Dana K. Savidge

A numerical study of the continental shelf circulation of the U.S. South Atlantic Bight during the autumn of 1987

Abstract The autumn circulation on the inner- and mid-shelf of the U.S. South Atlantic Bight (SAB) is examined numerically. Using data collected in 1987 during the Fall Experiment (FLEX) the alongshore structure of the currents and the coastal sea level fluctuations were found to be correlated to local winds which were strong and persistently northeasterly. The observed inshore distribution of freshwater during FLEX, characterized by the presence of a coastal front confined to the coast inside the 25 m isobath, reflects the local autumn discharge subjected to strong and persistent downwelling winds. The freshwater signal found outside the 25 m isobath is suggested to be the previous summers discharge advected northward by the summer winds subsequently returning south forced by the autumn winds.

The Hatteras Front: August 2004 velocity and density structure

The Hatteras Front is a persistent mesoscale cross-shelf oriented front off Cape Hatteras, North Carolina. It is the boundary between relatively cool, fresh Mid-Atlantic Bight shelf waters and warmer, saltier shelf waters of the South Atlantic Bight, which both converge along-shelf upon Cape Hatteras year round. The Frontal Interaction Near Cape Hatteras (FINCH) project was conducted in 2004-2005 to intensively sample the Hatteras Front with shipboard ADCP and undulating towed CTD. This paper documents velocity and density structures associated with the cross-shelf oriented zone of Hatteras Front during the August 2004 field season. Property gradients across the Hatteras Front are large, with temperature (T) and salinity (S) differences of ∼4-6°C, 2-5 psu, respectively over distances of 1-2 km. The T and S are not completely compensating, and a strong density (p) gradient also exists, with Ap of∼2 kg/m 3 across a gentler 10 km wide front. The density gradient results in a steric sea-level height gradient of ∼ 1-2 cm across the Front, which is in approximate geostrophic balance with a surface intensified jet, directed shoreward along the cross-shelf oriented Front. The velocity is sheared with depth at 3.0 × 10 -2 to 5.0 x 10- 2 s -1 in the upper 5 m of the jet; a rate consistent with the density gradient according to the thermal wind relationship. Shoreward transport of ∼4.8 x 104 m 3 /s results from the surface intensified jet. The structure of the velocity field associated with the Hatteras Front resembles that of a slope-controlled buoyant plume, as described by Lentz and Helfrich (2002). Velocity and density structures are similar during both advancing (southwestward) and retreating (northeastward) motion of the Front.

Numerical Investigation of Coastal Circulation Dynamics near Cape Hatteras, North Carolina in January, 2005.

A realistic regional ocean model is used to hindcast and diagnose coastal circulation variability near Cape Hatteras, North Carolina, in January 2005. Strong extratropical winter storms passed through the area during the second half of the month (January 15–31), leading to significantly different circulation conditions compared to those during the first half of the month (January 1–14). Model results were validated against sea level, temperature, salinity, and velocity observations. Analyses of along-shelf and cross-shelf transport, momentum, and kinetic energy balances were further performed to investigate circulation dynamics near Cape Hatteras. Our results show that during the strong winter storm period, both along-shelf (southward) and cross-shelf (seaward) transport increased significantly, mainly due to increases in geostrophic velocity associated with coastal sea level setup. In terms of momentum balance, the wind stress was mainly balanced by bottom friction. During the first half of month, the dominant kinetic energy (KE) balance on the shelf was between the time rate of KE change and the pressure work, whereas during the stormy second half of month, the main shelf KE balance was achieved between wind stress work and dissipation.

CASPER: Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research

CapsuleCASPER objective is to improve our capability to characterize the propagation of radio frequency (RF) signals through the marine atmosphere with coordinated efforts in data collection, data analyses, and modeling of the air-sea interaction processes, refractive environment, and RF propagation.

Airborne Remote Sensing of the Upper Ocean Turbulence during CASPER-East

This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established 63 km offshore by a large multi-platform CASPER-East experiment. The supersite was equipped with a range of in situ instruments resolving air-sea interface and underwater properties, of which a bottom-mounted acoustic Doppler current profiler was used extensively in this paper for the purposes of airborne current retrieval validation and interpretation. A series of water-tracing dye releases took place in coordination with aircraft overpasses, enabling dye plume velocimetry over 100 m to 10 km spatial scales. Similar scales were resolved by a Multichannel Synthetic Aperture Radar, which resolved a swath of instantaneous surface velocities (wave and current) with 10 m resolution and 5 cm/s accuracy. Details of the skin temperature variability imprinted by the upper ocean turbulence were revealed in 1–14,000 m range of spatial scales by a mid-wave infrared camera. Combined, these methodologies provide a unique insight into the complex spatial structure of the upper ocean turbulence on a previously under-resolved range of spatial scales from meters to kilometers. However, much attention in this paper is dedicated to quantifying and understanding uncertainties and ambiguities associated with these remote sensing methodologies, especially regarding the smallest resolvable turbulent scales and reference depths of retrieved currents.

Separation of Short Time Series of Currents into “Fluctuations,” “Tides,” and “Mean” Flow

AbstractWith standard low-frequency velocity data from current meter moorings, pressure gradient–driven mean flow is determined by low-pass filtering, while tides are estimated by fitting tidal constituents, with accuracy and numbers of constituents determined by record length. With the advent of higher-frequency measurements from cabled coastal ocean observatories, current data also include supertidal variability (fluctuation) associated with a variety of turbulent and internal wave processes. To examine the relationships of such fluctuations to variability of the tides and/or potentially time-variable mean flows within which they are embedded, it is highly desirable to find a method whereby these flow components can be separated over relatively short periods of intensified event-scale forcing. A method is presented that first isolates “fluctuations” and then separates the remaining longer time-scale variability into “tides” and a remaining “mean,” without recourse to extraction of a fitted tide with the...

VHF radar measurements of flow in a salt marsh creek

A VHF ocean radar system was deployed for one month in a salt marsh in coastal Georgia, USA. In this environment, radar backscatter from the marsh grass is highly attenuated and scatter from water in the narrow creeks has a broad band of Doppler shift due to the horizontal variation in water speed across the channel. By assuming a monotonic velocity profile from the edge to the middle of the channel, a theoretical relationship is derived to link the broadening of Bragg energy in the echo spectrum to varying water speed. A methodology is developed to derive the cross-creek profile of velocity variation in the main channel of the salt marsh.