About ROMS Zeta

[taru_model]

Respected ROMS Users,

In some earlier posts I found that ROMS Zeta is absolute SSH which can be considered equivalent to satellite derived MDT + SLA , where MDT is mean dynamic topography and SLA is Sea level anomaly.

In AVISO site I found MADT, which is map of absolute dynamic topography is again MDT + SLA .

I still found there is little difference in the range in MADT and ROMS ZETA.

Can you kindly help me in this regard whether I can compare MADT with ZETA or not ?
Can I consider both are same and compare with each other ? It is important for my research work.

Please help me.


Yours faithfully
Tarumay

[mengqingjun]

Respected ROMS Users,

In some earlier posts I found that ROMS Zeta is absolute SSH which can be considered equivalent to satellite derived MDT + SLA , where MDT is mean dynamic topography and SLA is Sea level anomaly.

In AVISO site I found MADT, which is map of absolute dynamic topography is again MDT + SLA .

I still found there is little difference in the range in MADT and ROMS ZETA.

Can you kindly help me in this regard whether I can compare MADT with ZETA or not ?
Can I consider both are same and compare with each other ? It is important for my research work.

Please help me.


Yours faithfully
Tarumay

I met the same problem. Can anyone clarify as wilkin said

ROMS zeta is "absolute SSH" in the sense that it includes time mean and time varying SSH with respect to a datum (i.e. zeta=0) that is a geopotential surface, i.e. if zeta=constant there is no associated pressure gradient force.

Altimetrists do not say "absolute" SSH so let me clarify:

In altimetry the equivalent quantity to ROMS zeta is close to the sum of mean dynamic topography (MDT) and sea level anomaly (SLA).

MDT is the height of the long-term time mean sea level with respect to a geopotential surface (the marine geoid). This is different from the Mean Sea Surface which has huge slopes in it because of variations in gravity due to the uneven distribution of the earth's mass. These geoid slopes are not relevant to ocean dynamics so MDT+SLA is the most ROMS-like quantity because the slope of MDT+SLA is almost equivalent to the slope of zeta.

Why do I say "almost"? Well there are some subtleties here.

Note that tides are usually removed from altimeter SLA, though you can choose to leave them in depending on how you are acquiring your altimeter data. ROMS has tides so remember that.

The inverse barometer effect is usually removed from altimetry BUT ALSO absent from ROMS (so no problem there).

Other high frequency sea level variability is not removed from SLA so just keep that in mind (coastal trapped waves for example).

But where we cannot immediately reconcile altimetry and ROMS is the steric effect associated with thermal expansion of the water column. This is real and observed by altimeters, so included in SLA. But ROMS is a Boussinesq model so this effect is neglected. On a seasonal time scale the steric effect might be significant in SLA but it always has long wavelengths so is seldom relevant to coastal dynamics. In our experience, mesoscale SLA variance is much larger and the steric effect disappears in the noise. But strictly speaking one should make some allowance for this missing dynamics in ROMS if it's relevant to the problem you are studying.

John.

I find that the value of ADT in AVIISO is bigger than that zeta in ROMS, actually the reference surface of ADT in AVISO is the geoid, but the reference surface of zeta in ROMS is the zero geopotential surface. So, could I compare the value of ADT and zeta to evluate the simulations ?
Thank you for reply !