The ocean’s surface rises and falls in a rhythmic pattern known as the tide, driven primarily by the gravitational pull of the Moon and the Sun. These changes do not occur simultaneously across the globe, but progress as immense waves traveling through the ocean basins. To accurately predict and visualize this time-dependent movement, oceanographers and navigators rely on specialized charts. These maps translate the traveling tidal crest into a visual format, allowing users to understand when high tide will arrive.
Defining Cotidal Lines
Cotidal lines are isolines used on tidal charts to represent geographic locations where the phase of the tide occurs simultaneously. A cotidal line connects all points on a map where high tide is reached at the exact same moment. These lines illustrate the progression of the tidal crest as it moves across an ocean or sea. They are fundamental to understanding the timing of the tide, depicting equal time instead of equal elevation, similar to contour lines on a topographic map.
The Amphidromic System
The intricate, radiating pattern of cotidal lines results from the Earth’s rotation and continental boundaries, which create an amphidromic system. In this system, the tidal wave rotates around a central point rather than simply moving back and forth. This rotation is caused by the Coriolis effect, which deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
The center of this rotating system is called the Amphidromic Point, where the cotidal lines converge. At this location, the tidal range—the difference between high and low tide—is virtually zero. Cotidal lines radiate outward from this center, typically marked at one-hour intervals, showing the time progression of the tidal wave as it revolves.
The tidal wave completes one rotation around the amphidromic point approximately every 12 hours, corresponding to the period of the principal semi-diurnal lunar tide. The tidal range progressively increases with distance from the central point, reaching its maximum amplitude along continental coasts.
Reading and Interpreting Cotidal Maps
Cotidal maps are labeled to determine the exact time of high tide at any location. The lines are commonly numbered in hourly increments, often representing the Mean High Water Interval (MHWI). On global charts, these times are referenced to a standard time meridian, such as Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC). The numbers indicate the hours passed since the high tide crest crossed that reference meridian.
The direction of the tidal wave’s propagation is determined by observing the sequence of the numbered lines. The tidal crest moves perpendicular to the lines, advancing from a lower-numbered line to the next higher-numbered line.
The spacing between the cotidal lines provides information about the speed of the tidal wave. Lines spaced far apart indicate the wave is propagating rapidly across the deep ocean basin. Conversely, tightly clustered lines show the wave’s progress is slowed, usually due to shallow water or constrictions near coastlines. While cotidal lines show the timing of the tide, they are often paired with corange lines, which show equal tidal range or height, to provide a complete picture.