Co-evolution of Air and Sea: How do Landfalling Tropical Cyclones Influence Coastal Ocean Circulation?

The average forward translation speed of Tropical Cyclones (TCs) has slowed down globally since the mid 20th century¹. The coastal ocean response to these changes in speed of landfalling TCs has not been investigated in depth. Exploring the coastal ocean dynamics of air and sea of landfalling storms will help forecasters and emergency managers assess threats they may have onshore. A slow moving storm can cause increased hazards about coastal regions for longer periods: increased storm surge, increased rain deposition and extreme flooding events. In addition, as we consider these destructive weather events, TC ‘stalling’ can be evaluated in tandem with a slowdown of tropical cyclone translation speed⁴; that is, slower moving TCs can be susceptible to stalling¹. A stalling storm is one that lingers or resides in an area for an extended period of time. There has been an increase in stalling events over the past 70 years near the North American coast². Although this is not a new phenomenon, there have been limited studies examining the behavior of the stall and the associated coastal ocean response. The coastal ocean response during a TC can be affected by the duration of wind forcing, the local inertial period, bathymetry, stratification, among other factors. Wind forcing that exceeds a local inertial period can develop a coastal Ekman response, which drives either upwelling or downwelling for down-or up-shelf aligned winds³. This will consequently alter coastal ocean circulation during TC passage, and impact coastal marine and land communities. Limited research has been carried out on the influence of coastal upwelling and downwelling on storm behavior; we know even less of their role in coastal impacts. This relationship is critical to study as stalling hurricanes are becoming more frequent and damaging to coastal regions¹. My work investigates stalling behavior of four TCs in the North Atlantic: Hurricane Harvey, Hurricane Florence, Hurricane Dorian and Hurricane Sally. Size, maximum sustained wind speed, wind stress, track and associated accumulated rainfall of a hurricane are evaluated with each case study. To quantify the correlation between stalled storms and coastal ocean response, I use tools such as coupled atmosphere and ocean models alongside observations. That is, this research analyzes observational datasets from High Frequency (HF) Radars and Slocum Autonomous Underwater Vehicles (gliders) and a comparison with coupled model simulations. Historical data from these instruments in a coastal region before, during and after storm stalling are used to examine the thermal structure and current velocities at the surface and within the water column. Additionally, we analyze variables such as translation speed and residence time used in Hall & Kossin 2019 and average inertial periods of these TCs within a designated coastal impact zone (i.e. a circular region with a radius of 200 km about a defined point along the coastline). The residence time metric from Hall & Kossin 2019, is compared against our calculated averaged local inertial periods to quantify coastal ocean responses and circulation during a TC event. This work aims to signify the relationship between stalled TCs and the increased presence of storm surge and extreme flooding events.

1. Hall, T.M., & Kossin, J.P. npj Clim Atmos Sci (2019). 2. Zhang, L. et al. Geophys. Res. Lett. (2023). 3. Zhang, F., Li, M., & Miles, T. J. Geohys. Res., C (2018). 4. Kossin, J.P. Nature (2018).