Near-Inertial and Diurnal Band Temporal Modulation

To investigate the temporal variability in the different frequency bands, we arbitrarily define the near-inertial band from $0.7$ to $0.9 cpd$ , the diurnal band from $0.9$ to $1.1 cpd$ , and the semidiurnal band from $1.9$ to $2.1 cpd$ . Time-depth plots of the variance in the three frequency bands are computed for temperature using 8 day subrecords. The variance time series show episodic events in the diurnal and the near-inertial frequency bands (e.g. days 340 or 375, figure 4.1c and d). The same events are also visible in the variance plots from the current record (Figure 4.2c and d). The semidiurnal band exhibits a fortnightly modulation, as expected for variability associated with the astronomical tide.

We use the results from the harmonic analysis and from the TPXO barotropic model of Egbert (1997) to compare the temporal evolution of the different frequency bands with the beating of tidal constituents (Figure 4.2). The TPXO model contains tidal frequencies $M_2$ , $S_2$ , $N_2$ , $K_2$ , $K_1$ , $O_1$ , $P_1$ , $Q_1$ , $MF$ and $MM$ . We find that the measured semidiurnal band variability is well explained by the tidal constituents. The variability in the diurnal band, however, is not well explained by the beating of the diurnal tidal constituents alone (Figure 4.2c). We interpret the event-like character of the diurnal variability and the weak correspondence with "predicted" tidal behavior as an indication that the diurnal band defined here comprises high-frequency NIWs. Assuming the NIWs are generated at the local inertial frequency at their generation site, we expect the diurnal band waves to have originated north of the islands between $26.5 ^\circ N$ and $33.5^\circ N$ where $0.9 cpd \le f \le 1.1 cpd$ . This can explain the intermittent nature of the diurnal band, and the overall higher levels of diurnal variability during the winter months (Figure 4.2c).

As expected, the near-inertial band is completely unrelated to the tidal constituents (Figure 4.2d). Observed time-series of temperature (Figure 4.3 a and b) illustrates the depth-dependence of the near-inertial signal. At the top of the DN mooring, between $100$ and $200 mab$ , (Figure 4.3a), the temperature oscillations are dominated by the semidiurnal tide, while near the bottom (Figure 4.3b), the temperature oscillates predominantly at periods between the diurnal and the inertial period of 32 hours. During this time period the vertical temperature gradient, calculated between pairs of sensors, shows two minima separated by 26-28 hours (Figure 4.3c). During these stratification minima, large overturns are observed, leading to high estimated dissipation (see chapter 6 for the details of the overturns analysis). The velocity observed near the bottom, at the depths of the temperature sensors shown in Figure 4.3b, also undergo oscillations at diurnal to inertial periods. The direction of the velocity field and the temperature record are consistent in time with the advection of water along the slope.