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Circulation characteristics of EP and CP ENSO and their impacts on precipitation in South China
Journal of Atmospheric and Solar-Terrestrial Physics 179 (2018) 405–415
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Circulation characteristics of EP and CP ENSO and their impacts on precipitation in South China
T
Jiangnan Lia,c,∗, Dazhen Huanga,b, Fangzhou Lia, Zhiping Wena a
School of Atmospheric Sciences, Sun Yat-Sen University, Guangzhou, 510275, PR China Zhejiang Sub-bureau of East China Regional Air Traffic Management Bureau of Civil Aviation Administration of China, Hangzhou, 311207, PR China c Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou, 510275, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: ENSO Precipitation South China Numeric simulation
EP and CP events have different effects on precipitation in South China (SC). In EP El Niño years, the winter precipitation increases and the summer precipitation is distributed in the form of a tri-pole; specifically, it increases in the northeastern and southwestern parts of SC and decreases in the central and northwestern parts. In EP La Niña years, the winter precipitation decreases in the most part of SC and the summer precipitation increases in the central part of SC and decreases in the eastern and southwestern parts. In CP El Niño years, the summer precipitation increases and the winter precipitation increases in the northeastern part of SC and decreases in the southwestern part. In CP La Niña years, the winter precipitation generally decreases and the summer precipitation decreases in the western and southeastern parts of SC but increases in the central and northeastern parts. The results of both numerical simulations and diagnostic analyses show that in EP El Niño winters, there is a northerly wind on the lower troposphere of SC conducive to the southward movement of cold air to SC, converging in the south and diverging in the north, in agreement with the distribution of the precipitation anomalies. In the lower troposphere of SC in summer, an abnormal southwester is dominant, facilitating the movement of warm and moist air in the southwest toward SC, which is conducive to an increase in precipitation there. In EP La Niña winter, there is a southwester anomaly in the lower troposphere of SC, which is adverse to the southward movement of cold air from the north to SC, with the divergence field diverging in the south and converging in the north, in agreement with the distribution of precipitation anomalies in this region. In EP La Niña summer, there is a northeaster anomaly in SC that is in the water vapor convergence area, which is conducive to precipitation in SC. In CP El Niño summer. There are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation.
1. Introduction The El Niño–Southern Oscillation (ENSO) is a strong signal of the tropical air–sea interaction, which has a significant impact on global climate anomalies (Wang et al., 2000; Turner, 2004; Ferday et al., 2008; Zhang et al., 2012, 2014; 2015; Yuan et al., 2012; Feng et al., 2010, 2011; Weng et al., 2011; Tedeschi et al., 2013; Taschetto and England, 2009; Kug et al., 2010; Chen et al., 2014; Jin et al., 2016). In the past 20 years, it has been observed that a warming phenomenon that is different from the traditional El Niño occurs frequently in the tropical Pacific and its warming center is not in the equatorial eastern Pacific (EP event) but rather in the central equatorial Pacific. Ashok et al. (2007) named it the “ENSO Modoki” (CP event); Li et al. (2010) suggested an improved CP El Niño index; and Wang et al. (2012) evaluated ∗
the El Niño and CP El Niño variability based on a new ocean reanalysis. Many studies have shown that the CP event also has a significant impact on global climate. For example, Ashok et al. (2009) simulated the CP El Niño event in 2004 and pointed out that the twin Walker circulation is an important cause of precipitation anomalies in tropical areas. Weng et al. (2011) compared the effects of three tropical systems, EP event, CP event, and the Indian Ocean Dipole (IOD), on summer climate in China. Among the three phenomena, CP El Niño has the strongest relationship with the western North Pacific summer monsoon. Feng et al. (2013) compared the differences between the influence of EP and CP events on precipitation in China during the decay stage. It is thought that the precipitation difference may be related to the variation in the anomalous anticyclone and the position difference in the northwestern Pacific, prompting further studies of the influence of CP event
Corresponding author. School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, 510275, China. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.jastp.2018.09.006 Received 26 January 2018; Received in revised form 11 September 2018; Accepted 12 September 2018 Available online 14 September 2018 1364-6826/ © 2018 Elsevier Ltd. All rights reserved.
Journal of Atmospheric and Solar-Terrestrial Physics 179 (2018) 405–415
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distribution characteristic of CP event. To distinguish the two different types of SST anomalies, the Niño3 index was defined to be ± 1 times its standard deviation in the EP event years and the EMI index was defined to be ± 0.9 times its standard deviation in the CP event years. The abnormal years from 1951 to 2010 are listed in Table 1.
on Hadley circulation. Via an analysis of satellite observation data, Lee et al. (2010) found that the CP event anomalies had an increasing tendency. In addition, Wang et al. (2014) found that CP event affected the frequency of typhoons in the South China (SC) Sea. Precipitation is abundant in SC; however, the interannual variation in the precipitation is obvious (Shen et al., 2014). In addition, sea surface temperature (SST) is an important factor affecting precipitation in SC (Chen et al., 2017). During the El Niño summer development phase, there is less precipitation in SC and North China but more precipitation in the Yangtze and Huaihe river basins (Huang et al., 1989). During EP El Niño winters, there are positive precipitation anomalies in southeastern China (Lu et al., 2017). In the El Niño late spring and early summer decaying phase, there is positive anomalous precipitation in SC (Zhang et al., 1999; Wang et al., 2000). Zhou et al. (2010) found that there is a significant correlation between the winter precipitation in SC and the SST in the equatorial regions of the central and eastern Pacific and the SC Sea. The summer precipitation in the Yangtze River Basin and SC shows significant asymmetry under the influence of the two types of ENSO (Karori et al., 2013). Hardiman et al. (2018) showed also that the Yangtze summer rainfall is not significant response following EP La Niña. However, this relationship between ENSO and SC precipitation is variable (Li and Ma, 2012), with a stronger correlation or a weaker correlation at different times. Therefore, there is still a lot of uncertainty. In summary, EP and CP event have different effects on regional climate. Most related studies consider SC as a whole to analyze its response to SST anomalies; this study primarily analyzes the different circulation characteristics of EP and CP events and focuses on its impact on winter and summer precipitation in SC, while simultaneously taking regional differences into account. This paper is organized as follows. Section 2 contains a simple description of the data. Section 3 presents an empirical orthogonal function (EOF) analysis of the SST in the tropical Pacific. Section 4 presents the effects of SST anomalies on precipitation in SC. Section 5 focuses on the circulation and anomalies. Section 6 contains the simulation experiments, and the results are summarized in Section 7.
4. Effects of EP and CP event on precipitation in SC To investigate the relationship between SST and precipitation in SC and to distinguish the influence of EP and CP event on precipitation in SC, the Niño3 index and the EMI index were used to analyze the partial correlation regression of precipitation in SC and to calculate the partial correlation coefficient (Fig. 2). The correlation between the Niño3 index and the precipitation in SC is good, and the entire area in winter has a positive correlation of more than 0.4 (Fig. 2a). There is a positive correlation in the eastern part of SC, a negative correlation in the middle and northwestern parts of SC, and a positive correlation in the southwestern part of SC during the summer (Fig. 2b); however, there are few areas with significant correlations. In winter, the EMI index is negatively correlated with the precipitation of South China over the southeast (Fig. 2c), and is positively correlated over the northwest. The significant region is in the north central; the summer EMI index is generally positively correlated with the precipitation of South China (Fig. 2d), and the significant region is in the southwest. According to Table 1, the EP and CP event years can be distinguished. Fig. 2a showed a positive correlation between the Niño3 index and the in winter precipitation in SC, the significant region is most of the SC region. The EMI index was generally positively correlated with the summer precipitation in SC (Fig. 2d), and the significant region is in the southwest. Fig. 3 showed more clearly that the winter precipitation in EP events is more than in CP events (Fig. 3a and c), but the summer precipitation in CP events is more than in EP events (Fig. 3b and d). This feature is clearer in El Niño years (Fig. 3a and b). 5. Circulation and anomalies
2. Data 5.1. 850 hPa wind field and divergence Average monthly precipitation data was gathered from 160 stations by the National Climate Center. The global SST 1° × 1° reanalysis data from 1951 to 2010 was acquired from the Hadley Centre (Rayner et al., 2003). We used the NCEP/NCAR monthly 2.5° × 2.5° reanalysis data (Kalnay et al., 1996). The CP event index (EMI) was calculated based on the expression of the SST anomalies in three sea areas selected by Ashok (2007):
EMI = 1.0Ta, A − 0.5Ta, B − 0.5Ta, C
In EP El Niño winters (Fig. 4a), there are southwesterly winds over SC and an anomalous anticyclone over the Philippines; SC is in the anomalous convergence zone, leading to additional precipitation anomalies. This teleconnection patterns agreed with Lu et al. (2017) too. In EP La Niña winters (Fig. 4c), there are northeasterly winds over SC, anomalous anticyclone circulation over the Yangtze River Basin, and weak anomalous cyclone circulation over the Philippines; most regions of SC are in the divergence zone, leading to less precipitation. In EP El Niño summers (Fig. 4b), there are weak southwesterly winds over SC, anomalous cyclone circulation over the northwest Pacific, and a strong wind shear over the eastern part of the Philippines; most regions of SC are in the divergence zone, leading to low precipitation. In EP La Niña summers (Fig. 4d), there are anomalous southwesterly winds over SC and weak cyclone circulation over the sea in the eastern part of the Philippines; the southwestern part of SC is in the divergence zone and the eastern part is in the convergence zone, which is consistent with the distribution of the precipitation anomalies. The comparison with Figs. 2 and 3 can be seen that the teleconnection patterns are more significant in winter. As well as the differences of summer precipitation in the Yangtze River Basin and SC between the two ENSO types (Karori et al., 2013) there is also asymmetry in the difference between La Nina and El Nino (Hardiman et al., 2018), Our conclusion also agreed with Hardiman et al. (2018). In CP El Niño winters (Fig. 5a), there are southwesterly winds over SC, an anomalous anticyclone over the Philippines, and anomalous
(1)
A (165° E−140° W, 10° S–10° N), B (110° W–70° W, 15° S–5° N), and C (125° E−145° E, 10° S–20° N). Ta,A 、Ta,B 、Ta,C represent SST anomalies of three regions of A、B、C, respectively. 3. EOF analysis of SST in the tropical Pacific A number of studies have shown that the SST had significant interdecadal variations in the late 1970s; therefore, to remove the influence of interdecadal variations, the SST time series were divided into two periods, 1951–1975 and 1979–2010, and the tropical Pacific SST anomalies were analyzed using EOF for the two periods separately. From 1951 to 1975, the variance contribution of the first mode is 40.9% (Fig. 1a) and belongs to the EP event. The variance contribution of the second mode is 9.5% (Fig. 1b), and the type of distribution is similar to CP event; however, the SST anomalies in the western Pacific are not obvious. From 1979 to 2010 (Fig. 1c and d), the variance contribution of the first mode is 45.3% and belongs to the EP event. The variance contribution of the second mode is 11.2% and shows a tri-pole 406
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Fig. 1. The first two modes of the EOF of the tropical Pacific SST anomaly (Unit: °C).). Table 1 SST anomaly years. CP event El Niño in boreal winter El Niño in boreal summer La Niña in boreal winter La Niña in boreal summer
1958, 1966, 1984, 1974,
1959, 1977, 1985, 1975,
EP event 1964, 1991, 1989, 1989,
1969, 1994, 1999, 1998,
1978, 2002, 2000, 1999,
1980, 1991, 2005, 2010 2004, 2009 2001, 2009 2008
1958, 1951, 1956, 1954,
1966, 1957, 1968, 1955,
1973, 1963, 1971, 1964,
1983, 1965, 1974, 1970,
1987, 1972, 1976, 1973,
1992, 1976, 1985, 1985,
1998, 2010 1982, 1983, 1987, 1997, 2009 2008 1988
Fig. 2. Distribution of the partial correlation coefficients of the Niño3 index, the EMI index, and precipitation in SC. The colored area indicates that a significance test with a confidence level of 90% was passed.
cyclonic circulation in northern SC. In addition, there is divergence over southwestern SC and convergence over northeastern SC, leading to low precipitation in southwestern SC and high precipitation in
northeastern SC. In CP La Niña winters (Fig. 5c), there are easterly winds in SC and anomalous cyclone circulation over the Philippines, Indonesia is in the strong divergence zone, and there are westerly winds
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Fig. 3. The standardized rainfall differences between EP and CP events during (a) El Niño winter, (b) El Niño summer, (c) La Niña winter, (d) La Niña l summer. The colored area indicates that a significance test with a confidence level of 90% was passed.
SC is in a divergence zone, while western SC is in a convergence zone, which is consistent with the distribution of the precipitation anomalies. The comparison with Figs. 2 and 3 can be seen that the teleconnection patterns are more significant in summer.
over the western–central equatorial Pacific. In CP El Niño summers (Fig. 5b), there are northerly winds over SC and an anomalous cyclone over the western Pacific at mid-latitude; western SC is in the convergence zone and there is divergent airflow in eastern SC, leading to high precipitation in the west and low precipitation in the east. In CP La Niña summers (Fig. 5d), there are northeasterly winds over SC; eastern
Fig. 4. Composite 850 hPa wind field and its divergence distribution in EP events years: (left) EP El Niño and (right) EP La Niña (unit: 10−6 s−1). 408
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Fig. 5. Composite 850 hPa wind field and its divergence distribution in CP events years: (left) CP El Niño and (right) CP La Niña (unit: 10−6 s−1).
over the western Pacific. In addition, there is an updraft over the equatorial Indian Ocean, a closed circulation in the upper atmosphere at 90° E, and a relatively weak closed circulation over the eastern Pacific and the American continent. In EP La Niña winters (Fig. 6c), the anomaly of the Walker circulation is roughly out of phase with that of El Niño, except that the closed circulation in the upper atmosphere at
5.2. Walker circulation In EP El Niño winters (Fig. 6a), there are three distinct vertical zonal circulation cells in the Walker circulation, with the strongest circulation cell over 180° in the equatorial central Pacific Ocean, the strong updraft over 180–70° W in the equatorial eastern Pacific, and the downdraft
Fig. 6. Composite Walker circulation anomaly in Niño3 anomaly years: (left) EP El Niño and (right) EP La Niña. The colored area indicates that the vertical velocity increased by 50 times (unit: Pa/s). 409
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Fig. 7. Composite Walker circulation anomaly in EMI anomaly years: (left) CP El Niño and (right) CP La Niña. The colored area indicates that the vertical velocity increased by 50 times (unit: Pa/s).
composite anomalous distribution of the SST. Over warmer SST regions, there is convergence in the lower layer and divergence in the upper layer, which results in anomalous updrafts; over colder SST regions, there is divergence in the lower layer and convergence in the upper layer, which results in downdrafts. Walker circulation anomalies cause the change of subtropical high intensity and position, thus affecting the precipitation in SC (Lu et al. (2017).
90° E moves to 120° E. In EP El Niño summers (Fig. 6b), there are also three distinct vertical zonal circulation cells in the Walker circulation with the strongest circulation cell over the western–central equatorial Pacific. In addition, there is a closed circulation in the middle atmosphere over the eastern Pacific and the American continent and a weak closed circulation over the Indian Ocean. In EP La Niña summers (Fig. 6d), there are a wide range of downdrafts in the Walker circulation over the equatorial eastern Pacific and a small range of updrafts at 180–150° W (maximum rising height: 300 hPa). In addition, at 150–180° E, there is a small range of downdrafts and an updraft over the equatorial Indian Ocean and the western Pacific. In CP El Niño winters (Fig. 8a), the strong ascending branch of the Walker circulation is over the central Pacific at 150° E−120° W and the downdraft is over the eastern and western Pacific. In CP La Niña winters (Fig. 7c), the anomaly distribution of the Walker circulation was out of phase with that of CP El Niño years. In CP event summers (Fig. 7b and d), the Walker circulation anomaly is over the equatorial Pacific and the vertical velocity of the airflow over the central Pacific is out of phase with that over the eastern and western Pacific. The vertical zonal circulation appears to be well correlated with the
6. Simulation experiments 6.1. Design of the simulation experiments The sixth generation community atmosphere model CAM4 (Neale et al., 2010) of the National Center for Atmospheric Research of the United States was used to conduct a simulation experiment. During the experiment, using the flux coupler CPL7, CAM4 was coupled to the global community land model CLM4, the Los Alamos sea ice model CICE4, and the digital ocean model DOCN used for the acquisition of the ocean flux. During the entire integration process, the ocean exerted forces on the atmosphere but the model atmosphere had no feedback
Fig. 8. EP events experiments: (left) 2 °C positive SST anomaly as the EP El Niño experiment and (right) 2 °C negative SST anomaly as the EP La Niña experiment. 410
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anomaly was in the central part of Guangdong Province, decreasing by 0.25 mm d−1. In the northern and northwestern parts of the province, the precipitation increased by 0.25 mm d−1 at most, centered on northern Guizhou. In summer (Fig. 9d), SSTAs led to a precipitation increase in eastern SC, with the center in Hunan and southern Jiangxi, where the precipitation increased by 0.8 mm d−1 and, in a small part of western SC, the precipitation decreased by 0.4 mm d−1. The modelled precipitation anomalies were in many areas consistent with the results of the observational analysis. With respect to EP El Niño years, in winter (Figs. 3a and 9a), the majority of the precipitation anomalies in SC were focused in Guangdong, showing a trend of decreasing progressively from the southeast to the northwest. The precipitation increase is more significant over the middle south. During the EP La Niña winters (Figs. 3c and 9c), the few precipitation anomalies in SC were in the east and an increase in precipitation occurred in the middle west, with a center featuring decreased precipitation anomalies in the central part of Guangdong. In winter in the EP El Niño experiment (Fig. 10a), the anomaly in SC was represented by a weak northeasterly wind, which was conducive to the southward movement of cold air from the north to SC, which was further beneficial to precipitation. The anomaly of the cross-equatorial flow above Somalia was demonstrated as a southerly wind, which indicates that the cross-equatorial flow was weakened when going southward in winter. In winter in the EP La Niña experiment (Fig. 10c), the anomaly in SC was shown by a southwesterly wind, weakening the winter monsoon as well as the cold air coming from the north, which was adverse to precipitation in SC. In EP El Niño summers (Fig. 10b), the anomaly in SC was shown by a southwesterly wind, while the anomaly in the northwest was represented by a westerly wind with an enhanced summer monsoon. In summer in the EP La Niña experiment (Fig. 10d), the anomalies of the mean wind field in SC were represented by a southwesterly wind with a weakened summer monsoon. Compared to the 500 hPa geopotential height field (Fig. 11), the geopotential height above the Northeast Pacific Ocean in winter in the EP El Niño experiment (Fig. 11a) was relatively low, and meanwhile, an abnormally high area was located above North America, with the
effects on the ocean. In the control (CTL) experiment, a finite element dynamic kernel was adopted with a horizontal resolution of 2.5° × 1.9° and 26 layers in the vertical direction. The model atmosphere was driven by a pre-set monthly mean SST field. The integration lasted for a total of 30 years, with time steps of 30 min. To alleviate impacts caused by adjustments of the model calculations, we removed 10 years of spin up, only the mean field of the 20 model years from the 11th year to the 30th year was chosen as the model climate state and recorded as the CTL experiment. With respect to the distribution of the EP event SST, two sensitivity experiments involving positive and negative SSTAs were designed (Fig. 8). The Niño3 sea area was selected, and based on the original sea surface temperature distribution of the climatic mean state, the SST was increased by 2 °C and was recorded as the EP El Niño experiment, while the case where the SST was decreased by 2 °C was recorded as the EP La Niña experiment. Other settings of the sensitivity experiments were identical to those of the CTL experiment. Two experiments were also designed for CP event SSTA, with SST anomalies (1 °C, 0.5 °C and 0.5 °C respectively) added to the original climatic distribution of SST in the central Pacific Ocean, western Pacific Ocean and eastern Pacific Ocean (Fig. 12), and other settings exactly the same as the control assimilation experiment (CTL). 6.2. Analysis of the experiment results According to the precipitation anomaly responses (Fig. 9), in the EP El Niño experiment, the SSTA caused a decrease in the precipitation in northern SC in winter (Fig. 9a), the center of which was in western Hunan, and in the meanwhile, the precipitation in the south increased and the largest precipitation anomaly on eastern Hainan Island was increased by 0.35 mm d−1. In summer (Fig. 9b), SSTAs in the El Niño experiment led to an increase in the precipitation in eastern SC, rising by more than 1 mm d−1 in the coastal areas of Guangdong, while precipitation decreased in western SC. In the EP La Niña experiment, the SSTAs caused a drop in the precipitation in southeastern SC in winter (Fig. 9c), in which, the largest
Fig. 9. Anomaly response of precipitation rate in South China: (left) the EP El Niño experiment and (right) the EP La Niña experiment (unit: mm/d). 411
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Fig. 10. Wind vector field at 850 hPa: (left) the EP El Niño experiment and (right) the EP La Niña experiment (unit: m/s).
Fig. 11. Anomalous response of geopotential height (unit: 10gpm) at 500 hPa: (left) the EP El Niño experiment and (right) the EP La Niña experiment. 412
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Fig. 12. CP events experiments: (left) −0.5 °C, +1.0 °C, −0.5 °C SST anomaly for three area as the CP El Niño experiment and (right) +0.5 °C, −1.0 °C, +0.5 °C SST anomaly for three area as the CP La Niña experiment.
convergence, and divergence in the lower troposphere. CP El Niño experiment always resulted in a general decrease in winter precipitation in SC (Fig. 13a), and a general increase in summer precipitation in the middle area and eastern area (Fig. 13b), which is consistent with Fig. 3b. CP La Niña experiment always led to an increase in winter precipitation in most parts of SC (Fig. 13c), but a decrease in precipitation only in the northern region. Summer precipitation in the south-central region of SC increased (Fig. 13d), which is consistent with Fig. 3d. In summer in the CP El Niño and CP La Niña experiments (Fig. 14c and d), there are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation. These simulation results were consistent with the results of observational analysis (Fig. 5). However, in winter with CP El Niño experiment (Fig. 13a), the simulation results were different from the results of observational analysis. The distribution of the partial correlation coefficient revealed that the relationship between CP event and precipitation in SC was not obvious (Fig. 2) in
anomaly in the center being up to 60 gpm. The geopotential height in a wide range of the middle and low latitudes was high, with anomaly centers existing above both China and the Central Pacific Ocean. This distribution of anomalies caused a south–north gradient increase in the geopotential height, which was conducive to the deepening of the East Asia Deep Trough and facilitated cold air moving southward to affect the precipitation in SC. In summer in the EP El Niño experiment (Fig. 11b), in the mean height field, differences at low altitudes were rare and, in Asia and the Pacific Ocean, the geopotential height at middle altitudes was low, with an abnormally low center above the Ural Mountains and a negative abnormal center above the Kamchatka Peninsula, the central value of which was up to 20 gpm. Abnormally high centers were located on the west and east coasts of North America. According to a comparison with the results of the observational analysis, the circulation field anomaly simulated in the sensitivity experiments could reflect the results of the observational analysis well; the two matched very well in terms of circulation patterns,
Fig. 13. Anomaly response of precipitation rate in South China: (left) the CP El Niño experiment and (right) the CP La Niña experiment (unit: mm/d). 413
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Fig. 14. Wind vector field at 850 hPa: (left) the CP El Niño experiment and (right) the CP La Niña experiment (unit: m/s).
During EP El Niño winters, convergence appeared in the lower layer and divergence in the upper layer in SC, which is favorable for increasing precipitation in SC. During EP La Niña winters, divergence appeared in the lower layer and convergence in the upper layer of SC, which is favorable to descent and reduces precipitation. During EP El Niño summers, divergence appeared in the lower layer and convergence in the upper layer in SC, reducing the precipitation. In EP La Niña summers, the situation in the northeastern and southwestern parts of SC was as follows: divergence in the lower level and convergence in the upper level, which caused the precipitation to correspond to a tripole distribution. At the same time, the low-velocity potential of the tropical area of EP El Niño displayed a binary distribution, that is, convergence in the tropical East Pacific and divergence in the western Pacific, while that of EP La Niña displayed the inverse phase, which was consistent with the distribution of the Walker circulation anomalies. During CP El Niño winters, there was an anomalous anticyclone in the lower layer near the Philippines and a distinct anomalous cyclone in the lower layer of the northeastern Pacific. The situation for the CP La Niña winters was opposite: an anomalous cyclone existed near the Philippines and an anti-cyclonic circulation anomaly appeared in the lower layer of the northeastern Pacific. During CP El Niño summers, anomalous cyclone circulation occurred in the low-layer ocean surface in the eastern part of the Philippines and there was an obvious cyclonic anomaly in the lower layer of the northeastern part of the Pacific. During CP La Niña summers, the low layer of the ocean surface east of the Philippines was still an anomalous cyclone, while the lower part of the northeastern Pacific was an anticyclone. At the same time, during the CP El Niño, the velocity potential in the lower layer of the tropic area was distributed in the form of a tri-pole, that is, divergence in the western Pacific, convergence in the central Pacific, and divergence in the eastern Pacific; meanwhile, the situation for the CP La Niña was opposite, which was consistent with the distribution of the Walker
winter, which might be the reason for the errors in the simulation experiments.
7. Summary In this paper, using a space–time decomposition of the SST in the tropical Pacific Ocean, two major distribution types of SSTAs were obtained, namely, ENSO and ENSO Modoki (EP and CP events). Meanwhile, a diagnostic analysis was conducted on the effects of the two SSTA distribution types on precipitation in SC, based on which, the community atmosphere model CAM4 of NCAR was used to conduct a simulation study on the physical mechanism of how ENSO SSTAs affect precipitation in SC. The two types of SST anomalies have different effects on precipitation in SC. In the EP events, the precipitation in SC increased in EP El Niño years and decreased in EP La Niña years in winter; this was obvious in the middle and eastern parts of SC. The precipitation in SC in summer was distributed in the form of a tri-pole, that is, the northeastern part of SC, central and northwest parts of SC, and southwest part of SC. The precipitation decreased in the central and northwestern parts. During EP La Niña years, there was more precipitation in the central part of SC and less in the northeastern and southwestern parts. Overall, the teleconnection patterns are more significant in winter. The influence of CP events on precipitation in SC was as follows: in winter, the precipitation in SC was greater in the northeast and less in the southwest during CP El Niño years and precipitation was generally less during CP La Niña years; in summer, precipitation in SC generally increased except in Fujian during CP El Niño years and precipitation decreased in the western and southeastern parts of SC and increased in the central and northeastern parts of SC during CP La Niña years. Overall, the teleconnection patterns are more significant in summer; the CP events sensitivity experiment also proved the mode. 414
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circulation anomaly and closely related to the SST anomalies. The CP events sensitivity experiment were consistent with the results of observational analysis in summer. The EP events sensitivity experiments simulated the anomalies of the atmospheric circulation caused by the SSTAs very well. When negative and positive SSTAs were added to the equatorial East Pacific Ocean, the precipitation in southeastern South China in winter increased while that in the northwest decreased, showing a trend of decreasing gradually from the southeast to the northwest; in summer, the precipitation had the same trend. When negative and positive SSTAs were added to the sea area of the equatorial East Pacific Ocean, the precipitation in eastern South China in winter decreased while that in the south increased, showing an overall precipitation trend decreasing to the east and increasing to the west; in summer, most of the precipitation was in the central part of South China, with little precipitation in the west. The CP El Niño event sensitivity experiment led to a general decrease in winter precipitation in SC, and a general increase in summer precipitation. The CP La Niña event sensitivity experiment led to an increase in winter precipitation in most parts of SC. There are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation in summer in the two CP events sensitivity experiments. These precipitation anomalies were in many areas consistent with the results of the observational analysis.
temperatures. J. Clim. 21 (15), 3687–3703. Hardiman, S.C., Dunstone, N.J., Scaife, A.A., Bett, P.E., Li, C., Lu, B., et al., 2018. The asymmetric response of Yangtze River basin summer rainfall to el niño/la niña. Environ. Res. Lett. 13, 024015. https://doi.org/10.1088/1748-9326/aaa172. Huang, R.H., Wu, Y.F., 1989. The influence of ENSO on the summer climate change in China and its mechanism. Adv. Atmos. Sci. 6 (1), 21–32. Jin, D., Hameed, S.N., Huo, L., 2016. Recent changes in ENSO teleconnection over the western Pacific impacts the eastern China precipitation Dipole. J. Clim. 29 (21), 7587–7598. Kalnay, E., Kanamitsu, M., Kistler, R., et al., 1996. The NCEP/NCAR 40-year reanalysis project. Bull. Am. Meteorol. Soc. 77 (3), 437–472. Karori, M.A., Li, J., Jin, F.F., 2013. The asymmetric influence of the two types of el niño and La niña on summer rainfall over southeast China. J. Clim. 26 (13), 4567–4582 2013. Kug, J.S., Ahn, M.S., Sung, M.K., et al., 2010. Statistical relationship between two types of El Niño events and climate variation over the Korean Peninsula. Asia-Pacific Journal of Atmospheric Sciences 46 (4), 467–474. Lee, T., Mcphaden, M.J., 2010. Increasing intensity of el niño in the central-equatorial Pacific. Geophys. Res. Lett. 37https://doi.org/10.1029/2010GL044007. L14603. Li, C., Ma, H., 2012. Relationship between ENSO and winter rainfall over Southeast China and its decadal variability. Adv. Atmos. Sci. 29, 1129–1141. Li, G., Ren, B.H., Yang, C.Y., Zheng, J.Q., 2010. Indices of el niño and el niño Modoki: an improved el niño Modoki index. Adv. Atmos. Sci. 27 (5), 1210–1220. https://doi.org/ 10.1007/s00376-010-9173-5. Lu, B., Scaife, A.A., Dunstone, N., Smith, D., Ren, H.-L., Liu, Y., Eade, R., 2017. Skillful seasonal predictions of winter precipitation over southeastern China. Environ. Res. Lett. 12, 074021. https://doi.org/10.1088/1748-9326/aa739a. Neale, R., Richter, J., Jochum, M., et al., 2010. Description of the NCAR Community Atmosphere Model (CAM 4.0). Technical Note (NCAR/TN-486-STR). National Center for Atmospheric Research. Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C., Kaplan, A., 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res. 108 (D14). https://doi.org/10.1029/2002JD002670. 4407. Shen, Han, Li, Jiangnan, Wen, Zhiping, Cai, Rongshuo, 2014. Numerical simulation of the impact of latent heat flux anomaly in the tropical western Pacific on precipitation over south China in junes. J. Trop. Meteorol. 20 (3), 236–241. Taschetto, A.S., England, M.H., 2009. El niño Modoki impacts on Australian rainfall. J. Clim. 22 (11), 3167–3174. Tedeschi, R.G., Cavalcanti, I.F.A., Grimm, A.M., 2013. Influences of two types of ENSO on South American precipitation. Int. J. Climatol. 33 (6), 1382–1400. Turner, J., 2004. The El Niño–southern oscillation and Antarctica. Int. J. Climatol. 24 (1), 1–31 2004. Wang, L., Guo, Z.L., 2014. Modulation of tropical cyclogenesis over the South China Sea by ENSO Modoki during boreal summer. J. Ocean Univ. China 13 (2), 223–235. Wang, B., Wu, R., Fu, X., 2000. Pacific–east Asian teleconnection: how does ENSO affect east Asian climate? J. Clim. 13 (9), 1517–1536. Wang, D.X., Qin, Y., Xiao, X., et al., 2012. El Niño and El Niño Modoki variability based on a new ocean reanalysis. Ocean Dynam. 62, 1311–1322. https://doi.org/10.1007/ s10236-012-0566-0. Weng, H., Wu, G., Liu, Y., Behera, Swadhin K., Yamagata, Toshio, 2011. Anomalous summer climate in China influenced by the tropical Indo-Pacific Oceans. Clim. Dynam. 36, 769–782. https://doi.org/10.1007/s00382-009-0658-9. Yuan, Y., Yang, S., 2012. Impacts of different types of el niño on the east Asian climate: focus on ENSO cycles. J. Clim. 25 (21), 7702–7722. Zhang, R.H., Sumi, A., Kimoto, M., 1999. A diagnostic study of the impact of El Niño on the precipitation in China. Adv. Atmos. Sci. 16 (2), 229–241. Zhang, W., Jin, F.F., Ren, H.L., et al., 2012. Differences in teleconnection over the north Pacific and rainfall shift over the USA associated with two types of el niño during boreal autumn. J. Meteorol. Soc. Jpn. 90 (4), 535–552. Zhang, W., Jin, F., Turner, A., 2014. Increasing autumn drought over southern China associated with ENSO regime shift. Geophys. Res. Lett. 41 (11), 4020–4026. Zhang, W., Wang, L., Xiang, B., et al., 2015. Impacts of two types of La Niña on the NAO during boreal winter. Clim. Dynam. 44, 1351–1366. Zhou, L.T., Tam, C.Y., Zhou, W., et al., 2010. Influence of South China sea SST and the ENSO on winter rainfall over south China. Adv. Atmos. Sci. 27 (4), 832–844.
Funding This study was funded by the National Key Research and Development Program of China under contract No. 2016YFA0602701, the National Key Basic Research Program of China under contract No. 2014CB953903, and the National Natural Science Foundation of China (Grant No. 41875168). References Ashok, K., Behera, S.K., Rao, S.A., Weng, H., Yamagata, T., 2007. El niño Modoki and its possible teleconnection. J. Geophys. Res. 112, C11007. https://doi.org/10.1029/ 2006JC003798. Ashok, K., Iizuka, S., Rao, S.A., Saji, N.H., Lee, W.J., 2009. Processes and boreal summer impacts of the 2004 El Niño Modoki: an AGCM study. Geophys. Res. Lett. 36, L04703. https://doi.org/10.1029/2008GL036313. Chen, Z.S., Wen, Z.P., Wu, R.G., Zhao, P., Cao, J., 2014. Influence of two types of El Niños on the East Asian climate during boreal summer: a numerical study. Clim. Dynam. 43, 469–481. Chen, J., Wen, Z., Wu, R., Wang, X., He, C., Chen, Z., 2017. An interdecadal change in the intensity of interannual variability in summer rainfall over southern China around early 1990s. Clim. Dynam. 48 (1–2), 191–207. https://doi.org/10.1007/s00382-0163069-8. Feng, J., Li, J., 2013. Contrasting impacts of two types of ENSO on the boreal spring Hadley circulation. J. Clim. 26 (13), 4773–4789. Feng, J., Wang, L., Chen, W., Fong, S.K., Leong, K.C., 2010. Different impacts of two types of Pacific Ocean warming on Southeast Asian rainfall during boreal winter. J. Geophys. Res. 115, D24122. https://doi.org/10.1029/2010JD014761. Feng, J., Chen, W., Tam, C.Y., et al., 2011. Different impacts of el niño and el niño Modoki on China rainfall in the decaying phases. Int. J. Climatol. 31 (14), 2091–2101. Ferday, D.R., Knight, J.R., Scaife, A.A., Folland, C.K., 2008. Cluster analysis of North Atlantic–European circulation types and links with tropical Pacific sea surface
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Circulation characteristics of EP and CP ENSO and their impacts on precipitation in South China
T
Jiangnan Lia,c,∗, Dazhen Huanga,b, Fangzhou Lia, Zhiping Wena a
School of Atmospheric Sciences, Sun Yat-Sen University, Guangzhou, 510275, PR China Zhejiang Sub-bureau of East China Regional Air Traffic Management Bureau of Civil Aviation Administration of China, Hangzhou, 311207, PR China c Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou, 510275, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: ENSO Precipitation South China Numeric simulation
EP and CP events have different effects on precipitation in South China (SC). In EP El Niño years, the winter precipitation increases and the summer precipitation is distributed in the form of a tri-pole; specifically, it increases in the northeastern and southwestern parts of SC and decreases in the central and northwestern parts. In EP La Niña years, the winter precipitation decreases in the most part of SC and the summer precipitation increases in the central part of SC and decreases in the eastern and southwestern parts. In CP El Niño years, the summer precipitation increases and the winter precipitation increases in the northeastern part of SC and decreases in the southwestern part. In CP La Niña years, the winter precipitation generally decreases and the summer precipitation decreases in the western and southeastern parts of SC but increases in the central and northeastern parts. The results of both numerical simulations and diagnostic analyses show that in EP El Niño winters, there is a northerly wind on the lower troposphere of SC conducive to the southward movement of cold air to SC, converging in the south and diverging in the north, in agreement with the distribution of the precipitation anomalies. In the lower troposphere of SC in summer, an abnormal southwester is dominant, facilitating the movement of warm and moist air in the southwest toward SC, which is conducive to an increase in precipitation there. In EP La Niña winter, there is a southwester anomaly in the lower troposphere of SC, which is adverse to the southward movement of cold air from the north to SC, with the divergence field diverging in the south and converging in the north, in agreement with the distribution of precipitation anomalies in this region. In EP La Niña summer, there is a northeaster anomaly in SC that is in the water vapor convergence area, which is conducive to precipitation in SC. In CP El Niño summer. There are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation.
1. Introduction The El Niño–Southern Oscillation (ENSO) is a strong signal of the tropical air–sea interaction, which has a significant impact on global climate anomalies (Wang et al., 2000; Turner, 2004; Ferday et al., 2008; Zhang et al., 2012, 2014; 2015; Yuan et al., 2012; Feng et al., 2010, 2011; Weng et al., 2011; Tedeschi et al., 2013; Taschetto and England, 2009; Kug et al., 2010; Chen et al., 2014; Jin et al., 2016). In the past 20 years, it has been observed that a warming phenomenon that is different from the traditional El Niño occurs frequently in the tropical Pacific and its warming center is not in the equatorial eastern Pacific (EP event) but rather in the central equatorial Pacific. Ashok et al. (2007) named it the “ENSO Modoki” (CP event); Li et al. (2010) suggested an improved CP El Niño index; and Wang et al. (2012) evaluated ∗
the El Niño and CP El Niño variability based on a new ocean reanalysis. Many studies have shown that the CP event also has a significant impact on global climate. For example, Ashok et al. (2009) simulated the CP El Niño event in 2004 and pointed out that the twin Walker circulation is an important cause of precipitation anomalies in tropical areas. Weng et al. (2011) compared the effects of three tropical systems, EP event, CP event, and the Indian Ocean Dipole (IOD), on summer climate in China. Among the three phenomena, CP El Niño has the strongest relationship with the western North Pacific summer monsoon. Feng et al. (2013) compared the differences between the influence of EP and CP events on precipitation in China during the decay stage. It is thought that the precipitation difference may be related to the variation in the anomalous anticyclone and the position difference in the northwestern Pacific, prompting further studies of the influence of CP event
Corresponding author. School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, 510275, China. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.jastp.2018.09.006 Received 26 January 2018; Received in revised form 11 September 2018; Accepted 12 September 2018 Available online 14 September 2018 1364-6826/ © 2018 Elsevier Ltd. All rights reserved.
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distribution characteristic of CP event. To distinguish the two different types of SST anomalies, the Niño3 index was defined to be ± 1 times its standard deviation in the EP event years and the EMI index was defined to be ± 0.9 times its standard deviation in the CP event years. The abnormal years from 1951 to 2010 are listed in Table 1.
on Hadley circulation. Via an analysis of satellite observation data, Lee et al. (2010) found that the CP event anomalies had an increasing tendency. In addition, Wang et al. (2014) found that CP event affected the frequency of typhoons in the South China (SC) Sea. Precipitation is abundant in SC; however, the interannual variation in the precipitation is obvious (Shen et al., 2014). In addition, sea surface temperature (SST) is an important factor affecting precipitation in SC (Chen et al., 2017). During the El Niño summer development phase, there is less precipitation in SC and North China but more precipitation in the Yangtze and Huaihe river basins (Huang et al., 1989). During EP El Niño winters, there are positive precipitation anomalies in southeastern China (Lu et al., 2017). In the El Niño late spring and early summer decaying phase, there is positive anomalous precipitation in SC (Zhang et al., 1999; Wang et al., 2000). Zhou et al. (2010) found that there is a significant correlation between the winter precipitation in SC and the SST in the equatorial regions of the central and eastern Pacific and the SC Sea. The summer precipitation in the Yangtze River Basin and SC shows significant asymmetry under the influence of the two types of ENSO (Karori et al., 2013). Hardiman et al. (2018) showed also that the Yangtze summer rainfall is not significant response following EP La Niña. However, this relationship between ENSO and SC precipitation is variable (Li and Ma, 2012), with a stronger correlation or a weaker correlation at different times. Therefore, there is still a lot of uncertainty. In summary, EP and CP event have different effects on regional climate. Most related studies consider SC as a whole to analyze its response to SST anomalies; this study primarily analyzes the different circulation characteristics of EP and CP events and focuses on its impact on winter and summer precipitation in SC, while simultaneously taking regional differences into account. This paper is organized as follows. Section 2 contains a simple description of the data. Section 3 presents an empirical orthogonal function (EOF) analysis of the SST in the tropical Pacific. Section 4 presents the effects of SST anomalies on precipitation in SC. Section 5 focuses on the circulation and anomalies. Section 6 contains the simulation experiments, and the results are summarized in Section 7.
4. Effects of EP and CP event on precipitation in SC To investigate the relationship between SST and precipitation in SC and to distinguish the influence of EP and CP event on precipitation in SC, the Niño3 index and the EMI index were used to analyze the partial correlation regression of precipitation in SC and to calculate the partial correlation coefficient (Fig. 2). The correlation between the Niño3 index and the precipitation in SC is good, and the entire area in winter has a positive correlation of more than 0.4 (Fig. 2a). There is a positive correlation in the eastern part of SC, a negative correlation in the middle and northwestern parts of SC, and a positive correlation in the southwestern part of SC during the summer (Fig. 2b); however, there are few areas with significant correlations. In winter, the EMI index is negatively correlated with the precipitation of South China over the southeast (Fig. 2c), and is positively correlated over the northwest. The significant region is in the north central; the summer EMI index is generally positively correlated with the precipitation of South China (Fig. 2d), and the significant region is in the southwest. According to Table 1, the EP and CP event years can be distinguished. Fig. 2a showed a positive correlation between the Niño3 index and the in winter precipitation in SC, the significant region is most of the SC region. The EMI index was generally positively correlated with the summer precipitation in SC (Fig. 2d), and the significant region is in the southwest. Fig. 3 showed more clearly that the winter precipitation in EP events is more than in CP events (Fig. 3a and c), but the summer precipitation in CP events is more than in EP events (Fig. 3b and d). This feature is clearer in El Niño years (Fig. 3a and b). 5. Circulation and anomalies
2. Data 5.1. 850 hPa wind field and divergence Average monthly precipitation data was gathered from 160 stations by the National Climate Center. The global SST 1° × 1° reanalysis data from 1951 to 2010 was acquired from the Hadley Centre (Rayner et al., 2003). We used the NCEP/NCAR monthly 2.5° × 2.5° reanalysis data (Kalnay et al., 1996). The CP event index (EMI) was calculated based on the expression of the SST anomalies in three sea areas selected by Ashok (2007):
EMI = 1.0Ta, A − 0.5Ta, B − 0.5Ta, C
In EP El Niño winters (Fig. 4a), there are southwesterly winds over SC and an anomalous anticyclone over the Philippines; SC is in the anomalous convergence zone, leading to additional precipitation anomalies. This teleconnection patterns agreed with Lu et al. (2017) too. In EP La Niña winters (Fig. 4c), there are northeasterly winds over SC, anomalous anticyclone circulation over the Yangtze River Basin, and weak anomalous cyclone circulation over the Philippines; most regions of SC are in the divergence zone, leading to less precipitation. In EP El Niño summers (Fig. 4b), there are weak southwesterly winds over SC, anomalous cyclone circulation over the northwest Pacific, and a strong wind shear over the eastern part of the Philippines; most regions of SC are in the divergence zone, leading to low precipitation. In EP La Niña summers (Fig. 4d), there are anomalous southwesterly winds over SC and weak cyclone circulation over the sea in the eastern part of the Philippines; the southwestern part of SC is in the divergence zone and the eastern part is in the convergence zone, which is consistent with the distribution of the precipitation anomalies. The comparison with Figs. 2 and 3 can be seen that the teleconnection patterns are more significant in winter. As well as the differences of summer precipitation in the Yangtze River Basin and SC between the two ENSO types (Karori et al., 2013) there is also asymmetry in the difference between La Nina and El Nino (Hardiman et al., 2018), Our conclusion also agreed with Hardiman et al. (2018). In CP El Niño winters (Fig. 5a), there are southwesterly winds over SC, an anomalous anticyclone over the Philippines, and anomalous
(1)
A (165° E−140° W, 10° S–10° N), B (110° W–70° W, 15° S–5° N), and C (125° E−145° E, 10° S–20° N). Ta,A 、Ta,B 、Ta,C represent SST anomalies of three regions of A、B、C, respectively. 3. EOF analysis of SST in the tropical Pacific A number of studies have shown that the SST had significant interdecadal variations in the late 1970s; therefore, to remove the influence of interdecadal variations, the SST time series were divided into two periods, 1951–1975 and 1979–2010, and the tropical Pacific SST anomalies were analyzed using EOF for the two periods separately. From 1951 to 1975, the variance contribution of the first mode is 40.9% (Fig. 1a) and belongs to the EP event. The variance contribution of the second mode is 9.5% (Fig. 1b), and the type of distribution is similar to CP event; however, the SST anomalies in the western Pacific are not obvious. From 1979 to 2010 (Fig. 1c and d), the variance contribution of the first mode is 45.3% and belongs to the EP event. The variance contribution of the second mode is 11.2% and shows a tri-pole 406
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Fig. 1. The first two modes of the EOF of the tropical Pacific SST anomaly (Unit: °C).). Table 1 SST anomaly years. CP event El Niño in boreal winter El Niño in boreal summer La Niña in boreal winter La Niña in boreal summer
1958, 1966, 1984, 1974,
1959, 1977, 1985, 1975,
EP event 1964, 1991, 1989, 1989,
1969, 1994, 1999, 1998,
1978, 2002, 2000, 1999,
1980, 1991, 2005, 2010 2004, 2009 2001, 2009 2008
1958, 1951, 1956, 1954,
1966, 1957, 1968, 1955,
1973, 1963, 1971, 1964,
1983, 1965, 1974, 1970,
1987, 1972, 1976, 1973,
1992, 1976, 1985, 1985,
1998, 2010 1982, 1983, 1987, 1997, 2009 2008 1988
Fig. 2. Distribution of the partial correlation coefficients of the Niño3 index, the EMI index, and precipitation in SC. The colored area indicates that a significance test with a confidence level of 90% was passed.
cyclonic circulation in northern SC. In addition, there is divergence over southwestern SC and convergence over northeastern SC, leading to low precipitation in southwestern SC and high precipitation in
northeastern SC. In CP La Niña winters (Fig. 5c), there are easterly winds in SC and anomalous cyclone circulation over the Philippines, Indonesia is in the strong divergence zone, and there are westerly winds
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Fig. 3. The standardized rainfall differences between EP and CP events during (a) El Niño winter, (b) El Niño summer, (c) La Niña winter, (d) La Niña l summer. The colored area indicates that a significance test with a confidence level of 90% was passed.
SC is in a divergence zone, while western SC is in a convergence zone, which is consistent with the distribution of the precipitation anomalies. The comparison with Figs. 2 and 3 can be seen that the teleconnection patterns are more significant in summer.
over the western–central equatorial Pacific. In CP El Niño summers (Fig. 5b), there are northerly winds over SC and an anomalous cyclone over the western Pacific at mid-latitude; western SC is in the convergence zone and there is divergent airflow in eastern SC, leading to high precipitation in the west and low precipitation in the east. In CP La Niña summers (Fig. 5d), there are northeasterly winds over SC; eastern
Fig. 4. Composite 850 hPa wind field and its divergence distribution in EP events years: (left) EP El Niño and (right) EP La Niña (unit: 10−6 s−1). 408
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Fig. 5. Composite 850 hPa wind field and its divergence distribution in CP events years: (left) CP El Niño and (right) CP La Niña (unit: 10−6 s−1).
over the western Pacific. In addition, there is an updraft over the equatorial Indian Ocean, a closed circulation in the upper atmosphere at 90° E, and a relatively weak closed circulation over the eastern Pacific and the American continent. In EP La Niña winters (Fig. 6c), the anomaly of the Walker circulation is roughly out of phase with that of El Niño, except that the closed circulation in the upper atmosphere at
5.2. Walker circulation In EP El Niño winters (Fig. 6a), there are three distinct vertical zonal circulation cells in the Walker circulation, with the strongest circulation cell over 180° in the equatorial central Pacific Ocean, the strong updraft over 180–70° W in the equatorial eastern Pacific, and the downdraft
Fig. 6. Composite Walker circulation anomaly in Niño3 anomaly years: (left) EP El Niño and (right) EP La Niña. The colored area indicates that the vertical velocity increased by 50 times (unit: Pa/s). 409
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Fig. 7. Composite Walker circulation anomaly in EMI anomaly years: (left) CP El Niño and (right) CP La Niña. The colored area indicates that the vertical velocity increased by 50 times (unit: Pa/s).
composite anomalous distribution of the SST. Over warmer SST regions, there is convergence in the lower layer and divergence in the upper layer, which results in anomalous updrafts; over colder SST regions, there is divergence in the lower layer and convergence in the upper layer, which results in downdrafts. Walker circulation anomalies cause the change of subtropical high intensity and position, thus affecting the precipitation in SC (Lu et al. (2017).
90° E moves to 120° E. In EP El Niño summers (Fig. 6b), there are also three distinct vertical zonal circulation cells in the Walker circulation with the strongest circulation cell over the western–central equatorial Pacific. In addition, there is a closed circulation in the middle atmosphere over the eastern Pacific and the American continent and a weak closed circulation over the Indian Ocean. In EP La Niña summers (Fig. 6d), there are a wide range of downdrafts in the Walker circulation over the equatorial eastern Pacific and a small range of updrafts at 180–150° W (maximum rising height: 300 hPa). In addition, at 150–180° E, there is a small range of downdrafts and an updraft over the equatorial Indian Ocean and the western Pacific. In CP El Niño winters (Fig. 8a), the strong ascending branch of the Walker circulation is over the central Pacific at 150° E−120° W and the downdraft is over the eastern and western Pacific. In CP La Niña winters (Fig. 7c), the anomaly distribution of the Walker circulation was out of phase with that of CP El Niño years. In CP event summers (Fig. 7b and d), the Walker circulation anomaly is over the equatorial Pacific and the vertical velocity of the airflow over the central Pacific is out of phase with that over the eastern and western Pacific. The vertical zonal circulation appears to be well correlated with the
6. Simulation experiments 6.1. Design of the simulation experiments The sixth generation community atmosphere model CAM4 (Neale et al., 2010) of the National Center for Atmospheric Research of the United States was used to conduct a simulation experiment. During the experiment, using the flux coupler CPL7, CAM4 was coupled to the global community land model CLM4, the Los Alamos sea ice model CICE4, and the digital ocean model DOCN used for the acquisition of the ocean flux. During the entire integration process, the ocean exerted forces on the atmosphere but the model atmosphere had no feedback
Fig. 8. EP events experiments: (left) 2 °C positive SST anomaly as the EP El Niño experiment and (right) 2 °C negative SST anomaly as the EP La Niña experiment. 410
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anomaly was in the central part of Guangdong Province, decreasing by 0.25 mm d−1. In the northern and northwestern parts of the province, the precipitation increased by 0.25 mm d−1 at most, centered on northern Guizhou. In summer (Fig. 9d), SSTAs led to a precipitation increase in eastern SC, with the center in Hunan and southern Jiangxi, where the precipitation increased by 0.8 mm d−1 and, in a small part of western SC, the precipitation decreased by 0.4 mm d−1. The modelled precipitation anomalies were in many areas consistent with the results of the observational analysis. With respect to EP El Niño years, in winter (Figs. 3a and 9a), the majority of the precipitation anomalies in SC were focused in Guangdong, showing a trend of decreasing progressively from the southeast to the northwest. The precipitation increase is more significant over the middle south. During the EP La Niña winters (Figs. 3c and 9c), the few precipitation anomalies in SC were in the east and an increase in precipitation occurred in the middle west, with a center featuring decreased precipitation anomalies in the central part of Guangdong. In winter in the EP El Niño experiment (Fig. 10a), the anomaly in SC was represented by a weak northeasterly wind, which was conducive to the southward movement of cold air from the north to SC, which was further beneficial to precipitation. The anomaly of the cross-equatorial flow above Somalia was demonstrated as a southerly wind, which indicates that the cross-equatorial flow was weakened when going southward in winter. In winter in the EP La Niña experiment (Fig. 10c), the anomaly in SC was shown by a southwesterly wind, weakening the winter monsoon as well as the cold air coming from the north, which was adverse to precipitation in SC. In EP El Niño summers (Fig. 10b), the anomaly in SC was shown by a southwesterly wind, while the anomaly in the northwest was represented by a westerly wind with an enhanced summer monsoon. In summer in the EP La Niña experiment (Fig. 10d), the anomalies of the mean wind field in SC were represented by a southwesterly wind with a weakened summer monsoon. Compared to the 500 hPa geopotential height field (Fig. 11), the geopotential height above the Northeast Pacific Ocean in winter in the EP El Niño experiment (Fig. 11a) was relatively low, and meanwhile, an abnormally high area was located above North America, with the
effects on the ocean. In the control (CTL) experiment, a finite element dynamic kernel was adopted with a horizontal resolution of 2.5° × 1.9° and 26 layers in the vertical direction. The model atmosphere was driven by a pre-set monthly mean SST field. The integration lasted for a total of 30 years, with time steps of 30 min. To alleviate impacts caused by adjustments of the model calculations, we removed 10 years of spin up, only the mean field of the 20 model years from the 11th year to the 30th year was chosen as the model climate state and recorded as the CTL experiment. With respect to the distribution of the EP event SST, two sensitivity experiments involving positive and negative SSTAs were designed (Fig. 8). The Niño3 sea area was selected, and based on the original sea surface temperature distribution of the climatic mean state, the SST was increased by 2 °C and was recorded as the EP El Niño experiment, while the case where the SST was decreased by 2 °C was recorded as the EP La Niña experiment. Other settings of the sensitivity experiments were identical to those of the CTL experiment. Two experiments were also designed for CP event SSTA, with SST anomalies (1 °C, 0.5 °C and 0.5 °C respectively) added to the original climatic distribution of SST in the central Pacific Ocean, western Pacific Ocean and eastern Pacific Ocean (Fig. 12), and other settings exactly the same as the control assimilation experiment (CTL). 6.2. Analysis of the experiment results According to the precipitation anomaly responses (Fig. 9), in the EP El Niño experiment, the SSTA caused a decrease in the precipitation in northern SC in winter (Fig. 9a), the center of which was in western Hunan, and in the meanwhile, the precipitation in the south increased and the largest precipitation anomaly on eastern Hainan Island was increased by 0.35 mm d−1. In summer (Fig. 9b), SSTAs in the El Niño experiment led to an increase in the precipitation in eastern SC, rising by more than 1 mm d−1 in the coastal areas of Guangdong, while precipitation decreased in western SC. In the EP La Niña experiment, the SSTAs caused a drop in the precipitation in southeastern SC in winter (Fig. 9c), in which, the largest
Fig. 9. Anomaly response of precipitation rate in South China: (left) the EP El Niño experiment and (right) the EP La Niña experiment (unit: mm/d). 411
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Fig. 10. Wind vector field at 850 hPa: (left) the EP El Niño experiment and (right) the EP La Niña experiment (unit: m/s).
Fig. 11. Anomalous response of geopotential height (unit: 10gpm) at 500 hPa: (left) the EP El Niño experiment and (right) the EP La Niña experiment. 412
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Fig. 12. CP events experiments: (left) −0.5 °C, +1.0 °C, −0.5 °C SST anomaly for three area as the CP El Niño experiment and (right) +0.5 °C, −1.0 °C, +0.5 °C SST anomaly for three area as the CP La Niña experiment.
convergence, and divergence in the lower troposphere. CP El Niño experiment always resulted in a general decrease in winter precipitation in SC (Fig. 13a), and a general increase in summer precipitation in the middle area and eastern area (Fig. 13b), which is consistent with Fig. 3b. CP La Niña experiment always led to an increase in winter precipitation in most parts of SC (Fig. 13c), but a decrease in precipitation only in the northern region. Summer precipitation in the south-central region of SC increased (Fig. 13d), which is consistent with Fig. 3d. In summer in the CP El Niño and CP La Niña experiments (Fig. 14c and d), there are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation. These simulation results were consistent with the results of observational analysis (Fig. 5). However, in winter with CP El Niño experiment (Fig. 13a), the simulation results were different from the results of observational analysis. The distribution of the partial correlation coefficient revealed that the relationship between CP event and precipitation in SC was not obvious (Fig. 2) in
anomaly in the center being up to 60 gpm. The geopotential height in a wide range of the middle and low latitudes was high, with anomaly centers existing above both China and the Central Pacific Ocean. This distribution of anomalies caused a south–north gradient increase in the geopotential height, which was conducive to the deepening of the East Asia Deep Trough and facilitated cold air moving southward to affect the precipitation in SC. In summer in the EP El Niño experiment (Fig. 11b), in the mean height field, differences at low altitudes were rare and, in Asia and the Pacific Ocean, the geopotential height at middle altitudes was low, with an abnormally low center above the Ural Mountains and a negative abnormal center above the Kamchatka Peninsula, the central value of which was up to 20 gpm. Abnormally high centers were located on the west and east coasts of North America. According to a comparison with the results of the observational analysis, the circulation field anomaly simulated in the sensitivity experiments could reflect the results of the observational analysis well; the two matched very well in terms of circulation patterns,
Fig. 13. Anomaly response of precipitation rate in South China: (left) the CP El Niño experiment and (right) the CP La Niña experiment (unit: mm/d). 413
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Fig. 14. Wind vector field at 850 hPa: (left) the CP El Niño experiment and (right) the CP La Niña experiment (unit: m/s).
During EP El Niño winters, convergence appeared in the lower layer and divergence in the upper layer in SC, which is favorable for increasing precipitation in SC. During EP La Niña winters, divergence appeared in the lower layer and convergence in the upper layer of SC, which is favorable to descent and reduces precipitation. During EP El Niño summers, divergence appeared in the lower layer and convergence in the upper layer in SC, reducing the precipitation. In EP La Niña summers, the situation in the northeastern and southwestern parts of SC was as follows: divergence in the lower level and convergence in the upper level, which caused the precipitation to correspond to a tripole distribution. At the same time, the low-velocity potential of the tropical area of EP El Niño displayed a binary distribution, that is, convergence in the tropical East Pacific and divergence in the western Pacific, while that of EP La Niña displayed the inverse phase, which was consistent with the distribution of the Walker circulation anomalies. During CP El Niño winters, there was an anomalous anticyclone in the lower layer near the Philippines and a distinct anomalous cyclone in the lower layer of the northeastern Pacific. The situation for the CP La Niña winters was opposite: an anomalous cyclone existed near the Philippines and an anti-cyclonic circulation anomaly appeared in the lower layer of the northeastern Pacific. During CP El Niño summers, anomalous cyclone circulation occurred in the low-layer ocean surface in the eastern part of the Philippines and there was an obvious cyclonic anomaly in the lower layer of the northeastern part of the Pacific. During CP La Niña summers, the low layer of the ocean surface east of the Philippines was still an anomalous cyclone, while the lower part of the northeastern Pacific was an anticyclone. At the same time, during the CP El Niño, the velocity potential in the lower layer of the tropic area was distributed in the form of a tri-pole, that is, divergence in the western Pacific, convergence in the central Pacific, and divergence in the eastern Pacific; meanwhile, the situation for the CP La Niña was opposite, which was consistent with the distribution of the Walker
winter, which might be the reason for the errors in the simulation experiments.
7. Summary In this paper, using a space–time decomposition of the SST in the tropical Pacific Ocean, two major distribution types of SSTAs were obtained, namely, ENSO and ENSO Modoki (EP and CP events). Meanwhile, a diagnostic analysis was conducted on the effects of the two SSTA distribution types on precipitation in SC, based on which, the community atmosphere model CAM4 of NCAR was used to conduct a simulation study on the physical mechanism of how ENSO SSTAs affect precipitation in SC. The two types of SST anomalies have different effects on precipitation in SC. In the EP events, the precipitation in SC increased in EP El Niño years and decreased in EP La Niña years in winter; this was obvious in the middle and eastern parts of SC. The precipitation in SC in summer was distributed in the form of a tri-pole, that is, the northeastern part of SC, central and northwest parts of SC, and southwest part of SC. The precipitation decreased in the central and northwestern parts. During EP La Niña years, there was more precipitation in the central part of SC and less in the northeastern and southwestern parts. Overall, the teleconnection patterns are more significant in winter. The influence of CP events on precipitation in SC was as follows: in winter, the precipitation in SC was greater in the northeast and less in the southwest during CP El Niño years and precipitation was generally less during CP La Niña years; in summer, precipitation in SC generally increased except in Fujian during CP El Niño years and precipitation decreased in the western and southeastern parts of SC and increased in the central and northeastern parts of SC during CP La Niña years. Overall, the teleconnection patterns are more significant in summer; the CP events sensitivity experiment also proved the mode. 414
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circulation anomaly and closely related to the SST anomalies. The CP events sensitivity experiment were consistent with the results of observational analysis in summer. The EP events sensitivity experiments simulated the anomalies of the atmospheric circulation caused by the SSTAs very well. When negative and positive SSTAs were added to the equatorial East Pacific Ocean, the precipitation in southeastern South China in winter increased while that in the northwest decreased, showing a trend of decreasing gradually from the southeast to the northwest; in summer, the precipitation had the same trend. When negative and positive SSTAs were added to the sea area of the equatorial East Pacific Ocean, the precipitation in eastern South China in winter decreased while that in the south increased, showing an overall precipitation trend decreasing to the east and increasing to the west; in summer, most of the precipitation was in the central part of South China, with little precipitation in the west. The CP El Niño event sensitivity experiment led to a general decrease in winter precipitation in SC, and a general increase in summer precipitation. The CP La Niña event sensitivity experiment led to an increase in winter precipitation in most parts of SC. There are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation in summer in the two CP events sensitivity experiments. These precipitation anomalies were in many areas consistent with the results of the observational analysis.
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