Sa

Sa is the solar annual tidal constituent, a long-period term with a period of one tropical year, about 8,766 hours or 365.24 days, and a speed near 0.0411 degrees per hour. Its small gravitational part comes from the eccentricity of Earth’s orbit, but the Sa signal in sea level is dominated by the seasonal, meteorological cycle: thermal expansion of the water column, seasonal changes in atmospheric pressure, wind, and river runoff. Because it is largely radiational, Sa amplitude varies widely between stations. It is included in harmonic analysis with its companion Ssa (semiannual) to remove the seasonal mean-sea-level cycle, which is why a tidal-datum epoch spans 19 years.