# Incorporating hadal depths/trenches in ACCESS

With ACCESS-OM2 running at 1/10th degree resolution was hoping someone had looked at this or how they could be incorporated, given there is high-resolution bathy available for some of these locations now! If not, a nudge in the right direction would be very appreciated!

Thanks!

Hi Jessica. Welcome to the ACCESS-Hive Forum!

I hope you don’t mind, but I’ve moved this topic to the COSIMA/Oceans category as that is probably the best place and you may get more feedback if it is located there. Having said which, traffic has slowed down a lot just before the holidays.

The topography for the 0.1 model is listed in the model config manifest, and is here:

/g/data/ik11/inputs/access-om2/input_20201102/mom_01deg/topog.nc


The maximum depth is 5809 metres. I believe it is truncated at that point as a trade-off between representing the deep ocean correctly without having too much overhead of additional layers.

How well are the hadal depths represented in the current bathymetry.

Thanks for your reply Aidan! Being cut off at that depth removes the hadal depths, which are most of the world’s trenches - which is what I am interested in. If I were to get started on tackling this fresh, would MOM would be a good option? Thinking I could reduce the depth range to > 4000 m? The Tonga/Kermadec trench and ridge system is likely my focus, so a regional scale

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I’m not sure what you’re interested in Jessica. However, keep in mind that the model has a very low vertical resolution in the deep ocean, roughly 200m at 4000m depth. It’s unclear how good a job it does representing very deep circulations.

Thanks for the comment. I hope to complement very deep ocean observations with modelling efforts to better understand heat and water-mass flux through/alongside trenches and associated abyssal plains. It’s likely I will have to start from scratch, which is no problem

Thanks Jessica, interesting question.

To within a scaling factor, the barotropic timestep is limited by the horizontal grid size divided by the long gravity wave speed \sqrt{gh}. So the barotropic timestep would need to be shorter if h is increased (perhaps by increasing barotropic_split), which would slow down the model. I believe this (and the wasted storage having so many grid cells below the seafloor in a z-coordinate model) is why models typically limit the maximum depth to omit the trenches, which occupy a very small fraction of the total ocean area.

It could be done, but at considerably increased computational expense (especially if the abyssal / hadal vertical resolution is increased). We have some tools to make generation of topography less painful: GitHub - COSIMA/domain-tools

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