Satellite Data Challenge Textbook Tide Model

Satellite observations covering more than 360,000 ocean locations have challenged the long-standing textbook explanation that the Moon’s gravitational pull creates two broad, opposing water bulges responsible for the rise and fall of ocean tides. Researchers led by Yongfeng Yang of the Water Resources Comprehensive Development Center of Shandong Province, Jiajia Yuan of Anhui University of Science and Technology and Mingyuan Fan of the Water Resources Research Institute of Shandong Province examined tidal observations from the Jason-3 satellite to test whether the predicted pattern of raised and depressed ocean water is actually evident on Earth’s surface.

Their findings, based on observations collected throughout 2021, indicate that the distribution of high and low tides does not correspond to the two-bulge model. The study analyzed tidal data from 362,370 ocean locations recorded by Jason-3 through the AVISO satellite-altimetry system. The researchers compared high- and low-tide occurrences with the lunar angle—the angle between a location on Earth and the Moon, measured relative to Earth’s centre. Of the 175,402 locations falling within lunar angles of 0°–60° and 120°–180°, the areas where the traditional model would expect water bulges, 56.84% recorded low tides, while only 43.16% recorded high tides.

The researchers found an opposing pattern in the 60°–120° range, which corresponds to the region that the conventional model would regard as a depressed area between the two bulges. Among 186,968 locations in this range, 56.38% experienced high tides. The researchers said the results challenge the idea that two physical water bulges exist on opposite sides of Earth. To determine whether the satellite observations reflected a broader phenomenon, the researchers also examined data from 166 tide-gauge stations for August 2014. Those observations showed a similar distribution, with low tides occurring more frequently at lunar angles of 0°–60° and 120°–180°, while high tides were more common between 60° and 120°.

The findings add observational evidence to long-standing scientific concerns about the simplified two-bulge explanation of tides. Although the model has traditionally been used to illustrate how the Moon’s gravitational influence produces two daily high tides, Earth’s oceans do not behave as a single uninterrupted body of water. Continents divide the oceans into separate basins, while the shape and depth of those basins, the Earth’s rotation and associated Coriolis effects, and friction between seawater and the ocean floor all influence tidal behaviour. As a result, actual tides vary considerably from one coastline and ocean basin to another.

The researchers further considered an alternative explanation in which tidal movements are closely linked to deformation of the solid Earth and the resulting response of ocean basins. Under this interpretation, the Moon’s gravitational force deforms the Earth, while the rotation of the deformed planet alters the depths and shapes experienced by ocean water. Changes in basin geometry can then drive the movement of seawater that produces recurring high and low tides. The researchers argue that this mechanism is compatible with their observations because areas where the solid Earth rises can become shallower and tend to correspond with lower tides, while areas experiencing greater compression can become deeper and favour higher tides.

The study does not mean that the Moon’s gravity is unimportant to tides. Rather, it challenges the use of two enormous, symmetrical bodies of ocean water as a literal physical description of what happens across Earth’s surface. The findings therefore highlight the distinction between a simplified educational model and the far more complicated behaviour of real oceans, where gravitational forces interact with Earth’s deformation, ocean-basin geometry, rotation and friction. The researchers’ analysis suggests that understanding tides requires looking beyond the familiar picture of two water bulges moving around the planet and considering the coupled behaviour of the solid Earth and its oceans.

By: Joyce Owusu