**7.3. Background currents**

The orbital tsunami current must be detected among the ambient background flows that depend on a number of site-specific factors. Tsunami detectability may be degraded if there are large variations in the ambient current field over the tsunami period, which for the Japan tsunami and the US East Coast meteotsunami was approximately 40 min.

Existing networks of coastal HF radars have been installed primarily for reporting real-time currents. Such currents vary with location and time. Temporal variations may be either predictable (e.g. tides) or unpredictable (due to many causes, such as storms). Their patterns within the radar coverage where tsunami detection is desired can be quite complex. Some background flow variations may be mistaken for a tsunami and hence produce a false alarm in the absence of a good pattern-recognition algorithm.
