The relationship between SST and the detail of heat flux is further analyzed. The result shows that the latent heat flux is the important factor, which warms the SST.
Fig. 21 shows the lead-lag regression map of SENP tendency and latent heat flux. The pattern of latent heat flux (fig. 21) is similar to that of total net heat flux (fig. 19). In fig.21g, the maximum positive latent heat flux accompanies the strongest cyclonic circulation with the southwesterly wind is located in the SENP region.
The climatology of low-level wind is the northeasterly wind in SENP and shows in Fig. 2. The background of low-level wind may reduce the latent heat flux to go to the atmosphere by Wind–evaporation–SST mechanism (Xie and Philander, 1994). The regression map shows a cyclonic circulation which is southwesterly wind in the SENP.
This southwesterly wind against the climatological northeasterly wind and reduce the wind speed, therefore, the latent heat flux warm the SST. The warm SST further enhances the southwesterly wind.
The SST warming in SENP is through Wind–evaporation–SST mechanism, which is similar to that of PMM. This is associated with the background of low-level wind.
The SST pattern of global warming also exhibits relative large warming in SENP. It is suggested that the SENP warming may be more important. We will discuss it in section 7.
Ch6 Impact of SENP on ENSO and TC
We will show the impact of SENP SST on ENSO and tropical cyclone (TC) activity in the two case studies, which hinders the development of 2014 El Niño (Wu et al., 2018a) and enhances the 2016 TC activity in western North Pacific (Wu et al., 2018b). Notably, the definition of SENP region is similar but not totally the same because it is case study. However the result is similar which is suggested the SENP SST is not sensitive.
6.1 ENSO: Meridional Dipole of SSTA and Associated Cross-Equatorial Flow in the Tropical Eastern Pacific in Terminating the 2014 El Niño Development
6.1.1 Comparison between 2014 Conditions and the 1997 El Niño Event
Fig. 22 presents a Hovmüller diagram (averaged over 2°S–2°N) of the SSTA (contour) and SSTA tendency (shaded) in 1997 and 2014. During the late boreal winter to the ensuing spring (February–April), the Niño 3 SST growth rates in 2014 and 1997 were approximately 0.35 and 0.45 K/month, respectively. The SSTA growth rate in 2014 was pronounced and was similar to that in 1997. The SSTA grew continually in the boreal summer of 1997; however, the SSTA growth terminated and SSTA tendency rapidly changed from positive to negative (−0.1 K/month) in the boreal summer of 2014. Such a decline in SSTA tendency is responsible for the prevention of El Niño development in the boreal summer of 2014. Fig. 23 depicts the spatial distributions of the anomalous SST and low-level wind in the boreal summer [i.e., June–August (JJA)] of 1997 and 2014. A significant positive SSTA in the equatorial EP and pronounced westerly wind anomalies in the equatorial western Pacific were clearly identified in 1997 (Fig. 23b). In contrast, a weak positive SSTA and westerly anomaly were observed in 2014 (Fig. 23a). Among the differences between 1997 and 2014 conditions, the north–south dipole of SSTA in 2014 [i.e., the
positive (negative) SSTA in the northeastern (southeastern) subtropical Pacific] was particularly notable (see the boxes of ENP and ESP in Fig. 23a). This distinction was further evidenced by the meridional profiles of SSTA in the JJA of 1997, 2014, and the composite El Niño (Fig. 24). The SSTA in both 1997 and the composite El Niño were symmetric to the equator; that is, the meridional SSTA gradient between the approximately located. The cross-equatorial flow in the ENP is an unstable mode of the air–sea coupled system in the tropical EP, which is closely associated with the annual cycle and the geometries of continents (Chang and Philander 1994; Li and Philander 1996). Details of the physical processes responsible for the asymmetry of the cross-equatorial flow were provided by Chang and Philander (1994). The enhanced cross-equatorial flow led to a rainfall surplus in the ENP (not shown), exceeding normal levels by approximately 30%. Further analysis revealed that the anomalous rainfall associated with diabatic heating generated a meridional overturning circulation anomaly, ascending (descending) in the eastern North (South) Pacific (not shown), in which the low-level returning flows had a positive feedback to enhance the cross-equatorial flow.
In the following, we show the relationship between the SSTA dipole and the associated cross-equatorial flow. Fig. 25a shows the surface wind stress regressed on the SSTA dipole index in boreal summer (i.e., JJA); here the SSTA dipole index was defined as the SSTA in the ENP minus the SSTA in the ESP (boxes in Fig. 23a). It is clearly shown that the SSTA dipole matched well with the cross-equatorial flow, characterizing by a southeasterly anomaly in the Southern Hemisphere and a southwesterly anomaly in the Northern Hemisphere. A westward low-level wind anomaly in the equatorial central–eastern Pacific was clearly observed during the
positive SSTA dipole phase. As shown in Fig. 25b, the SSTA dipole index and the cross-equatorial flow (whose strength is measured by the 925 hPa meridional wind anomaly averaged over 80°W–140°W, 10°S–10°N) show a positive correlation relationship (the correlation coefficient is approximately 0.65, which is at the 95% ESP but also the contribution of warm SSTA over ENP. Additionally, it is noted that the SSTA dipole index is statistically negative correlated with Niño 3 index (cr =
−0.43). This indicates that the oceanic conditions associated with the SSTA dipole might not favor the El Niño development.
6.1.2 Role of the SSTA Dipole and the Cross-Equatorial Flow in the EP
The role of cross-equatorial flow in suppressing the El Niño development through modulating the oceanic conditions was documented in this section. During the boreal spring, a marked positive temperature anomaly along the thermocline was clearly observed in 2014 and 1997 (Fig. 26a, b, respectively). The temperature perturbation in the thermocline was associated with a deepening D20 (i.e., depth at 20 °C) and eastward current anomalies in the upper ocean, providing the optimal oceanic conditions for El Niño development. Oceanic conditions favoring El Niño development were persistent in the boreal summer of 1997 (Fig. 26d); however, the oceanic conditions have changed in the boreal summer of 2014, e.g., the upper oceanic current anomalies turned from eastward to westward in response to the westward surface wind stress anomaly in the equatorial central–eastern Pacific (Fig.
26a, c). This westward surface wind stress anomaly may induce westward and
upwelling oceanic current anomalies in the equatorial central–eastern Pacific (Fig.
26c), blocking the Bjerknes positive feedback and hence preventing El Niño development. In the following, we conduct the oceanic mixed layer heat budget to reveal how the enhanced cross-equatorial flow hinder El Niño development through regulating the three-dimensional (3D) oceanic currents.
Firstly, corresponding to the decrease in the positive SSTA in JJA compared to that in MAM, the magnitude of the negative heat flux become weaker in JJA than MAM (i.e., larger positive SSTA generally corresponds to larger negative heat flux term, and vice versa), the thermodynamic term associated with the surface heat flux makes a negative contribution to the decrease of SSTA tendency (not shown) from boreal spring to summer. Thus, in the following we mainly focused on the dynamic terms, which are essential in determining the SSTA tendency during El Niño development (Li 1997; Jin and An 1999; Chen et al. 2015, 2016a). These important dynamic terms are associated with Bjerknes feedback processes, including the advection of mean temperature by anomalous zonal current ( −u'∂T / ∂x , hereinafter anomalous zonal advection term), the advection of anomalous temperature by mean upwelling
'/
w T z
− ∂ ∂ , hereinafter mean upwelling advection term), and the advection of mean temperature by anomalous upwelling ( −w'∂T / ∂z , hereinafter anomalous upwelling advection term).
Fig. 27 illustrates a comparison of the heat budget results of 1997 and 2014. The bars, from left to right, represent the mixed layer temperature tendency, the anomalous zonal advection term, anomalous upwelling advection term, mean upwelling advection term, and anomalous meridional advection term. The temperature tendency in the boreal spring of 2014 was approximately 0.35 K/month, showing a comparable magnitude to that of 1997. Here all the four dynamic terms made a positive contribution to the positive temperature tendency. Nevertheless, the four dynamic terms either changed from positive to negative or decreased substantially in the boreal summer of 2014, resulting in negative temperature tendency. In contrast, the positive temperature tendency and associated Bjerknes
feedback processes were sustained in the boreal summer of 1997. The weakening in the oceanic dynamic terms that lead to the termination of positive temperature tendency in the boreal summer of 2014, could be traced back to the anomalous cross-equatorial flow in the boreal summer of 2014. The chain of physical causation is described as follows. (1) The cross-equatorial flow induced westward surface wind stress anomaly near the equator, driving an anomalous westward current (u < ) and ' 0 upwelling (w > ) in the equatorial central–eastern Pacific. Consequently, negative ' 0 zonal (− ∂u T' /∂ <x 0) and vertical (−w T'∂ /∂ <z 0) temperature advections were produced. (2) D20 shoaled and returned to the climatological level because of the anomalous westward and upwelling currents; the temperature perturbation in the thermocline decreased, and thus the mean upwelling advection term (− ∂w T'/∂z) weakened substantially. (3) The northward current anomaly (v > ) that is also ' 0 induced by anomalous cross-equatorial flow, transported a substantial quantity of climatological cold water from the South Pacific to the equator ( −v'∂T / ∂y < 0 ), especially over the region east of 90°W (Fig. 28), contributing to a decrease in temperature tendency.
This explains how the cross-equatorial flow modulated the 3D ocean currents and then prevented the 2014 El Niño development. In next section, we further investigate such a possible mechanism through a set of numerical experiments.
6.1.3 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 and 1997–1998 on the western North Pacific monsoon and tropical cyclone activity: Role of subtropical eastern North Pacific warm SSTA
6.2.1 Distinctions between 2016 and 1998
6.2.1.1 Summer monsoon and TC activity in the WNP
Fig. 32 provides a comparison of the TC genesis number and summer monsoon intensity in the WNP between 1998 and 2016. We focused on the El Niño decaying summer because the major distinctions between the two strong El Niño events primarily occurred in decaying summer. A positive low-level (850 hPa) streamfunction anomaly (i.e., WNPAC, shading in Fig. 32a) was clearly observed in June, July and August (JJA) 1998. The WNP monsoon trough (black dashed line, Fig.
31a) was weakened and retreated westward to approximately 120°E in response to the WNPAC. In contrast to 1998, a negative streamfunction anomaly was observed in JJA 2016 (Fig. 32b). The monsoon trough was strengthened and extended eastward to 140°E because of the negative streamfunction anomaly in the WNP. The enhancement of the WNP monsoon trough provided a favorable environment for TC genesis, and it possibly resulted in an above-normal TC genesis number in JJA 2016 (Fig. 32c); contrastingly, the weakening of the WNP monsoon trough was unfavorable for TC genesis, and a below-normal TC genesis in JJA 1998 was observed. Among the distinctions, the increase of TC genesis number in August and September 2016 was particularly high, approximately 1 σ higher than the climatological mean. Because the boreal summer was the focus, the JJA mean was considered in the analysis. Overall, the distinctions of TC-genesis number between 1998 and 2016 in JJA, typhoon season mean (JJASON), and even the entire year exhibits similar results (Table 2).
In addition to the distinct TC activity, the WNP summer monsoon index (Wang and Fan, 1999), a measure of monsoon strength, exhibited a significant difference between 1998 and 2016 (Fig. 32d). Whereas both events showed consistency in the delay of monsoon onset, the WNP summer monsoon was much stronger in 2016 than in 1998.
Particularly, the WNP summer monsoon in 2016 was stronger than the climatological
mean after August. Because the Niño 3.4 index was approximately the same during both strong El Niño events, the SSTA other than the Niño 3.4 region, is likely a contributor if the oceanic forcing is a candidate for the distinct effects on the WNP’s climate. In the next section, we will demonstrate that the positive SSTA in the SENP is crucial for resulting in these distinctions.
6.2.1.2 Large scale circulation and SST anomalies
A comparison of the SSTA in the decaying summers of 1997–1998 and 2015–2016 revealed that a southwest–northeast-tilted positive SSTA in the SENP, which resembles the PMM, was apparent in 2016 (Fig. 33). This subtropical positive SSTA was accompanied by a southwesterly anomaly in the subtropical eastern North Pacific (~150°W, 20°N). The southwest–northeast-tilted SSTA was not observed in JJA 1998 (Fig. 33). It is evident that the major difference in large scale atmospheric and oceanic conditions between 2016 and 1998 was featured by a cyclonic anomaly in the WNP, which resembles the southwest–northeast-tilted PMM-like SSTA.
Because the PMM is an air–sea coupling mode phase-locked with the annual cycle (Chiang and Vimont, 2004), singular value decomposition (SVD) of the covariance of the SSTA and low-level wind in the region 80°–180°W, 30°S–40°N was analyzed to investigate the possible air–sea coupling processes correlated with these distinctions.
The leading mode exhibited an El Niño-like pattern, positive equatorial eastern SSTA-associated equatorial westerly anomalies in the tropical eastern Pacific (Fig.
34a, referred as ENSO mode hereafter). The second mode resembled the PMM structure [the correlation coefficient between PMM index (Chiang and Vimont, 2004) and PC2 is 0.91], a meridional dipole-like SSTA accompanied by a cross-equatorial flow in the eastern Pacific (Fig. 34b, referred as PMM mode hereafter). These two leading models explain approximately 88% of the total covariance of the SSTA and low-level wind.
The time series of the expansion coefficient of the leading and second modes (referred to as PC1 and PC2 hereafter) revealed that PC1 had approximately the same
amplitude during the lifecycle between 1997–1998 and 2015–2016 (Fig. 34c, 34e).
The amplitude of ENSO (PC1) was nearly identical between the two strong events,
The amplitude of ENSO (PC1) was nearly identical between the two strong events,