Remote Observation of a Small Meteotsunami in the Bight of Benin Using HF Radar Operating in Lower HF Band

High-frequency (HF) surface-wave (SW) radar systems have demonstrated their capability to capture the signals from seismic and non-seismic tsunami events using the estimation of surface current changes. Several ocean radar systems, operating in the upper HF band, were already optimized for tsunami alerting and installed for real-time tsunami monitoring at the coast. If the shelf extends tens of kilometers off the coast then the first appearance of tsunami waves can be monitored at the shelf quite early. On May 25, 2018, an HF SW phased-array radar system, operating in the lower HF band (6.9 MHz), detected strong changes in measured radial currents at distances up to 60 km off the coast. The system tracked the tsunami-like current pattern in the Bight of Benin. The wave propagation coincided with an atmospheric cold front passage. No earthquake occurred at that time but a strong atmospheric disturbance was present and it is the most likely reason for the detected disturbance. This localized event could be classified as a meteorological tsunami (meteotsunami). It is important to note that this is the very first detection of meteotsunami in the whole West African region. This detection showed the capability of the radar to measure unusual surface current velocities induced by tsunami waves using the lower part of the HF band. The detection of this tsunami-like event has shown good applicability of HF phased-array radar technology for offshore tsunami monitoring and providing safe navigation, in real-time. At the same time, it has been shown that a single HF SW radar system, originally optimized for vessel tracking at very long ranges using a proper output power, transmitter's noise optimization and integration time, can also be utilized as a component of Tsunami early warning systems (TEWS).