# NEMO\<-\>wave coupling: modified surface stress with ln\_taw and ln\_tauoc **URL:** \ **Category:** v4.2.x\ **Tags:** OASIS, SBC\ **Created:** [22 November 2024 09:04 UTC](https://nemo-ocean.discourse.group/t/nemo-wave-coupling-modified-surface-stress-with-ln-taw-and-ln-tauoc/788 "2024-11-22T09:04:20Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![r-hartmann](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@r-hartmann](https://nemo-ocean.discourse.group/u/r-hartmann)\ **Post date:** [22 November 2024 09:04 UTC](https://nemo-ocean.discourse.group/t/nemo-wave-coupling-modified-surface-stress-with-ln-taw-and-ln-tauoc/788/1 "2024-11-22T09:04:20Z")
Hi everyone, I have a question regarding the coupling of wave stress in NEMO. As I understand there are two options to do so: (i) component-wise with `ln_taw` for u and v stress (on u- and v-grids). (ii) as fraction based on the normalized wave stress with `ln_tauoc` (on T-grid). According to the NEMO manual (v4.2.1, Sec. 6.12.7) the surface stress felt by the ocean \tau\_{oc} is the atmospheric wind stress \tau\_a minus the stress taken up by the waves \tau\_w: \tau\_{oc} = \tau\_a - \tau\_w However, checking the source code, I see that for option (i), `ln_taw=.true.`, the stress components are updated as (`sbcmod.F90`, l.460): `utau(:,:) = utau(:,:) - tawx(:,:) + twox(:,:)` `vtau(:,:) = vtau(:,:) - tawy(:,:) + twoy(:,:)` While I assume that `tawx,y` are components of \tau\_w (sent to NEMO via OASIS variables O\_Tawx, and O\_Tawy), **what is stress field `twox,y` (sent to NEMO via OASIS variable O\_Twox, and O\_Twoy)?** Activating `ln_taw`, NEMO assumes to receive all the four components `tawx`, `tawy`, `twox`, and `twoy`, which - in case only `tawx,y` or `twox,y` are coupled - results in a unitialized 2nd field leading to Infinity values in `utau` and `vtau` and the termination of NEMO. On the other hand, for option (ii), `ln_tauoc=.true.`, the manual (Sec. 6.12.7) states that \tau\_{oc} = \tau\_a \times \tilde{\tau}, with the “normalized wave stress into the ocean” \tilde{\tau}. In the source code (`sbcmod.F90` l.447), \tilde{\tau} is `tauoc_wave` and directly received from OASIS (as O\_WStrf): `utau(ji,jj) = utau(ji,jj) * ( tauoc_wave(ji,jj) + tauoc_wave(ji-1,jj) ) * 0.5_wp` `vtau(ji,jj) = vtau(ji,jj) * ( tauoc_wave(ji,jj) + tauoc_wave(ji,jj-1) ) * 0.5_wp` Based on the equations above, I would assume that \tilde{\tau} = \frac{\tau\_a - \tau\_w}{\tau\_a} = 1 - \frac{\tau\_w}{\tau\_a}. Therefore, it seems that NEMO is not simply distinguishing normalized/not normalized and T-grid/UV component-wise between the two options `ln_taw` and `ln_tauoc`, but substantially expecting different fields. And especially some field that is not really decribed in the manual… Can someone resolve my confusion here? Which stresses is NEMO exptecting to receive for the two options? Thanks in advance! Robert ---
**Author:** ![amoulin](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@amoulin](https://nemo-ocean.discourse.group/u/amoulin)\ **Post date:** [27 November 2024 16:24 UTC](https://nemo-ocean.discourse.group/t/nemo-wave-coupling-modified-surface-stress-with-ln-taw-and-ln-tauoc/788/2 "2024-11-27T16:24:32Z")
Hi, For the first option ln\_taw=true, indeed it is not fully clear in the documentation. We will modify the next manual version. For this option, 4 fields are transferred by the wave model : - tawx and tawy which are the net wave supported stress in x and y directions - twox and twoy which are the wave to ocean momentum flux in x and y directions Those fields should be sent to the NEMO u and v grid via the OASIS namcouple file. Hope this helps! ---
**Author:** ![r-hartmann](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@r-hartmann](https://nemo-ocean.discourse.group/u/r-hartmann)\ **Post date:** [2 December 2024 10:20 UTC](https://nemo-ocean.discourse.group/t/nemo-wave-coupling-modified-surface-stress-with-ln-taw-and-ln-tauoc/788/3 "2024-12-02T10:20:48Z")
Hi amoulin, Thanks for your reply! I’ve also found this definition in the sorce code. However, this is what actually confuses me. To my understanding I would only need one of the two fields as \tau\_{oc}=\tau\_a -\tau\_w. So either I provide the components of net wave supported stress \tau\_w (the stress the waves subtract from the total atmospheric stress), which means that: ```auto utau(:,:) = utau(:,:) - tawx(:,:) vtau(:,:) = vtau(:,:) - tawy(:,:) ``` Or I directly provide the wave to ocean momentum flux \tau\_{oc} (the residual stress remaining from total atmospheric stress), which would mean that: ```auto utau(:,:) = twox(:,:) vtau(:,:) = twoy(:,:) ``` However, providing both in the current source code implementation would yield `utau=2*twox`, wouldn’t it? Please enlighten me, where my understanding of the terms and of the (supposed) usage of these fields is wrong. Thanks! Robert