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= ''' Direct Model Changes ''' =

== Introduction ==
Some modifications and additions are compulsory to integrate the Tangent and Adjoint Module to NEMO. They mainly consist in:
 * changing subroutines or parameters declaration from Private to Public for:
 * initialization
 * re-use
 * adding a subroutine to save the trajectory
 * adding parameters
 * updating namelist

== 1 - Non-Linear Trajectory ==

To evaluate the tangent and/or the adjoint at a time t, some actual parameters of the direct model are needed at the corresponding time t. To fulfill this purpose a subroutine, ''tamtrj_wri''  is added to write the trajectory. It is controlled with one logical parameter: ''ln_trjwri''. The trajectory is saved upon a given frequency "nittrjfrq" only if the logical parameter is set to TRUE.

=== Subroutine: tam_trj_wri ===

A new subdirectory is created into /OPA/ named '''TAM'''. The file is '''tamtrj.F90'''.
The purpose of this routine is to write to file the model state trajectory for use with 4DVar. It is called in ''opa_model'' routine to initialize tamtrj routine TAM and at in the ''stp'' routine. 

Currently the following parameters are saved:
|| '''Parameters''' || '''Associated ccp keys''' ||
||emp ||   ||
||emps ||   ||
||un ||   ||
||vn ||   ||
||tn ||   ||
||sn ||   ||
||ta ||   ||
||sa ||   ||
||tb ||   ||
||sb ||   ||
||avmu ||   ||
||avmv ||   ||
||aeiu || lk_traldf_eiv  ||
||aeiv || lk_traldf_eiv  ||
||aeiw || lk_traldf_eiv  ||
||uslp || key_ldfslp  ||
||vslp || key_ldfslp  ||
||wslpi || key_ldfslp  ||
||wslpj || key_ldfslp  ||
||avs || key_zdfddm  ||
||strdmp || key_tradmp  ||
||hmlp || key_tradmp ||

=== Specific management of the initial state ===
For the initial state of the model, we call the tam_trj_wri in 'step' routine just before the tracer block. We,then, have access to some parameters (tke and sbc for instance) as they are not yet computed in opa (outside 'step').


=== Linear Interpollation ===
For efficiency matters, the whole trajectory is of course not saved. Only a predefined steptime frequency is saved. The frequency ''nittrjfrq'' is controlled by namelist. The intermediate steptime will be estimated in TAM by linear interpollation.

=== Namelist ===
Parameters related to the model state saving is controlled by the namelist '''namtam''' with:

 * ln_trjwri: Logical switch for writing out state trajectory
 * nittrjfrq: saving frequency of the trajectory

== 2 - Routine modification ==

 * '''opa.F90''': 
   * include initilialization of trajectory saving handling (USE tamtrj)
 * '''step.F90''':
   * include calls for trajectory saving (CALL tam_trj_wri)
   * add temporary variables (zta, zsa) to save ta/sa (used as workspace in dynamic block)

== 3 - Declaration Changes ==
The following table describes the change from PRIVATE to PUBLIC of some routines and parameters.

|| '''Modules''' || '''Routines''' || '''parameters''' ||
||dynadv || dyn_adv_ctl  || ||
||daymod || day_mth  || ||
||lib_mpp ||          || mpi_comm_opa ||
||par_oce ||          || jpni,jpnj,jpnij ||
||zdftke || tke_rst   || ||
||eosbn2 ||          || neos_init||
||tradmp || cofdis, dtacof, dtacof_zoom  || ||
||  ||    ||strdmp, ttrdmp, resto ||
||traqsr ||    ||nksr, gdsr ||