Review of Regional Weather Conditions for First Fortnight of December 2018
1. Review of Regional Weather Conditions for First Fortnight of December 2018
1.1 Northeast Monsoon conditions prevailed over the ASEAN region in the first half of December 2018. In the northern ASEAN region, weather was generally dry. Prevailing winds blew mainly from the northeast or east in general except in Myanmar, where winds were more variable. In the southern ASEAN region, winds were generally light and variable except for the moderate to strong northeasterly winds over parts of Malaysia. Confluence of winds along the equatorial region gave rise to rainy weather and above-average rainfall in many areas.
1.2 The daily average rainfall and the percentage of average rainfall for the first fortnight of December 2018 are shown in Figures 1 and 2 respectively. The 5000ft average winds and wind anomalies in the first half of December 2018 are shown in Figures 3 and 4 respectively.
Figure 1: Daily average rainfall for the ASEAN region in the first fortnight of December 2018. (Source: JAXA Global Satellite Mapping of Precipitation)
Figure 2: Percentage of Average Rainfall for 1 – 15 December 2018. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)
Figure 4: 5000 ft average winds (left) and anomalies (right) for 1 – 15 December 2018. (Source: JMA)
1.3 The Madden – Julian Oscillation (MJO) transitioned from Phase 8 to Phase 4 in the first half of December 2018, with slightly stronger amplitude of signals registered in Phase 2 to Phase 4. The stronger MJO activities in Phase 2 to Phase 4 are also associated with the rainy conditions experienced in the Indian Ocean and many parts of the southern ASEAN region.
Figure 5: The MJO phase diagram for Nov – Dec 2018 (blue for December). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology)
1.6 Although above-average sea surface temperatures persisted across the equatorial Pacific Ocean, the atmospheric indicators of El Niño – Southern Oscillation (ENSO) remained close to average. Sustained El-Niño conditions have yet been established.