Guoqi Han

Seasonal-Mean Hydrography and Circulation in the Gulf of St. Lawrence and on the Eastern Scotian and Southern Newfoundland Shelves

Abstract The climatological seasonal-mean hydrography and circulation in the Gulf of St. Lawrence and on the eastern Scotian and southern Newfoundland shelves are studied by reconstructing high-resolution temperature, salinity, and density fields for four seasons and numerically computing the associated circulation fields. The current fields are obtained from a three-dimensional diagnostic model, forced by baroclinic pressure gradients, seasonal wind stresses, and additional barotropic inflows across the Strait of Belle Isle and southern Newfoundland shelf upstream boundaries. The hydrographic fields suggest strong gulf–shelf interconnections, including outflow of relatively fresh surface water from the gulf to the eastern Scotian shelf, penetration of slope water at depth onto the shelves and into the gulf, and flow into the gulf through the Strait of Belle Isle. The circulation is generally cyclonic in the gulf, reinforced by inflows of Labrador and Newfoundland shelf water through the Strait of Belle I...

Seasonal variation of the three-dimensional mean circulation over the Scotian Shelf

The seasonal-mean circulation over the Scotian Shelf is studied numerically by computing mean and tidal current fields for winter, spring, and summer using a three-dimensional nonlinear diagnostic model. The mean current fields are forced by seasonal-mean baroclinic pressure gradients, tidal rectification, uniform wind stresses, and associated barotropic pressure gradients. A historical hydrographic database is used to determine the climatological mean baroclinic forcing. Upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along-boundary elevation gradients or depth-integrated normal velocities. The numerical solutions for nominal bimonthly periods (January–February, April–May, and July–August) reveal the dominant southwestward nearshore and shelf-break flows of relatively cool and fresh shelf water from the Gulf of St. Lawrence and Newfoundland Shelf, with speeds up to about 20 cm/s. The seasonal intensification of the southwestward flows is reproduced by the model, with the transport increasing from 0.3 Sv in summer to 0.9 Sv in winter on the inner Halifax section. There are also pronounced topographic-scale influences of submarine banks, basins, and cross-shelf channels on the circulation, such as anticyclonic gyres over banks and cyclonic gyres over basins. Baroclinicity is the dominant forcing throughout the domain, but tidal rectification is comparable on the southwestern Scotian Shelf (e.g., about 0.2 Sv recirculating transport around Browns Bank for all the periods). The mean wind stress generates offshore surface drift in winter. The solutions are in approximate agreement with observed currents and transports over the Scotian Shelf, although there are local discrepancies.

Decline and partial rebound of the Labrador Current 1993-2004: Monitoring ocean currents from altimetric and conductivity-temperature-depth data

Monitoring and understanding of Labrador Current ariability is important because it is intimately linked to the meridional overturning circulation and the marine ecosystem off northeast North America. Nevertheless, knowledge of its decadal variability is inadequate because of scarcity of current meter data. By using a novel synthesis of satellite altimetry with conductivity-temperaturedepth (CTD) data we assess the Labrador Current variability north of the Hamilton Bank (56oN) over 1993-2004. Our analysis shows a decline of the surface-to-bottom transport of current by 6.3 ± 1.5 Sv (1 Sv =106 m3 s-1) in the 1990s (significant at the 99% confidence level) and a likely partial rebound of 3.2 ± 1.7 Sv in the early 2000s (significant at the 89% confidence level only). The inferred multiyear changes in the Labrador Current transport seem to be primarily barotropic and positively correlated (at the 99% level) with the North Atlantic Oscillation at zero lag implying a fast response of the regional circulation to the atmospheric forcing variability. The results compare favorably with direct current measurements and recent model-based findings on the multi-year variability of the subpolar gyre and its underlying mechanisms. The study demonstrates the feasibility of combining altimetry and CTD data for assessing the climatic variability of the boundary currents.

Three‐dimensional seasonal‐mean circulation and hydrography on the eastern Scotian Shelf

A three-dimensional nonlinear finite element model with an advanced turbulence scheme is used in conjunction with historical current, temperature, and salinity observations to describe seasonal-mean circulation and hydrography on the eastern Scotian Shelf (ESS). The model solutions consist of density-, wind-, and boundary-driven currents, and associated elevation, hydrographic, and turbulence fields. The M2 and K1 tides are simultaneously computed to include the effects of tidally induced turbulent mixing and bottom friction, and tidal rectification. The circulation is dominated by surface-intensified baroclinic nearshore and shelf-break currents, both directed southwestward with prominent seasonal and alongshelf changes. These currents are primarily associated with the equatorward inflow of relatively fresh water from the Gulf of St. Lawrence and Newfoundland Shelf/Slope. Pronounced and persistent influences of outer-shelf banks, inner-shelf basins, and cross-shelf channels are evident in the circulation fields. In particular, the Sable Gully features a year-round cyclonic partial gyre, with a net onshore flow that is greatest in spring. A relatively strong anticyclonic gyre over Sable/Western Bank contrasts a weak partial gyre over Banquereau Bank. The model solutions are in approximate agreement with observed transports and currents for the primary flow features.

Three‐dimensional modeling of tidal currents and mixing quantities over the Newfoundland Shelf

Major semidiurnal (M2, S2, and N2) and diurnal (K1 and O1) barotropic tides over the Newfoundland Shelf are computed using a three-dimensional nonlinear primitive equation model, with the vertical eddy viscosity calculated from a level 2.5 turbulence closure scheme. Computed elevation cotidal charts for the five constituents are generally consistent with previous knowledge for this region. Comparisons based on a statistical analysis of the differences between the computed elevations and currents and in situ observations indicate good agreement. While M2 tidal currents (up to 20–30 cm/s) are dominant, there are locally intensified diurnal currents (up to 5–10 cm/s) in some outer shelf locations. The diurnal current intensification is attributed to first-mode continental shelf waves. An examination is carried out for the vertical structure of the computed M2 current and for the temporal and spatial variability of model turbulent kinetic energy, mixing length scale, vertical eddy viscosity, and bottom friction velocity. The examination indicates that large vertical eddy viscosity magnitudes and bottom friction velocities are associated with strong currents in shallow regions, where strong vertical shears produce large turbulent kinetic energy. Solutions with both specified and evolving vertical stratification indicate that the stratification has a significant influence on the vertical profile of tidal mixing parameters and currents in shallow areas. Tidally induced turbulence is substantially reduced in the bottom boundary layer and completely suppressed above it. Tidal currents decrease in the log layer, increase significantly in the rest of the bottom boundary layer, and decrease in the upper and middle water column.

Annual variation of sea‐surface slopes over the Scotian Shelf and Grand Banks from Geosat altimetry

Abstract The Geosat radar altimeter data from ∼60 repeat cycles of the Exact Repeat Mission (ERM) over the period November 1986 to September 1989 have been analysed to show the annual variations of the sea‐surface slopes, corrected for ocean tides, over the Scotian Shelf and the Grand Banks. A coastal tidal model developed at the Bedford Institute of Oceanography, combined with the global tidal model of Schwiderski, is employed to remove the tidal signals from the sea‐surface heights over those regions. Linear regression is used to estimate the sea‐surface slopes over the inner shelf region, the outer shelf region, or a combination of the two along the Geosat ground tracks. Harmonic analysis is applied to the time series of sea‐surface slopes to derive the annual signals, showing that amplitudes are of order of 5 × 10‐7 (5 cm/100 km) with onshore slopes positive in winter and negative in summer. The largest annual cycles occur over the outer portion of the Laurentian Channel and the southern Grand Banks. ...

Annual Variations of Sea Surface Elevation and Currents over the Scotian Shelf and Slope

Abstract TOPEX/Poseidon (T/P) altimeter data over the period 1992–98 have been analyzed to examine annual variability of sea surface elevation and currents over the Scotian Shelf and Slope. A modified orthogonal response analysis is used to derive the annual cycle while simultaneously removing the residual tides and other dynamical processes at the appropriate T/P alias periods. An evaluation of the M2 and K1 alias variations is carried out, suggesting notable tidal correction errors off Cape Cod and over Georges Bank. The along-track sea surface slopes, which represent surface geostrophic current components normal to the track, are estimated on selected T/P ascending and descending ground tracks. The annual altimetric sea level harmonic is compared with steric height anomalies and wind-driven setup. The comparison indicates that the altimetric sea surface elevation variability is dominated by the baroclinic (and associated barotropic) component and supplemented by the wind-driven and remotely forced comp...

Interannual sea-level variations in the Scotia-Maine region in the 1990s

The TOPEX/Poseidon altimeter data over the period from 1992 to 1999 have been analyzed to examine interannual sea-level variability in the Scotia‐Maine region (defined as the outer Laurentian Channel, the Scotian Shelf, the Gulf of Maine, and the northern Middle Atlantic Bight). A modified orthogonal response analysis is used to simultaneously remove the annual cycle and residual tides (including the semi-annual cycle). Altimetric data reveal significant interannual sea-level variability of magnitude 5‐10 cm over the shelves in the 1990s, falling to the lowest level in 1994, rising to the highest level in 1997‐1998, and falling again. The second sea-level decrease shows an overall equatorward propagation. These results are generally consistent with those detected from detrended coastal tide-gauge data at Halifax, Nova Scotia. The interannual sea-level variability is thought to be forced by fluctuations of the Gulf Stream position, which seems to be related to the North Atlantic Oscillation, and the baroclinic Labrador Current transport.

Satellite Observations of Seasonal and Interannual Changes of Sea Level and Currents over the Scotian Slope

Abstract Seasonal and interannual sea level and current variations over the Scotian slope are examined using 10 years of Ocean Topography Experiment (TOPEX)/Poseidon (T/P) satellite altimeter data. Geostrophic surface current anomalies normal to ground tracks are derived from the along-track gradients of sea level anomalies. The altimetric current anomalies are combined with a climatological mean circulation field of a finite-element model to construct nominal absolute currents. The seasonal mean results indicate that the sea level is highest in late summer and lowest in late winter and that the surface slope circulation is strong in winter/autumn and weaker in summer/spring. The total transport associated with the westward shelf-edge current and with the eastward slope current, calculated by combining the T/P data with a climatological seasonal mean density field, reveals a substantial seasonal change dominated by the barotropic component. The present analysis reveals prominent interannual changes of the...

Sea level and surface current variability in the Gulf of St Lawrence from satellite altimetry

Sea level and surface current variability in the Gulf of St Lawrence have been investigated primarily using seven years of TOPEX/Poseidon altimeter data. An orthogonal response analysis is used to derive an annual cycle from 1-s altimetric data along satellite ground tracks, while simultaneously removing aliased residual tides and dynamic signals at alias tide frequencies. An examination of tidal-frequency variability points to the need for a better tide model for detiding altimetric data in order to study shorter (than seasonal) period processes in the Gulf of St Lawrence. Annual sea level amplitudes and phase fields are constructed from the along-track analysis results using a linear interpolation procedure. The altimetric annual harmonic has a magnitude of 2-5 cm in amplitude and is highest in fall. The altimetric sea level results agree well with independent tide-gauge data at coastal stations and can be accounted for mainly by steric height. Geostrophic surface current anomalies derived from the altimetric annual sea level anomalies are then discussed in conjunction with numerical solutions from a regional hydrodynamic model. Interannual sea level change in the Gulf of St Lawrence is also investigated from both altimetry and tide-gauge data, indicating a prominent gulf-wide sea level drop around 1997, with exact timing dependent on location. The interannual sea level variability is thought to be primarily associated with the Labrador Current transport variability (via both the Strait of Belle Isle and Cabot Strait) and the south-north fluctuation of the Gulf Stream position (via Cabot Strait).