Thompson, Keith R.
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Item Open Access Surface heat flux, horizontal advection, and the seasonal evolution of water temperature on the Scotian Shelf(1994) Umoh, JU; Thompson, K. R.Seasonal temperature variations on the Scotian Shelf penetrate to a depth of about 75 m. The net surface heat flux (Q) can explain about 85% of the annual cycle in the rate of change of mean temperature over this depth range. The long-term mean heat budget for a midshelf location shows that horizontal advection (-40 W m super(-2)) is almost exactly balanced by the combined contributions of Q (25 W m super(-2)), horizontal mixing (11 W m super(-2)), and vertical diffusion (6 W m super(-2)). The seasonal evolution of temperature structure on the mid-Scotian Shelf is modeled with a one-dimensional vertical diffusion equation, modified to include the effect of horizontal advection. The model features a vertical eddy diffusivity (K sub(v)) that varies with the background density stratification (N) according to the parameterization K sub(v) = K sub(0)(1 + alpha N super(p)) super(-1). The three free parameters (K sub(0), alpha and p) are estimated by best fitting, in a least squares sense, the predicted temperatures to observations at standard depths between the surface and 100 m. Typical values of K sub(v) lie in the range 0.2-20 x 10 super(-4) m super(2) s super(-1) with the highest values found in winter and close to the surface, as expected. The root mean square of the differences between observed and predicted monthly temperatures is small and equal to 1.0 degree C. The root mean square of the errors increases significantly (from 1.0 degree C to 3.1 degree C) on fitting with a constant K sub(v), highlighting the importance of allowing K sub(v) to vary with depth and time. The diffusion model is finally used to examine the role of Q and horizontal advection in the seasonal evolution of the temperature profile on the Scotian Shelf and, in particular, the cold intermediate layer which is a prominent feature of the hydrography on the midshelf.Item Open Access Effects of temperature and circulation on the population dynamics of Calanus finmarchicus in the Gulf of St. Lawrence and Scotian Shelf: Study with a coupled, three-dimensional hydrodynamic, stage-based life history model(2003-11) Zakardjian, BA; Sheng, JY; Runge, JA; McLaren, I.; Plourde, S.; Thompson, KR; Gratton, Y.We developed a physical-biological model for the Gulf of St. Lawrence (GSL) and Scotian Shelf (SS) by coupling a stage-based life-history model of the planktonic copepod Calanus finmarchicus to a three-dimensional ocean circulation model. The life-history model consists of 13 morphologically distinct life stages of C. finmarchicus, with stage-specific and temperature-dependent molting rates. The model also includes stage-specific vertical distribution and seasonally varying diapause, egg production, and stage-specific mortality rates. The model domain covers the eastern Canadian shelf from 55degreesW to 72degreesW and from 39degreesN to 52degreesN, including the Gulf of St. Lawrence, Scotian Shelf, and Gulf of Maine. A comparison of a 1-year simulation with observations indicates that the physical-biological model reasonably describes the observed abundance and distribution of C. finmarchicus in this region. To determine the effects of ocean circulation in the C. finmarchicus population dynamics, we divided the GSL-SS region into eight sub-areas and compared the net fluxes of C. finmarchicus across lateral boundaries to the net production in each sub-area. We found that the annual cross-boundary exchange rates constitute from <1% to 39% of the local net production, indicating that the horizontal transport of C. finmarchicus by the ocean currents can play a very important role in the dynamics of local C. finmarchicus populations. The results provide insights into the mechanisms of exchange in the GSL-SS system, as put forward in recent hypotheses.Item Open Access Assimilating long-term hydrographic information into an eddy-permitting model of the North Atlantic(American Geophysical Union, Washington, DC, 2006-09) Wright, D. G.; Thompson, K. R.; Lu, Y.No abstract available.Item Open Access Mapping the relationship between northern hemisphere winter surface air temperature and the Madden-Julian oscillation(American Meteorological Society, 2011-08) Zhou, Yang; Thompson, K. R.; Lu, YouyuA regression-based modeling approach is described for mapping the dependence of atmospheric state variables such as surface air temperature (SAT) on the Madden-Julian oscillation (MJO). For the special case of a linear model the dependence can be described by two maps corresponding to the amplitude and lag of the mean atmospheric response with respect to the MJO. In this sense the method leads to a more parsimonious description than traditional compositing, which usually results in eight maps, one for each MJO phase. An- other advantage of the amplitude and phase maps is that they clearly identify propagating signals, and also regions where the response is strongly amplified or attenuated. A straightforward extension of the linear model is proposed to allow the amplitude and phase of the response to vary with the amplitude of the MJO or indices that define the background state of the atmosphere-ocean system. Application of the approach to global SAT for boreal winter clearly shows the propagation of MJO-related signals in both the tropics and extratropics and an enhanced response over eastern North America and Alaska (further enhanced during La Nina years). The SAT response over Alaska and eastern North America is caused mainly by horizontal advection related to variations in shore-normal surface winds that, in turn, can be traced (via signals in the 500-hPa geopotential height) back to MJO-related disturbances in the tropics.Item Open Access Prediction of surface currents and drifter trajectories on the inner Scotian Shelf(2003-09) Thompson, KR; Sheng, JY; Smith, PC; Cong, LZThe predictive skill of a simple model of surface flow on the Scotian Shelf is assessed using oceanographic data collected in February 1996. The model is forced by wind stress, water density, and sea level along the open boundaries of the model domain. The skill of the model with respect to subtidal variations of alongshore current and bottom pressure is quantified by the ratio gamma(2) = Var(O - P)/Var(O), where Var(O - P) and Var(O) are the variance of the prediction errors and observations, respectively. Skill is highest for bottom pressure (gamma(2) = 0.2) followed, in order, by cross-shelf gradients in bottom pressure (gamma(2) = 0.5), horizontally averaged currents (0.4 < ?(2) < 0.7), and currents at individual current meters (0.6 < ?(2) < 1.4). The skill of the model with respect to drifter position is quantified by the search radius, centered on the predicted drifter position, that ensures a 50% chance of locating the drifter. Skill varies significantly with time but is generally highest when the drifter motion is strongest. We conclude with a comparison of the performance of the model against predictions based on the release point of the drifters and the operational scheme presently used to guide marine search and rescue in this region.Item Open Access Tropical Pacific Ocean and the Madden-Julian Oscillation: Role of wind and buoyancy forcing(2010-05) Zhang, Xu; Lu, Youyu; Thompson, Keith R.; Jiang, Jing; Ritchie, HalA global ocean circulation model is used to examine the dynamical response of the tropical Pacific Ocean to forcing associated with the Madden-Julian Oscillation (MJO). Model sensitivity experiments first reveal that MJO-related changes in sea level are caused primarily by changes in wind stress. Further, the MJO-related changes in sea surface temperature (SST) are mainly caused by buoyancy (heat) flux in the Indian Ocean, by wind stress in the central tropical Pacific, and by both buoyancy flux and wind stress in the eastern tropical Pacific. Additional model sensitivity studies quantify the tropical Pacific Ocean response to MJO wind forcing. The simulations reveal that the subsurface temperature variations associated with the MJO propagate eastward along the thermocline and rise to the surface in the eastern Pacific. Zonal advection plays a dominant role in SST variation in the central Pacific; vertical advection plays an important role in the evolution of subsurface and surface temperatures in the eastern Pacific. Finally, it is shown that MJO wind forcing can rectify SST variations through nonlinear interactions of the intraseasonal variations of the zonal currents and the zonal SST gradient.Item Open Access Time-Averaged Forms of the Non-Linear Stress Law(1983) WRIGHT, DG; THOMPSON, KRNo abstract available.Item Open Access Subtidal circulation on the Scotian Shelf: assessing the hindcast skill of a linear, barotropic model(American Geophys. Union, 1997-11/15) Thompson, K. R.; Sheng, JinyuCurrents measured during the winter of 1985-1986 by four moorings on the inner Scotian Shelf are used to assess the hindcast skill of a three-dimensional circulation model forced by local wind stress and coastal sea level. The model is linear and barotropic. The integration scheme is based on the modification of the Galerkin spectral method proposed recently by Sheng and Thompson 1993]. The skill of the model is measured by the variance of the hindcast errors divided by the variance of the observations (henceforth 2). The model is most effective within 30 km of shore (0.5120.66). At the mooring in the Nova Scotia Current, a surface intensified southwestward jet with its center approximately 50 km from shore, the model fails to capture the bulk of the variance at current meters within 70 m of the surface (0.8120.87). The skill of the model is lowest at the mooring 65 km from shore (0.9220.94). To put these measures of skill into perspective, the currents are hindcast using a linear statistical model with the same inputs as the circulation model. The statistical model is optimal in the sense no other linear model with these inputs can achieve a lower hindcast error variance. For half of the current meters within 30 km of shore and two in the Nova Scotia Current the skill of the circulation model is not significantly lower than that of the statistical model. The largest discrepancies in the skill of the two models are found at the offshore mooring. The authors show that the suboptimal performance of the circulation model is due in part to the assumption of a spatially uniform wind field. They speculate that another contributing factor is the assumed form of the sea level profile along the open boundary that is upstream in the sense of coastal trapped wave propagationItem Open Access Mean surface topography of the northwest Atlantic: Comparison of estimates based on satellite, terrestrial gravity, and oceanographic observations(American Geophysical Union, 2009-07/08) Thompson, K. R.; Huang, J.; Veronneau, M.; Wright, D. G.; Lu, Y.The accuracy of a new mean sea surface topography (MSST) of the northwest Atlantic is assessed. The MSST is estimated from 12 years of altimeter observations referenced with respect to a new regional geoid based on a blend of Gravity Recovery and Climate Experiment (GRACE), terrestrial, and altimeter-derived gravity data. The new MSST is first compared to a recently published mean surface topography calculated using an eddy-permitting model of the North Atlantic. Geostropic currents calculated from the GRACE-based MSST are next compared to mean surface currents in the northwest Atlantic estimated from the motion of near-surface drifters corrected for surface Ekman effects. Finally, the mean path of the Gulf Stream is compared to the line of zero skewness of sea level variability which provides a measure of the mean path of unstable, intense ocean currents. Overall the agreement amongst the various estimates of surface topography and circulation is excellent. There are, however, some significant differences that can be separately attributed to problems with the MSST and, in some cases, with the ocean model (in particular the ocean climatology to which it was nudged).Item Open Access Estimation of Low-Frequency Wind Stress Fluctuations Over the Open Ocean(1983) THOMPSON, KR; MARSDEN, RF; WRIGHT, DGNo abstract available.Item Open Access A model of the circulation on the outer Scotian shelf with open boundary conditions inferred by data assimilation(American Geophys. Union, 1998-12/15) Thompson, K. R.; Griffin, D. A.The circulation on Western Bank is described using data collected in spring 1991 and 1992 as part of an interdisciplinary study of the early life history of Atlantic cod (Gadus morhua). The mean circulation over the crest of the bank is weak, of the order of a few centimeters per second, and is flanked to the south and west by an anticyclonic flow with a speed of about 10 cm s-1. A thermal wind calculation shows the mean circulation is due primarily to horizontal variations in the density field. The M2 tidal constituent is dominant and reaches speeds of about 20 cm s-1. The tidal residuals have a standard deviation of about 10 cm s-1 and characteristic time and length scales of several days and tens of kilometers, respectively. In order to help collect and interpret biological data from Western Bank, we developed a hydrodynamic model of the near-surface flow that could be used operationally. The model is based on a decomposition of the flow into components driven by (1) local wind stress, (2) horizontal density gradients, and (3) flows through the open boundaries of the model. The wind-driven component is calculated using a simple slab model driven by the observed wind. The other two components are estimated through the assimilation of observed bottom pressures, dynamic heights, and currents into the hydrodynamic model. The hindcast skill of the model is quantified by cross validation and shown to be higher than that of four simple, statistically based schemesItem Open Access A robust method for diagnosing regional shelf circulation from scattered density profiles(1996-11) Sheng, JY; Thompson, KRWe present a straightforward method for estimating surface circulation on an f plane from a set of irregularly spaced vertical density profiles. The first step is to express bottom density rho(h) as the sum of a mean for a given water depth h and an anomaly, = rho(h) - . Sea level eta can then be decomposed into a dynamic height ij relative to a deep reference level and a correction term, eta' = eta - (n) over tilde. The dynamic height is estimated using a generalization of the method of Helland-Hansen [1934] for diagnosing flow through a cross-shelf section under the assumption of zero bottom geostrophic flow. The correction eta' satisfies a two-dimensional elliptic partial differential equation forced by the bottom density anomaly, wind stress and the open boundary conditions, Before calculating the density-driven component of eta' we first test if the rho(h)' are statistically different from uncorrelated noise. If they are not, the correction associated with the bottom density anomaly is set to zero. Thus the method has a degree of robustness to errors in the density observations. If the rho(h)' have well-defined spatial structure the elliptic equation is solved for eta' and sea level is equated to + eta'. Note that even if this last step is required the only gridding of the density data is two-dimensional. This makes the proposed method simpler to use than many of the existing diagnostic models which require a three-dimensional gridding of the observed density profiles, To test the method, we use it to diagnose the flow from an idealized density field overlying an isolated topographic feature. The predicted sea level and flow fields are then compared, and shown to be in good agreement, with results from the Princeton Ocean Model, The method is then used to diagnose the winter surface circulation on the Scotian Shelf from observed density profiles, The reliability of the diagnosed flow pattern is assessed by comparing it against all available near-surface current measurements. The differences between the observed and diagnosed currents are used to estimate the remotely forced circulation on the Scotian Shelf. It is shown that the main features of the circulation can be explained by gradients in the density field, Remote forcing is important near the coast and the shelf break, The effect of local wind is relatively weak.Item Open Access Modeling events of sea-surface variability using spectral nudging in an eddy permitting model of the northeast Pacific Ocean(2006-06) Stacey, Michael W.; Shore, Jennifer; Wright, Daniel G.; Thompson, Keith R.[ 1] Eddies are an important part of the current system that hugs the coasts of British Columbia and Alaska. The ability of "spectral nudging'' to improve the eddy statistics determined from model simulations of this current system is investigated. Spectral nudging differs from standard nudging in that only specified frequency and wave number bands of the simulated potential temperature and salinity fields are nudged toward the observed climatology. Therefore the simulated eddy field can develop and evolve with time while the model is prevented from drifting far from the observed climatology. The Parallel Ocean Program ( POP) is used to do the simulations, with 0.25 degrees horizontal resolution and 23 vertical levels. The simulated standard deviation and skewness fields for the sea surface height are compared with those estimated from ten years of TOPEX/ Poseidon altimetry observations. This comparison shows that spectral nudging allows the model to simulate the eddy statistics of the current system with significantly more accuracy than when the nudging is not used.Item Open Access A modified galerkin-spectral model for three-dimensional, barotropic, wind-driven shelf circulation(1993) Sheng, Jinyu; Thompson, K. R.The authors describe an efficient numerical scheme for calculating wind-driven currents on the continental shelf. Our scheme is based on the spectral approach introduced by Heaps and subsequently modified by Lardner. The basic idea behind Heaps' approach is to express the horizontal flow, u(x, y, z, t), as a linear combination of vertical structure functions, Phi sub(r)(z), and then solve numerically for the temporally and horizontally varying coefficients. To obtain an accurate representation of wind-driven flow, many Phi sub(r) are often required. Following Lardner, we reduce this number by subtracting from u and analytically defined flow field, u sub(p), prior to its expansion in terms of the Phi sub(r). Our choice of u sub(p) is steady Ekman flow in water of finite depth. This particular choice includes, as a special case, the u sub(p) used by Lardner. Using an idealized basin and time-harmonic wind forcing, we compared the convergence rate of the expansion of u-u sub(p) with u sub(p) taken to be zero, corresponding to Heaps' approach, flow with constant horizontal shear stress through the vertical, corresponding to Lardner's recent suggestion, and steady Ekman flow. We find that removal of steady Ekman flow generally leads to the most rapid convergence, particularly when the water depth is much greater than the Ekman depth, a condition often found on the middle and outer continental shelf.Item Open Access Predicting the frequency of storm surges and extreme sea levels in the northwest Atlantic(2006-10) Bernier, N. B.; Thompson, K. R.[1] A 40 year hindcast of storm surges in the northwest Atlantic and adjacent shelf seas is performed using a 2-D nonlinear barotropic ocean model forced by realistic 6 hourly winds and air pressures. This hindcast is used to generate spatial maps of the return level of storm surges and also to estimate the return period of extreme total sea levels. The accuracy of the hindcast is assessed in two ways. First, the standard deviation of the difference between the observed residuals (total sea level minus tide) and the hindcast is calculated at 24 tide gauge locations. A typical error standard deviation is 8 cm. Second, the 40 year return level of observed residuals is compared to that of the hindcast surges. The predicted 40 year return levels are typically within 10 cm of observed return levels at the 24 observation locations. A spatial map of the 40 year return level of surges is presented for the northwest Atlantic. It identifies the regions exposed to the largest surges. Total sea levels are reconstructed using (1) the hindcast surges and (2) tides and higher-frequency variability predicted from short, observed sea level records. An extremal analysis of the reconstructed total sea levels shows that their 40 year return levels are in good agreement (within about 10 cm) with the levels calculated from multidecadal observed sea level records. This means that given a short record anywhere within the model domain, or results from a good tidal model, 40 year return levels can be estimated.Item Open Access Madden-Julian Oscillation and sea level: Local and remote forcing(American Geophysical Union, 2010-01) Oliver, ECJ; Thompson, K. R.The Madden-Julian Oscillation (MJO) is the dominant mode of atmospheric variability in the tropical atmosphere on intraseasonal time scales (i.e., weeks to seasons). This study examines the connection between the MJO and global sea level measured by altimeters over the last 17 years. We first identify regions exhibiting a significant (both statistical and practical) relationship between sea level and the MJO. The first region consists of the equatorial Pacific and western coastal zones of North and South America. Consistent with previous studies, we identify wind-driven equatorially trapped Kelvin waves that propagate eastward along the equatorial Pacific and then transform into coastal trapped waves that propagate poleward along the western coasts of North and South America. The second region includes the shallow waters of the Gulf of Carpentaria (off Australia's north coast) and the adjacent Arafura and Timor seas. Setup by onshore winds is shown to be the dominant physical process. Finally, the northeastern Indian Ocean is shown to be a complex region involving a combination of equatorially trapped Kelvin waves, coastal trapped waves, and westward-propagating Rossby waves exhibiting characteristics of both local and remote forcing. The implications of the results for deep and coastal ocean forecasting are discussed.Item Open Access Return periods of extreme sea levels from short records(1986-10/15) Middleton, J. F.; Thompson, K. R.Extreme sea levels usually arise from a combination of the tides (assumed here to be deterministic) and storm surges (assumed stochastic). The authors show how tide and surge statistics derived from short (~1 year) records can be used to predict the occurrence of extremes with much longer return periods (~50 years). The method is based on an extension of the exceedance theory originally developed by Rice (1954) to study noise in electrical circuits. A comparison of predicted return periods with those obtained directly from a 50-year Markovian simulation of surge is used to validate the exceedance probability method. The method is next applied to the Canadian ports of Halifax and Victoria, which are dominated by semidiurnal and diurnal tides, respectivelyItem Open Access A description of water types on the Mackenzie Shelf of the Beaufort Sea during winter(1992) Moore, R. M.; Melling, H.; Thompson, K. R.For a number of years during the 1980s, observations of the physical and chemical properties of seawater in the southeastern Beaufort Sea have been acquired in late winter. The most complete data set, from 1987, has been used in a comparison of winter and summer (Macdonald et al., 1989) water properties in the area. Most obvious is an increase in the salinity of surface waters in winter. The magnitude of this increase varies dramatically from year to year. Part of the increase is a consequence of brine rejection during the growth of sea ice, and part is associated with an intrusion over the shelf of a water mass of high nutrient and low oxygen concentrations which is a feature of the entire western Arctic Ocean. Principal component analysis was used to allow all five chemical tracers to be combined and viewed simultaneously. The properties of the upper 120 m are found to lie, to a close approximation, on a plane. This leads us to simple interpretation based on a three-component mixing model involving river runoff, water from the nutrient maximum, and an offshore near-surface component. It is shown that the best fit plane occupied by arctic surface waters in the Beaufort Sea closely matches that defined by the influences of river inflow, of the freeze-melt cycle, and of photosynthesis and respiration. However, the effects of freezing/melting and of river inflow cannot be clearly distinguished using the chosen suite of tracers. It has been determined that if the waters of the upper 250 m are to be represented in the same manner, a fourth end member is required.Item Open Access Sea level and circulation variability of the Gulf of Carpentaria; influence of the Madden-Julian Oscillation and the adjacent deep ocean(American Geophysical Union, Washington, DC, 2011) Oliver, E. C. J.; Thompson, K. R.The Madden-Julian Oscillation (MJO) is a significant contributing factor to intraseasonal variability in both the tropical and extratropical atmosphere and ocean. Sea level and circulation variations in the Gulf of Carpentaria (northern Australia) and the coastal regions of the northeastern Indian Ocean and eastern Pacific have been shown to be related to the MJO. A nonlinear barotropic numerical model, validated with local tide gauge data, is used to study the Gulf of Carpentaria from 1979 to 2009. It is shown that the model reproduces well the seasonal cycle of sea level as well as intraseasonal variations and their seasonal modulations. Intraseasonal variability is shown to be driven by surface wind stress that is closely related to the MJO. The model is next used to remove the local wind effect from the tide gauge data, resulting in a low-frequency residual signal. This low-frequency signal is interpreted in terms of larger-scale modes of variability of the adjacent shelf seas and the Indian and Pacific oceans through comparison with climatological indices, correlations with regional sea level measured by altimeters, and Hovmoeller diagrams. It is shown that this signal is generated in the Pacific and related to the El Nino-Southern Oscillation. The implications for predictability and forecasting in the Gulf of Carpentaria on intraseasonal timescales are discussed.Item Open Access Predicting mesoscale variability of the North Atlantic using a physically motivated scheme for assimilating altimeter and Argo observations(American Meteorological Society, 2009-07) Liu, Yimin; Thompson, K. R.A computationally efficient scheme is described for assimilating sea level measured by altimeters and vertical profiles of temperature and salinity measured by Argo floats. The scheme is based on a transformation of temperature, salinity, and sea level into a set of physically meaningful variables for which it is easier to specify spatial covariance functions. The scheme also allows for sequential correction of temperature and salinity biases and online estimation of background error covariance parameters. Two North Atlantic applications, both focused on predicting mesoscale variability, are used to assess the effectiveness of the scheme. In the first application the background is a monthly temperature and salinity climatology and skill is assessed by how well the scheme recovers Argo profiles that were not assimilated. In the second application the backgrounds are short-term forecasts made by an eddy-permitting model of the North Atlantic. Skill is assessed by the quality of forecasts with lead times of 1-60 days. Both applications show that the scheme has useful skill.