Leonard J. Pietrafesa

The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields

A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (∼5–15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (∼10–40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2–10 days. Frictional wind-driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along-shelf depth-averaged flow over the inner to midshelf is linear, and depth-averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoyancy-driven currents at ∼5 m (calculated from surface density) and along-shelf geostrophic wind-driven currents (computed from a depth-averaged linear model) are comparable in magnitude (∼10–25 cm s−1).

Patterns, Mechanisms and Approaches to the Study of Migrations of Estuarine-Dependent Fish Larvae and Juveniles

Five species (<10%) contribute over 85% of the annual commercial catch of finfish off North Carolina. All five spawn in winter near the Gulf Stream and migrate about 100 km to major inlets in the barrier islands and then up to another 100 km to juvenile estuarine nursery areas. The vertical distribution of pelagic larvae of menhaden (Brevoortia tyrannus) differs significantly from that of more benthic-oriented larvae such as spot (Leiostomus xanthurus), croaker (Micropogonias undulatus) and flounder (Paralichthys lethostigma and P. dentatus). Analysis of the shelf current regime suggests that differences in vertical distribution and season will subject larvae to markedly different currents, and therefore, different mechanisms for migration are required. Calculations of water movements using Ekman’s original transport equations are probably wrong in relatively shallow shelf waters subject to cross-shelf winds and density currents. Major differences probably exist, therefore, between the pelagic larvae of the west coast (e.g. Pacific sardine, Sardinops sagax) and estuarine dependent species of the east coast—both with respect to the importance of microscale processes (e.g. food patchiness) and drift. Predator avoidance is likely to be a more important determinant of migration pattern than has been suggested to date. Problems of determining migration vectors and mechanisms from highly variable larval and juvenile fish distributions are discussed. A research strategy dealing with the above is outlined, and testable implications of hypotheses are presented.

The intrusion of Gulf Stream water across the continental shelf due to topographically-induced upwelling

Abstract Summer bottom temperatures along the continental shelf between Cape Hatteras and Cape Canaveral are abnormally low in regions where isobaths diverge. The regions are north of capes and shoals, which force the flow of shelf water to change vorticity and induce upwelling. Gulf Stream Water intrudes across the bottom during summer to replace the upwelled water, and accounts for the colder and more stratified water over the northern Florida and the North Carolina shelves.

A numerical study of wave‐current interaction through surface and bottom stresses: Wind‐driven circulation in the South Atlantic Bight under uniform winds

The influences of surface waves on ocean currents in the coastal waters of the South Atlantic Bight are investigated by using a coupled wave-current modeling system. The ocean circulation model employed is the three-dimensional Princeton Ocean Model (POM), and the wave model invoked is an improved third-generation wave model (WAM). The coupling procedure between the POM and the WAM and the simulated coastal ocean circulation driven by uniform surface winds are presented. The simulated results show that wind waves can significantly affect coastal ocean currents not only through an enhancement of wind stress but also through a modification of bottom stress. Wave-induced wind stress increases the magnitude of currents both at the surface and near the seabed. On the other hand, wave-induced bottom stress weakens the currents both at the sea surface and near the seabed. Therefore the net effect of surface wind waves on currents depends on the relative importance of current modulations by wave-induced wind stress and bottom stress. The results further indicate that at a fixed location, the relative importance of wave-induced surface and bottom shear stresses in coastal ocean circulation depends on the surface wind field. For the constant wind cases considered in this study, the effect of wave-induced bottom stress is more significant in along-shore wind conditions than in cross-shore wind conditions.

Climatology and Interannual Variability of North Atlantic Hurricane Tracks

The spatial and temporal variability of North Atlantic hurricane tracks and its possible association with the annual hurricane landfall frequency along the U.S. East Coast are studied using principal component analysis (PCA) of hurricane track density function (HTDF). The results show that, in addition to the well-documented effects of the El Nino–Southern Oscillation (ENSO) and vertical wind shear (VWS), North Atlantic HTDF is strongly modulated by the dipole mode (DM) of Atlantic sea surface temperature (SST) as well as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). Specifically, it was found that Atlantic SST DM is the only index that is associated with all top three empirical orthogonal function (EOF) modes of the Atlantic HTDF. ENSO and tropical Atlantic VWS are significantly correlated with the first and the third EOF of the HTDF over the North Atlantic Ocean. The second EOF of North Atlantic HTDF, which represents the “zonal gradient” of North Atlantic hurricane track density, showed no significant correlation with ENSO or with tropical Atlantic VWS. Instead, it is associated with the Atlantic SST DM, and extratropical processes including NAO and AO. Since for a given hurricane season, the preferred hurricane track pattern, together with the overall basinwide hurricane activity, collectively determines the hurricane landfall frequency, the results provide a foundation for the construction of a statistical model that projects the annual number of hurricanes striking the eastern seaboard of the United States.

Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

Abstract During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10 1 km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area. Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m − d −1 and 75 mg m − , respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO 3 -supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a −1 h −1 ), high mean α (14 mg C mg Chl a −1 Ein −1 m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters. The results imply a seasonal (June to August) middle shelf production of 150 g C m −1 , about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.

A Real-Time Hurricane Surface Wind Forecasting Model: Formulation and Verification

A real-time hurricane wind forecast model is developed by 1) incorporating an asymmetric effect into the Holland hurricane wind model; 2) using the National Oceanic and Atmospheric Administration (NOAA)/ National Hurricane Center’s (NHC) hurricane forecast guidance for prognostic modeling; and 3) assimilating the National Data Buoy Center (NDBC) real-time buoy data into the model’s initial wind field. The method is validated using all 2003 and 2004 Atlantic and Gulf of Mexico hurricanes. The results show that 6- and 12-h forecast winds using the asymmetric hurricane wind model are statistically more accurate than using a symmetric wind model. Detailed case studies were conducted for four historical hurricanes, namely, Floyd (1999), Gordon (2000), Lily (2002), and Isabel (2003). Although the asymmetric model performed generally better than the symmetric model, the improvement in hurricane wind forecasts produced by the asymmetric model varied significantly for different storms. In some cases, optimizing the symmetric model using observations available at initial time and forecast mean radius of maximum wind can produce comparable wind accuracy measured in terms of rms error of wind speed. However, in order to describe the asymmetric structure of hurricane winds, an asymmetric model is needed.

Water mass linkages between the Middle and South Atlantic bights

Abstract Time and frequency domain analyses are used to relate coastal meteorological data with 7 years of daily surface temperature and salinity collected at three coastal light stations; offshore of the mouth of Chesapeake Bay, Virginia, on Diamond Shoals, at Cape Hatteras, North Carolina and on Frying Pan Shoals, off Cape Fear, North Carolina. Salinity fluctuations at Diamond Shoals are highly correlated with alongshore wind stress, implying wind driven advection of the front between Virginia Coastal Water (VCW) and Carolina Coastal Water (CCW) across Diamond Shoals. The data collected at Diamond Shoals indicate that more than half the time there is significant encroachment of Mid Atlantic Bight water into the South Atlantic Bight around Cape Hatteras, contrary to the notion that VCW is entirely entrained into the Gulf Stream. In fact, VCW can appear as far south as Frying Pan Shoals, thereby extending across the entire North Carolina Capes inner to mid shelf. Temperature and salinity time series also indicate that water masses overlying Diamond Shoals respond quickly to cross-shelf winds. Cross-shelf wind stress is significantly correlated with surface water temperature at Diamond Shoals, for periods between 2 and 12 days. Changes in temperature can be brought about by wind-driven cross-shelf circulation and by wind-induced upwelling. Seasurface temperature satellite (AVHRR) imagery taken during the SEEP II confirm these concepts.

The effects of alongshore variation in bottom topography on a boundary current—(topographically induced upwelling)

Abstract A theory which describes the constant f -plane flow of a steady inviscid baroclinic boundary current over a continental margin with a bathymetry that varies slowly in the alongshore but rapidly in the offshore directions is developed in the parameter regime (L D /L) 2 ≤ Ro ≪ 1 , where L D is the internal deformation radius, L the horizontal length scale, and Ro the Rossby number. To lowest order in the Rossby number the flow is along isobaths with speed q o = V u (h,z)|Vh|/α , where V u (h,z) is the upstream speed, α the upstream bottom slope at depth h , and Vh the bottom slope downstream at depth h . The lowest order flow produces a variation in the vertical component of relative vorticity along the isobath as the magnitude and direction of Vh vary in the downstream direction. The variation of vorticity requires a vertical as well as a cross-isobath flow at first order in the Rossby number. The first order vertical velocity is computed from the vorticity equation in terms of upstream conditions and downstream variations of the bathymetry. The density, pressure, and cross-isobath flow at first order in the Rossby number are then calculated. It is shown that in the cyclonic region of current ( d/dh(V u /α) > 0 ), if the isobaths diverge in the downstream direction ( (∂/∂s)|Vh| ), then upwelling and onshore flow occur. The theory is applied to the northeastern Florida shelf to explain bottom temperature observations.

Sediment resuspension over the continental shelf east of the Delmarva Peninsula

Abstract Resuspension of sediment over the continental shelf east of the Delmarva Peninsula has been examined using records of light-beam attenuation, near-bottom current speed and surface-wave height spectra collected during 1988 and 1989. These data give evidence of a factor of three variation in the bottom stress threshold required for sediment resuspension at the outer shelf. This appears to be related to resuspension history as the largest thresholds are observed after lengthy periods without resuspension. Episodes of shelf-wide sediment resuspension are evidenced only during very intense atmospheric storms. A 7-month-long set of records from the 90 m isobath show storm-induced sediment resuspension on only three occasions. The failure of storms of modest intensity to effect resuspension at the outer shelf is largely due the decline of surface-wave currents with depth. High-frequency currents, presumably due to internal waves, are shown to be an important agent in initiating sediment motion at the shelf edge. On a number of occasions, supertidal currents pushed the near-bottom current speed measured near the seafloor at the 131 m isobath above the estimated level required for sediment resuspension. Numerous clouds of turbid water detected by the light-beam attenuation records could not be attributed to local sediment resuspension. A probability analysis indicates that some, but not all, of these could have resulted from sediment resuspension by bottom fishing.