Characterization of physical properties of a coastal upwelling filament with evidence of enhanced submesoscale activity and transition from balanced to unbalanced motions in the Benguela upwelling region
<p>We combine high-resolution in situ data (acoustic Doppler current profiler (ADCP), Scanfish, and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the Benguela upwelling region. The 30–50 km wide and about 400 km long filament p...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2024-01-01
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| Series: | Ocean Science |
| Online Access: | https://os.copernicus.org/articles/20/103/2024/os-20-103-2024.pdf |
| Summary: | <p>We combine high-resolution in situ data (acoustic Doppler current profiler (ADCP), Scanfish, and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the Benguela upwelling region. The 30–50 km wide and about 400 km long filament persisted for at least 40 d. Mixed-layer depths were less than 40 m in the filament and over 60 m outside of it. Observations of the Rossby number <i>Ro</i> from the various platforms provide the spatial distribution of <i>Ro</i> for different resolutions. Remote sensing focuses on geostrophic motions of the region related to the mesoscale eddies that drive the filament formation and thereby reveals <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>|</mo><mtext mathvariant="italic">Ro</mtext><mo>|</mo><mo><</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="48pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="0ae3ee547d919550f206c52ee6d6dfcc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-103-2024-ie00001.svg" width="48pt" height="13pt" src="os-20-103-2024-ie00001.png"/></svg:svg></span></span>. Ship-based measurements in the surface mixed layer reveal <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0.5</mn><mo><</mo><mo>|</mo><mtext mathvariant="italic">Ro</mtext><mo>|</mo><mo><</mo><mn mathvariant="normal">1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="07b8510dbc66456cc09b63917c23b2d2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-103-2024-ie00002.svg" width="67pt" height="13pt" src="os-20-103-2024-ie00002.png"/></svg:svg></span></span>, indicating the presence of unbalanced, ageostrophic motions. Time series of <i>Ro</i> from triplets of surface drifters trapped within the filament confirm these relatively large <i>Ro</i> values and show a high variability along the filament. A scale-dependent analysis of <i>Ro</i>, which relies on the second-order velocity structure function, was applied to the latter drifter group and to another drifter group released in the upwelling zone. The two releases explored the area nearly distinctly and simultaneously and reveal that at small scales (<span class="inline-formula"><15</span> km) <i>Ro</i> values are twice as large in the filament in comparison to its environment with <span class="inline-formula"><i>Ro</i>>1</span> for scales smaller than <span class="inline-formula">∼500</span> m. This suggests that filaments are hotspots of ageostrophic dynamics, pointing to the presence of a forward energy cascade. The different dynamics indicated by our <i>Ro</i> analysis are confirmed by horizontal kinetic energy wavenumber spectra, which exhibit a power law <span class="inline-formula"><i>k</i><sup>−<i>α</i></sup></span> with <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">α</mi><mo>∼</mo><mn mathvariant="normal">5</mn><mo>/</mo><mn mathvariant="normal">3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="40pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f8676d5b267cfca25309b4d70c6069da"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-103-2024-ie00003.svg" width="40pt" height="14pt" src="os-20-103-2024-ie00003.png"/></svg:svg></span></span> for wavelengths <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">2</mn><mi mathvariant="italic">π</mi><mo>/</mo><mi>k</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="833607d1229f883e1e2cfc67324b5c28"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-103-2024-ie00004.svg" width="27pt" height="14pt" src="os-20-103-2024-ie00004.png"/></svg:svg></span></span> smaller than a transition scale of <span class="inline-formula">15</span> km, supporting significant submesoscale energy at scales smaller than the first baroclinic Rossby radius (<span class="inline-formula"><i>Ro</i><sub>1</sub>∼30</span> km). The detected transition scale is smaller than those found in regions with less mesoscale eddy energy, consistent with previous studies. We found evidence for the processes which drive the energy transfer to turbulent scales. Positive Rossby numbers <span class="inline-formula">𝒪</span>(1) associated with cyclonic motion inhibit the occurrence of positive Ertel potential vorticity (EPV) and stabilize the water column. However, where the baroclinic component of EPV dominates, submesoscale instability analysis suggests that mostly gravitational instabilities occur and that symmetric instabilities may be important at the filament edges.</p> |
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| ISSN: | 1812-0784 1812-0792 |