Variability, coherence and forcing mechanisms in the New Zealand ocean boundary currents

Abstract

Several fast, narrow western boundary currents ow along the eastern margin of New Zealand forming part of the western boundary current system of the South Pacific. The currents transport heat, salt and nutrients influencing the structure of water masses and determining the local physical environment and thereby impacting biota including sheries of the east coast of New Zealand. In this study, I investigate the mean ow and the variability of the current transports using over 20 years of satellite altimeter time series and in situ data at seasonal, interannual and decadal time scales. The mean ow and the fluctuations of the currents are related to potential drivers of the circulation, such as local and basin-scale winds, and to the principal climate modes impacting the South Pacific. The barotropic and baroclinic responses of the currents to basin-scale wind forcing are investigated with Rossby wave models. In addition, the formation, distribution and variability of Subtropical Mode Water, a water mass formed in the western boundary region north of New Zealand, are compared to the variability in air-sea heat fluxes and transports of the northern subtropical current to assess any potential connection between the boundary current and the structure of the water mass. The currents are highly variable and there are no simple trends in the transports over the last two decades. There is little coherence in transport of the different currents along the boundary, suggesting a range of mechanisms drive the variability of the currents individually rather than the system as a whole. At interannual time scales, there is little correspondence between the transports of all the currents and climate indices for El Nino Southern Oscillation and the Southern Annular Mode. While sea level variability east of New Zealand varies in phase with incoming baroclinic Rossby waves, the transports are not responding to linear Rossby wave dynamics. At decadal time scales, only transports in the subantarctic region show a close correspondence with the variability of the wind stress curl averaged over the South Pacific. The seasonality in transports is significant only in two of the currents, with the annual cycle in transport likely driven by seasonality in local winds. The formation and variability of Subtropical Mode Water north of New Zealand are likely influenced by a combination of atmospheric and oceanic forcing, both preconditioning for deeper mixed layers and deepening the thermocline. Collectively, the analyses presented here on the coherence and variability of the currents and the structure of Subtropical Mode Waters in the western boundary region of New Zealand build a more comprehensive picture of the Southwest Pacific ocean circulation.