Sensitivity of the Fraser River Plume to Winds, Tides and Freshwater Flux

Jie Liu, Susan Allen, Nancy SoontiensEarth, Ocean and Atmospheric Sciences

The University of British Columbia

Mixing in the plume• Rivers carry more than

one-third of land-based precipitation to the ocean (Trenberth et al., 2007).

• Impact of the river plume on shelf circulation and ecosystem health partly depends on the vertical mixing dilution rate (A.R.Horner-Devine et al., 2015).

• Multiple forcing factors contribute to mixing in the river plume.

R.D.Hetland, 2005

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Objective

Examine the sensitivity of the mixing of the Fraser River plume to external forcing factors, especially tides and winds in a three-dimensional numerical model.

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Outline

• Model description• The Fraser River plume• Model runs• Analysis• Results• Conclusion and future work

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Model description• NEMO 3.4• ~500m horizontal 1-27m vertical• Improved from

Soontiens et al., 2015, with a long river and decreased background vertical eddy viscosity to sm 125-101

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The Fraser River plume

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Model runs

• Run 1 ： only Fraser River (baseline)• Run 2: Run1 + tides (no winds)• Run 3: Run1 + winds (no tides)• Run 4: Run1+ tides + winds (combine all)

• Period: September 25~October 31, 2014• Analysis: October 27~October 31, 2014

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Geometry of analysis

• Long river channel

• River flows in the last five days of October, 2014: ~2500 sm 13

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• only river case: strongest stratification

• Winds are more important than tides for vertical mixing at south of Sandheads, indicating it is in the far-field region.

ResultsRun1: only river Run2: river + tides

Run3: river + winds Run4: river + tides+ winds

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Rmix = SS sur

0

f

Where is the surface salinity and is the averaged salinity in the water column.

S surf

S 0

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(E.Masunaga et al., 2011)

Run1: only river Run2: river + tides

Run3: river + winds

Run4: river + tides+ winds

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Summary of Rmix along the transect

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Spatial distribution of RmixAll – (river + tides) = winds All – (river + winds) = tides

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Conclusion• Near-field and far-field mixing are reproduced in the model at a moderate

river flow period. Near-field region extends about 12 km from the river mouth of Steveston in the model.

Future work

• Investigate the mixing in the plume response to forcing factors under the low and high river flows conditions, respectively.• Isolate and quantify the total volume flux and fresh water transport at along/cross-strait transect advected by each forcing agent.

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ReferenceTrenberth, Kevin E., et al. "Estimates of the global water budget and its annual cycle using observational and model data." Journal of Hydrometeorology 8.4 (2007): 758-769.

Hetland, Robert D. "Relating river plume structure to vertical mixing." Journal of Physical Oceanography 35.9 (2005): 1667-1688.

Horner-Devine, Alexander R., Robert D. Hetland, and Daniel G. MacDonald. "Mixing and transport in coastal river plumes." Annual Review of Fluid Mechanics47 (2015): 569-594.

Soontiens, Nancy, et al. "Storm Surges in the Strait of Georgia Simulated with a Regional Model." Atmosphere-Ocean (2015): 1-21.

Masunaga, Eiji, Oliver B. Fringer, and Hidekatsu Yamazaki. "An observational and numerical study of river plume dynamics in Otsuchi Bay, Japan." Journal of Oceanography (2015): 1-19.

Thank you!Any questions?