Impact of tropical cyclones on Florida Current transport and associated coastal sea level fluctuations

The Florida Current (FC) is an integral part of the Gulf Stream system. With the mean northward transport of about 32 Sv flowing between Florida and the Bahamas, it provides the bulk of the northward transport of the upper limb of the Atlantic Meridional Overturning Circulation in the tropics. The importance of the FC in the ocean-atmosphere dynamical system as well as in climate and weather variability has long been recognized, resulting in the establishment in 1982 of a one-of-a-kind nearly continuous monitoring program to measure the FC transport and properties using a submarine telephone cable and hydrographic observations. These nearly four decade long observations have shown that the FC is rather stable with no significant long-term changes, but the bulk of its spectral energy is concentrated at synoptic frequencies. While the standard deviation of the FC transport is 3.4 Sv, the amplitude of weather-driven fluctuations can reach ±15 Sv. Such fluctuations may have important implications on sea level along the low-lying Florida eastern seaboard subject to nuisance flooding, since the physics of the FC flow sustains a sea level difference of over a meter between Florida and the Bahamas. In the first days of September 2019, category 5 Hurricane Dorian reached the northern Bahamas and was nearly stationary there for 2-3 days before turning northwestward. While Dorian was stationary over the Bahamas, the record minimum FC transport of 17.1 Sv was estimated using cable measurements on September 4, 2019. At the same time coastal sea level in Miami was over 40 cm above the expected tidal range, which also coincided with the second highest tide of the year (king tides). Extensive nuisance flooding was then observed throughout Miami. Interestingly, the old record minimum of 17.2 Sv was observed on October 28, 2012, when Hurricane Sandy was travelling over the Bahamas northward. Using a suite of in situ observations in the Florida Straits and an ocean model, we present a detailed analysis of how tropical storms may affect the FC/Gulf Stream transport and how this can cascade to remotely-forced coastal sea level changes. Using tide gauge records, we investigate to what extent the tropical storm forced changes in the FC transport affect the coastal sea level through geostrophic adjustment as opposed to storm surges driven directly by the wind.