Intensification and variability of the confluence of subtropical and subantarctic boundary currents east of New Zealand

Abstract

AbstractThe confluence of subtropical and subantarctic boundary currents east of New Zealand creates strong fronts. The fronts have clear signatures in sea surface height (SSH) and sea surface temperature (SST) which make the confluence a good region to investigate the variability of the boundary currents of the South Pacific. Analysis of the 20 year time series of the SSH is used to investigate the location and strength of fronts, measured as the gradient in SSH (∇SSH), and the eddy kinetic energy (EKE) and their relationship to local and large‐scale wind forcing. The intensity of the ∇SSH and the EKE have increased at a rate of 0.02 cm km−1 and 32 cm2 s−2 decade−1, respectively. There is a significant correlation (r = 0.7, p < 0.01) between the ∇SSH and EKE signals, reflecting baroclinic instabilities inherent in the fronts. Differences between northward and southward wind‐driven transports across the confluence from the Island Rule are also increasing at 7.5 Sv decade−1 along with an upward trend in the SST differences across the region. Time series of the Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) and local winds were compared to the frontal strength. Although the positive trend in the SOI coincides with increasing subtropical inflows, there is little correspondence of the indices and local winds with short‐term variability. While these results indicate a connection between the intensification of the confluence and South Pacific winds, there is little change in frontal location north of Bollons Seamount which suggests bathymetry influences the location of the confluence.