12 What is a "Coupler"?

The CCSM coupled model is a framework that divides the complete climate system into component models connected by a coupler. In this design the Coupler is a hub that connects four major component models - atmosphere, land, ocean, and sea-ice (see Figure 3. We'll ignore the river model component contained in the land model). Each component model is connected to the Coupler, and each exchanges data with the Coupler only. The CCSM is not a particular climate model, but a framework for building and testing various climate models for various applications. In this sense, more than any particular component model, the Coupler defines the high-level design of CCSM software.

Figure 3: The CCSM Coupled Model Framework in its most basic form.

The Coupler code has several key functions within the CCSM framework:

• It allows the CCSM to be broken down into separate components, atmosphere, sea-ice, land, and ocean, that are "plugged into" the Coupler. Each component model is a separate code that is free to choose its own spatial resolution and time step. Individual components can be created, modified, or replaced without necessitating code changes in other components. CCSM components run as separate executables communicating via message passing (MPI).

• It controls the execution and time evolution of the complete CCSM by synchronizing and controlling the flow of data between the various components.

• It communicates interfacial fluxes between the various component models while insuring the conservation of fluxed quantities. For certain flux fields, it also computes interfacial fluxes based on state variables. In general, the Coupler allows any given flux field to be computed once in one component, this flux field is then routed through the Coupler so that other involved components can use this flux field as boundary forcing data.