Review of Regional Weather Conditions for First Fortnight of May 2019

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1. Review of Regional Weather Conditions for First Fortnight of May 2019

1.1 Under inter-monsoon conditions, the prevailing winds over the equatorial region were mostly light and variable in direction over the past fortnight. Over the northern ASEAN region, there were anomalous westerly winds (Figure 1).

Figure 1 5000 ft average winds (left) and anomalies (right) for 1 – 15 May 2019 (Source: JMA)

Figure 2: Daily average rainfall for the ASEAN region in the first fortnight of May 2019. (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percentage of average rainfall for 1 – 15 May 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.2 There were scattered showers over the region in the first half of May (Figure 2), with the exception of Myanmar where generally dry weather prevailed. Myanmar and the northern parts of Lao PDR and Thailand received below-average rainfall, which could have been due to dry air from the Bay of Bengal that persisted over these areas early in the month. Many areas along the equatorial region (including southern parts of Peninsular Malaysia, Sumatra, and Borneo) received below-normal rainfall (Figure 3), and this could in part be attributed to the Madden-Julian Oscillation (MJO) activities from Phase 5 to Phase 8 during the fortnight.

Figure 4: The MJO phase diagram for March 2019-May 2019 (green for April). 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)

1.3 In the first half of May, although there was a slight cooling of the sea surface, the sea surface temperatures of the equatorial Pacific Ocean remained warmer than average. There was no consistent coupling between the sea surface temperatures and the atmosphere, indicating that full-fledged El Niño conditions have not developed.