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Subseasonal Weather Outlook and Review

Review of Regional Weather for July 2021

1. Overview

1.1 During July 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were over the coastal parts of southern Myanmar, southern Cambodia, northwestern Philippines, as well as the Maluku Islands for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies (drier conditions) were over the central Philippines. However, there were some discrepancies over the western and central Maritime Continent, where CMORPH-Blended recorded drier conditions than GSMaP-NRT.

1.2 The observed rainfall anomaly pattern (i.e. mix of below- to above-average rainfall) is broadly consistent with the predictions from the subseasonal weather outlooks for July 2021 (28 June – 11 July and 12 – 25 July). The only portion of the ASEAN region with a consistent outlook for July were the wetter conditions in the southeastern Maritime Continent, which agrees with the observed anomaly pattern.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for the month of July 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 ASEAN region experienced above-average temperatures during July 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over Lao PDR and the southern Maritime Continent, followed by central and northern Myanmar, Viet Nam, and central and southern parts of the Philippines. Elsewhere, near- to above-average temperature were recorded.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for the month of July 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 At the start of July 2021, a Madden-Julian Oscillation (MJO) signal was present in the Indian Ocean (Phases 2 and 3, Figure 3). In the second week of July the signal propagated eastwards over the Maritime Continent (Phases 4 and 5). The signal then weakened slightly as it moved over the Western Pacific (Phases 6 and 7), although it strengthened again entering the Western Hemisphere (Phase 8) at the end of July. The mix of below- to above-average rainfall for the ASEAN region is indicative of the near complete circumnavigation of the MJO signal: Phases 3 to 4 tend to bring wetter conditions to much of the region, while Phases 7 to 8 tend to bring drier conditions, and Phase 2 and Phase 5 bring a mix of drier and wetter conditions at this time of the year.

mjo_phase_diagram

 

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. A negative Indian Ocean Dipole (IOD) has developed in the Indian Ocean, with warmer sea surface temperatures southwest of Java during July. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.