On the dynamics of wind-driven shelf currents

1981 ◽  
Vol 302 (1472) ◽  
pp. 617-634 ◽  
Keyword(s):  
Sea Level ◽  
Momentum Equation ◽  
Bottom Friction ◽  
Empirical Orthogonal Functions ◽  
Rate Of Change ◽  
Time Dependent ◽  
Momentum Balance ◽  
Orthogonal Functions ◽  
Northwest Africa ◽  
Coastal Sea

Measurements of currents, wind, and coastal sea level from off Oregon, northwest Africa and Peru are used to examine characteristics of the depth-integrated momentum balance at mid-shelf locations. Attention is focused on determining the nature of the balance, as a function of frequency, of time-dependent terms in the alongshore momentum equation. Decomposition of the estimated terms into empirical orthogonal functions, and regression of terms on the wind stress to obtain the wind-forced component, are methods used in an attempt to assess objectively the type of balance present. It is found, for Oregon, that there is a relatively large amount of variance in the depth-integrated cross-shelf velocity which is not balanced by other estimated terms. In addition, a substantial component of the flow is wind-forced, with bottom friction insignificant for periods less than 11 days. For northwest Africa, the motion is strongly wind-driven, with bottom friction playing an important role for periods greater than 6 days and with a quasi-steady response evident for periods longer than 10 days. For Peru, the motion is dominated by an inviscid, unforced balance which, for the 5- to 11-day frequency band, is primarily between the alongshore pressure gradient and the rate of change with time of the alongshore velocity.

Currents off south-eastern Australia: results from the Australian coastal experiment

1988 ◽  
Vol 39 (3) ◽  
pp. 245 ◽  
Author(s):  
A Huyer ◽  
RL Smith ◽  
PJ Stabeno ◽  
JA Church ◽  
NJ White
Keyword(s):  
Sea Level ◽  
Continental Slope ◽  
Wind Stress ◽  
Continental Margin ◽  
Eddy Currents ◽  
Empirical Orthogonal Functions ◽  
Trapped Waves ◽  
Orthogonal Functions ◽  
Coastal Trapped Waves ◽  
And Sea Level

The Australian Coastal Experiment was conducted off the east coast of New South Wales between September 1983 and March 1984. The experiment was conducted with arrays of current meters spanning the continental margin at three latitudes (37.5�, 34.5�, and 33.0�S.), additional shelf moorings at 29� and 42�S. coastal wind and sea-level measurements, monthly conductivity-temperature-depth probe/expendable bathythermograph (CTD/XBT) surveys, and two satellite-tracked buoys. Over the continental shelf and slope, the alongshore component of the current generally exceeded the onshore component, and the subtidal (<0.6 cpd, cycles per day) current variability greatly exceeded the mean flow. Part of the current variability was associated with two separate warm-core eddies that approached the coast, causing strong (>50 cm sec-1), persistent (>8 days), southward currents over the continental slope and outer shelf. Temperature and geostrophic velocity sections through the eddies, maps of ship's drift vectors and temperature contours at 250 m, and the satellite-tracked drifter trajectories showed that these eddies were similar in structure to those observed previously in the East Australian Current region. Both eddies migrated generally southward. Eddy currents over the shelf and slope were rare at Cape Howe (37.5�S.), more common near Sydney (34.5�S.), and frequent at Newcastle (33.0�S.), where strong northward currents were also observed. Near Sydney, the eddy currents over the slope turned clockwise with depth between 280 and 740 m, suggesting net downwelling there. Repeated CTD sections also indicated onshore transport and downwelling at shallower levels; presumably, upwelling occurred farther south where the eddy currents turned offshore. Periodic rotary currents over the continental slope near Sydney and Newcastle indicated the presence of small cyclonic eddies on the flank of a much larger anticyclonic eddy. Between early October and late January, no strong southward currents were observed over the continental margin near Sydney. Data from this 'eddy-free' period were analysed further to examine the structure and variability of the coastal currents. Much of this variability was correlated with fluctuations in coastal sea-level (at zero lag) and with the wind stress (at various lags). The coherence and phase relationships among current, wind-stress, and sea-level records at different latitudes (determined from spectral analysis and frequency-domain empirical orthogonal functions) were consistent with the equatorward propagation of coastal-trapped waves generated by winds in phase with those near Cape Howe. Time-domain empirical orthogonal functions show that the current fluctuations decayed with distance from shore and with depth, as expected of coastal-trapped waves.

scholarly journals Data-Driven Interpolation of Sea Level Anomalies Using Analog Data Assimilation

2019 ◽  
Vol 11 (7) ◽  
pp. 858 ◽  
Author(s):  
Redouane Lguensat ◽  
Phi Huynh Viet ◽  
Miao Sun ◽  
Ge Chen ◽  
Tian Fenglin ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Sea Level ◽  
Large Scale ◽  
Empirical Orthogonal Functions ◽  
Data Driven ◽  
Optimal Interpolation ◽  
Observation System ◽  
Orthogonal Functions ◽  
Sea Level Anomalies ◽  
Recent Developments

From the recent developments of data-driven methods as a means to better exploit large-scale observation, simulation and reanalysis datasets for solving inverse problems, this study addresses the improvement of the reconstruction of higher-resolution Sea Level Anomaly (SLA) fields using analog strategies. This reconstruction is stated as an analog data assimilation issue, where the analog models rely on patch-based and Empirical Orthogonal Functions (EOF)-based representations to circumvent the curse of dimensionality. We implement an Observation System Simulation Experiment (OSSE) in the South China Sea. The reported results show the relevance of the proposed framework with a significant gain in terms of Root Mean Square Error (RMSE) for scales below 100 km. We further discuss the usefulness of the proposed analog model as a means to exploit high-resolution model simulations for the processing and analysis of current and future satellite-derived altimetric data with regard to conventional interpolation schemes, especially optimal interpolation.

scholarly journals ENSO components of the Atlantic multidecadal oscillation and their relation to North Atlantic interannual coastal sea level anomalies

Ocean Science ◽  
2013 ◽  
Vol 9 (3) ◽  
pp. 535-543 ◽  
Author(s):  
J. Park ◽  
G. Dusek
Keyword(s):  
Sea Level ◽  
North Atlantic ◽  
Southern Oscillation ◽  
Atlantic Multidecadal Oscillation ◽  
The United States ◽  
Empirical Orthogonal Functions ◽  
Water Levels ◽  
Total Contribution ◽  
Sea Level Anomalies ◽  
Coastal Sea

Abstract. The El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO) are known to influence coastal water levels along the East Coast of the United States. By identifying empirical orthogonal functions (EOFs), which coherently contribute from the Multivariate ENSO Index (MEI) to the AMO index (AMOI), we characterize both the expression of ENSO in the unsmoothed AMOI, and coherent relationships between these indices and interannual sea level anomalies at six stations in the Gulf of Mexico and western North Atlantic. Within the ENSO band (2–7 yr periods) the total contribution of MEI to unsmoothed AMOI variability is 79%. Cross correlation suggests that the MEI leads expression of the ENSO signature in the AMOI by six months, consistent with the mechanism of an atmospheric bridge. Within the ENSO band, essentially all of the coupling between the unsmoothed AMOI and sea level anomalies is the result of ENSO expression in the AMOI. At longer periods we find decadal components of sea level anomalies linked to the AMOI at three southern stations (Key West, Pensacola, Charleston), but not at the northern stations (Baltimore, Boston, Portland), with values of coherence ranging from 20 to 50%. The coherence of MEI to coastal sea level anomalies has a different structure and is generally weaker than that of the ENSO expressed AMOI influence, suggesting distinct physical mechanisms are influencing sea level anomalies due to a direct ENSO teleconnection when compared to teleconnections based on ENSO expression in the AMOI. It is expected that applying this analysis to extremes of sea level anomalies will reveal additional influences.

Modified Time-Dependent Penetration Length and Inlet Pressure Field in Rectangular and Cylindrical Channel Flows Driven by Non-Mechanical Forces

2011 ◽  
Vol 133 (11) ◽  
Author(s):  
Martin Ndi Azese
Keyword(s):  
Velocity Profile ◽  
Governing Equation ◽  
Control Volume ◽  
Momentum Equation ◽  
Rate Of Change ◽  
Creeping Flow ◽  
Time Dependent ◽  
Penetration Length ◽  
Wide Range ◽  
Accurate Expression

In this paper, we derive the governing equation for the time dependent penetration length of a fluid column in rectangular and cylindrical channels under the action of nonmechanical forces like capillary or electro-osmotic force. For this purpose, first we obtain the velocity profile for unidirectional unsteady flow by satisfying momentum equation in differential form. Then, we relate the rate of change of penetration length with volume flux to obtain the governing equation of the penetration length. As the velocity profile is exact, the analysis is devoid of any mathematical error. As a result, the theoretical results are valid irrespective of the Reynolds number of the system as long as the flow inside the cylindrical or rectangular conduit is laminar. We then use the new expressions of velocity fields of respective conduits to derive a more accurate expression of the entrance pressure by using a hemispherical model for the control volume for finite aspect ratio. As these channels are very common, our governing equations for penetration length will have a wide range of applicability. These applications especially include creeping flow in micro fluidic domain for which we have a simplified version of the derived equation.

scholarly journals The Effect of Signal-to-Noise Ratio on the Study of Sea Level Trends

2011 ◽  
Vol 24 (5) ◽  
pp. 1396-1408 ◽  
Author(s):  
B. D. Hamlington ◽  
R. R. Leben ◽  
R. S. Nerem ◽  
K.-Y. Kim
Keyword(s):  
Least Squares ◽  
Sea Level ◽  
Least Squares Method ◽  
Secular Trend ◽  
Signal To Noise Ratio ◽  
Empirical Orthogonal Functions ◽  
Signal To Noise ◽  
Orthogonal Functions ◽  
Geophysical Processes ◽  
Noise Ratio

Abstract Extracting secular sea level trends from the background ocean variability is limited by how well one can correct for the time-varying and oscillating signals in the record. Many geophysical processes contribute time-dependent signals to the data, making the sea level trend difficult to detect. In this paper, cyclostationary empirical orthogonal functions (CSEOFs) are used to quantify and improve the signal-to-noise ratio (SNR) between the secular trend and the background variability, obscuring this trend in the altimetric sea level record by identifying and removing signals that are physically interpretable. Over the 16-yr altimetric record the SNR arising from the traditional least squares method for estimating trends can be improved from 4.0% of the ocean having an SNR greater than one to 9.9% when using a more sophisticated statistical method based on CSEOFs. From a standpoint of signal detection, this implies that the secular trend in a greater portion of the ocean can be estimated with a higher degree of confidence. Furthermore, the CSEOF method improves the standard error on the least squares estimates of the secular trend in 97% of the ocean. The convergence of the SNR as the record length is increased is used to estimate the SNR of sea level trends in the near future as more measurements become available from near-global altimetric sampling.

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