Short period (1–4 h) sea level fluctuations on the Canterbury coast, New Zealand

Abstract

Data from six sea level sites on the Canterbury coast, New Zealand, were analysed for short period (1-4 h) waves. Persistent waves with periods of 3.4 h in Pegasus Bay and 2.4 h in the Canterbury Bight were found. Their amplitude and phase are highly variable. Edge waves of similar period were found in simulations using a 2-dimensional, harmonic, shallow water, numerical model. Numerical and analytical modelling showed that the offshore decay in amplitude may be approximated by Stokes zero-mode edge waves, but the numerical model revealed that the detailed structure of the offshore decay is more complicated than can be explained by analytical models using a semi-infinite sloping beach. Rotation appears to have little effect. In Pegasus Bay the edge waves are trapped laterally between the northern extent of the bay and Banks Peninsula and offshore by the shelf. In the Canterbury Bight they are trapped laterally between Banks Peninsula and the curvature of the coast and offshore by the shelf. The origin of the waves and the reason for their variability are unclear, but may be the result of the non-linear interaction between semidiurnal tides and meteorological effects.

Physical oceanography of New Zealand/Aotearoa shelf seas – a review

“…For example, Goring (1995) found an oceanic response to barometric pressure at higher frequencies 1 to 9 h −1 , leading to the conclusion that to a large extent variation in sea level is caused by waves propagating into the area. At slightly longer periods, coastal edge waves with periods of a few hours were seen to be caused by non-linear interaction between semidiurnal tides and meteorological effects along the Canterbury Coast (Goring and Henry 1998).…”

Response of New Zealand waters to the Peru tsunami of 23 June 2001

“…The waves in Detail 5 (which are 16 times the scale of those in Detail 1) are similar for the two sites, except that the Lyttelton waves are 1.7 times larger. These are the ubiquitous Pegasus Bay seiche waves of 3.4 h period reported by Goring & Henry (1998). Evidently, the tsunami generated additional seiche, but notice that it dissipated quickly and after just two waves the seiche resumed at its pre-tsunami level.…”

“…described in Goring & Bell (1996) in which the record is transformed to the frequency domain and a tapered boxcar transfer function is applied, then the resulting signal is transformed back into the time domain. We have centred the rising limb of the boxcar at 75° Ir 1 , which is well above the semidiurnal tidal frequencies (c. 30° Ir 1 ) and the main compound and overtide frequencies (c. 45° Ir 1 and c. 60° Ir 1 ), but is below the observed seiche frequencies (c. 100° Ir 1 ) reported by Goring & Henry (1998). As with all high-pass filters, there are end effects with this method, meaning that the initial and final 2 h of record must be rejected.…”

Colony growth and pup condition of the New Zealand fur seal (Arctocephalus forsteri) on the Kaikoura coastline compared with other east coast colonies

“…In 2004, when the Southern Oscillation Index was near neutral, Ohau Point pups were in significantly better condition and at a much higher mass than their Banks Peninsula counterparts. The two regions differ in that the Kaikoura colonies are close to a deep-sea canyon that may provide a more consistent pelagic food source within 2 km offshore, while for the Banks Peninsula colonies, the 500-m and 1000-m isobaths are 100-150 and 150+ km offshore, respectively (Goring and Henry 1998;Boren 2005). This difference may be enough to limit the effects of a mild El Niño along the Kaikoura coast but not on Banks Peninsula (Boren 2001).…”

“…These colonies are remote and not easily accessible by land or by sea. The edge of the continental shelf here is located ~100-150 km offshore (Goring and Henry 1998).…”