**6. The atmospheric wind field and extra-tropical cyclones**

The wind field as observed by the National Weather Service (NWS) at the Cape Hatteras Lighthouse station (not shown) was evaluated over the period September 1 through December 31, 1987. The wind's velocity vector, i.e., wind speed and direction, is measured and recorded every 3 hours. The Cape Hatteras wind vector time series data were chosen since no meteorological buoy data were available (from the region) for the Fall of 1987. The Hatteras winds were deemed more representative of outer shelf Onslow Bay wind conditions than were winds from Wilmington or Beaufort, NC. Weisberg and Pietrafesa [14] found that in the Carolina Capes, wind speed increases from 1.5 to 2.5 times in magnitude along the coastal mainland to several tens of kilometers offshore due to the larger boundary layer drag created by land vs. that of water, which slips with the wind. The net result is that the effective wind stress over water is 2.5–6.5 times larger than that over land, albeit in the same direction. Cape Hatteras winds are less affected by the frictional boundary layer created by the mainland, because they are collected on a barrier island more than 20 kilometers from the mainland and are thus deemed more representative of actual over-the-water winds. It is of note that the region surrounding Cape Hatteras is a spawning region for wintertime atmospheric low-pressure systems or cyclonic storms [15].

During the late fall, winter, and early spring period, Atlantic low pressure systems known variously as Nor'easters, Atlantic Lows, Cape Hatteras Lows, and Extra-Tropical Cyclones (ETCs) are omnipresent over the coastal zone principally from South Carolina (SC) to Virginia (VA) [15] but actually extend from 25° N latitude to 75° N. ETCs intensify, and often form, throughout this zone, centered about Cape Hatteras [15]. The ETCs can deepen, i.e., further intensify, or spawn through a process known as "cyclogenesis" [16] and develop rapidly along and off the coast. The SC to VA coastal region is unique in its position adjacent to the warm waters of the Gulf Stream. Its alignment is favorable to the generation of offshore flow in response to winds typically associated with the incursion of cold, dry air from the north and west, often referred to as cold-air outbreaks (CAOs). The oceanographic setting in the region between SC and VA is such that the Gulf Stream Front (GSF) is omnipresent along the shelf-break between 32.5° and 35.5° N. During occasions of incursions of cold dry air streaming into the area from the north, local air temperatures can drop to between 0°C and 10°C, hence a CAO and the formation or genesis of an ETC. Cione et al. [15] determined that the mean path of the ETCs was from the SW to the NE and located about 30–50 km offshore so that the winds on the coastward side of the storm were from the NE to SW. As it occurs, the wind field present on the NC/SC coasts (**Figure 9**) was created by the passages of a series of ETCs.

The ETC winds would have driven offshore waters shoreward as depicted in **Figure 10** (from [3]). The basic dynamic balance relating the onshore-offshore component of the flow field in the surface layer of the water column is described by invoking conventional Ekman theory, in which the onshore-offshore (diabathic) mass flux, Mx, in the surface layer, D, is related to the alongshore (parabathic) component, Wy, of the total wind-stress vector or as expressed via: Mx = Wy/fD (1), where: Mx = Vertical Integral of r(udz) from 0 to D, the water surface to the depth D, the depth of the surface Ekman Layer, r is the water density, u is the diabathic or cross shelf water velocity, and f is the local Coriolis frequency. Note that this relationship states that the net transport of the wind-driven surface layer will be directly onshore if the wind is blowing from the northeast.

*On the Possibility of Non-Local and Local Oil Spills Striking the Shores of North Carolina... DOI: http://dx.doi.org/10.5772/intechopen.106679*

#### **Figure 9.**

*The mean winds for the month of October 1987 from NARR winds (https://www.emc.ncep.noaa. gov›mmb›rreanl›index.html).*

#### **Figure 10.**

*Wind blowing with the coast to the right, creating a surface Ekman transport toward the NC and SC coasts.*

The surface layer shown in **Figure 10** cartoon will be of the order of 5–25 m thick as a function of wind speed, vertical density gradient, and vertical velocity gradient on the NC and SC shelfs. Thus, a positive Wy (a northeastward wind, not shown) yields a positive Mx (surface layer transport to the southeast or offshore) and a negative Wy (a southwestward wind, as shown) yields a negative Mx (surface layer

transport to the northwest or onshore). From October 9 through November 9, the wind velocity vector at Cape Hatteras was directed toward the southwest to south sector with essentially no reversals. From October 12 to 18, the winds were especially strong toward the south-southwest. On November 8th, the winds switched to become northeastward to northward. Over the entire 19-day period, October 9–27, the mean Wy, alongshore wind-stress component, was about 0.75 dynes/cm<sup>2</sup> , which suggests a surface Ekman layer, D, of approximately 12–15 m thick and a vertically averaged onshore Ekman layer speed of approximately 6.3 cm/sec (or 5.5 km/day).

During the October 9–27 period, the distance water parcels and/or passive drifters would have moved across the shelf in the surface layer, which is about 105 km (65 mi). To calculate the total trajectory of a water parcel located in the surface layer requires that we integrate the vertically averaged (mean) onshore velocity component over the total time, with the wind fluctuating but remaining favorable for a shoreward moving surface water layer. For example, from October 13 to 16, the wind blew toward the SSW with an effective stress of between 1 and 3 dynes/cm2 , causing an onshore displacement of the surface layer of some 52 km (32 mi), about 13 km/day. By October 19, the surface layer had moved an additional 16 km (10 mi) shoreward driven by the SW winds of 0.3–0.5 dynes/cm2 . At this point, a passively drifting, buoyant particle imbedded in the GSF prior to October 8 would have traversed some 76 km (47 mi) across the shelf. To evaluate the possibility of this having occurred, we check the AVHRR imagery of October 19.

In **Figure 11**, the NOAA AVHRR SST map created by Dr. S. Baig (AOML) is shown. It appears that the entire Gulf Stream Front system of three filaments, which were present on October 9 (**Figure 5** lower panel), were subsequently mechanically driven

#### **Figure 11.**

*NOAA AVHRR image of the Gulf Stream and GSF on October 19 1987 (courtesy of Dr. S. Baig, AOML). The magenta coloring is employed to depict the Gulf Stream surface waters that have been mechanically driven by the atmospheric winds towards the coasts of the Carolinas. The green coloring is employed to depict the Gulf Stream and its Frontal Filaments.*

*On the Possibility of Non-Local and Local Oil Spills Striking the Shores of North Carolina... DOI: http://dx.doi.org/10.5772/intechopen.106679*

or rather, advected, onshore. The thermal frontal feature located in midshelf waters suggests that frontal waters, which 10 days previous were part of three filaments, now blanket the mid to outer shelf of Raleigh and Onslow Bay NC and Long Bay NC/ SC. Amazingly, the warm-water front appears to have maintained its general outline, essentially intact, from 10 days earlier. From a comparison of **Figure 5** (lower panel) and 10, it is clear that the warm water boundary defining the filament front has moved more than 70 km across the shelf.

From October 17 to 27, the surface layer was advected another 26 km shoreward. By the latter date, the first warm water parcels that were mechanically detached from the Gulf Stream 19 days earlier would have reached the shoreline of mid Onslow Bay NC. By October 31, the entire Onslow Bay coastline could have been invaded by filament waters. Then, from October 31 to November 7, the southwestward winds would have blown an additional water mass 18 km wide in the onshore/offshore direction and 13 m thick toward the coast. In all, a block of water 100 km wide in the longshore direction, 13 m thick in the vertical, and 163 km wide in the cross-shelf direction moved across Onslow Bay coastal waters over the 30-day period. This is depicted in **Figure 11** as the rosette-colored water masses. So, every day, on the average, a block of water 40 feet thick, 62 miles long, and 3.3 miles wide was advected toward the coast. At least 12 of those blocks reached the NC and SC beaches. That scenario, depicted inferentially by winds and SST images, raises the question: Could this scenario be numerically modeled to validate the data-based explanation of the physical dynamics? We address that next.

An additional numerical model experiment, employing the National Weather Service Weather Research Forecast Model (WRF), described in Skamarock et al. [17] and ROMS. If oil were to be drilled in NC waters, and an oil spill occurred between September and March, when the winds are predominately out of the North to East Quadrant (Weisberg and Pietrafesa, 1983), then the oil would likely reach NC coast as shown in **Figure 12a**. If the winds were absent, but a GSF was passing by (e.g., **Figure 4b**), particularly where these filaments nearly hit the Outer Banks near Cape Hatteras NC, then the oil spill would be carried as shown in **Figure 12b**. SC has a ban on oil drilling, so it was not considered in the latter two experiments.

#### **Figure 12.**

*The numerically modeled trajectory of surface oil spills in central Raleigh Bay NC: (a) left panel, shows the surface trajectory of a virtual oil spill driven by an ETC from the middle of the northern bay to the southwest into the lower bay; (b) right panel, shows the surface trajectory of a virtual oil spill offshore and then entrained into a passing GSF. Both wind driven (a) and ocean feature (b) events, project oil being carried to the NC coast. The Red Dots are employed in both panels to depict the trajectories of the oil spilled offshore and carried via surface currents.*
