3.1, Features of Three MJO Types
Figure 1 gives the composite Hovm ller diagrams of OLR anomalies of three MJO types. For the type-I MJO (Fig. 1a), the convection initiates in the WIO and propagates eastward with intensified amplitude. Upon reaching the MC, the convection is weakened by the topographic blocking (Inness and Slingo 2003; Hsu and Lee 2005; Wu and Hsu 2009; Tan et al. 2020) and strong diurnal cycle over the MC (Sobel et al. 2010; Oh et al. 2012; Hagos et al. 2016). Then, it re-intensifies after crossing the MC to the western Pacific. This propagating characteristics of type-I MJO is consistent to the canonical MJO depicted in many previous studies (Madden and Julian 1972; Lau and Chan 1985; Knutson and Weickmann 1987; Matthews 2000) , or the propagating MJO as classified in some recent literatures (Feng et al. 2015; Kim et al. 2014; Hung and Sui 2018). To the east of the MJO convection, there is a robust eastward-propagating dry phase that was deemed to play a favorable role for the eastward propagation of MJO convection (Kim et al. 2014; Chen and Wang 2018; Wang et al. 2019).
For the type-II MJO (Fig. 1b), a relative weak convection initiates in the WIO and shifts eastward, but rapidly decays before reaching 125 E. The type-II MJO resembles the eastward-decaying events of Hirata et al. (2013) and DeMott et al. (2018). There are two unique features of the type-II MJO: (1), a robust precedent event with convection lingering around the dateline along with the type-II MJO convection over the IO; (2), a severe dry zone persists over the WP. The dry zone over the WP behaves quite differently for the type-I and type-II, which dissipates rapidly in the former case, but not for the latter. To some degree, the severe dry zone in the type-II may be enhanced by the westward-propagating transient dry disturbances, which were also deemed to halt the MJO convection over the IO to propagate into the WP (e.g., Zhu and Wang 1993; Roundy and Frank 2004; Feng et al. 2015; DeMott et al. 2018).
For the type-III MJO (Fig. 1c), the associated convection initiates over the MC instead of the WIO as in the type-I and type-II. The type-III MJO convection, once initiated, rapidly intensifies over the WP and propagates further east to the central Pacific (CP). Unlike the type-I and type-II, no robust dry zone presents to the east of type-III MJO convection. It is worthy to note that the type-III MJO can not only propagate around the globe but initiate another successive event in the WIO.
The life cycles of the convective signals of three MJO types show distinctive initiation characteristics and downstream evolutions over the MC. There are two favorable MJO initiation places: the WIO for the type-I and type-II; the MC for the type-III. This result is consistent with previous studies (e.g., Kiladis et al. 2014; Hong et al. 2017; Ling et al. 2017). The dry zones to the east of the convection of three MJO types also exhibit distinctive features and potentially impact the MJO evolutions in different ways. More detailed dynamic and thermodynamic processes shaping the initiations and evolutions of three MJO types will be investigated in the following subsections.
3.2, Diverse Controlling Processes of the Three MJO Types
3.2.1, A Large-scale Circulation Perspective
Upper-level velocity potential has long been used to represent the large-scale circulations associated with the MJO (e.g.,Lorenc 1984; Krishnamurti et al. 1985; Knutson and Weickmann 1987). Figs. 2, 3, and 4, respectively, give the composite spatial-temporal life cycles of the 200-hPa velocity potential (hereafter VP200), divergent wind vectors and OLR anomalies for the type-I, type-II, and type-III.
For the type-I MJO from day -25 to -15 (Figs. 2a, b, c), a dry phase develops over the IO, accompanied by the establishment of the upper-level convergent flows (VP200>0). During the same period, a weak convective anomaly along with upper-level divergent flows (VP200<0) in the east of New Guinea dissipates on its way eastward. By day -10 (Fig. 2d), the cross-MC overturning circulation with the descending (ascending) branch over the IO (WP) has evolved into a cross-Pacific overturning circulation with the descending (ascending) branch over the WP (equatorial South America). Together with the arrival of the upper-level divergence, the surface convergence induced by the dry phase over the MC (Zhao et al. 2013) forces a weak convection over the equatorial eastern Africa and the WIO (Fig. 2d).
At day -5 (Fig. 2e), the convection center of the type-I MJO has formed over the WIO with the upper-level divergence sitting around the African-WIO sector. The in-phase relationship between the upper-level divergence and the convection implies that in addition to the low-level forcing, the upper-level divergence also contributes to the development of the convection as suggested by Hendon and Salby (1994), Matthews (2000), and Roundy (2014). At day 0 (Fig. 2f), the MJO convection intensifies rapidly and propagates to the eastern Indian Ocean (EIO). The upper-level divergent center also quickly moves eastward to the EIO co-locating with the convective center. Along with the convective (suppressed) phase over the IO (WP), a regional cross-MC overturning circulation establishes again with the ascending (descending) branch on the western (eastern) side of the MC. During the next two pentads (from day +5 to +10 in Figs. 2g, h), the convection crosses the MC from the IO into WP as well as the upper-level divergence, while the dry zone and upper-level convergence over the WP steadily weaken on their way eastward to tropical South America. A basin-wide cross-Pacific overturning circulation, therefore, is reestablished with the descending (ascending) branch over the tropical South America (MC). From day +15 to +20 (Figs. 2j, k), the basin-wide cross-Pacific circulation turns into a regional cross-MC circulation along with the formation of a robust dry zone and upper-level convergence over the IO accompanied by the weakening convection and upper-level divergence over the WP.
For the type-II MJO from day -20 to -15 (Figs. 3a, b), a dry zone rapidly forms over the EIO along with the intensified convection over the MC-WP, establishing a regional cross-MC overturning circulation. From day -10 to -5 (Figs. 3c, d), these two anomalous convective signals weaken on their way eastward. At day -5 (Fig. 3d), as the suppressed phase moves to the MC, a weak type-II MJO convection emerges over the WIO. In this case, the convection forms basically due to the surface convergence driven by the dry zone over the MC without any favorable large-scale upper-level divergence. The VP200 anomaly over the IO is still dominated with large-scale convergence: a response to the dry zone over the MC instead of the circumnavigating divergence as in the type-I (Figs. 2d, e). At day 0 (Fig. 3e), the convection anomaly intensifies and slightly extends to the MC. At the same time, the dry zone and upper-level convergence over the WP rapidly amplifies locally rather than propagating eastward to the EP. Thus, unlike the type-I, the basin-wide cross-Pacific overturning circulation is absent in the type-II. Moreover, the strong dry zone and upper-level convergence stay over the MC-WP, together with a lingering convection and upper-level divergence around the dateline from day 0 to +5 (Figs. 3e, f). The strong dry zone over the MC-WP blocks the further eastward propagation of the type-II MJO convection, eventually resulting in its decaying over the IO.
For the type-III MJO from day -15 to -10 (Figs. 4a, b), a northwest-southeast tilting dry zone develops over the MC and moves southeastward to the south of New Guinea. At day -5 (Fig. 4c), the type-III MJO convection emerges over the MC in association with the eastward moving upper-level divergence (Figs. 4a, b, c). At day 0 (Fig. 4d), the convection along with an upper-level divergent center rapidly amplifies locally over the MC-WP. During the next two pentads (from day +5 to +10 in Figs. 4e, f), the convection and the associated upper-level divergence weakens and moves eastward to the American sector, leaving the suppressed phase and the robust upper-level convergent zone over the IO-WP sector. Different from the type-I and type-II, the initiation region of the type-III is over the MC-WP instead of the WIO. Moreover, in terms of the large-scale overturning circulation, the regional cross-MC overturning circulations can be seen at days -10, and +5 (Figs. 4b, e), while the basin-wide cross-Pacific ones exist at days -5, 0, and +10 (Figs. 4c, d, f).
3.2.2, A Tropical-extratropical Interaction Perspective
Many previous studies have shown that the tropical convection associated with the MJO has strong influences on the extratropical circulations through teleconnections (Sardeshmukh and Hoskins 1988; Hoskins and Ambrizzi 1993; Matthews and Kiladis 1999; Matthews et al. 2004; Li et al. 2009). At the same time, extratropical disturbances can also affect the initiation and the evolution of MJO convection in the tropics (Hsu et al. 1990; Chang and Chen 1992; Kemball-Cook and Weare 2001; Ray et al. 2009; Ray and Zhang 2010; Ling et al. 2013; Ray and Li 2013; Zhao et al. 2013; Lee et al. 2019). In this subsection, the diverse tropical-extratropical interactions of three MJO types are investigated with the 200-hPa stream function (hereafter SF200) and associated wave activity flux (hereafter WAF), as given in Figs. 5, 7, 8.
For the type-I at day -15 and -10 (Figs. 5a, b), a pair of cyclonic [SF200<0 (>0) in NH (SH)]) and anticyclonic gyres [SF200>0 (<0) in NH (SH)] straddling on the equator, respectively, develop on the west and east of a robust dry zone over the EIO-MC. Thesequadrupole cyclonic and anticyclonic gyres are classic Kelvin-Rossby-wave responses to the MJO convection (Gill 1980; Jin and Hoskins 1995; Matthews 2000; Seo and Wang 2010; Seo and Son 2012), leading to robust upper-level equatorial westerly and easterly, respectively, over the IO and WP (Fig. 6a). A Rossby-wave train is established over northern Pacific-American-Atlantic sector with alternative anticyclonic and cyclonic gyres residing over the tropical WP-CP, extratropical North Pacific, eastern Russia-to-Alaska and eastern North America (Figs. 5a, b). The last three “CAC” gyres over the Pacific-North American (PNA) region features a positive PNA pattern. In the Pacific sector, the WAF vectors indicate that extratropical waves propagate both northeastward to Alaska-North America and equatorward to eastern Pacific. In the Atlantic sector, wave propagation is primarily equatorward. Therefore, the upper-level easterly over the equatorial western hemisphere (Fig. 6a) results not just from eastward-propagating equatorial Kelvin-wave, but also the extratropical teleconnections in response to the dry phase over the MC (e.g., Sakaeda and Roundy 2015).
From day -5 to 0 (Figs. 5c, d), in association with the onset of type-I MJO convection over the IO and the dry zone moving to the WP, a new set of quadrupole gyres with reversed signs develops over the IO-WP sector. At day 0 (Fig. 5d), a pair of cyclonic gyres is established over the tropical WP. The previous anticyclonic pairs have moved along the equator to the western hemisphere and merged with the extratropical anticyclones. The robust equatorial easterly associated with the intensified anticyclones in the African-WIO sector provides a favorable downward forcing to the intensification of the convection over the IO. As the convection over the IO further intensifies and moves to the EIO-MC at day +5 (Fig. 5e), the quadrupole Kelvin-Rossby-wave response amplifies too. The associated extratropical teleconnections (Fig. 5e) form a “CACA” wave-train over the WNP, north Pacific, north America and north Atlantic. At day +10 (Fig. 5f), this pattern further evolves into a “CACACA” wave-train with one more “CA” pair extending to northern tropical Atlantic-African sector. During day +5 and +10 (Figs. 5e, f), a negative PNA pattern with the “ACA” wave-train has been established over the northern Pacific-American-Atlantic sector, which is about five days after the robust MJO convection develops in the IO (Fig. 5d). This result is consistent with previous findings (Hsu 1996; Mori and Watanabe 2008; Seo and Son 2012; Tseng et al. 2019; Chen 2021).
For the type-II MJO from day -15 to -10 (Figs. 7a, b), the upper-level quadrupole Kelvin-Rossby-wave response forms over the tropical IO-WP region associated with the eastward-moving convection (dry zone) over the MC (IO) as in the type-I. However, the zonal spans of the anticyclonic pairs over the WP are much smaller than that in the type-I (Fig. 6b and Fig. 7b). Instead of propagating eastward as in the type-I, the anticyclonic pairs over the WP rapidly disappear in next pentad (day -5 in Fig. 7c). During the same period (day -15 to -5 in Figs. 7a, b, c), the cyclonic pairs over the western hemisphere along with the equatorial westerly (Fig. 6b) rapidly move eastward to enhance the cyclonic pairs over the IO. As the dry zone moves to the MC-WP, the type-II MJO convection initiates over the IO with unfavorable upper-level westerly and convergence (Fig. 3d). At day 0 (Fig. 7d), both the convection over the IO and the dry zone over the MC-WP intensifies, but the upper-level anticyclonic pairs as a Rossby-wave response to the IO convection never manifest as in the type-I. From day -5 to 5 (Figs. 7c, d, e), a quasi-stationary elongated cyclonic gyre presents over the northern WP-CP as one lobe of a Pacific-Japan-like pattern (Kosaka and Nakamura 2006; Wu et al. 2020) and/or a persistent North Pacific pattern (Higgins and Mo 1997). The induced convection over the subtropical WP may help sustain the dry phase over the equatorial WP through a meridional overturning cell. Near the eastern end of this elongated cyclonic gyre, the enhanced equatorward wave activity maintains a lingering convection over the equatorial CP during the entire life cycle of the type-II MJO convection over the IO (Matthews and Kiladis 1999; Moore et al. 2010). The lingering convection over the CP keeps feeding the IO with upper-level westerly (Fig. 6b) and blocking the dry zone over the WP and associated upper-level easterly moving downstream. The lack of upper-level easterly “ventilating” flow and the blocking of enhanced dry zone over the MC-WP lead to the rapid decaying of the type-II MJO convection over the IO (Fig. 7e). During the entire life cycle of the type-II MJO, the extratropical downstream response is largely caused by the quasi-stationary cyclonic gyre over the subtropical WP instead of the tropical convection over the IO-WP. Therefore, a negative PNA-like pattern with “ACA” wave-train (similar as in Fig. 1f of type-I) has presented over the northern Pacific-American-Atlantic sector from day -5 to 5 (Figs. 7c, d, e).
For the type-III MJO from day -15 to -10 (Figs. 8a, b), a weak dry zone over the WP along with its Kelvin-wave-like response (i.e., a pair of anticyclones over the equatorial eastern Pacific) gradually moves eastward. The anticyclone sitting around 20oN in the African-Indian sector is intensified by the equatorward-propagating wave activity in association with the positive NAO pattern (Figs. 8b, c; Lin et al. 2009), thus establishing an apparent equatorial easterly on its southern flank (Fig. 6c). At day -5 (Fig. 8c). The precedent upper-level easterly over the equatorial IO is further intensified (Fig. 6c) by the arrival of the eastward-propagating Kelvin-wave-like anticyclonic pairs, providing an efficient “ventilating” flow for the rapid onset of type-III MJO convection around the MC. In response to this convection, a cyclonic gyre is supposed to form over the WNP as a Gill-type response (Rui and Wang 1990). However, there is an anticyclone over the WNP, which was propagated eastward from eastern China (Figs. 8a, b, c). At day 0 (Fig. 8d), the convection over the MC-WP amplifies rapidly along with the enhanced anticyclones in the WNP and southern IO. The associated equatorial easterly almost covers the entire eastern hemisphere (Fig. 6c). The NH cyclonic response to the convection over the MC-WP is shifted about 40° east of its SH counterpart due to the existence of a quasi-stationary anticyclone over the WNP. Therefore, the upper-level equatorial westerly (Fig. 6c) around the dateline is primarily driven by the SH cyclonic gyre. From day +5 to +10 (Figs. 8e, f), the convection over the WP rapidly weakens on its way eastward. The associated cyclonic pairs and equatorial westerly quickly move into the IO and foster the development of a dry zone, which eventually leads to another MJO cycle (Fig. 1c). In terms of the NH extratropical teleconnections, the situation is very similar as for the type-II. The downstream impacts largely originate from the quasi-stationary anticyclone over the WP-CP, instead of a direct response to the MJO-related convection. During the life cycle of the type-III MJO (Figs. 8c, d, e, f), the anticyclone in the northern extratropical Pacific gradually intensifies and moves to the CP. The associated downstream teleconnections over the northern Pacific-American-Atlantic sector also evolve from a negative PNA-like pattern (Figs. 8c, d) to a positive one (Figs. 8e, f).
In order to better understand the possible processes leading to the rapid development of the type-III MJO convection over the MC-WP around the day 0 (Fig. 8d), the associated 850-hPa wind and stream function (hereafter SF850) anomalies are examined in Figs. 9 and 10. As early as day -10 (Fig. 9b), there is an apparent, yet weak, equatorial easterly anomaly between 130°E and 180° in the north of Australia, which opposes the prevailing westerly along the equator, favoring the development of lower-level convergence (a hint can be seen in Fig. 11i). This equatorial easterly anomaly is forced by a pair of anticyclones straddling on the equator (Fig. 10b). In association with the “ventilating” upper-level easterly over the equatorial IO (Fig. 8c) and large lower-tropospheric convective instability due to tropospheric drying (Fig. 13i), the type-III MJO convection emerges around the MC at day -5. At day 0 (Figs. 9d, 10d), the convection is rapidly intensified by the northerly in the WNP associated with the arrival of a strong east-Asian cold surge. The arrival of the northerly along the east coast of Asia is accompanied by the build-up of a meridionally-oriented anticyclone at lower troposphere, as manifested by the positive SF850 anomaly spreading from eastern Russia to eastern Asia (from day -5 to 0 in Figs.10c, d). This scenario is consistent with previous findings (e.g., Hsu et al. 1990; Chang and Chen 1992; Pang et al. 2018; Wang et al. 2018) that the cold surge-related northerly favors the initiation, intensification and eastward propagation of the MJO convection over the MC-WP through enhancing the lower-level convergence.
3.2.3, A Regional Perspective
In preceding two subsections, the distinctive large-scale circulations in association with three MJO types have been examined. In present and next subsections, we will zoom into the active convection region over tropical Indo-western Pacific domain (from 30oE to 150oW) to investigate the regional dynamic and thermodynamic processes associated with three MJO types during their initiation and evolution periods. Some previous studies have been devoted to understanding the regional dynamic and thermodynamic processes in association with MJO diversity (Hsu and Li 2012; Zhao et al. 2013; Feng et al. 2015; Chen and Wang 2018; Wang et al. 2019). The potentially important regional processes include the discharge-recharge of convective instability (Blade and Hartmann 1993; Kemball-Cook and Weare 2001), the precedent easterly over the IO contributed by both upstream forcing of Kelvin wave and downstream forcing of Rossby wave (Matthews 2000; Seo and Kim 2003; Zhao et al. 2013), boundary-layer convergence, leading dry phase and front walker cell (Hsu and Li 2012; Hsu et al. 2014; Kim et al. 2014; Feng et al. 2015; Chen and Wang 2018), as well as intra-seasonal SST feedback (Li et al. 2008; Fu et al. 2015, 2017, 2018b). To what degree do these regional processes contribute to the initiations and evolutions of the convection associated with three MJO types? To answer this question, the vertical structures of divergence, moisture, equivalent potential temperature (EPT), apparent heat source and moisture sink anomalies (Q1 and Q2, Yanai et al. 1973) along with the associated circulations for three MJO types have been carefully examined in the following (Figs. 11, 12, 13, and 14).
For the type-I MJO (Figs. 11, 12, 13, 14a-d), the boundary-layer convergence, positive moisture and EPT anomalies appear as early as day -15 in the WIO before the emergence of negative OLR anomaly (Figures not shown). From day -10 to -5 (Figs. 11a, b), this boundary-layer convergence along with the positive moisture (Figs. 12a, b) and positive EPT (Fig. 13a, b) anomalies extend eastward to the EIO and upward to mid-troposphere. The associated convection also evolves from shallow and congestus to deep convection (Figs. 14a, b) with an apparent front walker cell to the east of the convection (Chen and Wang 2018). The higher altitude of maximum Q1 than Q2 (Fig. 14b) suggests that eddy vertical flux convergence contributes significantly to apparent heat source, indicating the MJO convection at a rigorous development stage (Johnson et al. 2015). In next two pentads (day 0 and +5, Figs. 11c, d), the deep convergence zone still resides over the EIO-MC with boundary-layer convergence along with positive moisture (Figs. 12c, d) and positive EPT (Fig. 13c, d) anomalies penetrating to the WP (120°E-160°E) already, thus establishing robust rearward-titling structures of the convergence zone, moisture and EPT. A front (backward) walker cell develops on the east (west) of the convection. The alignment of maximum Q2 and Q1 indicates that the MJO convection has transitioned from dominant convective to stratiform regime (Figs. 14c, d). The systematic positive (Q1-Q2) in the troposphere also suggests that radiative heating potentially plays an important role on sustaining the MJO convection. During the entire life cycle of type-I MJO convection (Figs. 11, 12, 13, 14a-d), the boundary-layer convergence, positive moisture and convective instability consistently lead the convection (negative OLR anomaly). This result testifies that the boundary-layer convergence (Hsu and Li 2012; Chen and Wang 2018; Wang et al. 2019), positive moisture (Lin and Johnson 1996; Raymond and Torres 1998; Hsu and Li 2012) and convective instability (Kemball-Cook and Weare 2001; Zhao et al. 2013; Wang et al. 2019) work in a cooperative way for the development and eastward propagation of the type-I MJO convection.
For the type-II MJO (Figs.11, 12, 13, 14e-h), weak and scattering boundary-layer convergence anomalies begin to appear over the tropical IO as early as day -10 (Fig. 11e) although there is still robust convergence (divergence) anomaly in the upper (lower) troposphere, which is consistent with the overwhelmingly positive VP200 in Fig. 3c. The boundary-layer convergence and the equatorial easterly (Fig. 6b) are likely forced by the dry zone around the MC (Zhao et al. 2013). At the same time, weak positive moisture (Fig. 12e) and EPT (Fig. 13e) anomalies also appear over the IO with robust convective instability (Fig. 13e), but without apparent convection (negative OLR anomaly, Fig. 14e). The robust convective instability (Fig. 13e) is primarily caused by mid-tropospheric drying rather than boundary-layer moistening (Figs. 12e and 13e). In the next pentad (day -5 in Fig. 11f), the low-level anomalous convergence intensifies over the IO and slightly extends upward, so do the positive moisture (Fig. 12f) and EPT (Fig. 13f) along with the onset of deep convection (Fig. 14f). A deep front (shallow backward) walker cell forms on the east (west) of the convection. At the same time, a deep divergence zone along with negative moisture and EPT anomalies develop in the WP (Figs. 11, 12, 13f), in association with the lingering convection near the dateline and quasi-stationary cyclonic gyre over the WNP (Fig. 7c). At day 0, fueled by the robust convective instability (Figs. 13f, g), the convection over the IO is further intensified by the positive feedback between the overturning circulation and the diabatic heating. The robust ascending motion along with positive moisture and EPT anomalies occupies the entire troposphere (Figs. 11, 12, 13g). The MJO convection transitions from dominant convective to stratiform regime (Figs. 14f, g). Different from the type-I MJO, the dry zone over the WP is further intensified (Figs. 12f, g) together with the enhanced convection over the IO instead of weakening (Figs. 12b, c). As the backward walker cell disappears, the front walker cell dominates. The enhanced drying and descending motion over the MC-WP hamper the development of leading boundary-layer convergence and moistening (Figs. 11, 12g). At day +5 (Figs. 11, 12, 13, 14h), the convection over the IO rapidly decays due to the lack of efficient upper-level “ventilating” outflows (Fig. 6b) and leading boundary-layer convergence and moistening as well as the intrusion of severe dry zone over the WP (Feng et al. 2015; DeMott et al. 2018) although there is robust leading convective instability over the MC-WP sector (Figs. 13g, h).
For the type-III MJO (Figs. 11, 12, 13, 14i-l), the initial convergence primarily appears over the WIO in the mid-troposphere (700-300-hPa) accompanied by upper-level divergence at day -10 (Fig. 11i). Weak but significant convergence can also be found in the mid-troposphere (700-500-hPa) around 105°E and near the surface around the MC associated with the low-level easterly (Fig. 9b) induced by the dry zone over the MC-WP. At the same time, weak positive moisture, EPT and negative OLR anomalies (Figs. 12, 13i) scatter over the IO, but without significant diabatic heating (Fig. 14i). Robust convective instability develops over a broad region from the WIO to the dateline (Fig. 13i) with that over the WP primarily due to the mid-tropospheric drying instead of boundary-layer forcing. At day -5 (Figs. 11, 12, 13, 14j), a broad zone of upper-level “ventilating” easterly (Fig. 6c) and divergence (Fig. 11j) is established in association with the rapid eastward extension of a pair of anticyclones (Fig. 8c). These favorable upper-level conditions and strong convective instability foster the rapid development of type-III MJO convection along with robust tropospheric and boundary-layer convergence (Fig. 11j) as well as positive moisture and EPT anomalies (Figs. 12 and 13j) over the IO-MC-WP sector. Contrary to the type-II MJO (Fig. 11f), a deep backward (shallow front) walker cell develops to the west (east) of the convection (Fig. 11j). The convection on the west and east sides of about 135oE are, respectively, dominated by stratiform and convective regimes (Fig. 14j). The robust in-situ convective instability (Fig. 13j), along with the arrival of the east-Asian cold surge (Figs. 9d, 10d, 13k), boosts the rapid intensification of deep convection over the WP at day 0 (Figs. 11, 12, 13k). The corresponding convection also shifts from convective to stratiform regime (Fig. 14k). The persistent positive (Q1-Q2) anomalies at the mature stages (with dominant stratiform rainfall) of the type-I (Figs. 14c, d), type-II (Fig. 14g) and type-III (Fig. 14k) MJO convections testify the important role of radiative heating on sustaining the MJO convection regardless the MJO types. Different from both the type-I and type-II, the type-III MJO has a predominant backward walker cell (Figs. 11j, k, l) instead of a dominant front walker cell (Figs. 11b, c, d and 11f, g), likely due to the lack of a robust deep dry zone preceding the type-III MJO convection (Figs. 12j, k, l). At day +5, the boundary-layer convergence (Fig. 11l), positive moisture (Fig. 12l) and convective instability (Fig. 13l) over the CP steer the type-III MJO convection to further propagate eastward.
3.2.4, A Moisture-budget Perspective
Many previous works have emphasized the essential roles of positive boundary-layer moisture anomaly and positive moisture tendency on leading the eastward propagation of MJO convection (e.g., Kemball-Cook and Weare 2001; Hsu and Li 2012; DeMott et al. 2014; and Feng et al. 2015). For the type-I and type-III, there are apparent positive boundary-layer anomalies leading the eastward-propagating MJO convection, but not for the type-II (Fig. 13). For different MJO events, the boundary-layer moisture controlling processes can be quite different (Kemball-Cook and Weare 2001; Mei et al. 2015). In this subsection, the major processes governing the boundary-layer moisture tendency for three MJO types are investigated through boundary-layer moisture budget analysis (Figs. 15, 16).
For the type-I (Fig. 15a), the positive boundary-layer moisture tendency leads both the initiation and eastward-propagation of MJO convection. The overall evolution of the moisture tendency resembles the horizontal advection term (Figs. 15a, d). Although the magnitudes of the vertical advection and the moisture sink (Figs. 15b, c) are five times larger than other terms, they almost cancel each other (Fig. 15e). Further decomposition of the horizontal advection term into zonal and meridional advection (Fig. 16a, b, c) shows that, during the initiation phase of the MJO, the positive advection between 30oE-90oE is primarily contributed by zonal advection (Zhao et al. 2013; Hung and Sui 2018). On the other hand, the meridional advection leads to the positive advection over the 110°E-180°, favoring the eastward propagation of the MJO (Kiranmayi and Maloney 2011; Kim et al. 2014; Feng et al. 2015; Wolding and Maloney 2015).
For the type-II MJO (Fig. 15f), the leading positive tendency only exists over the IO region, which is mainly contributed by the horizontal advection (Fig. 15i). The maximum positive tendency around 90°E at day -10 (Fig.15f) is also contributed by the moisture sink term although the negative vertical advection plays an opposite role (Figs. 15g, h, j). As in the type-I, the horizontal moisture advection generates positive boundary-layer moisture tendency before the onset of the type-II convection in the IO and to the east of the convection in the WP (Fig. 15i), which is supposed to favor the initiation of convection and its eastward propagation. However, the strong negative vertical advection induced by the dry zone over the WP (Fig. 15g and Figs. 12f, g, h) exceeds the positive moisture sink and horizontal advection (Figs. 15h, i), thus resulting in near-zero boundary-layer moisture tendency east of 105oE (Fig. 15f). The lack of apparent positive boundary-layer moisture tendency east of the IO convection hinders the eastward propagation of type-II MJO (Feng et al. 2015). Further decomposition shows that meridional advection plays a larger role than the zonal advection on the positive horizontal advection term (Figs. 16d, e, f).
For the type-III MJO (Fig. 15k), there is no obvious positive boundary-layer moisture tendency preceding the onset of convection over the MC (from 90°E to 120°E). The coherent positive moisture tendency over the WP (from 120°E to 170°E) around day -10 may favors the rapid eastward migration of the type-III MJO convection through moistening the boundary layer over the WP (Fig. 12j). This result suggests that the boundary-layer process only plays a minor role on the initiation of type-III MJO convection. It is the upper-level easterly “ventilating” flow (Fig. 6c and Figs. 8c, d; Roundy 2014) and large-scale convective instability induced by mid-tropospheric drying (Fig. 13i; Lavender and Matthews 2009) triggers the onset of type-III MJO convection over the MC. As for the Type-I and Type-II, the positive moisture tendency of the type-III over the 120°E-170°E is basically caused by horizonal advection (Fig. 15n). In fact, the coherent positive anomaly extending from the IO to WP has been generated by horizontal advection preceding the onset of the convection (Fig. 15n). It is the negative vertical advection and moisture sink (Figs. 15l, m) that oppose the positive horizontal advection, resulting in a near-zero overall tendency over the IO-MC. Further decomposition (Figs. 16g, h, i) shows that meridional (zonal) advection dominates the positive horizontal advection west (east) of 120°E.