Why Labuan Bajo’s Tidal Currents Can Reverse Twice Daily

Labuan Bajo — Studies of tidal currents in the Sape Strait, the main channel east of Labuan Bajo, indicate that surface currents commonly flow northward on the rising tide and southward on the falling tide, driven by differences in sea level between the southern and northern parts of the strait.

Researchers from Bandung Institute of Technology (ITB) describe the tide in the Labuan Bajo–Sape Strait area as a mixed, predominantly semi-diurnal regime, meaning the sea level and currents generally go through two flood and two ebb phases in roughly 24 hours, with unequal heights and periods between successive tides.

How the reversal works

An ITB study on sea currents in the Sape Strait explains that the main factor controlling current direction and speed in this area is the tidal component. According to the study, when the tide is rising, sea level in the southern part of the strait becomes higher than in the north, creating a pressure gradient that drives currents towards the north. As the tide falls, sea level in the north is higher than in the south, reversing the pressure gradient and producing a southward flow.

This mechanism means current direction is closely tied to tidal phase, rather than to daily wind forecasts alone. Boat operators who navigate only by general weather conditions may therefore encounter an unanticipated change in current direction as water levels transition from rising to falling, or vice versa.

The same ITB work and a separate analysis using numerical modelling and high-frequency radar data conclude that tidal forcing, particularly the semi-diurnal components, is the dominant driver of currents in the Sape Strait. These findings support the view that current reversals in the Labuan Bajo area are fundamentally a tidal-current effect.

Tidal pattern and current strength

In its introduction, the ITB research notes that tides around Labuan Bajo are classed as “mixed tide prevailing semi-diurnal”, a category also used in Indonesian hydrographic studies for locations that experience two high and two low waters per day with differing heights and intervals. In practical terms, that pattern tends to produce two periods of northward (flood) currents and two periods of southward (ebb) currents in a day, though exact timing depends on local topography and basin shape.

ITB’s detailed analysis of Sape Strait currents, based on data from November 2018 to October 2019, reports that tidal currents can be strong. During spring tides, model scenarios show northward currents of around 1.05 metres per second during the flood phase, when sea level in the south is significantly higher than in the north.

In the same simulations, southward currents during the ebb phase are calculated to be stronger, reaching about 2.3 metres per second, with sea level in parts of the strait falling well below mean sea level.

The research also examines different seasons, grouping data into periods that broadly correspond to west and east monsoon conditions. It finds that current speeds vary seasonally, with certain periods showing stronger currents, which the authors relate to the combined influence of tides and seasonal winds.

Seasonal bulletins from Indonesia’s Meteorology, Climatology and Geophysics Agency (BMKG) similarly describe surface current speeds in the southern Sape Strait ranging from around 0.3 to more than 1 metre per second, with dominant directions that can include southward flow in particular months.

Observation systems and practical implications

The ITB introduction notes that BMKG, through its Maritime Meteorology Centre, installed a high-frequency (HF) radar system in the Sape Strait in 2018 to increase the density of ocean observation along Indonesia’s coasts. HF radar systems can monitor surface currents over broad coastal areas, providing datasets that can be compared with current profilers, tide models and numerical simulations.

BMKG’s marine weather service now issues forecasts for ports including Labuan Bajo and Sape, listing expected current directions, speeds and tidal heights by time of day. For vessel operators, the key operational implication of the documented reversal is that the direction of pull on an anchored boat, or the set on a hull under way, may change as the tide moves between flood and ebb phases.

Anchoring or route plans that appear appropriate in one phase should be checked against local forecasts and observations for the full tidal cycle.

For small boats, snorkelling and diving activities around Labuan Bajo and the Sape Strait, the tidal current cycle underlines the need to account for both direction and strength. A northward drift during the flood may turn southward on the ebb, and stronger seasonal currents can increase the distance a swimmer or small vessel is carried in constricted channels.

Operators are therefore advised, in line with BMKG’s forecast use, to consult up-to-date tidal and current information for specific sites rather than rely only on the general pattern described in regional studies.

Sources

• Bab I Pendahuluan – ITB Digital Library
• Jurnal Hidrografi Indonesia Volume 07 Nomor 02 – Pushidrosal
• Informasi Meteorologi Maritim – BMKG Maritime, Edisi V Mei 2022