scholarly journals Influence of Eddy Momentum Fluxes on the Mean Flow of the Kuroshio Extension in a 1/10° Ocean General Circulation Model

2016 ◽  
Vol 46 (9) ◽  
pp. 2769-2784 ◽  
Author(s):  
Kunihiro Aoki ◽  
Atsushi Kubokawa ◽  
Ryo Furue ◽  
Hideharu Sasaki
Keyword(s):  
Reynolds Stress ◽  
Coriolis Force ◽  
General Circulation ◽  
Kuroshio Extension ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Upstream Region ◽  
Downstream Region ◽  
Momentum Budget ◽  
The Kuroshio

AbstractThis study explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, the zonal momentum budget in a high-resolution ocean general circulation model is examined on the basis of the temporal residual mean framework. The momentum budget analysis is performed for two control volumes: the upstream region of the KE jet flanked by the robust recirculations (33°–38°N and 142.2°–149.4°E) and the downstream region to the east (33°–38°N and 149.4°–160.0°E), both fully covering the meridional width of the KE jet. In both regions the KE jet decelerates to the east, which can be well accounted for by sum of zonal Reynolds stress and Coriolis force on mean ageostrophic flow; the former tends to decelerate the KE jet and the latter to accelerate it in the upstream region, respectively, but these effects are switched in the downstream region. The mean ageostrophic Coriolis force is partially balanced by the horizontal gradient of the eddy kinetic energy, which is the isotropic component of the Reynolds stress. The difference between these terms, that is, net ageostrophic Coriolis force, leads to the final deceleration of the KE jet in the downstream region, overwhelming the acceleration tendency of the anisotropic Reynolds stress. The authors also reinterpret the downstream decay process of an eastward jet in a previous quasigeostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment.

scholarly journals Southward Eddy Heat Transport Occurring along Southern Flanks of the Kuroshio Extension and the Gulf Stream in a 1/10° Global Ocean General Circulation Model

2013 ◽  
Vol 43 (9) ◽  
pp. 1899-1910 ◽  
Author(s):  
Kunihiro Aoki ◽  
Shoshiro Minobe ◽  
Youichi Tanimoto ◽  
Yoshikazu Sasai
Keyword(s):  
Heat Transport ◽  
Gulf Stream ◽  
General Circulation ◽  
Kuroshio Extension ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Global Ocean ◽  
Eddy Heat Transport ◽  
Ocean General Circulation ◽  
The Kuroshio

Abstract The present study investigates meridional heat transport induced by oceanic mesoscale variability in the World Ocean using a ° global ocean general circulation model (OGCM) running on the Earth Simulator. The results indicate prominent poleward eddy heat transport around the western boundary currents and the Antarctic Circumpolar Current, and equatorward eddy heat transport in the equatorial region, consistent with the previous studies using coarse-resolution OGCMs. Such poleward eddy heat transport in midlatitude oceans suggests that the eddies act to reduce meridional background temperature gradients across the currents, as would be expected based on baroclinic instability. Interestingly, however, along the southern flanks of the eastward jets of the Kuroshio Extension and the Gulf Stream, southward eddy heat transport occurs in subsurface layers. This is likely due to the southward migration of warm water cores originating from southern areas adjacent to these currents. Southward movement of these cores is caused by interactions with unsteady meanders and cold eddies detaching from the meanders. The potential impact on biological production in the subtropical surface layers of these southward-traveling warm water cores is also discussed.

scholarly journals Decadal Shifts of the Kuroshio Extension Jet: Application of Thin-Jet Theory*

2011 ◽  
Vol 41 (5) ◽  
pp. 979-993 ◽  
Author(s):  
Yoshi N. Sasaki ◽  
Niklas Schneider
Keyword(s):  
Time Scales ◽  
Potential Vorticity ◽  
General Circulation ◽  
Decadal Variability ◽  
Kuroshio Extension ◽  
Phase Speed ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Dynamic Framework ◽  
The Kuroshio

Abstract Meridional shifts of the Kuroshio Extension (KE) jet on decadal time scales are examined using a 1960–2004 hindcast simulation of an eddy-resolving ocean general circulation model for the Earth Simulator (OFES). The leading mode of the simulated KE represents the meridional shifts of the jet on decadal time scales with the largest southward shift in the early 1980s associated with the climate regime shift in 1976/77, a result confirmed with subsurface temperature observations. The meridional shifts originate east of the date line and propagate westward along the mean jet axis, a trajectory inconsistent with the traditionally used linear long Rossby waves linearized in Cartesian coordinates, although the phase speed is comparable to that in the traditional framework. The zonal scale of these westward propagation signals is about 4000 km and much larger than their meridional scale. To understand the mechanism for the westward propagation of the KE jet shifts, the authors consider the limit of a thin jet. This dynamic framework describes the temporal evolution of the location of a sharp potential vorticity front under the assumption that variations along the jet are small compared to variations normal to the jet in natural coordinates and is well suited to the strong jet and potential vorticity gradients of the KE. For scaling appropriate to the decadal adjustments in the KE, the thin-jet model successfully reproduces the westward propagations and decadal shifts of the jet latitude simulated in OFES. These results give a physical basis for the prediction of decadal variability in the KE.

scholarly journals Potential Predictability of Interannual Variability in the Kuroshio Extension Jet Speed in an Eddy-Resolving OGCM

2012 ◽  
Vol 25 (10) ◽  
pp. 3645-3652 ◽  
Author(s):  
Masami Nonaka ◽  
Hideharu Sasaki ◽  
Bunmei Taguchi ◽  
Hisashi Nakamura
Keyword(s):  
Interannual Variability ◽  
General Circulation ◽  
Degrees Of Freedom ◽  
North Pacific Ocean ◽  
Kuroshio Extension ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Propagation Time ◽  
Potential Predictability ◽  
The Kuroshio

AbstractVariability in the Kuroshio Extension (KE) jet speed has been considered to impact the upper-ocean ecosystem. This study investigates potential predictability of interannual variability in the KE jet speed that could arise from the propagation time of wind-driven Rossby waves as suggested by previous studies, through prediction experiments with an eddy-resolving ocean general circulation model (OGCM) under the perfect-model assumption. Despite the small number of experiments available because of limited computational resources, the prediction experiments with no anomalous atmospheric forcing suggest some predictability for not only broad-scale sea surface height anomalies (SSHAs) but also the frontal-scale KE jet speed. The predictability is confirmed in a 60-yr hindcast OGCM integration as a significantly high correlation (r = 0.68) of 13-month running mean time series of the anomalous KE jet speed with SSHAs that appear in the central North Pacific Ocean 3 yr earlier. Although with fewer degrees of freedom, the same lag relationship can be found between satellite-measured SSHAs and the geostrophically derived KE jet speed.

scholarly journals A global eddying hindcast ocean simulation with OFES2

2020 ◽  
Vol 13 (7) ◽  
pp. 3319-3336 ◽  
Author(s):  
Hideharu Sasaki ◽  
Shinichiro Kida ◽  
Ryo Furue ◽  
Hidenori Aiki ◽  
Nobumasa Komori ◽  
...  
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Sea Surface ◽  
Sea Surface Salinity ◽  
Tidal Mixing ◽  
Surface Salinity ◽  
Previous Version ◽  
Spatiotemporal Scales ◽  
The Kuroshio

Abstract. A quasi-global eddying ocean hindcast simulation using a new version of our model, called OFES2 (Ocean General Circulation Model for the Earth Simulator version 2), was conducted to overcome several issues with unrealistic properties in its previous version, OFES. This paper describes the model and the simulated oceanic fields in OFES2 compared with OFES and also observed data. OFES2 includes a sea-ice model and a tidal mixing scheme, is forced by a newly created surface atmospheric dataset called JRA55-do, and simulated the oceanic fields from 1958 to 2016. We found several improvements in OFES2 over OFES: smaller biases in the global sea surface temperature and sea surface salinity as well as the water mass properties in the Indonesian and Arabian seas. The time series of the Niño3.4 and Indian Ocean Dipole (IOD) indexes are somewhat better in OFES2 than in OFES. Unlike the previous version, OFES2 reproduces more realistic anomalously low sea surface temperatures during a positive IOD event. One possible cause of these improvements in El Niño and IOD events is the replacement of the atmospheric dataset. On the other hand, several issues remained unrealistic, such as the pathways of the Kuroshio and Gulf Stream and the unrealistic spreading of salty Mediterranean overflow. Given the worldwide use of the previous version and the improvements presented here, the output from OFES2 will be useful in studying various oceanic phenomena with broad spatiotemporal scales.

scholarly journals Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving Model Hindcast*

2007 ◽  
Vol 20 (11) ◽  
pp. 2357-2377 ◽  
Author(s):  
Bunmei Taguchi ◽  
Shang-Ping Xie ◽  
Niklas Schneider ◽  
Masami Nonaka ◽  
Hideharu Sasaki ◽  
...  
Keyword(s):  
Time Series ◽  
Rossby Wave ◽  
North Pacific ◽  
General Circulation ◽  
Kuroshio Extension ◽  
Low Frequency ◽  
Circulation Model ◽  
Global Ocean ◽  
Basin Scale ◽  
The Kuroshio

Abstract Low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidecadal (1950–2003) hindcast by a high-resolution (0.1°), eddy-resolving, global ocean general circulation model for the Earth Simulator (OFES). In both the OFES hindcast and satellite altimeter observations, low-frequency sea surface height (SSH) variability in the North Pacific is high near the KE front. An empirical orthogonal function (EOF) analysis indicates that much of the SSH variability in the western North Pacific east of Japan is explained by two modes with meridional structures tightly trapped along the KE front. The first mode represents a southward shift and to a lesser degree, an acceleration of the KE jet associated with the 1976/77 shift in basin-scale winds. The second mode reflects quasi-decadal variations in the intensity of the KE jet. Both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations. A linear Rossby wave model forced by observed wind successfully reproduces the time series of the leading OFES modes but fails to explain why their meridional structure is concentrated on the KE front and inconsistent with the broadscale wind forcing. Further analysis suggests that KE variability may be decomposed into broad- and frontal-scale components in the meridional direction—the former following the linear Rossby wave solution and the latter closely resembling ocean intrinsic modes derived from an OFES run forced by climatological winds. The following scenario is suggested for low-frequency KE variability: basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic modes of the KE jet and reorganizing SSH variability in space.

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