Characterization of the 2012 and 2013 Metro Manila “Enhanced Habagat” Heavy Rainfall Events

The strong southwest monsoon episodes in August 2012 and 2013, locally referred to as the “Enhanced Habagat ” 2012 and 2013, respectively, resulted in rainfall that exceeded the monthly mean rainfall of the affected regions along western Luzon, including Metro Manila. The prolonged heavy rainfall events were the result of tropical cyclone enhancement of the monsoon winds. The synoptic environment in the two events was characterized by the deepening of the Asian monsoon trough depicted by the zonally-oriented and eastward-extended 1000 hPa isobar. The deep troughs were caused each year by a combination of tropical cyclones located to the northeast of the Philippines – Typhoon Haikui in 2012 and Severe Tropical Storm Trami in 2013, and remnant tropical cyclones in the northern South China Sea region that formed several days prior to the enhanced Habagat episodes. The monsoon trough induced a low-level westerly jet in the South China Sea toward the western Luzon region that transported a narrow stream of moisture-laden air mass to Metro Manila and surrounding areas. Consequently, heavy precipitation ensued. Analysis showed the remnant lows are as important as the enhancing tropical cyclones Haikui and Trami in inducing the westerly jets. Removal of the enhancing tropical cyclones in numerical model simulations still showed a relatively deep monsoon trough that led to heavy rainfall along western Luzon. In addition, the intrusion of low potential vorticity area to the eastern and northern flanks of the tropical cyclones facilitated the strengthening of the steering ridge that resulted in their westward and slow translational motion, making the events last for several days and exacerbating the impacts. Furthermore, the Madden-Julian Oscillation possibly serves as a precursor to heavy rainfall events. Compounded with a populous megacity, understanding the mechanisms that lead to these extreme hazards is vital to future forecasting and disaster risk management of similar events.