scholarly journals Competition between baroclinic instability and Ekman transport under varying buoyancy forcings in upwelling systems: An idealized analog to the Southern Ocean.

2021 ◽  
Author(s):  
Soeren Thomsen ◽  
Xavier Capet ◽  
Vincent Echevin
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Coastal Upwelling ◽  
Baroclinic Instability ◽  
Heat Fluxes ◽  
Ekman Transport ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Mesoscale Dynamics ◽  
Offshore Transport

AbstractCoastal upwelling rates are classically determined by the intensity of the upper-ocean offshore Ekman transport. But (sub-)mesoscale turbulence modulates offshore transport, hence the net upwelling rate. Eddy effects generally oppose the Ekman circulation, resulting in so-called “eddy cancellation”, a process well studied in the Southern Ocean. Here we investigate how air-sea heat/buoyancy fluxes modulate eddy cancellation in an idealized upwelling model. We run CROCO simulations with constant winds but varying heat fluxes with and without submesoscale-rich turbulence. Eddy cancellation is consistently evaluated with three different methods that all account for the quasi-isopycnal nature of ocean circulation away from the surface. For zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest and leads, near the coast, to nearly full cancellation of the Ekman cross-shore circulation by eddy effects, i.e., zero net mean upwelling flow. With increasing heat fluxes eddy cancellation is reduced and the transverse flow progressively approaches the classical Ekman circulation. Sensitivity of the eddy circulation to synoptic changes in air-sea heat fluxes is felt down to 125 m depth despite short experiments of tens of days. Mesoscale dynamics dominate the cancellation effect in our simulations which might also hold for the real ocean as the relevant processes act below the surface boundary layer. Although the idealized setting overemphasis the role of eddies and thus studies with more realistic settings should follow, our findings have important implications for the overall understanding of upwelling system dynamics.

Numerical Simulation of Air–Sea Coupling during Coastal Upwelling

2007 ◽  
Vol 37 (8) ◽  
pp. 2081-2093 ◽  
Author(s):  
Natalie Perlin ◽  
Eric D. Skyllingstad ◽  
Roger M. Samelson ◽  
Philip L. Barbour
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Coastal Upwelling ◽  
Heat Fluxes ◽  
Lower Atmosphere ◽  
Ekman Transport ◽  
Internal Boundary ◽  
Surface Boundary ◽  
Oregon Coast ◽  
Upwelled Water

Abstract Air–sea coupling during coastal upwelling was examined through idealized three-dimensional numerical simulations with a coupled atmosphere–ocean mesoscale model. Geometry, topography, and initial and boundary conditions were chosen to be representative of summertime coastal conditions off the Oregon coast. Over the 72-h simulations, sea surface temperatures were reduced several degrees near the coast by a wind-driven upwelling of cold water that developed within 10–20 km off the coast. In this region, the interaction of the atmospheric boundary layer with the cold upwelled water resulted in the formation of an internal boundary layer below 100-m altitude in the inversion-capped boundary layer and a reduction of the wind stress in the coupled model to half the offshore value. Surface heat fluxes were also modified by the coupling. The simulated modification of the atmospheric boundary layer by ocean upwelling was consistent with recent moored and aircraft observations of the lower atmosphere off the Oregon coast during the upwelling season. For these 72-h simulations, comparisons of coupled and uncoupled model results showed that the coupling caused measurable differences in the upwelling circulation within 20 km off the coast. The coastal Ekman transport divergence was distributed over a wider offshore extent and a thinner ocean surface boundary layer, with consistently smaller offshore and depth-integrated alongshore transport formed in the upwelling region, in the coupled case relative to the uncoupled case. The results indicate that accurate models of coastal upwelling processes can require representations of ocean–atmosphere interactions on short temporal and horizontal scales.

scholarly journals Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

2021 ◽  
Vol 13 (19) ◽  
pp. 3989
Author(s):  
Hiroshi Kuroda ◽  
Takashi Setou
Keyword(s):  
Boundary Layer ◽  
Pacific Ocean ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Northwestern Pacific ◽  
Surface Boundary ◽  
Northwestern Pacific Ocean ◽  
Surface Boundary Layer ◽  
Westerly Jet ◽  
Oceanic Surface

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

scholarly journals Characteristics of the Near-Surface Boundary Layer within a Mountain Valley during Winter

2012 ◽  
Vol 51 (3) ◽  
pp. 583-597 ◽  
Author(s):  
Warren Helgason ◽  
John W. Pomeroy
Keyword(s):  
Boundary Layer ◽  
Heat Fluxes ◽  
Quadrant Analysis ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
Wind Speed Profile ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Mountainous Regions ◽  
Mass Fluxes

AbstractWithin mountainous regions, estimating the exchange of sensible heat and water vapor between the surface and the atmosphere is an important but inexact endeavor. Measurements of the turbulence characteristics of the near-surface boundary layer in complex mountain terrain are relatively scarce, leading to considerable uncertainty in the application of flux-gradient techniques for estimating the surface turbulent heat and mass fluxes. An investigation of the near-surface boundary layer within a 7-ha snow-covered forest clearing was conducted in the Kananaskis River valley, located within the Canadian Rocky Mountains. The homogeneous measurement site was characterized as being relatively calm and sheltered; the wind exhibited considerable unsteadiness, however. Frequent wind gusts were observed to transport turbulent energy into the clearing, affecting the rate of energy transfer at the snow surface. The resulting boundary layer within the clearing exhibited perturbations introduced by the surrounding topography and land surface discontinuities. The measured momentum flux did not scale with the local aerodynamic roughness and mean wind speed profile, but rather was reflective of the larger-scale topographical disturbances. The intermittent nature of the flux-generating processes was evident in the turbulence spectra and cospectra where the peak energy was shifted to lower frequencies as compared with those observed in more homogeneous flat terrain. The contribution of intermittent events was studied using quadrant analysis, which revealed that 50% of the sensible and latent heat fluxes was contributed from motions that occupied less than 6% of the time. These results highlight the need for caution while estimating the turbulent heat and mass fluxes in mountain regions.

scholarly journals High-resolution observations of the North Pacific transition layer from a Lagrangian float

2021 ◽  
Author(s):  
Alexis K. Kaminski ◽  
Eric A. D’Asaro ◽  
Andrey Y. Shcherbina ◽  
Ramsey R. Harcourt
Keyword(s):  
Mixed Layer ◽  
Transition Layer ◽  
Vertical Motion ◽  
Heat Fluxes ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Diverse Range ◽  
The North ◽  
Temperature Jumps ◽  
Temperature Inversions

AbstractAcrucial region of the ocean surface boundary layer (OSBL) is the strongly-sheared and -stratified transition layer (TL) separating the mixed layer from the upper pycnocline, where a diverse range of waves and instabilities are possible. Previous work suggests that these different waves and instabilities will lead to different OSBL behaviours. Therefore, understanding which physical processes occur is key for modelling the TL. Here we present observations of the TL from a Lagrangian float deployed for 73 days near Ocean Weather Station Papa (50°N, 145°W) during Fall 2018. The float followed the vertical motion of the TL, continuously measuring profiles across it using an ADCP, temperature chain and salinity sensors. The temperature chain made depth/time images of TL structures with a resolution of 6cm and 3 seconds. These showed the frequent occurrence of very sharp interfaces, dominated by temperature jumps of O(1)°C over 6cm or less. Temperature inversions were typically small (≲ 10cm), frequent, and strongly-stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles. These structures are consistent with scouring behaviour rather than Kelvin-Helmholtz-type overturning. Their net effect, estimated via a Thorpe-scale analysis, suggests that these frequent small temperature inversions can account for the observed mixed layer deepening and entrainment flux. Corresponding estimates of dissipation, diffusivity, and heat fluxes also agree with previous TL studies, suggesting that the TL dynamics is dominated by these nearly continuous 10cm-scale mixing structures, rather than by less frequent larger overturns.

The Seasonality of submesoscale variability in the Antarctic Seasonal Ice Zone

2020 ◽  
Author(s):  
Isabelle Giddy ◽  
Sarah Nicholson ◽  
Marcel Du Plessis ◽  
Andy Thompson ◽  
Sebastiaan Swart
Keyword(s):  
Sea Ice ◽  
Mixed Layer ◽  
Climate Models ◽  
Critical Role ◽  
Heat Fluxes ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Antarctic Sea Ice ◽  
The Antarctic ◽  
Seasonal Ice Zone

<p>The ocean surface boundary layer in the Southern Ocean plays a critical role in heat and carbon exchange with the atmosphere. Submesoscale flows have been found to be important in setting mixed layer variability in the Antarctic Circumpolar Current (ACC). However, sparsity in observations, particularly south of the ACC in the Antarctic Seasonal Ice Zone (SIZ) where the horizontal density structure of the mixed layer is influenced by sea ice melt/formation and mesoscale stirring, brings into question the ability of climate models to correctly resolve mixed layer variability. We present novel fine-scale observations of the activity of submesoscale variability in the ice-free Antarctic SIZ using three deployments of underwater gliders over an annual cycle. Salinity-dominated density fronts of O(1)km associated with strong horizontal buoyancy gradients are observed during all deployments. There is evidence that stratifying ageostrophic eddies, energised by salinity driven submesoscale fronts are active across seasons, with intermittent equivalent heat fluxes of the same order to, or greater than local atmospheric forcing. This study highlights the need to consider future changes of Antarctic sea-ice in respect to feedback mechanisms associated with salinity (sea-ice) driven submesoscale flows. </p>

scholarly journals Seasonal and Diurnal Cycles of Surface Boundary Layer and Energy Balance in the Central Andes of Perú, Mantaro Valley

Atmosphere ◽  
2019 ◽  
Vol 10 (12) ◽  
pp. 779 ◽  
Author(s):  
José Luis Flores-Rojas ◽  
Joan Cuxart ◽  
Manuel Piñas-Laura ◽  
Stephany Callañaupa ◽  
Luis Suárez-Salas ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Energy Balance ◽  
Bowen Ratio ◽  
Heat Fluxes ◽  
Surface Boundary ◽  
Sensible Heat ◽  
Surface Boundary Layer ◽  
Mountain Valley ◽  
Arid Conditions

The present study presents a detailed analysis of the diurnal and monthly cycles the surface boundary layer and of surface energy balance in a sparse natural vegetation canopy on Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m ASL), which is located in the central Andes of Perú (Mantaro Valley) during an entire year (May 2018–April 2019). We used a set of meteorological sensors (temperature, relative humidity, wind) installed in a gradient tower 30 m high, a set of radiative sensors to measure all irradiance components, and a set of tensiometers and heat flux plate to measure soil moisture, soil temperatures and soil heat flux. To estimate turbulent energy fluxes (sensible and latent), two flux–gradient methods: the aerodynamic method and the Bowen-ratio energy-balance method were used. The ground heat flux at surface was estimated using a molecular heat transfer equation. The results show minimum mean monthly temperatures and more stable conditions were observed in June and July before sunrise, while maximum mean monthly temperatures in October and November and more unstable conditions in February and March. From May to August inverted water vapor profiles near the surface were observed (more intense in July) at night hours, which indicate a transfer of water vapor as dewfall on the surface. The patterns of wind direction indicate well-defined mountain–valley circulation from south-east to south-west especially in fall–winter months (April–August). The maximum mean monthly sensible heat fluxes were found in June and September while minimum in February and March. Maximum mean monthly latent heat fluxes were found in February and March while minimum in June and July. The surface albedo and the Bowen ratio indicate semi-arid conditions in wet summer months and extreme arid conditions in dry winter months. The comparisons between sensible heat flux ( Q H ) and latent heat flux ( Q E ), estimated by the two methods show a good agreement (R 2 above 0.8). The comparison between available energy and the sum of Q E and Q H fluxes shows a good level of agreement (R 2 = 0.86) with important imbalance contributions after sunrise and around noon, probably by advection processes generated by heterogeneities on the surface around the Huancayo observatory and intensified by the mountain–valley circulation.

scholarly journals Jet–Topography Interactions Affect Energy Pathways to the Deep Southern Ocean

2017 ◽  
Vol 47 (7) ◽  
pp. 1799-1816 ◽  
Author(s):  
Alice Barthel ◽  
Andrew McC. Hogg ◽  
Stephanie Waterman ◽  
Shane Keating
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Baroclinic Instability ◽  
Deep Ocean ◽  
Ocean Model ◽  
Shear Instability ◽  
Horizontal Shear ◽  
Mean Flow ◽  
Flat Bottom ◽  
Turbulent Dissipation

AbstractIn the Southern Ocean, strong eastward ocean jets interact with large topographic features, generating eddies that feed back onto the mean flow. Deep-reaching eddies interact with topography, where turbulent dissipation and generation of internal lee waves play an important role in the ocean’s energy budget. However, eddy effects in the deep ocean are difficult to observe and poorly characterized. This study investigates the energy contained in eddies at depth, when an ocean jet encounters topography. This study uses a two-layer ocean model in which an imposed unstable jet encounters a topographic obstacle (either a seamount or a meridional ridge) in a configuration relevant to an Antarctic Circumpolar Current frontal jet. The authors find that the presence of topography increases the eddy kinetic energy (EKE) at depth but that the dominant processes generating this deep EKE depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography. In cases with high topography, horizontal shear instability is the dominant source of deep EKE, while a flat bottom or a strongly sheared inflow leads to deep EKE being generated primarily through baroclinic instability. These results suggest that the deep EKE is set by an interplay between the inflowing jet properties and topography and imply that the response of deep EKE to changes in the Southern Ocean circulation is likely to vary across locations depending on the topography characteristics.

scholarly journals Evidence for the Influence of Surface Heat Fluxes on Turbulent Mixing of Microplastic Marine Debris

2016 ◽  
Vol 46 (3) ◽  
pp. 809-815 ◽  
Author(s):  
Tobias Kukulka ◽  
Kara L. Law ◽  
Giora Proskurowski
Keyword(s):  
Turbulent Transport ◽  
Surface Heat ◽  
Transport Processes ◽  
Heat Fluxes ◽  
Marine Debris ◽  
Surface Boundary ◽  
Heating Cycle ◽  
Surface Cooling ◽  
Surface Boundary Layer ◽  
Surface Heat Fluxes

AbstractBuoyant microplastic marine debris (MPMD) is a pollutant in the ocean surface boundary layer (OSBL) that is submerged by wave-driven turbulent transport processes. This study analyzes observed MPMD surface concentrations in the Atlantic and Pacific Oceans to reveal a significant increase in concentrations during surface heating and a decrease during surface cooling. Turbulence-resolving large-eddy simulations of the OSBL for an idealized diurnal heating cycle suggest that turbulent downward fluxes of buoyant tracers are enhanced at night, facilitating deep submergence of plastics, and suppressed in heating conditions, resulting in surface-trapped MPMD. Simulations agree better with observations if enhanced mixing due to wave-driven Langmuir turbulence (LT) is included. The simulated time-dependent OSBL response results in hysteresis effects so that surface concentrations depend also on the phase of the diurnal heating cycle. The results demonstrate the controlling influence of surface heat fluxes and LT on turbulent transport in the OSBL and on vertical distributions of buoyant marine particles.

The Role of Eddies in Determining the Structure and Response of the Wind-Driven Southern Hemisphere Overturning: Results from the Modeling Eddies in the Southern Ocean (MESO) Project

2006 ◽  
Vol 36 (12) ◽  
pp. 2232-2252 ◽  
Author(s):  
Robert Hallberg ◽  
Anand Gnanadesikan
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Ekman Transport ◽  
Global Ocean ◽  
Intermediate Cell ◽  
Intermediate Water ◽  
Deep Waters ◽  
Significant Difference ◽  
The Mean ◽  
The Impact

Abstract The Modeling Eddies in the Southern Ocean (MESO) project uses numerical sensitivity studies to examine the role played by Southern Ocean winds and eddies in determining the density structure of the global ocean and the magnitude and structure of the global overturning circulation. A hemispheric isopycnal-coordinate ocean model (which avoids numerical diapycnal diffusion) with realistic geometry is run with idealized forcing at a range of resolutions from coarse (2°) to eddy-permitting (1/6°). A comparison of coarse resolutions with fine resolutions indicates that explicit eddies affect both the structure of the overturning and the response of the overturning to wind stress changes. While the presence of resolved eddies does not greatly affect the prevailing qualitative picture of the ocean circulation, it alters the overturning cells involving the Southern Ocean transformation of dense deep waters and light waters of subtropical origin into intermediate waters. With resolved eddies, the surface-to-intermediate water cell extends farther southward by hundreds of kilometers and the deep-to-intermediate cell draws on comparatively lighter deep waters. The overturning response to changes in the winds is also sensitive to the presence of eddies. In noneddying simulations, changing the Ekman transport produces comparable changes in the overturning, much of it involving transformation of deep waters and resembling the mean circulation. In the eddy-permitting simulations, a significant fraction of the Ekman transport changes are compensated by eddy-induced transport drawing from lighter waters than does the mean overturning. This significant difference calls into question the ability of coarse-resolution ocean models to accurately capture the impact of changes in the Southern Ocean on the global ocean circulation.

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