scholarly journals Turbulent Large-Eddy Momentum Flux Divergence during High-Wind Events

2017 ◽  
Vol 47 (6) ◽  
pp. 1493-1517 ◽  
Author(s):  
H. W. Wijesekera ◽  
D. W. Wang ◽  
E. Jarosz ◽  
W. J. Teague ◽  
W. S. Pegau ◽  
...  
Keyword(s):  
Wind Stress ◽  
Mixed Layer ◽  
Momentum Flux ◽  
Mixed Layer Depth ◽  
Vertical Component ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
High Wind ◽  
Flux Divergence ◽  
Wind Events

AbstractMomentum transport by energy-containing turbulent eddies in the oceanic mixed layer were investigated during high-wind events in the northern Gulf of Alaska off Kayak Island. Sixteen high-wind events with magnitudes ranging from 7 to 22 m s−1 were examined. Winds from the southeast prevailed from one to several days with significant wave heights of 5–9 m and turbulent Langmuir numbers of about 0.2–0.4. Surface buoyancy forcing was much weaker than the wind stress forcing. The water column was well mixed to the bottom depth of about 73 m. Spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10–30-min time scales. The ratio of horizontal scale to mixed layer depth was from 2 to 8. Turbulent shear stresses in the mixed layer were horizontally asymmetric. The downwind turbulent stress at 10–20 m below the surface was approximately 40% of the averaged wind stress and was reduced to 5%–10% of the wind stress near the bottom. Turbulent kinetic energy in the crosswind direction was 30% larger than in the downwind direction and an order of magnitude larger than the vertical component. The averaged eddy viscosity between 10- and 30-m depth was ~0.1 m2 s−1, decreased with depth rapidly below 50 m, and was ~5 × 10−3 m2 s−1 at 5 m above the bottom. The divergence of turbulent shear stress accelerated the flow during the early stages of wind events before Coriolis and pressure gradient forces became important.

scholarly journals Air–Sea Momentum Fluxes during Tropical Cyclone Olwyn

2019 ◽  
Vol 49 (6) ◽  
pp. 1369-1379 ◽  
Author(s):  
Joey J. Voermans ◽  
Henrique Rapizo ◽  
Hongyu Ma ◽  
Fangli Qiao ◽  
Alexander V. Babanin
Keyword(s):  
Tropical Cyclone ◽  
Wind Stress ◽  
Momentum Flux ◽  
Turbulent Stress ◽  
High Wind ◽  
Wind Speeds ◽  
Induced Stress ◽  
Induced Stresses ◽  
Turbulent Momentum ◽  
Wave Induced

AbstractObservations of wind stress during extreme winds are required to improve predictability of tropical cyclone track and intensity. A common method to approximate the wind stress is by measuring the turbulent momentum flux directly. However, during high wind speeds, wave heights are typically of the same order of magnitude as instrument heights, and thus, turbulent momentum flux observations alone are insufficient to estimate wind stresses in tropical cyclones, as wave-induced stresses contribute to the wind stress at the height of measurements. In this study, wind stress observations during the near passage of Tropical Cyclone Olwyn are presented through measurements of the mean wind speed and turbulent momentum flux at 8.8 and 14.8 m above the ocean surface. The high sampling frequency of the water surface displacement (up to 2.5 Hz) allowed for estimations of the wave-induced stresses by parameterizing the wave input source function. During high wind speeds, our results show that the discrepancy between the wind stress and the turbulent stress can be attributed to the wave-induced stress. It is observed that for > 1 m s−1, the wave-induced stress contributes to 63% and 47% of the wind stress at 8.8 and 14.8 m above the ocean surface, respectively. Thus, measurements of wind stresses based on turbulent stresses alone underestimate wind stresses during high wind speed conditions. We show that this discrepancy can be solved for through a simple predictive model of the wave-induced stress using only observations of the turbulent stress and significant wave height.

The Importance of Nonlinear Cross-Shelf Momentum Flux during Wind-Driven Coastal Upwelling*

2004 ◽  
Vol 34 (11) ◽  
pp. 2444-2457 ◽  
Author(s):  
Steven J. Lentz ◽  
David C. Chapman
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
Momentum Flux ◽  
Coastal Upwelling ◽  
Bottom Boundary Layer ◽  
Return Flow ◽  
Flux Divergence ◽  
Bottom Boundary ◽  
Shelf Circulation ◽  
The Cross

Abstract A simple theory is proposed for steady, two-dimensional, wind-driven coastal upwelling that relates the dynamics and the structure of the cross-shelf circulation to the stratification, bathymetry, and wind stress. The new element is an estimate of the nonlinear cross-shelf momentum flux divergence due to the wind-driven cross-shelf circulation acting on the vertically sheared geostrophic alongshelf flow. The theory predicts that the magnitude of the cross-shelf momentum flux divergence relative to the wind stress depends on the Burger number S = αN/f, where α is the bottom slope, N is the buoyancy frequency, and f is the Coriolis parameter. For S ≪ 1 (weak stratification), the cross-shelf momentum flux divergence is small, the bottom stress balances the wind stress, and the onshore return flow is primarily in the bottom boundary layer. For S ≈ 1 or larger (strong stratification), the cross-shelf momentum flux divergence balances the wind stress, the bottom stress is small, and the onshore return flow is in the interior. Estimates of the cross-shelf momentum flux divergence using moored observations from four coastal upwelling regions (0.2 ≤ S ≤ 1.5) are substantial relative to the wind stress when S ≈ 1 and exhibit a dependence on S that is consistent with the theory. Two-dimensional numerical model results indicate that the cross-shelf momentum flux divergence can be substantial for the time-dependent response and that the onshore return flow shifts from the bottom boundary layer for small S to just below the surface boundary layer for S ≈ 1.5–2.

scholarly journals Wind-Driven Mixing below the Oceanic Mixed Layer

2011 ◽  
Vol 41 (8) ◽  
pp. 1556-1575 ◽  
Author(s):  
Alan L. M. Grant ◽  
Stephen E. Belcher
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Mixed Layer ◽  
Momentum Flux ◽  
Layer Structure ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Turbulent Momentum ◽  
Bulk Model ◽  
Shear Production

Abstract This study describes the turbulent processes in the upper ocean boundary layer forced by a constant surface stress in the absence of the Coriolis force using large-eddy simulation. The boundary layer that develops has a two-layer structure, a well-mixed layer above a stratified shear layer. The depth of the mixed layer is approximately constant, whereas the depth of the shear layer increases with time. The turbulent momentum flux varies approximately linearly from the surface to the base of the shear layer. There is a maximum in the production of turbulence through shear at the base of the mixed layer. The magnitude of the shear production increases with time. The increase is mainly a result of the increase in the turbulent momentum flux at the base of the mixed layer due to the increase in the depth of the boundary layer. The length scale for the shear turbulence is the boundary layer depth. A simple scaling is proposed for the magnitude of the shear production that depends on the surface forcing and the average mixed layer current. The scaling can be interpreted in terms of the divergence of a mean kinetic energy flux. A simple bulk model of the boundary layer is developed to obtain equations describing the variation of the mixed layer and boundary layer depths with time. The model shows that the rate at which the boundary layer deepens does not depend on the stratification of the thermocline. The bulk model shows that the variation in the mixed layer depth is small as long as the surface buoyancy flux is small.

scholarly journals On the Emergence of the Atlantic Multidecadal SST Signal: A Key Role of the Mixed Layer Depth Variability Driven by North Atlantic Oscillation

2020 ◽  
Vol 33 (9) ◽  
pp. 3511-3531
Author(s):  
Ayako Yamamoto ◽  
Hiroaki Tatebe ◽  
Masami Nonaka
Keyword(s):  
Heat Flux ◽  
North Atlantic Oscillation ◽  
Mixed Layer ◽  
North Atlantic ◽  
Mixed Layer Depth ◽  
Vertical Component ◽  
Surface Heat ◽  
Ocean Dynamics ◽  
Layer Depth

AbstractDespite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.

scholarly journals Seasonal Variability of The Mixed Layer Depth in The Sulawesi Sea

2021 ◽  
Vol 6 (3) ◽  
pp. 163
Author(s):  
Mochamad Riza Iskandar ◽  
Prima Wira Kusuma Wardhani ◽  
Toshio Suga
Keyword(s):  
Wind Stress ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
World Ocean ◽  
Southwest Monsoon ◽  
Buoyancy Flux ◽  
Freshwater Flux ◽  
Layer Depth ◽  
Reanalysis Dataset ◽  
Mechanical Forcing

The Sulawesi Sea is a semi-enclosed basin located in the Indonesian Seas and considered as the one of location in the west route of Indonesian Throughflow (ITF). There is less attention on the mixed layer depth investigation in the Sulawesi Sea. Concerning that the mixed layer plays an important role in influencing the ocean in air-sea interaction and affects biological activity, the estimation of mixed layer depth (MLD) in the Sulawesi Sea is important. Seasonal variation of the mixed layer in the Sulawesi Sea between 115°-125°E and 0°-8°N is estimated by using World Ocean Atlas 2013. Forcing elements on the mixed layer in terms of surface-forced turbulent mixing from mechanical forcing of wind stress and buoyancy forcing (from heat flux as well as freshwater flux) in the Sulawesi Sea is provided by using a reanalysis dataset. The MLD is estimated directly on grid profiles with interpolated levels based on chosen density fixed criterion of 0.03 kg.m<sup>-3</sup> and temperature criterion of 0.5°C difference from the surface. The results show that mixed layer depth in the Sulawesi Sea varies both spatially and temporally. Generally, the deepest MLD was occurred during the southwest monsoon (JJA), and the lowest MLD was occurred during the first transition (MAM) and second transition monsoon (SON). Strengthening and weakening MLD are influenced by mechanical forcing from wind stress and buoyancy flux. In the Sulawesi Sea, the mixed layer deepening coincides with the occurrence of a maximum in wind stress, and low buoyancy flux at the surface. This condition is the opposite when mixed layer shallowing occurs.

scholarly journals Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction

2017 ◽  
Vol 47 (10) ◽  
pp. 2521-2530 ◽  
Author(s):  
Stephanie M. Downes ◽  
Clothilde Langlais ◽  
Jordan P. Brook ◽  
Paul Spence
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Intermediate Water ◽  
Freshwater Input ◽  
Westerly Winds ◽  
Anthropogenic Carbon ◽  
Westerly Wind Stress ◽  
Poleward Shift

AbstractSubduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.

scholarly journals Relative roles of energy and momentum fluxes in the tropical response to extratropical thermal forcing

2020 ◽  
pp. 1-74
Author(s):  
Yen-Ting Hwang ◽  
Hung-Yi Tseng ◽  
Kuan-Chen Li ◽  
Sarah M. Kang ◽  
Yung-Jen Chen ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Time Scale ◽  
Momentum Flux ◽  
Mixed Layer Depth ◽  
Hadley Cell ◽  
Slight Reduction ◽  
Layer Depth ◽  
Tropical Ocean ◽  
Momentum Fluxes ◽  
Energy And Momentum

AbstractThis study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time-scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: (1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. (2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the Intertropical Convergence Zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time-scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increase as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis 31 combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time-scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.

scholarly journals Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth

2009 ◽  
Vol 6 (1) ◽  
pp. 277-341 ◽  
Author(s):  
S. Elipot ◽  
S. T. Gille
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Transfer Function ◽  
Southern Ocean ◽  
Wind Stress ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Velocity ◽  
Layer Depth ◽  
Second Best

Abstract. Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth.) Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m2 s−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

scholarly journals Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth

Ocean Science ◽  
2009 ◽  
Vol 5 (2) ◽  
pp. 115-139 ◽  
Author(s):  
S. Elipot ◽  
S. T. Gille
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Transfer Function ◽  
Southern Ocean ◽  
Wind Stress ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Velocity ◽  
Layer Depth ◽  
Second Best

Abstract. Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth). Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m2 s−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103 m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

scholarly journals Sensitivity of the Simulated Annual Cycle of Sea Surface Temperature in the Equatorial Pacific to Sunlight Penetration

1998 ◽  
Vol 11 (8) ◽  
pp. 1932-1950 ◽  
Author(s):  
Edwin K. Schneider ◽  
Zhengxin Zhu
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Mixed Layer ◽  
Annual Cycle ◽  
General Circulation ◽  
Mixed Layer Depth ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Western Equatorial Pacific

Abstract The annual cycle of sea surface temperature (SST) in the equatorial Pacific is compared for two simulations with a coupled atmosphere–ocean general circulation model. The simulations differ only in the optical properties of the ocean: sunlight penetrates below the topmost layer of the ocean model in one case but is completely absorbed in the top layer in the other. The simulation without the sunlight penetration produces an unrealistic annual cycle of SST with a strong semiannual component in the equatorial Pacific, whereas the simulation with sunlight penetration is more realistic. The change in the character of the annual cycle results from an increase in the effective heat capacity of the ocean associated with an increase in the depth of the mixed layer directly forced by the sunlight penetration. This produces a smaller amplitude of the annual cycle of SST at latitudes close to but off the equator. The zone of intense tropical convection then remains closer to the equator, leading to a reduced semiannual cycle of zonal wind stress at the equator. The reduction in the unrealistic semiannual wind stress forcing leads to a more realistic annual cycle in SST. The simulation of the annual mean SST is also improved by the inclusion of the sunlight penetration, with a better simulation of the warm pool in the western equatorial Pacific and associated improvements in the atmospheric circulation. This improvement is also attributed to the increase in the mixed layer depth, which changes the ocean heat flux in the western equatorial Pacific by reducing the sensitivity of SST to upwelling.

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