David S. Ullman

Satellite‐derived sea surface temperature fronts on the continental shelf off the northeast U.S. coast

The distribution and variability of sea surface temperature (SST) fronts, over the shelf and slope along the east coast of North America from Cape Hatteras to Nova Scotia, are studied using a 12 year time series (1985-1996) of advanced very high resolution radiometer (AVHRR) images. After the masking of cloud- contaminated pixels, an edge-detection algorithm identifies surface temperature fronts in each image. Maps of the seasonal probability of detecting a front indicate substantial spatial and temporal variability in the occurrence of SST fronts. Over the continental shelf south of New England during spring through autumn, surface fronts are rare and observed only in scattered locations. North of Nantucket Shoals, both tidal mixing fronts and fronts associated with the Eastern Maine Coastal Current occur during the summer. A major finding of this study is the observation of fronts in winter over the inner and middle shelf from Cape Hatteras to the Bay of Fundy. These fronts, peaking during January-March, are characterized by cold SST on their inshore (shallow) side and appear to result from the influence of surface cooling on shallow nearshore waters. The shelfbreak front is found to vary strongly with season, being detected most frequently during spring and autumn. South of Hudson Canyon, it essentially disappears during the summer, while from Hudson Canyon to Northeast Channel, it weakens during summer but nevertheless remains detectable in SST.

Evaluation of Front Detection Methods for Satellite-Derived SST Data Using In Situ Observations

Sea surface temperature (SST) fronts detected in Advanced Very High Resolution Radiometer (AVHRR) data using automated edge-detection algorithms were compared to fronts found in continuous measurements of SST made aboard a ship of opportunity. Two histograms (a single-image and a multi-image method) and one gradient algorithm were tested for the occurrence of two types of errors: (a) the detection of false fronts and (b) the failure to detect fronts observed in the in situ data. False front error rates were lower for the histogram methods (27%‐28%) than for the gradient method (45%). Considering only AVHRR fronts for which the SST gradient along the ship track was greater than 0.18 Ck m 21, error rates drop to 14% for the histogram methods and 29% for the gradient method. Missed front error rates were lower using the gradient method (16%) than the histogram methods (30%). This error rate drops significantly for the histogram methods (5%‐10%) if fronts associated with small-scale SST features (,10 km) are omitted from the comparison. These results suggest that frontal climatologies developed from the application of automated edge-detection methods to long time series of AVHRR images provide acceptably accurate statistics on front occurrence.

Southern Ocean Gas Exchange Experiment: Setting the stage

[1] The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of U.S.‐led open ocean process studies aimed at improving the quantification of gas transfer velocities and air‐sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively stable water masses selected for large DpCO2 took place in the southwest Atlantic sector of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous surface ocean and atmospheric monitoring, and autonomous drifting instruments to study the evolution of chemical and biological properties over the course of the experiment. CO2 and DMS fluxes were directly measured in the marine air boundary layer with micrometeorological techniques, and physical, chemical, and biological processes controlling air‐sea fluxes were quantified with measurements in the upper ocean and marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered during the tracer patch observations, providing additional data to constrain wind speed/gas exchange parameterizations. In this paper, we set the stage for the experiment by detailing the hydrographic observations during the site surveys and tracer patch occupations that form the underpinning of observations presented in the SO GasEx special section. Particular consideration is given to the mixed layer depth as this is a critical variable for estimates of fluxes and biogeochemical transformations based on mixed layer budgets.

Continental shelf surface thermal fronts in winter off the northeast US coast

Analysis of 12 years (1985–1996) of sea surface temperature (SST) imagery covering the shelf and slope off the northeast US coast has revealed the presence of persistent fronts in winter over the middle shelf. Ongoing work shows that similar fronts occur in other coastal regions, suggesting that these fronts are of more than regional interest. The satellite data from the US east coast make clear that these fronts, which are found with highest frequency in the vicinity of the 50 m isobath, separate cool water inshore from warmer outer shelf water. The temperature step across the fronts, a measure of the frontal strength, is negatively correlated with estimates of heat flux (latent plus sensible) indicating that winter surface cooling plays an important role in their formation. Although, generally, the fronts are oriented parallel to the bottom topography, the region around Nantucket Shoals is a location where fronts oriented in the crossisobath direction occur more often than elsewhere, suggesting that this area is one of enhanced crossisobath flow. The cross-isobath flow manifests itself in the form of cold tongues extending south and west from the shallowest part of the shoals. Historical hydrographic data from the shelf in winter indicate that the fronts typically have a salinity signal, with water on the inshore sides of the fronts having lower salinity and resulting lower density due to the controlling influence of salinity on density. Weak vertical stratification is often present inshore of the fronts suggesting that the fronts may represent the offshore edge of a freshened coastal zone. # 2001 Elsevier Science Ltd. All rights reserved.

Surface Temperature Fronts in the Great Lakes

The distribution and variability of surface temperature fronts in the Great Lakes is studied using an 11 year time series (1985 to 1995) of Advanced Very High Resolution Radiometer (AVHRR) images. After the removal of cloud contaminated pixels as well as those potentially ice covered, an edge detection algorithm identifies surface temperature fronts in each image. The probability of detecting a front in the Great Lakes is highly variable from lake to lake as well as seasonally. Fronts during the winter months generally have cold water on their shallow side and appear in progressively deeper water as winter progresses. The properties of the winter fronts are consistent with a formation mechanism involving strong surface cooling over a sloping bottom. A spring transition occurs, whereby these fronts disappear and are replaced by nearshore thermal bar fronts with warm water found on their shallow side. The time of this transition depends upon the lake, occurring first in Lake Erie, next in Lakes Ontario, Huron, and Michigan, and finally in Lake Superior. These fronts are also observed to progress slowly into deeper water as nearshore waters warm. The classical thermal bar front (4°C) is seen in all of the lakes but most of the fronts during the spring peak in frontal activity have temperatures greater than 4°C, suggesting that the thermal bar marks the offshore edge of a more extensive frontal zone. Frontal activity declines during summer in all lakes except Superior.

Direct Observations of Along-Isopycnal Upwelling and Diapycnal Velocity at a Shelfbreak Front*

Abstract By mapping the three-dimensional density field while simultaneously tracking a subsurface, isopycnal float, direct observations of upwelling along a shelfbreak front were made on the southern flank of Georges Bank. The thermohaline and bio-optical fields were mapped using a towed undulating vehicle, and horizontal velocity was measured with a shipboard acoustic Doppler current profiler. A subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship. The float was released near the foot of the shelfbreak front (95–100-m isobath) and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2-day deployment. The floats average westward velocity was 0.09 m s−1, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m s−1. The float measured strong downward vertical velocities (in excess of 0.02 m s−1) associated with propagation of internal tidal solibores in...

Geographic window sizes applied to remote sensing sea surface temperature front detection

Abstract The effects of using a geographic window size with an existing edge-detection technique for the detection of thermal fronts in sea surface temperature (SST) imagery are investigated. The size of a geographic window is not constant but instead is determined by the correlation of the data surrounding the windows central point. Using this approach instead of a fixed window size, the investigation windows are optimized for the whole image, providing more reliable detection of edges within the windows. The new algorithm was run on several SST images from southern Lake Michigan and compared to runs of the original algorithm and a modification of the original algorithm optimized for this region. The results show that the geographic window improves edge detection most in the nearshore regions and to a lesser extent in the offshore regions.

The front on the Northern Flank of Georges Bank in spring: 2. Cross‐frontal fluxes and mixing

buoyancy fluxes are downward and most intense, reaching values of 5 � 10 � 7 W/kg, near the bottom at the edge of the bank and decrease both on- and off-bank. Horizontal, crossbank buoyancy fluxes are partitioned into mean, tidal pumping, and nontidal eddy components and are computed as a function of cross-isobath/vertical position by averaging in the along-isobath direction. The tidal pumping component is dominant over most of the cross-bank section with a peak value of � 1 � 10 � 4 W/kg, directed off-bank near the bank edge. A diagnosed tidal vertical velocity field is used with mean buoyancy gradients to compute the along-isopycnal skew flux. The horizontal component of this skew flux has similar spatial structure and magnitude to that of the observed tidal pumping flux. The divergent component of the skew flux, at depths above the bottom boundary layer, appears to be convergent north of the bank edge and divergent at the bank edge, suggesting that tidally driven advective processes drive buoyant bank water downward and off-bank at mid-depth and force the upwelling of deep, dense water near the bottom at the bank edge. INDEX TERMS: 4528 Oceanography: Physical: Fronts and jets; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4560 Oceanography: Physical: Surface waves and tides (1255); 4544 Oceanography: Physical: Internal and inertial waves; KEYWORDS: Georges Bank, tidal mixing front, GLOBEC, internal tide, skew eddy flux, turbulent mixing

Processing of Underway CTD Data

AbstractA processing methodology for computation of accurate salinity from measurements with an underway CTD (UCTD) is presented. The UCTD is a rapidly profiling sensor package lacking a pump that relies on instrument motion to produce flow through the conductivity cell. With variable instrument descent rate, the flow through the cell is not constant, and this has important implications for the processing. As expected, the misalignment of the raw temperature and conductivity is found to be a function of the instrument descent rate. Application of a constant temporal advance of conductivity or temperature as is done with pumped CTDs is shown to produce unacceptable salinity spiking. With the descent rate of the UCTD reaching upwards of 4 dbar s−1, the effect of viscous heating of the thermistor is shown to produce a significant salinity error of up to 0.005 psu, and a correction based on previous laboratory work is applied. Correction of the error due to the thermal mass of the conductivity cell is achieve...

Cross-shelf mixing and mid-shelf front dynamics in the Mid-Atlantic Bight evaluated using the radium quartet

Mid-shelf fronts (MSFs) are thought to be ubiquitous in shelf areas. However, their dynamical role in cross-shelf mixing has yet to be fully characterized. In January, February, and April of 2007, radium isotopes (223Ra, t1/2 = 11 d; 224Ra, t1/2 = 3.7 d; 226Ra, t1/2 = 1600 yr; 228Ra, t1/2 = 5.7 yr) were measured along a transect in the Mid-Atlantic Bight to constrain mixing rates at and around a MSF. Cross-shelf eddy diffusivities (Kx) were determined from 223Ra and 224Ra distributions using a variable-depth model. Two key assumptions – minimal advection and negligible benthic radium input – involving the use of 223Ra and 224Ra as tracers of mixing were quantitatively evaluated in order to assess the accuracy of the Kx estimates. Eddy diffusivities over the three-month sampling period range from 0.1 ± 0.05 − 1.6 ± 0.5 × 102 m2 s−1 for 223Ra and from 1.7 ± 0.4 − 2.2 ± 0.6 × 102 m2 s−1 for 224Ra. The temporal variability in Kx is low in comparison to the uncertainty of the derived values, indicating that eddy diffusivity in this area is relatively constant throughout the sampling period. Observations in the Mid-Atlantic Bight differ from theoretical data corresponding to the tidal dispersion frontogenesis model, suggesting that a different mechanism is responsible for MSF formation. Variability in supported 223Ra and 228Ra observed near the front indicates that cross-shelf mixing may be inhibited by MSFs. Conversely, along-shelf transport is enhanced by the front’s presence. These results indicate that the equatorward jet associated with the front is capable of effectively transporting dissolved chemicals over hundreds of kilometers.