ORCID ID
https://orcid.org/0009-0009-8641-3538
Date Awarded
2024
Document Type
Thesis
Degree Name
Master of Science (M.Sc.)
Department
Virginia Institute of Marine Science
Advisor
Harry Wang
Committee Member
Joseph Zhang
Committee Member
Mary C. Fabrizio
Committee Member
Courtney K. Harris
Abstract
As Hurricane Matthew of 2016 passed by the South Atlantic Bight (SAB), a storm tide reaching 1 – 2 m, an elevated post-hurricane abnormal water level (PHAWL), and a pro-longed Gulf Stream (GS) slowdown were recorded. In contrast to the short burst of storm surge by the hurricane, PHAWL produces a more gradual coastal sea level rise ranging between 0.5 – 0.8 meters, which lasts for 7 – 20 days SAB-wide and corresponds to a 15-25% reduction in the GS transport. A 3D unstructured grid, baroclinic model SCHISM was applied to the Western Atlantic Ocean where the open boundary condition was specified at longitude 60 degrees west based on HYCOM, and the atmospheric forcing and pressure were specified at the sea surface based on ECMWF. The 3D model results compared excellently with the 6 tidal gauge records in the SAB (R2 from 0.94 – 0.98). The simulation included an extra-tropical cyclone atmospheric forcing that arrived at the mid-Atlantic region shortly after Matthew downgraded to a tropical storm offshore of North Carolina, which generated a coastally trapped wave (CTW) propagating southward along the SAB. It is noted that the simulation was capable of modeling both phenomena: PHAWL and the GS slowdown. By parsing through model results before and after the event, we found that the dynamic height tilted upwards on the coastal side (downwards on the open ocean side), the thickness of the surface mixed layer deepened to around 60 m, the slope of the cross-shelf density gradient decreased, and strong vertical velocities ranging from 0.5 to 1.0 mm/s in both upwelling and downwelling were found in the interior of the ocean. By ageostrophic analysis, we identified a window that overlaps with the period of the hurricane and CTW when active ageostrophic activities were identified, which includes the frontogenesis of the density field, increased cross-shelf velocity with reversal at depth, and large alternations of the vertical shear, all of which are hallmarks of submesoscale processes. After the active period, most of the state variables return to quiescence (i.e., in geostrophic balance) with the exception of ageostrophic shear which stays negative to bring down the modeled vertical shear. The negative vertical shear indicates the GS has slowed down, consistent with the thermal wind equation. Further analysis of the flow field of the fine-grid model result during the ageostrophic peak reveals different sized eddies emerged spontaneously. These eddies are part of the ageostrophic motions with a horizontal scale of 20 – 50 km and a vertical extent of 500 m deep, and have signatures in temperature, salinity, and vorticity as well. They facilitated intense vertical velocities and lateral stirring resulting in horizontal mixing and dispersion of the density field. Based on the analyses, the evidence indicates that the hurricane and CTW disturbances served as triggers setting off a chain reaction of sub-mesoscale instability which created ageostrophic motions and lateral mixing causing the cross-shelf horizontal density gradient to slump, eventually leading to the slowdown of the GS.
DOI
https://dx.doi.org/10.25773/v5-aa53-ep98
Rights
© The Author
Recommended Citation
Maldonado, Breanna, "Numerical Modeling Of Coastal Sea Level Anomaly, Gulf Stream Slowdown, And Cross-Shelf Mixing In South Atlantic Bight During Hurricane Matthew" (2024). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1717521766.
https://dx.doi.org/10.25773/v5-aa53-ep98
