Why stronger stratification is changing ocean tides
Model and satellite M2 trends since 1993 point to stronger ocean stratification increasing tidal conversion, not just sea-level rise.
Source
A likely role for stratification in long-term changes of the global ocean tides
Study at a glance
- Design
- Computational / modelling — Yearly internal-tide-permitting MITgcm runs relaxed to reanalysis stratification, compared with altimetry and tide gauges
- N
- 1993–2020 simulations; coastal M2 trends matched about half of 10 tide-gauge cases
- Population
- Global ocean M2 tide, 1993–2020
- Outcome
- Strengthening stratification, not sea-level rise, accounts for much of the observed long-term M2 change
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Why stronger stratification is changing ocean tides
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Simulations typically capture 60% of the satellite M2 trend estimate.
Our simulations typically capture 60% of the satellite estimate, pointing to a residual that may arise from model limitations, subtle errors in the altimetry solution, or both (Methods).
Over what period are stratification-forced tide trends diagnosed?
Emphasis is on trends from 1993 to 2020, a period where we have suitable three-dimensional (3D) density data.
What they did
Authors ran yearly internal-tide-permitting MITgcm simulations relaxed to reanalysis stratification for 1993–2020 and compared M2 trends with altimetry, tide gauges, and a 2-D sea-level-rise tide model.
What they found
Stratification-driven coastal M2 trends are about 0.1 mm/yr and match roughly half of ten tide-gauge cases. Open-ocean M2 is mostly declining; the 3-D runs capture about 60% of satellite regional trends, while relative sea-level rise explains ≲20% at most open-coast sites.
The limits
What it doesn't show
German Bight decline is not explained by these runs; Northwest European Shelf results are sensitive to steric corrections, and altimetry still has uncorrected systematic errors.
Key terms
- M2 tide
- Principal lunar semidiurnal tidal constituent.
- Tidal conversion
- Transfer of barotropic tidal energy into internal tides at topography.
- Stratification
- Vertical density structure that supports internal waves.
- Barotropic tide
- Depth-averaged tidal motion, as opposed to internal (baroclinic) tides.
- MITgcm
- Ocean general circulation model used here in tide-permitting mode.
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Primary driver of 1993–2020 open-ocean M2 trends here:
Common questions
What years are modeled?
1993–2020, when 3-D density data exist.
Does sea-level rise explain most M2 trends?
No—typically ≲20% at open-coast sites.
Main proposed mechanism?
Enhanced tidal conversion over steep topography as stratification strengthens.
How much of satellite M2 decline is captured?
About 60% in four regional averages.
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