SPACE-TIME SPECTRAL ANALYSIS OF BORNEO VORTEX USING SPHERICAL HARMONICS TRANSFORM METHOD

• Main Author: Amri, Sayful
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/46795
• Institution: Institut Teknologi Bandung

Borneo vortex (BV) is a quasi-stationary vortex that usually develops over the South China Sea (SCS) during boreal winter, which may cause high-impact weather events in the region. BV occurrence is often associated with northerly cold air surges but it is known that equatorial waves can also trigger the formation of vortices, tropical depressions (TD), and tropical cyclones (TC). However, knowledge about the relationship between equatorial waves and BV is still limited. This study aims to obtain wavenumber-frequency signatures of BV through space-time spectral analysis of outgoing longwave radiation (OLR) anomaly that have been classified according to the duration of BV events. Wavenumber content of the spectra is obtained using spherical harmonics (SH) transform, whereas frequency is analyzed using Fourier transform methods. Spatial-temporal structures of the waves are reconstructed by inverse transforms after filtering in the spectral domain using the identified BV signatures. The results show that the spectral signatures of BV is characterized by enhancements of westward propagating (antisymmetric) mixed Rossby gravity (MRG) and (symmetric) equatorial Rossby (ER) with zonal wavenumbers of 5-10 and periods of 5-8 days and 12-31 days, respectively. It is also found that BV events can be initiated by the superposition of MRG and ER waves. Moreover, further evolution of BV is characterized by westward-moving convection that follows MRG (ER) propagation for BV events with a duration of 1-3 (4-7) day. The strong role of equatorial waves implies that the synoptic-scale components of high impact weather events in the Maritime Continent should be, at least partly, predictable.