**4. Discussion**

In the present work, we separated the different flavors of ENSO, but their physical backgrounds are unclear. Due to the limited space of this article, we give a short discussion. Figure 9 displays the seasonal evolution of sea surface temperatures and surface wind anomalies for the three El Niño types. For SEN and equatorial EN during the springtime (Figure 9a,i), significant westerly wind anomalies prevail in the western Pacific. On the one hand, the westerlies induce the upwelling of the subsurface water in the WP, cooling down the local SST. Conversely, the westerlies blow the warm waters in the western Pacific warm-pool eastward, warming up the equatorial central-eastern Pacific. Such a process can continue to function from summer to winter (upper and lower panels of Figure 9). During the early summer, the cooling in the WP increases the zonal gradient between the North IO (NIO), generating tropical easterlies across the NIO (Figure 9b,j), slowing down the background westerlies and continuing the warm SSTA in the NIO (Figure 9c,k), leading to a positive Indian Ocean Dipole (IOD)-like SSTA pattern in the autumn (Figure 9d,l). However, the IOD decays rapidly thereafter, and SEN and equatorial EN continue to force the IO, inducing an Indian Ocean basin-wide warm SSTA in the ensuing winter [74] (Figure 2a,c).

The SEN and equatorial EN, however, show significant discrepancies in the northwestern Pacific (NWP). For the SEN case, the intense equatorial central-eastern Pacific warming excites a Rossby wave west of the equatorial central Pacific. Anomalous northeasterly flow from North Pacific (NP) down to the tropical WP, speeding up the trades over the western NP (WNP). The resulting wind speed—stronger in the extra-tropical and WNP—induces SSTA changes in the WNP via the Wind-Evaporation-SST (WES) feedback [75]: the structure exhibits a strong east-west gradient (Figure 9d). In the equatorial EN case, the anomalous NWP cooling reaches its maximum in the early summer (Figure 9k) and decays in the autumn (Figure 9i). The weak WNP cold SSTA, going against the local air-sea interaction process proposed by Wang et al. [17], restricts the formation of the WNP anticyclone to accelerate the trade wind and then feedback to the local SST via the WES mechanism.

Nevertheless, the WSGEN exhibits a gradual enhancement of the west-east gradient in SST over the Pacific. The initial SST warming occurs in the equatorial EP, and cooling emerges in the subtropical WP during the early summer, with the westerlies prevailing over the equatorial CP (Figure 9f). The westerly wind anomalies become stronger in the late summer and autumn with the enhancement of the west-east gradient in SST over the Pacific (Figure 9g,h), reflecting a Bjerknes positive feedback process [41]. Moreover, the initial cool SSTA in the South China Sea (SCS) moves eastward from spring to autumn. During the summertime, the deficiency of the salient cool SSTA in SCS restricts the easterlies penetrating NIO and the development of the IOD. We also noticed that the dominant wind and SST anomalies in the extra-tropical NP, which cannot be seen in the equatorial EN cases, may contribute to the cooling of the WP. However, the origin of the circulation and SST anomalies in the NP during the WSGEN autumn call for further investigation.

**Figure 9.** SST (internal: 0.2 K) and horizontal wind at 1000-hPa (UV1000; m/s) composite differences under strong El Niño. (**a**) March–April–May (MAM), (**b**) May–June–July (MJJ), (**c**) July–August– September (JAS), and (**d**) September–October–November (SON). (**e**–**h**) same as (**a**–**d**), but for WSG-El Niño. (**i**–**l**) for equatorial El Niño. The shadings and vectors in each panel represent the region with anomalies significant at the 90% confidence level (Student's *t*-test).

For the La Niña events, the SGLN and equatorial LN coincide with the CP and EP La Niñas, respectively. Prior studies have revealed that the previous winter or spring North Pacific Oscillation (NPO) through seasonal footprint mechanism is largely responsible for triggering the CP ENSO [76–78]. Figure 10 shows the lead-lag composite of UV850 and SSTA for SGLN. An abnormal positive NPO-like pattern appears in the previous spring, driving the North Pacific SST and resulting in a tripolar SSTA pattern in the NP region (Figure 10a). Specifically, the anticyclonic anomalies occur over the subtropical to mid-latitude North Pacific, causing abnormal easterlies in the equatorial CP. The abnormal easterlies in the tropical CP induce convergence and give feedback to maintain the development of negative SSTAs (Figure 10b). Such an air-sea interaction process amplifies the equatorial zonal wind and SST anomalies, driving the evolution of La Niña from summer to autumn (Figure 10c,d). However, such a process cannot be seen for the equatorial LN (lower panel of Figure 10). It starts to develop in the late summer without the salient atmospheric signals. This oceanic process needs to be investigated.

We should also point out that dividing ENSOs based on intensity is still debatable. Alizadeh [79] grouped ENSOs into weak, moderate, strong, and very strong events. This classification is more elaborate but obtains smaller samples than our study does. Then, the unsupervised learning methods, such as clusters, self-organizing maps, and many others, should be examined to determine whether the different types of ENSO can be subdivided more objectively. Furthermore, we only apply the AGCM to mimic the various forcing effects of different types of ENSOs on the extra-tropical atmospheric anomalies. The lack of air-sea interaction process of AGCM makes it difficult to capture the potential effects of extra-tropical SSTAs (e.g., North Pacific), i.e., feedback to ENSO forcing, on the atmospheric anomalies over the North Atlantic. A better method would be to apply the samples of CMIP6 history experiments to verify our hypothesis.

**Figure 10.** The same as Figure 9, but for SG-La Niña and equatorial La Niña.

#### **5. Conclusions**

The previous research pointed out that the combined effect of ENSO and NAO usually results in even more significant impacts than their individual effects [80,81]. It is, therefore, necessary to clarify their potential linkage. In this study, we classified the ENSO events according to their amplitude and east-west gradients in the Pacific. Strong ENSOs commonly contain a sizeable east-west gradient, and only the weak-intensity ENSOs can be separated based on the east-west gradient of the WSG and equatorial events. For the El Niño events, the SEN features a salient east-west gradient in the Pacific with the warming SST in the tropical IO (Figure 2a). The WSGEN also exhibits a strong gradient in the Pacific but without the significant warm SSTA in the IO (Figure 2b). The equatorial EN is characterized by the salient warm SSTA in the equatorial CP and EP, as well as in the tropical IO (Figure 2c). For the La Niña events, their spatial patterns in SST are roughly opposite to their EN counterparts (Figure 2d–f). In addition, the maximum SSTA for SLN and WSGLN centers in the equatorial CP but moves to the EP for the equatorial LN.

Next, we demonstrated that atmospheric anomalies in response to different ENSO types exhibit large discrepancies over the North Atlantic region (Figures 5 and 6). Only the WSGEN and SGLN events can produce NAO-like patterns through the eastward elongation of the two branches of the jet streams. During the WSGEN (SGLN) winter, the salient warm (cool) SSTA in the CP and EP and the opposite anomalies in the WP induce enhanced (weakened) subtropical and weakened (enhanced) mid-latitude westerly jets, leading to a negative (positive) NAO-like anomaly. By contrast, the NAO-like anomaly cannot be detected in other ENSO cases. The further study implied that the warm SSTA in the IO (Figure 7), which triggers abundant precipitation in the tropical IO during the SEN and equatorial EN winters, generates a positive NAO pattern that offsets the effect of El Niño signals in the Pacific region, inducing an NAO-neutral anomalies pattern. However, the IO SSTA cannot modulate the relation of LN and NAO owing to the weak forcing of the cool IO SSTA on local precipitation and remote teleconnections. The SGLN's maximum SSTA is located over the CP, where the climatology SST exceeds the threshold for exciting deep convective motion. The negative SSTA in the CP consistently influences atmospheric circulation, resulting in a response over the North Atlantic that resembles a positive NAO. In comparison, the maximum SSTA of equatorial LN centers in the EP, where the climatology SST is below the threshold for deep convection. The atmospheric response over the North Atlantic shows a neutral pattern.

This study further illustrates the important roles ENSO diversity plays in affecting atmospheric variability over remote regions, especially over the North Atlantic. The atmospheric/coupled general circulation model may offer better analysis for the seasonal prediction of the NAO when considering the ENSO diversity according to its amplitude and the zonal gradient in SST over the Pacific.

Other than the roles of the troposphere, tropical warming excites a poleward-propagating Rossby wave train, which can also extend upward and reach the stratosphere, resulting in a weaker polar vortex that drives a negative NAO anomaly over the NA–Eurasia. In subsequent studies, we would like to examine whether or not SG-ENSO can affect the NAO through stratospheric pathways. Moreover, we will try to classify the summertime ENSO events and inspect their potential forcing effects on extra-tropical atmospheric anomalies.

**Author Contributions:** Conceptualization, P.Z. and Z.W.; methodology, P.Z.; software, P.Z.; validation, P.Z. and Z.W.; formal analysis, P.Z.; investigation, Z.W.; resources, P.Z.; data curation, P.Z.; writing—original draft preparation, P.Z.; writing—review and editing, Z.W.; visualization, P.Z.; supervision, Z.W.; project administration, Z.W.; funding acquisition, Z.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is jointly supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 91937302) and the Ministry of Science and Technology of China (Grant Nos. 2019QZKK0102, 2019YFC1509100 and 2017YFC1502302).

**Data Availability Statement:** The ERA-40 and ERA-interim datasets identified and investigated in this study are provided by ECMWF websites: https://www.ecmwf.int/en/forecasts/datasets/ browse-reanalysis-datasets; The SST data from: https://psl.noaa.gov/data/gridded/data.noaa.ersst. v5.html and https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html; The precipitation data from: https://psl.noaa.gov/data/gridded/data.prec.html. The ERA-40 data is accessed on 1 June 2017, other datasets are accessed on 20 March 2020.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

### **References**


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