Wind Stirring Research Articles

A modelling investigation of the distribution of stratification and phytoplankton abundance in the Irish Sea

Thermal stratification and phytoplankton abundance are modelled on a 5 km grid covering the Irish Sea. The water column is approximated by three layers. The top layer is uniformly mixed by wind stirring and the bottom by tidal energy, while linear gradients can occur in the middle layer. The model is forced with hourly meteorological data and mean tidal energies. Primary production is represented by a model with a single nutrient and a single phytoplankton population. The results from the model show good agreement with data collected on a Ministry of Agriculture, Fisheries and Food (MAFF) cruise in May 1992 and with historical data. When advection is included, driven by depth-averaged currents, the surface temperature patterns are improved but bottom temperatures in deep water are raised and high concentrations of chlorophyll are carried offshore from coastal regions. This indicates a limitation of using depth-averaged currents and a need to account for differences in phytoplankton species composition in coastal and offshore waters. Calculations demonstrate the importance of salinity variations to stratification and phytoplankton growth. Smoothing the wind mixing energy has the effect of delaying the onset of the spring bloom in areas where wind mixing is significant. Removing the diurnal cycle of solar heating also delays the spring bloom. The chlorophyll gradient in the middle layer has a large impact on the response of the model to short-term variability in the meteorological forcings.

Observations of the upper ocean response to storm forcing in the South Atlantic Roaring Forties

In the austral summer of 1992–1993 the passage of a storm system drove a strong upper ocean response at 45°S in the mid-South Atlantic. Good in situ observations were obtained. CTD casts revealed that the mixed layer deepened by ~40 m over 4 days. Wind stirring dominated over buoyancy flux-driven mixing during the onset of high winds. Doppler shear currents further reveal this to be intimately related to inertial dynamics. The penetration depth of inertial currents, which are confined to the mixed layer, increases with time after a wind event, matched by a downward propagation of low values of the Richardson number. This suggests that inertial current shear is instrumental in producing turbulence at the base of the mixed layer. Evolution of inertial transport is simulated using a time series of ship-observed wind stress. Simulated transport is only 30–50% of the observed transport, suggesting that much of the observed inertial motion was forced by an earlier (possibly remote) storm. Close proximity of the subtropical front further complicates the upper ocean response to the storm. A simple heat balance for the upper 100 m reveals that surface cooling and mixing (during the storm) can account for only a small fraction of an apparent ~1 °C mixed layer cooling.

A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models

Abstract A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning. The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turn...

Embedding a mixed layer model into an ocean general circulation model of the Atlantic: The importance of surface mixing for heat flux and temperature

A primitive equation ocean general circulation model (OGCM) of the Atlantic is coupled with an explicit mixed layer model (MLM). The coupled model is used to investigate the impact and possible improvement achieved by an MLM on the representation of temperature and heat flux. The MLM improves model performance in regions where mixed layer (ML) formation is governed by wind stirring, which is identified as a major missing process in the OGCM. It is shown that the widely used parameterization of Pacanowski and Philander for the vertical exchange coefficients is not able to properly represent this process. The deepening of the ML by wind stirring leads to temperature and heat flux being in closer agreement with climatology than is the case without MLM. Integrations with a coupled ocean‐atmosphere model are needed to show whether this improvement leads to a reduced climate drift in such models. Furthermore, momentum mixing improves the vertical profiles of velocity. The Ekman transport now occurs over the whole depth of the ML instead of just within the upper model layer. Convective adjustment as incorporated in the OGCM already tends to overestimate the depth of the mixed layer in buoyancy‐driven situations. Adding the MLM cannot remedy this failure. Thus under these circumstances there are only slight improvments of some model aspects while others become even worse. We conclude that the parameterization of convection has to be improved.

The Oceanographic and Biological Tidal Cycle Succession in Shallow Sea Fronts in the North Sea and the English Channel

Two major discrepancies between existing theories concerning the positioning and development of shallow shelf sea fronts and available data are that these models (1) do not adequately describe the neap-spring adjustment of frontal position and (2) these theories fail to explain the relatively small changes in the location of the front despite the rapid seasonal decay of the stabilizing surface heat flux.In the model presented here, these weaknesses in earlier attempts to describe frontal position are overcome. This is achieved partly by incorporating wind stirring in the heat and mixing balance for the frontal area and partly by allowing for a cross-frontal circulation. This new model defines two types of shallow sea fronts: tidal 'surface' and tidal 'bottom' fronts. Surface fronts are dominated by tidal and bottom fronts by wind stirring. The front type determines the circulation pattern on the stratified side of the front. The circulation pattern, in turn, governs the biological activity. The model describes the persistent but temporally varying redistribution of water from the bottom and surface layers to the thermocline layer which supplies nutrients from the deep (light limited) to the shallow regions where phytoplankton activity is stimulated. This new interpretation of the spring-neap adjustment also implies that, during the heating season, an increasing part of the potential energy to be overcome during a tidal cycle stems from the mixing of nutrient-rich dense bottom water and light surface water imported from offshore. The existence of this mechanism is supported by field observations.

A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific

The intertropical convergence zone (ITCZ) stays in the northern hemisphere over the Atlantic and eastern Pacific, even though the annual mean position of the sun is on the equator. To study some processes that contribute to this asymmetry about the equator, we use a two-dimensional model which neglects zonal variations and consists of an ocean model with a mixed layer coupled to a simple atmospheric model. In this coupled model, the atmosphere not only transports momentum into the ocean, but also directly affects sea surface temperature by means of wind stirring and surface latent heat flux. Under equatorially symmetric conditions, the model has, in addition to an equatorially symmetric solution, two asymmetric solutions with a single ITCZ that forms in only one hemisphere. Strong equatorial upwelling is essential for the asymmetry. Local oceanic turbulent processes involving vertical mixing and surface latent heat flux, which are dependent on wind speed, also contribute to the asymmetry.

The Clyde Sea: a Model of the Seasonal Cycle of Stratification and Mixing

Mooring observations in the fjordic system of the Clyde Sea show complete vertical mixing in the deep waters during late November 1990 as a result of cooling and wind stirring. ADCP observations before and during mixing indicate that the breakdown of stratification drastically modified the circulation and largely removed the vertical shear associated with the density driven flow. There are no previous documented cases of complete mixing though the available records are limited. A compilation of the historical data indicates that the Clyde system is usually stratified by a combination of thermal and freshwater buoyancy inputs with most pronounced stratification in the summer months when Δσ l may exceed 1·5 kg m -3. Winter stratification is generally weaker with temperature inversions occurring during the cooling phase. The processes controlling stratification are represented in a filling box model of the Clyde system in which the exchange flow with the North Channel is related to the surface density difference across the sill. The stratifying effect of buoyancy inputs as heat and freshwater is opposed by mechanical stirring due to (i) wind-stress and (ii) the barotropic tide. Additional stirring contributions from (iii) convective effects associated with deep-water replacement and (iv) the internal wave mechanism proposed by Stigebrandt (1976) may also contribute. Model runs using only mechanisms (i) and (ii) did not exhibit the rise in bottom temperature observed during the summer season and, as a consequence, over-estimated density stratification. Additional inputs from (iii) and, in particular (iv), using best estimates of the efficiency factors, produced a more realistic balance and gave a reasonable first order account of the seasonal cycle of density stratification. It also provides a fair simulation of the seasonal evolution of temperature and salinity including the temperature inversions observed in the November and December. The results point to a fine balance between the annual mean inputs of buoyancy and stirring in the Clyde system thus allowing the possibility of annual winter mixing.

Biological consequences of tidal stirring gradients in the North Sea

Tidal stirring gradients, interacting with seasonal variation in photosynthetically active radiation, sea-surface heating, and wind stirring, are proposed as the most important controls on plankton in the southern North Sea. The hypothesis, in the form of a numerical model, is tested against observations during 1988/89 of seasonal cycles and spatial variation in phyto- and zoo-plankton. The importance of the tidal mixing front, and the effects of residual circulation and nutrient-rich river discharges, are discussed, and estimates given of microplankton community production and its fate.

Diurnal variations of convective mixing and the spring bloom of phytoplankton

Abstract Turbulent stirring of the surface mixed layer extends to shallower depths during the day than in the night because the increased buoyancy resulting from solar heating inhibits the mixing associated with wind action and surface heat loss. Calculations using a simple model in which a mixed layer of constant depth is more strongly coupled to deeper layers at night than during the day indicate that the reduction of mixing by day may be critical to the onset of the spring phytoplankton bloom, for this occurs at a time when there is increasing solar warming in the day and yet considerable heat loss and wind stirring at night. If the Kraus-Turner model of the mixed layer is used to estimate the mixing rates occurring during darkness, the values obtained agree with those that give realistic simulations of the spring bloom. Diurnal observations of chlorophyll a , p CO 2 and oxygen saturation made at 60°N during the Lagrangian experiment carried out in 1989 as part of the U.K. Biogeochemical Ocean Flux Study can be modelled more successfully if the day-night changes in vertical mixing are included in the same manner as the single layer model. The calculations indicate that these changes may shift the timing of the bloom by about 1 week and may account for the depth of penetration of some spring blooms. This process needs to be considered when modelling the coupling between climate and phytoplankton.

Modelling the effects of motion on primary production in the mixed layer of lakes

A model of the time dependent relationship between productivity and light intensity following changes in light intensity is briefly described. The model incorporates two response timescales simulating initial response and photoinhibition, although additional timescales could easily be incorporated. The model is calibrated against one set of time dependent data, and applied to two simple models of motion in the upper mixed layer of a lake. The two models are: organised motion simulating Langmuir cells, and disorganised motion simulating the turbulent velocity field associated with surface wind stirring. The depth and therefore light histories for a number of photosynthesising particles are calculated by these models, and used by the productivity model to calculate mean productivities. The results show that the influence of the time dependent nature of the productivity relationship depends on the ratio of the mixed layer depth to the euphotic depth, and to a less extent, on the rate at which the particles circulate in the mixed layer.

Tidal Straining, Density Currents, and Stirring in the Control of Estuarine Stratification

Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations.

Richardson number profiles in laboratory experiments applied to shallow seas

Abstract A unified analysis is given of the critical conditions for the onset of stratification due to either a vertical or a horizontal buoyancy flux, with tidal or wind stirring. The critical conditions for the onset of stratification with a horizontal buoyancy flux are found to be of the form of ratios of the tidal slope, or wind setup, to the equivalent surface slope due to the lateral density gradient. These ratios, which are easily determined from sea data, indicate that the profiles of critical flux Richardson Number, averaged over the stirring cycle, are similar to those inferred from the laboratory experiments of Hopfinger and Linden (1982) in which there is zero mean shear turbulence with a stabilising buoyancy flux, and also that the efficiency for the conversion of kinetic energy to potential energy for tidal stirring is similar to that for wind stirring. The observed much greater efficiency for wind stirring, compared with tidal stirring with a vertical buoyancy flux, is also consistent with ...

A numerical investigation of sea surface temperature variability in the Arabian Sea

A numerical model is developed to study the thermodynamics of the Arabian Sea. The model consists of a surface mixed layer of thickness hm and temperature Tm, embedded in a dynamic layer of thickness h≥hm. Entrainment and detrainment in the mixed layer are determined by wind stirring and cooling at the oceanic surface, as in the Kraus and Turner (1967) model. The dynamic layer is a 1½‐layer model which allows entrainment due to shear instability at the base of the layer, similar to the Pollard et al. (1973) model. Monthly climatological data, along with the model sea surface temperature (SST), are used to determine the wind stress and heat fluxes forcing the model. The model is very successful in simulating the observed SST patterns, generally differing by no more than 0.5°C. During the Southwest Monsoon (June‐September), Tm decreases markedly near Somalia and the Arabian peninsula owing to the entrainment of cool subsurface water at the coast and its subsequent advection offshore; farther offshore in the central Arabian Sea it decreases owing to increased evaporative cooling. During the Northeast Monsoon (December‐March), the decrease in Tm is caused in part by reduced solar radiation and by increased evaporative cooling, with additional cooling by entrainment in the northern Arabian Sea. An interesting observed feature is that there is a net annual surface heat flux into the Arabian Sea. In the western Arabian Sea this flux is a direct result of coastal upwelling during the Southwest Monsoon, the cold water reducing the latent heat flux out of the ocean. In the central and eastern Arabian Sea it is caused by the southward advection of this upwelled water toward the equator. The annual heat and mass budgets are closed by equatorial currents: a westward undercurrent is the source of cool subsurface water that is entrained in the Arabian Sea, and a warm eastward surface current removes the excess heat and entrained water from the upper layer.

Hypolimnetic mixing in a weakly stratified lake

The evolution of the thermal structure of a weakly stratified lake, located in Venezuela, is studied by applying a hydrothermal model as a tool to analyze the observed data. Particular attention is given to the quantification of the mixing energies involved in the redistribution of heat content in the epilimnion and in the hypolimnion. Hence an analysis of the epilimnetic and hypolimnetic mixing dynamics is presented. As a result, expressions for epilimnetic (wind stirring and penetrative convection) and hypolimnetic mixing energies are obtained and it is concluded that for the lake in study, hypolimnetic mixing controls the local thermal structure and the overall heat transfer process.

UNDERSTANDING THE PHYSICAL STRUCTURE OF SOUTHERN AFRICA'S STANDING WATERS

SUMMARY An examination of the nature of the stratification in polymictic and monomictic Southern African reservoirs is presented. Wind stirring is the principal mode of heat transfer during spring heating. In high summer radiation from stable heated surface layers is responsible for the generation of turbulent kinetic energy (TKE) as a consequence of cascading of high density water downwards to neutral density levels. Entrainment of warm layers by inflow gradients in the reservoirs of the summer rainfall regions of the subcontinent is a further important mechanism setting up TKE in the epilimnion. It may become dominant during periods of high river inflow and often precedes autumnal turnover.

Fine‐structure variability observed in CTD measurements from the central equatorial Pacific

Conductivity, temperature, and depth data from the 15 quasi‐monthly cruises of the Hawaii‐Tahiti Shuttle Experiment are used to document fine‐structure variability in the upper 400 m of the central equatorial Pacific. The analysis reveals the existence of anomalously strong temperature and density variations on 0(1–10 m) vertical scales concentrated in the high mean shear zones of the equatorial current system and in the thermostad. These are regions where microscale measurements also indicate intense turbulent activity. The implication of this spatial correspondence between large‐, fine‐, and microscale signals is that there may be a cascade of energy from the largest to the smallest scales of variability that is mediated by fine‐structure processes. The highly structured fine‐scale signal suggests that the cascade would be most pronounced and persistent on certain density surfaces that lie within the regions of highest mean shear. Other processes, such as wind stirring, air‐sea buoyancy exchange, and intrusions, also contribute to the generation of fine‐structure variability and mixing at equatorial latitudes. There is evidence, for example, that intrusions across the sharp equatorial salinity front in the upper 150–200 m account for 10%–20% of the fine‐structure temperature variance and more than 50% of the fine‐structure salinity variance. The dominant vertical scale of these intrusions appears to be ≲10 m and the cross‐frontal scale < 100 km. A simple model calculation of cross‐frontal mixing indicates that they are important in transferring heat and salt laterally across the front. Intrusions in the vicinity of the Equatorial Undercurrent core may support salt fingering; hence they may contribute to vertical transports of salt and heat in a region where turbulent mixing is weak.

Development of Efficient, Dynamical Ocean-Atmosphere Models for Climatic Studies

Abstract Three ocean-atmosphere models are constructed and evaluated for further studies of climatic variability and climatic sensitivity. All the models are based on the two-layer, highly truncated spectral atmospheric model of Held and Suarez modified to have a seasonal cycle of solar forcing and to interact with simple geographical distributions of land and sea. For simplicity, a 120° sector configuration is adopted. The first coupled model has as its oceanic component two motionless layers, connected by vertical diffusion and convective adjustment. This is chosen to represent the seasonal cycle in the upper ocean more realistically than a constant-thickness mixed layer. The effect of wind stirring on mixed-layer thickness is added in the second coupled model. The third coupled model is one in which three-dimensional ocean circulation is included, using a relatively fine oceanic grid (1.5°). Long integrations of the three coupled models are carried out in order to understand their intrinsic dynamics. T...

The upper mixed zone of a salt gradient solar pond: Its dynamics, prediction and control

Parameterisation of the physical processes that occur at the surface of a salt gradient solar pond are used in an extended version of the numerical model DYRESM, to simulate the behaviour of such a pond. The model is run using meteorological data collected at the site of a 2000 m 2 solar pond at Alice Springs, Australia, and the comparison with actual pond data shows very good agreement. By breaking down the model output, the relative importance of each individual mixing mechanism on the depth of the surface mixed layer is calculated. For the case of Alice Springs it is shown that the evaporative effects completely dominate wind stirring effects. At another site at Halls Creek, Australia, wind stirring is seen to be dominant, however evaporative effects are still sufficient to cause an intolerable amount of deepening. A method of enhanced surface washing is proposed as a method of controlling surface layer deepening. Using the model, this is shown to be effective, irrespective of the cause of the deepening.

Lake Temperature Dynamics in Hybrid Cooling System

A transient thermal simulation model of a hybrid cooling lake is developed to assess the impact of cooling water withdrawals and cooling tower discharges on the thermal structure of the lake. The model is based on one‐dimensional mechanical and thermal energy balances and includes the physical processes of solar heating, wind stirring, vertical advection, and hypolimnetic diffusion. Temperature data were collected on two small lakes in northern Indiana, and the simulated temperatures are compared with the measured data for the lake stratification seasons of 1976 and 1978. It is shown that cooling water circulations significantly thicken the epilimnion due to vertical advection of heat. The simulation results are examined in terms of the values of the wind Richardson number, the withdrawal densimetric Froude number, and the dimensionless surface buoyancy flux. The sensitivity of the lake thermal structure to the densimetric Froude number is demonstrated.

The Role of Sediments in the Nitrogen Budget of Lower Green Bay, Lake Michigan

The extreme southern portion of Green Bay is a shallow (1 to 5 m depth), eutrophic water body which receives considerable nutrients from the Fox River and metropolitan Green Bay, Wisconsin. Research to evaluate the effect of sediments on nitrogen (N) in the bay entailed periodic sampling of waters and sediments at six sites over 20 months and laboratory investigations of the rates of nitrification, denitrification, mineralization, immobilization, and N 2 fixation. The monitoring data indicated that the N concentrations, approximately 0.6 and 0.8 mg/L of inorganic and organic N, respectively, in the bay waters are considerably higher than the threshold limits that may cause algal bloom and aquatic weed problems. Consideration of the available sediment N pool with respect to recognizable N inputs indicated that only 1.2 percent of the yearly N loading from the Fox River is present in the active sediment layer. Nitrification and subsequent denitrification at the sediment-water interface as a result of intermittent wind stirring could be a major sink for N, but presently it has a minor impact due to the high loading rate of N in this ecosystem. The study indicates that even if approximately 50 percent of the present point source loading of N were eliminated by pollution abatement, the N input from nonpoint sources (combined with existing concentrations of phosphorus in the bay waters) would be sufficient to maintain eutrophic conditions.