Differential impacts of conventional El Niño versus El Niño Modoki on Malaysian rainfall anomaly during winter monsoon

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 21 November 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.3873 Differential impacts of conventional El Niño versus El Niño Modoki on Malaysian rainfall anomaly during winter monsoon Ester Salimun, a Fredolin Tangang, a * Liew Juneng, a Swadhin K. Behera b and Weidong Yu c a Research Centre for Tropical Climate Change System (IKLIM), Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Malaysia b Frontier Research Center for Global Change/JAMSTEC, Yokohama, Japan c Center for Ocean and Climate Research, First Institute of Oceanography, SOA, Qingdao, China ABSTRACT: This study investigates distinct impacts of conventional El Niño and El Niño Modoki on Malaysian rainfall anomaly during the winter monsoon. Generally, during conventional El Niños, northern Borneo experiences significant rainfall deficit while over Peninsular Malaysia the impact is minimal. In contrast, most El Niño Modoki events favour deficit rainfall over both northern Borneo and Peninsular Malaysia. The level of impacts, particularly over northern Borneo, is higher during El Niño Modoki than conventional El Niño. The different patterns of anomalous rainfall distribution during the two types of El Niño are associated with differences in regional sea surface temperature anomalies, anomalous regional circulation and the different patterns of ascending and descending air associated with the nodes of Walker circulation over this region. However, the most prominent change is the weakening and westward shifting of the anti-cyclonic circulation over the western North Pacific region. KEY WORDS anomalous rainfall; conventional El Niño; El Niño Modoki; northern Borneo; Peninsular Malaysia Received 31 January 2013; Revised 12 September 2013; Accepted 17 October Introduction The El Niño phenomenon exerts a significant impact over the Southeast Asia (SEA) region, both in terms of regional climate anomalies and the socioeconomic wellbeing of its inhabitants (e.g. Juneng and Tangang, 2008; Feng et al., 2010). Malaysia is directly affected by El Niño-induced drought, where water shortages and serious haze episodes are the common aftermaths (Tangang and Juneng, 2004; Juneng and Tangang, 2005). During the strongest 1997/1998 El Niño, Malaysia experienced the worst ever drought condition and a haze episode that blanketed the sky not just over Malaysia but over the greater SEA region (Tangang et al., 2010). Generally, the drought-stricken region varies spatially and temporally according to the El Niño phases. Southern Peninsular Malaysia, Sumatra and southern Borneo are prone to severe drought during an El Niño summer. However, during winter and spring, the El Niño impact is more pronounced over northern Borneo than Peninsular Malaysia (Juneng and Tangang, 2005). Such a northeastward progression of the drought-affected region from southern Peninsular Malaysia in the early phases of an El Niño to * Correspondence to: Prof. F. Tangang, Research Center for Tropical Climate Change System (IKLIM), Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia. tangang@ukm.my northern Borneo during its decaying phases is modulated by the strengthening and weakening of low-pressure systems over the southern Indian Ocean and north western Pacific, respectively, during the summer (Juneng and Tangang, 2005). The air-sea coupled interaction over these regions is modulated by the monsoonal background flow (Wang et al., 2003). However, such understanding of the El Niño impacts over Malaysia refers to the conventional type of El Niño. The impact of El Niño Modoki over Malaysia is not well understood. Since late 1970s the atmosphere ocean coupled system in the tropical Pacific Ocean has experienced changes, with the appearance of the Modoki events, probably due to the increased background temperature associated with anthropogenic warming (Ashok et al., 2007; Yeh et al., 2009). In contrast to the conventional El Niño, an El Niño Modoki is attributed to the maximum surface warming in the central Pacific Ocean (Ashok et al., 2007; Kug et al., 2009; Chen and Tam, 2010). Such westward shifting of the location of maximum heating as compared to canonical El Niño imposes changes on the Walker circulation and produces different patterns of teleconnection across the tropical region and around the globe (Ashok et al., 2007; Kug et al., 2009; Chen and Tam, 2010; Feng et al., 2010; Kim et al., 2011). Some studies have demonstrated the different impacts arising from the conventional El Niño and El Niño Modoki over various regions (Wang and Hendon, 2007; Chang et al., 2008; Feng et al., 2010; 2013 Royal Meteorological Society

2 2764 E. SALIMUN et al. Feng and Li, 2011; Kim et al., 2011). Wang and Hendon (2007) suggested that Australian rainfall is more influenced by the anomalous sea surface temperature (SST) associated with El Niño Modoki than the conventional El Niño. However, Taschetto and England (2009) found that the conventional El Niño affects northeastern and southeastern Australia during September to November while the impacts of El Niño Modoki are more pronounced over northwestern and northern Australia during March April May (MAM). El Niño Modoki also influences the monsoon onset and termination over Australia (Wang and Hendon, 2007). Feng et al. (2010) also showed that El Niño Modoki tends to affect the SEA region differently than the conventional El Niño. During the winter season of the conventional El Niño, rainfall is suppressed over the Philippines, Borneo, Celebes and Sulawesi but enhanced over South China. However, during El Niño Modoki the dry condition extended westward to affect southern China, the Indo-China Peninsula and Peninsular Malaysia. Despite the explanation provided by Feng et al. (2010) for the different impacts of thesetwotypesofelniño over the SEA region, the level of understanding of El Niño Modoki s influence on climate variability over Malaysia is relatively limited. Therefore, the purpose of this study is to further investigate the different impacts of the conventional El Niño and El Niño Modoki over Malaysia during the boreal winter season. Climatologically, Malaysia receives about 26% of its total annual rainfall during the boreal winter period (December February). The rest of this article is arranged as follows: Section 2 presents the data and method; Section 3 reveals the differences in impact on anomalous rainfall distribution associated with the conventional El Niño and El Niño Modoki based on composite analysis; Section 4 discusses teleconnection patterns of anomalous atmospheric conditions during the conventional El Niño and El Niño Modoki; Section 5 presents the regression analysis; Section 6 discusses the 2002 and 2009 El Niño Modoki events that impacted the region differently; lastly, Section 7 gives the summary. 2. Data and method Several datasets that span the period from January 1950 to December 2009 were used in this study. The wind (horizontal and vertical velocity) and sea level pressure (SLP) datasets were taken from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NP/NCAR) reanalysis product with resolution (Kalnay et al., 1996). The moisture flux and its divergences were calculated from the components taken from the NP/NCAR reanalysis. The gridded SST is the HadISST of the United Kingdom Meteorological Office (UKMO) (Rayner et al., 2003), while the high-resolution ( ) monthly rainfall data were based on the Global Precipitation Climatology Centre (GPCC) that span a period from 1901 until present (Becker et al., 2013). For this study, we use Peninsular 16 8 Malaysia Northern Borneo Figure 1. The geographical location of the 17 stations over Malaysia. this dataset for a period from 1955 to Additionally, a rainfall gauge dataset that spans a period of 55 years ( ) for 17 meteorological stations maintained by the Malaysian Meteorological Department (MMD) were also considered (Figure 1). In this study, a composite technique was employed to quantify the impacts of the two types of El Niño. The years of El Niño and El Niño Modoki follow the definition of the Climate Diagnostics Center (CDC) and Ashok et al. (2007) (Table 1). We focused this investigation on the December January February (DJF) period. Climatologically, Malaysia s seasonal rainfall and its interannual variability peak during this season (Tangang and Juneng, 2004). A regression analysis was also carried out to highlight the co-variability pattern between the Malaysian rainfall anomaly and the SST anomaly over the tropical Table 1. The list of conventional El Niño and El Niño Modoki years used for composite analysis. The identification of conventional El Niño and El Niño Modoki is based on the CDC and Ashok et al. (2007). Rainfall phase patterns are defined according to Figure 4. Conventional El Niño 1 2 El Niño Modoki 1957/ /1980* 1963/1964* 1986/1987* 1965/ /1991* 1968/ /1992* 1972/ /1993* 1976/1977** 1994/1995* 1977/1978* 2002/ /1983* 2004/2005* 1987/1988*** 1997/1998 Without star: out-of-phase with enhanced rainfall over Peninsular Malaysia and deficit rainfall over northern Borneo. Single star: deficit rainfall over both Peninsular Malaysia and northern Borneo (negative in-phase). Two stars: out-of-phase with deficit rainfall over Peninsular Malaysia and enhanced rainfall over northern Borneo. Three stars: inphase with enhanced rainfall over both regions.

3 IMPACTS OF CONVENTIONAL EL NIÑO VS. EL NIÑO MODOKI ON MALAYSIAN RAINFALL 2765 Indian-Pacific Ocean. We first regressed the averaged rainfall anomaly over Peninsular Malaysia and northern Borneo to the SSTA over the tropical Indian and Pacific Oceans for three different periods, i.e. period prior to 1979 ( ) and period after 1979 ( ) in addition to the entire period ( ). The analysis was also repeated but regressing the Niño3.4 (Index for conventional El Niño) and the El Niño Modoki Index (EMI) to the anomalous rainfall. 3. Spatial patterns of Malaysian anomalous rainfall associated with El Niño and El Niño Modoki The composite patterns of anomalous rainfall over Malaysia corresponding to the two types of El Niño are presented in Figure 2. During a conventional El Niño, Peninsular Malaysia and northern Borneo experience different patterns of anomalous rainfall. Significant negative rainfall anomalies are observed over northern Borneo (Figure 2). In contrast, the anomalies over Peninsular Malaysia are not significant with only slightly wetter condition along the coastal region and drier condition in the central region. Hence, during mature phase of a conventional El Niño, northern Borneo suffers from a significant rainfall deficit while Peninsular Malaysia experience minimal impact, consistent with earlier studies (Tangang and Juneng, 2004, Juneng and Tangang, 2005). In contrast, during El Niño Modoki, the dry condition enhances over Peninsular Malaysia with significant negative anomalies dominating the northern region of Peninsular Malaysia and the southern region of Thailand (Figure 2). The dry condition over northern Borneo also enhances and the area affected by significant anomalies is broader than during that of the conventional El Niño. This implies that the drought condition over this region is more severe during El Niño Modoki, particularly over the northern Peninsular Malaysia. On the basis of these composite patterns, it is concluded that northern Peninsular Malaysia tends to experience significant impact of drier than normal condition during El Niño Modoki instead of near normal condition experienced during the conventional El Niño. These findings are different than those of Feng et al. (2010), which indicated the level of impacts over the region was weaker during El Niño Modoki compared with conventional El Niño, particularly over northern Borneo. The distinctive impacts of these two types of El Niños over Peninsular Malaysia and northern Borneo are reiterated by station-based rainfall composites, as shown in Figure 3. Most stations over Peninsular Malaysia experience weak anomaly during the conventional El Niño. However, stations in the southern region tend to show positive anomalies (Figure 3). Furthermore, during El Niño Modoki, the rainfall anomalies over northern Peninsular Malaysia were significantly negative and in-phase to those over northern Borneo, suggesting that both Peninsular Malaysia and northern Borneo experience deficit rainfalls during El Niño Modoki events (Figure 3). Figure 2. The composite of anomalous rainfall for events listed in Table 1. The shaded area indicates significance at the 95% level. Conventional El Niño and El Niño Modoki. Unit is mm month N 5 N 0 95 E 10 N 5 N E 105 E 110 E 115 E 120 E E 100 E 105 E 110 E 115 E 120 E Figure 3. The composite of anomalous rainfall stations for events listed in Table 1. Dark triangles indicate significance at the 95% level. Conventional El Niño and El Niño Modoki. Unit is mm month 1. The differences in the impacts of the conventional El Niño and El Niño Modoki on rainfall over Peninsular Malaysia and northern Borneo are further corroborated by the rainfall anomaly plot averaged over these two regions since 1955 (Figure 4). Prior to 1979, the averaged anomalies over these two regions during conventional El Niños are mostly out-of-phase with enhanced rainfall over Peninsular Malaysia and deficit rainfall over northern Borneo (Table 2). In contrast, during , 0.06

4 2766 E. SALIMUN et al. Averaged Rainfall Anomaly (mm/yr) EM Peninsular Malaysia Northern Borneo Years EM EM EM EM Figure 4. Averaged rainfall anomaly over Peninsular Malaysia and northern Borneo from 1955 to The notations and EM correspond to conventional El Niño and El Niño Modoki, respectively. Unit is mm year 1. influenced by frequent El Niño Modoki events, the rainfall anomalies over these two regions were negative in-phase. 4. Differences in patterns of regional anomalous SST and atmospheric circulation Figure 5 shows the spatial pattern of sea surface temperature anomaly (SSTA) composites of 10 conventional El Niños and eight El Niño Modoki events as listed in Table 1. Comparatively, these composites indicate distinct features of anomalous SST in the eastern Indian Ocean, South China Sea and western Pacific Ocean. The warmer SSTA in the South China Sea and Bay of Bengal are stronger during the conventional El Niño compared to that during El Niño Modoki. However, the SSTA along the equator in the Indian Ocean, western North Pacific and in the Java Sea south of Borneo and Makassar islands are much warmer during El Niño Modoki compared to that of the conventional El Niño. Moreover, the cooler SSTA in the western Pacific region is weaker during El Niño Modoki due to the warming in the central region. The different patterns of SSTA in these regions, especially in the South China Sea and in the western North Pacific, are consistent with the differences in regional circulation anomalous between the conventional El Niño and El Niño Modoki (Figure 6). The most striking one is the SSTA associated with anomalous anti-cyclonic circulation over the western North Pacific region. A EM EM EM prominent anti-cyclonic circulation dominates over the western North Pacific, positioned around 135 E and 12 N, during a conventional El Niño. Juneng and Tangang (2005) attributed this anomalous circulation to the drier than normal condition over northern Borneo during the boreal winter (Figure 6). According to Wang et al. (2003), the local air sea interaction drives a positive feedback between the SSTA dipole and the anticyclonic circulation, resulting in the strengthening of these anomalous features during this season. Northeasterlies in conjunction with this anti-cyclonic circulation blow over the western Pacific towards the Maritime Continent and later feed into the returning flow of strong southwesterlies over the South China Sea and western North Pacific. Similarly, southeasterlies over the western South Pacific that originate from northern Australia also feed into the southwesterlies over the South China Sea. At this time, strong easterlies dominate over the southeastern Indian Ocean. These easterlies are part of an anti-cyclonic circulation system that has its centre located off western Australia. Over the Bay of Bengal area, southwesterlies dominate which later feed into the anti-cyclonic circulation over the northern region of the South China Sea. Overall, the anomalous circulation over the western Pacific, South China Sea and eastern Indian Ocean are consistent with those of Juneng and Tangang (2005; Figure 5). Figure 7 shows the composites of low-level moisture flux and moisture flux divergences during the conventional El Niños, which are consistent with the vertically integrated quantities presented in Juneng and Tangang (2005). Positive and negative values correspond to drier and wetter than normal conditions, respectively. The moisture flux divergence pattern is also consistent with the anomalous low-level wind pattern shown in Figure 6. The eastern side of the anti-cyclonic circulation over the western North Pacific region is mostly dominated by positive moisture flux divergence patterns that encroach into the eastern Maritime Continent. The region stretching from Irian Jaya to northern Borneo is featured with large significant positive values of moisture flux divergence. Moreover, moisture is transported from this region to the southern region of the South China Sea as well as the eastern Indian Ocean, leaving this region experiencing drier than normal conditions during the mature phase of the conventional El Niño (Juneng Table 2. The number of El Niño and El Niño Modoki events together with rainfall phases over Peninsular Malaysia and northern Borneo for the periods of , and El Niño type Condition of impacts Conventional El Niño Negative in-phase Out-of-phase a Others b El Niño Modoki Negative in-phase Out-of-phase a Others b a Out-of-phase only refers to enhanced rainfall over Peninsular Malaysia and deficit rainfall over northern Borneo. b Others refer to enhanced rainfall over northern Borneo and deficit rainfall over Peninsular Malaysia or in-phase with enhanced rainfall over both regions.

5 IMPACTS OF CONVENTIONAL EL NIÑO VS. EL NIÑO MODOKI ON MALAYSIAN RAINFALL 2767 Figure 5. The composite of sea surface temperature anomaly (SSTA) for events listed in Table 1. The hatched area indicates significance at the 95% level. Conventional El Niño and El Niño Modoki. Unit is C. Figure 6. The composite of low-level wind (850 mb) anomaly for events listed in Table 1. The shaded contour indicates positive magnitude of wind speed anomaly. The darkened vector indicates significance at the 95% level. Conventional El Niño and El Niño Modoki. Unit is ms 1. and Tangang, 2005, Feng et al., 2010). The South China Sea and western Borneo experience significant negative moisture flux divergence while positive moisture flux divergence emerges over the eastern Indian Ocean and Gulf of Thailand. Noticeably, the northern and southern regions of Peninsular Malaysia experience slightly negative and positive moisture flux divergence, respectively, indicating near normal conditions over this region. However, significant negative moisture flux divergence is also established over the region east of the Philippines that stretches northward to cover Taiwan and the East Asia region. The moisture converges to this region from the South China Sea and Bay of Bengal area. The apparent contrast of moisture flux divergence between northern Borneo and Peninsular Malaysia is consistent with the composite of anomalous precipitation over these regions during the conventional El Niño (Figure 2). The anomalous circulations over these regions during El Niño Modoki are markedly different from those of the conventional El Niño (Figure 6). The anomalous anti-cyclonic circulation over the western North Pacific region is considerably weakened, with its centre displaced westward to a location over the Philippines. Moreover, the orientation of the centre is in a northwest-southeast direction with strong northerly flow that dominates the western North Pacific region. These northerly winds do not circulate back to the Maritime Continent but continue to flow south towards Australia in contrast to the anomalous northeasterlies circulation during the conventional El Niño. Over the South China

6 2768 E. SALIMUN et al. Figure 7. The composite of moisture flux (vectors) (kgm 2 s 1 ) and moisture flux divergence (shaded) (kgm 2 s 1 ) anomaly for events listed in Table 1. The hatched area and darkened vector indicate significance at the 95% level. Conventional El Niño and El Niño Modoki. Sea, the anomalous southwesterlies weaken considerably. Meanwhile, an anti-cyclonic flow reestablishes over the southern Indian Ocean with its southerly flow feeding into strong westerlies over the eastern equatorial region. Over the Bay of Bengal, the circulation reverses to a dominating cyclonic circulation. A relatively cooler SSTA in the Bay of Bengal during El Niño Modoki compared with the conventional El Niño enhances land sea temperature contrast that favours an anomalous cyclonic circulation near the land sea interface. The corresponding composites of low-level moisture flux and moisture flux divergence for the El Niño Modoki are shown in Figure 7. The western Pacific region is still dominated by positive moisture flux divergence in which the intensity is relatively higher compared to that of the conventional El Niño (Figure 7). The elongated positive moisture flux divergence that stretches from Irian Jaya westward to northern Borneo is still evident. However, the negative moisture flux divergence over the South China Sea is replaced with positive ones. The South China Sea and Peninsular Malaysia are entirely dominated by positive moisture flux divergence. Hence, in contrast to the conventional El Niño, both Peninsular Malaysia and northern Borneo experience positive moisture flux divergence during El Niño Modoki. This is consistent with the drier-than-normal condition experienced over these two regions during El Niño Modoki (Figure 2). Moreover, the significant positive moisture flux divergence over Java, southern Sumatra and the southern Indian Ocean are no longer present but replaced by negative moisture flux divergence during El Niño Modoki. The contrasting anomalous features over the Maritime Continent associated with the conventional El Niño and El Niño Modoki are also related to the strength and position of the ascending and descending nodes of the Walker circulation over this region. Figure 8 shows the anomalous vertical velocity averaged from 5 Sto5 N, where the positive and negative values represent the descending and ascending nodes of the Walker circulation, respectively. During a conventional El Niño, the descending node is dominantly located over the eastern part of the Maritime Continent between 110 E and 160 E (Figure 8). Moreover, as indicated by the vertical velocity at 500 mb, northern Borneo is dominated by a pattern of descending air during a conventional El Niño (Figure 9), consistent with dry conditions as shown in Figure 2. In contrast, Peninsular Malaysia is characterized by ascending air, as indicated by negative values of vertical velocity at 500 mb (Figure 9). This anomalous vertical velocity shows a weakening of the descending node of the Walker circulation (Figure 8) over the Maritime Continent (60 E 160 E) during El Niño Modoki events. The vertical velocity at 500 mb is basically similar over both Peninsular Malaysia and northern Borneo, which indicates descending air during El Niño Modoki over the whole region (Figure 9). This is consistent with the dry conditions experienced over these two regions as shown in Figure Regression analysis between rainfall anomaly and SSTA Previous sections showed different impacts of anomalous precipitation over Malaysia during the conventional El Niño and El Niño Modoki, indicating different covariability modes between precipitation anomaly and tropical Indian-Pacific SSTA during these events. In this section, we further explore such co-variability modes using regression analysis between averaged rainfall anomalies over the Peninsular Malaysia and northern Borneo (Figure 4) and the corresponding SSTA for

7 IMPACTS OF CONVENTIONAL EL NIÑO VS. EL NIÑO MODOKI ON MALAYSIAN RAINFALL 2769 Figure 8. The composite of vertical velocity averaged from 5S to 5 N for events listed in Table 1. The hatched area indicates significance at the 95% level. The y-axis is pressure level in hpa. Conventional El Niño and El Niño Modoki. Unit is Pa s 1. Figure 9. The composite of vertical velocity anomaly at 500 mb for events listed in Table 1. The hatched area indicates significance at the 95% level. Conventional El Niño and El Niño Modoki. Unit is Pa s 1. three different periods, i.e. the period prior to 1979 ( ), the period after 1979 ( ) and the entire period ( ). Prior to 1979, conventional El Niños dominated whereas after the 1979 El Niño Modoki occurs more frequently (Table 1; Ashok et al., 2007). Similarly, La Niña Modoki occurs mostly after 1979 (Yuan and Yan, 2013). Moreover, the co-variability mode depicted through this analysis is equally applicable for the negative phase of the conventional ENSO and ENSO Modoki (i.e. conventional La Niña and La Niña Modoki). Figure 10 shows the coefficients for the regression analysis for the three periods with their signs are flipped to reflect the SSTA pattern during an El Niño event. Hence, significant positive values correspond to warmer SSTA and deficit rainfall over northern Borneo. Interestingly, all three periods have similar patterns of regression coefficients that greatly resemble to the SSTA pattern during the matured phase of the conventional ENSO event (e.g. Rasmusson and Carpenter, 1982, Juneng and Tangang, 2005). The central-eastern Pacific Ocean is significantly dominated by warmer SSTA, with a cooler boomerangshaped anomaly pattern flanking it (Figure 10). Moreover, the Indian Ocean and the South China Sea are dominated by significant positive SSTA. Such a pattern is expected for the period of ( ) since it is dominated by the conventional El Niños without the occurrence of El Niño Modokis (Figure 10). Despite the domination of El Niño Modoki events after 1979 (Table 1), the pattern of the regression coefficients for the period of ( ) also closely resembles to that of the ( ) period (Figure 10). However, this is not surprising as northern Borneo is affected by both the conventional El Niño and El Niño Modoki (Figure 2). Indeed, similar pattern is also depicted for the entire period ( ) where both conventional El Niños and El Niño Modokis occurred (Figure 10).

8 2770 E. SALIMUN et al Figure 10. The coefficients of regression (the sign is flipped) of the averaged rainfall anomaly over northern Borneo to the SSTA for the periods , and The hatched area indicates significance at the 95% level. Unit is mm month 1 C 1. The corresponding patterns for the coefficients for regression of averaged rainfall anomalies over Peninsular Malaysia (Figure 4) to the SSTA are shown in Figure 11. Peninsular Malaysia is mainly impacted by the El Niño Modoki (Figure 2). Interestingly, the pattern for the period of ( ) shows no similarity to the typical patterns of El Niño Modoki as such event did not occur during this period (Figure 11). However, during the period of ( ), where El Niño Modoki dominates, the pattern resembles closely to the SSTA associated with El Niño Modoki with maximum warming occurring in the central Pacific Ocean (e.g. Ashok et al., 2007) (Figure 11). When the entire period is considered, the pattern weakens due to weak relationship between rainfall anomalies over Peninsular Malaysia and SSTA during the first half of the period. We repeated the regression analysis by regressing the Niño3.4 and the EMI to the rainfall anomalies. Consistent with Figure 10, the three periods show similar patterns of coefficients for regression of Niño3.4 to the rainfall anomalies with significant values over northern Borneo (Figure 12). This implies the conventional El Figure 11. The coefficients of regression (the sign is flipped) of the averaged rainfall anomaly over Peninsular Malaysia to the SSTA for the periods , and The hatched area indicates significance at the 95% level. Unit is mm month 1 C 1. Niño impacting northern Borneo regardless of the period. The regression of EMI and rainfall anomalies reveals interesting patterns (Figure 13). For the first half of the period ( ), the pattern is rather weak due to the absence of El Niño Modoki occurrences during this period (Figure 13). In contrast, during the period of ( ) the pattern enhances with significant values over both northern Borneo and northern Peninsular Malaysia, indicating the significant impacts of El Niño Modoki over both regions (Figure 13). For the entire period ( ), a similar pattern is depicted albeit slightly weaker (Figure 13) due to the absence of El Niño Modoki during the first half of the period. Moreover, these co-variability modes are equally valid for the conventional La Niña and La Niña Modoki but opposite in polarity. 6. The impact of the 2002 and 2009 El Niño Modoki events Table 2 highlights two El Niño Modoki events, i.e and 2009, that have out-of-phase rainfall anomaly patterns over Peninsular Malaysia and northern Borneo.

9 IMPACTS OF CONVENTIONAL EL NIÑO VS. EL NIÑO MODOKI ON MALAYSIAN RAINFALL 2771 Figure 12. The coefficients of regression of the Niño3.4 Index to the rainfall anomaly for the periods , and The hatched area indicates significance at the 95% level. Unit is Cmm 1 month 1. Figure 13. The coefficients of regression of the El Niño Modoki Index to the rainfall anomaly for the periods , and The hatched area indicates significance at the 95% level. Unit is Cmm 1 month 1. In this section, further evidences are provided to explain the out-of-phase rainfall anomaly pattern of these two events. Figure 14 depicts the composite pattern of 7 negative in-phase Modoki events during , which is consistent with the overall composite shown in Figure 2. However, the 2002 and 2009 El Niño Modoki events clearly indicate out-of-phase rainfall anomaly pattern between Peninsular Malaysia and northern Borneo (Figure 14 and ). During the 2002 event, enhanced rainfalls are pronounced over Sumatra and southern Peninsular Malaysia (Figure 14). Similarly, during the 2009 event, Sumatra and central Peninsular Malaysia experience wetter than normal condition (Figure 14). The striking differences between the out-of-phase and in-phase El Niño Modoki events do not confine to anomalous rainfall distribution pattern but also to SST and wind anomalies. Figure 15 shows the spatial pattern of SSTA for a wider region that covers Indian Ocean, South China Sea and the Pacific Ocean. The SSTA composite for the in-phase El Niño Modoki events indicates markedly different distribution pattern than the out-of-phase events with much weaker warming in the Indian and regional seas surrounding the Maritime Continent (Figure 15). Interestingly, the SSTA patterns of two out-of-phase events show greater resemblance with enhanced warming in the Indian Ocean and seas surrounding the Maritime Continent (Figure 15 and ). The two events also display consistent pattern of anomalous low-level wind (Figure 16 and ) that feature strong southwesterlies anomaly wind over Sumatra, Peninsular Malaysia and South China Sea. During the in-phase events, the anomalous winds are mostly northeasterly and weaker over this region (Figure 16). The strong southwesterlies over Sumatra, Peninsular Malaysia and South China Sea during the two outof-phase El Niño Modoki events imply weaker winter monsoon wind. Weaker monsoon wind over the South China Sea leads to warmer SSTA due to less extraction of latent heat from the surface and weaker surface mixing in the ocean (Figure 15 and ) (Wang et al., 2003). However, over land (Sumatra and Peninsular Malaysia), weaker winds may results in weaker vertical shear which in turn providing favourable condition for local convection to occur. We hypothesize that the condition of weaker vertical shear due to weak monsoon wind strengthens the diurnal cycle over this region which favours local convection, particularly in the form of late afternoon heavy rainfall (Sow et al., 2011). Aldrian and Susanto (2003) identified Sumatra

10 2772 E. SALIMUN et al. Figure 14. The spatial patterns of anomalous rainfall composite of seven negative in-phase Modoki events during , 2002 event and 2009 event. Unit is mm month 1. and west coast of Peninsular Malaysia as having a semi-monsoonal type of precipitation annual cycle in which the peaks occur during April May June (AMJ) and September October November (SON). These two seasons represent the inter-monsoon periods in which the wind and vertical shear are much weaker, providing favourable conditions for local convection to occur (Sow et al., 2011). 7. Summary This study investigates the differential impacts of conventional El Niño and El Niño Modoki on Malaysian rainfall variability during the boreal winter season. Malaysian boreal winter rainfall is impacted by both the conventional El Niño and El Niño Modoki but with different teleconnection patterns. Generally, during conventional El Niño, northern Borneo experiences significant rainfall deficit while Peninsular Malaysia receives little impact. In contrast, El Niño Modoki favours conditions with deficit rainfall over both regions. In contrast to Feng et al. (2010), the level of impacts of El Niño Modoki on rainfall anomaly over this region, particularly northern Borneo, is higher than during conventional El Niño. The different patterns of rainfall anomalies during the conventional El Niño and the El Niño Modoki represent the dominant modes of co-variability between rainfall anomalies over these regions and the SSTA in the tropical Indian and Pacific Ocean. Moreover, these covariability modes are also valid for the conventional Figure 15. The spatial patterns of sea surface temperature anomaly composite of seven negative in-phase Modoki events during , 2002 event and 2009 event. Unit is C. La Niña and La Niña Modoki but opposite in polarity. These distinctive conditions of anomalous rainfall are linked to differences in regional SST anomaly and anomalous circulation associated with those two climate modes of the tropical Pacific. During conventional El Niño, warmer SSTA dominates the eastern Indian Ocean, South China Sea and East China Sea and cooler SSTA dominates in the western north Pacific region. In conjunction with this SSTA pattern, the atmospheric circulation prominently features anti-cyclonic circulation over the western North Pacific region with the descending node of the Walker circulation located along 110 E 160 E. Northern Borneo dominantly features descending air as indicated by the mid-level vertical velocity. During El Niño Modoki, the positive SSTA that dominates the eastern Indian Ocean, South China Sea and East China Sea does exist, albeit with much reduced magnitude and relatively less significant anomalies compared to that of conventional El Niño. The dipole pattern over western North Pacific has weakened and the corresponding anti-cyclonic circulation that dominates the region in the conventional El Niño has also weakened, with its center displaced westward over to the Philippines. Both northern Borneo and northern Peninsular Malaysia experience

11 IMPACTS OF CONVENTIONAL EL NIÑO VS. EL NIÑO MODOKI ON MALAYSIAN RAINFALL 2773 Acknowledgements The first author would like to thank MOSTI for providing an NSF scholarship for her PhD program. The authors acknowledge the Malaysian Meteorological Department for providing station precipitation data. The GPCC data is obtained from Germany s National Meterological Service (DWD). This research is funded by the Malaysian Ministry of Higher Education LRGS/TD/2011/UKM/PG/01 and the Universiti Kebangsaan Malaysia DIP and DPP Figure 16. The spatial patterns of low-level wind (850 mb) anomaly composite of seven negative in-phase Modoki events during , 2002 event and 2009 event. Unit is ms 1. descending air. However, two El Niño Modoki events, i.e and 2009 exerted out-of-phase impact with enhanced (deficit) rainfall over Sumatra and Peninsular Malaysia (northern Borneo). Notably, the two events also displayed much warmer SST anomalies in Indian Ocean and regional seas in the Maritime Continent, particularly the South China Sea due to weaker monsoon wind. References Aldrian E, Susanto RD Identification of three dominant rainfall regions within Indonesia and their relationship to sea surface temperature. Int. J. Climatol. 23: Ashok K, Behera SK, Rao SA, Weng H, Yamagata T El Niño Modoki and its possible teleconnection. J. Geophys. Res. 112: C11007, DOI: /2006JC Becker A, Finger P, Meyer-Christoffer A, Rudolf B, Schamm K, Schneider U, Ziese M A description of the global land-surface precipitation data products of the Global Precipitation Climatology Centre with sample applications including centennial (trend) analysis from 1901 present. Earth Syst. Sci. Data, 5(1): 71 99, DOI: /essd Chang CWJ, Hsu HH, Wu CR, Sheu WJ Interannual mode of sea level in the South China Sea and the roles of El Niño and El Niño Modoki. Geophys. Res. Lett. 35: L03601, DOI: / 2007GL Chen G, Tam CT Different impacts of two kinds of Pacific Ocean warming on tropical cyclone frequency over the western North Pacific. Geophys. Res. Lett. 37: L01803, DOI: /2009GL Feng J, Li J Influence of El Niño Modoki on spring rainfall over South China. J. Geophys. Res. 116: D13102, DOI: /2010JD Feng J, Wang L, Chen W, Fong SK, Leong KC Different impacts of two types of Pacific Ocean warming on Southeast Asian rainfall during boreal winter. J. Geophys. Res. 115: D24122, DOI: /2010JD Juneng L, Tangang FT Evolution of ENSO-related rainfall anomalies in Southeast Asia region and its relationship with atmosphere ocean variations in Indo-Pacific sector. Climate Dynam. 25: Juneng L, Tangang FT Level and source of predictability of seasonal rainfall anomalies in Malaysia using canonical correlation analysis. Int. J. Climatol. 28: , DOI: /joc Kalnay E, Kanamitsu M, Kistler R, Colloins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woolen J, Zhu Y, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelewski C, Wang J, Leetma A, Reynolds R, Jenne R, Joseph D The NP/NCAR 40-year Reanalysis Project. Bull. Am. Meteorol. Soc. 77: Kim DW, Choi KS, Byun HR Effects of El Niño Modoki on winter precipitation in Korea. Climate Dynam. 38(7): , DOI: /s Kug JS, Jin FF, An SI Two types of El Niño events: cold tongue El Niño and warm pool El Niño. J. Climate 22: , DOI: /2008JCLI Rasmusson EM, Carpenter TH Variation in the tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/El Niño. Mon. Weather Rev. 110: Rayner NA, Parker DE, Horton EB, Folland CK, Alexander LV, Rowell DP, Kent EC, Kaplan A Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res. 108: D14, DOI: /2002JD Sow KS, Juneng L, Tangang FT, Hussin AG, Mahmud M Numerical simulation of a severe late afternoon thunderstorm over Peninsular Malaysia. Atmos. Res. 99(2): Tangang FT, Juneng L Mechanism of Malaysian rainfall anomalies. J. Climate 17: Tangang F, Latif MT, Juneng L The roles of climate variability and climate change on smoke haze occurrences in Southeast

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