Review of Regional Weather for January 2023


1. Overview

1.1 During January 2023, a mix of below- to above-average rainfall was recorded over the Maritime Continent, with much of the northern half of the region receiving above-average rainfall, and the much of the southern half receiving below-average rainfall (Figure 1). Near-average rainfall was recorded over most of Mainland Southeast Asia, except for parts of eastern Mainland Southeast Asia where above-average rainfall was recorded. The largest positive anomalies (wetter conditions) were recorded over the northeastern Maritime Continent for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates, along with parts of the western Maritime Continent for GSMaP-NRT. The largest negative anomalies (drier conditions) were recorded over parts of southern Borneo, southern Sulawesi, and Java, with CMORPH-Blended data showing relatively drier anomalies as compared to GSMaP-NRT data.

1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and the above-average rainfall for eastern Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for January 2023 (26 December 2022 – 8 January 2023, 9 – 22 January 2023 and 23 January – 5 February 2023).

Figure 1: Rainfall anomalies for January 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


1.3 Warmer-than-average temperatures were recorded over northwestern Mainland Southeast Asia and parts of southeastern Maritime Continent, whereas cooler-than-average temperatures were observed over much of central and eastern Mainland Southeast Asia. Elsewhere in the ASEAN region, near-average temperatures were observed. The warmest anomalies were recorded over Myanmar and the coolest over the eastern Mainland Southeast Asia.



Figure 2: Temperature anomalies for January 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal was present over the Western Pacific (Phases 6 & 7) at the start of January. This signal weakened in the first week of January, with no clear MJO signal present in based on the RMM diagram until the second half of the month. By the end of the third week of January, the MJO signal strengthened over the Indian Ocean (Phase 3). Typically, during this time of the year, Phase 7 tends to bring drier conditions to the western Maritime Continent, whereas Phase 3 tends to bring wetter conditions to the 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 La Niña conditions were present over the tropical Pacific. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were consistent with La Niña conditions, though there are signs of weakening. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the December to February period.