JUNE 2017 H A I E T A L . 849 Extreme Rainstorms that Caused Devastating Flooding across the East Coast of Peninsular Malaysia during November and December 2014 OOI SEE HAI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia AZIZAN ABU SAMAH National Antarctic Research Center, Institute of Postgraduate Studies, and Institute of Ocean and Earth Sciences, University of Malaya, Kuala Lumpur, Malaysia SHEEBA NETTUKANDY CHENOLI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia KUMARENTHIRAN SUBRAMANIAM Malaysian Meteorological Department, Petaling Jaya, Selangor, Malaysia MUHAMMAD YUNUS AHMAD MAZUKI National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, Kuala Lumpur, Malaysia (Manuscript received 2 September 2016, in final form 6 January 2017) ABSTRACT During the early boreal winter (northeast) monsoon (November–December), cold air frequently bursts out from intense Siberian highs toward the Chinese coast in response to the development and movement of a 500-hPa trough. The resultant strong low-level northwesterlies turn into northeasterlies across the South China Sea as ‘‘cold surges.’’ On interacting with the near-equatorial trough, mesoscale convective systems form north of the trough, normally giving rise to heavy downpours and severe flooding, mainly along the coastal stretch in the east coast states of Peninsular Malaysia. In November 2014, a 1-week-long episode of heavy downpours, producing more than 800 mm of rain, occurred along the coastal stretch of northeastern Peninsular Malaysia. However, during December 2014, two episodes of extreme rainfall occurred mostly over inland and mountainous areas of the east coast of Peninsular Malaysia, in particular across its northern sector. These two unusual events, which lasted a total of 11 days and delivered more than 1100 mm of precipitation, resulted in extreme and widespread flooding, as well as extensive damage, in many inland areas. Analysis shows that the stronger wind surges from the South China Sea due to very intense cold-air outbreaks of the Siberian high developed under ENSO-neutral conditions. In addition, the mesoscale convective systems that developed across the northeastern Indian Ocean (near northern Sumatra) in response to the propagation of a 500-hPa short-wave trough across the Indian subcontinent toward China were the combined factors for these unusual extreme rainfall and flooding events along the east coast of Peninsular Malaysia. 1. Introduction occurred along the coastal stretch of northeastern Peninsular Malaysia. However, from mid-December Heavy rain with its associated flooding is an annual onward, two episodes of extreme rainfall caused wide- occurrence along the east coast of Peninsular Malaysia spread flooding in Kelantan, Terengganu, and Pahang (Fig. 1) during the northeast monsoon. In November on the east coast of Peninsular Malaysia. The unusual 2014, an episode of heavy rainfall and ensuing flooding situation was that these long-lasting extreme rainfall events were concentrated over the catchment areas in Corresponding author e-mail: Sheeba Nettukandy Chenoli, the upper reaches of the Kelantan and Pahang River [email protected] basins instead of near the lower reaches, as normal. A DOI: 10.1175/WAF-D-16-0160.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/07/21 11:52 PM UTC 850 WEATHER AND FORECASTING VOLUME 32 oscillation. The relationship between the rainfall vari- ability over the Maritime Continent region and ENSO has been studied by Hendon (2003), McBride et al. (2003), and Tangang and Juneng (2004). These studies reveal that the winter monsoon is weak during El Niño years and is likely to be strong during La Niña years. Recently, Wu and Leung (2009), as well as Tubi and Dayan (2012), studied the impact of ENSO on the Si- berian high, the East Asian winter monsoon, and the modulation of rainfall over southern China and Hong Kong. Unlike the results from the above-mentioned previous studies, the cases examined in this paper, as revealed in section 3, show that the December 2014 episodes of unusual cold surge–induced extreme rainfall FIG. 1. Topography of Malaysia (Peninsular Malaysia and the states of Sarawak and Sabah). occurred mostly over inland and mountainous areas in the northern sector of the east coast of Peninsular Malaysia and were due to the influences of the intense mountainous hydrological station (Gunung Gagau) in Siberian high during an ENSO-neutral year along with Pahang recorded 624 mm of torrential rain from 15 to developing mesoscale convective systems in the north- 19 December and 1395 mm from 20 to 24 December eastern Indian Ocean. 2014. These rainfall amounts were claimed to have re- turn periods of 40 and 1000 yr, respectively (Sun Daily, 2. Data and methods 20 January 2015). The extreme rainfall situation was aggravated by the low water-holding capacity of the Surface and upper-air reanalysis data at 0000 UTC lower reaches, excessive logging and land clearing, as from the European Centre for Medium-Range well as sediment buildup in river areas. In fact, exces- Weather Forecasts (ECMWF) are used. These so- sive land clearing caused serious erosion and swift called ERA-Interim (Dee et al. 2011) data have a spa- drainage of water, leading to widespread flooding. River tial resolution of 0.75830.758 in latitude and longitude. levels during these two episodes far exceeded those of We complement the reanalysis data with a wide range of the 1967 and 2004 floods. At the height of the flood, satellite observations (Rahimi et al. 2015). We use more than 66 500 people were evacuated, at least 21 monthly, daily, and 3-hourly Tropical Rainfall Measur- people died, and 250 000 lost their homes. Estimated ing Mission (TRMM) precipitation at 0.258 resolution; damages cost about 1 billion Malaysian ringgits (RM), the data were obtained from the Goddard Earth Sci- almost U.S. $300 million (source: Malaysian National ences Data and Information Services Center (GES- Security Council). DISC) Interactive Online Visualization and Analysis The available records show that the above flood- Infrastructure (Giovanni). TRMM data (Bookhagen associated extreme meteorological events during 2010) are merely used to complement the precipitation December 2014 are the first to occur in Peninsular analysis in data-sparse land and sea areas throughout the Malaysia. Thus, understanding the features and pro- region. Furthermore, selected daily/hourly gridded cesses of these rare episodes is essential in advancing the Multifunctional Transport Satellite-2 (MTSAT-2) in- scientific knowledge for better flood mitigation to min- frared channel 1 (IR1) data, which are available from the imize the socioeconomic impacts in the future (Valipour Center for Environmental Remote Sensing (CEReS), 2012, 2016). This is the core objective and approach of Chiba University, Chiba, Japan, are also utilized. These this paper. Several recent studies carried out for the data, which have a resolution of 0.048, cover an area Malaysia region show that there has been an increase in from 608Sto608N and from 858 to 2058 longitude. The the frequency and intensity of extreme rainfall events measured blackbody temperatures (TBBs) of these in- (Suhaila et al. 2010; Zin et al. 2010). In addition, a few frared images are color enhanced by suppressing those previous case studies of extreme rainfall over Peninsular TBBs warmer than 2208C based on the convection in- Malaysia by Tangang et al. (2008) and Juneng et al. dex, as described by Chang et al. (2005). The coldest (2007) were mainly focused on the extreme precipitation cloud tops are associated with colors ranging from blue events caused by the interaction of northeasterly cold to orange to pink. The colder the cloud tops are, the surges with the Borneo vortex, as well as the impact on higher the clouds and, most likely, the heavier the the events by phenomena such as the Madden–Julian thunderstorms. For example, near the strongest IR Unauthenticated | Downloaded 10/07/21 11:52 PM UTC JUNE 2017 H A I E T A L . 851 TABLE 1. List of El Niño, La Niña, and neutral events in the index from Chang et al. (2005), which is chosen as the tropical Pacific based on the ONI. averaged 925-hPa meridional wind between 1108 and 8 8 El Niño years La Niña years Neutral years 117.5 E along 15 N(Fig. 5a, described in more detail below). By adapting the index definition from Chang et al. 1982 1983 1981 1986 1984 1985 (2005), we further define the easterly surge index [zonal 1987 1988 1989 wind surge due to strengthening or equatorward move- 1991 1995 1990 ment in the subtropical ridge of the northwestern Pacific 1994 1998 1992 (Raman et al. 1978) as a result of a Siberian high out- 1997 1999 1993 break] as the averaged 925-hPa zonal wind between 7.58 2002 2000 1996 8 8 2004 2007 2001 and 15 N along 120 E. A surge event occurs when either 21 2006 2010 2003 one of these indices exceeds 8 m s . The surge intensity is 2011 2005 classified into weak, moderate, and strong categories for a 2008 surge index between 8 and 10, 10 and 12, and greater than 2 2012 12 m s 1 [as adopted from Chang et al. (2005)], re- 2013 2014 spectively. Cold and/or easterly surges are necessary but not a sufficient condition to ensure the impingement and sustenance of surge-induced heavy rainfall events toward gradients at the leading edge of an enhanced image is the the east coast of Peninsular Malaysia. location of heavy rain with cumulonimbus clusters. TBBs from 2708 to 2808C indicate that the cloud top 3. Analysis and discussion has penetrated the tropopause level (above 100 hPa or a.