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 ocean's surface

Cyclogenesis in the deep ocean beneath the Gulf Stream 2. Dynamics

Strong cyclones in the deep ocean beneath the Gulf Stream have been observed during the period June 13, 1988, to August 7, 1990, near 68°W, 37°N in data from the Synoptic Ocean Prediction (SYNOP) Experiment. These cyclones developed in association with the evolution of large amplitude, quasi-stationary meander troughs in the Gulf Stream. It is likely that baroclinic instability is responsible for cyclone spin-up. The dynamical analysis of these cyclones indicates the process is analogous with atmospheric cyclogenesis from the perspectives of divergence and conservation of potential vorticity, but not in terms of the density field evolution. Large positive vertical velocities in the thermocline over developing low pressure centers at 3500 m are consistent with convergence at depth and divergence in the upper ocean, and with stretching of the lower water column and shortening of the upper water column. The stretching of the lower water column accounts for the generation of positive relative vorticity there. However, the evolution of the density field in the oceanic case does not resemble the atmospheric case. In the atmosphere, density field adjustments in the air column above the low pressure center at the Earth's surface are in the correct sense to account for decreasing pressure there. In the ocean, density field adjustments in the water column fail to account for the developing low pressure centers, so sea surface height depressions must be responsible. These depressions must have an approximate magnitude of 0.5 m depth over a 250 km horizontal extent (the cyclone's diameter)

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

CASPER coupled air-sea processes and electromagnetic ducting research

The Coupled Air\u2013Sea Processes and Electromagnetic Ducting Research (CASPER) project aims to better quantify atmospheric effects on the propagation of radar and communication signals in the marine environment. Such effects are associated with vertical gradients of temperature and water vapor in the marine atmospheric surface layer (MASL) and in the capping inversion of the marine atmospheric boundary layer (MABL), as well as the horizontal variations of these vertical gradients. CASPER field measurements emphasized simultaneous characterization of electromagnetic (EM) wave propagation, the propagation environment, and the physical processes that gave rise to the measured refractivity conditions. CASPER modeling efforts utilized state-of-the-art large-eddy simulations (LESs) with a dynamically coupled MASL and phase-resolved ocean surface waves. CASPER-East was the first of two planned field campaigns, conducted in October and November 2015 offshore of Duck, North Carolina. This article highlights the scientific motivations and objectives of CASPER and provides an overview of the CASPER-East field campaign. The CASPER-East sampling strategy enabled us to obtain EM wave propagation loss as well as concurrent environmental refractive conditions along the propagation path. This article highlights the initial results from this sampling strategy showing the range-dependent propagation loss, the atmospheric and upper-oceanic variability along the propagation range, and the MASL thermodynamic profiles measured during CASPER-East