Strength in Numbers: The Tail End of Typhoon Songda Combines with Local Cyclones to Generate Extreme Sea Level Oscillations on the British Columbia and Washington Coasts during Mid-October 2016

From 12 to 16 October 2016, a series of three major low pressure systems, including the tail end of Typhoon Songda, crossed the coasts of British Columbia (BC) and the state of Washington (WA). Songda was generated on 2 October and, after traveling northward along the coast of Japan, turned eastward toward North America. Once there, it merged with two extratropical cyclones moving along the coast of Vancouver Island. The combined lows generated pronounced storm surges, seiches, and infragravity waves off southern BC and northern WA. Here, we examine the event in terms of sea levels measured by tide gauges and offshore bottom pressure recorders, together with reanalysis data, and high-resolution air pressure and wind measurements from 182 meteorological stations. Surge heights during the event typically exceeded 80 cm, with maximum heights of over 100 cm observed at La Push (WA) and New Westminster (BC). At Tofino, on the west coast of Vancouver Island, there was a sharp 40-cm increase in sea level on 14 October in response to a marked air pressure disturbance; slightly lower sea level peaks were also observed at other outer coast locations. In all cases, the sea level response was 1.5–2.5 times as great as that expected from the inverted barometer effect, consistent with local topographic amplification. The sea level oscillations at Tofino had the form of a forced solitary wave (“meteorological tsunami,” or meteotsunami), whereas those on the southwestern shelf off Vancouver Island are well described by classical standing-wave theory. A numerical model closely reproduces the observed meteotsunami peaks and standing-wave oscillations.