The Impact of Tidal Forcing on the Oceanography of the Northern Continental Shelf of New Zealand
The northern continental shelf is a unique region of the Aotearoa New Zealand shelf seas. The shelf (< 500 m depth) features some of the largest tidal velocities in the country (> 2.0 m s-1) and despite lying adjacent to warm, poleward flows of the East Auckland Current, the shelf is often enveloped by a large (>1000 km2) relatively cold pool of water. To date, the processes that generate this cold pool have not been fully resolved, with “upwelling” of the East Auckland Current onto the shelf previously put forward as a likely mechanism. However, the presence of strong tidal currents moving over shallow bathymetry on the shelf led to an alternative hypothesis that the cold pool may be the result of vertical mixing of the water column driven by the regional tidal circulation. The current study uses a combination of a regional tidal solution, high-resolution satellite ocean temperature data, and a regional ocean circulation model, to investigate the role that tidal forcing plays in generating and modulating this cold- water pool. Analysis of a regional tidal solution shows that the M2, N2, and S2 are the three largest tidal constituents over the northern continental shelf. As the largest constituent, the characteristics of the M2 tide (amplitude, phase, velocity, and tidal ellipses) were examined in more detail. The amplitude of the M2 tidal velocity was also used to calculate a contoured map of the Simpson-Hunter (SH) parameter, as a proxy for the strength of stratification over the shelf. This map revealed low SH values (e.g. weak stratification) around the Three Kings Islands, Cape Maria van Diemen, above King Bank, and above Middlesex Bank. The SH values in these areas were similar to those found in other regions within New Zealand and globally that experience cool, well-mixed conditions due to tidal stirring. Charts of an adjusted SH parameter were also calculated at peak spring and neap tide, but these showed little change, due to the relatively small amplitude of the S2 relative to the M2 tidal constituent in this area. A seasonal composite analysis of satellite-derived sea surface temperature (SST) data revealed three distinct cold-water pools over the northern continental shelf. These are located around the Three Kings Islands (inner pool), Cape Maria van Diemen (cape pool), and over the remainder of the shelf (outer pool). All three pools are visible during summer, but only the inner pool was visible during winter. Compositing of satellite SST data over the spring-neap and semi-diurnal cycle was then used to quantify the variability of the cold pools at tidal frequencies. The inner pool cooled (warmed) significantly during spring (neap) tides and this change became more evident when the cooling (heating) lagged peaked spring (neap) tides by two to four days. The edges of both the inner and cape cold-water pools were also found to advect horizontally consistent with the pattern expected from the strength and direction of the M2 tidal ellipse on the shelf.Analysis of output from a regional ocean circulation model that included tidal forcing revealed similar patterns to the satellite observations on seasonal and spring-neap time scales. However, a model run without tidal forcing lacked any cold-water pool on the shelf. This highlights the dominant role played by tidal processes in generating the shelf cold pool. Full-depth transects extracted across the shelf showed significantly cooler waters in the upper and lower water column at spring compared to neap tides. Bottom water temperature on the shelf edge also appears to fluctuate at a semi-diurnal frequency consistent with forcing by the M2 tide and is suggestive of cold water being pumped onto and off the shelf by the internal tide.Collectively, these analyses suggest that the cold-water pools observed on the northern continental shelf of New Zealand are primarily generated by tidal forcing. A dominant process appears to be tidal mixing, where strong tidal currents vertically mix the water column above the shallow bathymetry. Several other processes, including horizontal advection by tidal currents, mesoscale eddies, boundary currents, and subsurface processes associated with the internal tide likely play an important role in controlling the cold pools' position and temperature. Distinguishing the processes driving and modulating these cold pools allows insight into the underlying biogeochemistry on the shelf. The nature of tidal mixing regimes can increase nutrient concentrations within the photic zone, increasing primary production. However, the patchy distribution of these nutrients and the pools' distribution across the shelf may cause high variability in primary productivity, both spatially and temporally.
Advisor: Smith, Robert Owain; Suanda, Sutara H.
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Keywords: New Zealand; Oceanography; Tide; cold-water; mixing
Research Type: Thesis