By Jeannette Bedard
With each ebb and flow the shoreline changes. Rocks and sand are exposed only to be hidden again a few hours later. Tides are rhythmic and predictable–a look at a tide chart will tell you when a beach will be exposed. What makes tides predictable is how they are created, yet how the tide manifests at a specific location is shaped by the local topography.
The sun and moon’s gravities pull at the earth, and since the oceans are a fluid they are free to respond to this pull in a very observable way. Tides start out uniform with depth since the pull from the sun and moon act on the entire water column at once. The wave created is thousands of miles long–the true tidal wave. It doesn’t appear like a wave to an observer the way lapping wind-driven waves do, instead, tides raise and lower the sea level, exposing more or less of the beach. Since tides are a type of wave, they take time to propagate. This means that low tide (or high tide) occurs at different times at different locations. How the tides propagate is determined by the shape of shorelines and roughness of the ocean floor. As tides flow onto the shallower continental shelf, they slow down and their energy builds up in a smaller volume, amplifying the rise and fall of the water.
Anyone living near a sea shore can observe tides on a daily basis and there are tidal records extending back to antiquity. To build our modern understanding, Lord Kelvin led the first systematic effort to resolve the fundamental frequencies in 1867 and much of this work still holds today. There are semidiurnal (twice-a-day), diurnal (once-a-day) and longer period components measured in days, months, and up to just over 18 years. To add complexity, these constituents can interact with each other, amplifying or damping local tides, however, the semidiurnal lunar tides dominate at most locations.
There are two main considerations with tides: how high the water gets (tidal height) and how fast the water moves (tidal currents). Maximum tidal heights vary greatly depending on location. On Hawaii, tides are tiny, reaching only 12 inches (30 cm), while in the Bay of Fundy tides reach up to nearly 40 feet (12 m). The Bay of Fundy is often cited as having the highest tide in the world, however, it has a competitor: Ungava Bay in northern Quebec, and it is still not determined which has the higher tides.
CONSTRICTION OF FLOW
Strong tidal currents are typically the result of a constriction on the flow. Consider a garden hose: if you turn it on the water comes out at a certain rate, if you cover part of the opening with your finger, water will come out faster. The same amount of water is coming out, just the opening has changed. Even though the Islands of Hawaii have tiny tides, there are huge currents in the area. The islands are actually part of a mountain chain with a height comparable to the Himalayas. Tidal waters are pushed up the slopes and between the islands, and like the flow from a partly blocked hose, the currents are forced to speed up.
In addition to exposing beaches to relax on, tides play an important role in ocean mixing, which is a ongoing area of investigation for physical oceanographers. In a coastal environment, tides keep the water mixed, bringing up nutrient rich water from the depths and removing waste.
Munk, W. and C. Wunsch (1998), Abyssal recipes II: energetics of tidal and wind mixing, Deep-Sea Research I, 45, 1977-2010.
Pinet, P.R. (2006), Invitation to Oceanography fourth edition, 594pp, Jones and Bartlett Publishers, Sudbury, MA, USA
St. Laurent, L., and C. Garrett (2002), The Role of Internal Tides in Mixing the Deep Ocean, Journal of Physical Oceanography, 32, 2882-2899.