Publications

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2019
Zeiden, KL, Rudnick DL, MacKinnon JA.  2019.  Glider observations of a mesoscale oceanic island wake. Journal of Physical Oceanography. 49:2217-2235.   10.1175/jpo-d-18-0233.1   AbstractWebsite

In this study, a 2-yr time series of velocity profiles to 1000 m from meridional glider surveys is used to characterize the wake in the lee of a large island in the western tropical North Pacific Ocean, Palau. Surveys were completed along sections to the east and west of the island to capture both upstream and downstream conditions. Objectively mapped in time and space, mean sections of velocity show the incident westward North Equatorial Current accelerating around the island of Palau, increasing from 0.1 to 0.2 m s(-1) at the surface. Downstream of the island, elevated velocity variability and return flow in the lee are indicative of boundary layer separation. Isolating for periods of depth-average westward flow reveals a length scale in the wake that reflects local details of the topography. Eastward flow is shown to produce an asymmetric wake. Depth-average velocity time series indicate that energetic events (on time scales from weeks to months) are prevalent. These events are associated with mean vorticity values in the wake up to 0.3f near the surface and with instantaneous values that can exceed f (the local Coriolis frequency) during periods of sustained, anomalously strong westward flow. Thus, ageostrophic effects become important to first order.

2018
Fine, EC, MacKinnon JA, Alford MH, Mickett JB.  2018.  Microstructure observations of turbulent heat fluxes in a warm-core Canada Basin eddy. Journal of Physical Oceanography. 48:2397-2418.   10.1175/jpo-d-18-0028.1   AbstractWebsite

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6 degrees C, significantly warmer than the surrounding -1 degrees C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy epsilon was quite low . Three different processes were associated with elevated epsilon. Double-diffusive steps were found at the eddy's top edge and were associated with an upward heat flux of 5 W m(-2). At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m(-2) downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m(-2). Integrating these fluxes over an idealized approximation of the eddy's shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1-2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

Ramachandran, S, Tandon A, MacKinnon J, Lucas AJ, Pinkel R, Waterhouse AF, Nash J, Shroyer E, Mahadevan A, Weller RA, Farrar JT.  2018.  Submesoscale processes at shallow salinity fronts in the Bay of Bengal: Observations during the winter monsoon. Journal of Physical Oceanography. 48:479-509.   10.1175/jpo-d-16-0283.1   AbstractWebsite

Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often <= 10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1-10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1-10) km scales at the radiator survey.