Combined impact of in-phase and out-of-phase variation between the northern East Asian low and western North Pacific subtropical high on East Asian summer rainfall

Abstract East Asian summer rainfall is affected by both the continental northern East Asian low (NEAL) and the western North Pacific subtropical high (WNPSH) in the lower troposphere. This study investigates the joint effect of the two circulation factors on East Asian summer rainfall. It is found that the rainfall in East Asia behaves differently in the years with in-phase and out-of-phase variation between the NEAL and WNPSH. When the NEAL and WNPSH vary in phase, i.e. when they are both stronger, the rainfall anomaly shows a dipole pattern in East Asia and displays opposite changes between north and south of 30°N. When the two circulation factors vary out of phase, the rainfall anomaly is concentrated in the Yangtze River valley.


Introduction
East Asian summer monsoons and rainfall are affected by the land-sea thermal contrast between continental East Asia and the North Pacific Ocean (Guo 1983;Shi and Zhu 1996;Zhao and Zhou 2005;Zhu et al. 2005). The thermal contrast is characterized by a continental low pressure system that is centered over northern East Asia (NEAL) and a subtropical high over the western North Pacific (WNPSH) in the lower troposphere (Figure 1(a)). Previous studies have found that the WNPSH and NEAL both have a crucial role in affecting East Asian summer rainfall (e.g. Li 2000a, 2000b;Lu 2001;Lu and Dong 2001;Gong and Ho 2002;Wu et al. 2010;Shen et al. 2011;Ren, Yang, and Sun 2013;Lin 2014;Huangfu, Huang, and Chen 2015;Lin and Wang 2016). For example, Li (2000a, 2000b) showed that a westwardly extended WNPSH can cause heavier rainfall along the Yangtze River valley and droughts in southeast China in May and June. Lin and Wang (2016) found that an enhanced NEAL can increase the rainfall in northern East Asia and shift the subtropical East Asian rain belt northward.
However, the relationship between the NEAL and WNPSH and their joint impact on East Asian summer rainfall have yet to be well studied. This study tries to explore this issue by investigating the impact of in-phase and outof-phase variation between the NEAL and WNPSH on East Asian summer rainfall. The arrangement of the text is as follows. Section 2 introduces the data and methods used in this study. Section 3 presents the results, and the key findings are summarized in Section 4.

Data and method
The monthly geopotential height data used in this study are derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets (Kalnay et al. 1996) at a resolution of 2.5° × 2.5°. The monthly precipitation data used include the TS3.22 data-set provided by the Climatic Research Unit (CRU), the University of East Anglia, UK, with a spatial resolution of 0.5° × 0.5° (Harris et al. 2014) and the 160-station observed rainfall for continental China provided by the National Climate Center of the China Meteorological Administration. All of the data used are from the period 1968-2013.
In this study, to obtain the interannual variability, both the long-term trends and decadal variations have been removed by eliminating the components with periods larger than eight years using Fourier harmonic analysis. Student's t-test is used to determine statistical significance.

In-phase and out-of-phase variation between the NEAL and WNPSH
To depict the interannual variations of the NEAL and WNPSH, we define the NEAL and WNPSH indices using the 850-hPa geopotential height (H850) following previous studies (Lu 2002;Lin and Wang 2016). The NEAL index (NEALI) is defined as the reversed H850 anomaly averaged over the region (45°-60°N, 110°-130°E) and H850 anomaly over the region (10°-30°N, 110°-150°E) for the WNPSH index (WNPSHI). The two core regions are depicted by the two boxes in Figure 1(a). The NEAL enhances when the
To investigate the combined effect of the two circulation factors, we classify the NEAL and WNPSH into four types according to the two indices ( Figure 2). Phases 1 and 3 represent the in-phase variation (simultaneous enhancement Asia and a positive H850 anomaly over the western North Pacific Ocean are identified, enhancing the NEAL and WNPSH, respectively. The enhanced NEAL and WNPSH can also be indicated by a southward expansion of the H850 contour of 1432 gpm (solid blue line) and a westward extension of the H850 contour of 1510 gpm (solid orange line), compared with the climatology (dashed lines). The pattern in Phase 3 is in opposition to Phase 1, with both the NEAL and WNPSH weakened (Figure 3(b)). The H850 difference between Phases 1 and 3 shows a significant negative anomaly centered over continental northern East Asia and a positive anomaly over the western North Pacific Ocean (Figure 3(e)). That is, Phases 1 and 3 depict an in-phase change of the NEAL and WNPSH, in which the NEAL and WNPSH simultaneously enhance in Phase 1 and weaken in Phase 3. or weakening) of the NEAL and WNPSH; Phases 2 and 4 represent the out-of-phase change (one enhancing, the other weakening) of the NEAL and WNPSH. The years included in each of the four phases are presented in Table 1. During 1968-2013, there are 11 and 9 years in Phases 1 and 3, and 11 and 15 years in Phases 2 and 4, respectively. The number of out-of-phase years (26) is slightly larger than that of in-phase years (20), which is consistent with the weak and negative correlation coefficient (-0.22) between the NEALI and WNPSHI. We also chose the cases based on the criterion of the NEALI and WNPSHI anomalies being stronger than ±0.5 times the standard deviations and obtained similar results (not shown). Figure 3 shows the spatial distribution of the composited H850 for the four phases. In Phase 1 (Figure 3(a)), a negative H850 anomaly over continental northern East WNPSH, and confirmed that they are manifested by the East Asia-Pacific pattern (not shown).
Therefore, we here focus on the impact of the in-phase variation between the NEAL and WNPSH. Based on the composite rainfall anomalies during the in-phase years, a rainfall dipole index is defined as the rainfall anomaly averaged over the region to the north of the Yangtze River (30°-40°N, 100°-120°E) minus that over the region to the south of the Yangtze River (20°-30°N, 100°-120°E). The regression of H850 anomalies on the rainfall dipole index shows a similar spatial pattern to the composite in-phase result (Figure 3(c)). The similarity confirms the combined effect of the in-phase change of the enhanced NEAL and WNPSH on the rainfall dipole pattern identified in Figure  4(a) and (c). The possible mechanism is that the increased geopotential height gradient caused by the enhanced NEAL and WNPSH leads to a stronger southwest wind, Similarly, the H850 anomalies in Phases 2 and 4 are shown in Figure 3(d) and (e), respectively. In Phase 2 ( Figure  3(d)), the NEAL weakens and the WNPSH is enhanced, indicated by a northward retreat of the 1432 gpm contour over continental northern East Asia and a westward extension of the 1510 gpm contour over the western North Pacific Ocean in comparison with the climatology (dashed lines), which is consistent with two strong positive H850 anomalies over the two regions. The H850 anomalies in Phase 4 are opposite to that in Phase 2, so the NEAL is enhanced while the WNPSH weakens. Their differences (Phase 2 minus Phase 4) show two significant positive anomalies over the two core regions (Figure 3(f )). Therefore, Phases 2 and 4 depict an out-of-phase change of the NEAL and WNPSH; i.e. the WNPSH is enhanced when the NEAL weakens in Phase 2, and the WNPSH weakens when the NEAL is enhanced in Phase 4.

Effect on the East Asian summer rainfall
Corresponding to different circulation patterns of the WNPSH and NEAL during in-phase and out-of-phase years, East Asian summer rainfall behaves differently (Figure 4). When the NEAL and WNPSH are both enhanced (Phase 1 minus Phase 3), the rainfall anomaly shows a dipole pattern over East Asia: rainfall increases north of 30°N and decreases to the south, similar to the second leading mode of summer rain in eastern China (e.g. Huang, Chen, and Huang 2007;Ye and Lu 2012). The dipole pattern is found using both the CRU (Figure 4(a)) and station (Figure 4(c)) rainfall data. In the out-of-phase years, rainfall increases significantly along the Yangtze River valley in response to the enhanced WNPSH and weakened NEAL (Phase 2 minus Phase 4) based on the CRU (Figure 4(b)) and station (Figure 4(d)) rainfall data.
These rainfall anomalies are consistent with the circulation anomalies. When the NEAL and WNPSH are both enhanced, the lower-tropospheric southerlies over eastern China would be stronger and extend northward into North China (estimated from Figure 3(c)), enhancing rainfall in North China and suppressing rainfall in the Yangtze River valley. In contrast, when the WNPSH is enhanced and the NEAL weakens, the lower-tropospheric southerlies would be stronger south of the Yangtze River valley but could not extend into North China (estimated from Figure 3(f )), leading to an increase of rainfall in the Yangtze River valley.
The out-of-phase variation of the NEAL and WNPSH corresponds well to the rainfall anomalies along the Yangtze River valley, which are well documented to be associated with the circulation anomalies characterized by the East Asia-Pacific pattern (Huang and Sun 1992) or Japan-Pacific pattern (Nitta 1987). We analyzed the circulation anomalies associated with the out-of-phase variation of the NEAL and which delivers more moisture transported northward and brings more rainfall to north of the Yangtze River ( Figure 5).
We examined the sea surface temperature (SST) anomalies and found that the SST anomalies in in-phase years were relatively weaker from the preceding winter to simultaneous summer than those in out-of-phase years (not shown). The SST anomalies in out-of-phase years showed a quick decaying of El Niño, similar to the result shown by Chen et al. (2012).

Conclusion and discussion
This study investigates the joint effect of the NEAL and WNPSH on East Asian summer rainfall. When the NEAL and WNPSH vary in phase, the rain anomaly in East Asia shows a dipole pattern, displaying opposite variation between north and south of the Yangtze River. That is, when both the NEAL and WNPSH are strong, rainfall increases north of the Yangtze River and decreases south of the Yangtze River. The opposite rainfall anomaly occurs when both circulation systems are weak. When the NEAL and WNPSH vary out of phase, the rainfall anomaly is concentrated in the Yangtze River valley. The rainfall over the region of the Yangtze River increases as the WNPSH is enhanced and the NEAL weakens but decreases when the WNPSH weakens and NEAL is enhanced.
The mean sea level pressure (MSLP) has been widely used to depict the land-sea thermal contrast between continental East Asia and the North Pacific Ocean. Therefore, we examined the MSLP anomalies during in-phase years, and found that they are very similar to the H850 anomalies shown in Figure 3(c). This result is different to the wellknown land-sea thermal contrast between East Asia and the North Pacific (e.g. Zhao and Zhou 2005), suggesting that the present results highlight the land-sea thermal contrast in the north-south direction, while the zonal contrast has been previously highlighted.