scholarly journals The Mean Upper-Layer Flow in the Central Gulf of Mexico by a New Method

2016 ◽  
Vol 46 (10) ◽  
pp. 2915-2924 ◽  
Author(s):  
Wilton Sturges
Keyword(s):  
Gulf Of Mexico ◽  
Numerical Models ◽  
Independent Set ◽  
Sea Surface ◽  
Return Flow ◽  
Mean Flow ◽  
The Mean ◽  
Layer Flow ◽  
A New Technique ◽  
Height Data

AbstractPrevious studies have found a puzzling disagreement between two large datasets and the results of numerical models in the central Gulf of Mexico. The observations suggest an upper-layer mean flow to the west of order 10 cm s−1, while the numerical models find no such mean flow. A new technique is used here, using 23 yr of satellite-derived sea surface height data, to estimate the mean flow. This third, independent set of data yields the same westward flow found in previous studies. These findings require that there be sinking in the western Gulf. The details of the return flow remain an intriguing problem.

On the Mean Flow in the Western Gulf of Mexico and a Reappraisal of Errors in Ship-Drift Data

2020 ◽  
Vol 50 (7) ◽  
pp. 1983-1988
Author(s):  
Wilton Sturges
Keyword(s):  
Gulf Of Mexico ◽  
Surface Flow ◽  
Return Flow ◽  
Loop Current ◽  
Mean Flow ◽  
The West ◽  
Near Surface ◽  
Southern Half ◽  
The Mean ◽  
Layer Flow

AbstractShip-drift data in the Gulf of Mexico have led to a perplexing result, that the near-surface flow in the west has a north–south mean, of the east–west flow, ~5–10 cm s−1 into a closed basin. Ship-drift data have been used in the past hundred years under the assumption that they are reasonably accurate; the present study examines that assumption carefully, finding that the standard deviation of individual observations is typically ~20 cm s−1. In a monthly mean composed of order 400 observations or more, as examined here, the standard error of the mean will be reduced accordingly. In the southern part of the western Gulf of Mexico, the observed upper-layer flow is clearly to the west and is consistent with our expectations. In the northern part, however, the apparent flow as reported by ship drift in deep water is not significantly different from zero. Thus, the puzzling result remains: three different datasets in the southern half of the basin clearly show flow to the west, with speeds of 10 cm s−1 or more, yet there is no clear evidence of a near-surface return flow back to the east. The convergent wind stress forces downwelling of the upper layer; its return flow could be at some intermediate depth. The transport to the west from Loop Current rings is possibly returned in a deep boundary flow driven by the rectification of deep topographic Rossby waves.

scholarly journals A Puzzling Disagreement between Observations and Numerical Models in the Central Gulf of Mexico

2013 ◽  
Vol 43 (12) ◽  
pp. 2673-2681 ◽  
Author(s):  
Wilton Sturges ◽  
Alexandra Bozec
Keyword(s):  
Gulf Of Mexico ◽  
Numerical Models ◽  
Surface Flow ◽  
Return Flow ◽  
Mean Flow ◽  
The West ◽  
Near Surface ◽  
Direct Observations ◽  
Order Of Magnitude ◽  
The Difference

Abstract Two large, independent sets of direct observations in the central Gulf of Mexico show a mean near-surface flow of ~10 cm s−1 to the west, concentrated in the northern and southern Gulf. Numerical models that the authors have examined do not produce this mean westward flow. The observed speeds appear to be almost an order of magnitude larger than the estimated errors; this paper studies the observations to estimate carefully the possible errors involved and compares the observations with model results. The flow to the west in the southern Gulf is presumably wind driven on the shallow parts of the shelf, and, in slightly deeper water at the outer edges of the shelf, is possibly the result of southward Sverdrup interior flow driven by the negative curl of the wind stress. In another possibly related issue, long-term deep current-meter observations in the northern Gulf at ~1000 m and below find flow to the west, whereas some models find flow to the east. The flow proposed here assumes a mean flow to the west above roughly 300 m, with a required return flow in deep water. The difference between the deep observations and the models will produce a slope of pressure surfaces of the opposite sign below 1000 m, reversing the direction of upper-layer geostrophic flow in the models.

scholarly journals Deep Cyclonic Circulation in the Gulf of Mexico

2005 ◽  
Vol 35 (10) ◽  
pp. 1801-1812 ◽  
Author(s):  
Christopher J. DeHaan ◽  
Wilton Sturges
Keyword(s):  
Gulf Of Mexico ◽  
Accurate Result ◽  
Sea Water ◽  
Cyclonic Circulation ◽  
Loop Current ◽  
Mean Flow ◽  
Eastern Gulf Of Mexico ◽  
Geostrophic Shear ◽  
Layer Flow ◽  
Yucatan Channel

Abstract The anticyclonic Loop Current dominates the upper-layer flow in the eastern Gulf of Mexico, with a weaker mean anticyclonic pattern in the western gulf. There are reasons, however, to suspect that the deep mean flow should actually be cyclonic. Topographic wave rectification and vortex stretching contribute to this cyclonic tendency, as will the supply of cold incoming deep water at the edges of the basin. The authors find that the deep mean flow is cyclonic both in the eastern and western gulf, with speeds on the order of 1–2 cm s−1 at 2000 m. Historical current-meter mooring data, as well as profiling autonomous Lagrangian circulation explorer (PALACE) floats (at 900 m), suggest that vertical geostrophic shear relative to 1000 m gives a surprisingly accurate result in the interior of the basin. The temperature around the edges of the basin at 2000 m is coldest near the Yucatan Channel, where Caribbean Sea water is colder by ∼0.1°C. The temperature increases steadily with distance in the counterclockwise direction from the Yucatan, consistent with a deep mean cyclonic boundary flow.

scholarly journals Use of a Mixed-Layer Model to Investigate Problems in Operational Prediction of Return Flow

2007 ◽  
Vol 135 (7) ◽  
pp. 2610-2628 ◽  
Author(s):  
John M. Lewis
Keyword(s):  
Sensitivity Analysis ◽  
Gulf Of Mexico ◽  
Boundary Conditions ◽  
Mixed Layer ◽  
Initial Conditions ◽  
Sea Surface ◽  
Return Flow ◽  
Control Vector ◽  
Layer Model ◽  
Mixed Layer Model

Abstract Inaccuracy in the numerical prediction of the moisture content of return-flow air over the Gulf of Mexico continues to plague operational forecasters. At the Environmental Modeling Center/National Centers for Environmental Prediction in the United States, the prediction errors have exhibited bias—typically too dry in the early 1990s and too moist from the mid-1990s to present. This research explores the possible sources of bias by using a Lagrangian formulation of the classic mixed-layer model. Justification for use of this low-order model rests on careful examination of the upper-air thermodynamic structure in a well-observed event during the Gulf of Mexico Experiment. The mixed-layer constraints are shown to be appropriate for the first phase of return flow, namely, the northerly-flow or outflow phase. The theme of the research is estimation of sensitivity—change in the model output (at termination of outflow) in response to inaccuracies or uncertainties in the elements of the control vector (the initial conditions, the boundary conditions, and the physical and empirical parameters). The first stage of research explores this sensitivity through a known analytic solution to a reduced form of the mixed-layer equations. Numerically calculated sensitivity (via Runge–Kutta integration of the equations) is compared to the exact values and found to be most credible. Further, because the first- and second-order terms in the solution about the base state can be found exactly for the analytic case, the degree of nonlinearity in the dynamical system can be determined. It is found that the system is “weakly nonlinear”; that is, solutions that result from perturbations to the control vector are well approximated by the first-order terms in the Taylor series expansion. This bodes well for the sensitivity analysis. The second stage of research examines sensitivity for the general case that includes moisture and imposed subsidence. Results indicate that uncertainties in the initial conditions are significant, yet they are secondary to uncertainties in the boundary conditions and physical/empirical parameters. The sea surface temperatures and associated parameters, the saturation mixing ratio at the sea surface and the turbulent transfer coefficient, exert the most influence on the moisture forecast. Uncertainty in the surface wind speed is also shown to be a major source of systematic error in the forecast. By assuming errors in the elements of the control vector that reflect observational error and uncertainties in the parameters, the bias error in the moisture forecast is estimated. These bias errors are significantly greater than random errors as explored through Monte Carlo experiments. Bias errors of 1–2 g kg−1 in the moisture forecast are possible through a variety of systematic errors in the control vector. The sensitivity analysis also makes it clear that judiciously chosen incorrect specifications of the elements can offset each other and lead to a good moisture forecast. The paper ends with a discussion of research approaches that hold promise for improved operational forecasts of moisture in return-flow events.

scholarly journals What Makes an Annular Mode “Annular”?

2017 ◽  
Vol 74 (2) ◽  
pp. 317-332 ◽  
Author(s):  
Edwin P. Gerber ◽  
David W. J. Thompson
Keyword(s):  
Numerical Models ◽  
Atmospheric Model ◽  
External Forcing ◽  
Eof Analysis ◽  
Mean Flow ◽  
Orthogonal Function ◽  
The Mean ◽  
High Degree ◽  
Symmetric Statistics ◽  
Variance Explained

Abstract Annular patterns with a high degree of zonal symmetry play a prominent role in the natural variability of the atmospheric circulation and its response to external forcing. But despite their apparent importance for understanding climate variability, the processes that give rise to their marked zonally symmetric components remain largely unclear. Here the authors use simple stochastic models in conjunction with an atmospheric model and observational analyses to explore the conditions under which annular patterns arise from empirical orthogonal function (EOF) analysis of the flow. The results indicate that annular patterns arise not only from zonally coherent fluctuations in the circulation (i.e., “dynamical annularity”) but also from zonally symmetric statistics of the circulation in the absence of zonally coherent fluctuations (i.e., “statistical annularity”). It is argued that the distinction between dynamical and statistical annular patterns derived from EOF analysis can be inferred from the associated variance spectrum: larger differences in the variance explained by an annular EOF and successive EOFs generally indicate underlying dynamical annularity. The authors provide a simple recipe for assessing the conditions that give rise to annular EOFs of the circulation. When applied to numerical models, the recipe indicates dynamical annularity in parameter regimes with strong feedbacks between eddies and the mean flow. When applied to observations, the recipe indicates that annular EOFs generally derive from statistical annularity of the flow in the midlatitude troposphere but from dynamical annularity in both the stratosphere and the mid–high-latitude Southern Hemisphere troposphere.

scholarly journals On the non-geostrophic appearance of some mean surface velocity observations in the Gulf of Mexico

2021 ◽  
Author(s):  
WILTON STURGES
Keyword(s):  
Gulf Of Mexico ◽  
Surface Flow ◽  
Sea Surface ◽  
Surface Velocity ◽  
Geostrophic Balance ◽  
Sea Surface Heights ◽  
The Mean ◽  
Surface Drifters ◽  
Good Agreement

AbstractA previous study of currents in the Gulf of Mexico by the author used long-term means from three independent data sources. Ship-drift results are in good agreement with surface drifters, but these two do not agree with satellite sea-surface heights (SSH). The agreement between the first two suggested the possibility that there could be errors in the SSH or that the mean surface flow is not in geostrophic balance. The present results, using the addition of a fourth long-term mean from hydrographic data, which agrees with the SSH, resolves the issue. The lack of agreement between different long-term means is from inadequate coverage in space and time in data from ship drifts and drifters.

Exponential roughness layer and analytical model for turbulent boundary layer flow over rectangular-prism roughness elements

2016 ◽  
Vol 789 ◽  
pp. 127-165 ◽  
Author(s):  
Xiang I. A. Yang ◽  
Jasim Sadique ◽  
Rajat Mittal ◽  
Charles Meneveau
Keyword(s):  
Boundary Layer ◽  
Analytical Model ◽  
Rectangular Prism ◽  
Mean Flow ◽  
Mean Velocity ◽  
Roughness Elements ◽  
Normal Distance ◽  
The Mean ◽  
Turbulent Boundary Layer Flow ◽  
Layer Flow

We conduct a series of large-eddy simulations (LES) to examine the mean flow behaviour within the roughness layer of turbulent boundary layer flow over rough surfaces. We consider several configurations consisting of arrays of rectangular-prism roughness elements with various spacings, aspect ratios and height distributions. The results provide clear evidence for exponential behaviour of the mean flow with respect to the wall normal distance. Such behaviour has been proposed before (see, e.g., Cionco, 1966 Tech. Rep. DTIC document), and is represented as $U(z)/U_{h}=\exp [a(z/h-1)]$, where $U(z)$ is the spatially/temporally averaged fluid velocity, $z$ is the wall normal distance, $h$ represents the height of the roughness elements and $U_{h}$ is the velocity at $z=h$. The attenuation factor $a$ depends on the density of the roughness element distribution and details of the roughness distribution on the wall. Once established, the generic velocity profile shape is used to formulate a fully analytical model for the effective drag exerted by turbulent flow on a surface covered with arrays of rectangular-prism roughness elements. The approach is based on the von Karman–Pohlhausen integral method, in which a shape function is assumed for the mean velocity profile and its parameters are determined based on momentum conservation and other fundamental constraints. In order to determine the attenuation parameter $a$, wake interactions among surface roughness elements are accounted for by using the concept of flow sheltering. The model transitions smoothly between ‘$k$’ and ‘$d$’ type roughness conditions depending on the surface coverage density and the detailed geometry of roughness elements. Comparisons between model predictions and experimental/numerical data from the existing literature as well as LES data from this study are presented. It is shown that the analytical model provides good predictions of mean velocity and drag forces for the cases considered, thus raising the hope that analytical roughness modelling based on surface geometry is possible, at least for cases when the location of flow separation over surface elements can be easily predicted, as in the case of wall-attached rectangular-prism roughness elements.

scholarly journals The structure of turbulence in a rapid tidal flow

2017 ◽  
Vol 473 (2204) ◽  
pp. 20170295 ◽  
Author(s):  
I. A. Milne ◽  
R. N. Sharma ◽  
R. G. J. Flay
Keyword(s):  
Numerical Models ◽  
Tidal Flow ◽  
Tidal Energy ◽  
Tidal Channels ◽  
Mean Flow ◽  
Mean Velocity ◽  
Simple Geometry ◽  
The Mean ◽  
The Uk ◽  
A Site

The structure of turbulence in a rapid tidal flow is characterized through new observations of fundamental statistical properties at a site in the UK which has a simple geometry and sedate surface wave action. The mean flow at the Sound of Islay exceeded 2.5 m s −1 and the turbulent boundary layer occupied the majority of the water column, with an approximately logarithmic mean velocity profile identifiable close to the seabed. The anisotropic ratios, spectral scales and higher-order statistics of the turbulence generally agree well with values reported for two-dimensional open channels in the laboratory and other tidal channels, therefore providing further support for the application of universal models. The results of the study can assist in developing numerical models of turbulence in rapid tidal flows such as those proposed for tidal energy generation.

scholarly journals Eddies in Numerical Models of the Antarctic Circumpolar Current and Their Influence on the Mean Flow

1999 ◽  
Vol 29 (3) ◽  
pp. 328-350 ◽  
Author(s):  
S. E. Best ◽  
V. O. Ivchenko ◽  
K. J. Richards ◽  
R. D. Smith ◽  
R. C. Malone
Keyword(s):  
Antarctic Circumpolar Current ◽  
Numerical Models ◽  
Mean Flow ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Turbulent Mixed Convection Over an Isothermal Horizontal Flat Plate

1990 ◽  
Vol 112 (1) ◽  
pp. 124-129 ◽  
Author(s):  
N. Ramachandran ◽  
B. F. Armaly ◽  
T. S. Chen
Keyword(s):  
Mixed Convection ◽  
Flat Plate ◽  
Free Stream ◽  
Surface Heat ◽  
Heated Plate ◽  
Force Parameter ◽  
Mean Flow ◽  
Local Surface ◽  
The Mean ◽  
Layer Flow

Turbulent boundary layer flow adjacent to an isothermal horizontal flat plate is analyzed for the mixed convection regime. Results are presented for both air (Pr = 0.7) and water (Pr= 7) flowing above the heated plate at various velocities over a range of temperature differences between the plate and the free stream. Closure of the governing, time-averaged, turbulent equations for the mean flow quantities is attained by using a modified k–ε model that accounts for the influence of buoyancy-induced forces on the turbulent quantities. It is found that the local surface heat flux increases with increasing buoyancy force parameter. The numerical results are in qualitative agreement with available experimental measurements.

Sign in / Sign up

Export Citation Format

Share Document