Measurement and Analysis of Ocean Current using High- Frequency (HF) Radar Observation in the Bali Strait

High-Frequency (HF) Radar is an instrument using radio waves to measure ocean currents and waves remotely. This technology has many advantages, including has unprecedented spatial and temporal resolution, can operate in any weather condition, and is not dangerous for the environment. However, HF Radar's research is still limited in Indonesia. This research aimed to analyze the tidal and residual current in the Bali Strait in July 2020. Radial velocity from two HF Radar sites is combined to obtain the total currents. Current data from HF Radar were compared with Acoustic Doppler Current Profiler (ADCP) data to investigate its accuracy. Surface current data were analyzed using harmonic analysis to separate tidal and residual currents. Comparison between HF Radar and ADCP data are in good agreement for meridional current with a very high correlation of 0.813 and a small RMSE value of 0.22 m/s. Harmonic analysis shows that the dominant currents are tidal currents. The current direction was northward (southward) at flood (ebb), with maximum northward (southward) velocities are 2.17 m/s (2.97 m/s), respectively. The residual current has a random pattern, slightly faster northward than southward, and has similar spectral with the wind.

Deriving Six Components of Reynolds Stress Tensor from Single-ADCP Data

Acoustic Doppler current profilers (ADCP) are widely used in geophysical studies for mean velocity profiling and calculation of energy dissipation rate. On the other hand, the estimation of turbulent stresses from ADCP data still remains challenging. With the four-beam version of the device, only two shear stresses are derivable; and even for the five-beam version (Janus+), the calculation of the full Reynolds stress tensor is problematic currently. The known attempts to overcome the problem are based on the “coupled ADCP” experimental setup and include some hard restrictions, not to mention the essential complexity of performing experiments. In this paper, a new method is presented which allows to derive the stresses from single-ADCP data. Its essence is that interbeam correlations are taken into account as producing the missing equations for stresses. This method is applicable only for the depth range, for which the distance between the beams is comparable to the scales, where the turbulence is locally isotropic and homogeneous. The validation of this method was carried out for convectively-mixed layer in a boreal ice-covered lake. The results of computations turned out to be physically sustainable in the sense that realizability conditions were basically fulfilled. The additional verification was carried out by comparing the results, obtained by the new method and “coupled ADCPs” one.

Character, spatial and temporal variation of turbidity currents on a source-to-sink scale.

<p>Seafloor avalanches of sediment called turbidity currents are one of the principle mechanisms for moving sediments across our planet. However, turbidity currents are notoriously difficult to monitor directly in action, and we still mainly depend on their sedimentary deposits as well as physical and numerical models to understand their temporal and spatial evolution. In recent years, multiple studies have successfully made direct measurements within active turbidity currents at multiple sites along their pathway. However, these direct measurements are often limited to the upper reaches of submarine systems, only cover relatively short (few months to a couple of years) time scales, or have very few measurement stations (<3). To capture the full range of turbidity current types and recurrence times we need to combine direct monitoring with longer-term archives in sedimentary deposits. Here we present an unusual data set that extends from the submarine channel on the delta, to the final deposits in the deep basin. The dataset combines short-term (< 1 year) direct measurements of flows with long-term sediment deposits (dating back to about 100 years). This combination of data types allows us to understand turbidity current frequencies, runouts, heights and characteristics along an entire submarine system.  </p><p>We analyse data from Bute Inlet, which is a fjord in British Colombia, Canada. The entire turbidity current system stretches out for 80 km, with an incised submarine channel extending for 45 km. 46 Cores have been collected between 2015 and 2018. Simultaneously, direct measurements of the currents have been obtained in 2016 and 2018 using Acoustic Doppler Current Profilers (ADCPs) in the submarine channel.</p><p>Our objective is two-fold. First, we look at flow frequency over time and space. Visual logs of the sediment cores, as well as sediment accumulation rates for a selection of cores, are used to infer flow frequencies. We then use the ADCP data to understand more frequent and recent flows at 6 places along the channel. These ADCP measurements are used to infer frequencies which are not necessarily recorded in the deposits, and give additional insights into current-day activity. This allows us to reconstruct the change in frequency over space and time.</p><p>Second, we consider the variation in turbidity current character to understand how flows evolve along the channel. Facies determination and grain size data are used to infer turbidity current character. Cores along the channel, on terraces and in the deep basin are used to understand the spatial variation. Finally, comparison of deposits and monitoring (ADCP) data shows how submarine flows are recorded by their deposits.</p>

Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean

<p>The Bellingshausen Sea, located between the West Antarctic Peninsula and the Amundsen Sea, is poorly observed, compared with its neighbours. The Antarctic Slope Front (ASF), that rings the continental slope of Antarctica, supports a westward current (the Antarctic Slope Current). The structure and variability of this current affect exchange processes close to Antarctica such as the transport of warm Circumpolar Deep Water onto the Antarctic continental shelf. This water mass is responsible for the transport of heat across the shelf and therefore the basal melting of ice shelves. Due to the lack of observations, it is still unclear if the ASF even exists in the Bellingshausen Sea or if there are other processes moderating the transport of warm water onto the shelf.</p><p>We present ship-based and glider-based CTD data collected in 2007 and 2019, which in total provide 7 cross-slope sections in the Bellingshausen Sea. Geostrophic velocities are referenced to lowered ADCP data, shipboard ADCP data and the Dive Average Current. Cumulative transports show remarkable differences between the years 2007 and 2019. The sections of 2007 provide cumulative transports of up to 3.5 Sv eastward. In contrast, the sections in 2019 have cumulative transports up to 2 Sv westward. The sections from 2007 and 2019 are in very similar locations, indicating a temporal change rather than a spatial change.</p><p>We compare the cross-slope sections from the observations with sections from the NEMO 1/12 ° model output. A time series of cumulative transports from the model, covering the years from 2000 to 2010, allows us to identify seasonality and interannual variability in this current system.</p>