FY 2025

Elevated mixing estimates and trapped near-inertial waves on the inshore flank of the New Guinea coastal undercurrent from sustained glider observations in the Solomon Sea

Johnston, T.M.S., D.L. Rudnick, and W.S. Kessler

J. Geophys. Res., 129(10), e2024JC021626, doi: 10.1029/2024JC021626, View open access article at AGU/Wiley (external link) (2024)

The Solomon Sea is a major contributor to (a) the volume transport into the Equatorial Undercurrent and (b) the associated heat transport, which has an order one effect on interannual temperature variability on the equator according to previous work. The narrow western boundary current (New Guinea Coastal Undercurrent, NGCU) merges with the broad, shallow North Vanuatu Jet within 100 km of the southern entry to the sea, which implies mixing. Existing estimates from observations suggest mixing is larger than in models with different mixing parameterizations, which produce disparate properties of these exiting waters. Here, we use sustained underwater glider measurements across the Solomon Sea from 2007 to 2020 to examine the spatial variability of mixing estimates and internal waves. We estimate diffusivity via a finescale parameterization using the vertical strain of isopycnal displacements from internal waves. A typical accuracy of this parameterization when compared to turbulence measurements is within a factor of 2–3. Our results and previous observations in this area agree within this factor. Our main results are: (a) vertical diffusivity estimates are about 10^–4 m² s^–1 on the inshore, anticyclonic side of the NGCU, which are 10–100 times higher than offshore and (b) elevated near-inertial internal wave (NIW) amplitudes are also found inshore. Taken together, these results suggest trapping of NIW by the anticyclonic vorticity of the NGCU leads to the elevated mixing within 100 km of the entry to the Solomon Sea.

Plain Language Summary. Two currents enter the Solomon Sea and merge within about 100 km, which implies mixing. The salinity change that occurs in this area indicates elevated mixing. This mixing contributes to the properties of waters exiting the sea, a major source of the Equatorial Undercurrent, which contributes to El Niño and other longer period variability on the equator. From 2007 to 2020 underwater gliders crossed the broad, shallow North Vanuatu Jet in the middle of the basin and the narrow boundary current on the western side of the basin (the New Guinea Coastal Undercurrent, NGCU). Internal gravity wave variability is measured by the vertical displacement of density surfaces and mixing is estimated based on measured vertical stretching and compressing between these density surfaces. This method is not actually measuring turbulence, but estimating it based on the energy in the measured internal waves. Our main result concerns the spatial pattern of the estimated mixing. Stronger internal gravity waves with frequencies near the inertial frequency (near-inertial internal waves or NIW) are noted on the inshore side of the NGCU. Mixing is also elevated in this area, which suggests NIW are trapped there and provide an energy source for the mixing.