Listening to Tidal Dynamics

Figure 1: In this comparative chart, the tidal heights for Narvaez Bay in 2012 are shown in blue against a vertical (metric) scale. The black line represents the phases of the moon from new (low) to full (high), and the red line, the sun, its zenith rising and falling over the year. One can follow the entire musical piece here in one page, noting tidal transitions, spring and neap tides, and seasonal variations.

by Dr. Richard Dewey

Dr. Dewey is a physical oceanographer at the School of Earth and Ocean Sciences, University of Victoria, and the associate director of science for Ocean Networks Canada.

The rhythmic ebb and flow, rise and fall, and wax and wane of the tides is beautifully captured in the choral piece Narvaez Bay: Tidal Predictions 2012, a conceptual work by Mark Timmings, co-composed with Stephen Morris. Narvaez Bay is located on the south shore of Saturna Island, among the Gulf Islands of the Salish Sea. The Salish Sea experiences complex tides, as the ocean waters enter and exit through Juan de Fuca Strait from the Pacific Ocean. The tides are caused by the gravitational pull of the sun and moon on the ocean waters, as they sway back and forth in the oceanic basins of the rotating earth.

Listening to Tidal Predictions or reading the work’s musical notation, we can follow the transitions of the tides (alto chorus) as the moon (soprano solo) cycles from its new to full phase* and back again, once every 29.5 days (a “lunar month” or “moonth”). The notation provides a measure for each day. The sun (bass drone) proceeds to climb in the sky from the depth of our northern-hemisphere winter, peaking through the summer solstice, then returns to its faint glow by late December. The music is captivating and soulful, almost ethereal. But it is, in fact, most natural, as are the tides themselves.

An oceanographer, as I listen to the music I cannot help but identify the physics and dynamics behind the tides: the interactions of the moon and the sun, the spring and neap tide cycles,** and the phase transitions of the moon as it first lags and then leads the sun in pulling the ocean to and fro.

As you listen, hear the moon solo start its first climb up the musical scale to a bright crescendo at the highest note, when the moon is full. It then descends, slowly, ultimately hiding in its own shadow as a new moon, when it becomes inaudible. This cycle repeats twelve more times during the year. At new and full moon phases, the sun and moon are aligned and working together. Here the tidal range is often at its greatest, with the highest highs, but perhaps most notably, with the deepest lows. We hear the chorus become agitated. These larger tides at new and full moon are referred to as spring tides. In Narvaez Bay, spring tides typically have two highs and only one, very deep, low tide.

During the lunar rise from new to full moon, we follow the moon through a waxing crescent to a first-quarter phase. At this time, the moon and sun appear at 90 degrees to one another (in “quadrature”), and their tidal forces are working at cross-purposes. The moon and sun are also at quadrature at the third quarter, during the waning phase from full to new moon. These periods are referred to as neap tides. During a neap tide, the compression of the tidal range is audible in the chorus, and we often find that there are two almost equal high tides and two low tides (a “semi-diurnal sequence”).

As the moon cycles up and down between new and full phases every lunar month, the sun climbs steadily. Tidal Predictions begins just following winter solstice, with the sun represented by a single-note drone, when it is low in the sky and shining down in the southern hemisphere. Then, the Narvaez Bay tides feel the sun more at midnight than at midday. The drone amplifies to two notes at the spring equinox. On June 20 at the summer solstice, the drone augments again to its strongest resonance of three notes. The sun has progressively migrated northward and now at noon stands high over Narvaez Bay. In the Salish Sea, we experience the lowest tides of the year near the solstices, with a midnight low in December, and a midday low in June. These are the moments when the chorus is audibly its most agitated.

Additional, subtle tidal dynamic characteristics are pervasive and can be discerned as the musical piece wanes into the fall and the drone drops to two notes. As we progress out of one spring tide into the next neap tide, the daily deep low tide always precedes the daily high tide. As we progress from each neap into the next spring tide, the daily high tide precedes the daily low tide. At each spring and neap tide in the music, there is a phase transition that seems to stall the daily rhythm of the tide until it is established again as a more rhythmic high-low sequence. The winter gradually returns and the sun’s drone diminishes to one note at the solstice. In the final December 2012 movement, the moon cycles through a last low, new moon phase, coinciding perfectly with the deepest spring tides of the year. The piece ends with the crescendo of a full moon, waning into a beautiful harmonic chord.

* The “full moon” is the phase in which the moon’s whole disk is illuminated. The “new moon” is the phase when the moon is in conjunction with the sun and invisible from earth.

**“Spring tides” are exceptionally high and low tides that occur at the time of the new moon or the full moon when the sun, moon, and earth are approximately aligned. “Neap tides” occur just after the first or third quarters of the moon, when there is the least difference between high and low water.