OS32B-01 10:20h
The Western North Atlantic Shelfbreak Current System
Twelve years of historical hydrographic data, spanning the period 1990-2001, were analyzed to examine the alongstream evolution of the western North Atlantic shelfbreak front and current, following its path from Cape Farewell, Greenland, to its termination near Cape Hatteras, North Carolina. Over 700 synoptic sections were used to construct a mean three-dimensional description of the structure of the shelfbreak front, and quantify changes in properties, including frontal strength and grounding position. The hydrographic sections were also used to examine the evolution of the baroclinic velocity field and its associated volume transport. Our detailed view reveals that there are actually two fronts in the northern part of the domain - an inshore front located near the shelfbreak, and a deeper front centered in the core of Irminger Sea Water over the upper slope. However, we were only able to distinguish a single baroclinic velocity core. The deeper Irminger Front disappears with the erosion of the Irminger Water near the Tail of the Grand Banks while the inshore front is identifiable through the Middle Atlantic Bight. The two fronts vary differently with downstream distance but, regardless of the width of the shelf and the depth of the shelfbreak, the foot of the shelfbreak front remains within 20 km of the shelfbreak. This suggests that, although the position of the surface outcrop of the front varies substantially from region to region, the foot of the front is tied to the shelfbreak.
OS32B-02 10:35h
From the CMV Oleander Project: A Study of the Shelfbreak Front of the Middle Atlantic Bight From Long-term ADCP and Hydrographic Data
Utilizing the first decade of shipboard ADCP data as well as XBT and surface salinity data obtained from the CMV Oleander, this study is focused on the mean structure, and seasonal and interannual variability of the frontal zone at the edge of the shelf of the Middle Atlantic Bight. The early analysis showed that more than half the data in the frontal zone were influenced by warm core rings and that removing the confounding influence of the rings was vital if the true structure of the front was to emerge. From the culled data set of 128 transects of the front with sufficient coverage we have proceeded to generate a velocity description following the core of the frontal jet showing a maximum, surface intensified velocity of more than 0.25 ms-1, a vertical extent of roughly 80 m, a half-amplitude width of about 20 km and an alongshore transport of ~0.34 Sv. The maximum mean relative vorticity of the jet is 0.56*f. The alongshore jet is accompanied by a substantial surface intensified convergent flow that implies a maximum down-welling in the center of the jet of ~30 m/day. The seasonally the shelfbreak jet has its minimal velocities during the summer months, increasing to maximal velocities during the winter before decreasing agin in the spring. An interesting feature that emerges from the ADCP data is that while the shelfbreak frontal jet is usually assumed to consist of a single high-speed core, in fact, the jet often exhibits multiple high velocity extrema, the existence of which appears to undergo a seasonal progression.
OS32B-03 10:50h
Cold Event in the South Atlantic Bight During Summer of 2003: Anomalous Hydrographic and Atmospheric Conditions
Unusually cold seawater temperatures were observed along much of the U.S. eastern seaboard during the summer of 2003. Large scale wind patterns were upwelling favorable during this period. In the South Atlantic Bight, the presence of salinity stratification in late spring, due to larger than average river discharge, could have preconditioned the shelf waters to favor shoreward penetration of upwelled cold water. The resulting thermal stratification had significant effects on the dynamical and biological processes in the shelf. The characteristics of the upwelled water corresponded with water coming from the deeper part of the Gulf Stream. A set of modeling studies is developed using hydrographic characteristics observed during the summer of 2003 as well as realistic atmospheric forcing. The goal of the modeling studies is to quantify cross-shelf and along-shelf transport associated with the cold water event, and to assess the relative importance of different forcings: upwelling favorable winds, stratification, river discharge and position and intensity of the Gulf Stream. The latitudinal location of the source of cold water into the shelf through shelf-break frontal eddies interaction is discussed.
http://www.unc.edu/~hseim/sablam/summer03/index.html
OS32B-04 11:05h
The Heat Balance and the Breakdown of Density Stratification on the New England Inner Shelf During Fall 2003
We describe the regional evolution of stratification in the Martha's Vineyard Coastal Observatory (MVCO) region from July~15 to October~25,~2003. The temperature, salinity, and density stratification are highly variable on time scales from hours to months. We concentrate on synoptic and longer timescales, including the ``fall breakdown'' of stratification in the inner shelf region. We contrast the behavior of the stratification at different moorings in a cross-shore line stretching from the 12- to the 27-m isobath: near the coast the stratification is less strong than at the offshore site. Decreases in stratification are episodic, are often followed by increases in stratification, and are often associated with downwelling-favorable winds. We discuss the relative importance of surface cooling, wind-driven mixing, alongshore advection, and wind-driven cross-shore advection (upwelling and downwelling circulation) to the maintenance, variability, and eventual breakdown of the stratification in this shallow inner continental shelf area. We use temperature, salinity, and velocity data from six moorings in an alongshore and cross-shore array, along with surface heat flux and wind stress data, to estimate the alongshore and cross-shore temperature, salinity, and density gradients and also the heat storage, surface heating, and advective terms in the heat budget for each of three of the cross-shore moorings. We look at the temporal behavior of different terms in the heat budget as an aid for interpreting changes in the stratification. The time-integrated heat budget is far from a one-dimensional balance in this inner shelf region: the heat content at the 12- to 27-m sites is relatively constant in time, rather than increasing in a direct response to the surface heat flux. On time scales of weeks to months, the primary balance is between the surface and cross-shelf heat fluxes. On time scales of a few days, however, the variations in heat content and cross-shelf heat flux are large compared to variations in the surface heat flux.
OS32B-05 11:20h
Non-Linear Internal Tides Move Cool sub-Surface Waters From the mid-Shelf to the Beach
In the summer of 2001, a dense array of moorings measured currents, salinity and temperature over the entire water column for 4 months across the southern San Pedro shelf in southern California. An associated hydrographic program periodically monitored synoptic temperature and salinity distributions over the entire shelf. The measurement program was designed in part to determine the characteristics of the semidiurnal internal tide in the region. Initial results show that although the semidiurnal barotropic tidal currents were oriented alongshelf, the baroclinic, or internal, tidal currents were oriented cross-shelf. Approximately 6 times during the summer, the energy in the internal tides over the outer shelf was much larger than normal for the region. The energetic internal tides usually consisted of a series of amplified tidal pulses that lasted for a few days to more than a week. On average, the cross-shelf excursions associated with the energetic internal tides moved water and suspended material less than 2 km across the 7 km wide shelf. However, during three of the energetic internal tidal events, measurements from the moored array and hydrographic program showed that cool, subthermocline water from the mid-shelf was carried 2-3 km into the nearshore region. Subsequent mixing/shoaling processes transported these cool nearshore waters into the surfzone and onto the beach. Surf zone temperature measurements showed cool waters in the surfzone for the several days that the energetic internal tidal pulses were present in the nearshore. While near-bottom water from the shelf break was carried toward shore, this did not reach the nearshore. Subtidal currents over the shelf were always strongly downcoast when cool waters were observed at the beach. When the subtidal currents were not downcoast during a multiday energetic event, the internal tide pulses did not deliver cool water to the nearshore. Presumably, the uptilted thermocline associated with the downcoast subtidal flows is necessary to allow this process to exist.
OS32B-06 11:35h
Limitations in the Application of Tidal Harmonic Analysis to a Coastal Temperature and Velocity Field.
Tidal harmonic analysis is widely used to describe the regular oscillations observed in sea-level. The technique uses a least-squares approach to calculate the amplitude and phase of the variance at specific frequencies corresponding to astronomical, meteorological and other forcing mechanisms. Here tidal harmonic analysis is used to calculate the tidal constituents for each year of a 10-year timeseries of sea-level data from Santa Barbara on central California coast. Both the amplitude and phase of the constituents are stable between years at the dominant tidal frequencies. The calculated constituents can be used to reconstruct a predicted sea-level which describes 99% of the observed variance. This is consistent with the numerous other observations of sea-level in coastal regions. In contrast, tidal harmonic analysis is not successful in describing the tidal-band oscillations of temperature and velocity in the same coastal region. We calculate the diurnal- and semidiurnal-band variance in the temperature and velocity from 10-year timeseries measured at four locations in the Santa Barbara Channel region. Tidal harmonic analysis is then used to calculate the amplitude and phase at each of the tidal frequencies for each year of data. In the diurnal frequency band, the amplitude of the dominant temperature tidal constituents can vary between years by $O(10)$ and the associated phase by $150^{o}$. Reconstructing the timeseries from these constituents only accounts for between 15% and 50% of the observed temperature variance and between 30% and 60% of the observed velocity variance. Tidal harmonic analysis similarly fails to resolve the observed semi-diurnal temperature and velocity variance. We conclude that, unlike sea-level, the temperature and velocity fields in this region are not predictable using the tidal harmonic technique. We suggest that this limitation results from spectral broadening of the tidally-driven temperature and velocity variance into narrow-band peaks which cannot be effectively resolved as variance at a selection of specific frequencies.
OS32B-07 11:50h
Topographic Effects in a Nonlinear Model of Coastal Upwelling
Exact time dependent solutions to a nonlinear model of coastal upwelling have recently been discovered. Included in the model is cross-shore advection of density and momentum, continuous density stratification, and a geostrophic alongshore flow. The model solutions capture the drawing up of deep interior isopycnals into the surface layer in response to surface Ekman transport divergence. An equatorward velocity at the surface is driven by the Coriolis force accompanying the rapid near-surface onshore flow, and a deep poleward undercurrent develops only if there is a poleward pressure gradient present. We present new results on steady solutions of this model with variable bottom topography. A critical condition is found for the existence of the steady solution that depends on the topographic curvature. Thus, the presence of topography resembling a continental shelf break induces important differences in the qualitative behavior of the solutions. How such a topography affects the vertical structure of interior cross-shelf velocity and the associated alongshelf flow will be addressed.
OS32B-08 12:05h
Wind-driven coastal circulation response to the presence of a shallow submarine bank
The coastal upwelling region off Oregon is studied using both observations and numerical modeling. Repeated mesoscale surveys of waters over the shelf and slope were conducted during spring and summer of 2001 to study the spatial structure of the velocity and hydrographic fields. The results show that the presence of a shallow submarine bank (Heceta Bank) increases the cross-isobath transport, as the coastal upwelling jet is driven offshore leading to separation. The wind stress plays an important role in the separation, as it speeds up the currents making the jet more inertial. Comparisons between the wind stress and the position of the upwelling front derived from satellite images support that idea. Bank geometry and stratification are also important in the jet separation process. A three-dimensional numerical model (POM) is used to explore the relative importance of these parameters. Preliminary results using idealized bank topography suggest that both the Rossby number and the Burger number (formed using the radius of curvature of the topography as the length scale) are important. For strong wind conditions, maximum separation occurs for Burger numbers between 0.3 and 0.7. An increase in the Rossby number also leads to more separation. Future simulations will explore variations in other parameters/geometries, including the use of realistic bottom topography for Oregon's shelf and slope.