OGCM For The Earth Simulator Research Articles

Seasonal variability of eddy kinetic energy in the East Australian current region

The East Australian Current (EAC) is an important western boundary current of the South Pacific subtropical Circulation with high mesoscale eddy kinetic energy (EKE). Based on satellite altimeter observations and outputs from the eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), the seasonal variability of EKE and its associated dynamic mechanism in the EAC region are studied. High EKE is mainly concentrated in the shear-region between the poleward EAC southern extension and the equatorward EAC recirculation along Australia's east coast, which is confined within the upper ocean (0-300 m). EKE in this area exhibits obvious seasonal variation, strong in austral summer with maximum (465±89 cm² s-²) in February and weak in winter with minimum (334±48 cm² s-²) in August. Energetics analysis from OFES suggests that the seasonal variability of EKE is modulated by the mixed instabilities composed of barotropic and baroclinic instabilities confined within the upper ocean, and barotropic instability (baroclinic instability) is the main energy source of EKE in austral summer (winter). The barotropic process is mainly controlled by the zonal shear of meridional velocities of the EAC southern extension and the EAC recirculation. The poleward EAC southern extension and the equatorward EAC recirculation are synchronously strengthened (weakened) due to the local positive (negative) sea level anomalies (SLA) under geostrophic equilibrium, and the barotropic instability dominated by zonal shear is enhanced (slackened), which results in a high (low) level of EKE in the EAC region.

Long-term mean circulation in the Japan Sea as reproduced by multiple eddy-resolving ocean circulation models

The capacity of four eddy-resolving ocean circulation models—HYCOM (HYbrid Coordinate Ocean Model), MRI.COM (Meteorological Research Institute Community Ocean Model), OFES (Ocean General Circulation Model for the Earth Simulator), and NEMO (Nucleus for European Modeling of the Ocean)—to simulate the long-term mean hydrographic conditions and circulation patterns in the Japan Sea is investigated in this study. The assessment of this study includes the evaluation of mean vertical profiles and time series of temperature and salinity at the representative monitoring stations. Different model products from 1993 to 2015 are compared with in situ measurements provided by historical cruises and monitoring stations. After that, we compared the observed and simulated surface current velocities over the basin and volume transports through the key straits in the Japan Sea. Simulated current velocities are validated against 15 years of Acoustic Doppler Current Profiler (ADCP) measurements near the longshore and offshore branches of the East Korea Warm Current (EKWC). Furthermore, the atmospheric forcing data of the four ocean circulation models are validated against the satellite wind product. We found that the vertical profiles and long-term variations of temperature and salinity reproduced by MRI.COM and HYCOM are closer to in situ measurements. All models simulate temperature well in upper ocean, but salinity simulations are of lower quality from OFES and NEMO at several stations. Simulated current velocities predominantly lie within the standard deviation of ADCP measurements at two locations. However, the sea surface currents are underestimated by four models compared with Drifter data. Although simulated hydrographic profiles agree well with in situ observations, the mean circulation patterns greatly differ between the models, which highlight the need for additional evaluation and corrections based on the long-term current measurements. Because of the lack of ocean current measurements, only the baroclinic velocities simulated by each model are reliable. The substantial part of the differences in barotropic velocities among the simulate result of four models is explained by the differing wind velocities from the corresponding atmospheric forcing datasets.

Role of wind forcing and eddy activity in the intraseasonal variability of the barrier layer in the South China Sea

In addition to widely discussed seasonal variability, the barrier layer (BL) of the South China Sea (SCS) also exhibits significant intraseasonal variability (ISV) and plays an important role in the upper heat and salt balances. The characteristics and mechanisms of spatiotemporal variations in the BL are investigated using an eddy-resolving ocean model OFES (OGCM For the Earth Simulator) ouput and related atmospheric and oceanic processes. The active intraseasonal BL variability in the SCS occurs mainly during the late summer/autumn and winter and exhibits remarkable differences between these two periods. The BL ISV in late summer/autumn occurs in the southern basin, while in winter, it is limited to the northwestern basin. To further discuss the evolution and driving thermodynamic mechanisms, we quantify the processes that control the variability of intraseasonal BL. Different mechanisms for the intraseasonal BL variability for these two active periods are investigated based on the case study and composite analysis. During late summer/autumn, the active BL in the southern basin is generated by advected and local freshwater, and then decays rapidly with the enhanced wind. In winter, anticyclonic eddy activity is associated with the evolution of the BL by affecting the thermocline and halocline variations, while wind stress and wind stress curl have no obvious influence on BL.

Energetics of the Brazil Current in the Rio Grande Cone region

The energetics of the Brazil Current (BC) in the region of the Rio Grande Cone (RGC, 30–35.5°S), a topographic rise in the southwest portion of the Brazilian continental margin, are analyzed using 16 years of numerical data from the Ocean General Circulation Model (OGCM) for the Earth Simulator (OFES). The main focus of this study is the eddy-mean flow interactions of the BC and the local energy budgets in the study region. The kinetic and potential energy balance equations are derived for mean and eddy flows, and the resulting terms are presented and discussed. The eddy-mean flow interactions exhibit complex spatial distributions, and the intensities of the energy budgets decrease with increasing depth. However, only the mean potential energy (MPE) budget decreases southward. Eddy kinetic energy (EKE) and eddy potential energy (EPE) exhibit similar horizontal distribution patterns. Additionally, the baroclinic and barotropic conversion rates increase downstream of the bump, where the eddy energy field exhibits along-stream variability that increases southward. Barotropic conversion is more intense between 50 and 200m, where mean kinetic energy (MKE) and EKE are concentrated, and it exhibits a horizontal cross-stream variation pattern, with mean-to-eddy energy conversion observed on the offshore side of the BC. This result indicates that the turbulence associated with the stream jet increases as the BC moves away from the coast, with the conversion term acting to stabilize the flow. Baroclinic conversion exhibits a high intensity below 300m (where MPE and EPE display peaks), and it has a greater influence on the eddy-mean flow interaction than does the barotropic conversion. The RGC directly affects the local dynamics of the BC by increasing the eddy field as soon as the BC reaches the bump. The energy diagrams illustrate a stream characterized by evolving barotropic and baroclinic instability processes throughout the water column. This result indicates an intrinsically unstable jet in the study region. Moreover, baroclinic instability is the main source of EKE in the RGC region.

A mechanism for the latitudinal dependence of peak‐spectrum sea surface height variability

AbstractPrevious studies have shown that the power spectrum of satellite‐observed sea surface height (SSH) variability peaks at a certain frequency (or a wave number) band at a given latitude. Lin et al. (2008) attributed this latitudinal dependence to the critical frequency of the first baroclinic mode Rossby waves in the tropical and subtropical oceans. Their study was based on the linear Rossby wave theory and focused on SSH variability in the tropical and subtropical oceans since the altimetry data do not adequately resolve lengths of baroclinic Rossby waves at and near the critical frequency in high latitudes. In this study, we expand their analysis to high‐latitude oceanic basins and to include nonlinear eddy effects, by using a linear wave model and a high‐resolution model output from the OGCM for the Earth Simulator (OFES). It is found that the linear wave mechanism by and large remains valid in the tropical and subtropical oceans. In higher latitudes as well as in some regions in the western tropical and subtropical oceans, other mechanisms, like nonlinear eddy, play more important role in determining the SSH variability.

Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model

Abstract The advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30–40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10–25 yr it takes the 3D numerical supertrajectories to complete the journey.

Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model

AbstractThe advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas th...

Sharing the results of a high-resolution ocean general circulation model under a multi-discipline framework—a review of OFES activities

Quasi-global, eddy-resolving ocean general circulation model experiments, so-called OFES (ocean general circulation model for the Earth Simulator) project, have been conducted with a basic concept of sharing results among scientists in a multi-discipline framework. Studies using such OFES results extend widely from the physical aspects of ocean circulations and energy considerations to biogeochemical tracer distributions and marine ecosystem dynamics. This article reviews the OFES activities during a period from 2004 to 2009 and highlights some interesting scientific results that emerged from the OFES project. Future plans of the project are also presented.

Characteristics of coastal trapped waves along the southern and eastern coasts of Australia

The spatial structures and propagation characteristics of coastal trapped waves (CTWs) along the southern and eastern coasts of Australia are investigated using observed daily mean sea level data and results from a high-resolution ocean general circulation model (OGCM), and by conducting sensitivity studies with idealized numerical models. The results obtained from the sea level observations show that shortterm variations, with a typical period of 1 to 2 weeks, dominate the sea level variability in the southern half of Australia. The signal propagates anticlockwise around Australia with a propagation speed of 4.5 m/s or faster in the western and southern coasts and 2.1 to 3.6 m/s in the eastern coast. Strong seasonality of the wave activity, with large amplitude during austral winter, is also observed. It turns out that the waves are mainly generated by synoptic weather disturbances in the southwestern and southeastern regions. The numerical experiment with idealized wind forcing and realistic topography confirms that the propagating signals have characteristics of the CTW both in the southern and eastern coasts. Sensitivity experiments demonstrate that the difference in the phase speed between the coasts and reduction of the amplitude of the waves in the eastern coast are attributed to the different shape of the continental shelf in each region. The structures and the propagation characteristics of the CTWs around Australia are well reproduced in OFES (OGCM for the Earth Simulator) with dominant contribution from the first mode, although meso-scale eddies may modify the structure of the CTWs in the eastern coast. It is also found that generation or reinforcement of the waves by the wind forcing in the southern part of the eastern coast is necessary to obtain realistically large amplitude of the CTWs in the eastern coast.

The Kuroshio Extension Northern Recirculation Gyre: Profiling Float Measurements and Forcing Mechanism

Abstract Middepth, time-mean circulation in the western North Pacific Ocean (28°–45°N, 140°–165°E) is investigated using drift information from the profiling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (∼145°E) and the Shatsky Rise (∼156°E), and bordered to the north by the subarctic boundary along ∼40°N. This northern RG, which is simulated favorably in the eddy-resolving OGCM for the Earth Simulator (OFES) hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, the authors find the eddy dissipation effect to be important as well.

Decadal Sea Level Variability in the South Pacific in a Global Eddy-Resolving Ocean Model Hindcast

Abstract Sea level variability and related oceanic changes in the South Pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), along with sea level data from tide gauges since 1970 and a satellite altimeter since 1992. The first empirical orthogonal function mode of sea level anomalies (SLAs) of OFES exhibits broad positive SLAs over the central and western South Pacific. The corresponding principal component indicates roughly stable high, low, and high SLAs, separated by a rapid sea level fall in the late 1970s and sea level rise in the late 1990s, consistent with tide gauge and satellite observations. These decadal changes are accompanied by circulation changes of the subtropical gyre at 1000-m depth, and changes of upper-ocean zonal current and eddy activity around the Tasman Front. In general agreement with previous related studies, it is found that sea level variations in the Tasman Sea can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies, if the impact of New Zealand is taken into account. The corresponding atmospheric variations are associated with decadal variability of El Niño–Southern Oscillation (ENSO). Thus, decadal sea level variability in the western and central South Pacific in the past three and half decades and decadal ENSO variability are likely to be connected. The sea level rise in the 1990s, which attracted much attention in relation to the global warming, is likely associated with the decadal cooling in the tropical Pacific.

Stationary mesoscale jet‐like features in the ocean

XBT and float data of the World Ocean Database 2005 are used to validate new stationary jet‐like features (striations) recently found in the high‐resolution mean dynamic ocean topography (MDOT). Two study areas were selected in the eastern parts of the North and South Pacific subtropical gyres free of any strong currents. Appropriate horizontal high‐pass filtering applied to the ensemble‐mean temperature at 100 m depth revealed the striation signal coherent with MDOT. The features are also distinct in the data of depth of 12°C isotherm lying close to the thermocline. The striations are oriented nearly zonally and coherent vertically at least through 700 m depth. Their wavelength is approximately 400 km, and the azimuth of wavevector is −13° (9°) in the northern (southern) domains. Correspondence between the striations in observations and OFES (OGCM for the Earth Simulator) model as well as the nature of the striations are discussed.