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In section 4, the impacts from both Niño-4 and tropical western Pacific SSTA on the WNPSM have been analyzed. Through the comparison among Figs. 2c, 2e, 2g, and Figs. 3c, 3e, 3f, it is evident that the SSTA related to the WNPSM, located in Niño-4 and tropical western Pacific, do not always work in accordance, namely not always with a significant negative (positive) SSTA in the tropical western Pacific and positive (negative) SSTA in the Niño-4 area. It then becomes necessary to further analyze when both have worked in coherence and when the tropical western Pacific SSTA and Niño-4 area worked independently to exert an influence on the WNPSM, respectively.
As shown in Table 1, we define the year with a value (below) beyond (minus) 0.5 standard deviation as a (negative) positive anomaly year for the TWPI and Niño-4 series, respectively. Because the SSTAs in the Niño-4 and tropical western Pacific regions always present a dipole pattern in the climatological mean, there is no need to study "Negative & Negative”, “Positive & Positive”, and “Normal & Normal” types of SSTA associated with the TWPI and Niño-4. Rather, the necessary combinations are displayed in Table 1. During the years 1994, 1997, 2002, 2004, 2015, and 2019, the negative SSTAs in the tropical Western Pacific and positive SSTAs in Niño-4 area could work in accordance to influence the WNPSM; during the years 1996, 1998, and 2010, the positive SSTA in the tropical Western Pacific and negative SSTA in Niño-4 area, also could work in accordance to influence the WNPSM. During 2013, 2014, 2016, and 2017, the positive SSTA anomaly in the tropical western Pacific could have worked alone to influence the WNPSM; during the years 1999, 2008, and 2011, the negative SSTA anomaly in Niño-4 area could have worked alone to exert impacts on the WNPSM. Due to an absence of (or too few) samples, the types of “Negative & Normal” and “Normal & Positive” cases are not discussed.
Types Years Negative & Positive 1994, 1997, 2002, 2004, 2015, 2019 Positive & Negative 1996, 1998, 2010 Negative & Normal No Positive & Normal 2013, 2014, 2016, 2017 Normal & Negative 1999, 2008, 2011 Normal & Positive 2009, 2018 Table 1. The six combinations of years with different anomalies of the TWPI and Niño-4, positive (negative) anomalies are defined as beyond (below) the (minus) 0.5 standard deviation.
Based on the years 1994, 1997, 2002, 2004, 2015, and 2019, we perform composite analyses of the SST and horizontal wind fields to explore the combined effect of the negative SSTA in the tropical Western Pacific and positive SSTA in the Niño-4 area on the WNPSM (Fig. 8). As shown in Fig. 8a, a positive SSTA in the Niño-4 area is reproduced, but the SSTAs are negative in the tropical western Pacific during 1994, 1997, 2002, 2004, 2015, and 2019 years.
The wind anomalies in Figs. 8b and 8c bear a great resemblance with those displayed in Figs. 5d and 5e, respectively, which are also more similar to Figs. 7b and 7c, compared to those displayed in Figs. 6b and 6c. Relative to the 500-hPa geopotential height anomalies and 2-m temperature anomalies, the circulation anomaly in Fig. 8d is less similar to that in Fig. 5f, with a less significant temperature anomaly and a more significant geopotential height anomaly in the northern-most pole of PJ-like pattern. From the circulation anomalies in Figs. 8b and 8c, both the positive phase PJ-like pattern and Gill response can be discerned. The anomalous years 1994, 1997, 2002, 2004, and 2015 are all El Niño developing years, except for 2019, based on the criteria that the SSTA must be less (greater) –0.5°C (0.5°C) for six consecutive months, including June, July, and August. So, during the El Niño developing year in P2, the SSTA is positive in Niño-4 and negative in tropical Western Pacific, both of which can work in coherence to influence the WNPSM by an atmospheric Gill response connected with the SSTA in Niño-4 and an anomalous PJ-like circulation driven by the western tropical Pacific. As shown in Fig. 8e, an anomalous anti-Walker circulation, located between the Niño-4 area and the tropical western Pacific, helps to maintain the negative SSTA in the tropical western Pacific and a positive SSTA in the Niño-4 area. As displayed in Fig. 9, the statistical analyses can also be reproduced by the sensitivity experiment when the SSTAs are added in the tropical Western Pacific and Niño-4 area (marked in Fig. 8a). The PJ-like pattern and the Gill response can also be approximately reproduced at 850 hPa (Fig. 9a); however, only the Gill response can be reproduced at 200 hPa (Fig. 9b).
Figure 8. The composite analyses of the (a) SST, (b) 850-hPa horizontal wind (units: m s–1), (c) 200-hPa horizontal wind (units: m s-1), (d) 500-hPa geopotential height (units: gpm) and 2-m temperature (units: °C), (e) u-component of wind and omega (multiplied by –30) based on 1994, 1997, 2002, 2004, 2015 and 2019 years. Red vectors, red contours, and dots represent statistical significance at the 95% confidence level. Light-to-dark shadings represent statistical significance at the 90%, 95%, and 99% confidence levels. Violet shading denotes the Tibetan Plateau.
Figure 9. The difference between the sensitivity experiment and the control experiment, when the negative SSTA is added in the tropical western Pacific and the positive SSTA is added in Niño-4 (marked by a rectangle in Fig. 8a). For a detailed description, see section 2. Horizontal wind differences at (a) 850 hPa and (b) 200 hPa (units: m s–1) are shown. Red vectors indicate key anomalies.
Based on 1996, 1998, and 2010, we conducted the composite analyses of SST and horizontal wind fields to explore the combined effect of the positive SSTA in the tropical western Pacific and negative SSTA in Niño-4 area on the WNPSM (Fig. 10). As shown in Fig. 10a, the reproduced SSTA is negative in the Niño-4 area, but positive in the tropical western Pacific during 1996, 1998, and 2010.
The wind anomalies in Figs. 10b and 10c present the negative phase of a PJ-like pattern, with the opposite phase to those displayed in Figs. 8b and 8c. Most notably, the negative PJ-like circulation anomalies at 200 hPa do not satisfy a 95% confidence level and also lack the polarity near the equator (Fig. 10c). In terms of the 500-hPa geopotential height anomaly and 2-m temperature anomaly, the PJ-like pattern is much too weak (Fig. 10d). As shown in Fig. 10e, an anomalous Walker circulation can be seen between the tropical western Pacific and tropical central Pacific, that helps to maintain the positive SSTA in the tropical western Pacific and negative SSTA in Niño-4 area. The anomalous years are La Niña developing years, except for 1996, thus in La Niña developing years in P2, the SSTA is negative in Niño-4 and positive in the tropical western Pacific, both of which can work in coherence to influence the WNPSM, showing obvious symmetry, compared to Fig. 8. Because of the prominent symmetry, there is no need to carry out the opposite sensitivity experiment to Fig. 9.
As shown in Table 1, 2013, 2014, 2016, and 2017 are positive anomalous years of the tropical western Pacific SSTA and Niño-4 normal year. Based on these years, the composite analyses help to isolate the independent factors responsible for its influence on the tropical western Pacific on the WNPSM. As shown in Fig. 11a, anomalous SSTAs existed in the tropical western Pacific during these years, and no significant anomalies over the Niño-4 area were reproduced. Figure 11b shows that there is also a negative phase PJ-like wave at 850 hPa when there is a positive SSTA in the tropical western Pacific. At 200 hPa, Fig. 11c also bears some similarities to Figs. 5e and 7c, only out of phase. The geopotential height and temperature anomalies in Fig. 11d can also reproduce the out-of-phase circulation pattern anomalies, compared to Fig. 5f. As shown in Fig. 11e, anomalous ascending motion appears in the tropical western Pacific. The analyses about the “Positive & Negative” type, namely a positive SSTA in the tropical western Pacific and a normal SSTA in Niño-4, show that tropical SSTA can influence the WNPSM by PJ-like wave. When the SSTA in the tropical western Pacific is positive (negative), a negative (positive) phase PJ-like wave appears and weakens (strengthens) the WNPSM (Figs. 7a–f, 10a–e).
As shown in Table 1, 1999, 2008, and 2011 are normal years for a tropical western Pacific SSTA and negative SSTA in Niño-4. Composite analyses based on these years help isolate the independent, influential factors from Niño-4 on the WNPSM. As shown in Fig. 12a, negatively anomalous SSTAs in the Niño-4 region with no significant anomalies over the tropical western Pacific are reproduced during these years. Figure 12b shows an abnormal anticyclone to the north and south of the equator at 850 hPa; however, at 200 hPa, these features develop into cyclones (Fig. 12c), which further confirms that the Niño-4 influences the WNPSM by a Gill response, consistent with the results obtained in Figs. 6b and 6c. The geopotential height and temperature anomalies in Fig. 12d also present weaker circulation anomalies compared to Fig. 6d. At the same time, the anomalous Walker circulation is very weak, with weak descending motion over the Niño-4 area (Fig. 12e). The years of 1999, 2008, 2011 are featured CP La Niña developing events, in which the negative SSTA in Niño-4 area works alone on the WNPSM. When the SSTA in Niño-4 is negative (positive), the abnormal 850-hPa anticyclone (cyclone) in WNP can weaken (enhance) the WNPSM (Figs. 6a–f, 12a–e). Besides, a BBC-like wave appears within 40°–80°N, along with the occurrence of Gill response (Fig. 12b, c), and whether the BBC plays a role in influencing the WNPSM warrants further investigation in future studies.
On the whole, during the El Niño (La Niña) developing year in P2, the negative (positive) SSTA in the tropical western Pacific and positive (negative) SSTA in the Niño-4 area, could work together to influence the WNPSM, by enhancing (weakening) the WNPSM, by a positive (negative) phase PJ-like wave, and/or through a Gill response. In some years, the positive SSTA in the tropical western Pacific can work alone to influence the WNPSM in P2, weakening the WNPSM by the negative phase of a PJ-like wave. During CP La Niña developing years, the negative SSTA anomaly in the Niño-4 area can work alone to impact the WNPSM in P2 by weakening the WNPSM by a Gill response associated with an anomalous anticyclone in the WNP. The above analyses further confirm that SSTA in the tropical western Pacific (Niño-4) can exert an influence on the WNPSM through a PJ-like wave (Gill response), consistent with the results displayed in section 4.
Types | Years |
Negative & Positive | 1994, 1997, 2002, 2004, 2015, 2019 |
Positive & Negative | 1996, 1998, 2010 |
Negative & Normal | No |
Positive & Normal | 2013, 2014, 2016, 2017 |
Normal & Negative | 1999, 2008, 2011 |
Normal & Positive | 2009, 2018 |