Submarine groundwater fluxes across the seafloor facilitate important hydrological and biogeochemical exchanges between oceans and seabed sediment, yet few studies have investigated spatially distributed groundwater fluxes in deep‐ocean environments such as continental slopes. Heat has been previously applied as a submarine groundwater tracer using an analytical solution to a heat flow equation assuming steady state conditions and homogeneous thermal conductivity. These assumptions are often violated in shallow seabeds due to ocean bottom temperature changes or sediment property variations. Here heat tracing analysis techniques recently developed for terrestrial settings are applied in concert to examine the influences of groundwater flow, ocean temperature changes, and seabed thermal conductivity variations on deep‐ocean sediment temperature profiles. Temperature observations from the sediment and bottom ocean water on the Scotian Slope off eastern Canada are used to demonstrate how simple thermal methods for tracing groundwater can be employed if more comprehensive techniques indicate that the simplifying assumptions are valid. The spatial distribution of the inferred groundwater fluxes on the slope suggests a downward groundwater flow system with recharge occurring over the upper‐middle slope and discharge on the lower slope. We speculate that the downward groundwater flow inferred on the Scotian Slope is due to density‐driven processes arising from underlying salt domes, in contrast with upward slope systems driven by geothermal convection. Improvements in the design of future submarine hydrogeological studies are proposed for thermal data collection and groundwater flow analysis, including new equations that quantify the minimum detectable flux magnitude for a given sensor accuracy and profile length.

There has been a progressive deepening of winter convection in the Labrador Sea since 2012, with the individual profile maximum depth exceeding 1800 m since 2014 and reaching 2100 m in 2016. This increase, during repeated positive phases of the winter North Atlantic Oscillation (NAO), resembles that during the formation of the record depth (2500 m) Labrador Sea Water (LSW) class in 1987-1994, attributed to repeated positive NAO forcing having provided critical preconditioning. The 2012-2016 LSW class is one of the deepest and most persistent ever observed (back to 1938). Year-round observations from profiling Argo floats since 2002 complemented by annual surveys are providing novel information on the seasonal-to-decadal evolution of LSW, such as its variable density, the recent multiyear preconditioning, and its 2016 density being the highest since the mid-1990s. These findings should help international observation programs and numerical model studies investigating LSW influences on the subpolar North Atlantic and Atlantic Meridional Overturning Circulation.

Winter convective overturning in the Labrador Sea reached an aggregate maximum depth of 1700 m in 2015the deepest since 1994with the resulting Labrador Sea Water (LSW) year class being one of the deepest and thickest observed outside of the early 1990s. Argo float, annual survey, and moored measurements in recent decades provide an unprecedented view of important seasonal, interannual, and longer-term LSW variability in the Labrador Sea region. During the 2002-2015 Argo era, the average winter LSW pycnostad volume was about 70% larger in relatively strong convection years than in relatively weak ones. However, the winter-to-fall LSW disappearance volume was 180% larger, pointing to a factor of 2.8 difference in the potential LSW export rates from the region between relatively strong and weak convection years. Intermittently recurrent deep convection is contributing to predominant decadal-scale variations in intermediate-depth temperature, salinity, and density in the LS, with implications for decadal-scale variability across the subpolar North Atlantic and potentially in the Atlantic Meridional Overturning Circulation. Comparison of the LS ocean heat content changes and cumulative surface heat losses during the fall-winter cooling seasons indicates that anomalously strong winter atmospheric cooling, associated at least in part with the North Atlantic Oscillation, is continuing to be a major forcing of the recurrent convection.

As part of a broader study of ocean downscaling, the seasonal and tidal variability of the Gulf of Maine and Scotian shelf, and their dynamical interaction, are investigated using a high-resolution (1/36 degrees) circulation model. The model's seasonal hydrography and circulation, and its tidal elevations and currents, are compared with an observed seasonal climatology, local observations, and results from previous studies. Numerical experiments with and without density stratification demonstrate the influence of stratification on the tides. The model is then used to interpret the physical mechanisms responsible for the largest seasonal variations in the M-2 surface current that occur over, and to the north of, Georges Bank. The model generates a striation pattern of alternating highs and lows, aligned with Georges Bank, in the M-2 surface summer maximum speed in the Gulf of Maine. The striations are consistent with observations by a high-frequency coastal radar system and can be explained in terms of a linear superposition of the barotropic tide and the first-mode baroclinic tide, generated on the north side of Georges Bank, as it propagates into the Gulf of Maine. The seasonal changes in tidal currents in the well-mixed area on Georges Bank are due to a combination of increased sea level gradients, and lower vertical viscosity, in summer.

Key physical variables for the Northwest Atlantic (NWA) are examined in the "historical" and two future Representative Concentration Pathway (RCP) simulations of six Earth System Models (ESMs) available through Phase 5 of the Climate Model Intercomparison Project (CMIP5). The variables are air temperature, sea-ice concentration, surface and subsurface ocean temperature and salinity, and ocean mixed-layer depth. Comparison of the historical simulations with observations indicates that the models provide a good qualitative and approximate quantitative representation of many of the large-scale climatological features in the NWA (e.g., annual cycles and spatial patterns). However, the models represent the detailed structure of some important NWA ocean and ice features poorly, such that caution is needed in the use of their projected future changes. Monthly "climate change" fields between the bidecades 1986-2005 and 2046-2065 are described, using ensemble statistics of the changes across the six ESMs. The results point to warmer air temperatures everywhere, warmer surface ocean temperatures in most areas, reduced sea-ice extent and, in most areas, reduced surface salinities and mixed-layer depths. However, the magnitudes of the inter-model differences in the projected changes are comparable to those of the ensemble-mean changes in many cases, such that robust quantitative projections are generally not possible for the NWA.

Past trends and future projections of key atmospheric, oceanic, sea ice, and biogeochemical variables were assessed to increase our understanding of climate change impacts on Canadian Arctic marine ecosystems. Four subbasins are evaluated: Beaufort Sea, Canadian Arctic Archipelago, Baffin Bay/Davis Strait, and Hudson Bay Complex. Limited observations, especially for ecosystem variables, compromise the trend analyses. Future projections are predominately from global models with few contributions from available marine basin scale models. The assessment indicates a significant increase in air temperature, slight increases in precipitation and snow depth, and appreciable changes in atmospheric circulation patterns. Projections suggest an increase in storm strength and size, leading to enhanced storm surges and coastal erosion, a slight increase in wave heights, increases in gustiness, and small changes in mean wind speed. An Arctic-wide decrease in the extent of multiyear ice and a spatial and temporal increase in ice-free waters in summer have been observed and are projected to continue into the future. Limited observations of ocean properties show local freshening (Beaufort Sea) and summer warming (Baffin Bay). These trends are projected to continue along with localized strengthening in stratification. Increased ocean acidification has been observed and is projected to continue throughout the Canadian Arctic, leading to severely decreased saturation states of calcium carbonate (aragonite and calcite). Qualitative analysis of biological observations indicate large regional differences. Increased primary production and double bloom development is seen in areas of sea ice retreat where nutrient supply is sufficient, and unchanged or reduced production is seen where nutrients are low or suppressed in response to enhanced stratification. Future primary production projections show inconsistent results, with light-dependent increase or nutrient-limited decrease dominating, dependent on the model. For the next decade, natural intradecadal variability is expected to be of similar importance as longer-term trends. To improve our capacity to assess and project climate change adaptation in marine ecosystems, more consistent observations are needed, especially over marine areas and for biogeochemical variables. Higher resolution basin-scale models are also required to provide locally applicable projections relevant for Arctic communities and management units.

Simulations with a coarse-resolution global ocean model during 1958-2004 are analyzed to understand the inter-annual and decadal variability of the North Atlantic. Analyses of Empirical Orthogonal Functions (EOFs) suggest relationships among basin-scale variations of sea surface height (SSH) and depth-integrated circulation, and the winter North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) indices. The linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability. In the subpolar region, variations of the gyre strength, SSH in the central Labrador Sea and the NAO index are highly correlated. Surface heat flux is important in driving variations of SSH and circulation in the upper ocean and decadal variations of the Atlantic Meridional Overturning Circulation (AMOC). Surface momentum flux drives a significant barotropic component of flow and makes a noticeable contribution to the AMOC. In the subtropical region, momentum flux plays a dominant role in driving variations of the gyre circulation and AMOC; there is a strong correlation between gyre strength and SSH at Bermuda. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.

Historical variability in sea surface temperature (SST) in the North Atlantic (NA) is examined using trend and Empirical Orthogonal Function (EOF) analyses of annual and summer means from three interpolated monthly datasets: Hadley Centre Sea Ice and Sea Surface Temperature (HadISST1), Extended Reconstruction of SST (ERSST), and Centennial in situ Observation-Based Estimates (COBE). Comparisons with time series of upper-ocean temperature from four monitoring sites off Atlantic Canada reveal substantial similarity in the interannual to multi-decadal variability but notable differences in the longer-term trends. The magnitude of decadal-scale variability is comparable to, or greater than, the long-term changes in all of the datasets; together with the trend discrepancies, this needs to be considered in climate change applications. Averaged over the NA, the annual means have a long-term increasing trend and a pronounced multi-decadal variation, resembling those in global mean (land-ocean) surface temperature and the Atlantic Multi-decadal Oscillation (AMO). There is remarkable similarity in the spatial and temporal variability of the three leading EOF modes from the different gridded datasets, with the first highly correlated with the AMO, the second modestly correlated with the winter North Atlantic Oscillation, and the third apparently related to ocean circulation variability. Trends since 1981 are generally two to three times larger than those since 1900 and 1950, which is at least partly related to the phase of the AMO. Trends in the summer means are generally larger than in the annual means. Overall, the results provide support for both anthropogenic global warming and decadal-scale natural variations making important contributions to ocean climate variability in the Northwest Atlantic.

Measurements of ocean bottom pressure, particularly on the continental slope, make an efficient means of monitoring large-scale integrals of the ocean circulation. However, direct pressure measurements are limited to monitoring relatively short time scales (compared to the deployment period) because of problems with sensor drift. Measurements are used from the northwest Atlantic continental slope, as part of the Rapid Climate Change (RAPID)-West Atlantic Variability Experiment, to demonstrate that the drift problem can be over-come by using near-boundary measurements of density and velocity to reconstruct bottom pressure differences with accuracy better than 1 cm of water (100 Pa). This accuracy permits the measurement of changes in the zonally integrated flow, below and relative to 1100 m, to an accuracy of 1 Sv (1 Sv equivalent to 10(6) m(3) s(-1)) or better. The technique employs the "stepping method," a generalization of hydrostatic balance for sloping paths that uses geostrophic current measurements to reconstruct the horizontal component of the pressure gradient.

TOPEX/Poseidon (T/P) sea level data are used to investigate tidal and non-tidal sea level variability over the Scotian Shelf. Eight major diurnal and semi-diurnal tidal constituents are derived from the September 1992 to December 2001 T/P data. The present altimetric tides agree generally with in situ observations and show an error reduction of over 30 degrees in phase over Han et al.'s [Han, G., Ikeda, M. and Smith, P.C., 1996, Oceanic tides on the Newfoundland and Scotian Shelves from TOPEX/POSEIDON altimetry. Atmosphere-Ocean, 34, pp. 589-604] results for the diurnal tides. De-tided T/P sea level data are used to provide geo-referenced seasonal-mean sea surface topography during the period September 1992 to July 2002. The T/P data are explored to estimate seasonal-mean sea level along the Nova Scotia coast. Comparisons with sea level measurements from coastal tide gauges are carried out to evaluate the potential of altimetry data for coastal sea level monitoring. Seasonally the sea level is highest along the coast in late autumn/early winter (associated with the peak of the Nova Scotian Current), and over the Scotian Slope in summer. On the interannual scale it was low in the early 1990s, high in the mid 1990s, and close to normal in the late 1990s and early 2000s.

A May 2008 oceanographic survey of the Labrador Sea and recent Argo float profiles have revealed that convective overturning extended to a depth of about 1600 m during the winter of 2008, resulting in the production of a large “year class” of Labrador Sea Water. This convection was the deepest since 1994, and substantially exceeded the convection to 500–1100 m in the past few years in both the Labrador and Irminger Seas. The resultant 0.2°C cooling of the intermediate‐depth waters in the Labrador Sea has disrupted a steady warming of these waters since 1994. The cumulative heat loss from the ocean to the atmosphere during the 2007–2008 cooling season was the largest since the mid‐1990s and exceeded the 2000–2007 mean by about 50%. This indicates that enhanced atmospheric cooling, apparently associated with below‐normal air temperatures in the region, was the predominant factor contributing to the enhanced LSW production in 2008.

Flocculation of the fine particulate portion of drilling mud during discharge into the ocean provides a pathway for an impact on the benthic environment in that the higher settling velocity of flocculated particles leads to higher concentrations near the sea floor. The benthic boundary layer transport model, bblt , has been developed for predicting the dispersion and transport of suspended sediment in the benthic boundary layer on the continental shelf. Past bblt applications aimed at impact zone assessment of drill mud discharges on the Atlantic Canadian shelf have used a constant settling velocity to describe particle settling. However, recent observations on the shelf suggest that natural flocs break up when the turbulent shear stress exceeds 0.1 Pa. We incorporate a variable settling velocity into the bblt model to investigate how stress-dependant flocculation affects the magnitude and frequency of high drill mud concentrations near the sea floor. Applications to the Hibernia drill site on the Grand Banks and the Cohasset site near Sable Island are considered.