Interactions between tides and other frequencies in the Indonesian seas

Interactions of tidal constituents and the transfer of energy from the tidal frequencies to other frequencies are investigated using 3-D tidal simulations for the Indonesian seas, focusing on an area of active internal tides. Semidiurnal tides strongly affect diurnal tides; however, semidiurnal tides are essentially unaffected by diurnal tides. The semidiurnal and diurnal constituents interact with each other through non-linear interference, both destructive and constructive. Semidiurnal tides generate harmonics at nearly the diurnal frequency and higher vertical wavenumbers. In Ombai Strait, these harmonics are out of phase with the diurnal tides and interact destructively with the diurnal tides, effectively negating the diurnal response in some locations. However, this is not a general response, and interactions differ between locations. Energy is also transferred from both semidiurnal and diurnal tides to other frequencies across the spectrum, with more energy originating from semidiurnal tides. These energy transfers are not homogeneous, and the spectral responses differ between the Makassar and Ombai Straits, with the region east of Ombai showing a more active surface response compared to a more intense benthic response in Makassar. In deep water away from topography, velocity spectra generally follow the Garrett–Munk (GM) relation. However, in areas of internal tide generation, spectral density levels exceed GM levels, particularly between 4 and 8 cycles per day (cpd), indicating increased non-linear interactions and energy transfer through resonant interactions. The model indicates strong surface trapping of internal tides, with surface velocity spectra having significantly higher energy between 4 and 8 cpd even 100 km away from the prominent sill generating the internal tides.     

Mechanisms controlling the seasonal mixed-layer temperature and salinity of the Indonesian seas

We examine the seasonal mixed-layer temperature (MLT) and salinity (MLS) budgets in the Banda–Arafura Seas region (120–138° E, 8–3° S) using an ECCO ocean-state estimation product. MLT in these seas is relatively high during November–May (austral spring through fall) and relatively low during June–September (austral winter and the period associated with the Asian summer monsoon). Surface heat flux makes the largest contribution to the seasonal MLT tendency, with significant reinforcement by subsurface processes, especially turbulent vertical mixing. Temperature declines (the MLT tendency is negative) in May–August when seasonal insolation is smallest and local winds are strong due to the southeast monsoon, which causes surface heat loss and cooling by vertical processes. In particular, Ekman suction induced by local wind stress curl raises the thermocline in the Arafura Sea, bringing cooler subsurface water closer to the base of the mixed layer where it is subsequently incorporated into the mixed layer through turbulent vertical mixing; this has a cooling effect. The MLT budget also has a small, but non-negligible, semi-annual component since insolation increases and winds weaken during the spring and fall monsoon transitions near the equator. This causes warming via solar heating, reduced surface heat loss, and weakened turbulent mixing compared to austral winter and, to a lesser extent, compared to austral summer. Seasonal MLS is dominated by ocean processes rather than by local freshwater flux. The contributions by horizontal advection and subsurface processes have comparable magnitudes. The results suggest that ocean dynamics play a significant part in determining both seasonal MLT and MLS in the region, such that coupled model studies of the region should use a full ocean model rather than a slab ocean mixed-layer model.     

Regional energetics over the North Pacific,South China Sea and the Indonesian seas during winter

The energy equation was applied to four limited regions to investigate the basic mechanisms through which area-averaged eddy kinetic energy is maintained during the northern winter. The regions selected for this study are as follows: extratropical North Pacific (24.2°N–44.6°N, 130°E–150°W), tropical eastern North Pacific (0°–19.6°N, 170°W–110°W), South China Sea and. Bay of Bengal (0°–19.6°N, 80°E–140°E), and Timor Sea and eastern Indian Ocean (0°–19.6°S, 80°E–140°E). The zonally averaged upper flows over the first region were found to be barotropically stable. In contrast, they were barotropically unstable over the second region; namely, eddy motions over the tropical eastern North Pacific are maintained by receiving energy from zonal flows via barotropic interaction. The third and fourth regions are characterized by the importance of the conversion process between eddy available potential and eddy kinetic energy.Contribution No. 77-5, Department of Meteorology, University of Hawaii, USA.     

A numerical study of the barotropic tides and tidal energy distribution in the Indonesian seas with the assimilated finite volume coastal ocean model

The tides and tidal energetics in the Indonesian seas are simulated using a three-dimensional finite volume coastal ocean model. The high-resolution coastline-fitted model is configured to better resolve the hydrodynamic processes around the numerous barrier islands. A large model domain is adopted to minimize the uncertainty adjacent to open boundaries. The model results with elevation assimilation based on a simple nudge scheme faithfully reproduced the general features of the barotropic tides in the Indonesian Seas. The mean root-mean-square errors between the observed and simulated tidal constants are 2.3, 1.1, 2.4, and 1.5 cm for M₂, S₂, K₁, and O₁, respectively. Analysis of the model solutions indicates that the semidiurnal tides in the Indonesian Seas are primarily dominated by the Indian Ocean, whereas the diurnal tides in this region are mainly influenced by the Pacific Ocean, which is consistent with previous studies. Examinations of tidal energy transport reveal that the tidal energy for both of the simulated tidal constituents are transported from the Indian Ocean into the IS mainly through the Lombok Strait and the Timor Sea, whereas only M₂ energy enters the Banda Sea and continues northward. The tidal energy dissipates the most in the passages on both sides of Timor Island, with the maximum M₂ and K₁ tidal energy transport reaching about 750 and 650 kW m^–1, respectively. The total energy losses of the four dominant constituents in the IS are nearly 338 GW, with the M₂ constituent dissipating 240.8 GW. It is also shown that the bottom dissipation rate for the M₂ tide is about 1–2 order of magnitudes larger than that of the other three tidal components in the Indonesian seas.     

Seismogram Analysis of Indonesian Earthquakes at DAV Observation Station

The S-wave velocity across the earth structure under Indonesia for Indonesia earthquakes has been investigated through seismogram analysis, simultaneously in the time domain and three Cartesian components. The data were recorded at DAV observational station, the Philippines. The main data set is the seismogram comparison between the measured and synthetic seismogram, instead of travel time data, as commonly used in other seismological research. The synthetic seismogram is calculated using the GEMINI method, which is equivalent to Mode Summation. The above seismogram comparison shows that the global earth mantle of PREMAN gives a deviating synthetic seismogram and has earlier arrival times than those of the measurement. The gradient of β_h in the upper mantle layers is altered into a positive, rather than negative slope as stated in the PREMAN model, and negative corrections are imposed to the zero order of the polynomials coefficients in all earth mantle layers. The excellent fitting, as well as travel time or waveform, is obtained from the surface waves of Love and Rayleigh, surface wave to the S and SS mantle waves as well as the core reflected waves. This result expresses that part of the earth mantle, due to a collision between India and Asia tectonic released zones, has a negative anomaly in S-wave velocity and vertical anisotropy in all of the earth mantle layers.     

Odyssey of Aleutian eddies

Ocean Dynamics - Long-lived and large-scale anticyclonic Aleutian eddies (AEs), detaching from the Alaskan Stream to the west of 180∘ and propagating southwestward or almost zonally in the...     

Hydrodynamic modelling of mesoscale eddies in the Black Sea

A three dimensional structure of mesoscale circulation in the Black Sea is simulated using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System. A number of sensitivity tests reveal the response of the model to changes in the horizontal resolution, time steps, and diffusion coefficients. Three numerical grids are examined with x-fine (3.2 km), fine (6.7 km) and coarse (25 km) resolution. It is found that the coarse grid significantly overestimates the energy of the currents and is not adequate even for the study of basin-scale circulation. The x-fine grid, on the other hand, does not give significant advantages compared to the fine grid, and the latter is used for the bulk of simulations. The most adequate parameters are chosen from the sensitivity study and used to model both the basin-scale circulation and day-to-day variability of mesoscale currents for the months of May and June of 2000. The model is forced with actual wind data every 6 h and monthly climatic data for evaporation, precipitation, heat fluxes and river run-off. The results of the fine grid model are compared favourably against the satellite imagery. The model adequately reproduces the general circulation and many mesoscale features including cyclonic and anticyclonic eddies, jets and filaments in different parts of the Black Sea. The model gives a realistic geographical distribution and parameters of mesoscale currents, such as size, shape and evolution of the eddies.     

A vortex-tube model of eddies in the inertial range

Abstract

A vortex-tube geometry of the cascade of energy to small-scale eddies, in the inertial range of fully-developed turbulence, is proposed. The model is a special case of the beta model of Frisch, Sulem and Nelkin (1978). We require that the cascade conserve the principal invariants of inviscid, incompressible flow, namely volume, topological knottedness, circulation, and, at discrete times marking the termination of steps in the cascade, energy. The process terminates in a finite time, as in any beta model, leaving behind a self-similar network of “inactive” tubes. We associate a self-similar scaling dimension D with the structure, equal to the Hausdorff dimension of the set of “active” tubes at the termination of the cascade. Because circulation Λ plays a key role in the analysis of the cascade, we refer to these vortex-tube geometries as “gamma models”. The viewpoint throughout is entirely deterministic.

We describe two examples of gamma models. In the ring geometry, an eddy is a vortex ring, and the cascade produces “rings upon rings”, so we allow cutting and fusing of tubes while conserving total helicity. In the preferred helical model, no cutting is needed, and the cascade produces an infinite progression of braided “coils upon coils”. We suggest that latter geometry as a candidate for the topology of a singularity of the inviscid limit of a Navier-Stokes flow, when modeled by discrete vortex tubes.

A crucial ingredient of a gamma model, not explicitly present in a beta model, is the possibility of “splitting” a vortex tube into sub-tubes carrying smaller circulation. We suggest a dynamical basis for this process, as an instability of tubes whose cores violate the Rayleigh criterion.

The parameters describing a gamma model are not uniquely determined by our study, but there is a “simplest” helical gamma model, involving minimal splitting and distortion of tubes. The dimension D of the structure is 13/5, with a scale factor Λ = 2^?5/4. This value of D agrees with that suggested by Hentschel and Procaccia (1982), by analogy with established results for certain branched polymers.     

Convective and absolute instability of baroclinic eddies

Abstract

The long time response to an arbitrary unstable disturbance in a two layer model on an f-plane is sought. It has been found that depending on the ratio of the shear to the average speed of the mean flow two types of baroclinic instabilities exist: convective and absolute. When the system supports convective instabilities the long time response to an initial pulse excitation decreases with time at a fixed point in space. When such a system is excited by a wave maker the steady state frequency of response of the system corresponds to a spatially amplifying wave oscillating with the frequency of the wave maker. If the dispersion relation yields a saddle point of the frequency in the wave number complex plane with positive imaginary part of the frequency the system supports absolute instabilities. The response of the system at any point in space excited by an arbitrary signal grows exponentially with time at a rate determined by the properties of the system at the saddle point. This response is different from that of unstable normal modes. It is conjectured that absolute instability may be responsible for local cyclogenesis activity in certain geographical regions.     

Observation of baroclinic eddies southeast of Okinawa Island

In the region southeast of Okinawa, during May to July 2001, a cyclonic and an anticyclonic eddy were observed from combined measurements of hydrocasts, an upward-looking moored acoustic Doppler current profiler (MADCP), pressure-recording inverted echo sounders (PIESs), satellite altimetry, and a coastal tide gauge. The hydrographic data showed that the lowest/highest temperature (T) and salinity (S) anomalies from a 13-year mean for the same season were respectively -3.0/ 2.5℃ and -0.20/ 0.15 psu at 380/500 dbar for the cyclonic/anticyclonic eddies. From the PIES data, using a gravest empirical mode method, we estimated time-varying surface dynamic height (D) anomaly referred to 2000 dbar changing from -20 to 30 cm, and time-varying T and S anomalies at 500 dbar ranging through about ±2 ℃ and ±0.2 psu, respectively. The passage of the eddies caused variations of both satellite-measured sea surface height anomaly (SSHA) and tide-gauge-measured sea level anomaly to change from about –20 to 30 cm, consistent with the D anomaly from the PIESs. Bottom pressure sensors measured no variation related to these eddy activities, which indicated that the two eddies were dominated by baro-clinicity. Time series of SSHA map confirmed that the two eddies, originating from the North Pacific Subtropical Countercurrent region near 20°―30°N and 150°―160°E, traveled about 3000 km for about 18 months with mean westward propagation speed of about 6 cm/s, before arriving at the region southeast of Okinawa Island.     

Volcanic lineaments in the Indonesian island arcs

More than four hundred linear arrangements of nearly all active volcanic loci in the Indonesian island arcs have been subdivided into small (occurring on the same volcano), medium (occupying the same volcanic range), and large (interpreted connections between volcanic loci on separate cones or ranges). Two additional size-classes,i.e. small to medium and medium to large volcanic lineaments contain the transitory cases. Analyzing the orientations of the volcanic lineaments with respect to the regional structural trends and by using the most widely accepted angle of failure of 25° 30°, it was found that more than seventy percent of the lineaments can be classified as first and second order shear, tension, and extension directions. The tension direction occurs predominantly in the large size-class contrarily to the extension direction, which is rarely large. Instead, the latter direction is most frequent as small lineaments. There are no significant differences in the number of lineaments among the six directions of failure. Almost three quarters of the remaining unclassifiable volcanic lineaments belong to the small and small to medium size-classes, which very probably rellect the influence of local structural conditions. These data indicate conclusively that most volcanic lineaments occur along narrow zones of weakness which are genetically related to the regional structure.     

Decadal variability of the Kuroshio Extension: mesoscale eddies and recirculations

An eddy-resolving multidecadal ocean model hindcast simulation is analyzed to investigate time-varying signals of the two recirculation gyres present respectively to the north and south of the Kuroshio Extension (KE) jet. The northern recirculation gyre (NRG), which has been detected at middepth recently by profiling float and moored current meter observations, is a major focus of the present study. Low-frequency variations in the intensity of the recirculation gyres are overall highly correlated with decadal variations of the KE jet induced by the basin-wide wind change. Modulation of the simulated mesoscale eddies and its relationship with the time-varying recirculation gyres are also evaluated. The simulated eddy kinetic energy in the upstream KE region is inversely correlated with the intensity of the NRG, consistent with previous observational studies. Eddy influence on the low-frequency modulation of the NRG intensity at middepth is further examined by a composite analysis of turbulent Sverdrup balance, assuming a potential vorticity balance between the mean advection and the convergent eddy fluxes during the different states of the recirculation gyre. The change in the NRG intensity is adequately explained by that inferred by the turbulent Sverdrup balance, suggesting that the eddy feedback triggers the low-frequency modulation of the NRG intensity at middepth.     

High-resolution ensemble forecasting for the Gulf of Mexico eddies and fronts

High-resolution models and realistic boundary conditions are necessary to reproduce the mesoscale dynamics of the Gulf of Mexico (GOM). In order to achieve this, we use a nested configuration of the Hybrid Coordinate Ocean Model (HYCOM), where the Atlantic TOPAZ system provides lateral boundary conditions to a high-resolution (5 km) model of the GOM . However, such models cannot provide accurate forecasts of mesoscale variability, such as eddy shedding event, without data assimilation. Eddy shedding events involve the rapid growth of nonlinear instabilities that are difficult to forecast. The known sources of error are the initial state, the atmospheric condition, and the lateral boundary condition. We present here the benefit of using a small ensemble forecast (10 members) for providing confidence indices for the prediction, while using a data assimilation scheme based on optimal interpolation. Our set of initial states is provided by using different values of a data assimilation parameter, while the atmospheric and lateral boundary conditions are perturbed randomly. Changes in the data assimilation parameter appear to control the main position of the large features of the GOM in the initial state, whereas changes in the boundary conditions (lateral and atmospheric) appears to control the propagation of cyclonic eddies at their boundary. The ensemble forecast is tested for the shedding of Eddy Yankee (2006). The Loop Current and eddy fronts observed from ocean color and altimetry are almost always within the estimated positions from the ensemble forecast. The ensemble spread is correlated both in space and time to the forecast error, which implies that confidence indices can be provided in addition to the forecast. Finally, the ensemble forecast permits the optimization of a data assimilation parameter for best performance at a given forecast horizon.     

Eddy energy sources and mesoscale eddies in the Sea of Okhotsk

Based on eddy-permitting ocean circulation model outputs, the mesoscale variability is studied in the Sea of Okhotsk. We confirmed that the simulated circulation reproduces the main features of the general circulation in the Sea of Okhotsk. In particular, it reproduced a complex structure of the East-Sakhalin current and the pronounced seasonal variability of this current. We established that the maximum of mean kinetic energy was associated with the East-Sakhalin Current. In order to uncover causes and mechanisms of the mesoscale variability, we studied the budget of eddy kinetic energy (EKE) in the Sea of Okhotsk. Spatial distribution of the EKE showed that intensive mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. We revealed a pronounced seasonal variability of EKE with its maximum intensity in winter and its minimum intensity in summer. Analysis of EKE sources and rates of energy conversion revealed a leading role of time-varying (turbulent) wind stress in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk in winter and spring. We established that a contribution of baroclinic instability predominates over that of barotropic instability in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the simulated circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. In April, the mesoscale anticyclonic eddies are observed along the western boundary of the Sea of Okhotsk. The role of the sea ice cover in the intensification of the mesoscale variability in the Sea of Okhotsk was discussed.