Add SfcCoupling class as an interface layer to the coupler

components/omega/doc/devGuide/SfcCoupling.md

@@ -0,0 +1,83 @@
+(omega-dev-sfc-coupling)=
+
+# Surface Coupling
+
+The `SfcCoupling` class manages the variables exchanged to (`o2x`) and from
+(`x2o`) the coupler. It handles import/export of the raw coupler data,
+application of imported fields to the [`Forcing`](#omega-dev-forcing) object,
+and accumulation of export fields over the coupling interval. It is possible
+to have multiple `SfcCoupling` instances; every instance has a name and is
+tracked in a static C++ map called `AllSfcCoupling`.
+
+## Initialization
+
+The static method:
+```c++
+OMEGA::SfcCoupling::init(CouplingInitParams);
+```
+initializes the default `SfcCoupling` given a `CouplingInitParams` struct
+(number of import/export fields, name-to-index maps, coupling time step, and
+coupling `Layout`, i.e. `MCT` or `MOAB`) supplied by the coupler. A pointer to
+the default instance can be retrieved at any time using:
+```c++
+OMEGA::SfcCoupling* DefSfcCoupling = OMEGA::SfcCoupling::getDefault();
+```
+
+## Creation of non-default surface coupling objects
+
+A non-default instance can be created with the static `create` method,
+which returns a pointer to the newly created object:
+```c++
+OMEGA::SfcCoupling* NewSfcCoupling = OMEGA::SfcCoupling::create(
+    Name, Mesh, NImportFields, NExportFields, ImportIdxMap, ExportIdxMap,
+    Stepper, CouplingTimeStep, Layout);
+```
+Given its name, a pointer to a named instance can be obtained at any time:
+```c++
+OMEGA::SfcCoupling* NewSfcCoupling = OMEGA::SfcCoupling::get(Name);
+```
+
+## Data exchange
+
+Before import/export, unmanaged views must be attached to the raw coupler
+data pointers:
+```c++
+SfcCoupling.attachData(CplToOcnData, OcnToCplData);
+```
+To copy data from the coupler into the `CplToOcn` fields, and from the
+`OcnToCpl` fields back out to the coupler:
+```c++
+SfcCoupling.importFromCoupler();
+SfcCoupling.exportToCoupler();
+```
+Imported fields are applied to a `Forcing` object with:
+```c++
+SfcCoupling.applyImportFields(ForcingPtr);
+```
+Export fields are accumulated (running average) each ocean time step with:
+```c++
+SfcCoupling.updateExportFields(State, TracerArray);
+```
+
+## Units
+
+`OcnToCplFields`'s averaged temperature is Conservative Temperature (deg C)
+on device (`AvgSfcTemperature`), invariant at all times. `copyToHost()`
+converts it to in-situ temperature in Kelvin (via TEOS-10, when in use)
+into the host mirror `AvgSfcTemperatureH`, once per coupling interval.
+All other averaged fields keep the same units and value on host and device.
+
+To keep this invariant, `OcnToCplFields`'s averaged device arrays are
+private: they can only be written via `updateAverages()` (called each
+ocean time step from `updateExportFields()`) and read via `copyToHost()`.
+
+## Removal of surface coupling objects
+
+To erase a specific named instance use `erase`:
+```c++
+OMEGA::SfcCoupling::erase(Name);
+```
+To clear all instances do:
+```c++
+OMEGA::SfcCoupling::clear();
+```

components/omega/doc/index.md

@@ -54,6 +54,7 @@ userGuide/Timing
 userGuide/VerticalMixingCoeff
 userGuide/VertAdv
 userGuide/Forcing
+userGuide/SfcCoupling
 ```
 
 ```{toctree}
@@ -100,6 +101,7 @@ devGuide/Timing
 devGuide/VerticalMixingCoeff
 devGuide/VertAdv
 devGuide/Forcing
+devGuide/SfcCoupling
 ```
 
 ```{toctree}

components/omega/doc/userGuide/SfcCoupling.md

@@ -0,0 +1,7 @@
+(omega-user-sfc-coupling)=
+
+## Surface Coupling
+
+The `SfcCoupling` class manages the exchange of surface variables with the
+coupler (MCT or MOAB) between Omega and the other components of E3SM. There are
+no user-configurable options.