(Nearly) periodic oscillations are frequently attributed to tides, although diurnal sea breezes can also induce periodic upwelling 10– 13.Īt mid-latitudes, diurnal tides are sub-inertial, and strong currents have been frequently reported, e.g. Regarding the forcing of these waves, tides and wind are typically cited as factors.
CTWs may be bottom-intensified 9, which enhances their near-bottom impact. On the other hand, under sufficiently strong stratification, topography may be neglected and waves become very similar to Kelvin waves (KWs), where the sloping bottom has a purely geometric effect. When stratification is negligible, oscillations can be attributed to barotropic continental shelf waves (CSWs), for which topography provides the restoring mechanism. At sub-inertial frequencies, coastal-trapped waves (CTWs) 8 are often cited. Several explanations have been proposed for these observations, depending on the frequency of the oscillations, either above or below the local Coriolis frequency. To date, the largest documented oscillations include a recent report of near-tidal-front fluctuations of up to 7 ☌ with a semi-diurnal frequency at depths of around 60 m over Georges Bank (North-west Atlantic Ocean) 6 and diurnal and semi-diurnal frequency fluctuations of 6 ☌ at 15 m depth along the Californian coast 7. In the nearshore region of broad continental shelves, seabed temperature fluctuations are generally weaker. Strong seabed temperature fluctuations with ranges of 6–8 ☌ have been reported around reefs or submarine cliffs with variable frequencies: mainly semi-diurnal frequencies near the French Polynesian islands 1, mixed diurnal and semi-diurnal frequencies in the South China Sea 2 and in the Adriatic Sea 3, 4 or at subtidal frequencies in the Andaman Sea 5. They generally occur in stratified environments and have a broad, high-frequency spectrum typically ranging from several hours to a few days. Large seabed temperature oscillations have been observed in shallow waters at many locations around the world. In addition, bottom friction acts to rotate the axes of the diurnal tidal current ellipses with depth, and this combination of effects explains the large range of observed bottom temperature oscillations. Simplified calculations show that cross-shore motions are bottom-amplified. Our data demonstrate that these coastal-trapped waves propagate clockwise around the archipelago in roughly two days, and thus approximate an azimuthal, mode 2 pattern. We argue that the archipelago is nearly resonant for island-trapped waves at near-diurnal frequencies. They appear to be the manifestation of an internal wave, triggered by the diurnal surface tide and trapped by the bathymetric configuration of the area. In addition to their remarkable range, they exhibited a near-diurnal period centred on the O1 tidal component (~26 h), contrasting with the dominant semi-diurnal sea-level periodicity in the area. The oscillations coincided with the seasonal stratification period. These oscillations were detected on velocity and temperature profiles from moorings in Miquelon Bay and on an array of near-bottom temperature sensors around the archipelago.
Here, we report large, near-daily oscillations of near-bottom temperatures, with ranges of up to 11.5 ☌ at depths of 30–60 m in September 2011 around the Saint Pierre and Miquelon archipelago (south-eastern Canada).