Yu-Heng Tseng

Effects of coastal geometry and the formation of cyclonic/anti-cyclonic eddies on turbulent mixing in upwelling simulation

This paper presents a combination of theory and simulation on coastal upwelling with the aim of understanding the origin and nature of the structures found. Cyclones/anti-cyclones and thin filaments observed in satellite infrared images in upwelling regions are rather well reproduced. The instabilities are confirmed to be of mixed baroclinic–barotropic and modified Rayleigh–Taylor types. Nonlinear interactions limit the growth of the large scale structures and generate ‘fish-hook’ structures. The Rayleigh–Taylor and mixed instabilities and fish-hook structures cause sharp increases in mixing. Mixing and stirring are quantified using a mixedness parameter and energy budgets. Coastal perturbations modify the coherent structures which travel in the windward direction, changing their structure. The mechanisms of generation of these structures has been studied with simplified models but is not completely understood. We present animations derived from the simulations to investigate the process of formation of c...

Mixing and available potential energy in stratified flows

Mixing plays an important role in atmospheric and oceanic flows. It occurs on the small scales, is due to molecular diffusion, and is irreversible. On the other hand, stirring is a kinematic process that enhances mixing but is reversible. Budgets of the available potential energy, which require that the reference potential energy be computed, are used to study these processes. We develop an approach for calculating the available potential energy from the probability density function that is more efficient than existing methods, especially in two and three dimensions. It is suitable for application to both numerical simulations and experiments. A new length scale is defined which quantifies stirring and provides a measure of the strength of overturns resulting from stirring as well as their size. Simulations of lid-driven cavity flow and stratified homogeneous turbulent shear flow provide illustrations of the method. The new length scale is similar to Thorpe scale in lid-driven cavity flow and closely rela...

Evaluation of Multi-Scale Climate Effects on Annual Recruitment Levels of the Japanese Eel, Anguilla japonica, to Taiwan

Long-term (1967–2008) glass eel catches were used to investigate climatic effects on the annual recruitment of Japanese eel to Taiwan. Specifically, three prevailing hypotheses that potentially explain the annual recruitment were evaluated. Hypothesis 1: high precipitation shifts the salinity front northward, resulting in favorable spawning locations. Hypothesis 2: a southward shift of the position of the North Equatorial Current (NEC) bifurcation provides a favorable larval transport route. Hypothesis 3: ocean conditions (eddy activities and productivity) along the larval migration route influence larval survival. Results of time series regression and wavelet analyses suggest that Hypothesis 1 is not supported, as the glass eel catches exhibited a negative relationship with precipitation. Hypothesis 2 is plausible. However, the catches are correlated with the NEC bifurcation with a one-year lag. Considering the time needed for larval transport (only four to six months), the one-year lag correlation does not support the direct transport hypothesis. Hypothesis 3 is supported indirectly by the results. Significant correlations were found between catches and climate indices that affect ocean productivity and eddy activities, such as the Quasi Biennial Oscillation (QBO), North Pacific Gyre Oscillation (NPGO), Pacific Decadal Oscillation (PDO), and Western Pacific Oscillation (WPO). Wavelet analysis reveals three periodicities of eel catches: 2.7, 5.4, and 10.3 years. The interannual coherence with QBO and the Niño 3.4 region suggests that the shorter-term climate variability is modulated zonally by equatorial dynamics. The low-frequency coherence with WPO, PDO, and NPGO demonstrates the decadal modulation of meridional teleconnection via ocean–atmosphere interactions. Furthermore, WPO and QBO are linked to solar activities. These results imply that the Japanese eel recruitment may be influenced by multi-timescale climate variability. Our findings call for investigation of extra-tropical ocean dynamics that affect survival of eels during transport, in addition to the existing efforts to study the equatorial system.

The Victoria mode in the North Pacific linking extratropical sea level pressure variations to ENSO

The Victoria mode (VM) represents the second dominant mode (empirical orthogonal function, EOF2) of North Pacific variability, independent of the Pacific Decadal Oscillation and is defined as the EOF2 of SST anomalies in the North Pacific poleward of 20°N. The present study indicates that the VM is closely linked to the development of El Nino–Southern Oscillation (ENSO). The VM may effectively act as an ocean bridge (or conduit) through which the extratropical atmospheric variability in the North Pacific influences ENSO. The VM can trigger the onset of ENSO via the following two dominant processes: (1) surface air-sea coupling associated with the VM in the subtropical/tropical Pacific and (2) evolution of subsurface ocean temperature anomalies along the equator associated with the VM. These two processes may force sufficient surface warming to occur in the central eastern equatorial Pacific from spring to summer, which in turn initiates an ENSO event. The VM influence on ENSO relies on a basin-scale air-sea interaction dynamic, as opposed to more local-scale dynamics typically associated with the seasonal footprinting mechanism or Pacific meridional mode. The majority of VM events are followed by ENSO events. These ENSO events triggered by VM include El Nino Modoki (EM) as well as conventional El Nino. There is no evidence that the VM tends to be more conducive to the initialization of EM than conventional El Nino.

Large-eddy simulation of turbulent wavy boundary flow—illustration of vortex dynamics ∗

Large-eddy simulation (LES) is used to investigate three-dimensional turbulent flow over a wavy boundary. An efficient immersed boundary method (IBM) for simulating turbulent flows in complex geometries is presented. The method is based on a finite-volume approach on a non-staggered Cartesian mesh and a fractional-step method. A force is applied on the body surface through an immersed ghost-cell method. Both steady and unsteady flows are simulated. A steady flow simulation provides not only the mean and turbulence statistics but also visualization of the Gortler vortices. The simulation shows that Gortler vortices are formed by a sequential vortex reconnection process and broken down due to vortex separation. The unsteady oscillatory flow displays the three-dimensional vortex formation/transport cycle which is shown to be important for sediment transport. The turbulent structure is fully three-dimensional and is clearly seen from the current animations. This paper was chosen from Selected Proceedings of t...

Influence of the North Pacific Victoria mode on the Pacific ITCZ summer precipitation

This study demonstrates the close connection between the second dominant mode of spring sea surface temperature anomalies (SSTAs) in the North Pacific poleward of 20°N, referred to as the Victoria mode (VM), and the Pacific Intertropical Convergence Zone (ITCZ) precipitation during the following summer. Our analysis shows that strong positive VM cases are followed by positive precipitation anomalies over the central-eastern Pacific ITCZ region, in association with negative precipitation anomalies over the ITCZ regions of the tropical western Pacific and eastern North Pacific. The hypothesized physical mechanism through which the spring VM induces the Pacific ITCZ summer precipitation is similar to but slightly different from the seasonal footprinting mechanism. During strong positive VM cases, SSTAs in the subtropics associated with the spring VM persist until summer and develop toward the equator, where low-level convergence and divergence caused by SSTA gradients give rise to enhanced precipitation over the central-eastern Pacific ITCZ region and to reduced precipitation over the ITCZ regions of the tropical western Pacific and eastern North Pacific. The thermodynamic ocean-atmosphere coupling between the ITCZ and SSTAs associated with the VM may play a vital role in the initiation of El Nino–Southern Oscillation (ENSO) events. The VM influence on tropical Pacific summer precipitation can be passed on to the next year through its influence on ENSO. A VM-based linear model is established to predict the tropical Pacific summer precipitation, which yields skillful forecasts for summer precipitation across almost the entire tropical Pacific.

Entrainment and Transport in Idealized Three-Dimensional Gravity Current Simulation

A purely z-coordinate Dietrich/Center for Air Sea Technology (DieCAST) ocean model is applied to the Dynamics of Overflow Mixing and Entrainment (DOME) idealized bottom density current problem that is patterned after the Denmark Strait. The numerical results show that the background viscosity plays a more important role than the chosen coordinate system in the entrainment and mixing if the background viscosity is not small enough. Both higher horizontal viscosity and coarser resolution leads to slower along-slope propagation. Reducing vertical mixing parameterization also leads to slower along-slope propagation with thicker plume size vertically. The simulation gives consistent results for the moderate- and fine-resolution runs. At a very coarse grid the dense water descends more slowly and is mainly dominated by diffusion. Time-averaged downstream transport and entrainment are not very sensitive to viscosity after the flow reaches its quasi-steady status. However, more realistic eddies and flow structures are found in low-viscosity runs. The results show good convergence of the resolved flow as expected and clarify the effects of numerical dissipation/mixing on overflow modeling. Larger numerical dissipation is not required nor recommended in z-coordinate models.

Variation in the Kuroshio intrusion: Modeling and interpretation of observations collected around the Luzon Strait from July 2009 to March 2011

This study analyzes the observed subtidal currents, 1/12° global HYCOM model results, and the observed time series to interpret seasonal and interannual patterns in the behavior of the Kuroshio intrusion around the Luzon Strait (LS). The observations include current measurements conducted at mooring station N2 (20°40.441′N, 120°38.324′E) from 7 July 2009 to 31 March 2011, surface geostrophic currents derived from the merged absolute dynamic topography, and the trajectory of an Argo float during the winter of 2010–2011. Results from mooring station N2 confirmed the seasonal changes in the Kuroshio intrusion and the variation of the Kuroshio intrusion during El Nino event from July 2009 to April 2010 and La Nina even from June 2010 to March 2011. The strongest Kuroshio intrusion occurs in the winter, with successively weaker currents in spring, autumn, and summer. Comparison of relative differences (Δmax (z)) in the maximum absolute value of monthly average zonal velocity components |Umax (z)| showed that the Kuroshio intrusion was stronger during the 2009–2010 winter (El Nino) than the 2010–2011 winter (La Nina). Furthermore, the relative differences (Δmax (z)) in deeper layers exceed those of the surface layer. Circulation patterns in surface geostrophic currents and the Argo float trajectory confirmed the results of mooring station N2. The Kuroshio intrusion velocity variation modeled using the 1/12° global HYCOM model resembled the observation on both seasonal to interannual scales. Modeled variation in the zonal mean velocity anomaly was also consistent with Nino3, Nino4, and North Equatorial Current (NEC) bifurcation latitude indices, indicating concurrent impacts of the ENSO influence. Monsoon winds strongly affect the seasonal variation while the weak upstream Kuroshio transport induced by El Nino, strongly affects the interannual variation, such as 2009–2010 winter. In 2010–2011 winter, the impact of winter monsoon forcing still exists in the LS. However, the stronger upstream Kuroshio transport during this period did not allow the Kuroshio to penetrate into the LS deeply. This explains why the 2009–2010 winter Kuroshio intrusion (El Nino event) was stronger than that of the 2010–2011 winter (La Nina event).

Nonhydrostatic simulations of the regional circulation in the Monterey Bay area

[1] The regional circulation in the vicinity of Monterey Bay is complex and highly variable. We use a one-way coupled, nonhydrostatic version of the Dietrich Center for Air Sea Technology (DieCAST) ocean model to simulate the regional circulation. Seasonally varying local wind stress, topographic irregularities, coastal upwelling, and forcing from the open ocean are all important in this region. Satellite imagery often shows a cyclonic eddy inside the bay and an anticyclonic eddy outside the bay. The offshore anticyclonic eddy is also associated with a year-round anticyclonic eddy over the Monterey Submarine Canyon (MSC). The offshore eddy is better organized during winter. It is found that the California Undercurrent (200-400 m) does not enter the bay itself but is diverted offshore past the entrance of the bay, presumably to reform farther north along the coast. The main branch flows northward contributing to the deep anticyclonic eddy located approximately 50 km offshore of Monterey Bay. The simulations show that vertical motion is greater during summer than winter, as expected. During spring upwelling, the deep waters often upwell along the walls of the canyon and then spread and mix with surrounding waters. The deep circulation enhances mixing significantly due to the topography. We further investigate the regional circulation by comparing it with the cases where the deep canyon was filled gradually. Vortex stretching over the canyon just beyond the entrance to Monterey Bay and along the adjacent continental slopes contributes to cyclonic circulation at deeper levels. Vertical sections of velocity along the axis of MSC indicate horizontal and vertical patterns of flow that are generally consistent with past observations on the circulation of Monterey Bay.

Short communication: Software development of the TaIwan Multi-scale Community Ocean Model (TIMCOM)

The recently developed TaIwan Multi-scale Community Ocean Model (referred as TIMCOM), evolving from the DieCAST (Dietrich Center for Air Sea Technology) model, provides an accurate, efficient, and user-friendly framework to study a broad spectrum of oceanic flows, ranging from bays and coastal to global oceans. The model employs the finite volume concept and discretizes the primitive equations using the modified leapfrog scheme and fourth-order spatial approximation. The pressure Poisson equation is efficiently solved by the error vector propagation (EVP) method. Adaptive grid-coupling technique is further adopted to provide the required resolution for the targeted region without excessive computation. Besides, a user-friendly interface is introduced to simplify user customization. Two practical applications, global and dual-grid North Pacific Ocean modeling frameworks which are used to simulate the global and regional ocean-climate variability, clearly show the robustness, efficiency, and accuracy of the TIMCOM software.