The Southland Front, New Zealand: Variability and ENSO correlations

A detailed analysis of the Southland Front, a shelf-break system off the southeast coast of South Island, New Zealand is presented. The position, temperature, temperature range and width of the front are determined using a new statistical front detection algorithm and 21 years worth of Advanced Very High Resolution Radiometer satellite sea surface temperature (SST) data. Overall, the front is strongest (highest SST gradients) in the summer and winter, and the across front gradient decreases northward in all seasons, consistent with an equatorward decrease in stability and divergence of isobaths. The surface expression of the front moves further offshore during the winter months and is found closest inshore in the summer. Seasonality of the front is strongly controlled by the annual cycle of subtropical and subantarctic water mass temperatures. Both the temperature and strength of the front are interannually variable, and correlated with the El Niٌo-Southern Oscillation (ENSO); they both decrease during El Niٌo, and increase during La Niٌa events. ENSO indices lead changes in the fronts temperature by up to 6 months. Conversely, the gradient may change up to 6 months in advance of peak ENSO indices. The strength and sign of correlations is seasonally dependent.