anomalous cyclone over southern Japan. The occurrence of a strong anomalous cyclone in August 2016 is a direct Rossby wave response to a PMM-like positive SSTA pattern over the tropical and subtropical Pacific. The anomalous cyclone and associated enhanced convection may have a two-way interaction with suppressed convection in the IO.

6. Summary and discussion

In this study, we reveal distinctive precipitation evolution patterns during a regular and super El Niño group. For the regular El Niño composite, there is a steady northward propagation of a positive precipitation anomaly over East Asia, in a way similar to the climatological rainband movement. In contrast, a positive precipitation anomaly appears over the YRV region in June and July and then turns to be negative in August in the super El Niño composite.

The cause of the distinctive rainfall patterns is further investigated through both observational analyses and numerical modeling. It is found that the northward progression of the anomalous rainband is closely related to the northward movement of an anomalous anticyclone in the WNP. To reveal the relative roles of the mean state and the anomalous heating in causing the northward movement, three sets of experiments are carried out. In the first experiment (SEN_R), both the mean state and the anomalous heating vary month to month, while in the second and third experiments (SEN_R_M and SEN_R_A), either the anomalous heating or the mean state is fixed in time, respectively. The result shows that the mean-state change plays a more important (55%) role in causing the northward movement of the southerly anomalies associated with the WNPAC than the anomalous heating (45%) (Fig. 3k).

The precipitation evolution patterns in individual super El Niño events are further examined. It is found that the rainfall evolution feature in East Asia in summer 1998 resembles that in the regular El Niño composite (Fig. 4), while the feature in summer 1983 and 2016 resembles that of the super El Niño composite (Figs. 6 and 9).

To investigate the relative contributions of the mean state and tropical SSTA in summer 1998, we conduct additional two experiments. In the first experiment (SEN_98), both the mean state and the anomalous forcing change with time. In the second experiment (SEN_98_A) we keep the July mean-state condition while varying with time the SSTA forcing. Consistent with the simulation result of the regular El Niño, the mean state plays a dominant role in promoting the northward movement of southerly anomalies associated with the WNPAC (Fig. 5d).

Compared to the regular El Niño composite, a marked difference for the 1982/83 and 2015/16 super El Niño events was in the low-level wind field in August in the WNP (Figs. 6 and 9). In August 1983, due to the combined effect of a cold SSTA belt in the Pacific and the mean-state impact, an anomalous anticyclone formed in the WNP withdrawn eastward. Southwesterly anomalies on the northwestern flank of the anomalous anticyclone transported moisture northward, leading to a positive heating anomaly and thus an anomalous cyclone in southern Japan (Fig. 6e). Numerical model experiments confirm the mean-state effect in the eastward retreat of the anomalous anticyclone and the formation of the anomalous cyclone. In August 2016, both a PMM-like warm SSTA pattern in the Pacific and a negative heating anomaly in the IO contributed to the formation of a strong cyclonic anomaly in the WNP, as demonstrated by the following three sets of sensitivity experiments. In the first experiment (SEN_16), both the anomalous heating over the IO and the PMM-like SSTA pattern over the Pacific are specified. In the second and third experiments (SEN_16_I and SEN_16_P), either the IO heating anomaly or the Pacific SSTA is specified. The modeling results indicate that the PMM-like SSTA pattern played a more important role (56%) while the IO forcing also played a role (44%) (Table 2).

The present study, for the first time, quantitatively assesses the relative contributions of the mean state and the anomalous heating to the northward movement of the precipitation anomaly over East Asia during ENSO decaying summer in the regular El Niño composite. The finding of the distinctive precipitation evolution patterns in East Asia between the regular and super El Niño groups has important implication for seasonal operational forecast. It reminds forecasters that a detailed pattern of anomalous SST and precipitation fields matters. Special attention should be paid to the anomalous SST–precipitation phase relation. Different from previous comparisons of individual super El Niño events (e.g., C. F. Li et al. 2017; Liu et al. 2017; Paek et al. 2017; Chen et al. 2019), here we focused on the contrast between the super and regular El Niño groups, emphasizing the month-to-month change of the circulation field. It turns out that August is a special month. Given a similar SSTA forcing in July and August, the atmospheric circulation and rainfall response could be very different, particular over East Asia.

An interesting question is why the climatological monsoon trough did not have a large impact on the anomalous anticyclone in regular El Niño and other super El Niño cases but only in the decaying summer of the 1982/83 El Niño event. Note that in the regular El Niño composite, the WNPAC in August also shifts eastward compared to that in July (Fig. 2), though such a shift is not as strong. There are two reasons for the weaker zonal shift. First, a positive SSTA over the IO, through its positive heating effect, forces the WNPAC and prevents it from shifting eastward. Second, by August the WNPAC has moved to the northernmost latitudinal location. As a result, the monsoon trough modulation effect becomes weaker. For 2016 super El Niño case, both a PMM-like warming SSTA in the subtropical Pacific and a negative heating anomaly in the IO in August 2016 were responsible for the occurrence of an anomalous cyclone in the WNP. Comparing to the 1998 and 1983 super El Niño cases, a major difference lies in the longitudinal location of cold SST anomalies in the western Pacific. In 1998 the cold SSTA only penetrated westward to 160°E, while in 1983 it penetrated into 140°E. This led to a stronger monsoon trough modulation in 1983.

In the current study we focus on the impact of the mean state and anomalous SST or heating over the tropical Indian and Pacific Ocean on the northward movement of the WNPAC and associated rainfall anomalies in East Asia. In addition to these processes, other factors may also affect the WNPAC. For instance, Rong et al. (2010) suggested that the tropical Atlantic SSTA could exert a remote impact to circulation anomalies in the WNP through a Kelvin wave response. Feng et al. (2014) found that the PDO could modulate the WNPAC subseasonal evolution during the El Niño decaying summer. Feng and Chen (2021) further noted that the tropical North Atlantic (TNA) SST warming could shift the WNPAC location and impact the WNPAC propagation. Therefore, further in-depth observational and modeling studies are needed to investigate physical mechanisms behind the remote impacts and the relative importance of the remote and local forcing.

Acknowledgments.

This work was jointly supported by NSFC Grant 42088101, NSF AGS-2006553, and Postgraduate Research and Practice Innovation Program of Jiangsu Province KYCX20_0909. This is SOEST Contribution Number 11611, IPRC Contribution Number 1587, and ESMC Number 396. We acknowledge the High Performance Computing Center of Nanjing University of Information Science and Technology for their support of this work.

Data availability statement.

All the observational and reanalysis datasets used in this paper can be downloaded from the websites as follows. 1) The SST data from HadISST and ERSST datasets are available from https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html and https://psl.noaa.gov/data/gridded/data.noaa.ersst.v3.html, respectively. 2) The ERA-I data are from https://apps.ecmwf.int/datasets/. 3) CMAP and GPCP datasets are available from https://www.psl.noaa.gov/data/gridded/data.cmap.html and https://psl.noaa.gov/data/gridded/data.gpcp.html, respectively. 4) NCEP–NCAR reanalysis data are from https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html.

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