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The MJO is a large-scale eastward-propagating circulation in the atmosphere. (Madden and Julian, 1971) found a 40-50-day oscillation when analyzing the zonal wind anomalies of Canton Island. They pointed out that the MJO is characterized by the planetary scale of wavenumber 1 and eastward propagation (Madden and Julian, 1972). (Yasunari, 1980) confirmed that zonal wind has low-frequency oscillation at the time scale of 40-50 days. (Weickmann et al., 1985) and (Knutson et al., 1986) showed that the MJO also has a vertical baroclinic structure. Many subsequent studies have demonstrated that the active area of the MJO is in the South Asia monsoon region, tropical western Pacific and eastern Pacific (Madden and Julian, 1994; Zhang, 2005).
Since the MJO is the most dominant signal in tropical intraseasonal variability, and ENSO is the major source of interannual variability in the tropics, many previous studies have investigated the interaction between the MJO and ENSO. For example, the MJO can trigger El Niño through sea-air interaction (Lau and Chan, 1986). Before El Niño occurs, MJO activity is greatly enhanced over the tropical western Pacific (Li and Zhou, 1994), suggesting an impact of the MJO on the occurrence of El Niño events. MJO activity in late boreal spring is favorable to the development of El Niño in the subsequent autumn and winter (Hendon et al., 2007). On the other hand, ENSO can also influence the activity of the MJO. During El Niño, MJO amplitude is relatively weak, implying a weakening effect of El Niño on MJO intensity (Li and Zhou, 1994). During warm ENSO episodes, MJO convective activity shifts eastward to the central and eastern Pacific, with decreased intensity across the eastern Indian Ocean and Maritime Continent (Hendon et al., 1999). Moreover, the strong warm SST anomaly in the central Pacific promotes rapid growth of the MJO in the western Pacific convective phases (Marshall et al., 2016). These previous studies indicate a two-way interaction between enhanced MJO activity and the development of El Niño (Kessler and Kleeman, 2000; Marshall et al., 2016). Besides, the MJO extends further east to the central Pacific during El Niño events, which is associated with the warmer SST beneath (Kessler, 2001; Tam and Lau, 2005). Further study suggests that the lifetime of the MJO is also dependent on the state of ENSO: the MJO propagates faster through the Maritime Continent and western Pacific during El Niño (Pohl and Matthews, 2007).
Recently, a different type of El Niño, characterized by a warm SST anomaly in the central Pacific, has been widely discussed (Larkin and Harrison, 2005; Ashok et al., 2007). There are many terms to describe this phenomenon, such as "dateline El Niño" (Larkin and Harrison, 2005), "El Niño Modoki" (Ashok et al., 2007), and "central Pacific El Niño" (Kao and Yu, 2009). In this study, we name the two types of El Niño events as "eastern Pacific El Niño" (EP El Niño) and "central Pacific El Niño" (CP El Niño), respectively. During EP El Niño, SST, precipitation and wind anomalies all display dipolar patterns; whereas, they all display tripole patterns during CP El Niño (Ashok et al., 2007; Weng et al., 2007; Kao and Yu, 2009). It is important to note that some studies have raised doubt about the independence of CP El Niño events (Trenberth and Stepaniak, 2001; Trenberth et al., 2002); however, the two types of El Niño do seem to exert different influences on both regional climate and global climate via teleconnection (Larkin and Harrison, 2005; Feng and Li, 2011; Chen et al., 2014). Moreover, recent studies have shown that CP El Niño events have occurred more frequently since the beginning of the 1990s, as compared to EP El Niño events (Yeh et al., 2009; Zhang et al., 2011). Some studies even suggest that the frequency of CP El Niño occurrence will keep on increasing in the 21st century (Kim and Yu, 2012). Therefore, it is necessary to examine the different responses of the MJO to the two types of El Niño events.
However, few studies have thus far compared the MJO's activity between the two types of El Niño events, although many have examined the interaction between the MJO and ENSO. (Hendon et al., 1999) showed that enhanced MJO activity occurs along with an SST anomaly pattern like CP El Niño. (Gushchina and Dewitte, 2012) demonstrated that the MJO is intensified prior to the peak of EP El Niño, while it is increased during the mature and decaying phases of CP El Niño. (Yuan et al., 2015) further showed the seasonal changes of MJO kinetic energy during the two types of El Niño events. However, these studies mainly focused on comparing MJO intensity during the evolution of the two types of El Niño. It remains unclear whether the two types of El Niño have different influences on MJO simultaneously——not only on MJO intensity, but also on its eastward propagation. Besides, previous studies have generally used the zonal wind at 850 hPa to describe the MJO, which may be greatly constrained by the underlying surface.
The purpose of the present work is to explore whether the MJO's activity——including its intensity, eastward propagation and active phases——is distinct against the background of the two different types of El Niño events. Since the most active MJO events occur in December-February (Wheeler and Hendon, 2004), we focus on studying the differences during this season, i.e., boreal winter. Instead of 850-hPa zonal wind, we use OLR and 200-hPa velocity potential to depict the MJO. The remainder of the paper is organized as follows: Section 2 described the datasets and methods used in this work. Section 3 presents the impacts on MJO intensity during the two types of El Niño winters. Section 4 compares the MJO's eastward propagation, and conclusions are given in section 5.
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The datasets used in this study include:
(1) Daily NCEP-2 reanalysis horizontal wind data (resolution: 2.5°× 2.5°) from 1979 to 2012;
(2) Daily OLR (horizontal resolution: 2.5°× 2.5°), provided by NOAA, from 1979 to 2012;
(3) Monthly SST data (resolution: 1°× 1°), obtained from NOAA, from 1979 to 2012;
(4) Real-time Multivariate MJO (RMM) index (Wheeler and Hendon, 2004), obtained from the Australia Meteorological Bureau (
http://www.bom.gov.au/climate/mjo/graphics/rmm.74toRealtime.txt ), from 1979 to 2012.Boreal winter in this paper is defined as the period from December to February. For the sake of simplicity, we use the year of December to represent the year for a particular winter. For example, the "1979 winter" indicates the period from December 1979 to February 1980. All the daily data are dealt to 365 days in each year, which means the data on 29th February in leap years are removed.
The 30-60-day filtered OLR and 200-hPa velocity potential data are utilized to depict the spatial pattern of the MJO. In particular, the intensity of the MJO is quantified by the variance of these two variables. The RMM index is used to describe the phases of propagation. This index is based on a multivariable EOF analysis of daily OLR, 200-hPa and 850-hPa zonal wind anomalies. The principal components of the first two EOFs (RMM1 and RMM2) can be plotted on a phase-space diagram. It is generally divided into eight phases, and each phase corresponds to a particular stage of the MJO life cycle.
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To separate the characteristics of the MJO during the two types of El Niño events, the Butterworth bandpass filter and composite analysis are used. An F-test is used to compute the confidence level for the composite of variance anomaly. The degrees of freedom are n-2, where n is the number of cases.
Following former studies (e.g., Ashok et al., 2007; Weng et al., 2007), the Niño3 index and El Niño Modoki index (EMI) are used to classify EP El Niño and CP El Niño events: Niño3 index is defined as the mean SST anomaly averaged over the equatorial eastern Pacific [(5°S-5°N, 150°-90°W)]; and \begin{equation} {EMI}={SSTA}_{C}-0.5{SSTA}_{E}-0.5{SSTA}_{W} , (1)\end{equation} where SSTA C, SSTA E, and SSTA W represent the area-mean SST anomaly, averaged over the central Pacific [(10°S-10°N, 165°E-140°W)], eastern Pacific [(15°S-5°N, 110°-70°W)] and western Pacific [(10°S-20°N, 125°-145°E)], respectively.
Figure 1 shows the standardized Niño3 index and EMI in boreal winter from 1979 to 2011. A typical EP (CP) El Niño event is defined when the Niño3 index (EMI) is greater than or equal to one standard deviation, which is represented by the dotted line in Fig. 1. There are two years (1991 and 2009) that meet the criterion of both indices, and thus they are not taken into consideration in this study. Based on the above criteria, there are three EP El Niño years (1982, 1986 and 1997) and four CP El Niño years (1990, 1994, 2002 and 2004).
Figure 1. Standardized Niño3 index (black line) and EMI (red line) averaged during boreal winter.
Figure 2. Composite anomalies of 30-60-day OLR variance (units: W$^2$ m$^-4$; color-shaded) and SST (units: $^\circ$C; red contours) during (a) EP El Niño winters and (b) CP El Niño winters, and (c) the differences in the 30-60-day OLR variance between EP and CP El Niño winters (dots indicate regions that are statistically significant at the 99% confidence level).
2.1. Data
2.2. Methods
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In order to quantify the MJO intensity, three categories of index——cloudiness, dynamical, and combined cloudiness and dynamical——were generalized by (Straub, 2013). In this study, we utilize OLR data as the cloudiness index and upper-tropospheric zonal winds as dynamical indices to explore the differences of the MJO in response to the two types of El Niño events. The variance of 30-60-day OLR and 200-hPa velocity potential in the tropics are calculated to identify the MJO intensity.
Figure 2 shows the distribution of the variance anomaly of 30-60-day OLR, and the SST anomaly, which is represented by the contour lines of 1°C and 2°C, during the two types of El Niño winters. The differences between EP and CP El Niño winters are also presented. In the EP El Niño winters (Fig. 2a), the negative variance anomaly appears over the west of the dateline, including the Indian Ocean and the western Pacific, and the positive one appears over the east of the dateline. The negative center lies over the Maritime Continent, while the positive center lies over the central Pacific (near 130°W), which agrees with the results of (Hendon et al., 1999). By contrast, for the CP El Niño winters (Fig. 2b), the positive anomalies appear over the west of the Maritime Continent and the central Pacific near the dateline. Moreover, the warm SST anomaly corresponds to the enhanced MJO convective anomaly during both types of El Niño winters. The difference between EP and CP El Niño events (Fig. 2c) is significantly negative from the tropical eastern Indian Ocean to the western Pacific, which exceeds the 99% confidence level. Thus, it can be concluded that the intensity of 30-60-day OLR from the tropical eastern Indian Ocean to the western Pacific is weaker during EP El Niño winters, while it is stronger during CP winters.
Figure 3. As in Fig. 2 but for velocity potential (units: 10$^-12$ m$^4$ s$^-2$) at 200 hPa.
Figure 3 shows the variance anomaly of 30-60-day velocity potential at 200 hPa during the two types of El Niño winters. The negative variance anomaly occurs over almost the whole of the tropics, and the strongest negative center lies over the eastern Indian Ocean during EP El Niño winters (Fig. 3a). However, when CP El Niño occurs (Fig. 3b), three positive centers of variance anomaly appear over the tropical ocean. The strongest two lie over the eastern Indian Ocean and western Pacific, respectively. A weak negative anomaly over the tropical north-central Pacific is also seen. The distinct difference of 30-60-day velocity potential between the two types of El Niño winters can be seen in the eastern Indian Ocean and western Pacific (Fig. 3c). The same conclusion that 30-60-day velocity potential is weakened during EP El Niño winters and strengthened during CP El Niño winters, can be derived.
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Generally speaking, MJO-related convection emerges over the tropical western Indian Ocean, then weakens over the Maritime Continent, strengthens again over the western Pacific, and finally quickly dies out over the dateline (e.g., Madden and Julian, 1971; Yuan et al., 2014).
Figure 4. Composite anomalies of 30-60-day OLR (units: W m$^-2$) during EP El Niño winters by phase.
Figure 5. As in Fig. 4 but for CP El Niño winters (units: W m$^-2$).
Figure 6. Composite anomalies of 30-60-day OLR (units: W m$^-2$) in the tropics (averaged over 10$^\circ$S-10$^\circ$N) during (a) EP El Niño winters and (b) CP El Niño winters.
To understand the influence of the two types of El Niño events on the eastward propagation of MJO-related convection, composite patterns of 30-60-day OLR for each of the eight MJO phases are shown in Figs. 4 and 5, based on the RMM index. The intensity and propagation of the MJO are quite different between the two types of El Niño events. During EP El Niño winters (Fig. 4), the MJO emerges over the tropical eastern Indian Ocean, then develops from the Maritime Continent to the western Pacific, and finally weakens over the central Pacific. The convection can spread to the tropical central Pacific (near 120°W). By contrast, during CP El Niño winters (Fig. 5), the MJO emerges over the western Indian Ocean, and can maintain or even enhance its intensity over the region west of 120°E, which is in agreement with previous studies (Kessler, 2001; Tam and Lau, 2005). However, the propagation tends to be concentrated to the west of the dateline, and convection anomalies become greatly reduced to the east of 180°.
These eastward-propagation features of the MJO are also illustrated by Fig. 6, which shows the composite tropical (10°S-10°N) OLR anomalies based on the eight phases of the MJO. During EP El Niño winters (Fig. 6a), the MJO occurs near 60°E in phase 2. When the 30-60-day convection moves to 120°E in phase 4, it reaches its strongest intensity. It then weakens in phases 5 and 6, and strengthens again in phase 7. Finally, it dies out near 120°W. During CP El Niño winters, the MJO starts from the west of 60°E in phase 1. It continuously intensifies from phases 2 to 4, and then weakens and maintains its intensity until 180°. Comparing the extent of eastward propagation, the MJO can spread to 120°W during EP El Niño winters and stop propagating near 180° during CP El Niño winters. Another difference between EP and CP El Niño winters is that the MJO during CP El Niño winters tends to have a standing oscillation feature over the eastern Indian Ocean and western Pacific, which is mainly the result of the MJO in 1990 [Fig. S1 in Electronic Supplementary Material (ESM)], when this feature of standing oscillation was predominant.
Therefore, the above results suggest that the two types of El Niño may have different impacts on the eastward propagation of the MJO. During EP El Niño winters, the abnormally warm sea area is situated in the eastern Pacific, and the MJO can propagate to the eastern Pacific. By contrast, during CP El Niño winters, with the SST positive anomaly moving to the central Pacific, the MJO can only propagate to the dateline.
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Figure 6 also demonstrates that the phase speed of the MJO displays different features during the two types of El Niño winters. During EP El Niño, the MJO moves slowly in phases 2 and 3 (roughly 0.16° d-1), but rapidly in phases 4 and 5 (0.74° d-1). This means that the phase speed of the MJO is significantly slower over the Indian Ocean and faster over the Maritime Continent during EP El Niño. However, during CP El Niño, the MJO propagates relatively quickly in phases 2 and 3 (0.43° d-1), but slowly in phases 4 and 5 (0.21° d-1). These phase speeds are estimated by the longitudes of MJO propagation and corresponding days
In addition, we have counted the numbers of MJO days and calculated the proportion in each phase during the two types of El Niño winters, and compared them with normal winters, i.e., the winters of both Niño3 index and EMI anomalies being lower than one standard deviation (Table 1). Overall, the occurrence distributions in different phases is relatively equal, but slightly more frequent in phases 6 and 7 during normal winters, which accounts for 30% of total occurrence. However, it decreases sharply when El Niño occurs, especially during EP events (only 20%). The frequency of MJO occurrence is relatively high in phases 2 and 3 during EP El Niño winters, in which it approaches 35% of the total occurrence. Phases 4 and 5 (only 18%) show the least frequent occurrence. In contrast, the MJO occurs more often in phases 4 and 5 (nearly 40%) during CP El Niño winters, and less frequently in phases 8 and 1 (less than 15%). This suggests that the MJO may occur more frequently over the tropical Indian Ocean during EP El Niño winters, while it may favor the Maritime Continent during CP El Niño winters.
4.1. Extent of MJO eastward propagation
4.2. Phase occurrences of MJO eastward propagation
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The impacts of two types of El Niño on the MJO during boreal winter are investigated in this study. It is found that the characteristics of MJO activity are quite different between the two types of El Niño.
The variance of 30-60-day OLR and 200-hPa velocity potential are applied to identify the MJO intensity. Composites of MJO intensity are presented for EP El Niño and CP El Niño, as well as their differences. It is found that the strongest difference occurs between the tropical eastern Indian Ocean and western Pacific. Both variables lead to the conclusion that the intensity of the MJO is weak during EP El Niño, but stronger during CP El Niño.
Additionally, the propagation features of MJO-related convection are contrasted between the two types of El Niño. The composite of 30-60-day OLR for the eight MJO phases is based on the RMM index. The evolution of the MJO presents the extent of eastward propagation during the different types of El Niño. For EP El Niño, MJO-related convection emerges over the eastern Indian Ocean and can propagate further eastward into the central Pacific (to nearly 120°W). During CP El Niño winters, MJO-related convection emerges over the western Indian Ocean and can only propagate to the dateline, and there are no clear convection anomalies to the east of 180°. The implication of this finding is that the propagation extent of the MJO may be bounded to the abnormally warm area over the tropical Pacific.
We also find that the frequency in the eight phases of the MJO differs between the two types of El Niño. In general, MJO-related convection appears more frequently over the western Pacific during boreal winter (Lafleur et al., 2015). However, the occurrence of MJO-related convection is relatively high (nearly 35%) over the Indian Ocean (phases 2 and 3), while it is low (only 18%) over the Maritime Continent (phases 4 and 5) during EP El Niño. During CP El Niño winters, the most frequent occurrence lies over the Maritime Continent (nearly 40%) and the least frequent occurrence lies over the Western Hemisphere (less than 15%).
This study does not investigate why there are differences in MJO intensity, propagation and occurrence against the background of the two types of El Niño. Our hypothesis, however, is that the intensity of the MJO may be linked to the anomalous convection. During EP El Niño winters, there are positive OLR anomalies over the eastern Indian Ocean and western Pacific (Fig. S2 in ESM), which may weaken the 30-60-day convection. During EP El Niño winters, the enhanced convection appears over the tropical eastern Pacific, which supports the eastward propagation of 30-60-day convection. However, during CP El Niño winters, the enhanced convection moves to the central Pacific, which limits the eastward propagation. In addition to these possible effects of convection on MJO intensity and propagation, MJO occurrence may also be affected by convection. (Pohl and Matthews, 2007) hypothesized that the moisture conditions may influence the propagation speed of the MJO. In this study, during EP El Niño, humidity in the lower troposphere is higher over the western Indian Ocean, and lower over the Maritime Continent (Fig. S3 in ESM). According to the hypothesis of (Pohl and Matthews, 2007), these moisture anomalies may induce slower propagation speeds over the Indian Ocean and higher speeds over the Maritime Continent, thus resulting in greater and less occurrence over these two regions, respectively. During CP El Niño, however, the humidity anomalies are much weaker in the Indian Ocean and Maritime Continent, implying that other mechanisms may be at work. In summary, the mechanisms responsible for the differences in MJO intensity, propagation and occurrence between the two types of El Niño should be further studied.
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