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  • View in gallery

    Composites of 20–90-day filtered OLR anomalies (shading, W m−2) and 850-hPa wind anomalies (vectors) for different MJO phase categories: (a) phase 1 + 8, (b) phase 2 + 3, (c) phase 4 + 5, (d) phase 6 + 7, (e) no MJO, and (f) total for TC genesis in neutral years. Only anomalies exceeding 95% confidence based on the Student’s t test are shown. Circles in the figure denote the positions of cyclogenesis, while the brackets denote the number of TCs formed in each phase. Dashed (solid) contours in (f) represent negative (positive) anomalies of SST (interval 0.08°C).

  • View in gallery

    As in Fig. 1, but for El Niño years.

  • View in gallery

    As in Fig. 1, but for La Niña years.

  • View in gallery

    Composites of different environmental fields: (a),(b) 850-hPa relative vorticity anomalies (10−6 s−1); (c),(d) 500-hPa omega anomalies (0.5 × 10−2 Pa s−1); (e),(f) 600-hPa relative humidity anomalies (%); and (g),(h) 850-hPa streamlines for the inactive (2 + 3) and active (6 + 7) MJO phases in 23 neutral years. Dashed (solid) contours represent negative (positive) values, while shadings denote values exceeding 95% confidence. Circles denote the positions of cyclogenesis.

  • View in gallery

    Composites of different environmental fields: (a),(b),(c) 850-hPa relative vorticity anomalies (10−6 s−1); (d),(e),(f) 500-hPa omega anomalies (0.5 × 10−2 Pa s−1); (g),(h),(i) 600-hPa relative humidity anomalies (%); and (j),(k),(l) 850-hPa streamlines for the inactive (2 + 3) and active (6 + 7) phase as well as the mean state for seven El Niño years. Dashed (solid) contours represent negative (positive) values, while shadings denote values exceeding 95% confidence. Circles denote the positions of cyclogenesis.

  • View in gallery

    Differences for (a) 850-hPa relative vorticity (10−6 s−1), (b) 500-hPa omega (0.5 × 10−2 Pa s−1), and (c) 600-hPa relative humidity (%) for El Niño years between the active and inactive phase. Dashed (solid) contours represent negative (positive) values; shading denotes values exceeding 95% confidence.

  • View in gallery

    Composites of seasonal-mean MJO activity, represented by the rms value of the 20–90-day filtered OLR anomalies for (a) neutral, (b) El Niño, and (c) La Niña years during the TC season, June–November.

  • View in gallery

    As in Fig. 5, but for the inactive (2 + 3) and active (4 + 5) phase as well as the mean state for the six La Niña years.

  • View in gallery

    As in Fig. 6, but for La Niña years.

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Asymmetric Modulation of Western North Pacific Cyclogenesis by the Madden–Julian Oscillation under ENSO Conditions

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  • 1 Guy Carpenter Asia-Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
  • 2 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, and Guy Carpenter Asia-Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
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Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia-Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong 00852, China. E-mail: wenzhou@cityu.edu.hk

Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia-Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong 00852, China. E-mail: wenzhou@cityu.edu.hk

1. Introduction

The western North Pacific (WNP) is one of the most active regions for cyclogenesis, contributing about 30% of the global tropical cyclone (TC) formation (Neumann 1993). It has long been recognized that the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) (Madden and Julian 1971) are the two major modes affecting TC genesis on interannual and intraseasonal time scales in the WNP (Chan 2000; Wang and Chan 2002; Chia and Ropelewski 2002; Liebmann et al. 1994; Kim et al. 2008).

The MJO is a dominant mode of tropical intraseasonal variability characterized by planetary-scale eastward propagating signals of convection and circulation with a period of about 30–60 days. It originates from the Indian Ocean and propagates eastward across the Maritime Continent to the Pacific Ocean. The active and inactive phases of the MJO are associated with the passage of enhanced and suppressed convection (Madden and Julian 1971; Zhang 2005; Zhou and Chan 2005).

Previous studies have revealed that TC formation in various ocean basins is strongly modulated on the intraseasonal time scale. Gray (1979) first found the possible linkage between the MJO and global TC activity. He discovered that cyclogenesis occurs in clusters, with 1–2 weeks of active TC formation followed by a similar period of quiescence. A number of subsequent studies have further clarified the MJO–TC relationship. For example, Liebmann et al. (1994) found that enhanced (suppressed) cyclogenesis appears during the active (inactive) phase of the MJO over the Indian Ocean and the WNP. Sobel and Maloney (2000) proposed that wave accumulation is an important mechanism for the development of tropical-depression-type disturbances, which leads to an increase in TC formation during the active phase of the MJO. Hall et al. (2001) noted that the MJO strongly modulates TC genesis in the Australian region, with strengthening of the MJO–TC relationship during El Niño periods. Similar modulations of cyclogenesis by the MJO were also found in the North Atlantic Ocean (Maloney and Hartmann 2000) and the south Indian Ocean (Bessafi and Wheeler 2006), as well as the Fiji region (Chand and Walsh 2010). These modulations could be similarly attributed to the enhancement or suppression of favorable conditions for TC genesis during different phases of the MJO. More recently, Kim et al. (2008) looked into the variation of TC genesis by dividing the WNP into four quadrants and proposed a possible intensified MJO–TC relationship during El Niño events. Wang and Zhou (2008) further discovered that TCs with rapid intensification also exhibit strong intraseasonal variability. In addition, Mao and Wu (2010) performed a case study for summer 1991 and found that the unstable zonal flow in the monsoon trough region satisfies barotropic instability and leads to significant enhancement in cyclogenesis during the active phase in the WNP.

Previous works focusing on the impact of ENSO on cyclogenesis (Wang and Chan 2002; Chia and Ropelewski 2002; Kim et al. 2011) have pointed out that the location of TC genesis in the WNP shifts with ENSO phases. The number of TCs increases (decreases) remarkably in the southeast quadrant and decreases (increases) in the northwest quadrant during the warm (cold) phase of the ENSO. This can be attributed to changes in wave activity and the mean circulation, such as the low-level wind and vorticity under different ENSO conditions (Sobel and Maloney 2000; Wang and Chan 2002; Chia and Ropelewski 2002; Zhou and Chan 2007). Apart from the above parameters, ENSO is also found to have impacts on the monsoon trough, which is an important factor affecting cyclogenesis in the WNP. The monsoon trough is the region where monsoon westerlies and trade easterlies converge such that cyclogenesis is greatly favored. McBride (1995) found that more than 75% of TCs form in the monsoon trough region. Recently, Chen and Huang (2008) suggested that the TC distribution in the WNP is related to the thermal state of the warm pool, which is linked to the monsoon trough. The westward-retreating monsoon trough during the warm years is favorable for cyclogenesis in the northwest of the WNP, while the eastward penetration of the monsoon trough during the cold years results in more TC formation in the southeast of the WNP.

The above studies have certainly advanced our understanding of the relationship between the interannual and intraseasonal variations in TCs. However, many of them tend to discuss the effect of MJO or ENSO on TCs separately, despite the view that ENSO and MJO are related to each other (Zhang and Gottschalck 2002; Pohl and Matthews 2007). How MJO modulates cyclogenesis under different ENSO conditions is still unclear. In addition, the view of Kim et al. (2008) that the MJO–TC relationship is strengthened under ENSO conditions has not yet reached a consensus. Therefore, by using a real-time multivariate MJO index, the present study systematically examines the coupled impact of the ENSO with MJO on TC formation in the WNP. Beyond that, the spatial variations of cyclogenesis and the associated large-scale environmental parameters are also investigated with respect to different MJO phases and ENSO conditions to give a more thorough understanding of their relationship, which is lacking in the previous studies. Explanations are then given to account for the asymmetric modulation of TCs under different ENSO conditions.

The paper is organized as follows. The data and methodology are described in section 2. Section 3 presents the main statistical results including the MJO–TC relationship under neutral, El Niño, and La Niña years, and section 4 discusses and gives explanations in accordance with different environmental factors. Finally, a discussion and summary are presented in section 5.

2. Data and methodology

a. TC and atmospheric data

The present study is confined to the TC season (June–November) in the WNP, which accounts for 83% of the total annual number of TCs (853 out of 1032) during the period 1975–2010. The TC dataset over the WNP was archived from the website (http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/best_tracks/) of the Joint Typhoon Warning Center (JTWC) at 6-h intervals, and the position of TC genesis is taken as the first noted record in the dataset. In this study, all of the TCs forming in the WNP (0°–30°N, 100°E–180°) during the TC season are counted. It should be mentioned that the results of this study are unaffected if the number of tropical storms (TCs with maximum sustained wind reaching at least 34 kt) is counted instead.

Based on the availability of datasets, daily averaged outgoing longwave radiation (OLR), which is used as a proxy for deep convection, was obtained from the National Oceanic and Atmospheric Administration polar-orbiting satellites on a 2.5° latitude–longitude grid for the period 1975–2010 (Liebmann and Smith 1996). A 20–90-day Lanczos filter was then applied to the OLR data to extract the intraseasonal signals. Wind, relative humidity, and omega datasets for the same period were taken from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996). Traditionally, the NOAA monthly 2° × 2° extended reconstructed sea surface temperature (SST) V3b dataset (Smith and Reynolds 2004) in boreal winter is used to determine the ENSO events. Since the present study focuses on the WNP TC season, the ENSO years are selected based on the Niño-3.4 index averaged over June–November. El Niño (La Niña) years are chosen when the Niño-3.4 index is greater (less) than one standard deviation in the TC season. In this way, a total of seven El Niño years (1982, 1986, 1991, 1994, 1997, 2002, and 2009) and six La Niña years (1975, 1988, 1998, 1999, 2007, and 2010) are selected, which represent the developing stages of the ENSO events. The remaining 23 years are classified as neutral years.

b. MJO index and statistical test

In this study, the real-time multivariate MJO (RMM) index (Wheeler and Hendon 2004) obtained from the Center for Australian Weather and Climate Research is used to determine the phase of the MJO. It is a well-known index and has been successfully used for MJO phase identification (Leroy and Wheeler 2008; Chand and Walsh 2010). The RMM index is expected to increase the signal-to-noise ratio compared to that of a single field (Wheeler and Hendon 2004) and it is derived based on the first two empirical orthogonal functions (EOFs) of the combined fields of near-equatorially averaged 850-hPa zonal wind, 200-hPa zonal wind, and OLR data from the period 1979–2001. The time series of the two leading principal components are called the Real-time Multivariate MJO series 1 (RMM1) and 2 (RMM2), respectively. Together they define the two-dimensional phase space with different phases corresponding to different locations of the enhanced convective MJO signal (refer to Fig. 7 in Wheeler and Hendon 2004). In the current study, the MJO cycle is divided into four categories labeled as “1+8,” “2+3,” “4+5,” and “6+7,” each covering a quarter of the MJO cycle in the phase space specified by RMM1 and RMM2. An additional phase, denoted “No MJO,” is also included when the amplitude of the index (RMM12 + RMM22)1/2 is less than one. The active phase is defined as the phase that shows the largest increase in TC number, while the inactive phase is when the TC suppression is strongest.

A binning procedure is then applied by assigning each TC in the WNP to one of the five MJO categories based on its genesis date and composites of different environmental fields are performed based on different MJO phases. Following Hall et al. (2001), a statistical test is used to examine the MJO–TC relationship. The null hypothesis is that TCs are distributed uniformly, that is, there are no modulations across all the MJO categories. The relevant test statistic is given by
eq1
where Pe and P are the expected and observed daily genesis rates (DGR), respectively. The observed DGR (P) in each MJO phase category is defined as the ratio of the TC number to the number of MJO days N in the respective phase category, while the expected DGR (Pe) is the ratio of the total number of formed TCs to the total number of MJO days. The test statistic Z follows a Gaussian distribution with zero mean and unit standard deviation. It is tested using a two-tailed test with the critical values of Z = ±1.645 and Z = ±1.96 at 90% and 95% confidence, respectively. In addition, an enhancement to the suppression ratio (ESR), defined as the ratio of the maximum DGR to the minimum DGR of the four MJO categories, is used to denote the strength of the MJO–TC modulation. A large ESR means that a sharp difference in the number of TCs exists between the active and inactive phases and, thus, represents a stronger MJO modulation.

3. Statistical results

a. MJO–TC relationship during neutral years

To isolate the effect of the ENSO, the MJO–TC relationship is first examined for neutral years. Figure 1 shows the locations of cyclogenesis for different MJO categories in neutral years, while Table 1 summarizes the corresponding TC genesis statistics. There is a total of 538 TCs out of 4026 MJO days during the TC season of 1975–2010, giving rise to an expected DGR of 13.36%. Compared to the expected DGR, examination of the MJO phase categories depicts a significant difference in the individual DGR, with significant suppression in cyclogenesis in phases 1 + 8 and 2 + 3 and enhancement in phases 4 + 5 and 6 + 7. This yields an ESR of 1.85, indicating that TCs are 1.85 times more likely to form during the active phase 6 + 7 than in the inactive phase 2 + 3. Such a difference is consistent with the MJO basic state in the WNP and agrees with previous studies (Liebmann et al. 1994; Kim et al. 2008).

Fig. 1.
Fig. 1.

Composites of 20–90-day filtered OLR anomalies (shading, W m−2) and 850-hPa wind anomalies (vectors) for different MJO phase categories: (a) phase 1 + 8, (b) phase 2 + 3, (c) phase 4 + 5, (d) phase 6 + 7, (e) no MJO, and (f) total for TC genesis in neutral years. Only anomalies exceeding 95% confidence based on the Student’s t test are shown. Circles in the figure denote the positions of cyclogenesis, while the brackets denote the number of TCs formed in each phase. Dashed (solid) contours in (f) represent negative (positive) anomalies of SST (interval 0.08°C).

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

Table 1.

Tropical cyclone genesis statistics in each MJO phase category for neutral years. Here, N denotes the number of days in each MJO phase category, while WNP TCs refers to the number of tropical cyclones formed in the WNP region (0°–30°N, 100°E–180°). DGR refers to the daily genesis rate, defined as the ratio of the number of TCs to the number of MJO days in each phase. Phase categories where TC numbers are statistically enhanced (suppressed) at the 90% and 95% significance levels are indicated by + and ++ (* and **).

Table 1.

As shown in Fig. 1, the change in cyclogenesis closely follows the migration of the filtered OLR and circulation anomalies. In the inactive phase 2 + 3, suppressed MJO-related convection dominates in the WNP, which is unfavorable for TC formation (Fig. 1b). As the enhanced convection propagates both eastward and northward from the Indian Ocean to the WNP, an OLR pattern that is in sharp contrast to the inactive phase appears in the WNP (Fig. 1d), resulting in statistically more TC formation in phase 6 + 7. Such reversed OLR distribution can also be found in phases 1 + 8 and 4 + 5 over the South China Sea (SCS) and the Maritime Continent (Figs. 1a and 1c), though the modulation is a bit weaker (ESR = 1.50). The result agrees with the Gray (1979) finding that cyclogenesis occurs in clusters. There are one to two weeks of active TC formation during the MJO active phase followed by a similar period of quiescence during the MJO inactive phase. This is also consistent with previous studies (Liebmann et al. 1994; Kim et al. 2008). In addition, the No MJO category (Fig. 1e) is included to represent the cyclone activity in the absence of the MJO. The DGR in the No MJO phase (14.12%) is not significantly different from the average DGR (13.36%), as the modulation impact of the active and inactive phases is considered to have cancelled out after performing the composite. The associated TC-related parameters and the modulation mechanism will be examined in detail in the next section.

b. Enhanced MJO–TC relationship during El Niño years

Figure 2 shows the TC distributions and the composites of filtered OLR and lower-level wind anomalies for different MJO categories in El Niño years. The corresponding statistics are summarized in Table 2. Similar to neutral years, the strongest TC suppression (enhancement) occurs in phase 2 + 3 (6 + 7) in El Niño years. The overall DGR in El Niño years (14.21%) does not reveal a significant difference from that in neutral years (13.36%), but a much greater ESR (3.73) is found that is twice the value of that in neutral years (1.85). The DGR of the inactive phase 2 + 3 reduces from 9.67% for neutral years to 5.70% for El Niño years, while that of the active phase 6 + 7 increases from 17.88% to 21.28%. Obviously, the impact of MJO on cyclogenesis is strengthened during El Niño periods. This supports the view that the MJO–TC relationship tends to be intensified when only El Niño conditions are included (Kim et al. 2008; Hall et al. 2001; Chand and Walsh 2010). Apart from the degree of modulation, another difference worth noting here is the location of the MJO-related convection. In El Niño years, the positive and negative center of the MJO-related convection extends farther eastward and crosses 150°E (Figs. 2b and 2d), which is different from that in neutral years when the convective center is confined west of 150°E (Figs. 1b and 1d). In accordance with this, the positions of cyclogenesis also migrate southeastward in El Niño years, consistent with many previous studies (Wang and Chan 2002; Chia and Ropelewski 2002; Kim et al. 2011). In other words, the interannual shift in the locations of cyclogenesis is also associated with concomitant extension of the MJO-related convection. As will be shown in the next section, the elongated envelope of the MJO-related convection is closely linked to the eastward extension of TC-related parameters and eventually leads to a strengthened MJO–TC relationship in El Niño years.

Fig. 2.
Fig. 2.

As in Fig. 1, but for El Niño years.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

Table 2.

As in Table 1, but for El Niño years.

Table 2.

c. Weakened MJO–TC relationship during La Niña years

Similar analysis is made for La Niña years to examine the MJO–TC relationship. The locations of cyclogenesis with the composites of filtered OLR and wind anomalies for different MJO categories are displayed in Fig. 3 and the corresponding statistics are given in Table 3. The average DGR for La Niña years is 12.11%, which is smaller than that in neutral (13.36%) and El Niño years (14.21%), though the difference is insignificant. Among the four MJO categories, cyclogenesis in phase 2 + 3 (4 + 5) shows the largest suppression (enhancement), with a DGR of 7.33% (16.88%). This gives an ESR of 2.30, which is comparable to that in neutral years (1.85) but significantly smaller than that in El Niño years (3.73). From the above, it is clear that the impact of MJO on TCs under different ENSO conditions is asymmetric. The MJO modulation in La Niña years is merely normal in contrast to the greatly strengthened modulation during El Niño years.

Fig. 3.
Fig. 3.

As in Fig. 1, but for La Niña years.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

Table 3.

As in Table 1, but for La Niña years.

Table 3.

Examination of the MJO-related convection in La Niña years (Fig. 3) reveals a similar pattern to that in neutral years (Fig. 1), with the convective center located west of 150°E. This is consistent with the above statistical results that the MJO modulations in neutral and La Niña years are comparable to each other. However, the MJO-related signals in La Niña years are relatively weaker and less organized when compared with the well-developed and far-eastward-extending signals in El Niño years. This again reveals the asymmetric character of MJO in warm and cold ENSO episodes, which is closely related to the asymmetric TC modulation under different ENSO conditions.

4. Possible modulation mechanism

This section investigates the different dynamic and thermodynamic parameters suggested by Gray (1979) in association with the MJO and ENSO to account for the above statistical results. These parameters include low-level vorticity anomalies, position and strength of the monsoon trough, vertical wind shear, and the midlevel omega as well as relative humidity anomalies. The focus is mainly on the active and inactive phases in which the largest enhancement and suppression of cyclogenesis are found.

a. Neutral years

For neutral years, the largest enhancement (suppression) of cyclogenesis occurs in phase 6 + 7 (2 + 3), resulting in an ESR of 1.85. To account for such a difference, composites of different environmental parameters in these two phases are examined. Figure 4 shows composites of the anomalies of 850-hPa relative vorticity, 500-hPa omega, 600-hPa relative humidity, and orientation of the monsoon trough for phases 2 + 3 and 6 + 7 during neutral years. In phase 6 + 7 (2 + 3), negative (positive) convection anomalies and cyclonic (anticyclonic) flow associated with the MJO occur in the SCS and extend eastward to about 150°E (Fig. 1), which results in a strengthening (weakening) of the monsoon trough (Figs. 4g and 4h). The trough axis shows a dramatic extension from the SCS in phase 2 + 3 to about 150°E in phase 6 + 7. As the monsoon trough has long been recognized to be a region favorable for zonal wave energy accumulation and for TC formation (Chen and Huang 2008; Maloney and Hartmann 2001; McBride 1995), the strengthening and weakening of the trough will lead to changes in TC-related environmental parameters. Coincident with the strengthening of the trough, a concomitant increase in positive vorticity and relative humidity, as well as a decrease in omega, can be observed in phase 6 + 7 around the monsoon trough region (Figs. 4b,d,f,h), while the reverse unfavorable conditions appear in phase 2 + 3 (Figs. 4a,c,e,g). Vertical wind shear, on the other hand, shows only a slight insignificant increase in phase 6 + 7 and thus exerts a minor impact on cyclogenesis compared to other TC-related factors (figure not shown). Therefore, consistent with the propagation of the MJO-related convection, distinctive differences in the monsoon trough and other TC-related parameters explain why there is a significant difference in cyclogenesis during the active and inactive MJO phases. These results agree well with previous studies (Kim et al. 2008; Huang et al. 2011).

Fig. 4.
Fig. 4.

Composites of different environmental fields: (a),(b) 850-hPa relative vorticity anomalies (10−6 s−1); (c),(d) 500-hPa omega anomalies (0.5 × 10−2 Pa s−1); (e),(f) 600-hPa relative humidity anomalies (%); and (g),(h) 850-hPa streamlines for the inactive (2 + 3) and active (6 + 7) MJO phases in 23 neutral years. Dashed (solid) contours represent negative (positive) values, while shadings denote values exceeding 95% confidence. Circles denote the positions of cyclogenesis.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

b. El Niño years

Similar analysis is performed to find out why the MJO–TC relationship is enhanced for El Niño years. Figure 5 shows composites of the anomalies of the different TC-related parameters for the active and inactive MJO phases as well as the climatology for El Niño years. Climatologically, the large-scale westerly anomalies associated with El Niño cause a strengthening of the background monsoon trough. As a result, the background monsoon trough is much stronger and extends southeastward to ~170°E in El Niño years (Fig. 5l). This is similar to the results of Chen and Huang (2008), who found that a strengthened (weakened) monsoon trough corresponds to an abnormal cold (warm) pool known as a seesaw with the eastern Pacific SST anomaly. Since the strong MJO westerly anomalies in the active phase will also enhance the monsoon trough, the strengthening of the monsoon trough in the active MJO phase is synchronous with the background strengthening impact associated with El Niño events. Therefore the trough in the active phase (Fig. 5k) in El Niño years is much stronger and deeper than in neutral years (Fig. 4h). This is also true for the vorticity, relative humidity, and omega anomalies (Figs. 5b,e,h). Favorable background signals (anomalous low-level cyclonic vorticity, higher relative humidity, and smaller omega) associated with El Niño in the southeast part of the WNP are synchronous with the corresponding MJO-induced signal. This causes further strengthening and eastward extension of the favorable signals. For example, the cyclonic vorticity and negative omega anomalies in phase 6 + 7 extend eastward to the date line and thus intensify the enhancement effect on TC formation in the active phase. The increasing MJO influence in El Niño years is indeed consistent with the interannual shift of the MJO-related convection and the TC genesis position (Fig. 2). Therefore, the modulation of cyclogenesis by the MJO is more prominent and the highest DGR occurs during the active phase of El Niño years.

Fig. 5.
Fig. 5.

Composites of different environmental fields: (a),(b),(c) 850-hPa relative vorticity anomalies (10−6 s−1); (d),(e),(f) 500-hPa omega anomalies (0.5 × 10−2 Pa s−1); (g),(h),(i) 600-hPa relative humidity anomalies (%); and (j),(k),(l) 850-hPa streamlines for the inactive (2 + 3) and active (6 + 7) phase as well as the mean state for seven El Niño years. Dashed (solid) contours represent negative (positive) values, while shadings denote values exceeding 95% confidence. Circles denote the positions of cyclogenesis.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

In the inactive phase, though the El Niño events contribute to some westerly anomalies, the monsoon trough in the WNP is still very weak because of the strong MJO-induced easterlies. It can be seen from Figs. 5j and 4g that the monsoon trough in the inactive phase during El Niño years shows no distinctive difference from that of neutral years, with both troughs disappearing in the WNP. Similar consistency can be observed for the relative humidity and omega anomalies. In other words, the WNP is still dominated by the strong MJO-induced unfavorable signals in the inactive phases (Figs. 5a,d,g), though the El Niño events contribute favorable background conditions. As shown in Fig. 6, significant differences in different TC-related factors between the active and inactive phase appear in the majority of the WNP. This may be attributed to the stronger and more eastward MJO-related signal during El Niño years (Fig. 2). To further substantiate this point, Fig. 7 shows the mean MJO activity, represented by the root-mean-square value of the filtered OLR anomalies, during the TC season. It indicates the variation in the MJO. A remarkable feature here is the stronger MJO activity east of 150°E during El Niño years. The eastward-extending MJO activity then results in widespread differences in TC-related parameters (Fig. 6) and thus enhances TC modulation during El Niño years.

Fig. 6.
Fig. 6.

Differences for (a) 850-hPa relative vorticity (10−6 s−1), (b) 500-hPa omega (0.5 × 10−2 Pa s−1), and (c) 600-hPa relative humidity (%) for El Niño years between the active and inactive phase. Dashed (solid) contours represent negative (positive) values; shading denotes values exceeding 95% confidence.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

Fig. 7.
Fig. 7.

Composites of seasonal-mean MJO activity, represented by the rms value of the 20–90-day filtered OLR anomalies for (a) neutral, (b) El Niño, and (c) La Niña years during the TC season, June–November.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

In summary, synchronization and reinforcement of the El Niño–related and the MJO-induced favorable conditions during the active phase largely enhance the WNP TC formation, while the enlarged unfavorable conditions related to the MJO dominate in the inactive phase. Overall, the MJO activity in El Niño years is stronger and more widespread, causing significant differences in cyclogenesis parameters in most parts of the WNP, thus enhancing the MJO–TC relationship during El Niño years.

c. La Niña years

During La Niña years, however, the large-scale easterly anomalies weaken the background monsoon trough, which leads to negative vorticity and relative humidity anomalies as well as anomalous descending motion east of 150°E (Figs. 8c,f,i). As a result, TC formation in the southeastern part of the WNP is greatly suppressed during La Niña periods, as revealed in many previous studies (Wang and Chan 2002; Chia and Ropelewski 2002; Kim et al. 2011). Nevertheless, the region west of 150°E is still dominated by strong MJO-induced signals (Figs. 8b,e,h). In the active phase, the MJO exerts its enhancement impact mainly west of 150°E, which is similar to neutral years. Therefore, the DGR in the active phase of La Niña years (16.88%) is comparable to that of neutral years (17.88%) but significantly smaller than that of El Niño years (21.28%).

Fig. 8.
Fig. 8.

As in Fig. 5, but for the inactive (2 + 3) and active (4 + 5) phase as well as the mean state for the six La Niña years.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

As for the inactive phase, there is a strengthening of the unfavorable conditions due to synchronization with the background La Niña–related unfavorable signals (Figs. 8a,d,g). Nonetheless, when compared to El Niño years that exhibit widespread differences in TC-related parameters (Fig. 6), significant differences are confined mainly west of 150°E during the La Niña period and the corresponding magnitudes are smaller (Fig. 9). This is consistent with the activity of the MJO, which also appears west of 150°E (Fig. 7). As depicted by Fig. 7, the MJO activity of La Niña years is comparable to that of neutral years but weaker and less extended compared to that of El Niño years. This agrees well with our previous statistical results. Obviously, the asymmetric features of MJO activity and other TC-related parameters in warm and cold ENSO episodes lead to different degrees of TC modulation. This explains why TC modulations in La Niña and neutral years are significantly weaker than in El Niño years.

Fig. 9.
Fig. 9.

As in Fig. 6, but for La Niña years.

Citation: Journal of Climate 25, 15; 10.1175/JCLI-D-11-00337.1

5. Discussion and summary

Using the NCEP–NCAR reanalysis data, the real-time multivariate MJO (RMM) index, and the JTWC tropical cyclone dataset for the period 1975–2010, the present study examines the different impacts of the MJO on cyclogenesis in the TC season (June–November) over the WNP during neutral, El Niño, and La Niña periods. Results indicate stronger modulation of TC genesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other, which extends our knowledge of the MJO–TC relationship.

Consistent with previous studies (Liebmann et al. 1994; Kim et al. 2008), cyclogenesis is significantly modulated by the MJO during neutral years with more (fewer) TC genesis in the active (inactive) phase. During the active phase, the strong MJO westerlies strengthen the monsoon trough, which leads to enhanced cyclonic vorticity, relative humidity, and anomalous rising motion in the WNP that are favorable for TC formation. The situation is reversed in the inactive phase such that TC formation is inhibited. This yields a moderate ESR of 1.85 in neutral years. Further examination of the rms filtered OLR anomalies reveals a high MJO activity west of 150°E in the warm pool region, coinciding with the changes in cyclogenesis and TC-related parameters.

A key finding of this study is the asymmetric modulation of the MJO on cyclogenesis under different ENSO conditions. Consistent with the results of Kim et al. (2008), a strengthened MJO–TC relationship is found over the WNP during El Niño years. The ESR is 3.73, which is twice the value of neutral years (1.85), suggesting a stronger influence of the MJO on cyclogenesis during El Niño years. In contrast, the modulation in La Niña years is moderate, with an ESR of 2.30. It is comparable to the ESR of neutral years but is significantly smaller than the greatly enhanced ESR of El Niño years.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation during El Niño and La Niña years. First, MJO activity is intensified and extends eastward to the date line during El Niño years instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of the MJO is stronger and more widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In the active phase, cyclogenesis is further enhanced due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression of TC formation. This leads to an overall enhancement in the MJO–TC relationship during El Niño years. On the other hand, the westward-confined MJO signals lead to changes in TC-related parameters mainly in the westward region, which contributes to the comparatively weaker TC modulations in neutral and La Niña years.

This work shows that the ENSO modifies the background MJO state and thus affects the degree of MJO modulation on TC formation, which explains and extends the results of previous studies (Liebmann et al. 1994; Kim et al. 2008; Mao and Wu 2010). However, it should be noted that apart from the MJO and ENSO, other factors such as equatorial Rossby waves, mixed Rossby–gravity waves, or tropical-depression-type waves should also be taken into account since they might exert influences on cyclogenesis in the WNP (Frank and Roundy 2006). The MJO is merely one of the many modulating factors with spatial and temporal differences that will affect cyclogenesis. Therefore, the interactions of different types of waves should be considered in future MJO–TC studies. In addition, Huang et al. (2011) have recently found that the modulation of the MJO also exhibits seasonal variation. Though this topic is outside the scope of this study, research is currently under way to investigate the seasonal variation of the asymmetric modulation.

Another noteworthy point is that the ENSO years chosen for this study all belong to the developing stages of ENSO. Since previous studies have demonstrated distinctive differences in WNP circulation and weather patterns during the developing and decaying phases of ENSO (Wang et al. 2010; Wu et al. 2009, 2010; Zhou et al. 2007), it is also reasonable to research how the MJO–TC relationship varies during the decaying phase of ENSO events. El Niño (La Niña) decay years are taken as the years after the El Niño (La Niña) events selected in the present study. Preliminary results suggest that the MJO shows only moderate to weak modulation during the decaying phases of ENSO (Tables 4 and 5). The strengthened MJO modulation in ENSO developing years compared to ENSO decaying years is consistent with the previous studies that found stronger MJO events frequently occur during the developing stages of El Niño (Zhang and Gottschalck 2002; Zhang and Dong 2004; Pohl and Matthews 2007), though the modulation mechanisms may be different and deserve further investigation.

Table 4.

As in Table 1, but for El Niño decaying years.

Table 4.
Table 5.

As in Table 1, but for La Niña decaying years.

Table 5.

Acknowledgments

This research is supported by the City University of Hong Kong Croucher Foundation Grant (9220055). The authors are grateful to the three reviewers for their constructive comments and suggestions.

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