Topographic Influence on the MJO in the Maritime Continent

3. Convection, near-surface circulation, and topography

The Maritime Continent is then call because of the fragmental distribution of islands and seas ( Fig. 1a ). Narrow and high mountains on the islands of Sumatra, Borneo, Sulawesi, and New Guinea create sharp terrain variations in this knowledge domain. several of these islands have mountains higher than 3000 m. Long batch ranges exist on elongate islands, such as Sumatra, Java, and New Guinea. The largest one is the northwest–southeast batch range, which extends over 1600 kilometer with the highest batch at 4884 thousand, on the universe ’ s second largest island, New Guinea. It will be demonstrated late that these narrow and farseeing batch ranges seem to have a significant impression on the MJO. Intraseasonal discrepancy accounts for 40 % –50 % and 15 % –30 % of sum variation in 850-hPa zonal wind and TRMM surface precipitation, respectively. The spatial distribution of intraseasonal variation presented in Fig. 1b exhibits distinct land–sea contrast in the Maritime Continent, particularly in the vicinity of larger islands such as Sumatra, Borneo, and New Guinea. In the boreal winter, the zonal tip discrepancy is larger south of the equator than north of the equator. Furthermore, division over ocean is generally larger than over land. For example, local zonal wind variance maximum occupy over the oceanic distribution channel between islands in the Maritime Continent such as Java, New Guinea, and the australian landmass. The other utmost locates at the northeast side of New Guinea. These maxima distinctively contrast the local minimum variances along a zonal band covering Borneo, Sulawesi, and New Guinea. Another noteworthy feature is the maximum–minimum–maximum distribution from the Torres Strait to the region north of New Guinea, along the 130°–150°E longitudinal bands. The TRMM precipitation discrepancy exhibits like land–sea contrast. large variances are found confederacy of Sumatra and Java, over the Java Sea, the Arafura Sea, the Gulf of Carpentaria, and the western Pacific. Same as for zonal tip, the haste discrepancy over the oceanic regions union and south of New Guinea is much larger than the variability over New Guinea. As noted by Hsu and Lee ( 2005 ), these division maximum lay along the propagation path of the MJO through the Maritime Continent. other utmost are located near the Malay Peninsula, the South China Sea, and the Philippines. These are places where convection bodily process fluctuates during the MJO passage. The TRMM open precipitation ( shading ) and OLR ( contour ) anomaly maps between 12.5°N and 12.5°S from phase 1 to phase 7 are shown in Fig. 3. note that only those statistically significant at the 0.1 level are shown. Although the temporal role TRMM and OLR coverage overlaps by only three winters ( 1998/99–2000/2001 ), the respective phase evolution and dominant features are quite alike. The consistency between the OLR and TRMM composites indicates that different temporal coverage does not affect our observation about the MJO because the major characteristic of the MJO is rather stationary in time, at least in the past few decades. The close association between positive TRMM precipitation anomaly and negative OLR anomaly suggests that the TRMM precipitation anomaly can be interpreted as the variation of deep convection. In the come discussion the positive haste anomaly, which besides has finer spatial resolution for revealing details, will be interpreted as active bass convection.

The eastbound propagation leaning of the MJO is clearly observed in both TRMM precipitation and OLR anomalies in Fig. 3, but with the apparent charm of the land–sea contrast in the Maritime Continent. During the period from phase 1 to phase 3, the major MJO convection anomaly propagates along the equator from 75° to 100°E. When reaching the northwest–southeast-elongated Sumatra at phase 4, the convection anomaly moves southeastward along the southwest edge of Sumatra to the Timor Sea, while anomalous convection besides develops over the Java Sea, the Banda Sea, and over the ocean to the northwesterly of New Guinea. The relatively unblock convection over the neighbor islands is apparent. At phase 5, a region of anomalous convection flares suddenly over the ocean to the northeast of New Guinea, while the convection is silent active over the ocean to the west of New Guinea but is inactive over New Guinea. This sudden development between phase 4 and phase 6 shifts the major convection center from 130° to 150°E and closer to the equator. The newly formed convection anomaly east of New Guinea continues its southeastward travel to the westerly Pacific. The eastward bowel movement of the MJO convection apparently skirts around the island, explaining the large intraseasonal discrepancy over the ocean. The presence of the island chain from Sumatra to New Guinea in the Maritime Continent seems to have an impression in detouring the motion of the MJO deep convection, beginning south when entering the Maritime Continent and then back to near the equator when leaving. As noted in previous studies ( for example, Ichikawa and Yasunari 2008 ), diurnal fluctuation is potent over the islands in the Maritime Continent, obviously because of the heating effect of the country come on. conversely, the mountainous islands tend to prevent the convection from occurring over lands in the intraseasonal time scale. The universe of the high-mountain island apparently leads to the hedge and rise of the major convection around the islands. An examination of near-surface circulation may provide more clues to this interest characteristic. Horizontal maps of TRMM precipitation ( contours/shadings ) and 10-m tip anomalies ( vectors ) from phase 2 to phase 7 are shown in Fig. 4. During the period when the tip form changes from the prevailing easterly anomaly over the integral Maritime celibate at phase 2 to the prevailing prevailing westerly anomaly at phase 6, the development of the westerly anomaly occurs chiefly over the oceanic regions, and following close the movement of the deep convection it appears in and besides to the west of the deep convection region. The westerly anomaly tends to split toward the north and south when it approaches the mountainous islands and flows around the major terrains. This can be seen distinctly to the west of Sumatra at phase 3, to the north and south of Java at phase 4, and to the north and south of Sulawesi and New Guinea at phases 5–7. firm westbound anomalies are observed both to the north and south of these islands, but very faint winds are observed atop the islands. The separate flow around these islands does not occur alone in the prevailing westerly phase. It is besides apparent at phase 2 when the easterly anomaly prevails in the Maritime Continent. exchangeable results were besides observed in the pressure-level wind anomaly in the lower troposphere, such as 850 hPa. More details will be discussed in the latter sections .

4. Vorticity and divergence

figure 5 shows vorticity and discrepancy anomalies at 850 hPa from phase 2 to phase 7, respectively. Before the arrival of the MJO convection center at Sumatra ( phase 2 ), the meridional structure of vorticity design between 120° and 150°E is basically characterized by four zonally elongate bands of anomalies : negative to the north of 5°N, positive near the equator ( except Borneo ), negative between the equator and 10°S, and convinced between 10° and 20°S. These zonally banded patterns are distorted around cragged islands, where the rending flows are observed in Fig. 4. The most matter to ones are the positive–negative vorticity dipoles over Sulawesi and New Guinea. At phase 2 when the east wind anomaly prevails, positive and negative vorticity anomalies are located in the north and south of these two islands, respectively. conversely, the vorticity dipole reverses phase and becomes negative in the north and positive in the confederacy at phase 4 and 5 when the prevail scent is westerly anomaly. The universe of the vorticity dipole can be attributed to the parry and frictional effect of the mountains on the prevail hang. Both easterly and westerly anomalies at 850 hPa, the same as the 10-m wind shown in Fig. 4, are identical fallible near the mountainous islands and increasingly stronger away from the islands ( not shown ). This shearing effect that will be further discussed in Fig. 5 would result in the being of the vorticity dipoles over Sulawesi and New Guinea. Such a dipole may besides exist around the elongated mountainous Java Island. however, data in a much higher spatial resolution will be needed to reveal the structure because of the narrow-mindedness of Java. Another matter to sport is the vorticity anomaly over Borneo. At phase 2, easterly anomaly curves clockwise around southern Borneo ( as in Fig. 4a ), likely because of the blocking effect of cragged Borneo, and results in negative vorticity anomaly. On the contrary, convinced vorticity anomaly appears at phase 5 when westerly anomaly curves counterclockwise around southerly Borneo ( for example, Fig. 4d ). broadly speaking, a MJO is characterized by the Kelvin–Rossby wave packet, with counterpart cyclonic circulation and equatorial westerly to the west of deep convection and equatorial easterly to the east. Such a large-scale design is distorted when the MJO moves into the Maritime Continent because of the being of the mountainous islands. The distortion of the east wind anomaly can be seen clearly in Fig. 4a. To the east of New Guinea ( for example, 150°E ) where few islands exist, the easterly anomaly is about uniformly distributed and weakens away from the equator, exhibiting the Kelvin wave characteristics. conversely, the east wind anomaly in the Maritime Continent prevails only over the oceanic channels between islands. This land–sea contrast is besides observed in the vorticity field. For example, the positive vorticity anomaly associated with the anticyclonic circulation anomaly south of the equator at phase 2 is meridionally wider over the region from the amerind Ocean to the west of Australia, but it becomes much narrower in the Maritime Continent and restricted in the oceanic transmit between the islands and Australia where a stronger east wind anomaly is observed. This land–sea contrast impression results in the twist vorticity distribution, whose shape more or less follows the land–sea distribution. This characteristic is besides apparent in the later phases ( for example, phases 4–6 ) when the westerly and negative vorticity anomalies prevail. The circulation at phases 5 and 6 is in the equatorial Rossby wave government, which should be characterized by positive and negative vorticity to the north and south of the equator, respectively. however, this vorticity pair is interrupted by the vorticity dipoles existing over Sulawesi and New Guinea and becomes a quadruple model in the Maritime Continent as discussed before. The discrepancy anomalies presented in Fig. 5 read good relationships with the airfoil precipitation in Fig. 4, where overlap zones coincide with large precipitation regions. When the MJO convection anomaly moves closer to the west of Sumatra at phase 2, anomalous overlap is observed at 90°–100°E and anomalous deviation at 140°–160°E, implying the horizontal scale of the initial vertical circulation is about 70° longitude. Embedded in this large-scale convergence–divergence dipole are local convergence and divergence pairs near mountainous islands. For model, convergence anomalies are found near Sulawesi and at the western end of New Guinea Island ( for example, 120° and 135°E near the equator ), while a divergence anomaly is found at the east side of New Guinea Island ( for example, 140°E ). This local east–west convergence–divergence copulate is associated with the north–south vorticity dipoles over Sulawesi and New Guinea. It can be understand as follows : the topographical blocking effect on the easterly flow leads to local deviation and convergence anomaly at the windward ( easterly ) and lee ( western ) side of topography, respectively. The accelerate of splitting easterly anomaly around the topography decreases toward the batch terrain and creates local counterclockwise–clockwise circulation to form the north–south vorticity dipoles. After phase 2, the convergence anomaly to the west of Sumatra moves southeastward along the southwest edge of Sumatra and Java. This southerly fault of convergence anomalies, which is reproducible with the movement of the precipitation anomaly shown in Figs. 3 and 4, is probable ascribable to the blocking effect of the elongated mountainous Sumatra. By phase 5 when the overlap anomaly moves to the west and south of New Guinea, the convergence anomaly on the east side of New Guinea becomes well developed and merges with the southern design between 140° and 160°E. At this time, the large-scale convergence–divergence dipole switches sign, with anomalous convergence and divergence at the eastern and western sides of the Maritime Continent, respectively. The divergence–convergence couple over Sulawesi and New Guinea besides change signs with the divergence and convergence anomalies at the westerly and easterly ends, respectively. The vorticity dipoles straddling the islands reverse their signs at the same time. The development of the low-level divergence–convergence anomaly is besides strongly influenced by the topography like other variables .

5. Cross sections of dynamical fields

a. Meridional profiles at specific longitudes

The topographical effect on the MJO circulation and convection is demonstrated further in this part by examining the meridional upright cross department at longitudes where mountainous islands are located. zonal fart ( shadings ), vorticity ( contour ), and circulation ( vectors ) anomalies at 100°E during phase 4 ( Fig. 6a ), at 120° and 140°E during phase 6 ( Figs. 6b, coulomb ), and at 160°E ( Fig. 6d ) during phase 7 are shown. These specific phases are chosen when a firm westerly anomaly, rather of convection, prevails near the mountainous islands to clearly reveal the topographical effect on the hang. At phase 4 when strong westerly anomalies reach Java and a cyclonic circulation forms to the confederacy of 10°S ( Fig. 4c ) in the southeast indian Ocean, the traverse section at 100°E ( Fig. 6a ) is characterized by a pair of deep vorticity anomalies, positive in the Northern Hemisphere and negative in the South Hemisphere. This vorticity anomaly pair may be viewed as the Rossby wave located to the west of deep convection, which is now located south of Sumatra and Java. The asymmetry between the two vorticity components is apparent. For model, the southerly damaging one exhibits a deep upright structure with maximum amplitudes in the lower troposphere, while the northern positivist one is an elevated deep structure with maximum amplitudes in the middle troposphere. Furthermore, a shallow ( below 850 hPa ) plus vorticity anomaly is found between the southern negative vorticity anomaly and the cragged Sumatra ( dark-shaded sphere in the trope ). At the like fourth dimension, a westbound anomaly with the maximal below 600 hPa is found between the positive–negative vorticity pair south of Sumatra. The weave travel rapidly anomaly decreases cursorily away from the center. The strong zonal weave fleece at both sides of the maximum wind anomaly apparently results in the near-surface vorticity anomaly. In comparison with the menstruation convention at 120° and 140°E, which will be discussed belated, the shifting of the zonal wind anomaly at 100°E to the Southern Hemisphere is likely ascribable to the blocking effect of the elongated cragged Sumatra, which besides results in the near-surface positive anomaly between 5°S and the equator. Accompanying the vorticity match and the prevailing westerly wind anomaly in the Southern Hemisphere is a vertical circulation with the descent over and to the south of Sumatra and the ascent around 10°S. The ascent corresponds to the deep convection embedded in the MJO ( Figs. 4b, coulomb ). The descending motion is apparently due to the blocking effect of the elongated mountainous Sumatra, which results in the bifurcation of the westbound hang to the north and south ( Figs. 4b, speed of light ) and the deviation in the lower troposphere ( Figs. 5b, speed of light ). It is interesting to note that the near-surface northerly anomaly in this vertical circulation may act to accelerate the prevailing westerly anomaly through the Coriolis force. The situation is unlike at 120°E where the topographical effect is tied more interesting. topography in this area includes Sulawesi ( the equator to 5°S ), Sumba of the Lesser Sunda Islands ( 10°S ), and northerly Australia ( south of 20°S ). Shown in Fig. 6b is the hybridization section at phase 6 when the topographical effect is most discernible. When the westerly anomalies start prevailing at phase 5, the westerly anomaly flows over the cragged Sulawesi. The overall distribution of vorticity anomaly is negative in the Southern Hemisphere and positive in the Northern Hemisphere. The distortion of this vorticity pair by the topography is obvious. While the prevailing westerly anomaly flows over the mountains, specially over Sulawesi, its travel rapidly decreases toward the mountains both meridionally and vertically and results in an archlike distribution over Sulawesi and Sumba of the Lesser Sunda Islands. As a leave, shallow negative and positive vorticity anomalies appear over northern and southerly Sulawesi, respectively. A comparison between total vorticity ( ∂υ/∂x – ∂u/∂y ) and fleece vorticity ( −∂u/∂y ) confirms that the vorticity dipole owes its universe to the strong zonal wreathe shear near the topography. This leave indicates again the importance of the topography in modifying the MJO structure. A alike inclination is besides seen over Sumba of the Lesser Sunda Islands. The impression of the batch ranges in New Guinea on the MJO is demonstrated below. When the MJO reaches the central Maritime Continent at phase 5, the westerly anomaly around New Guinea splits into two branches, as seen in Fig. 4d. This phenomenon can be seen clearly at phase 6, shown in the vertical cross section at 140°E ( Fig. 6c ). A westerly anomaly extending from the come on to 400 hPa exists both north and south of New Guinea. The utmost wind instrument speed is observed over the equator and around 10°S in the low–middle troposphere and decreases meridionally away from the utmost fart region, leaving a weak westerly wind anomaly atop New Guinea. The boastfully zonal weave shear at both sides of the mountain results in the negative and convinced vorticity anomaly to the north and south of New Guinea, respectively. The vorticity anomaly exhibits basically an equivalent barotropic vertical structure. To the north and confederacy of this vorticity dipole around New Guinea, there exists another pair of vorticity anomalies ( incontrovertible in the north and negative in the south ) good like in early cross-section plots. The overall vorticity distribution is characterized by a quartet convention, which is seen besides in Figs. 5d, einsteinium. The presence of mountainous New Guinea obviously results in the appearance of the quadruple vorticity convention, which is a deep structure extending from surface to the upper troposphere. anomalous ascent is found to the north of New Guinea and between 10° and 20°S ( i, the Gulf of Carpentaria in northern Australia ), corresponding to the positive precipitation anomaly, while descent is observed over New Guinea and immediately south of it. A meridional vertical circulation is distinctly seen to the south of New Guinea and the equate near-surface northerly may have an impression in accelerating the westerly fart anomaly through the Coriolis force.

To contrast the topographical effect presented above, the cross section at 160°E, where entirely a small cragged island exists and therefore the topographical effect is minimum, is besides presented in Fig. 6d. The westerly anomaly is found in the lower–middle troposphere largely south of the equator. This tendency for the prevailing westerly anomaly to appear in the Southern Hemisphere in the western Pacific is one of the major characteristics of the MJO in the boreal winter. The maximal tip travel rapidly region is much wider than those in the previous three intersect sections where bombastic topography exists. The prevailing westerly anomaly region is sandwiched between a match of vorticity anomaly associated with the cyclonic circulation off the equator. This pattern exhibits the distinctive characteristics of the circulation, which is located to the west of the deep convection in a MJO. In contrast, the typical cyclonic copulate in the MJO is either falsify or modified by the topography in the former three cross sections .

b. Zonal profiles of 5° and 10°S

Figures 7a–d show the cross sections of anomalous erect circulation along 5°S, from phase 3 to phase 6. They demonstrate the kinship between the upright circulation and the topography along the major path of the MJO. The high terrains hindering the enactment of the MJO through the Maritime Continent are Sumatra ( 105°E ), Sulawesi ( 120°E ), and New Guinea ( 140°–150°E ). note that the vertical speed has been multiplied by 100 to show more intelligibly the match fluctuation. At phase 3, the major up and down gesture is located to the west of 120°E and east of 150°E, respectively. In an idealize situation ( e.g., an aquaplanet or a homogeneous background country ), the east–west overturn circulation would look like a smooth Walker circulation, without much pause between the upward branch around 90°–100°E and the down branch east of 150°E. however, wavelike perturbations are observed in the vicinity of the major terrain. The ascending region to the west of 120°E, which corresponds to the major convection region in the MJO in Fig. 4b, is interrupted by the remission ( below 700 hPa ) at the windward side of Sumatra. This settling is reproducible with the rending flow and the near-surface discrepancy in the region due to the blocking impression of Sumatra on the prevailing prevailing westerly anomaly in the lower troposphere. In the following phase, the major ascending region shifts east to 135°E, corresponding to the eastbound propagation of deep convection over the Java Sea ( Fig. 4c ), and occupies the western half of the world. Interestingly, another ascending region is found to the east of the high-rising batch in New Guinea, located between 140° and 165°E, where the east wind anomaly prevails. At this stage, the wholly sphere is dominated by anomalously up gesture and deep convection, except the lineage near major mountains, particularly the constrict descending region near 120°E and immediately to the west of New Guinea between 135° and 140°E. The phase 5 is characterized by two counterclockwise overturning circulations, with the ascend and descending at the easterly and western ends, respectively. The western overturning circulation is already observed at phase 4, but nowadays with a smaller zonal scale, because of the moving in of the descending branch at the western limit of the domain. The second revolutionize circulation is a new one, which is developed atop and to the east of New Guinea. The appearance of this vertical circulation corresponds to the sudden exploitation of the deep convection to the northeast of New Guinea at phase 5 seen in Fig. 4d. generally speaking, the zonal circulation in the domain is characterized by crinkled disturbances, with alternating ascending and descending anomalies at the windward and lee sides of mountainous islands between 100° and 160°E. An exception is found in the area around New Guinea, where descent and ascent are observed at the windward and lee sides of New Guinea, respectively. The descent has been there since phase 4, but became stronger and more organized at phase 5. Different hang characteristics may be caused by unlike mechanisms. The batch range in New Guinea is much larger and higher than the mountains in other islands. The blocking effect of New Guinea is probable to be more significant. For example, the descending region is where the bifurcation of 10-m wind and near-surface discrepancy occurs, as seen in Fig. 4d. The meridional profiles of zonal fart speed at 140°E bespeak that the stream bifurcation is not barely a near-surface phenomenon ; rather, it extends all the way to the upper troposphere ( Fig. 6c ). The blocking effect of the high-rising batch in New Guinea may force the low-level wind to split and flow around the elongate batch range. It may in turn induce settling to the west of the island, where the near-surface divergence occurs ( Fig. 5d ). On the contrary, the ascending area occurs to the east of New Guinea, where stream converges. This distinct characteristic is besides apparent at phase 6 ( Fig. 7d ). The major difference between phases 4 and 5 is the dampen of the deep convection in the western Maritime Continent and the sudden appearance of the deep convection to the east of New Guinea. In the following phase, major circulation features observed at phase 5 stay discernible, while the convection to the west of New Guinea starts weakening. A careful examination of the circulation from phase 3 to phase 5 reveals that strong rise and descending anomalies tend to occur at the like regions, for model, the ascending regions west of Sumatra ( 100°E ), west of Sulawesi ( 120°E ), over the Java Sea where a hard convergence occurs, and east of New Guinea. Overall, the ascending and descending motion tends to occur at the windward and downwind sides of the mountains in Sumatra and Sulawesi, but origin and ascent happen at the windward and lee sides of New Guinea, respectively. ( This remainder between Sumatra–Sulawesi and New Guinea will be discussed late. ) consequently, an ascending area in the westerly anomaly government may become a descending region in the easterly anomaly government. The crinkled structures tend to occur in specific regions and show little propagation leaning. This suggests the quasi-stationary nature of the crinkled structures, which owe their being to the complex terrains in the region because of the raise and blocking effects of the mountains. While the vertical circulation along 5°S is strongly affected by the mountains in the region, the vertical circulation along 10°S, which is largely over the narrow oceanic channel between the islands in the Maritime Continent and the australian mainland, is much less affect by the topography. Compared with its counterpart along 5°S, the crossbreed section along 10°S presented in Figs. 7e–h exhibits the characteristics of a much smoother Walker-type circulation with a larger zonal scale. At phase 2, an ascending motion occurs at the western end of the domain, while a descending gesticulate occurs at the eastern end, indicating a zonal scale of about 80° longitude. adverse to the combination of stationary and eastward-propagating features along 5°S, the upright circulation along 10°S moves at about a constant rush from the western to eastern Maritime continent from phase 2 to phase 6. The inclination for the erect movement to occur in certain regions is a lot less discernible. The contrast between cross sections at 5° and 10°S prove how importantly the MJO circulation and convection can be influenced by topography. The MJO is normally explained in terms of large-scale tropical wave ( e.g., wavenumber 1–2 ) and is often treated as global beckon blueprint in some diagnostic studies. The results shown above argue that the MJO in the Maritime Continent is significantly modified by local land–sea contrast and topography. One may wonder whether a spatial smooth will remove these local effects and yield a large-scale pattern, as documented in many previous studies. To remove these small-scale features, 9-point zonal average was applied twice to the parameters shown in Fig. 7, including the topography. The results are shown in Fig. 8. The topography along 5° and 10°S is indeed smoothed out via this method acting, and the vertical circulation patterns look like a smooth Walker circulation without break as seen in Fig. 7. Figures 8e–h show the smooth propagation of the Walker circulation accompanied with the MJO convection at 10°S, which is about american samoa idealized as may be expected from theoretical studies. The up arm of the circulation moves into the domain at phase 3, and the whole structure of the overturning circulation shows up in the Maritime Continent at phase 6 with a zonal scale of about 80° longitude. In contrast, the circulation contortion due to the bearing of topography at 5°S is still apparent ( Figs. 8a–d ). A saddle point at 800 hPa is found near 140°E at phase 4, which is a separation point for the easterly and westerly anomalies and besides the up and down gesture. The presence of the high-rising mountain range in New Guinea is apparently the campaign for the appearance of the saddle point. The curvature bending up in the lower part of the Walker circulation at phase 6 is obviously the footprint of the topography-induced wave structures. like results were obtained by retaining only wavenumber 1–3 components. These results imply that the topographical effect induced by complex topography and land–sea contrast is heavily to remove via spatial smooth or retaining only the longest spatial Fourier components. An analysis based on spatially smoothed data may distillery retain the influence of local topographical effect. This spatial polish or filtering approach has been a common practice in many studies ( for example, Hendon and Salby 1994 ) to interpret the intraseasonal signals in terms of idealized equatorial waves ( Matsuno 1966 ) in an aquaplanet. The results presented here suggest the likely limit of this type of approach in comparing the real world with the idealize wave radiation pattern .

6. Summary and discussion

This cogitation explores the likely consequence of the topography on the generation and characteristics of the MJO in the Maritime Continent. It is demonstrated that the passage of the MJO through the Maritime Continent is not a smooth propagation, as expected in an idealize situation ( e.g., an aquaplanet ). The cragged islands wield “ blocking effect ” on the MJO. As a resultant role, the east motion of deeply convection and near-surface weave anomalies in the MJO skirt around islands. Because of these effects on the associated vorticity/convergence development, the deep convection and westerly anomaly shift south of the equator, rather of along the equator as in the indian Ocean, and propagate eastbound over the Java Sea, the oceanic region off the southern slide of Sumatra and Java, and the Timor Sea. The generation is further stall west of cragged New Guinea. The prevailing westerly anomaly splits and flows around the island and converges at the oceanic region to the northeast of New Guinea. This newly developed convection area, now located near the equator, becomes the major trench convection region of the MJO and moves eastbound. The distribution of mountainous islands in the Maritime Continent seems to result in the southbound detour of the eastward-propagating MJO and the sudden shift of deep convection from one region to another. In addition to the blocking effects on the subordinate tip, mountain-wave-like structures are besides observed in the particular longitudes near the eminent terrains of Sumatra, Sulawesi, and New Guinea. The being of topography seems to create extra lift and sinking within the large-scale circulation, and therefore the convective systems are observed to diminish or generate on different sides of the tropical topography. Compared with the theoretical horizon of the frictional wave-conditional instability of the moment kind ( CISK ) mechanism, which is often applied to explain the MJO, the topographical effect is likely to play an important function in determining the placement of deep convection and the flow distribution in fine scales. The topographical impression creates quasi-stationary features and breaks up the larger-scale convection in places. The end consequence is the stalling propagation and weakening lastingness of the MJO after it reaches the Maritime Continent. The flow bifurcation around New Guinea and Sulawesi is particularly apparent. This phenomenon bears certain similarities to the theoretical characteristics of the flow around an idealize obstacle in the gloomy Froude count circumstance ( Hunt and Snyder 1980 ; Smolarkiewicz and Rotunno 1989 ; Sha et aluminum. 1998 ). The major discrepancy is as follows. In the theoretical study, a pair of lee vortices occurs behind the obstacle. In this analyze the copulate of vorticity anomalies is found straddling the mountain ranges because of the decreasing wind speed toward the mountains. It is hypothesized that frictional impression weakens the wreathe travel rapidly and creates anomalous shear vorticity at the both sides of the mountains. The bifurcation of the flow in the windward side besides creates anomalously downward motion, while the flow convergence in the lee side creates anomalously up apparent motion. Our results suggest that the topographical effects and the mountain-wave-like perturbations may have a combine effect on the MJO, with different degrees of influence at different regions. It is difficult to quantitatively assess the relative contribution of the forget and wave-making effect by the topography in the present diagnostic study. The topographical effect on the flow is very complicate and is significantly different from island to island. Well-designed numeric studies are needed to amply understand the topographical effect of each cragged island. This study entirely provides qualitative discussion as follows : overall, more spatially elongated and higher mountainous islands exert stronger blocking effect on the incoming menstruate. The blocking effect of the elongate and high-rising New Guinea is so strong that it causes a complete flow bifurcation from the surface to above 500 hPa. The long island chain of Sumatra and Java not alone causes the flow bifurcation but besides results in the southerly deflection of the incoming westerly anomaly. On the early hired hand, the lower terrain of Sumatra allows the westerly anomaly to flow over and creates vertical crinkled perturbation in the downriver. While less spatially extend islands such as Sulawesi and Borneo exert entirely a place block effect, they seem to cause significant downstream crinkled disturbance in the vertical. Flow bifurcation induces local convergence–divergence around the island. The topographically induce erect crinkled perturbation besides induces ascending–descending branches near and downstream of the islands. Both blocking and wave-making effects induce excess enhancement–suppression to the MJO convection natural process and consequently modify the demeanor of the MJO in the Maritime Continent.

The relative importance of blocking and wave-making effects seems to result in different anomalous ascent–descent distribution near the mountainous islands. In the prevail anomalous westbound, the impregnable blocking effect of the elongate and high-rising New Guinea results in local divergence and convergence anomalies near the coat at the westerly and eastern ends of New Guinea, which are responsible for the local settling and rise, respectively. In the casing of Sumatra and Sulawesi, the blocking effect is much weaker, partially because of the lower terrain. The prevailing westerly anomaly that is alone partially blocked is able to flow over the terrain and induces ascent and descent in the windward and lee english of the mountains. far theoretical and numerical studies are required to understand the demand physical mechanism. The results presented above incriminate that a high-resolution model, which can resolve the detail topographical effects, may be required to simulate the realistic characteristics of the MJO in the Maritime Continent, even though high gear resolution does not necessarily guarantee successful model, as demonstrated by Rajendran et aluminum. ( 2008 ) .

Acknowledgments

This analyze was supported by the National Science Council in Taiwan under Grant NSC-95-2111-M-002-010-MY3. Authors appreciate the precious comments from the three anonymous reviewers .