Observing ENSO Reversals using RegCM4 and Satellite Data in the Philippines during the Southwest Monsoon Season

Abstract

Introduction: Investigations into the temporal shifts and heightened intensity of the southwest monsoon (SWM) are of equal importance to the Philippines based to the fundamental impact of the SWM on the country's precipitation patterns. As a principal determinant of the rainfall climatology, the SWM significantly influences the distribution and availability of water resources, which are indispensable for various sectors including agriculture, industry, and commerce. Given that the SWM serves as a primary source of water supply in the Philippines, elucidating its alterations is imperative for informed decision-making and adaptive measures to address potential challenges posed by changing climatic conditions. Moreover, such research endeavors contribute to enhancing resilience and sustainability across key sectors, facilitating effective resource management strategies and fostering socio-economic development in the region.

Objectives: This research investigated whether the behavior of the possible indicators (precipitation, wind, sea level pressure and sea surface temperature) can be captured by a regional climate model (RegCM4).

Methods: The methodology employed in this study involved the analysis of various indicators to assess changes in rainfall patterns, particularly focusing on the southwest monsoon (SWM) season in the Philippines. Four key indicators—wind, sea surface temperature (SST), sea level pressure, and large-scale precipitation—were scrutinized using data sets from multiple sources. Specifically, wind, SST, and sea level pressure data were obtained from the ECMWF ERA-Interim reanalysis data set spanning from 1979 to the present. This data set provided comprehensive coverage with 60 vertical levels and a spatial resolution of 0.75° latitude x 0.75° longitude. Additionally, SST data from the NOAA Optimum Interpolation SST version 2 (V2) and mean sea level pressure data from the NCEP/NCAR Reanalysis data set were utilized. Furthermore, the study incorporated large-scale precipitation data from the TRMM data set to assess rainfall behavior over a broader spatial context. Moreover, the methodology employed a combination of observational data analysis and numerical modelling to comprehensively investigate changes in SWM rainfall patterns and associated atmospheric dynamics in the Philippines.

Results: Results showed that the model captured the general characteristics of the observed indicators (sea level pressure and SST), especially during EN and LN events but fails to adequately represent the corresponding effect on precipitation. In addition to this, the inability of the model to capture the wind patterns result into failure in capturing the behavior of precipitation. The results showed that the underestimation (overestimation) of winds made the precipitation becomes drier (wetter) in condition and these findings needs a future study.

Conclusions: The conclusion of the study indicates that while the numerical experiment successfully replicates certain observed data patterns, such as the precipitation anomaly during El Niño (EN) events, discrepancies exist, particularly in capturing anomalies in the western part of the country during normal years and wind patterns. Despite these shortcomings, the model aligns well with observed behaviors of sea surface temperature (SST) and sea level pressure during the ENSO signal reversal. Overall, the findings underscore the significance of SST, wind, and pressure as indicators linked to rainfall behavior in the western regions of the Philippines during the southwest monsoon (SWM) season.