Review of Regional Weather for June 2021

1. Overview

1.1 During June 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). Much of the western and central Maritime Continent (except for parts of eastern Borneo), and the north-eastern parts of Mainland Southeast Asia experienced above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over central Java regions and northern Viet Nam (due to Tropical Storm Koguma made landfall in mid-June) for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). A band of below-average rainfall between 8°N and 14°N was recorded, in particular parts of southern Mainland Southeast Asia. Elsewhere, the land masses of the eastern Maritime Continent predominately experienced below-average rainfall, except for central parts of the Philippines which experienced above-average rainfall due to Tropical Storm Choi-Wan in early June.

1.2 The observed rainfall anomaly pattern (i.e. below-average rainfall over southern and northern parts of the Philippines , above-normal rainfall in the Java region) is broadly consistent with the predictions from the subseasonal weather outlooks for June 2021 (31 May – 13 June, 14 – 27 June, 28 June – 11 July), apart from the positive anomalies (wetter conditions) for central Sumatra and the north-eastern parts of Mainland Southeast Asia not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of June 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


1.3 Most of the Mainland Southeast Asia experienced near- to above-average temperatures during June 2021, in particular warmer temperature for northern Myanmar, central Thailand, northern Cambodia and northern Viet Nam (Figure 2). Above-average temperatures were also experienced over northern Philippines and southern Maritime Continent. Elsewhere in the Maritime Continent, near-average temperatures were experienced in most of the region, with below-average temperatures for northern Sumatra and parts of Peninsular Malaysia.



Figure 2: Temperature anomalies for the month of June 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO, Figure 3) signal was incoherent for most of the first three weeks of June 2021 due to interference with other tropical waves. In the last week of June, an MJO signal emerged over the Western Hemisphere and Africa (Phase 1) and propagated eastwards to over the Indian Ocean (Phase 2). Phase 2 tends to bring wetter conditions to western Maritime Continent.



Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).


2.2 The equatorial Pacific Ocean is currently in an ‘ENSO neutral’ state. There are some signs of a negative Indian Ocean Dipole (IOD) developing in the Indian Ocean, with warmer sea surface temperatures south-west of Java. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.