Measurements of an Oceanic Front Using a Front-Following Algorithm for AVHRR SST Imagery

Abstract

The Subtropical Front (STF) is a global front which extends around the Southern Ocean and is the boundary where the Subantarctic Surface Water mass (SAW) converges with the Subtropical Surface Water mass (STW). The Southland Front (SF) is part of the STF, which lies off the east coast of the South Island, New Zealand. The SF is narrow, approximately 8 km, with a temperature difference of approximately 1.8°C which can be detected using remote sensing Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data. The work presented here is an application of remote sensing for the first detailed study of the surface spatial and temporal variability of the SF. The variability of the SF was quantified using an algorithm developed to follow the Front using AVHRR SST imagery. The algorithm used a new approach to determine the position, mean SST, SST difference, width, and gradient across the Front. Three time scales of variability were examined: long-term (3 years), annual, and seasonal. The algorithm efficiently followed the SF and consistently showed the 3-year mean position was stable and constrained by the bathymetry of the continental slope. Seasonally, the front moved inshore during summer. The temperature gradient across the front was strongest and the front narrowest in winter. The decrease in SST gradient during the 3-year data set coincided with the decrease in Southern Oscillation Index (SOI).