The Response of Buoyant Coastal Plumes to Upwelling-Favorable Winds*

2004 ◽  
Vol 34 (11) ◽  
pp. 2458-2469 ◽  
Author(s):  
Steven Lentz
Keyword(s):  
Wind Stress ◽  
Vertical Mixing ◽  
Wind Forcing ◽  
Ekman Transport ◽  
Entrainment Rate ◽  
Buoyant Plume ◽  
Cross Sectional ◽  
Buoyancy Forcing ◽  
Density Anomaly ◽  
Variable Wind

Abstract To better understand the response of a buoyant coastal plume to wind-induced upwelling, a two-dimensional theory is developed that includes entrainment. The primary assumption is that competition between wind-driven vertical mixing and lateral buoyancy forcing in the region where the isopycnals slope upward to intersect the surface results in continual entrainment at the offshore edge of the plume. The theory provides estimates of the buoyant plume characteristics and offshore displacement as a function of time t, given the wind stress, the characteristics of the buoyant plume prior to the onset of the wind forcing, and a critical value for the bulk Richardson number (Ric). The theory predicts that, for t̂ ≡ t/ts, the plume density anomaly decreases as (1 + t̂)−1, the thickness increases as (1 + t̂)1/3, the width increases as (1 + t̂)2/3, and the plume average entrainment rate decreases as (1 + t̂)−2/3. Here ts = 2Ao/(RicUE) is the time for entrainment to double the cross-sectional area of the plume Ao at the onset of the wind forcing, where UE is the Ekman transport. The theory reproduces results from 20 numerical model runs by Fong and Geyer, including their estimates of the plume-average entrainment rate (correlations greater than 0.98 and regression coefficients approximately 1 for plume characteristics and 1.7 for the entrainment rate). The theory, modified to allow for time-variable wind stress, also reproduces the observed response of the buoyant coastal plume from Chesapeake Bay during an 11-day period of upwelling winds in August 1994.

scholarly journals Eddy Heat Flux in the Southern Ocean: Response to Variable Wind Forcing

2008 ◽  
Vol 21 (4) ◽  
pp. 608-620 ◽  
Author(s):  
Andrew Mc C. Hogg ◽  
Michael P. Meredith ◽  
Jeffrey R. Blundell ◽  
Chris Wilson
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Wind Stress ◽  
Wind Forcing ◽  
Ekman Transport ◽  
Eddy Heat Flux ◽  
Circumpolar Current ◽  
Quasigeostrophic Model ◽  
Flux Response ◽  
Variable Wind

Abstract The authors assess the role of time-dependent eddy variability in the Antarctic Circumpolar Current (ACC) in influencing warming of the Southern Ocean. For this, an eddy-resolving quasigeostrophic model of the wind-driven circulation is used, and the response of circumpolar transport, eddy kinetic energy, and eddy heat transport to changes in winds is quantified. On interannual time scales, the model exhibits the behavior of an “eddy saturated” ocean state, where increases in wind stress do not significantly change the circumpolar transport, but instead enhance the eddy field. This is in accord with previous dynamical arguments, and a recent observational study. The instantaneous response to increased wind stress is to cool temperatures through increased northward Ekman transport of cool water. But, in the longer term, the enhanced eddy state is more efficient at transporting heat, leading to a warming of the ocean. The total eddy heat flux response is greater than the Ekman transport heat flux in this model by a factor of 2, indicating that coarse (non eddy resolving) models may fail to adequately capture the key processes. The authors also test the model response to long-term changes in wind forcing, including steadily increasing circumpolar wind strength over a 30-yr period. The model shows a response in eddy heat flux, and a change in ocean temperature not dissimilar from observed Southern Ocean warming. These findings suggest that eddy heat flux, energized by increasing wind stress, may be a significant contributor to the observed warming of the Southern Ocean.

scholarly journals The Influence of Wind Forcing on the Chesapeake Bay Buoyant Coastal Current*

2006 ◽  
Vol 36 (7) ◽  
pp. 1305-1316 ◽  
Author(s):  
Steven J. Lentz ◽  
John Largier
Keyword(s):  
Chesapeake Bay ◽  
Wind Stress ◽  
Vertical Mixing ◽  
Propagation Speed ◽  
Wind Forcing ◽  
Coastal Current ◽  
Plume Front ◽  
Shelf Flow ◽  
Weak Wind

Abstract Observations of the buoyant coastal current that flows southward from Chesapeake Bay are used to describe how the thickness, width, and propagation speed vary in response to changes in the along-shelf wind stress. Three basic regimes were observed depending on the strength of the wind. For weak wind stresses (from −0.02 to 0.02 Pa), the buoyant coastal current was relatively thin, the front slope was not steep, and the width was variable (1–20 km). For moderate downwelling (southward) wind stresses (0.02–0.07 Pa), wind-driven cross-shelf advection steepened the front, causing the plume to narrow and thicken. For stronger downwelling wind stresses (greater than 0.07 Pa), vertical mixing dominated, bulk Richardson numbers were approximately 0.25, isopycnals were nearly vertical, and the plume front widened but the plume width did not change. Plume thickness and width were normalized by the theoretical plume scales in the absence of wind forcing. Normalized plume thickness increased linearly from 1 to 2 as downwelling wind stresses increased from 0 to 0.2 Pa. Normalized plume widths were approximately 1 for downwelling wind stresses from 0.02 to 0.2 Pa. The observed along-shelf propagation speed of the plume was roughly equal to the sum of the theoretical propagation speed and the wind-driven along-shelf flow.

scholarly journals The Importance of Wind and Buoyancy Forcing for the Boundary Density Variations and the Geostrophic Component of the AMOC at 26°N

2014 ◽  
Vol 44 (9) ◽  
pp. 2387-2408 ◽  
Author(s):  
Irene Polo ◽  
Jon Robson ◽  
Rowan Sutton ◽  
Magdalena Alonso Balmaseda
Keyword(s):  
Density Gradient ◽  
Time Scales ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Western Boundary ◽  
Decadal Time Scale ◽  
Buoyancy Forcing

Abstract It is widely thought that changes in both the surface buoyancy fluxes and wind stress drive variability in the Atlantic meridional overturning circulation (AMOC), but that they drive variability on different time scales. For example, wind forcing dominates short-term variability through its effects on Ekman currents and coastal upwelling, whereas buoyancy forcing is important for longer time scales (multiannual and decadal). However, the role of the wind forcing on multiannual to decadal time scales is less clear. Here the authors present an analysis of simulations with the Nucleus for European Modelling of the Ocean (NEMO) ocean model with the aim of explaining the important drivers of the zonal density gradient at 26°N, which is directly related to the AMOC. In the experiments, only one of either the wind stress or the buoyancy forcing is allowed to vary in time, whereas the other remains at its seasonally varying climatology. On subannual time scales, variations in the density gradient, and in the AMOC minus Ekman, are driven largely by local wind-forced coastal upwelling at both the western and eastern boundaries. On decadal time scales, buoyancy forcing related to the North Atlantic Oscillation dominates variability in the AMOC. Interestingly, however, it is found that wind forcing also plays a role at longer time scales, primarily impacting the interannual variability through the excitation of Rossby waves in the central Atlantic, which propagate westward to interact with the western boundary, but also by modulating the decadal time-scale response to buoyancy forcing.

scholarly journals Effects of Wind Stress and Surface Cooling on Cross-Shore Exchange

2018 ◽  
Vol 48 (11) ◽  
pp. 2627-2647 ◽  
Author(s):  
Xiaodong Wu ◽  
Douglas Cahl ◽  
George Voulgaris
Keyword(s):  
Heat Loss ◽  
Wind Stress ◽  
Loss Rate ◽  
Baroclinic Instability ◽  
Wind Forcing ◽  
Ekman Transport ◽  
Surface Cooling ◽  
Inner Shelf ◽  
Heat Loss Rate ◽  
Shelf Slope

AbstractThe formation of coastal dense shelf water in winter provides the available potential energy (APE) to fuel baroclinic instability. The combined effects of baroclinic instability and wind forcing in driving cross-shelf exchange are investigated using idealized numerical simulations with varied bottom slope, wind stress, and heat loss rate. The results show that under upwelling-favorable winds, the intensity of the instability decreases as the wind stress increases. This is caused primarily by enhanced turbulence frictional dissipation. Under downwelling-favorable winds, an increase in wind stress and/or a decrease in heat loss rate tends to constrain the baroclinic instability, leading to a circulation resembling that driven purely by wind forcing. In the latter case, once a critical value of cross-shore density gradient is reached, isopycnal slumping is initiated, leading to increased vertical stratification and narrowing of the inner shelf. The change in depth of the inner-shelf outer boundary, defined as the location corresponding to the maximum cross-shore gradient of the surface Ekman transport, is proportional to an empirically derived multiparametric quantity , where a2 is a dimensional constant, B0 is a constant heat loss rate, γ = 0.43, f is the Coriolis parameter, α is the shelf slope, B is the heat loss rate, and τ is the wind stress. This relationship is found to hold for cases when instabilities are present.

scholarly journals SEASONAL PATTERN OF WIND INDUCED UPWELLING OVER JAVA-BALI SEA WATERS AND SURROUNDING AREA

2010 ◽  
Vol 5 (1) ◽  
Author(s):  
Siswanto ◽  
Suratno
Keyword(s):  
Wind Stress ◽  
Coastal Upwelling ◽  
Sea Water ◽  
Mixed Layer Depth ◽  
Surface Wind ◽  
Ekman Transport ◽  
Monsoon Circulation ◽  
Wide Field ◽  
Surrounding Area ◽  
Noaa Avhrr

The influence of monsoonal wind to coastal upwelling mechanism which is generated by Ekman transport was studied here by analyzing wind stress curl (WSC) distribution over Java-Bali Sea waters and its surrounding area. Surface wind data were used as input data to calculate curl of wind stress in barotropic model. Confirmation with Corioli effect in the Southern Hemisphere, it could be known that negative curl value has relation with vertical motion of sea water as resulted by Ekman transport. Result of analysis showed that negative curl near coast over Java Sea which is stretching to Lombok Sea occurred in December to April when westerly wind of the North West Monsoon actives. It can be guidance and related with season of coastal upwelling in the region. Reversal condition, the occurrance of coastal upwelling in the south coast of JAva island related with the negative value of WSC that occurs since easterlies wind take place in May to August as a part of South East Monsoon episode. Generally, upwelling occurrance in the field of study is a response to the Monsoon circulation. This study with related data such as sea surface temperature, chlorophyll concetration and mixed layer depth that derived from satellite imaging data National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA-AVHRR), Aqua/Modis and sea viewing Wide Field-of-view Sensor(Sea WiFS) shows as magnificent confirmation pattern. So applying WSC to recoqnize upwelling zone is alternatively way as climatic approach to maps potential fertilizing of sea water in maritime-continent Indonesia. Key words: coastal upwelling, Ekman transport, Java-Bali Sea, Monsoon circulation, upwelling.

scholarly journals Rapid Generation of Upwelling at a Shelf Break Caused by Buoyancy Shutdown

2015 ◽  
Vol 45 (1) ◽  
pp. 294-312 ◽  
Author(s):  
Jessica Benthuysen ◽  
Leif N. Thomas ◽  
Steven J. Lentz
Keyword(s):  
Boundary Layer ◽  
Vertical Mixing ◽  
Shelf Break ◽  
Bottom Boundary Layer ◽  
Ekman Transport ◽  
Bottom Boundary ◽  
Middle Atlantic ◽  
Time Period ◽  
Rapid Generation ◽  
Offshore Transport

AbstractModel analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf’s bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day−1 within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.

scholarly journals Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼  30° S) based on a high-resolution atmospheric regional model (WRF)

Ocean Science ◽  
2016 ◽  
Vol 12 (5) ◽  
pp. 1049-1065 ◽  
Author(s):  
Luis Bravo ◽  
Marcel Ramos ◽  
Orlando Astudillo ◽  
Boris Dewitte ◽  
Katerina Goubanova
Keyword(s):  
High Resolution ◽  
Wind Stress ◽  
Seasonal Variability ◽  
Coastal Upwelling ◽  
Ekman Transport ◽  
Northern Chile ◽  
Ekman Pumping ◽  
Relative Contribution ◽  
Alongshore Wind Stress ◽  
Alongshore Wind

Abstract. Two physical mechanisms can contribute to coastal upwelling in eastern boundary current systems: offshore Ekman transport due to the predominant alongshore wind stress and Ekman pumping due to the cyclonic wind stress curl, mainly caused by the abrupt decrease in wind stress (drop-off) in a cross-shore band of 100 km. This wind drop-off is thought to be an ubiquitous feature in coastal upwelling systems and to regulate the relative contribution of both mechanisms. It has been poorly studied along the central-northern Chile region because of the lack in wind measurements along the shoreline and of the relatively low resolution of the available atmospheric reanalysis. Here, the seasonal variability in Ekman transport, Ekman pumping and their relative contribution to total upwelling along the central-northern Chile region (∼  30° S) is evaluated from a high-resolution atmospheric model simulation. As a first step, the simulation is validated from satellite observations, which indicates a realistic representation of the spatial and temporal variability of the wind along the coast by the model. The model outputs are then used to document the fine-scale structures in the wind stress and wind curl in relation to the topographic features along the coast (headlands and embayments). Both wind stress and wind curl had a clear seasonal variability with annual and semiannual components. Alongshore wind stress maximum peak occurred in spring, second increase was in fall and minimum in winter. When a threshold of −3  ×  10−5 s−1 for the across-shore gradient of alongshore wind was considered to define the region from which the winds decrease toward the coast, the wind drop-off length scale varied between 8 and 45 km. The relative contribution of the coastal divergence and Ekman pumping to the vertical transport along the coast, considering the estimated wind drop-off length, indicated meridional alternation between both mechanisms, modulated by orography and the intricate coastline. Roughly, coastal divergence predominated in areas with low orography and headlands. Ekman pumping was higher in regions with high orography and the presence of embayments along the coast. In the study region, the vertical transport induced by coastal divergence and Ekman pumping represented 60 and 40 % of the total upwelling transport, respectively. The potential role of Ekman pumping on the spatial structure of sea surface temperature is also discussed.

Symmetric and Baroclinic Instability in Dense Shelf Overflows

2019 ◽  
Vol 49 (1) ◽  
pp. 39-61 ◽  
Author(s):  
Elizabeth Yankovsky ◽  
Sonya Legg
Keyword(s):  
Baroclinic Instability ◽  
Gravity Current ◽  
The Arctic ◽  
Neutral Buoyancy ◽  
Buoyancy Forcing ◽  
Dense Water ◽  
Density Anomaly ◽  
Eurasian Basin ◽  
Thermohaline Stratification ◽  
2D And 3D

AbstractIn this study, we revisit the problem of rotating dense overflow dynamics by performing nonhydrostatic numerical simulations, resolving submesoscale variability. Thermohaline stratification and buoyancy forcing are based on data from the Eurasian basin of the Arctic Ocean, where overflows are particularly crucial to the exchange of dense water between shelves and deep basins, yet have been studied relatively little. A nonlinear equation of state is used, allowing proper representation of thermohaline structure and mixing. We examine three increasingly complex scenarios: nonrotating 2D, rotating 2D, and rotating 3D. The nonrotating 2D case behaves according to known theory: the gravity current descends alongslope until reaching a relatively shallow neutral buoyancy level. However, in the rotating cases, we have identified novel dynamics: in both 2D and 3D, the submesoscale range is dominated by symmetric instability (SI). Rotation leads to geostrophic adjustment, causing dense water to be confined within the forcing region longer and attain a greater density anomaly. In the 2D case, Ekman drainage leads to descent of the geostrophic jet, forming a highly dense alongslope front. Beams of negative Ertel potential vorticity develop parallel to the slope, initiating SI and vigorous mixing in the overflow. In 3D, baroclinic eddies are responsible for cross-isobath dense water transport, but SI again develops along the slope and at eddy edges. Remarkably, through two different dynamics, the 2D SI-dominated case and 3D eddy-dominated case attain roughly the same final water mass distribution, highlighting the potential role of SI in driving mixing within certain regimes of dense overflows.

scholarly journals The Effects of Surface Wind Stress and Buoyancy Flux on the Evolution of a Front in a Turbulent Thermal Wind Balance

Fluids ◽  
2020 ◽  
Vol 5 (2) ◽  
pp. 87
Author(s):  
Matthew N. Crowe ◽  
John R. Taylor
Keyword(s):  
Wind Stress ◽  
Numerical Solutions ◽  
Surface Wind ◽  
Vertical Mixing ◽  
Spreading Rate ◽  
Buoyancy Flux ◽  
Thermal Wind ◽  
Surface Wind Stress ◽  
Shear Dispersion ◽  
Surface Buoyancy

Here we consider the effects of surface buoyancy flux and wind stress on a front in turbulent thermal wind (TTW) balance using the framework of Crowe and Taylor (2018). The changes in the velocity and density profiles induced by the wind stress and buoyancy flux interact with the TTW and can qualitatively change the evolution of the front. In the absence of surface-forcing, Crowe and Taylor (2018) found that shear dispersion associated with the TTW circulation causes the frontal width to increase. In many cases, the flow induced by the surface-forcing enhances the spreading rate. However, if the wind stress drives a cross-front flow which opposes the frontal buoyancy gradient or the buoyancy flux drives an unstable stratification, it is possible to obtain an up-gradient cross-front buoyancy flux, which can act to sharpen the front. In certain conditions, an equilibrium state develops where the tendency for the TTW circulation to spread the front is balanced by the frontogenetic tendency of the surface forces. We use numerical solutions to a nonlinear diffusion equation in order to test these predictions. Finally, we describe the connection between surface-forcing and vertical mixing and discuss typical parameters for mid-ocean fronts.

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