Seasonal and Interannual Variability of the Subtropical Front in the New Zealand Region

Abstract

The meridional variability of the Subtropical Front (STF) and the drivers of variability on interannual time scales in the New Zealand region are analyzed using a multi-decadal eddy-resolving ocean hindcast model, in comparison with Argo data. The STF marks the water mass boundary between subtropical waters and subantarctic waters, and is defined as the southern-most location of the 11°C isotherm and 34.8 psu isohaline between 100 and 500 m. The STF shifts up to 650 km (6°) meridionally on seasonal timescales. In addition to seasonal variability, shifts of around 200 km (2°) occur on interannual time scales. These shifts are connected to regional wind stress curl anomalies in the eastern Tasman Sea and east of New Zealand, which trigger Ekman convergence/divergence and Rossby waves and result in meridional transport of heat and salt into/out of the Tasman Sea. The net transports across the northern boundary of the Tasman Sea show the largest sensitivity to these wind stress curl anomalies. During periods of positive wind stress curl anomalies and Ekman convergence, the heat and salt content increases shifting the position of the STF southward. The opposite tendency occurs during periods of negative wind stress curl anomalies. The migration of the STF does not appear to be directly linked to regional climate oscillations.