Quasi-Biweekly Extensions of the Monsoon Winds and the Philippines Diurnal Cycle

The impact of quasi-biweekly variability in the monsoon southwesterly winds on the precipitation diurnal cycle in the Philippines is examined using CMORPH precipitation, ERA5 data, and outgoing longwave radiation (OLR) fields. Both a case study during the 2018 Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign and a 23-yr composite analysis are used to understand the effect of the quasi-biweekly oscillation (QBWO) on the diurnal cycle. QBWO events in the west Pacific, identified with an extended EOF index, bring increases in moisture, cloudiness, and westerly winds to the Philippines. Such events are associated with significant variability in daily mean precipitation and the diurnal cycle. It is shown that the modulation of the diurnal cycle by the QBWO is remarkably similar to that by the boreal summer intraseasonal oscillation (BSISO). The diurnal cycle reaches maximum amplitude on the western side of the Philippines on days with average to above-average moisture, sufficient insolation, and weakly offshore prevailing wind. This occurs during the transition period from suppressed to active large-scale convection for both the QBWO and BSISO. Westerly monsoon surges associated with QBWO variability generally exhibit active precipitation over the South China Sea (SCS), but a depressed diurnal cycle. These results highlight that modes of large-scale convective variability in the tropics can have a similar impact on the diurnal cycle if they influence the local-scale environmental background state similarly.

Automated summary

The study found that the quasi-biweekly oscillation has a significant impact on the precipitation diurnal cycle in the Philippines, mainly by increasing moisture, cloudiness, and westerly winds during the transition to active large-scale convection.