When the equatorial climate is not so equitable.

The Straits of Malacca and Singapore (SOMS) form one of the world’s busiest shipping lanes, carrying about one third (prior to Covid-19) of the world’s traded goods.  The SOMS connects the Indian and Pacific Oceans and adopts a 250nm long narrow traffic separation system for ships up to 300,000 DWT with some 81000 transits annually. It is surrounded by the Indonesian island of Sumatra, the Malay Peninsula and Singapore which form a natural bottleneck at the 1.5nm wide Phillips Channel. This area is close to the equator and consists of warm shallow seas, mountainous terrain and hundreds of islands of varied size.

The climate is dominated by two monsoon seasons, the southwest (SW) monsoon (June–September) and the northeast (NE) monsoon (December–March), with the inter-monsoon seasons (October–November and April–May) delivering a more varied wind pattern. The main variable of the local climate is not temperature or air pressure, but rainfall.  This region of South Asia has a hot tropical climate where the relative humidity is quite high, and ranges from 70-90%. 

Despite lying in the tropics tropical storms are extremely rare over the SOMS.  The last one occurred in 2001 when Typhoon Vamei became a Pacific tropical cyclone that formed closer to the equator than any other tropical cyclone on record.  Vamei was the last storm of the 2001 Pacific typhoon season, and developed on 26th December at 1.4N just off Borneo in the South China Sea (SCS).  This broke the previous record of Typhoon Sarah in 1956 which reached tropical storm strength at 2.2N. 

One of the key ingredients for a tropical cyclone to form is sufficient Coriolis effect which does not occur near the equator, so until the formation of Vamei happened this was previously considered not possible this close to the equator. There needed to be some other source for the spin for Vamei to develop. This spin was generated by strong NE winds funnelling in to the narrowing SCS and the Malayan Peninsula channelling the flow in to an anticlockwise pattern of winds together with a commensurate local cluster of showers and thunderstorms.  There is a natural vortex that appears every winter along the NW coast of Borneo which is maintained by the interaction between monsoonal winds and the local topography.  The vortex generally remains near the coast and rarely moves over the equatorial waters. The conditions which resulted in the formation of Vamei – a slow moving vortex enhanced by a monsoon surge for more than five days – are believed to occur only once every 100–400 years; so tropical storms are a very rare event in this region.

The most common destructive weather event throughout the seasons in the SOMS are Sumatra Squalls or ‘Sumatras’ which can bring wind gusts to 60 knots, extremely heavy rainfall and can last up to 20 hours or so. The highest wind speed recorded was 78 knots (equivalent to category 1 hurricane force) on 25th April 1984.  Sumatra Squalls are organised lines or bands of thunderstorms that have a lifespan that is longer than a single cell thunderstorm. They form the dominant mesoscale convective systems during the intermonsoon and SW monsoon seasons, which move east from Sumatra and typically occur predawn to morning time over the SOMS. Sumatras are usually hundreds of kilometres in length and should be considered an extreme weather event for shipping when in confined and narrow seas.

Their formation is due to the location of the Indonesian island of Sumatra to the west and the Malaysian Peninsular to the east. During the SW Monsoon, southwesterly winds blowing over the mountain range of Sumatra creates mountain waves on the leeward side, resulting in an unstable atmosphere. Relatively warmer waters of the Malacca Straits provide a source of moisture for convective development with thunderclouds (cumulonimbus clouds) forming late in the night which grow in-situ and merge into squall lines. These generally move northeastwards steered by the SW or westerly winds towards the west coast areas of the Malaysia Peninsular and the SOMS before dissipating over the SCS. They can be also perturbed and influenced by large scale synoptic systems such as tropical cyclones, Kelvin waves, Madden‐Julian Oscillation (MJO), etc. 

Sumatra Squalls can develop at any time of the year, but are uncommon during the NE Monsoon. There is a strong seasonal variability of formation with the most frequent months for squalls occurring in the intermonsoon period (October–November and April–May) with typically 6-7 squalls per month. The SW monsoon season also experiences 5-6 monthly which  reduces to 2-3 per month in the NE Monsoon season.

Tropical weather can be more difficult to forecast compared to mid-latitude weather. The tropics have a relatively homogenous air mass and fairly uniform distribution of surface temperature and air pressure compared to the mid-latitudes. Therefore local and mesoscale effects are more dominant that synoptic (large scale) influences, except for tropical cyclones.

Strong convection and moist air in the tropics give rise to frequent thunderstorms whose lifespan and small size presents a forecasting challenge in terms of accuracy and lead time. Furthermore, current understanding of these squall lines with their convective diurnal cycle is limited. Current NWP models have low skills in predicting tropical convection, and the complex dynamical processes that influence weather and climate in the tropics is still poorly understood.  More research is needed to better understand these systems, together with using a local high resolution fast update model. 

Meantime, for those at sea access to satellite imagery and (on board) radar observations form the best way to detect and track these systems when in season. Under the GMDSS scheme SOMS lies within METAREA XI with cover provided by China (Hong Kong) and Japan.  Associated marine forecasts mostly refer to generic local thunderstorms/severe weather but do not provide/predict detail on individual Sumatra Squalls. 

Local expertise can help, for example, the Singapore Meteorological Service (SMS) provides an example satellite and radar animation of a Sumatra Squall affecting Singapore in the predawn hours, as well as the latest high resolution satellite imagery.  SMS also produce a 24hr and 4 day outlook that includes a local wind forecast. They also produce a fortnightly outlook which currently (for the period 1-15 Sept 2020) states “In addition, widespread thundery showers with gusty winds due to the passage of Sumatra squalls are expected on one or two mornings.”

Stay connected and safe.

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