Long Term Upper Ocean Study Research Articles

A modification of the classical Ekman model accounting for the Stokes drift and stratification effects

A modification of the classical Ekman model of oceanic wind-driven currents including the Stokes drift and stratification effects is discussed. The modification is formulated as an application of turbulence mechanics accounting for the curvature effect of velocity fluctuation streamlines. It is shown that similar to the Stokes drift effect, the presence of a density jump layer (pycnocline) decreases the veering of the flow velocity vector at the surface from the direction of the wind stress. It is shown also that in the pycnocline the decrease of the norm of the velocity vector as well as its rotation with depth is smaller than in the regions adjacent to the pycnocline. If the Stokes drift and stratification effects are neglected, the model reduces to the classical Ekman solution with the coefficient of the turbulent shear viscosity replaced by an effective viscosity coefficient. The vertical distributions of velocity predicted by the modified model are compared with the velocity data measured in the Drake Passage and within the Long-Term Upper Ocean Study (LOTUS) in the North Atlantic.

A Parameterization of the Roughness Length for the Air-Sea Interface in Free Convection

The response of the upper ocean to the parameterization of the roughness length z0 on the air side of the air-sea interface is studied using a one-dimensional mixed-layer model. In particular, it is shown that in the free convection limit when both the wind speed and the friction velocity approach zero, the familiar Charnock formula for the momentum roughness, which relies solely on wind gen- eration, can be modified to account for contributions arising from the thermally generated turbulence. Therefore, a new parameterization is proposed for the momentum roughness length which extends the Charnock formula down to zero friction velocity. The value of a parameter which enters in the new formulation is determined by making use of exsisting free convection surface flux parameterizations. The effect of the new parameterization on the model performance is tested using data from the ocean weathership station Papa (OWS P), and data from the Long-Term Upper-Ocean Study (LOTUS) experiment. Simulations were carried out using a recently developed one-dimensional, second-order, turbulence closure scheme over diurnal as well as seasonal time scales. The findings suggest that the new momentum roughness parameterization improves the overall agreement between the observed and simulated sea-surface temperature (SST).

Observations of Seasonal Variation in the Ekman Layer

Abstract The seasonal variation in the Ekman layer is examined using observations from the Long-Term Upper Ocean Study (LOTUS), including surface meteorology, current, and temperature. The near-surface current is found to be coherent with the wind at low frequencies (periods greater than one day). Using a wind-relative averaging method, the authors find the observed near-surface volume transport to be consistent with Ekman transport. The mean current profile has a spiral shape that is flat compared to the classic Ekman spiral in that it rotates less with depth than does the Ekman spiral. The mean current e-folds over 12 m in the summer and 25 m in the winter, with a decay in amplitude that implies eddy viscosities of 60 × 10−4 and 250 × 10−4 m2 s−1, respectively. Diurnal cycling is the dominant mode of variability in the summer and it determines the vertical structure of the spiral. In the winter, diurnal cycling is almost nonexistent due to greatly reduced solar insolation and stronger winds. A persisten...

Evolution model of the vertical structure of the turbulent active layer of the sea

An one‐dimensional differential model of the vertical structure of the turbulent active layer of the sea based on the theory of the rotationally anisotropic turbulence is proposed. The present article consists of two parts. The first part gives the definition of the class of rotationally anisotropic turbulence (RAT) and presents the basics of the theory of RAT. In the second part the theory is used to describe the evolution of the vertical structure of the turbulent active layer of the ocean within the framework of the traditional model assumption about the horizontal homogeneity of all the fields under consideration. A set of submodels (including the analogue to the classical mixing length model) is derived and discussed. The model is used under different initial and boundary conditions. Depending on the actual conditions the model describes various effects: the appearance of the upper mixed layer with a steep thermocline below it, the switching between the convective and diffusive mixing regimes, the staircase‐like structure of the thermocline (caused by unsteady shear flow or by double‐diffusion instabilities), and the modulation of the velocity field by the period of the oscillation of the heat flux. The set of the model coefficients is determined on the basis of sea surface temperature data from the ocean weather station (OWS) Papa. The results of the following computations of temperature distribution show a reasonable similarity between the model results and the corresponding data from the OWS Papa and the Long‐Term Upper Ocean Study (LOTUS).

The Upper-Ocean Response to Surface Heating

Abstract Moored observations of atmospheric variables and upper-ocean temperatures from the Long-Term Upper-Ocean Study (LOTUS) and the Frontal Air-Sea Interaction Experiment (FASINEX) are used to examine the upper-ocean response to surface heating. FASINEX took place between January and June 1986 at 27°N, 7°M while LOTUS took place between May and October 1982 at 34°N, 7°W. The frequency-domain transfer function between rate of change of heat and the net surface heat flux is consistent with a one-dimensional heat balance between heating and convergence of vertical turbulent heat flux at timescales longer than the inertial. The observations satisfy the vertically integrated one-dimensional heat equation and indicate that the response to surface heating has been successfully isolated. Within the internal waveband, upward phase propagation in the response is inconsistent with a one-dimensional balance and the vertically integrated heat balance fails. The internal waveband response is explained as a balance ...

Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization

If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind‐driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double‐diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long‐Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Observations of Superinertial and Near-Inertial Wind-driven Flow

Abstract The superinertial and near-inertial wind-driven flow in the western North Atlantic is examined using data from two recent experiments. The Frontal Air–Sea Interaction Experiment (FASINEX) took place at 27°N, 70°W during 1986. The Long-Term Upper-Ocean Study (LOTUS) took place at 34°N, 70°W during 1982. Each experiment included moored measurements of meteorological variables that allowed estimation of the wind stress and oceanic currents. The directly wind-driven flow is isolated from other sources of variability, such as internal waves and mooring motion, using a transfer function between geocentric acceleration and wind stress. The transfer function is examined in rotary spectral bands bounded by periods of 36 and 12 h, and 12 and 2 h. For surface-moored observations, wind-driven mooring motion is found to cause a response that extends at least to 1000 m (much deeper than the frictional layer of direct wind forcing). Once this artifact is removed, the directly wind-driven flow is identified. Thi...

A simple eddy kinetic energy model for simulations of the oceanic vertical mixing: Tests at station Papa and long‐term upper ocean study site

A simple eddy kinetic energy parameterization of the oceanic vertical mixing is presented. The parameterization scheme is based on recent works on atmospheric turbulence modeling. It is designed to simulate vertical mixing at all depths, from the upper boundary layer down to the abyss. This scheme includes a single prognostic equation for the turbulent kinetic energy. The computation of the turbulent length scales is diagnostic, rather than prognostic. In weakly turbulent regions the simulated vertical diffusivity is inversely proportional to the Brunt‐Vaïsala frequency. In the first validation experiments presented here, the vertical mixing scheme is embedded into a simple one‐dimensional model and used for upper ocean simulations at two very different test sites: the station Papa in the Gulf of Alaska and the Long‐Term Upper Ocean Study (LOTUS) mooring in the Sargasso Sea. At station Papa the model successfully simulates the seasonal evolution of the upper ocean temperature field. At LOTUS the focus is on a short 2‐week period. A detailed analysis of the oceanic heat budget during that period reveals a large bias in the bulk‐derived surface heat fluxes. After correction of the fluxes the model does well in simulating the evolution of the temperature and wind‐driven current. In particular, the large observed diurnal cycles of the sea surface temperature are well reproduced. During the second (windy) week of the selected period the model accounts for about two thirds of the kinetic energy of the observed upper ocean currents at periods larger than 6 hours. The local wind forcing thus appears to be the dominant generation mechanism for the near‐inertial motions, which are the most energetic. The velocity simulation is especially good at the low frequencies. During the second simulated week the model accounts for as much as 78% of the kinetic energy at subinertial frequencies. The simulated mean velocity profile is reminiscent of an Ekman spiral, in agreement with the observations.

The determination of the latent and sensible heat fluxes at the sea surface viewed as an inverse problem

The determination of the latent and sensible heat fluxes at the air‐sea interface is formulated here as an inverse problem: given an upper ocean model, one seeks the successive values of the “nonsolar” surface heat flux Fnsol (sum of the latent, sensible, and net infrared fluxes) that yield a nearly exact simulation of the observed evolution of the sea surface temperature (SST). The model is otherwise forced by surface insolation and wind stress data. Provided that an estimate of the infrared flux is available, the sum of the latent and sensible heat fluxes is directly obtained as the difference between Fnsol and the infrared flux. A major interest of this approach is that all surface inputs (stress, SST, long‐wave and shortwave radiation) can be measured from a satellite. In this preliminary exploration of the inverse flux estimation technique, attention is restricted to situations where the oceanic advection is weak so that a one‐dimensional ocean model can be used. The inverse problem, in a rudimentary form, is solved using a simple iterative shooting method. The inverse flux estimation technique is tested using a 2‐year data set from station P, in the Gulf of Alaska, and a 2‐week data set from the Long‐Term Upper Ocean Study (LOTUS) experiment, in the Sargasso Sea. The first tests are performed using regularly sampled in situ measurements of the stress, SST, and insolation, rather than actual satellite data. At both test sites the inverse‐derived estimates of Fnsol prove to be consistent with the observed changes of the upper ocean heat content over periods of a few days. On the other hand, the inverse‐derived heat fluxes exhibit a large diurnal variability that is not present in classical bulk‐derived estimates. This difference in the short‐term variability of the two flux estimates is as yet unexplained. The final tests at LOTUS involve actual satellite data for the isolation and SST. Using the few available satellite measurements, the inverse technique still provides a realistic mean flux estimate. The simulated evolution of the SST in between the satellite observations agrees well with in situ observations, and a fair simulation of the deeper temperatures is obtained.

Observations and EOF Analysis of Low-Frequency Variability in the Western Part of the Gulf Stream Recirculation

Abstract This study of low-frequency oceanic variability is based on data collected during the Long Term Upper Ocean Study (LOTUS), which was a two year program of (mainly) moored meteorological and oceanographic measurements. The mooring arrays were centered at 34°N, 70°W over the Hatteras Abyssal Plain. With a distance of about 300 km to the mean Gulf Stream axis and the continental slope, LOTUS was the most northern and western long-term mooring site in the Gulf Stream recirculation region to date. The observed low-frequency variability is dominated by zonally elongated motions of the secular time scale (periods <100 days) even great depths (4000 m). In contrast to observations in other parts of the recirculation region, the spectral shapes are strongly depth dependent. The vertical structure of the variability was examined by EOF analysis. Different kinds of EOFs were tested; the best representation of the observed variability was obtained by an EOF representing a undirectional nonrotating (with depth...

Variability in the Upper-Ocean Internal Wave Field at a Sargasso Sea Site

Abstract Two 3.5 month time series records of upper-ocean current and density profiles collected in opposite seasons as part of the LOTUS (Long-Term Upper-Ocean Study) project at 34°N, 70°W indicate substantial variation in the shape of horizontal current spectra in the internal wave frequency range. The near-inertial peak in these records shifts by as much as 10% in frequency and varies by a factor of 5 in height across the depth range of the seasonal pycnocline. Near-inertial currents are weaker and less strongly circularly polarized in the spring-summer than in the fall-winter record. Spectral frequency and vertical wavenumber dependences differ from those predicted by the Garrett-Munk internal wave model. Energy fluxes are vertically asymmetric and internal wave energy varies linearly with rms subinertial current speed. Several aspects of the observed variability can be understood through two simple models. Depth dependence of near-inertial motions can be explained as a characteristic of linear respon...

Observations of the seasonal cycle of upper ocean structure and the roles of advection and diapycnal mixing

Two 3.5‐month profile time series of temperature, salinity, and current in the seasonal thermocline of the northwest Sargasso Sea are used to estimate the contributions of density advection by geostrophic currents and diapycnal mixing to the observed density changes. The two records were collected in opposite seasons (spring‐summer and fall‐winter) at 34°N, 70°W as part of the Long‐Term Upper Ocean Study (LOTUS). These records show the formation and decay of the seasonal pycnocline and the accompanying changes in upper ocean temperature‐salinity relationships. Geostrophic advection is an important contributor to density variability at vertical wavelengths of 100 m and greater at subinertial frequencies. Diapycnal diffusion accounts for observed cooling over time scales of a week and longer when currents are sufficiently weak that advective effects do not overwhelm the mixing signal.

Preliminary results from the long-term upper-ocean study (LOTUS)

The Long-Term Upper-Ocean Study (LOTUS) is a two-year experiment at 34°N, 70°W, in deep water over the Hatteras abyssal plain, 330 km from major topography and the mean path of the Gulf Stream. The observations began in May 1983. The principal measurements are from an array of three subsurface moorings and one surface mooring carrying meteorological instrumentation as well as near-surface current meters. Some basic meteorological and engineering data are telemetered back via the ARGOS system, but most of the data are self-recorded. Other observations in the program include XBT sections, CTD profiles, and cooperative efforts with other investigators to obtain satellite imagery and other in situ measurements. The main scientific goals of the experiment are: to obtain a description of the local response of the upper ocean to a potpourri of atmospheric forcing in a variety of environmental situations, such as in the presence of Gulf Stream rings or near oceanic fronts, and even to try and describe the statistical response to the fluctuating forcing; to investigate the energy level of the internal wave field over several seasons and during the passage of various forcing and environmental events as mentioned above; and to obtain the complete profile of eddy kinetic energy. We display here selected results from the first year of LOTUS, especially those that do not require the analysis of the complete array of measurements and supporting data. Of particular interest is the eddy energy kinetic energy profile showing a nearly constant energy in the top 500 m; the abyssal eddy energy is also about half the expected level for the site. The high-frequency internal wave field at 200–500 m depths shows a seasonal variation (high in the winter, low in the early fall) that cannot be ascribed to changes in the hydrography; at greater depths the winter maximum is still visible. At each depth the energy varies from a half or a third to 2 or 3 times the mean energy at that depth. Some of the energetic events in the near-inertial field are apparently unconnected to local 3ind events, and may instead be related to the edge of strong mesoscale oceanic currents; perhaps they are being radiated during the process of geostrophic adjustment by the mesoscale features.