Blair J.W. Greenan

Mixing in a coastal environment: 2. A view from microstructure measurements

[1] During the Coastal Mixing and Optics Experiment in 1996 and 1997, an integrated dye and microstructure experiment was done to measure and compare mixing rates on the continental shelf. The results of the dye experiment are presented in the companion paper by Ledwell et al. [2004]. In this paper, we explore the results from microstructure measurements using a vertical profiling instrument. We measure temperature and velocity microstructure and, along with simultaneous measurements of salinity and temperature as well as a shipboard acoustic Doppler current profiler (ADCP), are able to estimate the vertical diffusivities of heat, mass, and momentum. In three of four dye injections performed, we were able to make a comparison of the diffusivity from both dye and microstructure measurements. Although the mixing rates were quite small (vertical diffusivity of heat, K T < 10 -5 m 2 s- ), the two techniques yielded consistent results. A comparison of the vertical diffusivities K T and K ρ (the vertical diffusivity for density) allowed us to determine a flux Richardson number of R f = 0.16 ± 0.03. R f showed little dependence on either the buoyancy frequency, N, or gradient Richardson number, R i . A clear relationship was found between the ratio of diffusivities, K m /K T and R i consistent with K m /K T = 5 R i . Turbulence levels were extremely low, with Cox numbers in one experiment of about 20 and in the other three of about 5 (i.e., K T about 20 and 5 times molecular diffusion, respectively).

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for understanding this linkage is compelling since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic (OSNAP), to provide a continuous record of the trans-basin fluxes of heat, mass and freshwater and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the RAPID/MOCHA array at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014 and the first OSNAP data products are expected in the fall of 2017.

Stirring by Small-Scale Vortices Caused by Patchy Mixing

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m 2 s 1 . Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m 2 s 1 . The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

Estimates of Dissipation in the Ocean Mixed Layer Using a Quasi-Horizontal Microstructure Profiler

Abstract Some recent measurements of the mixed layer in oceans and lakes have indicated that the rate of the dissipation of turbulent kinetic energy, e, is much higher than expected from a purely shear-driven wall layer. This enhancement has usually been attributed to wave breaking. In this study, measurements of dissipation in the open-ocean mixed layer on the continental shelf off Nova Scotia are integrated with air–sea flux estimates and directional wave spectra to further study this issue. A microstructure profiler gliding quasi-horizontally provides estimates of e starting within 2 m of the ocean surface as it slowly descends through the mixed layer. Dissipation rates were found to be enhanced relative to the wind stress production and indicated that ∼6% of the wind energy at 10 m is dissipated in the ocean mixed layer. In addition, results from this experiment demonstrate that the WAVES scaling for e, based on wind and wave parameters, is valid for the case of a simple windsea in which the swell can...

Comparison of Canadian daily ice charts with surface observations off Newfoundland, winter 1992

Abstract Forecast ice drift rates and thicknesses displayed on daily ice charts and forecast winds for the Canadian east coast are compared to on‐ice observations made during the second Canadian Atlantic Storm Program (CASP II) of March 1992. Observed and 24‐hour forecasts of daily ice drift rates were weakly correlated even though long‐term means closely matched observations. Daily drift rates have an RMS error of 13 cm s‐1 relative to a 15 cm s‐1 mean in addition to an RMS direction error of 50 degrees. Contributions towards daily drift uncertainties were: the estimation of winds, unmodelled physics of ocean and ice cover processes; and the inconsistency in the methods used by the ice forecaster. Correlation coefficients between forecast winds and on‐ice observed winds decreased from 0.8 at 0‐hour to 0.7 for the 30‐hour forecast. Similar results were found between ice drift rates from forecast winds. Histograms of ice thicknesses observed along narrow swaths using a helicopter‐towed electromagnetic sens...

An abrupt shift in the Labrador Current System in relation to winter NAO events

The behavior of the Labrador Current during the period from 1990 to 2007 is investigated with an eddy-resolving circulation model for the North Atlantic Ocean. An EOF analysis of the model output suggests that the variability in the Labrador Current can be partitioned into a western Labrador Current (WLC; from the 300-2500 m isobaths), and an eastern Labrador Current (ELC; from the 2500-3300 m isobaths). The model results demonstrate that the WLC transport experienced an abrupt increase during 2000-2002, consistent with data. This differed significantly from the ELC transport which was strong during the high winter NAO (North Atlantic Oscillation) years (1990-95) and then steadily declined. This ELC trend is consistent with changes in the modelled Atlantic Meridional Overturning Circulation and convection depth. Our study proposes that the change in the WLC is due to a southwestward shift of the atmospheric circulation pattern starting in 2001, coincident with a change in the 2001 NAO index, and also in a westward shift of the action centers of the winter NAO events. This article is protected by copyright. All rights reserved.

Modelling study of three-dimensional circulation and particle movement over the Sable Gully of Nova Scotia

The Sable Gully is a broad deep underwater canyon located to the east of Sable Island on the edge of the Scotian Shelf. Being the home of many marine species including the endangered Northern Bottlenose Whale, the Gully was designated as a marine protected area (MPA) in 2004. Better understanding of physical environmental conditions over this MPA is needed for sustainable ecosystem management. In this study, a multi-nested ocean circulation model and a particle tracking model are used to examine the three-dimensional (3D) circulation and movement of particles carried passively by the flow over the Sable Gully. The 3D circulation model is driven by tides, wind, and surface heat/freshwater fluxes. The model performance is assessed by comparing the results with the previous numerical tidal results and current meter observations made in the Gully. The simulated tidal circulation over the Gully and adjacent waters is relatively strong on shallow banks and relatively weak on the continental slope. Below the depth of the Gully rim ( ∼ 200 m), the tidal currents are constrained by the thalweg of the Gully and amplified toward the Gully head. The simulated subtidal circulation in the Gully has a complex spatial structure and significant seasonal variability. The simulated time-dependent 3D flow fields are then used in a particle tracking model to study the particle movements, downstream and upstream areas, and residence time of the Gully. Based on the movements of particles released at the depth of the Gully rim and tracked forward in time, the e-folding residence time is estimated to be about 7 and 13 days in February and August 2006, respectively. The Gully flanks are identified as high retention areas with the typical residence time of 10 and 20 days in February and August 2006, respectively. Tracking particles with and without tides reveals that tidal circulation reduces the value of residence time in the Gully, particularly along the Gully flanks.

Hydrography and Coastal Circulation along the Halifax Line and the Connections with the Gulf of St. Lawrence

ABSTRACT Acoustic Doppler Current Profilers and underwater gliders were simultaneously deployed as part of the Ocean Tracking Network to continuously monitor the Halifax Line (HL) and the Nova Scotia Current (NSC) between 2008 and 2014. The HL transects the Scotian Shelf, which connects dynamically important areas, such as the Grand Banks, the Gulf of Maine, and the Gulf of St. Lawrence (GSL). The oceanographic measurements made at the HL during this period provide a unique opportunity to study the temperature, salinity, and alongshore current conditions and variability at both seasonal and interannual time scales. The analysis of observations reveals that the water over the Scotian Shelf is mainly composed of water coming from the Gulf of St. Lawrence (Cabot Strait subsurface water) in the upper layer (30 to 50 m, 81%) and Warm Slope Water below 100 m (77%), highlighting the connectivity between the GSL and the Scotian Shelf. The temperature–salinity characteristics of the Cold Intermediate Layer (CIL) observed along the HL and located mainly between 50 and 100 m, is indistinguishably influenced by both water coming from the Inshore Branch of the Labrador Current and CIL water formed in the GSL. These proportions stay similar over interannual time scales, suggesting that the 2012 warm anomaly observed over the Scotian Shelf is primarily driven by the advection of already anomalously warm water coming from offshore regions. The analysis of glider data also reveals that most of the alongshore transport over the Scotian Shelf occurs within the first 60 km from the coast, where the NSC is located. It was found that the freshwater discharge from the St. Lawrence River at Québec and the alongshore transport across the NSC have a significant covariance at a 9-month lag. The Empirical Orthogonal Function (EOF) analysis demonstrates that most of the current variability (between 78 and 92%) can be explained by the first EOF, which represents the baroclinicity resulting from the freshwater outflow coming from the GSL. Part of the second EOF is associated with the local wind forcing and explains between 4 and 14% of the NSC variability.

Wind Forcing of Ice Cover in the Labrador Shelf Marginal Ice Zone

Abstract Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r2 = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (∼10 m) winds from the Canadian Meteorological Centres Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%. The response of the ocean ...

Variability and wind forcing of ocean temperature and thermal fronts in the Slope Water region of the Northwest Atlantic

Subsurface temperatures in the Slope Water region of the Northwest Atlantic from Argo profiling floats and on the adjacent continental shelf from ship-based measurements are compared with the latitudinal position of the Shelf-Slope Front (SSF) and the Gulf Stream North Wall (GSNW). The Slope Water and shelf temperature anomalies at 200 m depth are in agreement for the period, 2002-2015. For the period 1978-2015, shelf temperatures are significantly correlated with the SSF position, and to a lesser extent with the GSNW position. Annual SSF position anomalies near the Grand Banks at 50-55°W lead anomalies to the west at 65-75°W by 1-2 years. Wind stress curl is compared with the annual change in the SSF and GSNW latitudinal positions, rather than with the positions directly. Changes in the mean position of the SSF are related to the wind stress curl pattern in the mid-Atlantic, with an 8-month lag. It is suggested that a wind pattern favoring a southward shift of the SSF is associated with a southward shift of the zero-curl line near 40°W, resulting in an expanded subpolar gyre and enhanced flow of Labrador Current Water westward from the Tail of the Grand Banks. However changes in the GSNW position are related to an NAO-like wind stress curl pattern in the eastern Atlantic in the winter-spring period, in agreement with other studies. High sea surface temperatures in the Gulf of Maine and on the Scotian Shelf in recent years can be largely attributed to positive local onshore wind anomalies.