A multi-scale investigation of rainfall drivers over Metro Manila, Philippines using empirical orthogonal function analysis

The Philippines is a country vulnerable to worsening climate, especially its rapidly urbanizing capital, Metro Manila (MM). However, the understanding of rainfall in this region is still poor due to the complex interplay of synoptic and local factors, alongside data scarcity and methodological limitations. This research aims to identify key drivers behind observed long-term rainfall trends over MM. Empirical orthogonal function analysis was applied to daily ERA5 geopotential data to identify recurring synoptic patterns over the Philippines between 1991 and 2021. For each pattern, annual precipitation amount (PA), frequency (PF), and intensity (PI) values were calculated from available PAGASA station data in MM. The Mann-Kendall trend test was used to identify long-term trends in PA, PF, and PI for each pattern. These trends were then correlated to MM urban expansion over recent decades. A similar analysis was made for variables linked to rainfall formation (i.e. surface temperature, CAPE, vertical moisture advection, and evaporation.) Results reveal that rainfall over MM is primarily driven by synoptic conditions, particularly the southwest monsoon (SWM) and passing low pressure systems. However, rainfall is occurring more frequently in the days leading up to the SWM season and when there is a high pressure system present over the northern region of the Philippines. This was linked to MM urban expansion, which seems to create ideal conditions for rainfall initiation. The results of this study may inform the decisions of policy makers and key stakeholders in the Philippines involved in disaster risk management and urban planning. • The southwest monsoon and tropical cyclones drive rainfall over Metro Manila, Philippines. • Rainfall occurrence is increasing significantly for calm days over the study area. • Temperature, CAPE, vertical moisture advection, and evaporation are increasing as well. • The observed trends show significant correlation to urban expansion.