John M. Bane

Highlights of Coastal Waves 1996

Some of the highlights of an experiment designed to study coastal atmospheric phenomena along the California coast (Coastal Waves 1996 experiment) are described. This study was designed to address ...

Offbank transport of carbonate sands along open, leeward bank margins: Northern bahamas

Abstract Vigorous offbank transport of carbonate sands occurs along the west-facing, leeward, open margins of Little and Great Bahama Bank. Large, offbank-oriented sand waves, thick sand bodies covering reefs, and shallow-water sands (mostly non-skeletal) obtained from adjacent deep flanks, all demonstrate the existence of this shallow-to-deep pathway of sediment transport. A variety of data suggests that transport occurs during storms and not during normal tidal-current fluctuations. An abundance of composite grains as well as vertical sequences of submarine cemented horizons (seen via seismic profiling within the marginal sand bodies) indicate cyclic periods of relative quiescence followed by moments of intense sediment movement. Current-meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle. These flows coupled with higher-frequency bottom currents (generated by surface gravity waves) winnow the sands but do not provide significant net offbank transport. Offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea-level highstands. Each sea-level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs, covered by a thick non-skeletal calcarenite, capped by a subaerially exposed, diagenetically altered surface. Shallow-water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts.

Gulf Stream path and thermocline structure near 74°W and 68°W

The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W modred observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime; small amplitude propagating and growing meanders characterized the variability there. The Central Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level streamfunctions, all with mesoscale resolution. Near 70°W the path envelope exhibited a relative node, confined within a 40-km band 55% of the time. Near 68°W the path envelope was over 3 times as wide, due to several elongated (“steep”) meander troughs and relatively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68°W, steepened, and persisted for one to several months. Two cases evolved into “S-shaped” paths and subsequently formed rings. Even the time-averaged fields showed a small trough in the mean path and thermocline structure. Whereas meanders of 20- to 60-day periods had similar spectral levels throughout 70°–67°W, meanders with long periods (>85 day) accounted for the local minimum in variance at 70°W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s−1) abyssal eddies; the time-averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough. The cross-stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form.

Coastally Trapped Wind Reversals: Progress toward Understanding

Abstract Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm—season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993—98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advanc...

Atmospheric forcing of the Oregon coastal ocean during the 2001 upwelling season

larger-scale and longer-term conditions. Southward wind stresses of 0.05� 0.1 N m � 2 occurred roughly 75% of the time, with a sustained period of dominantly southward stress from mid-June through July. Wind variations were correlated with variations in the largescale Aleutian Low and North Pacific High pressure centers; correlations with the continental Thermal Low were small. Intraseasonal oscillations in alongshore wind stress (periods near 20 days) correlated with the north-south position of the jet stream. These stress oscillations drove 20 day oscillations in upper ocean temperature, with a lag of roughly 5 days for maximum correlation and amplitudes near 4� C. The sum of sensible and latent air-sea heat fluxes was generally into the atmosphere through June, then weakly into the ocean thereafter, with fluctuations on synoptic timescales. Semidiurnal fluctuations in surface air temperature were observed at two northern moorings, apparently forced indirectly by nonlinear internal ocean tides. The diurnal cycle of wind stress was similar for both southward and northward wind conditions, with the diurnal alongshore fluctuation southward in the evening and northward in the morning. During southward winds the marine atmospheric boundary layer (MABL) was typically defined clearly by a strong temperature inversion, and a shallow stable internal boundary layer often formed within the MABL over cool upwelled waters, with surface air temperature roughly 1� C lower inshore than offshore. During northward winds, essentially no low-level temperature stratification was observed.

Observations and Analysis of the 10–11 June 1994 Coastally Trapped Disturbance

Abstract A coastally trapped disturbance (CTD), characterized by southerly flow at the surface on 10–11 June 1994, was observed from the California Bight to Bodega Bay during a field experiment along the California coast. (North–south approximates the coast-parallel direction.) Data from a special observational network of wind profilers, radio acoustic sounding systems, special surface data, balloon ascents, and a research aircraft were used with satellite and synoptic data to explore both the CTD structure and the regional-scale changes before the event. The disruption of the climatological northerly flow along the central California coast, which preconditioned the area for the development of a CTD, began with the eastward movement of a surface high into Washington and Oregon and the amplification of a thermal low in northern California. As with most CTDs in the region, this occurred over the 2–3 days preceding the CTD’s initiation. These large-scale changes caused westward advection of warm continental ...

Gulf Stream flow field and events near 68°W

The SYNoptic Ocean Prediction (SYNOP) experiment was designed to provide an accurate understanding of the energetic mesoscale processes in the Gulf Stream. The Central Array measured velocity and temperature throughout the water column, with horizontal extent large enough nearly to span the meander envelope and Eulerian mean structure of the jet at 68°W. The 55- to 70-km mooring spacing resolved mesoscale eddy interactions with the Gulf Stream, and the 26-month duration allowed stable estimation of long-term mean fields. Six steep meander troughs propagated into or developed within the array, each lasting around 30-60 days, thus impressing a small mean trough near 68°W in the predominantly eastward currents at jet level (1000 m and above). At the deep level (3500m) the mean flow was southwest at the shoreward sites shallower than 4300 m, but it flowed cyclonically around a mean low-pressure anomaly affecting all the deeper offshore sites. The eddy kinetic energy per unit mass (E K ) decreased by a factor of about 2.5 with each depth increment from 400 to 700 to 1000 m but was only a factor of 2 smaller at 3500 m than at 1000 m. Values of E K in the upper central jet (400 m) were 100 to 230 mJ kg -1 and were 4-13 mJ kg -1 at 3500 m. Overall, E K in the upper 1000m at 68°W was higher than previously published values at 55°W. Two extended case studies of meander propagation through the array demonstrate the development and intensification of deep cyclonic and anticyclonic flows beneath the Gulf Stream. The cyclonic flow at 3500m, associated with amplifying meander troughs, often exceeded 0.35 ms -1 , which was much larger than the typical 0.05 ms -1 deep mean velocities.

Diagnosing the warming of the Northeastern U.S. Coastal Ocean in 2012: A linkage between the atmospheric jet stream variability and ocean response

[1] The temperature in the coastal ocean off the northeastern U.S. during the first half of 2012 was anomalously warm, and this resulted in major impacts on the marine ecosystem and commercial fisheries. Understanding the spatiotemporal characteristics of the warming and its underlying dynamical processes is important for improving ecosystem management. Here, we show that the warming in the first half of 2012 was systematic from the Gulf of Maine to Cape Hatteras. Moreover, the warm anomalies extended through the water column, and the local temperature change of shelf water in the Middle Atlantic Bight was largely balanced by the atmospheric heat flux. The anomalous atmospheric jet stream position induced smaller heat loss from the ocean and caused a much slower cooling rate in late autumn and early winter of 2011–2012. Strong jet stream intraseasonal oscillations in the first half of 2012 systematically increased the warm anomalies over the continental shelf. Despite the importance of advection in prior northeastern U.S. continental shelf interannual temperature anomalies, our analyses show that much of the 2012 warming event was attributed to local warming from the atmosphere.

Wind and Gulf Stream influences on along‐shelf transport and off‐shelf export at Cape Hatteras, North Carolina

Along-shelf transports across three cross-shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992–1994. The along-shelf convergence has been used to infer off-shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along-shelf transport variability is primarily wind-driven and highly correlated with sea level fluctuations at the coast. Both winds and along-shelf transport exhibit a near-annual period variability. Along-shelf transport is not well correlated with Gulf Stream offshore position. Along-shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along-shelf convergence by hours to a few days. Long-period variability of 14–16 months and 1–3 months is apparent in both Gulf Stream position and transport convergence. Variability in along-shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along-shelf transport convergence is that the Gulf Stream is directly influencing cross-shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along-shelf divergence and shoreward cross-shelf transport exist (∼10% of the time just north of Cape Hatteras and ∼34% of the time just south of Cape Hatteras during episodes of up to 3–8 days duration). Implied onshore flows of a few cm s−1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along-shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf.

Cyclogenesis in the deep ocean beneath the Gulf Stream: 1. Description

One of the primary scientific results of the Synoptic Ocean Prediction (SYNOP) observational program was the discovery of strong cyclones in the deep ocean beneath the large amplitude Gulf Stream meander troughs that routinely form at about 68°W. These strong well-organized cyclones extend to at least 3500 m below the sea surface and are an important component of the overall dynamical variability of the Gulf Stream and adjacent deep waters. Typically, a small amplitude Gulf Stream meander “stalls” near 68°W and begins to amplify. As the amplitude of troughs in the Gulf Stream jet increases, the currents at 3500 m strengthen and turn, forming a cyclonic circulation pattern. During SYNOP, six well-defined instances of meander trough amplification and deep cyclogenesis occurred. The cyclones were characterized by strong swirl speeds (up to 0.5 m s−1) and were long-lived (typically lasting 6–9 weeks) frequent occurrences (present 35% of the time during the 25 month deployment period). The structure of the cyclones at 3500 m is characterized by increasing velocity from the cyclone center out to some radius of maximum velocity and decreasing velocity beyond that radius. This structure was robust over the lifetime of an event and from event to event. Cyclone radius and the radius to maximum velocity were consistently ≃130 km and ≃55 km, respectively. Evidence of cyclones at upper measurement levels and the low vertical shear values apparent in the deep water below the thermocline indicate that the cyclones extended throughout the entire water column; from the benthic boundary layer, through the thermocline, and to the oceans surface.