Interference pattern for low-mode internal wave propagation

Most of the energy put into the internal wave field, by either the wind generating inertial oscillations in the mixed layer or the tides flowing over topography, radiates away in modes 1 and 2. However, direct observation of these propagating waves is complicated by the presence of interference patterns. Multiple sources of internal tides interfere constructively and destructively to produce interference patterns in sea surface height and energy flux. To study this effect around the Hawaiian Ridge and interpret the HOME farfield measurement, we used the output of a high-resolution primitive equation model. It makes apparent that different modes are generated with different amplitudes along complex topography, and that these different waves interfere with each other. Nonetheless, a simple wave propagation model with four line sources on the Hawaiian Ridge reproduces well the sea surface height and energy flux fields from the numerical model for modes 1-2.

Baroclinic SSH due to the mode-1 M2 internal tide from tahe numerical model. The locations of R/P FLIP (stars), acoustic moorings (circles), and the 6 acoustic paths (lines) are plotted. Wave crests (thick black lines) and troughs (thin black lines) from a simplistic model with 4 sources (Kauai and Kaulakahi Channels, and near the Nihoa and Hawaii islands) are also plotted.

Interaction of internal waves with mesoscale eddies

I am interested in understanding the mechanisms affecting the propagation of the low-mode internal tide in the ocean are discussed. In addition to latitude, stratification, and depth, the mesoscale currents (derived from satellite altimetry) are a dominant factor affecting the paths, travel speeds, and the phases of all modes. Using ray models, I have found that the mesoscale field makes modes 3 and higher completely incoherent within a few tens of km from their source. Using ray-tracing models, we can therefore interpret altimetric measurements of the sea-surface manifestation of the internal tide. For example, at the Hawaiian Ridge, the M2 internal tide does not seem to lose much energy as it propagates southward, but its phase rapidly becomes random. The northward propagating internal tide should remain phase-locked but appears to rapidly lose energy.

Example of ray tracing of a mode 1 through a mesoscale eddy field (gray lines are contours of mesoscale SSH, separated by 0.05 m). Thick black lines represent wave crests (constant phases of baroclinic SSH), which are deformed relative to the case without currents (thick blue line). Convergence and divergence (shown by thin black lines) modify the amplitude of the wave (colorscale).

Interaction of internal waves with large-scale circulation

Intense and spatially coherent shear layers were detected in and under the Kuroshio in an April 2000 (ASIAEX) survey in the East China Sea. The sloping layers, revealed by shipboard Doppler sonars on the R/V Roger Revelle, appeared to cross isopycnal surfaces. Except in a small region near the Kuroshio shelf-break front, the rms finescale shear associated with the layers significantly exceeded the geostrophic shear. An April 2002 follow-on cruise was organized to establish whether these motions were propagating internal waves. Both shipboard and lowered ADCPs were operated from the R/V Melville. In addition to CTD-sonar transects, a 30-h time series of currents and shear was obtained in the core of the Kuroshio near the island of Kyushu, Japan.

The shear structures were indeed found to be propagating, with both up and down-going internal wave motions present. Compared to the nearby open-ocean, the finescale (< 160 m) vertical shear variance is increased by a factor of 3 in the Kuroshio, and by a factor of 6 in the region between the shelf break and the Kuroshio - suggesting a potentially very active mixing region. We conjecture that the geostrophic vorticity associated with the Kuroshio acts as a barrier, impeding the seaward propagation of internal waves generated at the shelf-break onshore of the Kuroshio front.

Velocity and shear across the Kuroshio in the East China Sea (ASIAEX 2000)

Episodic forcing, internal waves, and water mass evolution

Mooring and float observations collected during the Kuroshio Extension System Study (KESS) are used to study the variability of the Subtropical Mode Water (STMW) on timescales from days to seasons. Across the entire western Pacific, the depth of bottom of the STMW is tightly related to the SSH signature of mesoscale eddies. The top of the STMW as well as the mixed layer depth seem to be mostly affected by episodic events and increase in discrete steps. The cooling associated with cold air outbreaks plays an important role. The mechanical mixing due to the generation of inertial motions by the wind stress propagating as internal waves is found to be a large and episodic.

Top: Hourly (gray) and 3-day smoothed (black) wind speed time series measured at KEO, a surface mooring close to the southernmost KESS mooring. Bottom: Depth-time map of the magnitude of inertial shear. The STMW (low potential vorticity) is contoured in black and mixed layer depth shown in gray. Large wind events generating near-inertial waves and eroding the top of the STMW are shown by the black arrows.