A hub of tidal stillness
An amphidromic point is a location in an ocean basin where the tidal range—the vertical difference between high and low tide—is zero. These points exist for each harmonic component of the tide, such as the principal lunar semi-diurnal tide, known as M2. The phenomenon occurs because of the interference of tidal waves within the basin, combined with the Coriolis effect from the Earth's rotation. This interaction creates a rotary wave system that circulates around the central amphidromic point.
Imagine the tide as a vast wave sloshing around an ocean basin. The Earth's rotation deflects the moving water, causing the wave to pivot around a single point. This pivot point, or node, experiences no vertical movement. From this center, lines of constant tidal phase, called co-tidal lines, radiate outwards, connecting all locations that experience high tide at the same time. The tidal wave crest rotates around the amphidromic point, typically taking about 12 hours to complete a cycle. In the Southern Hemisphere, this rotation is clockwise. While the water level at the amphidromic point itself remains constant, the tidal range increases with distance from it.
Lombok's oceanographic intersection
The Lombok Strait, a 20 km wide passage at its narrowest point between the islands of Bali and Lombok, is a site of intense water dynamics. The strait is a primary channel for the Indonesian Throughflow (ITF), a massive current that transports water from the Pacific to the Indian Ocean. The total volume of the ITF is estimated to be around 15 Sverdrups, which is 15 million cubic meters of water per second.
The amphidromic point in this region forms due to the unique bathymetry and the collision of different tidal systems. The strait is a choke-point where the semi-diurnal tides from the Indian Ocean interact with the mixed tides flowing from the Pacific. This interaction, influenced by the underwater topography like the 250-meter deep sill, creates the rotational tidal pattern with a still point at its center. The strong tidal currents, reaching speeds of 3.5 meters per second, generate significant internal waves with amplitudes up to 100 meters.
This strait is a major biogeographical boundary known as the Wallace Line, first identified by naturalist Alfred Russel Wallace. He noted the stark differences between the fauna of Asia to the west (Bali) and those of Australian origin to the east (Lombok). The deep, fast-flowing water of the strait created a barrier that few species could cross, even during ice ages when sea levels were lower and landmasses were more connected.