Comment on gmd-2021-51

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> While the variations of tidal range are large and fairly well known across Australia (less than 1 <span class="inline-formula">m</span> near Perth but more than 14 <span class="inline-formula">m</span> in King Sound), the properties of the tidal currents are not. We describe a new regional model of Australian tides and assess it against a validation dataset comprising tidal height and velocity constituents at 615 tide gauge sites and 95 current meter sites. The model is a barotropic implementation of COMPAS, an unstructured-grid primitive-equation model that is forced at the open boundaries by TPXO9v1. The mean absolute error (MAE) of the modelled M2 height amplitude is 8.8 <span class="inline-formula">cm</span>, or 12 % of the 73 <span class="inline-formula">cm</span> mean observed amplitude. The MAE of phase (10<span class="inline-formula">^∘</span>), however, is significant, so the M2 mean magnitude of vector error (MMVE, 18.2 <span class="inline-formula">cm</span>) is significantly greater. The root sum square over the eight major constituents is 26 % of the observed amplitude. We conclude that while the model has skill at height in all regions, there is definitely room for improvement (especially at some specific locations). For the M2 major axis velocity amplitude, the MAE across the 95 current meter sites, where the observed amplitude ranges from 0.1 to 156 <span class="inline-formula">cm s^−1</span>, is 6.9 <span class="inline-formula">cm s^−1</span>, or 22 % of the 31.7 <span class="inline-formula">cm s^−1</span> observed mean. This nationwide average result is encouraging, but it conceals a very large regional variation. Relative errors of the tidal current amplitudes on the narrow shelves of New South Wales (NSW) and Western Australia exceed 100 %, but tidal currents are weak and negligible there compared to non-tidal currents, so the tidal errors are of little practical significance. Looking nationwide, we show that the model has predictive value for much of the 79 % of Australia's shelf seas where tides are a major component of the total velocity variability. In descending order this includes the Bass Strait, the Kimberley to Arnhem Land, and southern Great Barrier Reef regions. There is limited observational evidence to confirm that the model is also valuable for currents in other regions across northern Australia. We plan to commence publishing “unofficial” tidal current predictions for chosen regions in the near future based on both our COMPAS model and the validation dataset we have assembled.