1. Introduction
The Indochina Peninsula (ICP) consists of several agriculture-based and densely populated countries, including Vietnam, Laos, Cambodia, Myanmar, and Thailand. The regional agriculture, ecology, economy, and people’s livelihood are all closely associated with the spring precipitation (Kanae et al. 2001; Tanaka et al. 2008; de Bruyn et al. 2014; Mandapaka et al. 2017). Moreover, the ICP is a typical transitional region between dry and wet climates in spring, and thus the local land surface condition exerts evident effects on atmosphere (Gao et al. 2018, 2019). Being located in the upstream region of the East Asian summer monsoon (Huang and Yan 1999; Wen et al. 2004; Ding and Chan 2005), the ICP exhibits an evident effect of spring soil moisture on the downstream summer monsoon circulation (Gao et al. 2019, 2020a). Meanwhile, the local precipitation in spring is the main factor affecting the soil moisture anomaly over the ICP (Gao et al. 2019). Therefore, the interannual variability of spring precipitation over the ICP is important for the local economic development and human life as well as the variation in the downstream East Asian summer monsoon system.
Being the most prominent feature of coupled ocean–atmosphere variability on the interannual time scale, El Niño is characterized by anomalously warm sea surface temperature (SST) in the equatorial eastern Pacific and has global climatic teleconnections, with major damage to ecosystems, agriculture, fisheries, and economy (Rasmusson and Wallace 1983; Lyon and Barnston 2005; Wang et al. 2017; Ng et al. 2018; Luo and Lau 2019; Ren et al. 2020). El Niño can lead to considerable disruptions of the weather and climate worldwide (Dai and Wigley 2000; Wang et al. 2000; Juneng and Tangang 2005; Hu and Huang 2010). For example, induced by the local SST cooling as a Rossby wave response to El Niño, an anomalous lower-tropospheric anticyclone occurs over the western North Pacific from the El Niño mature winter to the El Niño decaying summer (Matsuno 1966; Wang et al. 2000, 2003; Yang et al. 2007; Xie et al. 2009; Kosaka et al. 2013; Stuecker et al. 2013, 2015; Li et al. 2019). Studies have proposed that the atmosphere–ocean interaction over the Indo-Pacific sector sustains and amplifies the low-level anticyclonic anomaly (Lau and Nath 2003; Wang et al. 2003; Juneng and Tangang 2005). This anomalous anticyclone can exert considerable climatic effects on both the South and East Asian summer monsoons (Huang and Wu 1989; Chang et al. 2000; Chowdary et al. 2011; Mishra et al. 2012; Chowdary et al. 2013) during the El Niño decaying year, affecting the Asian summer climate. In addition, the anomalous anticyclone can convey excessive water vapor to the Southern China during the decaying spring, which results in a local spring precipitation surplus (Jiang et al. 2019).
As a part of Asia, the ICP is located between the Indian and Pacific Oceans, and the local climate is featured by the South and East Asian monsoons, which are both closely associated with the El Niño variations. For instance, Nguyen et al. (2014) indicated that the interannual variabilities of precipitation and temperature over the Vietnam (one of the ICP countries) are strongly affected by the El Niño events. Luo and Lau (2017) and Lin et al. (2018) systematically explored the extreme climate events over the whole ICP and revealed that the local heat waves are obviously amplified by the El Niño events. Chen and Yoon (2000) have also shown that the summer monsoon precipitation over the ICP is closely linked with the El Niño–related SST anomalies over the tropical eastern Pacific. In addition, Ge et al. (2017) suggested that the tropical cyclones modulated by the El Niño events can significantly contribute to the summer monsoon precipitation variations over the ICP on the interannual time scale. In particular, before the rainy season, the effect of El Niño on the ICP precipitation in spring is relatively less studied in the past. Recently, our study (Li et al. 2021) found that the El Niño events can also significantly modulate the interannual variabilities of precipitation and soil moisture over the ICP in spring (before the rainy season) by remotely affecting the Asian atmospheric circulation.
However, the El Niño events have multiple SST anomaly patterns. The traditional El Niño events are featured by the highest SST anomalies located in the equatorial eastern Pacific (Ashok and Yamagata 2009; Kao and Yu 2009). Under the background of climate change, the anomalous warm SSTs for the El Niño events are observed more frequently in the equatorial central Pacific since the early 1990s (Ashok et al. 2007; Kug et al. 2009). In addition, Yeh et al. (2009) have also suggested that such central Pacific El Niño events would occur much more frequently in the projected future warming scenarios. As a result, the satellite observations display an increasing intensity of the El Niño events in the equatorial central Pacific (Lee and McPhaden 2010; McPhaden et al. 2011).
The El Niño–induced atmospheric circulation anomalies are sensitive to the location and intensity of the El Niño–related SST anomalies, and thus cause various regional climate responses. With the interdecadal change in the El Niño events, the relationships between the El Niño and regional climate systems, especially ones over the western North Pacific and East Asia, also experienced interdecadal changes (Wu and Wang 2002; Wang et al. 2008; Chowdary et al. 2012). Chen and Zhou (2014) suggested that the periodicity of the Pacific–Japan teleconnection pattern exhibited an obvious interdecadal change since the early 1990s. Besides, the evident climate shifts in the El Niño–affected regions are accompanied by the interdecadal change of El Niño (Ashok et al. 2007; Feng and Li 2011). Interdecadal changes have also been found in the relationship between the El Niño and Asian precipitation since the early 1990s (Yim et al. 2014; Jin et al. 2016; Piao et al. 2020). Therefore, one issue is proposed here: whether/how the influence of El Niño on the following spring precipitation over the ICP varies with the changing El Niño activity?
Indeed, the present study finds that the effect of El Niño on the decaying spring precipitation over the ICP has experienced an evident interdecadal strengthening: since the early 1990s, the El Niño–related SST anomalies could cause an evident precipitation deficit over the ICP in the decaying spring, while the ICP precipitation response to the El Niño–related SST anomalies is much weaker before the early 1990s. This is attributed to a stronger intensity and longer duration of the warm SST anomalies over the tropical central Pacific in the El Niño events since the early 1990s, which could lead to a westward extension of the abnormal atmospheric circulation, affecting the ICP precipitation more effectively.
The rest of this paper is arranged as follows. Section 2 introduces the data and methods. Section 3 reveals the linkages between El Niño and the following spring precipitation over the ICP. Section 4 explores the interdecadal change in the relationship of the following spring ICP precipitation with El Niño. The interdecadal change in the atmospheric circulation responses is investigated in section 5. Section 6 presents the conclusion and discussions.
2. Data and methods
The observed monthly precipitation dataset with a high resolution is provided by the Climatic Research Unit at the University of East Anglia (horizontal resolution: 0.5° × 0.5°; time span: 1901–2019; https://crudata.uea.ac.uk/cru/data/hrg/cru_ts_4.04/). The monthly SST dataset is collected from the Hadley Center Sea Ice and SST dataset (horizontal resolution: 1° × 1°; time span: 1870–2019; https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html). The monthly atmospheric fields, including the horizontal and vertical wind velocities, and humidity, are gathered from the Japanese 55-year reanalysis dataset provided by the Japan Meteorological Agency (horizontal resolution: 1.25° × 1.25°; time span: 1958–2019; https://jra.kishou.go.jp/JRA-55/index_en.html). The present study period is 1958–2019, which covers the shared time period of all the datasets.
The oceanic Niño index (ONI) provided by the Climate Prediction Center of the United States in the mature winter is employed in our study to denote the intensity of El Niño (Li et al. 2021; for detailed information, please see http://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php). We select 20 El Niño events with the ONI great than 0.5 for the composite analyses, and their decaying years are listed in Table 1. Besides, 17 years with absolute value of the ONI less than 0.5 are chosen as the “quasi-normal” tropical Pacific SST states for the present study period (list not shown). The deviation apart from the mean state of a given variable in the quasi-normal years denotes its composite anomaly. To avoid the possible influences of climate change (such as global warming), the long-term trends of all data are removed before the composite analyses.
List of the El Niño decaying years during 1958–2019 used in the composite analysis.
3. Spring precipitation over the ICP and its relationship with El Niño
During the springtime, the ICP is a dry–wet transitional climatic zone owing to its relatively moderate precipitation (Gao et al. 2019). As shown in Fig. 1a, the multiyear averaged spring precipitation over the ICP is featured by about 3.0–4.0 mm day−1, which is much more than the dry India and Mongolia, and less than the wet Southern China. In addition, the spring ICP precipitation exerts a relatively large interannual variability by a standard deviation of about 0.7–1.0 mm day−1, except for the southeastern corner (Fig. 1b). This almost reaches the magnitude of the wet Southern China. The interannual variability of the ICP precipitation reaches about 30% of the total local precipitation in spring (Fig. 1c). Considering that the ICP is densely populated and largely dependent on agriculture, such large interannual variations in local spring precipitation are worthy of attention.
(a) Mean precipitation (mm day−1) in spring (March–May) during 1958–2019. (b) Standard deviation (mm day−1) of spring precipitation for the period of 1958–2019. (c) Proportion (%) of the standard deviation of precipitation in mean precipitation in spring during 1958–2019. The red boxes denote the ICP region (95°–110°E, 8°–22°N).
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
As the most important climatic forcing factor worldwide on the interannual time scale, the El Niño events play a key role in modulating the atmospheric circulations and tropical precipitation in the decaying spring (Ropelewski and Halpert 1987; Tangang and Juneng 2004; Juneng and Tangang 2005; Tangang et al. 2017; Supari et al. 2018). Figure 2a shows the spatial distribution of the correlation between the winter tropical SST anomalies and the following spring precipitation averaged over the ICP. The pattern shows a strong negative correlation over the tropical eastern Pacific, flanked by strong positive correlations over the northwestern and southern Pacific. This indicates an obvious El Niño–related SST anomalies in preceding winter associated with the abnormal spring ICP precipitation. We further look into the relationship between the averaged ICP precipitation in spring and the preceding winter ONI (Fig. 2b), and their correlation coefficient reaches −0.47 during 1958–2019, which is statistically significant with p < 0.001. In specific, when the tropical eastern Pacific SST is abnormally warmer in winter (i.e., an El Niño event), the ICP usually exhibits a precipitation deficit in the following spring. This is similar to our recent finding (Li et al. 2021).
(a) Correlation coefficient of the preceding winter (December–February) SST anomalies with the spring precipitation anomalies averaged over the ICP for the period of 1958–2019. The dotted areas are significant with p < 0.1. (b) Time series of the standardized spring precipitation anomaly over the ICP and preceding winter ONI during 1958–2019. The green line is their 21-yr moving correlation coefficient, and the dashed lines denote the significance level of p < 0.01.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
The precipitation anomalies over Asia are often in association with anomalous atmospheric circulations induced by the abnormal El Niño SST (Ju and Slingo 1995). Figure 3a demonstrates the circulation responses at the upper layer of the troposphere to the El Niño SST anomalies by the correlation distributions of the 200 hPa velocity potential and divergent wind anomalies in the decaying spring with the preceding winter ONI. The result exhibits evident convergence and divergence regions at the upper troposphere located in the western and eastern Pacific during the El Niño decaying spring, respectively. It also implies that there are abnormal descent and ascent movements over the western and eastern Pacific accompanied by an abnormally higher SST over the tropical eastern Pacific, respectively. This could be owing to a Walker circulation system adjustment related to the tropical SST anomalies. Generally, a warmer SST over the tropical eastern Pacific during the El Niño years can weaken the Walker circulation, leading to evident vertical velocity anomalies over the tropical area (Philander 1990). The ICP is located in the ascending branch of the Walker circulation, and thus the abnormal downward motion during the El Niño decaying spring would hinder the local precipitation.
(a) Correlation coefficients of velocity potential [contours; contour interval (CI): 0.2; the shaded areas are significant with p < 0.1] and divergent wind (arrows; only arrows significant with p < 0.1 are shown) anomalies in spring at 200 hPa with the preceding winter ONI for the period of 1958–2019. (b) As in (a), but for the spring streamfunction (contours; CI: 0.2; the shaded areas are significant with p < 0.1) and rotational wind (arrows; only arrows significant with p < 0.1 are shown) anomalies at 850 hPa. The dashed and continuous lines denote the negative and positive values, respectively. The zero contours are not shown. The red boxes denote the ICP region.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
The low-level circulation anomalies are usually accompanied by the abnormal vertical motion. During the decaying phase, an anomalous anticyclone at lower troposphere is usually maintained over the western North Pacific as a response to the El Niño–related SST anomalies (Wang et al. 2000; Wang and Zhang 2002). Figure 3b further shows the correlation distributions of the abnormal streamfunction and rotational wind anomalies at 850 hPa during the decaying spring with the preceding winter ONI. Obviously, an anomalous anticyclone exists around a center to the east of the ICP associated with a warmer SST over the tropical eastern Pacific. This indicates that the ICP would be under the effects of such a low-level circulation anomaly. The southwesterly wind anomaly would guide the water vapor transporting to the Southern China excessively (Wang et al. 2000; Jiang et al. 2019), which hints a less moisture convergence over the ICP in spring. This is also an adverse climatic condition for the spring ICP precipitation.
4. The intensified response of the decaying spring ICP precipitation to El Niño
However, the present study finds that the relationship between the spring ICP precipitation and the El Niño SST anomalies has underwent an interdecadal change during the present study period. As shown in Fig. 2b, the 21-yr moving correlation of the spring ICP precipitation with the preceding winter ONI is not statistically significant in the early stage. Since the year of 1982 (the result for the period of 1972–92), such a negative relationship between the El Niño intensity and following spring precipitation over the ICP is dramatically strengthened, and the 21-yr moving correlation has the significant level of p < 0.01. Besides, the 21-yr moving correlation is relatively stable during 1982–99. In other words, the correlation between the spring ICP precipitation and the preceding winter ONI exhibits an interdecadal change and is only statistically significant in the recent decades (1992–2019). This implies a strengthening effect of El Niño on the following spring ICP precipitation since the early 1990s.
To further clarify the difference in the effect of the El Niño SST anomalies on the spring ICP precipitation during different periods, we divide the present study period into two subperiods: the previous epoch (1958–91) and post epoch (1992–2019). Figures 4a–d further show the means and variabilities of precipitation in spring during the two epochs. Generally, the multiyear mean precipitation remains the same magnitude during 1958–91 and 1992–2019. However, the interannual variability over most of the ICP, except for the southeastern ICP, is obviously strengthened during the post epoch. The enhanced El Niño effect may play an important role in such a change. As exhibited in Fig. 4e, spring is the transitional season from the dry winter to the wet summer for the ICP, and precipitation in spring is much less than that in the rainy summer. Nevertheless, the interannual variability of precipitation in spring and summer is mainly the same, which implies a rather importance of investigating the strengthening effect of El Niño on the spring ICP precipitation.
Mean precipitation (mm day−1) in spring during (a) 1958–91 and (b) 1992–2019. Standard deviation (mm day−1) of spring precipitation during (c) 1958–91 and (d) 1992–2019. (e) Seasonal evolutions of the ICP precipitation for the periods of 1958–91 and 1992–2019, and the short lines denote the seasonal interannual variabilities (standard variance).
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
Figure 5 shows the correlation distribution of spring precipitation over the ICP with the preceding winter ONI for the different periods. First, during 1958–2019, the strongest El Niño–induced precipitation anomalies are mainly located in the center of the ICP, and the negative correlation covers most of the ICP land except for the southeastern corner (Fig. 5a). However, the results in the two subperiods illustrate a gigantic discrepancy. In 1958–1991, the El Niño SST anomalies exert little influence on the spring ICP precipitation reflected by weak correlations over the ICP (Fig. 5b), while there are strong negative correlations dominating the whole ICP in 1992–2019 (Fig. 5c).
Correlation coefficients between the spring precipitation anomalies and the preceding winter ONI for the periods of (a) 1958–2019, (b) 1958–91, and (c) 1992–2019. The dotted areas are significant with p < 0.1.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
In turn, Fig. 6 shows the different winter SST anomaly patterns related to the following spring ICP precipitation anomaly during the two subperiods. In 1958–91, the spring ICP precipitation exhibits relatively weaker correlations in the tropical Pacific (Fig. 6a). Significant negative correlations with p < 0.1 are only located in the eastern Pacific near the South American continent. In contrast, the correlations are much stronger during the post epoch (1992–2019; Fig. 6b). The tropical eastern Pacific is dominated by significant negative correlations (p < 0.1) with a strong correlation center near the central Pacific. Besides, there are strong positive correlations over the western North Pacific and South Pacific. Such an interdecadal change in the precipitation-related SST anomaly patterns further confirms that the El Niño events in different periods exert different effects on the spring ICP precipitation.
As in Fig. 2a, but for the periods of (a) 1958–91 and (b) 1992–2019.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
The scatterplots between the regional-averaged spring ICP precipitation anomalies and the preceding winter ONI in different periods are shown in Fig. 7. The correlation coefficient between the spring ICP precipitation and preceding winter ONI is −0.47 (p < 0.001) for the whole period of 1958–2019 (Fig. 7a). This correlation coefficient is only −0.18 (not statistically significant) during the previous epoch (Fig. 7b), while it reaches −0.70 (p < 0.001) during the post epoch (Fig. 7c). Furthermore, the correlation coefficients of the preceding winter ONI with spring precipitation anomalies are −0.42 and −0.44 during the years with ONI < 0 in 1958–91 and 1992–2019, respectively, which are both statistically significant at the p < 0.1 level. The above results indicate that while the effect of El Niño on the following spring ICP precipitation has strengthened since the early 1990s, the La Niña effect exhibits little change before and after the 1990s.
Relationships between spring precipitation anomalies over the ICP and the preceding winter ONI for the periods of (a) 1958–2019, (b) 1958–91, and (c) 1992–2019. The r is the correlation coefficient between the spring ICP precipitation anomalies and the preceding winter ONI. The precipitation data are standardized.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
The Indian Ocean SST anomalies in association with El Niño can also affect the Asian climate by regulating the atmospheric circulation (Xie et al. 2016; Chen et al. 2018; Amirudin et al. 2020). As shown in Figs. 2 and 6, the Indian Ocean also exhibits significant correlations between the winter SST anomaly and the following spring ICP precipitation. Here we calculate the Indian Ocean basin (IOB) index and Indian Ocean dipole (IOD) index based on the IOB (40°–100°E, 20°N–20°S) SST anomaly in the decaying spring (Xie et al. 2016) and the difference in SST anomaly between the tropical western Indian Ocean (50°–70°E, 10°N–10°S) and the tropical southeastern Indian Ocean (90°–110°E, 10°N–0°) in preceding autumn (Saji et al. 1999), respectively. We further examine the relationship between the preceding winter ONI and the spring ICP precipitation anomalies after removing the IOB and IOD signals in the two epochs (figure not shown). The results (similar to Fig. 7) also support our conclusion that the influence of El Niño on the following spring precipitation over the ICP has strengthened since the early 1990s.
5. The interdecadal changes in El Niño and its remote effect
So far, this study demonstrates that the abnormally higher SST over the tropical eastern Pacific (i.e., the El Niño events) could cause a precipitation deficit over the ICP in the following spring through modulating the remote atmospheric circulation. In addition, the effect of El Niño on the spring ICP precipitation has an interdecadal change during the past six decades: it is getting strong and stable since the early 1990s, while it is much weaker before the early 1990s. Due to the climate change in the past decades, the El Niño SST anomalies also experienced an interdecadal change since the early 1990s (Lee and McPhaden 2010; McPhaden et al. 2011). Here we turn our attention to investigate the interdecadal changes in El Niño and its effect on the decaying spring atmospheric circulation. The analyses in this section suggest that the El Niño events in the post epoch (1992–2019) have a relatively stronger intensity and longer duration over the tropical central Pacific than the ones in the previous epoch (1958–91). This would induce a westward extension of the atmospheric circulation anomalies associated with the El Niño SST anomalies during 1992–2019, which causes a stronger influence on precipitation over the ICP during the decaying spring.
To check whether the changing SST–precipitation relationship is in association with the interdecadal change of the El Niño intensity, we calculate the 21-yr moving standard deviations of the preceding winter SST anomalies in the Niño-3.4 region (170°–120°W, 5°N–5°S) along with the 21-yr moving correlations of the spring ICP precipitation and the preceding winter ONI (Fig. 8). The 21-yr moving standard deviations of SST anomalies are from below 0.9°C before the early 1990s to almost 1.2°C in the recent decades, suggesting an intensified El Niño since the early 1990s. In particular, the correlation coefficient between the 21-yr moving correlations and the standard deviations highly reaches −0.72 (p < 0.001), which implies that the interdecadal change in the El Niño–ICP precipitation relationship is closely linked to that in the El Niño intensity.
The 21-yr moving correlation coefficient (gray line) of the preceding winter ONI with the spring precipitation anomalies over the ICP, and 21-yr moving standard deviation (red line) of the preceding SST anomalies (units: °C) over the Niño-3.4 region (170°–120°W, 5°N–5°S). The r is their correlation coefficient. Note that years in the x axis denote the central year of the 21-yr window.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
To reveal the related mechanisms, we look into the interdecadal differences of the El Niño SST anomalies from the developing summer to the decaying spring (Fig. 9). The SST anomaly patterns of El Niño in two subperiods are generally consistent with each other in the developing phase: the abnormal warm SST begins to occur over the equatorial eastern Pacific in the developing summer for the El Niño events in both the previous and post epochs (Figs. 9a,b) and extends westward during the developing autumn (Figs. 9c,d). In the mature winter, there are evident differences. The coverage of SST anomalies with over 1°C is relatively larger for the El Niño events in the post epoch than that in the previous epoch (Figs. 9e,f). Particularly, an abnormal center with over 1.5°C SST anomaly exits near the tropical central Pacific in the El Niño events during 1992–2019. Due to the strong memory of SST anomalies, the abnormally higher temperature sustains to the decaying spring (Figs. 9g,h). Similar to the mature winter, the SST anomalies over the tropical central Pacific are evidently higher in the El Niño decaying spring during 1992–2019 than that during 1958–91, reflected by an obvious SST anomaly center of above 0.5°C (Fig. 9h). This indicates that the tropical central Pacific SST anomalies for the El Niño events in 1992–2019 are stronger with a longer duration than the ones in 1958–91.
The composite anomalies of the (a) summer, (c) autumn, (e) winter, and (g) spring SST (°C) for the El Niño events in 1958–1991. The white dashed and solid lines denote the negative and positive values, respectively, and the zero contours are not shown (CI: 1°C). The numeral “−1” in the parentheses denotes the developing year of the El Niño events. All data are linearly detrended. The dotted areas are significant with p < 0.1. (b),(d),(f),(h) As in (a), (c), (e), and (g), but for the El Niño events during 1992–2019.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
Figure 10 further shows the monthly evolutions of the regional averaged SST anomalies over the Niño-3 (150°–90°W, 5°N–5°S) and Niño-4 (160°E–150°W, 5°N–5°S) regions in the El Niño events at the monthly scale. In the Niño-3 region, the SST anomalies are higher in the previous epoch than that in the post epoch during the developing summer (Fig. 10a). The situation turns to be opposite during the developing autumn and mature winter. Generally, the durations of SST anomalies in the El Niño events for the two subperiods are similar in the Niño-3 region. Both the El Niño SST anomalies significantly (p < 0.1) persist to March of the decaying spring. Meanwhile, in the Niño-4 region, the situation is slightly different compared to the Niño-3 region (Fig. 10b). First, the El Niño SST anomalies are evidently stronger in the post epoch than the ones in the previous epoch from the developing phase to the decaying phase. Moreover, the duration of SST anomalies for the El Niño events in 1992–2019 is relatively longer: the significant (p < 0.1) El Niño SST anomalies in post epoch sustain until April of the decaying spring, while that in previous epoch can only sustain until March. On one hand, the El Niño events are intensified in the tropical central Pacific since the early 1990s, which is similar to previous findings (Lee and McPhaden 2010). On the other hand, the abnormally higher SST over the central Pacific persists for a relatively longer time in the post epoch. In other words, the El Niño SST anomalies shift westward since the early 1990s. This hints that the response of the abnormal atmospheric circulation system may extend westward accordingly.
(a) Composite evolutions of SST anomalies (SSTA; °C) averaged over the Niño-3 region (150°–90°W, 5°N–5°S) since the developing summer to the decaying summer for the El Niño events during 1958–91 (denoted by before) and 1992–2019 (denoted by after). (b) Same as in (a), but for the Niño-4 region (160°E–150°W, 5°N–5°S). All data are linearly detrended. The solid bars are significant with p < 0.1.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
To clarify our hypothesis, Fig. 11 shows the abnormal wind fields at 850 hPa along with the SST anomalies in the decaying spring of the El Niño events for the two subperiods. For the El Niño events in the previous epoch, it is obvious that an anomalous anticyclone wind field exists over the western North Pacific, and its west rim is to the east of the ICP (Fig. 11a). For the El Niño events in the post epoch, with a relatively warmer SST over the equatorial central Pacific, the abnormal wind field extends westward compared with the previous epoch and covers the ICP region (Fig. 11b). This implies a stronger effect of El Niño on the atmospheric circulation related to the ICP precipitation in the decaying spring since the early 1990s. Additionally, during the El Niño events in 1992–2019, there is an evident anomalous anticyclonic circulation over the southern Indian Ocean (Fig. 11b). This suggests a stronger response of the atmospheric circulation to the El Niño–induced IOB SST anomaly in the decaying phase of El Niño since the early 1990s, which may induce different effect on the precipitation anomalies over the Southeast Asia and Maritime Continent regions (Juneng and Tangang 2005; Supari et al. 2018). This is beyond our current objective and could be further investigated in the future.
Composite anomalies of the decaying spring SST (colors; °C) and wind (arrows; m s−1; only arrows significant with p < 0.1 are shown) at 850 hPa for the El Niño events in (a) 1958–91 and (b) 1992–2019. All data are linearly detrended. The red boxes denote the ICP region.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
Generally, the low-level wind anomalies are closely linked to both horizontal and vertical atmospheric circulation anomalies. Figure 12 shows the abnormal rotational winds at 850 hPa along with the vertical velocity anomalies at 500 hPa in the decaying spring. In the El Niño events during 1958–91, the middle layer of troposphere exhibits an evident downward motion band over the western North Pacific, and its west side is far to the east of the ICP (Fig. 12a). This is owing to the response of the Walker circulation to El Niño in the decaying spring. Theoretically, the low-level airflow divergences with the abnormal downward motion, and forms an abnormal anticyclone. Therefore, there is a clockwise abnormal wind field located to the east of the ICP in the decaying spring at 850 hPa (Fig. 12a), which exerts little influences on the ICP region. For the El Niño events during 1992–2019, the abnormal descent region at middle troposphere shifts westward (Fig. 12b), which covers the ICP land area. Under such a condition, the abnormal low-level rotational wind field moves westward affecting the ICP climate.
Composite anomalies of the decaying spring vertical velocity (colors; 1 ×10−2 Pa s−1; the white lines denote the areas significant with p < 0.1) at 500 hPa and rotational wind (arrows; m s−1; only arrows significant with p < 0.1 are shown) at 850 hPa for the El Niño events in (a) 1958–91 and (b) 1992–2019. All data are linearly detrended. The red boxes denote the ICP region.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
Water vapor in the air is mainly concentrated in the middle and lower layers, and thus the low-level wind variation plays the decisive role in the air moisture transport. According to the above findings, the different El Niño–induced wind anomalies in different cases should lead to a difference in the water vapor flux over the ICP in spring. Figure 13 shows the tropospheric water vapor flux anomalies in spring corresponding to different El Niño cases. During 1958–91, similar to the 850 hPa wind anomaly, the significant (p < 0.1) abnormal air moisture transport appears to the east of the ICP, while there is little change in moisture flux over the ICP (Fig. 13a). On the contrary, the water vapor flux is significantly enhanced over the ICP for the El Niño events during 1992–2019. The abnormal wind guides excessive moisture transporting to the Southern China from the ICP in spring, resulting in the water vapor convergence and divergence over the Southern China and the ICP, respectively (Fig. 13b). Therefore, the westward shift of the El Niño–induced anomalous wind fields during 1992–2019 suppresses spring precipitation over the ICP.
Composite anomalies of the decaying spring water vaper flux (arrows; kg m−1 s−1; black arrows are significant with p < 0.1) integrated from 300 to 1000 hPa and its divergence (colors; 1 × 105 kg m−2 s−1) for the El Niño events in (a) 1958–91 and (b) 1992–2019. All data are linearly detrended. The red boxes denote the ICP region.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
From the aspect of the vertical motion, the abnormal downward movement in troposphere is also one of unfavorable conditions for the precipitation. Figure 14 further illustrates the longitude–vertical section of vertical velocity anomalies averaged between the latitudes covering the ICP (8°N–22°N) in the decaying spring of different El Niño cases. The abnormal descent motion associated with El Niño SST anomalies in 1958–91 is mainly located to the east of the ICP, while there is a weak abnormal ascent above the ICP (Fig. 14a). This indicates that El Niño has little effects on the vertical motion above the ICP in the decaying spring during 1958–91. For the El Niño events during 1992–2019, the abnormal descent area extends westward excessively, covering the ICP (Fig. 14b). This would further hamper the local precipitation. The evident discrepancy in the downward movement above the ICP in the El Niño decaying springs for the different periods also suggests an intensified effect of El Niño on the ICP precipitation by inducing abnormal downward movement in troposphere since the early 1990s (Fig. 14c).
Composite anomalies of the decaying spring vertical velocity averaged over 8°–22°N (arrows and colors; 1 × 10−2 Pa s−1; only arrows significant with p < 0.1 are shown) for the El Niño events in (a) 1958–91 and (b) 1992–2019. (c) The difference between (b) and (a). All data are linearly detrended. The red boxes are above the ICP region.
Citation: Journal of Climate 34, 14; 10.1175/JCLI-D-20-0940.1
6. Conclusions and discussions
The ICP is composed of the densely populated agricultural countries and is vulnerable to the local precipitation variations in spring. In addition, the recent studies (Gao et al. 2019; Yang et al. 2019; Gao et al. 2020a,b,c) have documented that the spring soil moisture anomaly induced by local precipitation is an important seasonal predictor for the East Asian summer monsoon and extreme climate. Hence, the present study investigates the cause of interannual variability of the spring ICP precipitation by using the observed and reanalyzed datasets. The precipitation anomalies over the ICP are closely associated with the El Niño in the decaying spring on interannual time scales. During the past six decades (1958–2019), the regional averaged spring precipitation over the ICP and the preceding winter ONI are evidently correlated by a correlation coefficient of −0.47 (p < 0.001). This indicates that there is usually a precipitation deficit over the ICP in spring when the tropical eastern Pacific SST is anomalously warmer in the preceding winter, that is, an El Niño event. Such an El Niño effect on the spring ICP precipitation is generally attributed to the El Niño–induced atmospheric circulation anomalies. During the El Niño decaying phase, the SST anomalies over the equatorial Pacific motivate an anomalous low-level anticyclone over the western North Pacific, guiding the air moisture transporting to the Southern China excessively from the ICP. The weakened Walker circulation in the El Niño decaying years hampers the spring ICP precipitation by an abnormal descent motion above the ICP.
In particular, the present study suggests that the response of the spring ICP precipitation to El Niño exhibits an interdecadal change. The negative El Niño SST–ICP precipitation relationship is strong and stable since the early 1990s, while it is much weaker during 1958–91. This is due to the interdecadal changes in the El Niño intensity and duration over the tropical central Pacific. During 1992–2019, the El Niño events in the mature winter are relatively stronger with a relatively larger and warmer center of the SST anomalies over the tropical central Pacific compared with the previous epoch (1958–91). The tropical central Pacific SST is evidently warmer in the El Niño decaying spring for the period of 1992–2019, which indicates a longer duration of SST anomalies in the post epoch than that in the previous epoch. This interdecadal strengthening of the El Niño SST anomalies over the equatorial central Pacific since the early 1990s is similar to the previous findings (Lee and McPhaden 2010; McPhaden et al. 2011). Under such a condition, the El Niño–induced anomalous atmospheric circulations also exhibit interdecadal changes. During 1958–91, the El Niño–induced anomalous anticyclone circulation is to the east of the ICP in the decaying spring, causing little influence on the ICP precipitation. During 1992–2019, the anomalous low-level anticyclone in the decaying spring shifts westward covering the ICP region, inducing a stronger effect on transporting water vapor to the Southern China from the ICP. Besides, the abnormal downward movement over the western North Pacific responding to the El Niño SST anomalies also extends westward and covers the ICP in the decaying spring during 1992–2019, while it is to the east of the ICP during 1958–91. Therefore, El Niño exerts a much stronger effect on the ICP precipitation in the decaying spring since the early 1990s.
The tropical Pacific SST anomaly pattern is very important for regional climate variabilities (Li et al. 2021), simulations (Li and Xie 2014; Li et al. 2015, 2019), and projections (Li et al. 2016, 2017). In this study, we emphasize an intensified effect of El Niño on the decaying spring precipitation over the ICP. Such an interdecadal change in El Niño affecting the spring ICP precipitation is crucial for the seasonal climate prediction over the ICP. The correlation coefficient of the spring ICP precipitation with the preceding winter SST anomaly over the Niño-3.4 region is from −0.49 for the whole study period (1958–2019) to −0.71 during 1992–2019. In other words, the seasonal predictability of spring precipitation over the ICP based on the preceding Niño-3.4 SST anomaly can increase from about 24% to about 50% if we only focus on the recent decades. The improvement of the seasonal precipitation prediction would benefit millions of people living over the ICP region.
Furthermore, our recent studies have revealed that the ICP has a strong land–atmosphere interaction in spring because it is located in a dry–wet transitional climate zone (Gao et al. 2019). Specifically, the spring dry–wet state on the ICP land surface, that is, the soil moisture, can exert evident effects on local temperature and thus the downstream East Asian summer monsoon circulation due to the strong persistence of soil moisture anomalies (Yang et al. 2019). As a result, both the mean and extreme climates in the following summer over East Asia are evidently affected by the spring precipitation over the ICP (Gao et al. 2019, 2020a). From this perspective, studying the spring ICP precipitation provides a better understanding of the East Asian summer monsoon and climate variabilities. The present findings suggest that El Niño can exert an intensified effect on the ICP precipitation during the decaying spring since the early 1990s, which would also provide an important implication for the seasonal predictions of the summer monsoon and extreme climate over East Asia.
Acknowledgments
This work was supported by the Natural Science Foundation of China (41975097, 41831175, 41905054, and 41861144013), the Fundamental Research Funds for the Central Universities (B210201015 and B210201029), the National Key Research and Development Program of China (2018YFC1506002), the Start-up Research Fund for the High-level Talents of Jinling Institute of Technology (jit-b-202114), the Open Research Fund of the State Key Laboratory of Loess and Quaternary Geology of China (SKLLQG2035), and the Open Research Fund of the State Key Laboratory of Tropical Oceanography (South China Sea Institute of Oceanology, Chinese Academy of Sciences) (LTO2110).
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