Deep near-inertial waves in the Samoan Passage

Abstract

The Samoan Passage Abyssal Mixing Experiment (SPAM-Ex) is a unique data set comprised of a deep mooring array and a long-term mooring time series in the Samoan Passage ($10^\circ{}$S, $170^\circ{}$W), an abyssal constriction through which the majority of deep and abyssal waters in the North Pacific pass. As near-inertial waves (NIW) are episodic and propagating, they are difficult to observe. Deep observations of NIW are even more difficult and as a result there are a limited number of deep observations of NIW. This work hopes to provide a detailed description of such an observation. Depth-time plots of velocity and shear, in addition to rotary spectra at each depth, show a NI event between 4000 and 4200\,m on the mooring array. An idealized model is formed to describe the shear by using a plane wave method fitting the best fit. Plane wave solutions found independently for each mooring and show spatial coherence across the array. The idealized model found the wave to be comprised of both a downward propagating and an upward propagating wave, used for all subsequent research. Wavelet analysis shows multiple NIW over the course of the long-term, 17-month mooring deployment. These events correspond to an increase in the magnitude of the v velocity and a decrease in the magnitude of the u velocity. In addition, on average the 27.786 kg/m$^3$ isopycnal is depressed 14\,m on average during NI events. In calculating the various variables describe the observed NIW, the inclusion of the horizontal Coriolis term ($\tilde{f}$) is found to be important. This solution was expanded across latitudes and values of $N^2$ near the inertial limit. It was found that $\tilde{f}$ should be included in the dispersion relation at low latitudes ($<10^\circ$) and low stratification ($N<0.001\,s^{-1}$) and for groups speeds at all latitudes at low stratification ($N<0.003\,s^{-1}$). As a result, inclusion of $\tilde{f}$ specifically in the group speed calculation is particularly important during ray tracing. Increased mixing was found to correspond to the NIW but only at one of the 4 moorings in the array. Both meridional shear above the interface and below the interface are found to have a high correlation to the near-inertial variation in depth integrated dissipation rate. This corresponds to a decrease, but not reversal in the magnitude of the velocity. Low Richardson numbers (below 0.25) correspond to increased turbulence indicating the observed mixing is shear driven.