Abstract
Understanding the detailed nature of water masses and their boundaries is problematic due to the methods available for observing the physical properties of the ocean. This is of particular concern in regions of the world’s oceans where water masses meet, mix and exchange nutrients. An example of such a dynamic region lies southeast of the South Island of New Zealand where Sub-Tropical Water meets Sub-Antarctic Water at the Sub-Tropical Front (referred to here as the Southland Front)
The reprocessing of three multichannel seismic lines acquired perpendicular to the front, has allowed detailed imaging of thermohaline structures by using seismic waves to image the water column. This technique, Seismic oceanography, has become increasingly popular since 2003 and this thesis presents its first application in New Zealand waters.
Clear images of the reflective patterns have led to the interpretation of the water-mass boundary between Sub-Tropical and Sub-Antarctic water masses, the Southland Front, and also as the transition into the underlying Antarctic Intermediate Water.
The seismic lines are from two petroleum industry data sets, one from DUN-06 collected in March of 2006 and two from OMV-08 collected during the summer of 2007-2008. The lines are closely spaced and parallel, thereby enabling a temporal comparison. The Southland Front is shown to lie further inland in the summer of 2007/8 than in the autumn of 2006 by around 20 km.
The seismic lines feature several mesoscale and submesoscale lenses which have been interpreted as spin-off eddies from the northward-flowing Southland Current. These eddies appear below the sea surface at depths of around 200 m and are not seen in contemporaneous sea surface temperature satellite data. The imaged eddies have been compared to all those which have been previously published using the newly developing technique of Seismic oceanography and are found to be, on average, at shallower depths with smaller dimensions.