Charles Tang

Modeling the seasonal variation of sea ice in the Labrador Sea with a coupled multicategory ice model and the Princeton ocean model

We use a multicategory sea ice model coupled to the Princeton ocean model, which is driven by monthly climatological atmospheric forcing, to study the seasonal variation of ice cover in the Labrador Sea. Initial ocean conditions are derived from a gridded, objectively analyzed temperature-salinity data set that provides improved resolution of gradients in the vicinity of the shelf break. The model produces a realistic seasonal variation of sea ice. There is ice growth over the inner shelf and ice melt over the outer shelf and slope. Over the inner shelf, advection and diffusion decrease the ice mass; over the outer shelf, advection and diffusion increase the ice mass, which maintains the location of the ice edge. Near the offshore ice edge the melt rate exceed s1mp er month, and the heat to melt ice together with the heat lost to the atmosphere exceeds 500 Wm 22 . The heat lost at the ocean surface is compensated for by advection of heat from an offshore convective region. The dominant heat source for the spring retreat of ice in the south is shortwave radiation over the open water fraction. Marginal ice zones are regions of dynamic interaction be- tween atmosphere, ice, and ocean. In the Labrador marginal ice zone the ice edge is located near the Labrador Current and its associated shelf break front. Wintertime convection in the western Labrador Sea is thought to form Labrador Intermedi- ate Water (Clarke and Gascard, 1983). Since the ice edge bor- ders the convective region, it is important to clarify the pro- cesses that limit the ice extent and to examine the heat and salt fluxes in the vicinity of the ice edge. In this study we investigate the seasonal evolution of sea ice over the Labrador shelf with a coupled ice-ocean model. Our objective is to assess the capability of the model in simulating the evolution of the ice cover and to identify model deficien- cies. We examine the processes that limit the ice extent. Labrador sea ice has been the subject of a number of studies with numerical models. Most recently, Ikeda et al. (1996) cou- pled a two-category ice model (Hibler, 1979) with the Geo- physical Fluid Dynamics Laboratory (GFDL) ocean model (Bryan, 1969; Cox, 1984). There was overall agreement with the seasonal cycle as well as with interannual variations. The re- sults showed that shoreward advection of relatively warm off- shore water is an important source of heat to melt ice. How- ever, the model ice decayed too rapidly, partly because the two-category ice model underestimates ice concentration. There are a number of differences between the present model and the model of Ikeda et al. (1996). We have imple- mented a multicategory ice model and used the sigma coordi- nate Princeton ocean model (Blumberg and Mellor, 1987; Mel- lor, 1996) with its embedded turbulence closure submodel. We use a climatology from the National Centers for Environmen- tal Prediction (NCEP)/National Center for Atmospheric Re-

The Impact of Waves on Surface Currents

Abstract Ocean models usually estimate surface currents without explicit modeling of the ocean waves. To consider the impact of waves on surface currents, here a wave model is used in a modified Ekman layer model, which is imbedded in a diagnostic ocean model. Thus wave effects, for example, Stokes drift and wave-breaking dissipation, are explicitly considered in conjunction with the Ekman current, mean currents, and wind-driven pressure gradient currents. It is shown that the wave effect on currents is largest in rapidly developing intense storms, when wave-modified currents can exceed the usual Ekman currents by as much as 40%. A large part of this increase in velocity can be attributed to the Stokes drift. Reductions in momentum transfer to the ocean due to wind input to waves and enhancements due to wave dissipation are each of the order 20%–30%. Model results are compared with measurements from the Labrador Sea Deep Convection Experiment of 1997.

Storm-Forced Baroclinic Near-Inertial Currents on the Grand Bank

Abstract Current meter data for six mouths from the Grand Bank are analyzed to study inertial currents generated by moving storms. It is found that during periods of strong winds, but no well-defined storm system, the inertial motion exhibits no simple relationship to the local wind. During intense storms inertial currents up to 0.5 m s−1 were observed both in and below the mixed layer. Upper and lower layer currents are roughly equal in amplitude, but are 180° out of phase. To explain this observation, a two-layer, one-dimensional model is developed that successfully simulates the observed inertial currents. We show that under the conditions encountered during the storms only baroclinic inertial motion can be generated. The pressure gradient effect is not important, and the current below the mixed layer is produced by mass continuity. Wavelength computed from the continuity equation is consistent with that predicted by first-order linear theory. For inertial motion generated during periods of strong wind...

Barotropic Response of the Labrador/Newfoundland Shelf to a Moving Storm

Abstract The barotropic response of the Labrador and Newfoundland shelves to a moving storm over the Labrador Sea is investigated using a linear barotropic ocean model with realistic coastline and topography. The model results show that the storm generates motions of different time–space scales. Four types of motions are identified:directly wind-forced motion, shelf waves with distinctive frequency and wavelength, low-frequency shelf waves, and trapped inertio–gravity waves. The strongest currents are directly wind-forced currents occurring in areas of maximum wind stress over the shelf. The spatial pattern and temporal change of the current field are strongly influenced by the time history of the storm and the geometry of the coastline. Continental shelf waves are generated in the shelf region south of the storm track. Maximum amplitude occurs along the shelf edge at a wavelength of 800 km and a period of 20 h. This wavelength and period are close to the maximum frequency point of the dispersion curve fo...

The formation and maintenance of the North Water Polynya

Abstract This paper investigates the formation and maintenance of the North Water Polynya, Baffin Bay in winter using a multi‐category sea‐ice model coupled with the Princeton ocean model. Monthly climatological atmospheric data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis provides the forcing. An objectively‐analysed climatology provides the initial ocean temperature and salinity. Wind stress drives the ice in a cyclonic gyre around northern Baffin Bay. Localized regions of thin ice form where wind drives ice away from coastlines or fast ice. The regions of thin ice are characterized by enhanced ice growth, exceeding 1.2 m mo−1. In the regions of thin ice, surface ocean heat flux is also enhanced and is between 30–60 W m−2. Surface heat flux is, in part, attributable to convective mixing and entrainment driven by ice growth. The surface heat flux reflects advection of the warm West Greenland Current. Heat and salt balances show that horizontal advective exchange counterbalances surface fluxes of heat and salt.

Modeling the Mean Circulation of the Labrador Sea and the Adjacent Shelves

Abstract A linear three-dimensional diagnostic model is used to study several aspects of the mean circulation of the Labrador Sea and the adjacent shelves: volume transport, three-dimensional structure of currents, mesoscale features produced by JEBAR, winter surface current derived from ice drift data, difference between summer and winter circulation, and effects of buoyancy and local wind on the circulation. The summer circulation is forced by a sea surface elevation at the northern boundary, tuned to produce a 35 Sv (Sv≡106 m3 s−1) total southward transport (including the shelf, the Labrador Current, and the deep sea transports) across the Hamilton Bank section. A comparison of the model results with data collected over the shelves indicates that the model is able to reproduce the major features of the observations. Buoyancy effects on the circulation are found to be more important in the southern Labrador Sea than in the northern Labrador Sea. The subbasin-scale currents give rise to an alongshelf var...

Modelling the mean circulation of Baffin Bay

Abstract The Princeton Ocean Model is applied to the Baffin Bay and Labrador region in order to model the summer mean circulation and transport of Baffin Bay. Several aspects of the mean circulation are investigated, but are restricted to the month of September when hydrographic data are available. These include the horizontal and vertical structure of the currents, the sensitivity of the volume transport to the boundary forcing and the effect of the local wind forcing. The model results are shown to be in general agreement with observations. The model reproduces the strongest currents: (a) offshore from the coast of Baffin Island and Ellesmere Island, and (b) at the mouth of Lancaster Sound. The model results also show the presence of the relatively strong southward current along the shelf break on the eastern side of the bay. Strong topographic control is evident in Davis Strait and in the vicinity of deep canyons on the continental shelf of western Greenland. Model sensitivity studies show that the Baffin Bay outflow through western Davis Strait is controlled mainly by the inflow from the archipelago. The inflow through eastern Davis Strait is controlled by both the archipelago inflow and the transport in the Labrador Sea gyre. The local wind stress plays a relatively unimportant role.

Low-frequency currents at the northern shelf edge of the Grand Banks

The structure and generation mechanisms of low-frequency currents at the northern shelf edge region of the Grand Banks are investigated using data from a bottom mounted acoustic Doppler current profiler (ADCP) and an array of conventional current meters collected during the Canadian Atlantic Storms Program II. The vertical structure of the current is determined from an empirical orthogonal function analysis of the ADCP data. The analysis shows that over the shelf edge the low-frequency currents are predominantly barotropic and polarized in the long-shelf direction. There is little coherence between the local wind and the barotropic current, while baroclinic currents are moderately coherent with the local wind. In the interior of the shelf, the currents and the local wind are highly coherent. The difference in response to wind forcing over the shelf edge and in the shelf interior suggests that currents in the shelf interior are wind driven, while currents over the shelf edge are generated by mechanisms other than direct wind forcing. Possible generation mechanisms for the barotropic long-shelf low-frequency currents are discussed, including meanders and eddy formation in the Labrador Current and continental shelf waves generated by distant storms.

Seesaw structure of subsurface temperature anomalies between the Barents Sea and the Labrador Sea

[1] Using a coupled ocean-sea ice model of the pan-Arctic and North Atlantic Ocean, we investigate the response of the Arctic and subarctic thermohaline structure to seasonal extreme atmospheric forcing associated with the winter Arctic Oscillation (AO). During the positive phase of AO, significant surface cooling occurs in the Labrador Sea, but there is no substantial surface warming in the Barents Sea, i.e., no seesaw pattern in sea surface temperature (SST) anomaly between the Barents Sea and Labrador Sea. A possible explanation is that Arctic sea ice export into the Barents Sea melts locally and lowers the SST. However, a seesaw structure in subsurface (below the mixed layer: 40-100 m) water temperature anomaly between the two regions is found, exceeding the 95% significance level. Corresponding to the positive phase of the winter AO, a significant warming of the subsurface water in the Barents Sea and a concurrent cooling in the northwestern Labrador Sea are seen in the model results, which is analogous to the seesaw structure in both surface air temperature (SAT) and sea ice extent anomalies. The mechanism leading to the anomalous subsurface temperature seesaw is consistent with the northward advection of the warm Atlantic Water into the Barents Sea and the southward advection of the cold Arctic and sub-arctic water into the Labrador Sea from the David Strait and Baffin Bay. Hydrographic data are analyzed and the resulting temperature distribution supports this new finding.

A modeling study of upper ocean winter processes in the Labrador Sea

A coupled ice-mixed layer model is developed to study winter processes in the Labrador Sea. The seasonal change of mixed-layer properties, air-sea fluxes, and ice coverage is examined. The effects of severe atmospheric conditions and salinity change on the upper ocean are investigated. The model is integrated from November to June to cover the entire winter period. Objectively analyzed temperature and salinity fields for November are used as the initial conditions of the model ocean. Meteorological parameters derived from the NCEP reanalysis are used as forcings. The results show that heat loss of the ocean reaches a maximum at the end of January. High heat loss, 250 to 400 W m−2, occurs in the northern Labrador Sea between 56°N and 63°N in ice-free waters near the ice edge. Pack ice significantly reduces the heat loss with a typical value of 50 W m−2. In the northern Labrador Sea, surface cooling causes the mixed layer to deepen continuously through winter and reaches a maximum of 150–450 m in late March. A decrease of surface salinity by 1% (for example, from 34.35 to 34) results in a decrease of mixed-layer depth by 17–55%. The high stratification associated with the low surface salinity has little effect on ice coverage, since the reduced level of vertical mixing cannot lower the surface temperature sufficiently to increase the ice area. This suggests that freshening of the Labrador Sea during the Great Salinity Anomaly in the late 1960s to early 1970s did not promote ice production. An increase of wind speed by 40% and a decrease of air temperature up to 4°C, representing the conditions of cold winters, doubles the mixed-layer depth to a maximum of 900 m and extends the southern ice limit by 200 km on the northern Grand Banks. This suggests that interannual variation of convection and ice extent are mainly controlled by local meteorological conditions.