Velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles (Figure 1). However, the onset of springtime upwelling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 m to 40 m (Figure 2). The time-variable stratification must be accounted for, since it significantly affects the energy, energy flux, the vertical modal structures and the energy distribution among the modes. The internal tide from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals (Figure 3), suggesting more reflection from up canyon of the moorings during the first period. Turbulence, computed from Thorpe scales, is greatest in the bottom 50-150 m and shows a spring/neap cycle. Depth-integrated dissipation is three times greater toward the end of the record, reaching 60 mW/m2 during the last spring tide (Figure 4). Dissipation near a submarine ridge is strongly tidally modulated, reaching 10 W/kg (10-15 m overturns) during spring tide, and appears to be due to breaking lee waves. However, the phasing of the breaking is also aected by the changing stratication, occurring when isopycnals are deected downward during the first period and upward during the second period.
Figure 1. Time series of the moored semidiurnal energy and flux (a-c) and barotropic forcing (d-e): (a) HKE; (b) APE; (c) flux magnitude; (d) semidiurnal barotropic tidal height; and (e) depth-averaged currents at MP2 (toward 60 true). In (d, e), the times of the four spring tides SP1-SP4 are highlighted in gray. In (a-c), the vertical lines give the maximum value of each quantity at each spring tide. In (b), the vertical dashed lines indicate large uncertainties in the time of the maximum because of a poorly-dened peak.
Figure 2. Background wind, stratification and currents. (a) Wind vectors at NOAA buoy #46042 (see map Figure for location), showing upwelling-favorable northwestly wind beginning yearday 65, except for a 5-day relaxation yearday 96-99 following a storm. (b) Temperature, (c) salinity and (d) stratication measured at MP2 and smoothed by a 2.1-day sliding window. (e) Eastward and (f) northward velocity low-pass ltered by a fourth-order Butterworth lter with a 5-day cuto period.
Figure 3. Mode-1 standing/progressive wave calculations. (a) The HKE to APE ratio. The horizontal black line indicates the theoretical value of 2.2. (b) The ratio of flux magnitude to total energy, estimated from the moorings (colored lines). The gray band indicates the theoretical group speed, which is a function of time owing to the varying stratification. The vertical spread accounts for the different water depth at the moorings, on which cg also depends. (c) Greenwich phase of mode-1 M2 displacement.
Figure 4. Turbulent dissipation rate at MP2 and MP3. (a) Semidiurnal and diurnal barotropic tidal forcing. (b) Semidiurnal energy flux. (c) Depth-integrated dissipation rate at MP2 and MP3. (d) Turbulent dissipation rate measured at MP2, and smoothed using 1-day running window. (e) As in (d) but for MP3. In (d, e) the y axis is height above the bottom (HAB) in meters.
Spatial survey results