19.3.1 Default Surface boundary conditions

The default set of surface boundary conditions and ordering for the MOM test case is constructed by the following code in driver.F:

      m = 0
      call setup_sbc ('taux', m)
      call setup_sbc ('tauy', m)
      call setup_sbc ('heatflux', m)
      call setup_sbc ('saltflux', m)
      numosbc = m

      call setup_sbc ('sst', m)
      call setup_sbc ('sss', m)
      numasbc = m - numosbc
      numsbc  = numasbc + numosbc

where ``numosbc'' is the number of surface boundary conditions for the ocean, ``numasbc'' is the number of surface boundary conditions for the atmosphere, and ``numsbc'' is the total number. Given the above ordering of calls, the index for heatflux ``i'' can be determined from

      i = index_of_sbc ('heatflux')

and the value of i will be i=3. The above six boundary conditions within and will be indexed as follows:

• m=1 for ``taux'' which references $\vec{\tau}\cdot \hat{\lambda}$which is the eastward windstress exerted on the ocean in units of dynes/cm². If the wind in the atmosphere model is positive (blowing towards the east) then the ocean and land exert a westward stress which decelerates the atmosphere. From the atmosphere point of view, this stress is negative. However, the ocean and land are being accelerated by the transfer of momentum through the boundary layer. Therefore the correct sign for the stress acting on the ocean is positive (towards the east).

• m=2 for ``tauy'' which references $\vec{\tau}\cdot \hat{\phi}$which is the northward windstress exerted on the ocean in units of dynes/cm². A northward windstress is positive.

• m=3 for ``heatflux'' which references heat flux in units of langley/sec where 1 langley = 1 cal/cm². A positive heatflux means heat is being pumped into the ocean.

• m=4 for ``saltflux'' which references a salinity flux into the ocean in units of grams/cm²/sec. In MOM, the rigid lid approximation implies that ocean volume is constant^19.26. Therefore, when it rains, salt must change since ocean volume cannot. If the atmosphere model supplies a fresh water flux, this should be converted to a salt flux = -(P-E+R) S_ref where P-E+R represents a precipitation minus evaporation plus runoff rate in  cm/sec, and S_ref is a reference salinity in units of grams of salt per gram of water (units of parts per part such as 0.035). Depending upon the application of interest, S_ref may be a constant over the entire model domain or the locally predicted salinity of the uppermost model level. If the intent is for a global average P-E+R flux of zero to imply a zero trend in the ocean salt content, then S_ref must be chosen as a constant. A positive precipitation minus evaporation into the ocean means that fresh water is being added to the ocean which decreases the salinity. This implies that the salt flux is negative.

• m=5 references ``sst'' which is sea surface temperature in units of deg C.

• m=6 references ``sss'' which is sea surface salinity in units of (S-35.0)/1000.

Other surface boundary conditions are possible as defined within the module list. For instance ...

      call setup_sbc ('short wave', m)

sets up a solar short wave flux in units of  langley/sec where 1 langley = 1 cal/cm². Normally this effect is included in the surface heat flux. However, solar short wave penetration into the ocean is a function of wavelength. The default clear water case (Jerlov turbidity type I) assumes energy partitions between two exponentials as follows: 58% of the energy decays with a 35 cm e-folding scale; 42% of the energy decays with a 23 m e-folding scale. If the thickness of the first ocean level dzt_k=1 = 50 meters, then shortwave penetration wouldn't matter. However, for dzt_k=1 = 10meters, the effect can be significant and may be particularly noticeable in the summer hemisphere. See Paulson and Simpson (1977), Jerlov (1968) and Rosati (1988).

Surface boundary conditions such as solar shortwave ``short wave'' or fresh water flux ``fresh wtr'' require datasets that are not part of MOM. It is the responsibility of the researcher to supply these datasets.

When option restorst is enabled, surface tracers are restored to prescribed data t $^\star_{i,j,n,time}$^19.27 using a Newtonian damping time scale dampts in units of days input through a namelist. Refer to Section 14.4 for information on namelist variables. Note that the damping time scale may by set differently for each surface tracer. The Newtonian damping term is actually converted into a surface tracer flux as described under option restorst in Section 28.2.9.