Extreme low and high waters due to a large and powerful tropical cyclone: Hurricane Irma (2017)

Hurricane Irma’s (2017) storm tide in Florida (USA) was extreme in varied ways, breaking records of water-level extrema around Florida (15 gauging stations) with low water of − 2.34 m relative to mean sea level (MSL) and estimated return period of 283 years on the gulf coast (Cedar Key), and high water of 1.81 m (MSL) and estimated return period of 110 years on the east coast (Mayport). Five other weather systems that caused similarly extreme low and high waters around Florida are identified in the observed records. A common causal factor associated with extreme storm surge around Florida is large total storm size where tropical storm-/hurricane-force winds act over scales of 100–400 km. The peninsular shape of Florida creates a geographical setting where simultaneous extreme low and high waters can occur. The continental shelf is generally wide for the gulf coast and variable with latitude for the east coast. The curvature afforded by the gently cuspate features of the coastline (scales of 50–200 km) and protruding points and capes lend local environments to amplify the storm tide. Depending on the wind direction, the coastal and shelf geometry traps and accumulates water (extreme high waters) or it provides an openness for winds to blow out the water (extreme low waters), where both are exacerbated by the long duration of winds due to large-scale storm systems. Sea-level rise, interannual variability, seasonality and astronomic tides are all shown to be positive contributing factors of water-level extrema.