| Western Pacific heading for rough weather and increased landslide risk |
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| Wednesday, 07 July 2010 | |||||||||
On August 2009, Typhoon Morakot (Typhoon Kiko in the Philippines) wreaked greater havoc than any other typhoon in Taiwan for the past half-century, and left the island with 461 dead, 192 missing and also feared dead, and around US$ 3.3 billion in damages.
The enormous amounts of rainfall that bore down on the island, peaking at 2,777 millimeters, triggered massive mudslides and flooding. The entire town of Xiaolin was buried.
Last 22-25 June 2010, nearly a thousand scientists gathered at the Taipei International Conference Center to discuss the current understanding about the factors that induce such extreme events. Recent findings from the work of around 4,000 scholars were presented at the Western Pacific Geophysics Meeting in Taipei in the desire to understand and help with more appropriate adaptation and mitigation strategies. The factors of risk can be viewed from three different scales:
There is still much to understand about how the climate system works. Comprehensive records in the last 40 years show that there occurs, on average, about 90 typhoons a year in the world.[1] It seems that this number has not changed, but the severity of the storms has increased. Even when we cannot entirely explain the complex interactions at work, the effects of the changes in the patterns are all too evident. To understand this and related concerns, we need to look at the phenomena in the Pacific and the larger linkages.
The changes in global weather affecting the Western Pacific The ITCZ Traditionally, we know the trade winds as the prevailing pattern of easterly surface winds found in the tropics and steer the flow of tropical storms forming over the ocean to make landfall in North America, Southeast Asia, and India. As the trade winds weaken, more rainfall is expected in neighboring areas. The merging of the trade winds from the northern and southern hemispheres is known as the Intertropical Convergence Zone (ITCZ), which is the area encircling the earth's equator. Two types of El Niņo Almost 25 years ago when the scientists learned that the anchovy harvest failure off Peru's coast was linked to broader climate patterns, we learned of the phenomenon of El Niņo. We understood El Niņo was linked to the Southern Oscillation (ENSO) and is causing a shift in weather patterns in the Pacific and elsewhere in the world. Since then we have learned there are two kinds of El Niņo. The standard one originates in the Eastern Pacific (EP El Niņo) and is characterized by weak trade winds that leave the upper ocean warm by failing to drive cold upwelling, which is an oceanographic phenomenon that involves the rising of deeper colder water to shallower depths. It usually occurs in cycles of three to eight years. For a schematic diagram of El Niņo, non-El Niņo, and La Niņa conditions, you may check out this webpage from the National Oceanic and Atmospheric Administration (NOAA) In more recent decades, scientists observed a non-canonical (which means not conforming to well-established rules or patterns) El Niņo forming in the central Pacific known as the Central Pacific Warming (CPW) or CP El Niņo.[2] Here, the cooler waters of the east and west flank the warmer central Pacific, and since the 1990s has become a common phenomena. The El Niņo of 2010 is of this kind. The effects of CPW are different from the EP El Niņo, and are thought linked to global warming. They are considered to have more damaging effects over a broader area, as warmer waters in the Central Pacific are associated with higher storm frequency.[3] The North Pacific Oscillation (NPO) precedes both forms of El Niņo and is characterized by large-scale fluctuation of atmospheric pressure and sea surface temperatures in the North Pacific.[4] Further, low values are associated with the southerly airflow along the west coast of North America, which tends to advect (or to transfer heat through the horizontal flow of air) warmer southern air into the region. High values are associated with the northerly flow, with corresponding advection of colder, sub-polar air into the region.[5] In knowing this, we have a six-month advance warning of a potential El Niņo in the Western Pacific. Feedback from the North Pacific The feedback loops are far-reaching as NPO highlights the different regions and specific changes in the Pacific and beyond.The Pacific contributes to certain processes that also affect the Atlantic Ocean. The amount of snow on the Tibetan plateau is a significant loop and may play a critical role. As the land warms in the spring, the air rises above the land causing a pressure gradient that drives the monsoon. More snow on the plateau in spring or early summer means the sun's heat is slower in warming up the land. And so, the less snow there is in winter, the stronger the monsoon the following summer.[6] It is now thought that the Indian Ocean feeds the NPO as the warming over the Tibetan plateau results in the advance of the monsoon rains from the Indian subcontinent two months earlier than the usual July or August arrival.[7] Such shift influences the geographical latitude of rain belts, distribution of typhoon track, flood, drought and other events in the Pacific. The Intra Seasonal Oscillation (ISO), also known as Madden-Julian Oscillation or MJO, significantly affects the weather and climate in the Western North Pacific during the boreal summer. ISO is an equatorial travelling pattern of anomalous rainfall that is planetary in scale. It is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Oceans.[8] This sub-monthly variability is dominated by a wave-like pattern that increases north northwestward from the northeast of Papua New Guinea to the East China Sea. The ISO is multi-scale in nature, both in time and space. The ISO essentially exhibits two spectral peaks: 30-60 days and 10-25 days. The 30-60-day ISO tends to grow northwestward from the Philippine Sea to subtropical East Asia, while the 10-25-day ISO tends to increase westward from the Philippine Sea to the Indo-China Peninsula and the Bay of Bengal.
The formation of typhoons in the changing pattern of the Western Pacific Typhoon formation tends to cluster in the timescales of the ISO in the tropical Western North Pacific, as it is closely associated with the monsoon trough fluctuation. This is the contraction and expansion of the trough in the east-west direction and the meridional shifts of the trough. These fluctuations modulate the typhoon activity in the tropical Western North Pacific.[9] The monsoon trough extends southward into the tropical Western Pacific, where the sea surface is higher than 26 degrees C in the boreal summer, and the moisture is abundant in the lower troposphere. It is in this region where the cyclonic relative vorticity and anti-cyclonic relative vorticity are present. Vorticity is a concept in fluid dynamics that describes the tendency of elements in the fluid to spin. The vertical shear of horizontal winds is smaller than the surrounding regions, which is among the favourable environmental factors for the tropical cyclone formation. This monsoonal circulation or monsoon trough draws in the ITCZ and is characterized with erratic weather patterns with stagnant calms and violent thunderstorms.[10] Typhoons can be embedded in such a large-scale monsoon trough and are called a gyre. A gyre is any large system of rotating ocean currents and is caused by the Coriolis effect. This is an inertial force described by the 19th century French engineer-mathematician Gustave-Gaspard Coriolis as an apparent deflection of moving objects when they are viewed from a rotating reference frame. The planetary vorticity, along with the horizontal and vertical friction, determine the circulation patterns from the wind curl. The gyre has a clockwise pattern and has four prevailing ocean currents in the Pacific: the North Pacific Current to the north, California Current to the east, North Equatorial Current to the south, and Kuroshio Current to the west.[11] The monsoon trough is a portion of the ITCZ that extends into or through a monsoon circulation. The term is commonly used in monsoonal regions of the Western Pacific such as Asia and Australia. The migration of the ITCZ/monsoon trough into a landmass heralds the beginning of the annual rainy season during summer months. Depressions and tropical cyclones often form in the vicinity of the monsoon trough, with each capable of producing a year's worth of rainfall in a relatively short time frame. The term monsoon stems from seasonal variations in winds but now it is more applied to tropical and subtropical seasonal reversals in both the atmospheric circulation and associated precipitation.[12] Traditionally, monsoon is defined as a seasonal reversing wind accompanied by seasonal changes in rainfall, but now it is used to describe seasonal changes in atmospheric circulation and rainfall. The northeastern monsoon (called amihan in the Philippines) that affects Southeast Asia begins during the months of September and October and ends sometime May and June. It is dominated by the trade winds, which are experienced as cool northeast winds with moderate temperatures and little or no rainfall.[13] The weakening of the monsoons has been attributed to tropical warming. Furthermore, there has been a noted increase in the number of heavy rain events and an increase in temperatures in the Pacific. Warm temperatures in the upper ocean are strongly correlated with stronger typhoons during the subsequent La Niņa. Warm temperatures also raise the altitude of the top of the storm-cloud formations resulting in a typhoon with significant power. From this warming, a sequence of typhoons may form that may feed off each other and strengthen their impact.[14] Also, the presence of aerosols has a cooling effect that masks the actual warming due to the change in climatic pattern. Aerosols are fine particles from man-made and natural sources that are released into the atmosphere around which water droplets form, creating smaller but a greater number of cloud droplets. Soot, carbon particles resulting from incomplete combustion of hydrocarbon, is the most significant aerosol source.[15]
Typhoon Morakot, Taiwan, August 2009 Typhoon Morakot was embedded in such a large-scale convection region with a monsoon circulation of different time scales in the tropical Western North Pacific. Morakot's landing on Taiwan occurred concurrently with the arrival of a large-scale cyclonic circulation in a submonthly wave pattern (10-30-day) during the cyclonic phase of the 40-50 day ISO.[16] Three typhoons were linked in a gyre: Goni, Morakot and Etau. Typhoon Morakot slowed down over Taiwan for 64 hours. As it left Taiwan, most rain fell to the south where a rain shadow was drawn in from the ocean and from the earlier typhoon. Typhoon Morakot triggered well over 40,000 landslides in Taiwan, although most were no more than 10 meters deep and averaged a few hectares in size. The mountainous terrain of Taiwan induces rainfall from the high clouds coming from over the ocean. The prior typhoon soaked the ground wet, allowing the succeeding storm to trigger the landslides. In the village of Shiaolin (Xiaolin) in the central mountains, the rains were heaviest, at least 3,000 millimeters in 72 hours. This, combined with the area being along a fault line, allowed for washouts as deep as 80 meters, amounting a total of 1010 kilos (that is 100,000,000,000) or 100 million metric tons of earth material. This particular landslide was strong enough to have a measurable impact equal to a magnitude 5 earthquake and traveling at a peak speed of 45 meters per second. Modeling work reconstructing the event showed that it was subsequent flows in the river valley that washed out the second half of the village. What does this mean to the rest of the Western Pacific region? On the scale of the Western Pacific, what is happening in Taiwan applies to Southern China, Hong Kong, Vietnam, Thailand, and the Philippines. We remember the recent massive flooding in China (June 2010) that displaced over 1.4 million people and with over 300 deaths. Ginsaugon in Leyte Island, Philippines, was a village buried under a massive landslide in February 2006. The landslide had a platform area of approximately 3,000,000 square meters that brought with it much of the mountain weakened by the fault line. The infiltrated mountainsides in these areas were affected by the typhoon series of increased intensity. These areas were saturated for a week or so before heavy rains of a second or third typhoon delivered half a year's average rainfall or more during. And frequently, in the most critical mountain areas, these events result in landslides, temporary dams on the rivers, and subsequent debris floods. What adds to this is that the Chichi earthquake of 1999[17] "prepared the mountainous landscape of Taiwan. And with the exception of the massive landslide immediately due to the earthquake, left much of the rest of the rock formation faulted and fractured, allowing chemical weathering over the last decade all the more available to flow when saturated by continuous rain and driven by torrential downpour." Responses and actions needed Engineering will have to cope with higher expected water levels. The impacts on engineering in the region could be massive given the continued tunnelling and damming of water for lowland use and the major investments on stabilizing mountain slopes. The interest of insurance companies is clear in the area of investment and infrastructure and is the one sector with a growing interest in what the scientists have today. Unfortunately, the burden is greatest for the marginal communities, who are not simply the poor, but those marginal to the political and economic centers of decision-making. They will absorb the most devastating hits to life and livelihood. They are the ones faced with relocation and economic and social insecurity if they leave their lands. Villages in the mountain valleys like Xiaolin and Guinsaugon are usually the communities of the vulnerable. These are communities without planning or infrastructure to withstand such events. These areas need more comprehensive work. Relocation of people in these areas is one of the biggest challenges in responding to their needs to stay in the traditional areas and, at the same time, not to use these evacuations as a basis to start further infrastructure projects for urban centers. Agriculture in the uplands will include the promotion of erosion-controlling crops, development and promotion of efficient water use in the uplands, localized weather-based early warning systems to follow a dynamic crop calendar, agricultural insurance, landslide and flood preparation, methods for localized seasonal climate forecast system, and development of farming systems in inundated areas. Long term adaptation includes breeding drought-resistant, pest and disease resistant, and flood-tolerant varieties, and the use of knowledge-based crop forecasting. In the Philippines, the Climate Change Act of 2009 was passed into law after two devastating typhoons, Ondoy and Pepeng (known internationally as Ketsana and Parma respectively in 2009), resulted in massive flooding and landslides in Metro Manila and other areas in northern Luzon. More than a thousand people perished. The damage to crops, especially rice, was extensive and is not expected to improve if the country will not be prepared to adapt to the changes needed. This is most critical as Filipinos now eat more rice than it can grow, and the Philippines now imports 10 percent of its rice from Vietnam and Thailand. Food security is a major concern as extreme weather events bear down on the country. In reviewing the lessons learned in the Pacific region and understanding the disasters, more effective action can be designed and planned. Disaster preparedness begins with identifying such areas in the Western Pacific that are most at risk to the consequences of landslides and debris floods. Because these areas will have vulnerable slopes and soils that are quicker to liquefy upon wetting and saturation, they will have a history of mass wasting.
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| Last Updated ( Thursday, 09 February 2012 ) | |||||||||
