Longitudinal contrast in turbulence along a ∼ 19° S section in the Pacific and its consequences for biogeochemical fluxes
<p>Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific <span class="cit" id="xref_paren.1">(<a href="#bib1.bibx32">Moutin and Bonnet</a>, <a href="#bib1.bibx32">2015</a>...
| Main Authors: | , , , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-12-01
|
| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/7485/2018/bg-15-7485-2018.pdf |
| Summary: | <p>Microstructure measurements were performed along the
OUTPACE longitudinal transect in the tropical Pacific
<span class="cit" id="xref_paren.1">(<a href="#bib1.bibx32">Moutin and Bonnet</a>, <a href="#bib1.bibx32">2015</a>)</span>. Small-scale dynamics and turbulence in the first
800 m surface layer were characterized based on hydrographic and current
measurements at fine vertical scale and turbulence measurements at
centimeter scale using a vertical microstructure profiler. The possible
impact of turbulence on biogeochemical budgets in the surface layer was also
addressed in this region of increasing oligotrophy to the east. The
dissipation rate of turbulent kinetic energy, <span class="inline-formula"><i>ϵ</i></span>, showed an
interesting contrast along the longitudinal transect with stronger
turbulence in the west, i.e., the Melanesian Archipelago, compared to the
east, within the South Pacific Subtropical Gyre, with a variation of
<span class="inline-formula"><i>ϵ</i></span> by a factor of 3 within [100–500 m]. The layer with enhanced
turbulence decreased in vertical extent travelling eastward. This spatial
pattern was correlated with the energy level of the internal wave field,
higher in the west compared to the east. The difference in wave energy mostly
resulted from enhanced wind power input into inertial motions in the west.
Moreover, three long-duration stations were sampled along the cruise
transect, each over three inertial periods. The analysis from the western
long-duration station gave evidence of an energetic baroclinic near-inertial
wave that was responsible for the enhanced <span class="inline-formula"><i>ϵ</i></span>, observed within a
50–250 m layer, with a value of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">8</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">9</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="42pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7ab3425fe6839fc6c9d3c2f71f932374"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-7485-2018-ie00001.svg" width="42pt" height="14pt" src="bg-15-7485-2018-ie00001.png"/></svg:svg></span></span> W kg<span class="inline-formula"><sup>−1</sup></span>, about 8
times larger than at the eastern long-duration stations. Averaged nitrate
turbulent diffusive fluxes in a 100 m layer below the top of the
nitracline were about twice larger west of 170<span class="inline-formula"><sup>∘</sup></span> W due to the
higher vertical diffusion coefficient. In the photic layer, the
depth-averaged nitrate turbulent diffusive flux strongly decreased
eastward, with an averaged value of 11 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="normal">µ</mi><msup><mi mathvariant="normal">molm</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="da616652040692bb628e11c98a455ad0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-7485-2018-ie00002.svg" width="62pt" height="15pt" src="bg-15-7485-2018-ie00002.png"/></svg:svg></span></span> west of
170<span class="inline-formula"><sup>∘</sup></span> W compared with the 3 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="normal">µ</mi><msup><mi mathvariant="normal">molm</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="640de40df36e813b435790306f198a58"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-7485-2018-ie00003.svg" width="62pt" height="15pt" src="bg-15-7485-2018-ie00003.png"/></svg:svg></span></span> averaged
value east of 170<span class="inline-formula"><sup>∘</sup></span> W. Contrastingly, phosphate turbulent diffusive
fluxes were significantly larger in the photic layer. This input may have an
important role in sustaining the development of <span class="inline-formula">N<sub>2</sub></span>-fixing organisms
that were shown to be the main primary contributors to the biological pump in
the area. The time–space intermittency of mixing events, intrinsic to
turbulence, was underlined, but its consequences for micro-organisms would
deserve a dedicated study.</p> |
|---|---|
| ISSN: | 1726-4170 1726-4189 |