OS31E-01 INVITED
Autonomous Glider Observations from the Oregon Shelf: The New Vagaries of Fortune
Since April 2006, we have maintained a small fleet of autonomous gliders, sampling cross-shelf transects of hydrography, currents and bio-optical properties within the central Oregon coastal ocean. The benefits of autonomous sampling are well known; the cost relative to comparable ship-time is minuscule, sampling is not curtailed by strong winds or large waves, and by maintaining a continuous presence in the ocean, the chances of observing intermittent, unpredictable (possibly important) processes are increased. For example, the ongoing observations off Oregon have found finescale structures composed of relatively warm, high- chlorophyll water subducting along the outcropping isopycnals of the seasonal upwelling front – consistent with the interactions between turbulent stresses and frontal dynamics seen at open ocean fronts. The subducting tongues of chlorophyll range from 2-20 m thick, can persist for more than 2 days, and extend 20 km or more offshore, offering a potentially new mechanism for thin-layer formation.
OS31E-02 INVITED
Lessons Learned From a Coordinated Observing and Modeling Program on the West Florida Shelf
A program of study that coordinates coastal ocean observations with numerical circulation models has been
in place for several years on the West Florida Continental Shelf (WFS). Applications to the coastal ocean
circulation and to multidisciplinary topics such as red tide evolution and fish larvae transport have led to a set
of lessons learned that may be of value for coastal ocean observing systems in general. These lessons will
be discussed along with selected findings on the synoptic, seasonal, and interannual variability of the broad,
gently sloping WFS. For instance, a wind and buoyancy related seasonal circulation exists, preferentially
upwelling in fall through spring months and conversely in summer months. Across shelf transport in the
bottom Ekman layer accounts for observations of ecological importance at the coast, and interannual
variations in the circulation can account for interannual variations in the WFS ecology, as demonstrated
using examples of red tides behaviors of 2005 through 2007.
http://ocgweb.marine.usf.edu/
OS31E-03
Coastal Radar Observations of Wind Induced Vortices in Microtidal Regimes
The coastal radar represents a new means of autonomous observation which comes as a complement to the traditional measures, and which allows to obtain maps of the surface circulation of the ocean. Two examples of radar campaign in the North Western Mediterranean are presented: in the Gulf of La Spezia (Ligurian coast, coverage 20km, resolution 300m) with a VHF radar and in the Gulf of Lions with a HF-radar (French coast, coverage 100km, resolution 3km). While the coastal circulation of the North-Western Mediterranean Sea is dominated by the cyclonic large Northern current, the microtidal onshelf circulation is mostly driven by intense forcings, i.e. strong wind conditions and river discharge. The sensitivity of the coastal flow to the wind forcing both of mesoscale and sub-mesoscale (Rossby radius scale) has been investigated using surface currents measurements and process oriented modelling. In the experiment in the Gulf of La Spezia, a comparison is made between the radar observations and data from surface drifter clusters, and a preliminary study of the circulation based on the analysis of the atmospheric forcing is presented. The results are highly encouraging for practical applications regarding transport and tracer spreading (biological or pollutant releases). In the Gulf of Lions case, the generation and driving mechanisms of observed sub-mesoscale eddies are analysed by means of a numerical model (Ifremer MARS3D) , from an idealized to a more realistic configuration. The relative influences and interaction of various possible forcings are analysed: wind, onshelf intrusion of the Northern slope current, Rhone river plume front, complex bathymetry and freshwater buoyancy effects.
OS31E-04
Combined effects of wind-driven upwelling and internal tide on the continental shelf
Internal tide on the continental shelf can be intermittent as a result of changing hydrographic conditions associated with wind-driven upwelling. In turn, the internal tide can affect transports associated with upwelling. To study these processes, simulations in an idealized, alongshore uniform set-up are performed utilizing the hydrostatic Regional Ocean Modeling System (ROMS) with conditions corresponding, as close as possible, to the central Oregon shelf. "Wind-only" (WO), "tide-only" (TO) and "tide-and-wind" (TW) solutions are compared, utilizing cases with constant upwelling favorable wind stress as well as with time-variable observed stress. The tide is forced by applying cross-shore barotropic flow at the offshore boundary with intensity sufficient to generate an internal tide with horizontal velocity amplitudes near observed values of 0.15 m/s. Variability in stratification associated with upwelling can affect barotropic-to-baroclinic energy conversion on the continental slope by changing the classification of the slope from nearly critical to supercritical, such that less barotropic tidal energy is converted to baroclinic and a larger fraction of the baroclinic energy is radiated into the open ocean. The internal tide affects the subinertial circulation, mostly through the changes in the bottom boundary layer variability, resulting in a larger bottom stress and weaker depth-averaged alongshore current in case TW compared to WO. The spatial variability of cross-shore volume transport is also affected. Increased vertical shear in the horizontal velocity resulting from the superposition of the upwelling jet and the internal tide results in intermittent patches of intensified turbulence in the mid-water column. Internal waves cause high-frequency variability in the turbulent kinetic energy in both the bottom and surface boundary layers, causing periodic restratification in the inner-shelf zone.
OS31E-05
Sea breeze driven ocean Poincare wave propagation and mixing near the critical latitude with application to the Gulf of Mexico
Near the vicinity of 30°N, the coincidence of the period of sea breeze forcing and inertial period of the ocean leads to a maximum near-inertial ocean response to sea breeze. In this paper, we investigate the generation and propagation of sea-breeze driven near-inertial motions and their effects on vertical mixing using a nonlinear numerical ocean model. Three-dimensional idealized simulations show different behaviors of ocean response to sea breeze on either side of 30°N. North of 30°N, the coastal oceanic response to sea breeze is trapped to the forcing area because the frequencies of these waves are less than the local inertial and free internal waves are inhibited. However, south of 30°N, this energy can propagate offshore as internal Poincare waves. Three-dimensional simulations with the real bathymetry of the Gulf of Mexico (GOM) confirms that basin-wide ocean response to coastal sea-breeze forcing is established in the form of Poincare waves. The near-zero group speed of Poincare waves limits the energy flux near the critical latitude, and therefore most of the near-inertial energy dissipates locally and promotes vertical mixing on the northern shelf of the GOM, consistent with previous observations. Enhanced vertical mixing by sea breeze can deepen the mixed layer on the northern shelf and may provide a mechanism to ventilate the frequently occurring near-bottom hypoxic waters in the summer months. Comparison of the three-dimensional model with two- and one- dimensional models shows some severe limitations of these simplified models of sea breeze simulations near the critical latitude.
OS31E-06
Simulation of Water Age and Residence Time in the New York Bight
Aiming at investigating the time scale of transporting biogeochemical tracers in the New York Bight (NYB), this work looks into the time scale associated with freshwater propagation in NYB. The Constituent-oriented Age and Residence-time Theory is applied in Regional Ocean Modeling System and then verified. Three-year mean age and two-year mean residence time simulations are carried out. Comparison between snapshots of modeled surface freshwater mean age and satellite measured channel ratio, an empirical proxy of age, shows agreement on the general patterns. Least square fit gives the first order estimation of the relationship between channel ratio and mean age. Time series show temporal and spatial variation in mean age, and seasonal averages demonstrate seasonality of surface mean age consistent with surface circulation. Correlation between surface mean age and wind shows major effects wind in different directions has on mean age. Time series of the mean residence time exhibits strong temporal fluctuation in the scale of days, and seasonal averages show seasonality in surface mean residence time, too. The surprising high value of mean residence time along the Long Island coast in spring and summer is caused by the reentry of previously exited water from the eastern boundary after wind changes direction. Correlation between mean residence time and wind shows major effects wind has on the time freshwater and tracers spend in the New York apex area. Results obtained here are very useful for coastal management and studies of local biogeochemical processes and larval dispersal given the ecological and economical importance of the New York Bay.
OS31E-07
Upwelling-driven buoyant plume dynamics in northern Monterey Bay
Northwesterly winds along the central California coastline lead to the occurrence of a strong upwelling plume that originates at Point Ano Nuevo and flows southward across the mouth of Monterey Bay. Within the bay, solar heating leads to the development of a buoyant water mass often called an upwelling shadow (Graham and Largier 1993), and a convergent front with temperature gradients of up to 7° C over one to a few hundred meters develop between cold upwelling plumes offshore, and warm nearshore waters. This nearshore convergent front is poorly resolved by remote sensing, due to the high frequency of regional cloud and fog cover as well as the failure of remote sensing algorithms near the coast. Oceanographic conditions were monitored using 4 cross-shore mooring arrays consisting of 12 total moorings and hydrographic surveys along a 10-km stretch of coast extending along the central California coast, from the shoreline out to the 60-m isobath during most of the summer upwelling season of 2007 (May – September) in order to evaluate the forcing mechanisms driving the location and movement of this nearshore front and its effects on biological communities in northern Monterey Bay. During the upwelling season, the upwelling shadow front behaved as a coastally- and surface-trapped buoyant feature moving up and down the coast up to several kilometers each day in response to wind forcing. Regional-scale wind relaxation events allowed the warm upwelling shadow waters to move northwestward along the coast, sometimes being completely flushed from the bay, and transporting phytoplankton, larvae, and potentially pollutants up the coast. The presence and location of the front along the coast was driven by regional wind forcing, leading to an alongshore pressure gradient, and local wind forcing (diurnal sea breeze) occurring on daily time scales.
OS31E-08
Estuarine Plume of a Small River Under Wind Forcing: A Case Study
It has been generally recognized that buoyant continental discharges from even relatively small rivers can have significant impacts on shelf circulations. Of various dynamical aspects of river plumes, perhaps the least well understood is the response of a surface-advected plume to wind stress variability. In this paper, we report a case study of a river plume in the Russian sector of the Black Sea. The Vulan River mouth in the northeastern part of the Sea is a non-tidal estuary with moderate discharge rates of about 10 m3/s on long-term average. Three fine scale hydrographic surveys (3-6 days each) of the shelf area adjacent to the estuary were undertaken by Shirshov Institute of Oceanology in 2006, 2007, and 2008. The measurements included CTD and ADCP profiling; water sampling for nutrients; and high resolution ultraviolet lidar probing for Chl-a and DOM; accompanied with continuous recording of meteorological data by a portable meteostation. We analyzed the observed variability of the plume in conjunction with the wind conditions. Furthermore, we applied POM to simulate the behavior of the plume for different scenarios of the wind forcing.