28.1.4 levitus_ic

This option accesses three dimensional temperature and salinity data which have previously been prepared by scripts in PREP_DATA for the resolution specified by module grids as discussed in Section 3.2. The particular month from the Levitus (1982) climatology which is to be used as initial conditions must be accessed by pathname from within script run_mom. The researcher should supply the pathname.

The flag values in the Levitus analyzed dataset have been replaced by values extrapolated from valid Levitus ocean points. The extrapolation was accomplished by solving the following two-dimensional elliptic boundary value problem on land points for each horizontal level within the Levitus dataset.

$$\nabla_{h}^{2} \, T(\lambda,\phi) 0 \qquad (\lambda,\phi) \in \mbox{land},$$ = (28.4)

The reason for the above was to provide data values everywhere within the dataset to ease the procedure of interpolating Levitus data to ocean points within arbitrary model grid resolutions. As long as the boundary between ocean and land cells in the model domain is not too different than the boundary in the Levitus dataset, the extrapolation yields reasonably good values. However, if model topography is modified by any means to produce ocean cells far from Levitus ocean points, the extrapolation may not be good enough. It is up to the researcher to verify that data is reasonable for particular configurations of model geometry and topography.

Ideally, the above procedure will not result in any extrema. The two-dimensional approach used in this algorithm can be limiting. For example, say one needs to prescribe data inside of a deep canyon for a high resolution model, or perhaps inside of a wide seamount whose top has been chopped off in order to smooth topography. In both cases, the two-dimensional approach will effectively extrapolate from values horizontally outside the canyon or seamount, through the rock, to the point of interest. No information from adjacent vertical points are used. A more complete algorithm would involve solving a three-dimensional Laplacian in order to incorporate vertical information as well. The three-dimensional approach has not been taken due to the added computational complexity.

The importance of these issues depends on how important the initial conditions are to the model experiment. For many climate models, initial conditions are almost irrelevant since the models are integrated for many thousands of years to equilibrium. For higher resolution models which are not integrated for so long, initial conditions can be important and something more general than the above procedure might be relevant.