What caused the mass floods in Manila in the summer of 2012?

Introduction

            During the summer of 2012, Manila, Philippines experienced a meteorological phenomenal event that devastated the capital with 2.4 million people affected and 326,300 people evacuated from their homes and their livelihoods. The capital city received five days of continuous rainfall surpassing previous records with 455mm of precipitation in 24 hours. The accumulation and synergy of the Southwest Asian monsoon, associated typhoons in the region, and the La Nina ENSO event were all factors in the devastation and extreme flooding of the vulnerable Manila area. Although these catastrophic events individually are common and not significantly dangerous, the accretion of the many events happening at the same time in the same place was disastrous. It was quite literally the perfect storm.

Southwest Asian Monsoon

             Southwest monsoon is a cog in the larger East Asian Monsoon and is a large scale circulation caused by the contrast in heating between land and ocean. As the Asian landmass heats up, in the northern hemisphere summer, air begins to rise over the continent forming low pressure. The surrounding oceans which heat up much less appear relatively cool, setting up a giant sea breeze where cool moist air flows off the ocean towards lower pressure over the continent. As a result, southwest winds flow around Indo-China and over the South China Sea, pulling up moisture along the way going South (NASA 2012). In the case of flooding of Manila 2012 the Southwest monsoon in which the locals call “Habagat” was extraordinarily stronger than previous monsoons and may have had a factor in the flooding. Summer monsoons are characterized by a strong, general west breeze that is responsible for bringing significant rainfall to the Asian subcontinent and to South and East Asia. The significant Southwest monsoon rainfall is a by-product of air passing over large areas of warm equatorial ocean, stimulating increased levels of evaporation from the oceans service and the southwest monsoon air now laden with water vapour, cools as it moves north and as it rises over land, at some point the air is no longer able to retain moisture and precipitates large volumes of water (OCHA Report 2012). Although they have links, monsoons are not directly related to the Inter-tropical Convergence Zone or the ITCZ. Monsoon regimes affecting the Philippines area characterized by wet and dry seasons, the wet season is hot with humid winds, the dry season is cooler with winds of lower humidity (Drake 2000). In spring, the continental interior warms up creating a thermal low, warm moist air is then drawn in from the oceans and into continental interior, here the air is risen releasing precipitation and latent heat and this helps maintain the system. On that note, it is important to know that the monsoon system is a crucial aspect of a good livelihood and lifestyle in Asia and does have plenty of benefits in which go beyond the scope of this paper. However, as we’ve seen this past summer in Manila, when monsoon systems are amplified by other meteorological phenomenon, disaster is imminent. Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan plateau after the collision of the Indian Sub-continent and Asia around 50 million years ago (OCHA Report 2012). It is believed that the Southwest monsoon is much governed by the movement of the sub-tropical jet stream. From the south Himalayan plateau to the North in late May which allows the monsoon rains to burst into the interior. This was a major factor in the flooding of Manila.

 

Typhoons in the Region (Saola and Haikui)

            Typhoons played a large role in the intensification of the monsoon that devastated the Philippines and flooded the capital city. The typhoons that catalyzed the monsoon in the summer of 2012 were quite predictable due to the fact that it is not the first time these two weather systems have met. Typhoon Ketsana in 2009 is the most well known example of the confrontation between typhoon and monsoon and caused the most damage and rainfall to the region. The two typhoons that intensified the Southwest monsoon were typhoon Saola and Typhoon Haikui, with the latter having the largest effect, the experts say (NASA 2012). These typhoons enhanced the effects of the monsoon although they did not hit manila directly, they did catalyze the events. The southerly flow of typhoon Haikui majorly affects the pattern that the monsoon regime followed and typhoon Saola had the same effect one week earlier. They both boosted the monsoon flow into northern Manila causing the same effect of increased precipitation and an increase in extreme weather (NASA 2012). According to NASA, the East Philippines got little rain due to the fact that it was int he lee of the terrain with respect to the monsoon flow. Although located hundreds of kilometres away from the Philippines, the southerly flow from typhoon Haikui enhanced the southwest monsoon across much of Manila. As a result, widespread rains impacted regions still recovering from Saola less than a week earlier. Saola’s impact had previously saturated the soil and ground in the region before Haikui’s impact had occurred and thus flooding was imminent and unpreventable (NASA 2012). Soil and ground in Manila has an average capacity to absorb 100-150mm of water. This amount was exceeded within the first 6 hours of the downpour. In some areas, spoil absorption only has the capacity to hold 40-60mm and thus all soils were fully saturated prior to Haikui and the worst of the precipitation. The region received a record breaking 455mm of rain in 24 hours. These were all major factors for the flooding in Manila in the summer of 2012; however these typhoons are regular events in the region, the combination of these typhoons with other weather events in the area increased the flooding in the area and the dramatic increase in precipitation.

La Nina and ENSO

            In El-Nino years the monsoon rains are weakened and there is less precipitation. So what happens during cold ENSO events or La-Nina events? The opposite of course. La Niña causes a hot water shift to the West Pacific (Southeast Asia) this can result in more precipitation and the extremity and catalyst for extreme weather (Drake 2000). La Niña brings rainy weather and floods, during La-Nina years, the eastern pacific is cooler than average, while the western pacific is warmer. Air over the ocean is warmer, more buoyant and more humid; it rises higher and forms more intense storms (NASA 2012). La Niña has been classified by NASA as being a warming pattern that has additive effects on intensifying the monsoon system in Asia. The La Niña is also known as the Cold-El Niño event, and sometimes referred to as a normal period. Under non El-Niño conditions, also known as La Niña, a pool of warm water is located in the western equatorial pacific, bringing warm sea surface temperatures and precipitation to Australia and Indonesia, as it provides moisture to the atmosphere through evaporative processes, which leads to the formation of convective activity and rain-producing clouds in that region (Glantz 1996). The La Niña usually brings beneficial weather to an area that heavily relies on precipitation for crops and agriculture, however when it is accumulated with the Asian monsoon system and the typhoons in the region, it can enhance meteorological catastrophe and ended up adding to the effect of the floods in Manila.

Conclusion

            The flooding in Manila, Philippines was not a result of a singular meteorological event. Although the Southwest Asian monsoon played quite a significant role in the flooding, it was essentially an accumulation of the monsoon regime, the associated typhoons in the region at the time and just prior to the monsoon and the La Niña event that was occurring simultaneously to these events. Individually, these weather events create prosperous wealth for the region, though together, these natural phenomena resulted in the flooding of the Manila region.

References

NASA, NASA. “Unusually Intense Monsoon Rains : Natural Hazards.” NASA Earth Observatory : Home. N.p., n.d. Web. 9 Aug. 2012. <http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=45177&gt;.

GOV, NASA. “Monsoon, Tropical Cyclones Bring Massive Flooding to Manila | Precipitation Measurement Missions.” Precipitation Measurement Missions | An international partnership to understand precipitation and its impact on humankind.. N.p., 10 Aug. 2012. Web. 20 Nov. 2012. <http://pmm.nasa.gov/mission-updates/trmm-news/monsoon-tropical-cyclones-bring-massive-flooding-manila&gt;.

Drake, Frances. Global warming: the science of climate change. London: Arnold ;, 2000. Print

Glantz, Michael H. Currents of Change: El Niño’s Impact on Climate and Society. Cambridge: Cambridge UP, 1996. Print.

Report, OCHA 2012. “The Philippines – Southwest Monsoon Floods.” Reliefweb.int. United Nations Office for the Coordination of Humanitarian Affairs, 17 Aug. 2012. Web. 17 Nov. 2012. <http://reliefweb.int/sites/reliefweb.int/files/resources/full%20sitrep.pdf&gt;

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