tidal_forcing Code by Robert Hallberg, August 2005, based on C-code by Harper Simmons, February, 2003, in turn based on code by Brian Arbic. The main subroutine in this file calculates the total tidal contribution to the geopotential, including self-attraction and loading terms and the astronomical contributions. All options are selected with entries in a file that is parsed at run-time. Overall tides are enabled with the run-time parameter 'TIDES=True'. Tidal constituents must be individually enabled with lines like 'TIDE_M2=True'. This file has default values of amplitude, frequency, Love number, and phase at time 0 for the Earth's M2, S2, N2, K2, K1, O1, P1, Q1, MF, and MM tidal constituents, but the frequency, amplitude and phase ant time 0 for each constituent can be changed at run time by setting variables like TIDE_M2_FREQ, TIDE_M2_AMP and TIDE_M2_PHASE_T0 (for M2). In addition, the approach to calculating self-attraction and loading is set at run time. The default is to use the scalar approximation, with a coefficient TIDE_SAL_SCALAR_VALUE that must be set in the run-time file (for global runs, 0.094 is typical). Alternately, TIDAL_SAL_FROM_FILE can be set to read the SAL from a file containing the results of a previous simulation. To iterate the SAL to convergence, USE_PREVIOUS_TIDES may be useful (for details, see Arbic et al., 2004, DSR II). With TIDAL_SAL_FROM_FILE or USE_PREVIOUS_TIDES,a list of input files must be provided to describe each constituent's properties from a previous solution.