11.3 The Memory Window

The memory window (MW) provides the framework for solving all tracer and baroclinic equations in a uni-tasking as well as a multi-tasking environment. It is not used for the barotropic equation since the total storage for 2D arrays is small compared to 3D arrays. In precursors to MOM, an array of data along longitude and depth was indexed as A(i,k). Within the MW, the same array would be indexed as A(i,k,j) where the ``j'' index is a latitude index local to the MW. This generalized approach is capable of simulating the older method but allows for much greater flexibility. Some of the advantages of the MW are:

• Higher order finite difference schemes and parameterizations which require access to more than three latitude rows can be implemented in a straight forward manner.

• There is a reduction in the number of names required for variables. For example, in MOM 1 and previous incarnations, tracers required three names: one for the row being computed, one for the row to the north and another for the row to the south. However, there is only need for one name: the row being computed is accessed by local index  j and rows to the north and south are accessed by local row indices  j-1 and j+1.

• Essentially all prognostic variables are subscripted by three spatial indices (i,k,j) as if infinite central memory were available. However, the actual memory needed is controlled by the size of the memory window jmw. Equations and coding looks the same, regardless of how large or small the memory window is. In fact, the memory window can be opened all the way so that all dataflow between disk and memory vanishes. However, this is wasteful of memory when the number of latitude rows per processor is large since the memory window requires three time levels and only two time levels are needed on disk (or ramdrive).

• Increases in speed can be realized when opening up the memory window on a single processor. This is the case when lots of diagnostic options are enabled. The reason is that less redundant computations are required when the memory window is opened up.

• All 3D parameterizations are easily parallelizable with a minimum of communication calls.

The only disadvantage of opening the MW up beyond the minimim size is that memory will be aggressively consumed.