Historical Observations of Coastal Upwelling Events along the North Carolina Coastline

Jenelle J. Forde Saint Augustine’s College jjforde@hotmail.com

Bilan Howard North Carolina A&T State University bh015825@ncat.edu

Mario Oliver Virginia State University oliver_mario@hotmail.com

ABSTRACT. Coastal upwelling occurs when coast-parallel winds drive surface waters offshore causing cold, nutrient rich, water to displace warm surface water. If seasonally persistent, these events and their associated blooms of microorganisms can ultimately lead to a transient enrichment of local marine life, but also have the potential to produce subsequent depressed levels of dissolved oxygen due to increased rates of organic consumption and decay.  The anoxic conditions thus created can be very harmful to indigenous benthic biota. This study examines sea surface temperature image data to determine the presence of upwelling events along the coastline of North Carolina, specifically at Cape Hatteras and Duck Pier, and establishes correlation with archived wind data and topographic highs as revealed by the most current NOAA Hydrographic charts of the two locations. The wind direction and average wind speed were found to be good indicators of upwelling events; although, wind direction was found to have greater influence on the occurrence of coastal upwelling. The portion of the North Carolina coast, from Cape Hatteras to Duck Pier, is oriented slightly southeast to northwest; therefore, coastal upwelling were driven by seasonal winds out of the South to Southeast. There exist a highly significant correlation between upwelling events and persistent wind direction as well as with topographic highs, which are means to alert marine authorities, marine agricultural interests, and the public to the possible risk of anoxic events which might do considerable harm to t he local shell or sport fishing industry.

INTRODUCTION

   Observations of coastal upwelling of cold, nutrient rich, oceanic water have been reported along the Mid Atlantic Bight, specifically the coastlines of New Jersey and Long Island (Neuman, 1996, & Glenn et al. 1996). Analysis has indicated that upwelling along the coast of NJ and other coastal regions, wherein cold bottom water displaces warmer surface water, occur on the down-slope side of topographic “highs” or shallow ocean bottom ridge-lines (Glenn et al, 1996). The events themselves are apparently triggered by persistent strong surface winds blowing out of the South with a component parallel to the local shoreline.  In the case of the southwest to northeast running of the New Jersey coast, this is most typically seen during periods of southwesterly winds. In the case of the East to West oriented Long Island bight, upwelling seems to accompany winds out of the West.

   The portion of the North Carolina Bight South of the Chesapeake Bay entrance and northward of Cape Hatteras, is oriented slightly southeast to northwest therefore, coastal upwelling is expected to be driven by seasonal winds out of the South to Southeast.  If such events do occur, they might also occur downwind of topographic highs in similar to the upwelling events seen further north.  Another geographic similarity between the Jersey Shore and Northeast North Carolina coastline that may impact upwelling phenomena is the proximity to the large drainage plumes of the Hudson River and the Chesapeake Bay that flow out into continental shelf waters and may influence coastal upwelling dynamics.

   Coastal upwelling events dramatically affect water temperature and enhance biological productivity. Considerable changes in water temperature will affect survival of taxons. Enhanced biological productivity can lead to massive phytoplankton or algal blooms resulting in anoxic conditions. There is a lack of published data on the occurrence of upwelling events and of the biological implications along the North Carolina coast. The current study is the foundation for future research at this location. This study examines historical sea surface image data to determine the presence of local upwelling events and establishes correlation with archived wind data and bathymetric data and proposes that wind direction takes prevalence over wind speed in determining upwelling events.

BACKGROUND

Generally on the east coast of the United Sates, parallel-shore southerly winds initiate coastal upwelling during summer months. This wind   (Neuman, 1996).

FIGURE 1. The net flow of water to the right of the wind in the Northern Hemisphere and to the left of the wind in the Southern Hemisphere as a result of the Coriolis effect. EKMAN TRANSPORT

Along shore winds drives net movement of water offshore resulting in the displacement of cold, subsurface waters to the surface (Figure 2).

METHODOLOGY

   Level-three sea surface temperature image data for Cape Hatteras and Duck Pier were obtained from the Advanced Very High Resolution Radiometer (AVHRR) on board the NOAA-12, 15, 16, and 17 orbiting satellites. These data can be found on Rutgers University Coastal Ocean Observation Laboratory (COOL) website. LView Pro image processing software was used to determine the sea surface temperature approximately three kilometers East of Cape Hatteras and Duck Pier by matching the RGB color values of those points on the image to the RGB color values on the image’s temperature scale. Sea surface temperature data for Cape Hatteras and Duck Pier were also obtained from the National Data Buoy Center (NDBC) at Station 41025 and Station DUCN7, respectively. Water temperature data for Duck Pier were obtained from the Field Research Facility (FRF). These data were computed from 6 temperature sensors mounted at the End-of-Pier (EOP) in nominal water depths of 1, 2, 3, 4, 5, and 7 meters. Although the NOAA AVHRR sea surface temperature data was used as the primary source, additional sea surface temperature data were obtained to verify accuracy.

   Daily wind direction and average wind speed data for Cape Hatteras and Duck Pier were obtained from NOAA National Weather Service (NWS).  Wind data for the two locations were also obtained from NDBC. The focus of this study is the months of June, July, and the first fifteen days of August of 2000. Wind data were used to establish correlation between persistent wind direction and upwelling events by supporting or

repudiating alleged upwelling events that were solely based on sea surface temperature data. The duration, sea surface temperature, and wind data were recorded for each upwelling event.

   Finally, topographic data were obtained for Cape Hatteras and Duck Pier from NOAA Bathymetry charts #11555 and #12204, respectively.  Bathymetry charts were analyzed for ridge-lines and topographic highs.

RESULTS

Satellite-derived, sea surface temperature at each location were plotted for each day of the study period along with the angle off shoreline data, which was derived using the average wind speed data. All of this data was obtained from the State Climate Office of North Carolina (Figure 3). A comparison of the angle off the shoreline with sea surface temperature indicates that the two are directly associated with the occurrence of upwelling events; however, the upwelling events were more prone to occur near Duck Pier than Cape Hatteras. Based on the sea surface temperature and the angle off shoreline five upwelling events were identified during May, June, and August.

   The first upwelling event occurred on June 1, and was detected 2 km off the coast of Cape Hatteras (Figure 4a).This event was associated with three days in which the angle off shoreline was less than 90° degrees. Because the sea surface temperature for these three days ranged from 19°C- 21°C and the angle off shoreline did not change appreciably, it is believed that this upwelling lasted until June 3.

   An ideal angle off shoreline reading along with the sea surface temperature for June 26 indicated the occurrence of another possible upwelling event approximately 2 km east of Duck Pier (Figure 4b). Angle off the shoreline for June 26 was 37°, 47 ° on June 27, and 37° again on June 28.

   A low temperature reading of 19°C along with the angle off shoreline being indicated as 38° at Duck Pier indicated the occurrence of a third upwelling event on July 8, 2000 (Figure 4c). This event lasted from July 8 until July 11.  During this time there were no dramatic change in the sea surface temperature or the angle off shoreline helping to indicate the presence of this upwelling.

            The fourth upwelling event also occurred in the month of July at Duck Pier. It was thought to start on the 14 and lasted until July 16 (Figure 4d). During this time period the sea surface temperature was directly in the perfect range as to indicate the presence of an upwelling. The sea surface temperature was measured at 17ºC on July 15, one day after the presumed upwelling was believed to have started and on the day it was believed to have ended, July 16, the temperature was 16ºC.

            The final upwelling event thought to have occurred was on August 8-11 again at Duck Pier (Figure 4e). This event was characterized by particularly favorable winds. The winds recorded during this time averaged to be17ºC which is precisely in the range that is favorable for upwelling occurrences to occur. Along with this factor the angle off shoreline was less than 52º.  

DISCUSSION

The occurrence of upwelling events in June, July, and August correlated with wind direction and average wind speed data. There were stronger correlations between coastal upwelling events and wind direction data rather than with average wind speed data.

The frequency of coastal upwelling at each location was linked to the topographic data.

ACKNOWLEDGMENTS

             We would like to thank our mentor, Dr. Malcolm LeCompte, for his shared wisdom and guidance. We are thankful to Jennifer Bosh, Director of Satellite Operations at the Rutgers Institute of Marine and Coastal Sciences for supply us with information on sea surface temperatures.  We would also like to thank Karen Glow from the Federal Research Facility for providing us with national buoy data. This project was made possible by the dedicated and hardworking Dr. Linda Hayden and through funding by the National Oceanic and Atmospheric Administration (NOAA) as well as the Office of Naval Research (ONR).

LITERATURE CITED

Neuman, Melissa. 1996. Evidence of Upwelling along the New Jersey Coastline and the South Shore of Long Island, New York. Bull. N.J. Acad. Sci. 41(1): 7-13

Crowley, M.F. Glenn, S.M., Haidvogel, D.B. and Song, Y.T. 1996. Underwater Observatory Captures Coastal Upwelling Events off New Jersey. Trans. American Geophysical Union. Vol. 77(25): 233- 236.