Influence of the Madden Julian Oscillation on Indonesian rainfall variability in austral summer

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: (2010) Published online 29 September 2009 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.2005 Influence of the Madden Julian Oscillation on Indonesian rainfall variability in austral summer Rahmat Hidayat a,b * and Shoichi Kizu a a Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai , Japan b Department of Geophysics and Meteorology, Bogor Agricultural University (IPB), Kampus IPB Darmaga, Bogor 16680, Indonesia ABSTRACT: The impact of the Madden Julian Oscillation (MJO) on Indonesian rainfall variability in austral summer is analysed by using the daily station rain gauge data and Tropical Rainfall Measuring Mission precipitation data for the periods of 1979 through 1990 and 1998 through 2006, respectively. Composite analysis of 21 and 16 MJO events identified in former respective periods shows that the rainfall variability over Indonesia is significantly affected by the phase of eastward-propagating MJO. Excess rainfall is brought during wet phase, when the convective activity reaches its maximum with enhanced low-level wind convergence over the region. In addition, the impact of MJO tends to be more profound over the surrounding seas than over the large islands of this region. The positive rainfall anomaly over the eastern Indian Ocean and Java Sea during the wet phase is up to 5 mm/day (60% of the long-term mean) as a pentad mean, while it is about 1 3 mm/day (10 30%) over Borneo and Java. Copyright 2009 Royal Meteorological Society KEY WORDS MJO; TRMM; Indonesian rainfall Received 8 September 2008; Revised 23 April 2009; Accepted 25 July Introduction Indonesia is a large area of land sea complex consisting of more than ten thousand islands surrounded by ocean. Many islands of this region are mountainous (Figure 1) and located in the equatorial area where the convective activity is the highest. The tropical deep convection is the engine that drives atmospheric circulation through the uptake and release of significant amount of latent heat of vaporisation. Undoubtedly, severe drought and flood, which cause hardship for several hundred million people of this region, are strongly influenced by rainfall variability. It has been well documented that the extreme dry event leads to devastating drought with crop failure. On the contrary, frequent heavy rainfall can lead to severe flood. Therefore, understanding of rainfall variability and its impact to the region is essential to improve the quality of climate forecasting to reduce the risk of natural disaster and make better strategic decisions in agricultural sectors. There have been numerous studies on the rainfall variability over Indonesia and its relation to the diurnal change (Mori et al., 2004; Seto et al., 2004; Ichikawa and Yasunari, 2006; Tian et al., 2006; Qian, 2008), the annual change like monsoon (Hamada et al., 2002; Aldrian and Susanto, 2003; Chang et al., 2005) and the interannual large-scale climatic phenomena such as * Correspondence to: Rahmat Hidayat, Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai , Japan. rahmat@pol.geophys.tohoku.ac.jp the El Niño-Southern oscillation (Kirono et al., 1999; Haylock and McBride, 2001; McBride et al., 2003; Aldrian and Susanto, 2003; Hendon, 2003; Chang et al., 2004). It is also widely known that the Madden Julian Oscillation (MJO; e.g. Madden and Julian, 1972) is a dominant component of intraseasonal (20 90 day) variability over the Tropics. During the last two decades, a number of studies have investigated roles of the MJO in the rainfall variability over the Tropics to higher latitudes (Jones et al., 2004; Matthews and Li, 2005; Barlow et al., 2005; Souza and Ambrizzi, 2006; Tian et al., 2006; Pohl and Camberlin, 2006; Donald et al., 2006). Some of those studies briefly showed that the MJO influences Indonesian rainfall, as a part of showing its global impact, but how the MJO modulates the rainfall variability over the entire region of Indonesia has not been described in detail. Therefore, the principal objective of this study is to further clarify the influence of the MJO on the rainfall variability over the whole Indonesia and its relationship with large-scale circulation by using both in situ and remote sensing data. The paper is organised as follows. The data sets and methods used in this study are described in Section 2. Results and discussion are presented in Section 3 and our summary is provided in Section Data and methods We use daily rainfall data obtained during the period from 1979 to 1990 at 31 rain gauge stations distributed Copyright 2009 Royal Meteorological Society

2 INFLUENCE OF THE MADDEN JULIAN OSCILLATION ON INDONESIAN RAINFALL 1817 Figure 1. Topography of Indonesian archipelagos. Colour indicates elevation above sea level. Unit is metres. Six boxes labelled from a through f denote the areas for composite analysis in Figure 7. over the Indonesia, extending from 95 E to 140 E in longitude and from 6 N to12 S in latitude. This data set was compiled by Hamada et al. (2002). Pentad-averaged daily rainfall is defined by dividing one analysis year into 73 pentads. Data gap of 2 days at maximum is allowed in each pentad when pentad mean is calculated. The length and continuity of data are very different from station to station, and we used only 31 stations out of the total 157 originally provided by Hamada et al. (2002), where data gap is less than 50% of the 12 years. The weakness of the rain gauge network is its limitation to some parts of land. In order to quantify the variation of precipitation rate over the whole region including surrounding seas, we also used Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation version 6 data from 1998 to 2006, which has horizontal resolution of Detail information about this data can be found at the web page of Goddard Earth Sciences Data and Information Services Center of National Aeronautics and Space Administration ( README/TRMM 3B42 readme.shtml). Unfortunately, there is no overlapping period between the TRMM and the rain gauge data. Also used are the zonal (u), meridional (v) and vertical (ω) wind component at nine pressure levels (1000, 925, 850, 700, 600, 500, 400, 300 and 200 hpa) provided by the National Centre for Environmental Prediction (NCEP) Department of Energy R-2 (NCEP-2) field (Kalnay et al., 1996; Kanamitsu et al., 2002). As a proxy of convective activity, we used the outgoing long-wave radiation (OLR) data (Liebmann and Smith, 1996) from 1982 to 2006 that were given at longitude and latitude grid by National Oceanic and Atmospheric Administration. Pentad-mean daily values are defined for each of TRMM precipitation, OLR and NCEP-2 reanalysis variables in the same manner as for the rain gauge data. In order to obtain mean annual cycle, the original pentadmean values are averaged over the whole period of individual data set (i.e. 12 years for the rain gauge, 9 years for the TRMM and 25 years for the OLR and NCEP-2 reanalysis data) at each station or grid point. The anomalies of the variables that will appear later are deviation from this mean annual cycle. The intraseasonal signals are obtained from the original gridded data by applying a fourth-order Butterworth band-pass filter with cut-off frequencies at 20 and 90 days. The filter is applied to pentad anomalies at each grid point for the entire period of each data. The phase and amplitude of the MJO are defined according to the MJO index based on Wheeler and Hendon (2004; hereafter WH04). The WH04 index is defined by the two principal components (RMM1 and RMM2) of the near-equatorial (15 S 15 N) daily zonal winds at 850 and 200 hpa, and OLR (see WH04 for further details). Here, eight MJO phases are defined. Phase 1 denotes the period, when the centre of the convective activity (i.e. low-level convergence) is located near Africa, phases 2 and 3 over the Indian Ocean, phases 4 and 5 over the Maritime Continent, phases 6 and 7 over the western Pacific and phase 8 over the eastern Pacific. In order to identify major MJO events to be used for later composite analysis, we used two criteria: the amplitude of the WH04 index needs to be greater than one and its phase needs to sequentially rotate from phases 1 to 8. The focusses are placed on austral summer, defined as a period from October through April of subsequent calendar year, when the most parts of Indonesia experience the rainy season. These months are also the period when the MJO shows strongest signal in the course of its seasonal variation (Gutzler and Madden, 1989; Wang and Rui, 1990). Twenty-one MJO events are identified in the rain gauge period (i.e. from 1979 through 1990) and sixteen in the TRMM period (i.e. from 1998 through 2006). Composites are made from the anomalies of each variable according to the MJO phase. 3. Result and discussion 3.1. Rainfall anomaly pattern associated with the MJO A common feature of Indonesian rainfall variability during austral summer is shown in Figure 2(a). The highest precipitation rate over this region generally occurs over high topography or in areas adjacent to coastlines especially of large islands such as Sumatra (Sumatera), Borneo, Java, Sulawesi and Irian. The pentad-to-pentad variability of this precipitation (Figure 2(b)) shows pattern similar to that of the mean precipitation, suggesting that the higher variability of rainfall occurs in areas of high average rainfall. Next, we investigate how MJO modulates the rainfall variability over Indonesia, by using the OLR and the wind reanalysis data. Figure 3 shows variations of OLR and wind vector anomalies with respect to the MJO phase during austral summer. The negative (positive) OLR

3 1818 R. HIDAYAT AND S. KIZU Figure 2. Spatial distribution of (a) mean TRMM precipitation rate (mm/day) in austral summer (October April) and (b) its standard deviation for the period 1998 through anomalies over the region indicate enhanced (reduced) convective activity. The figure presents well-known large-scale structures and evolution of the MJO within the Tropics (Wheeler and Hendon, 2004; Morita et al., 2006) whose life cycle has a period of about 40 days (i.e. eight pentads). During phase 1 (Figure 3(a)), reduced convection is observed over most part of Indonesia while enhanced convection of a growing event appears over the western Indian Ocean and that of decaying one does over the central Pacific. At this stage, westerly (easterly) wind anomalies exist over the Pacific (the Indian Ocean). Sequentially in phases 2 and 3 (Figure 3(b) and (c)), an area of high convective activity over the Indian Ocean grows and moves eastward as easterlies strengthens and passes over the region. The organised convection reaches its maximum during phases 4 and 5 (Figure 3(d) and (e)) when stronger westerly and easterly wind anomalies meet over Indonesia. The centre of convection further migrates to eastern part of Indonesia during phase 6 (Figure 3(f)), while the central part of the Indian Ocean is covered by convectively suppressed area. Eventually in phase 8 (Figure 3(h)), Indonesia is mostly covered with convectively suppressed area Modulation of the rain gauge precipitation by the MJO The variation of station rainfall with respect to the MJO phase is examined by using a four-phase categorisation, by averaging two succeeding phases into a new single phase, in order to keep sufficient sample size for each phase in later statistical test. For example, phase A, representing the dry phase, is obtained by averaging phases 8 and 1. Similarly, B is obtained from phases 2 and 3; C, the dry phase, from phases 4 and 5 and D from phases 6 and 7. Figure 4(a) (d) shows the composite rainfall anomalies during each MJO phase obtained from the 21 MJO events during austral summer from 1979 through Figure 4(a) shows negative rainfall anomalies that accompany over most part of the region. A few stations over Java and north Sulawesi, however, present positive rainfall anomalies. As areas of high convective activity move to the eastern Indian Ocean (Figure 4(b)), the number of stations that record positive rainfall anomalies increases. Then, the positive rainfall anomalies reach their maxima when convective activity is prominent over the Maritime Continent during the wet phase (Figure 4(c)). Eventually, as the areas of enhanced convection reach the western Pacific, positive rainfall anomalies move eastward and are mostly found over the southern part of Indonesia, namely over Java, chained islands of Bali and Nusa Tenggara and southeast Borneo (Figure 4(d)). At this stage, Sumatra, Malay Peninsula and some areas in north Sulawesi and Maluku Islands start drying. Although the prominent MJO signals are found in the rainfall anomalies over most of the regions, there are a few stations which show only very small anomalies in all phases, particularly in southern Maluku Islands. A small correlation (r = 0.2) between precipitation at Ambon (3.4 S, 128 E) and OLR of the grid including the site indicates that rainfall variability of this region is not effectively controlled by large-scale convection but rather by local factors. Aldrian and Susanto (2003) pointed out that seasonal variability of rainfall over Maluku Islands tends to be affected by local oceanic condition. The difference between rainfall anomalies in phase C and those in phase A (i.e. wet minus dry) is significantly positive at 26 of the total 31 stations according to the standard t-test (Figure 4(e)). Five exceptions are in north and central Java, and north Sulawesi, which have small or even negative difference. These results suggest that the influence of the MJO is generally single signed but highly inhomogeneous among places. These composite results indicate that the MJO-induced rainfall variability is clearly identified by the in situ measurement of rainfall. However, insufficient spatial coverage of the rain gauge network limited our analysis to only a small portion of the region. In order to identify the impact of the MJO over wider land area of the region and surrounding seas, the TRMM data are used next MJO impacts observed by TRMM precipitation We again use the eight-phase system to describe the cycle of the sixteen MJO events occurred during the TRMM period. Figure 5 depicts the composite of TRMM precipitation anomalies at each MJO phase. The eastward propagation of the MJO, described by using OLR and wind vector anomalies in Figure 3, is well captured in the TRMM precipitation pattern. During phase 2 (Figures 3(b) and 5(b)), when convective activity develops in the eastern Indian Ocean,

4 INFLUENCE OF THE MADDEN JULIAN OSCILLATION ON INDONESIAN RAINFALL 1819 Figure 3. Composite OLR (shade) and 850-hPa wind (arrows) anomalies in each MJO phase during austral summer: (a) phase 1, (b) phase 2, (c) phase 3, (d) phase 4, (e) phase 5, (f) phase 6, (g) phase 7 and (h) phase 8. Dark (light) shaded areas denote positive (negative) OLR anomalies in the unit of W/m 2. The reference vector is below the panels. positive rainfall anomalies are observed along the west coast of Sumatra, the eastern part of Borneo, the western Java, the north Sulawesi and some areas of Irian. In phase 3 (Figure 5(c)), positive rainfall anomalies tend to reach their maxima over the Indian Ocean and large land masses of the western part of Indonesia, which is designated by enhanced convection over the eastern Indian Ocean and over Sumatra Island (Figure 3(c)), as easterlies become stronger over the central Pacific. Furthermore, major parts of Indonesian seas are accompanied by positive rainfall anomalies during phase 4 (Figure 5(d)). At this phase, there is an enhancement of convective activity over Indonesia due to the strengthening of lowlevel westerly and easterly wind anomalies (Figure 3(d)) associated with the MJO. The well-organised convection brings abundant rainfall over most part of the region, and hence these phases are termed as wet phase over Indonesia. In phases 5 through 8 (Figure 5(e) (h)), the variation of precipitation exhibits a pattern similar to that of phases 1 through 4 (Figure 5(a) (d)), but with reversed sign.

5 1820 R. HIDAYAT AND S. KIZU Figure 4. Rainfall anomalies (mm/day) during austral summer for (a) phase A, (b) phase B, (c) phase C and (d) phase D. (e) Difference in rainfall anomalies between the wet phase (phase C) and the dry phase (phase A). Dark (light) shaded circles show stations with significant positive (negative) difference at 90% confidence level based on Student s t-test. Negative (positive) OLR anomalies and their eastward propagation are in good agreement with positive (negative) rainfall anomalies and their eastward propagation. These consistent results obtained from two independent data sets and for two different periods indicate that the large-scale structure of the MJO convection affects the intraseasonal rainfall variability over major portion of Indonesia. In addition, Figure 5 shows highly inhomogeneous spatial pattern of rainfall anomalies and distinct contrast between land and ocean. Figure 6 shows the spatial distribution of rainfall anomalies relative to the multi-year-averaged rainfall in austral summer. The rainfall is higher than the longterm seasonal mean by 10 30% during the wet phase over the land (Figure 6(c) (e)) and lower than mean by 10 20% during the dry phase (Figure 6(a) and (g) (h)). Note that increase of rainfall is observed over the west coast of Sumatra, the western Java and the eastern side of Borneo, all of which include high topography, during the dry phase (phase 8; Figure 6(h)). In contrast, the relative increase (decrease) of rainfall over the sea in the wet (dry) phase of the MJO is up to 60 70% (50 60%), which is much larger than over the islands. In order to further identify the land sea contrast in the MJO impact, six areas are chosen from both land and sea, as shown in Figure 1, to evaluate the regional characteristics in more detail. Figure 7 shows the evolution of the rainfall anomalies by TRMM as a function of the MJO phase over Borneo, Java Sea, Banda Sea, the Island of Java and the Indian Ocean. The relative impact of MJO cycle is largest in the Indian Ocean (Figure 7(a)), with a maximum rainfall anomaly of about 5 mm/day during phase 3, followed by Banda Sea and Java Sea (Figure 7(b) and (c)) with a maximum rainfall anomaly of about 5 mm/day during phase 4, respectively. Note that these numbers are pentadmean values. In contrast, the MJO impact over the west Borneo and that over the western Java (Figure 7(d) and (f)) are similar, with maximum rainfall anomaly in the range of 1 3 mm/day during phases 3 and 4 and minimum in phase 6. The variation over the east Borneo (Figure 7(e)) is out of phase with that of the west Borneo (Figure 7(d)). Figures 5 7 show that rainfall over the surrounding ocean tends to be more clearly controlled by the MJO than that over the large land masses.

6 INFLUENCE OF THE MADDEN JULIAN OSCILLATION ON INDONESIAN RAINFALL 1821 Figure 5. Composite TRMM precipitation anomalies in each MJO phase (a h). Dark (light) shading denotes positive (negative) rainfall anomalies by TRMM. Only anomalies significant at 95% confidence level are shaded. Unit is mm/day. The dominance of the MJO signal in rainfall over the ocean is in accordance to the description by Zhang (2005) that the convective component of the MJO over the Maritime Continent is generally much weaker than over the surrounding oceans mainly because of the strong diurnal cycle due to diurnal solar heating, topography and reduced surface evaporation over the land. Roles of wind variation may be essential to explain the contrast between rainfall anomalies over the land and those over the sea. Qian (2008) has recently noted that the diurnal wind variation plays an important role for maintaining the diurnal rainfall variation over Java by applying a regional climate model MJO circulation anomalies relation The clear phasing of the rainfall variability with respect to the MJO cycle can also be explained by using anomalies of zonal (u) and vertical (ω) wind component at various

7 1822 R. HIDAYAT AND S. KIZU Figure 6. Increase or decrease of rainfall in each MJO phase relative (a h) to its long-term mean during austral summer. Unit is percentage (%). atmospheric levels. For the 16 MJO events identified during the TRMM period, vertical cross section of these wind anomalies along the equator (5 N 5 S) is presented in Figure 8 for a region from 80 E to 150 E. Starting from phase 3 (Figure 8(c)) when a prominent negative OLR anomalies develops in the eastern Indian Ocean and the middle part of Sumatra Malay Peninsula (Figure 3(c)), anomalous easterlies and the deep upward motion (i.e. deep convection) are clearly observed over the eastern Indian Ocean (Figure 8(c)). This pattern is consistent with rainfall anomalies that show maxima during this phase over the region (Figure 5(c)). During phases 4 and 5 (Figure 8(d) and (e)), strong deep upward motion develops over the eastern Indian Ocean. This is associated with enhanced convective activity where negative OLR (Figure 3(d)) and positive

8 INFLUENCE OF THE MADDEN JULIAN OSCILLATION ON INDONESIAN RAINFALL 1823 Figure 7. Composite rainfall anomalies by TRMM (mm/day) in each MJO phase, averaged in six rectangular areas. (a) 2 S 1 S and 94 E 97 E over Indian Ocean. (b) 6.5 S 4.5 S and 126 E 230 E over Banda Sea. (c) 6 S 4.5 S and 113 E 118 E over north Java Sea. (d) 1 S 1 N and E 112 E over west Borneo. (e) 1 S 1 N and E 117 E over east Borneo. (f) 7.5 S 6.5 S and 107 E E over west Java Island. rainfall anomalies (Figure 5(d)) are observed. However, convectively suppressed areas are observed in lower troposphere over 100 E 110 E (e.g. Sumatra). This patch of reduced rainfall may be caused by the topography of Sumatra Island, as described by Hsu and Lee (2005). By using a simple aquaplanet general circulation model (GCM), Inness and Slingo (2006) pointed out that the main factor that reduces the strength of convective signal over the Maritime Continent is the topography of the islands. Hence, the presence of Sumatra Island seems to be effective for blocking and weakening signal of the eastward-propagating MJO. The descending motion is observed at lower levels and becomes intensified over the longitude around 100 E 110 E. It seems that this localised patch of subsidence anomalies leads to decrease rainfall during the wet phase that generally brings excess rainfall to a major part of Indonesia. A second patch of deep upward motion is extended over 120 E 140 E at mid-to-upper levels during this phase. This vertical motion leads to strong convection and produces positive rainfall anomalies over the region. At phase 5 (Figure 8(e)), the upward motion over the Indian Ocean and the western Indonesia starts to weaken, and intensified descending motion is prominent over 100 E 110 E during phase 6 (Figure 8(f)). Another patch of downward motion is observed over 130 E 150 E at phase 7 (Figure 8(g)), and it tends to persist during phase 8 (Figure 8(h)) when most of the regions are drying. Such a pattern is in good accordance with that of OLR and rainfall anomalies during the corresponding phases (Figures 3 and 5). These facts consistently indicate that the intraseasonal rainfall variability over the region is effectively controlled by deep convection associated with the MJO. 4. Summary We have shown that the MJO gives sizable impact on rainfall variability over Indonesia, based on analysis of data from rain gauge stations for 1979 through 1990 and TRMM for 1998 through At 26 of the 31 stations, the difference of rainfall anomalies between the wet phase

9 1824 R. HIDAYAT AND S. KIZU Figure 8. Vertical cross section of vertical (ω) and zonal (u) wind anomaly along the zonal region of 5 N 5 S from 1998 to 2006 for each MJO phase (a h). The ordinate is pressure (hpa). The reference vector is 10 ms 1. Dark (light) shading denotes downward (upward) wind anomalies whose vertical components are statistically significant at 95% confidence level. Note that the scale of the vertical wind (ω) is amplified by a factor of 100 for clarity. and the dry phase is significantly positive. Both of the rain gauge data set and TRMM precipitation clearly showed coherent eastward propagation of precipitation anomalies during the passage of the MJO. Rainfall anomalies associated with the MJO have amplitude of 1 3 mm/day (10 30% of climatological mean) over the islands and about 5 mm/day (60 70%) over the seas. These results suggest that the MJO controls a considerable fraction of the total precipitation over Indonesia and that the rainfall variability over the surrounding ocean is more clearly controlled by the MJO compared to over the large land masses. The rainfall over the equatorial Indian Ocean, Banda Sea and Java Sea is significantly influenced by the MJO, but the impact of the MJO cycle over Borneo and land of Java is smaller in the total rainfall variability.

10 INFLUENCE OF THE MADDEN JULIAN OSCILLATION ON INDONESIAN RAINFALL 1825 The impact of the MJO is shown to be largely inhomogeneous especially over the islands. A small and sometimes even negative impact of the MJO in some parts of the land area (e.g. over Sumatra Island) may be explained by local descending motion of the air during the wet phase. Land sea distribution with complex topography should play an important role in order to investigate the MJO-induced rainfall variation over this region in more detail. Further analysis will be necessary for clarifying orographic modulation of wind system and influence by variations in ocean atmosphere field. Acknowledgements We are grateful to Dr Jun-Ichi Hamada for providing rain gauge data over Indonesia. We also thank anonymous reviewers for their thoughtful comment for improving this article. 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