Ocean Current Time Series Research Articles

Measurements of mid-frequency acoustic signal variability in the Western Barents Sea

Two Vertical Line Arrays (VLAs) and one source moorings were deployed to study along- and cross-shelf acoustic propagation in the Western Barents Sea during October 12–19, 2022. An ITC 2010 source was used to transmit Linear Frequency-Modulated (LFM) acoustic signals covering a frequency band of 0.7–4.2 kHz. Several oceanographic moorings were also deployed to measure time series of ocean currents and sound-speed profiles. Analysis of the acoustic data showed strong effects of tidal currents and surface roughness on mid-frequency acoustic propagation. Along- and cross-shelf acoustic propagation data showed semidiurnal travel-time fluctuations due to tidal currents. In addition, a 15 dB increase in Transmission Loss was observed during the storm events due to rough surface scattering. Finally, an assessment of the Polar Front and its effects on acoustic propagation (both travel-time and amplitude) are studied. Several data and simulated examples are provided to demonstrate the 3D effects of a Polar front and nonlinear internal waves on mid-frequency acoustic propagation. [Work supported by the ONR.]

Ocean Variability at Greenland's Largest Glacier Tongue Linked to Continental Shelf Circulation

AbstractIncreased ocean‐to‐ice heat fluxes play a key role in the accelerated mass loss of Greenland’s marine‐terminating glaciers. Ocean current variability leads to variations in this heat flux. A year‐long time series of ocean currents at all gateways to the ocean cavity under Greenland’s largest remaining floating ice tongue at the Nioghalvfjerdsfjorden Glacier (79NG) was analyzed. The variability of the exchange flow at intra‐annual to near‐daily timescales was characterized. The currents exhibit considerable variability with standard deviations exceeding the time mean flow strength by a factor of 2. The inflow of warm Atlantic Intermediate Water into the cavity and the outflow via the northernmost calving front were directly coupled on intra‐annual timescales (periods, T > 30 days) with enhanced fluctuations in the winter months. A strong correlation between the variability of the deep inflow and currents in the subsurface boundary current on the continental shelf suggests a link between cavity and continental shelf circulation. Variability on higher frequencies (T < 30 days) in the outflow was only partly induced by the inflow variability. Two export branches of the cavity circulation were identified, which were potentially constrained by subglacial meltwater channels. The relative importance of the two export branches varies on monthly time scales. This research has provided evidence that the large intra‐annual ocean current variability at the 79NG is strongly influenced by the continental shelf circulation. Temporally varying preferred export routes increase the complexity of the cavity circulation.

Seasonal characteristics of internal tides on the continental shelf in the northern South China Sea

Three across‐shelf Acoustic Doppler Current Profiler (ADCP) moorings were deployed on the continental shelf in the northern South China Sea in 2006 and 2007, in order to obtain time series of ocean currents. During each of the three observational periods, in spring, autumn, and winter seasons, the observation was sustained for at least 1 month. Conductivity‐temperature‐depth data were also collected in the vicinity of these ADCP moorings. The two diurnal baroclinic constituents (O1 and K1) are found to be more prominent than the semidiurnal baroclinic ones (M2 and S2) at each mooring site, which are different from the barotropic tides. The highest diurnal (semidiurnal) baroclinic kinetic energy density exceeded 10 (2.7) kJ/m2 during the observational periods. In each observational period, the vertical structures of baroclinic tidal ellipses indicate mode‐1 characteristics for O1 and K1, whereas some of the semidiurnal internal tides show mode‐2 characteristics in autumn. Propagation directions of the diurnal and semidiurnal internal tides are basically across‐shelf, and their phase speeds estimated from phase changes and buoyancy frequencies indicate deceleration of internal tides, when the internal tidal waves propagate from deeper shelf break to shallower shelf. The increased stratification in autumn is favorable for maintaining internal tidal waveform and preserving internal tidal energy, while the intensity of the internal tidal flow decreases significantly in winter.

Ocean current and temperature time series at Brothers volcano

Time series of currents and temperatures were acquired near surface and at depth on three moorings above Brothers volcano (34° 52.25′S, 179° 04′E), ∼25 km west of the Kermadec Ridge and ∼300 km northeast of the North Island, New Zealand, from September 2004 to May 2005. Ocean current time series were previously unavailable within 300 km of this site, and few such measurements exist anywhere near magmatically or hydrothermally active volcanoes such as Brothers. Energy in near surface currents (163–527 m depth) was primarily subtidal (64%). Subtidal motions to 527 m depth are shown to be reasonably well correlated with geostrophic currents derived from satellite sea level anomalies. Although the moorings were separated laterally by only 11 km, the upper current meters also recorded significant differences in M2 tidal currents, indicating the presence of significant internal tides at this site. Below volcano summit depth (1220 m), the kinetic energy of currents was primarily tidal (54%). Longer period currents at depth evidenced the influence of volcano topography. Temperature time series showed a 3‐month period of sharply decreased temperature that extended in depth from 163 m to at least 1260 m. Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature data show colder surface water immediately above and north of Brothers over the same time period. These data together with the velocity records suggest an anomalous, deep, cold water mass arrived and lingered over Brothers for 3 months in the austral late spring and early summer of 2004–2005.

Soft and hard computing approaches for real-time prediction of currents in a tide-dominated coastal area

The prediction of tidal currents in the coastal region on a real-time or online basis is useful in taking operation- and planning-related decisions such as towing of vessels and monitoring of oil slick movements. Currently, however, this is done in offline mode on the basis of the statistical method of harmonic analysis involving fitting of harmonic functions to measured data. Alternatively, numerical solutions of hydrodynamic models can also provide spatial and temporal information on currents. Owing to the complex real sea conditions, such methods may not always yield satisfactory results. This paper discusses a few alternative approaches based on the soft computing tools of artificial neural networks (ANNs) and genetic programming (GP), as well as the hard mathematical approaches of stochastic and statistical methods. The suggested schemes use only a univariate time series of currents to forecast their future values. The measurements of coastal currents made at two locations in the Gulf of Khambhat along the west coast of India have been analysed. The current predictions over a time step of 20 min, a few hours, and a day at the specified locations were carried out. It was found that the soft computing schemes of GP and ANN performed better than the traditional hard technique of harmonic analysis in the present application. This work should initiate more application of GP in coastal engineering. Addressing the problem of current predictions in real-time mode based on analysis of observed time series of ocean currents is a specialty of this work.

Fourier transform filtering: A cautionary note

Many different filters exist to remove unwanted signals from time series of ocean currents and sea levels, and in this paper we examine two: a tidal harmonic “filter” and a discrete Fourier transform (DFT) filter. DFT filters are particularly easy to use if working in the frequency domain, but as with all filters it is necessary to understand the effects of a DFT filter before it can be used with complete confidence. A number of sample time series are used, some artificial and some real, to test DFT filters. It is shown that “ringing” in the retransformed time series can be minimized by careful choice of the filter bandwidth and the amount of tapering of the sides of the filter.

Some statistical properties of ocean currents

A study of 43 time series of ocean currents shows that the typically skewed distribution of speed becomes Gaussian when plotted against the logarithm of speed to the base ten. This is true in 34 of the 43 cases at the 0·05 level of significance or greater. The logarithmic standard deviation, σ L , ranged between about 0·15 and 0·32 in sufficiently long series. It is demonstrated that short series have smaller standard deviations, but different oceanic areas differ markedly in the quantitative aspects of this phenomenon. The log-normal distribution in time is shown to be equivalent to a normal distribution in a water-travel coordinate. The author's data, sampled every 10 min, is shown to be autocorrelated. Reliable statistics result when the speeds are sampled four or more times less frequently. Thus, currents measured for determination of the speed distribution alone require much less frequent sampling than is customary. The skirts of the probability distribution are often distorted, presumably partly by inherent current-meter errors. This distortion presently makes it impossible to determine to what degree extrapolation of the distribution is legitimate.