Generation of Internal Tidal Beam in Tasman Sea Shaped by Intrusion of Subtropical Front Waters

Macquarie Ridge south of New Zealand is a strong generator of internal tidal waves (ITs) which later form a prominent internal tidal beam. The mechanism of barotropic energy conversion into baroclinic field is investigated by means of numerical experiments with prescribed mesoscale conditions corresponding to Januaries of 2012-2015. The obtained energy transfer rates into IT mode-1 show variability both in spatial distribution and its magnitude. These changes are associated with propagation of remotely generated ITs with an easterly source at the Campbell Plateau. Their pathway is drastically different from the progressive plane waves due to interference with the ridge originating waves and consequent formation of a standing Poincare wave. This complex internal tide environment is modulated by the ratio of the respective East and West travelling components. The west part associated with the plateau experiences variability following the upper ocean stratification. Under 2014 conditions the Tasman Sea subtropical frontal zone was located northward of the region, leading to weaker stratification, lower amplitude waves, a more developed standing wave and hence, less energy transfer into the internal tidal beam. Conversely, in 2015 an intrusion of warm subtropical waters created conditions for stronger generation resulting in high across Macquarie ridge energy transport and larger conversion rates. This mechanism of variability in response to the upper ocean processes suggests a seasonal cycle in the generation of the internal tidal beam, leading to weaker energy levels during austral winter and stronger in the summer.