<p>Coastal river plumes and nonlinear internal waves (NLIWs) can both effectively mix salinity and temperature-stratified seas. Although river plumes themselves can generate NLIWs, direct measurements of turbulent mixing within the cooccurring phenomena remain unreported. Here, we identify NLIWs within the Merrimack River plume (MRP) during a ship-based observational campaign. Concurrent microstructure and acoustic Doppler current profiler (ADCP) sampling allowed for direct estimates of mixing type and strength at multiple points in time as NLIWs propagated beneath the sampling vessel. We observe NLIWs to initiate an ephemeral sequence of convective mixing on leading and trailing ends of counter-rotating wave cores and shear-dominated mixing within them. The superimposed NLIW and plume currents allow Froude numbers, <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="12237_2025_1575_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="24" /> </InlineMediaObject> <EquationSource Format="TEX">$Fr$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">Fr</mi> </mrow> </math></EquationSource> </InlineEquation>, to always equal or exceed unity, deviating from former work linking shear instability with <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="12237_2025_1575_Article_IEq2.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="54" /> </InlineMediaObject> <EquationSource Format="TEX">$Fr<1$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mi>r</mi> <mo><</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> and convection with <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="12237_2025_1575_Article_IEq3.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="54" /> </InlineMediaObject> <EquationSource Format="TEX">$Fr>1$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mi>r</mi> <mo>></mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>. We find that the offshore plume region influenced by NLIWs maintains vertical turbulent buoyancy flux magnitudes comparable to the energetic, inshore plume, indicating the waves augment offshore plume mixing. This work presents further evidence of NLIWs impacting coastal ocean processes and river plume mixing.</p>

Nonlinear Internal Wave Mixing Observations Within the Interior of a River Plume

期刊论文
发表日期：

2025年7月30日

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Preston Spicer,
Kimberly Huguenard,
Nicole L. Jones,
Agata Piffer-Braga,
Daniel G. MacDonald,
Michael M. Whitney,
Kelly L. Cole

摘要

Coastal river plumes and nonlinear internal waves (NLIWs) can both effectively mix salinity and temperature-stratified seas. Although river plumes themselves can generate NLIWs, direct measurements of turbulent mixing within the cooccurring phenomena remain unreported. Here, we identify NLIWs within the Merrimack River plume (MRP) during a ship-based observational campaign. Concurrent microstructure and acoustic Doppler current profiler (ADCP) sampling allowed for direct estimates of mixing type and strength at multiple points in time as NLIWs propagated beneath the sampling vessel. We observe NLIWs to initiate an ephemeral sequence of convective mixing on leading and trailing ends of counter-rotating wave cores and shear-dominated mixing within them. The superimposed NLIW and plume currents allow Froude numbers, $Fr$ $Fr$ , to always equal or exceed unity, deviating from former work linking shear instability with $Fr<1$ $F r < 1$ and convection with $Fr>1$ $F r > 1$ . We find that the offshore plume region influenced by NLIWs maintains vertical turbulent buoyancy flux magnitudes comparable to the energetic, inshore plume, indicating the waves augment offshore plume mixing. This work presents further evidence of NLIWs impacting coastal ocean processes and river plume mixing.