Richard Pawlowicz

Professor

Research Classification

Oceans and Inland Waters

Research Interests

ocean physics
properties of seawater
geophysical fluid dynamics
Nonlinear waves

Relevant Degree Programs

 

Research Methodology

field observations

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Time-dependent inverse box-model for the estuarine circulation and primary productivity in the Strait of Georgia (2011)

During 2002–2006, a comprehensive set of observations covering physical, biological, radiative and atmospheric parameters was obtained from the southern Strait of Georgia (SoG), Western Canada by the STRATOGEM program. Monthly time series of estuarine layer transports over 2002–2005 were estimated using a time-dependent 2-box model in a formal inverse approach. These transports are then consistent with the temperature and salinity fields, as well as riverine freshwater inflow (R) and atmospheric heat fluxes. Uncertainty was analyzed by resampling observations using bootstrap methods. The transport time series were then combined with observations of nutrient concentrations to construct monthly time series of nutrient uptake for nitrate, phosphate, and silicic acid.Analysis of these time series suggests that the SoG estuarine circulation is not very sensitive to the seasonal changes of R. Comparison of the surface layer transport (U₁) and R yields the first observational relationship between the SoG estuarine circulation and R. This relationship (U₁=2.68 m²s⁻²/³× 10³ R¹/³) is consistent with estuarine theories. Although the flows change slightly with the freshet, a 5-fold change in R results only in a 40% change in U₁. Based on the calculated sink of near-surface nutrients, net primary productivity is estimated to be 212 gC m⁻²yr⁻¹, which is similar to values obtained differently in similar estuaries. Comparison of the nitrate and phosphate uptake rates suggests that the primary productivity (PP) is mainly new PP during spring and summer. Thus, PP is mainly controlled by the upwelling supply of nutrients through deep inflow and entrainment. The uptake of silicic acid (Si) is almost two times larger than the uptake of nitrate during diatom spring blooms, while it is similar during the summer blooms. Such a high Si uptake suggests that spring diatoms form heavier frustules or that heterotrophic silicoflagellates compete with diatoms for Si. Speculative considerations based on comparison of the estimated production rate of near-surface oxygen and new PP also suggest that the regenerated PP is small. In addition, the summer heterotrophic respiration might be in excess by as much as2 gO m⁻² d⁻¹ relative to the net PP.

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Multi-timescale analysis of the salinity and algal biomass of the Fraser River plume from repeated ferry transects (2009)

An instrumented ferry made eight transects per day across the Fraser River plume over the years 2003 - 2006 as part of the STRATOGEM program to study biophysical coupling in the Strait of Georgia. Water temperature, salinity, chlorophyll-a fluorescence, nitrate concentration, and dissolved oxygen were measured. This thesis utilizes salinity and chlorophyll-a fluorescence to study mixing in the plume, and the impact of the plume on algal biomass. First, the effects of river discharge and tides on plume salinity and surface area are quantified. Tidal fluctuations are caused by advection of the estuarine salt field, while fortnightly variations are caused by modulation of mixing in the estuary. Tidal and fortnightly variations are strongest at high river discharge and weakest at low discharge. Plume salinity decreases quasi-linearly with river discharge. Plume surface area increases with river discharge, from about 300 km² at low river flow to about 1,200 km² at high river flow, and can be predicted by scaling the river mouth deformation radius. Second, the plume fresh water flushing time is estimated and a salinity budget is constructed. Fresh water flushing time is 2.2 days, independent of river discharge. The quasi-steady budget predicts a vertical entrainment flux which varies with river discharge. The discharge-dependent vertical entrainment velocities in the estuary and plume implied by the entrainment flux are consistent with other methods. Flow speeds at the edge of the plume estimated from this method are too weak to maintain a plume front, suggesting fronts are transient and created on tidal time-scales. Third, a time series of surface and depth-integrated chlorophyll-a biomass is constructed. Chlorophyll-a fluorometer data are corrected for fluorescence quenching with a parameterization specific to the region, and then calibrated with extracted samples. Instantaneous along-track differences in surface chlorophyll-a can be large, however, averaged over the whole time series, the distribution is nearly uniform. In contrast, depth-integrated values are about 35% lower on average in the plume compared to surrounding waters. Interannual variability in biomass is partly due to the magnitude and duration of the spring bloom, which is itself influenced by wind mixing and grazing.

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Master's Student Supervision (2010 - 2018)
Spatial and temporal variability of double diffusive structures in Powell Lake, British Columbia (2016)

The spatial and temporal properties of naturally occurring double diffusive (DD) structures present in the bottom waters of Powell Lake, British Columbia were investigated. Observations were obtained from four annual surveys consisting of vertical cm-resolution conductivity-temperature-depth (CTD) profiles along the 9 km length of the lake, and from a month-long mooring consisting of thirty-eight temperature sensors and two current meters. DD layers were identified by isolating clusters on temperature-salinity (T-S) diagrams, and tracked spatially and temporally throughout each of the CTD surveys. The layers were observed to be persistent over four years, and horizontally coherent over the entire lake length at depths of 336-347 m. In this region the vertical density ratio (a non-dimensional measure of the relative strengths of vertical temperature and salinity gradients), Rρz, and buoyancy frequency, N, were near constant at Rρz = 2.2 ± 0.2 and N = (2.3 ± 0.3) x 10‐³ s‐¹, and the Rayleigh number reached a peak at Ra ≈ 10⁷. Layers just above and below this region were less horizontally-coherent and with larger values of Rρz and N. Spatial variations in layer depth and the background temperature/salinity distribution showed persistent trends throughout the study period. These trends indicated that layer slope and horizontal property gradients are linked and that the horizontal density ratio may be an indicator of the mean layer slope. Linear fits to the layer properties indicated that a horizontal density ratio of Rρx = -0.35 ± 0.17 was accompanied by a mean layer slope of ∆z/ ∆x = 0.05 ± 0.02 m/km. An individual DD step within one of the stable and horizontally coherent DD layers was identified within the moored temperature time series and tracked over the course of a week. The convective regime within the DD step was observed to be composed of intermittent thermal plumes emitted from the bottom diffusive interface. The features appeared as a common peak in the mean DD step temperature and horizontal velocity power spectra. The plumes had a period of ∼22 minutes (coinciding well with the buoyancy period within the diffusive interface), a temperature scale of T' ≈ 0.2 m°C, and horizontal and vertical velocity scales of u' = w' ≈ 0.5 mm/s.

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Oxygen budgets and productivity estimates in the Strait of Georgia from a continuous ferry-based monitoring system (2015)

The oxygen budget in the top 50 m of lower Strait of Georgia, British Columbia is investigated using high resolution measurements of dissolved oxygen concentration and other oceanographic and meteorological properties from an instrumented ferry. An budget equation is established to describe the oxygen balance in the surface Strait of Georgia. The budget equation consists of 4 parts, which includes (1) the storage rate term, which is calculated with the ferry oxygen measurements using a 2-point differential scheme; (2) advective and vertical transport, which is estimated using a box model; (3) air-sea gas transfer, which is estimated using a bulk parametrization of air-sea gas flux; and (4) net community productivity, which is estimated by taking the residual of the budget equation. To further investigate the productivity level in the Strait of Georgia, daily community respiration rate is estimated by extracting the diurnal variation signal of oxygen, and gross productivity is estimated by combining net community productivity with the community respiration rate. Results suggest that gross productivity in the lower Strait of Georgia varies from 1.4 to 11.8 gC.m-²day-¹ and averages at 4.4 gC.m-²day-¹, slightly higher than historical measurements.

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Double diffusion in Powell Lake : new insights from a unique case study (2013)

High resolution measurements of temperature and electrical conductivity in Powell Lake, British Columbia provide an extensive set of layer and interface observations of a double diffusive staircase found between 325–350 m depth. Powell Lake is an ex-fjord with a quiescent salt layer at thermal steady state in which double diffusion is naturally isolated from turbulent and advective processes. Layers are coherent on the basin scale and their characteristics have a well defined vertical structure. The steady state heat flux is estimated from the large-scale temperature profile and agrees with an earlier estimate of the flux in thesediments. These estimates are compared to a 4/3 flux parameterization which agrees with the steady state flux to within a factor of 2. The discrepancy is explained by testing the scaling underlying the parameterization directly, and it is found that the assumed power law deviates systematically from the observations. Consequently, a different scaling which better describes the observations is presented. The assumption that interfacial fluxes are dominated by molecular diffusion is tested by comparing the interfacial gradient to that expected from the steady state heat flux; at low density ratios, the average interfacial gradient is not sufficiently large to account for transport by molecular diffusion alone, indicating that double diffusive fluxes cannot generally be estimated from bulk interface properties. Salinity interfaces are only marginally (9%) smaller than temperature interfaces, and a simple model to describe the observed difference is presented and shown to be consistent with the observations.

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Net physical transports, residence times, and new production for Rivers Inlet, British Columbia (2011)

A hydrographic dataset from the 2008-2009 Rivers Inlet Ecosystem Study (RIES) field program was used (a) to provide a more complete oceanographic description of Rivers Inlet, British Columbia and (b) to develop the first quantitative estimates of estuarine circulation and new production for this system. Water column observations show a highly stratified two-layer estuarine structure, particularly in the spring and summer months when river discharge and atmospheric heat inputs were high. The net air-sea heat flux had a seasonal range of approximately 220 Wm⁻² and peaked almost a month earlier in 2008 than in 2009. The main source of river input comes from the Wannock River. As temperatures begin to rise in the spring, the river discharge can suddenly increase by an order of magnitude (from about 100 m³ s⁻¹ to almost 1000 m³ s⁻¹) in less than two weeks. Residence times (ie. first-order estimates of estuarine circulation) were estimated for every cruise using salinity and temperature budgets in a two-layer box model parameterization of the flow structure. The results show that upper box residence times vary seasonally with river discharge; dropping from about 14 days in the winter to as low as 4 days in the spring at the freshet onset. An earlier flushing event in 2009 caused residence times to drop earlier and could have caused higher advection losses for phytoplankton in the early spring. Overall, residence times averaged to about 7 days for the upper layer and about 165 days for the lower layer during periods of high river discharge, and about twice that during periods of low river discharge. Deep water in the lower layer below the sill was renewed almost once a year in summer and was affected only by vertical diffusion during the rest of the year. Finally, a spring/summer new production estimate of 0.6-1.7 gCm⁻²d⁻¹ (which implies about 110-300 gCm⁻²y⁻¹ assuming no production during the other months) was obtained by combining transport estimates with observations of nutrients to infer a surface nitrate sink. This range compares well with independent estimates made in nearby regions.

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