Abstract
Abstract. A 4th order advection scheme is applied in a nested eddy-resolving Hybrid Coordinate Ocean Model (HYCOM) of the greater Agulhas Current system for the purpose of testing advanced numerics as a means for improving the model simulation for eventual operational implementation. Model validation techniques comparing sea surface height variations, sea level skewness and variogram analyses to satellite altimetry measurements quantify that generally the 4th order advection scheme improves the realism of the model simulation. The most striking improvement over the standard 2nd order momentum advection scheme, is that the southern Agulhas Current is simulated as a well-defined meandering current, rather than a train of successive eddies. A better vertical structure and stronger poleward transports in the Agulhas Current core contribute toward a better southwestward penetration of the current, and its temperature field, implying a stronger Indo-Atlantic inter-ocean exchange. It is found that the transport, and hence this exchange, is sensitive to the occurrences of mesoscale features originating upstream in the Mozambique Channel and southern East Madagascar Current, and that the improved HYCOM simulation is well suited for further studies of these inter-actions.
Highlights
Modelling of the greater Agulhas Current regime, in particular of the very energetic retroflection region, is challenging
In comparison this agrees better with the spatial distribution of variability observed from altimetry suggesting that, the higher-order momentum advection scheme seems to underestimate the variability for the region compared to observations, it is able to simulate more variable eddy trajectories in the ring shedding corridor
In particular the strength and distribution of the mean flow and variability (∇) in the southern Agulhas Current associated with the transition from a more stable current upstream to a strongly meandering and retroflecting current downstream appears to be more reliable in Hybrid Coordinate Ocean Model (HYCOM) O4 when compared to altimetry
Summary
Modelling of the greater Agulhas Current regime, in particular of the very energetic retroflection region, is challenging. Model numerics may be improved by applying higher-order finite difference approximations This would reduce the truncation error, much like increasing the model grid resolution, but at a lower computational cost (Sanderson, 1998). Winther et al (2007) conclude that with model grid spacing which is smaller than the Rossby radius of deformation, higher-order approximation need only be applied to the momentum scheme in order to yield beneficial results This is achieved at a fraction of the computational cost of increasing the grid resolution. In light of improving the model numerics, the QUICK scheme, with the bi-harmonic viscosity modified to minimise the viscosity coefficient (as in Winther et al, 2007), was applied to momentum advection in the Agulhas HYCOM. The main findings of the intercomparisons are summarised in the conclusion
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