Numerical experiment

To test the role of the north–south dipole of SSTA in preventing the 2014 El Niño development, a series of numerical experiments were conducted by employing a climate system-coupled general circulation model (CGCM). This CGCM was named FGOALS-g2 (Li et al. 2013) and was applied in the Coupled Model Intercomparison Project Phase 5 (CMIP 5); it showed excellent performance in ENSO simulation among both CMIP3 and CMIP5 models (Chen et al. 2013, 2016b; Bellenger et al.

2014; Chen and Yu 2014; Yu et al. 2014).

Through an SSTA restoring method (e.g., Luo et al. 2005; Yan et al. 2009, 2010), the SSTA field of the CGCM was nudged to the OISST daily SSTA field at each time step from 1982 to 2014. Through this coupled nudging approach, the model could represent realistic SSTA field and relatively reasonable ENSO-related anomaly fields (e.g., zonal wind stress anomaly and thermocline depth anomaly). Employing the outcome from the aforementioned nudging approach as the initial condition field at a certain time, namely May 1, 2014, the CGCM was freely integrated without the SSTA-nudging scheme (i.e., the nudging scheme is closed when running the CGCM) since May 1, 2014, which is referred to as the control hindcast experiment (hereinafter CTL). In the CTL hindcast experiment, the predicted Niño 3 SSTA increased during May 2014, but decreased since June 2014 (solid blue curve in Fig. 30), indicating that this CGCM is able to reproduce the transition feature of the SSTA (i.e., the SSTA started to decrease from June 2014).

Parallel to the CTL experiment, a sensitivity experiment, named “non-dipole”, was performed. All steps in non-dipole were same as those in CTL, except for the generation of the initial condition in the nudging step. Specifically, in the non-dipole experiment, the SSTA field of the CGCM was nudged to a modified SSTA field as shown in Fig. 29b. The modified SSTA field was same as the previously observed daily SSTA field used for nudging in CTL (Fig. 29a), except that the north–south dipole of SSTA was removed during March–April in 2014. Therefore, we obtained a new initial field on May 1, 2014, in which the warming over the eastern equatorial Pacific remained the same as that of CTL, but the meridional asymmetric SSTAs over ENP and ESP were removed (similar to the SSTA pattern in Fig. 29b). Using this new initial field to integrate the CGCM from May 1, 2014, we conducted the non-dipole experiment. Notably, the Niño 3 SSTA increased during May 2014 and continued to grow throughout 2014 (solid red curve in Fig. 30). This SSTA evolution feature in the non-dipole experiment was distinctly different from the SSTA transition feature in the CTL experiment, indicating that the SSTA dipole played a critical role in the

prevention of the 2014 El Niño event.

The relative contribution of the southeastern-pole and northeastern-pole SSTAs in hindering the Niño3 SST growth rate was evaluated by two additional experiments, namely, non-ENP-pole and non-ESP-pole. The non-ENP-pole (non-ESP-pole) experiment was conducted in the same way as that for the non-dipole experiment, except that the SSTAs over ENP (ESP) were removed from the target nudging field, as shown in Fig. 29c, d. Details of the design of the numerical experiments are shown in Table 1. The numerical experiment results revealed that the southeastern-pole SSTA alone appeared to be insufficient in terminating the El Niño development (see non-ENP-pole experiment: dashed purple curve in Fig. 30) compared with the result in CTL experiment; the northeastern-pole SSTA had a considerable effect on suppressing the SST growth rate in Niño 3 (see non-ESP-pole experiment: dashed brown curve in Fig. 30). The impact of northeastern pole SSTA on suppressing the El Niño’s growth seems to be comparable to that due to the southeastern pole (comparing the dashed brown curve with the dashed purple curve in Fig. 30).

Additionally, the ENP pole’s dynamical role was examined through comparing the non-ENP-pole experiment with CTL experiment (non-ENP-pole minus CTL). It is found that without the ENP pole’s role, the cross-equatorial flow in the summer was suppressed especially to the north of the equator and the easterly wind was weakened near the equator (not shown). This indicates the warm SSTA over ENP plays a role in reinforcing the cross-equatorial flow, which could modulate the oceanic temperature advection terms and regulate the ocean temperature. To sum up, the experiment results indicate that both the warm SSTA over ENP and the cold SSTA over ESP make a contribution to the suppression of El Niño’s development in boreal summer of 2014.

It’s worth mentioning that one recent study (Zhu et al. 2016) also focused on the role of off-equatorial SSTA in the 2014 El Niño’s growth. Through conducting a series of hindcast experiments with CFSv2 (Saha et al. 2010; Zhu and Shukla 2013;

overestimated amplitude) in peak phase of 2014 El Niño is due to the lack of prediction of negative SSTA over southeastern Pacific. In the view of the impact of the negative SSTA over ESP on suppressing the 2014 El Niño’s development, the experiment results in the present study is generally consistent with that in Zhu et al.

(2016). In addition, Zhu et al. (2016) also found that the positive SSTA over the tropical western North Pacific (120°E–180°E, 10°S–20°N; hereafter TNWP) region could cause the increase of predicted amplitude (this means the positive SSTA over TNWP may be favorable for 2014 El Niño development). However, such TNWP region is entirely different from the ENP region analyzed in this study.

Another relevant study (Su et al. 2014) suggested the termination of 2012 Pacific warming is attributed to the cold SSTA over both the northern and the southern subtropical Pacific. The role of cold SSTA over the southeastern Pacific in suppressing the 2012 Pacific warming is generally consistent with the results in the present study. Despite of cold SSTA during boreal summer of 2012 over the northern subtropical region (which is mainly confined to the west of 125°W), a warm SSTA signal is noted over the region of 80°–125°W, 10°–20°N (see Fig. 4c, d in Su et al.

2014), which overlaps most of the ENP region in this study (80°–140°W, 10°–20°N).

Therefore, more in-depth study with idealized numerical experiments may be needed to carefully examine the specific impacts of the different SSTA signals over different northern off-equatorial regions on the special case in 2012.

The mechanism of meridional dipole of SSTA terminating 2014 El Niño was presented in the section 6.1. The main results are also illustrated in the schematic diagram (Fig. 31). In next subsection, we will show another case, the TC activity, which was influenced by SENP warm SST.

6.2 Tropical cyclone: Distinct effects of the two strong El Niño events in 2015–2016