Islands in the Stream

story by Helen Hill for MITGCM

 

A group from National Taiwan University has been using MITgcm to study submesoscale island wakes in the Kuroshio Current off Taiwan.

Like a sailboat on a calm sea, a canoe on a lake, or indeed a vessel on any body of water, islands too can produce wakes.

To study submesoscale island wake formation, researchers Ming-Huei Chang and Chih-Lun Liu from the Institute of Oceanography, National Taiwan University, have been using MITgcm to carry out three-dimensional numerical simulations of the circulation around a neighboring island to enable them, given sparse observational data, to carry out a detailed fluid dynamical analysis of the system.

The Kuroshio, a strong western boundary current in the northwestern Pacific Ocean, flows northward along Taiwan’s east coast with a width of 100–150 km and a maximum speed of 1–1.5 m s–1. Around 50km offshore to the southeast lies Green Island, a ~7 km diameter conical feature, the first in a series of features that form a submarine ridge which extends north to Taiwan. With average current speeds at a depth of ~ 200 m reaching > 0.5 ms-1, the waters in the lee of Green Island provide a natural laboratory for studying submesoscale dynamics of island wakes.

Using a hydrostatic implementation of MITgcm, Liu and Chang set up a domain 80km square by just under 300m deep, prescribing (constant) inflow on the southern and western walls, with outflow at the northern and eastern walls, and a representative topography based on Ocean Data Bank data for the region (See the paper for full details).

Liu and Chang’s experiments successfully reproduce an island wake formed of sequentially detached recirculations, each containing a region of upwelling cold water, which propagate downstream via advection to form an along stream wake reminiscent of a von Kármán vortex street (VKVS).

Analysis of the circulation shows the shedding frequency (which is a function of the horizontal eddy viscosity) exhibits a trend analogous with classical wakes, that wake behavior exhibits the appropriate dependence on Reynolds number, and that the wake aspect ratio is similar to that predicted for classical VKVS.

Unlike classical VKVS, however, Liu and Chang report finding their wake features being adapted by inertial and barotropic instabilities. In particular they found that the inertial instability had a large growth rate that tended to slightly destabilize anticyclonic recirculation, leading them to propose that barotropic instability may act as a secondary process generating eddy kinetic energy downstream.

To find out more about this work contact Ming-Huei

Story image: https://commons.wikimedia.org/wiki/File:Vortex-street-1-90deg.jpg

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Do you have news about research using MITgcm? We are looking for contributions to these pages. If you have an interesting MITgcm project (ocean, atmosphere, sea-ice, physics, biology or otherwise) that you want to tell people about, get in touch. To make a post, contact Helen