Response of the Kuroshio Extension to Rossby Waves Associated with the 1970s Climate Regime Shift in a High-Resolution Ocean Model*

Abstract The response of the Kuroshio Extension (KE) to large-scale Rossby waves remotely excited by wind stress changes associated with the 1970s climate regime shift is studied using a high-resolution regional ocean model. Two ensemble simulations are conducted: The control run uses monthly climatological forcing while, in the second ensemble, anomalous forcing is imposed at the model eastern boundary around 165°E derived from a hindcast of decadal changes in subsurface temperature and salinity using a coarser-resolution model of the Pacific basin. Near the KE, ocean adjustment deviates strongly from the linear Rossby wave dynamics. Most notably, the eastward acceleration of the KE is much narrower in meridional extent than that associated with the incoming Rossby waves imposed on the eastern boundary. This KE acceleration is associated with an enhanced potential vorticity (PV) gradient across the front that is consistent with the inertial western boundary layer theory: the arrival of the Rossby waves at the western boundary causes the eastward current to accelerate, leading to enhanced advection of low (high) PV water of subtropical (subarctic) origin along the western boundary layer. The meridional dipole of PV anomalies results in a pair of anomalous recirculations with a narrow eastward jet in between. A three-layer quasigeostrophic model is used to demonstrate this inertial adjustment mechanism. Finally, transient eddy activity increases significantly and the eddy momentum transport acts to strengthen the mean flow response. The result that ocean physical response to broad-scale atmospheric forcing is large near the KE front has important implications for fisheries research.

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Modulation of Rossby waves on the Pacific North Equatorial Current bifurcation associated with the 1976 climate regime shift

Journal of Geophysical Research Oceans ◽

10.1002/2014jc010233 ◽

2014 ◽

Vol 119 (10) ◽

pp. 6669-6679 ◽

Cited By ~ 5

Author(s):

Li-Chiao Wang ◽

Chau-Ron Wu ◽

Bo Qiu

Keyword(s):

Regime Shift ◽

Rossby Waves ◽

North Equatorial Current ◽

Climate Regime ◽

Climate Regime Shift ◽

The Pacific ◽

Equatorial Current ◽

North Equatorial Current Bifurcation

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A Comparison of Mesoscale Eddy Heat Fluxes from Observations and a High-Resolution Ocean Model Simulation of the Kuroshio Extension

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0150.1 ◽

2013 ◽

Vol 43 (12) ◽

pp. 2563-2570 ◽

Cited By ~ 8

Author(s):

Stuart P. Bishop ◽

Frank O. Bryan

Keyword(s):

High Resolution ◽

Model Simulation ◽

Kuroshio Extension ◽

Mesoscale Eddy ◽

Ocean Model ◽

Heat Fluxes ◽

Dynamical Mechanism ◽

Eddy Heat Flux ◽

Conversion Rates ◽

The Kuroshio

Abstract For the first time estimates of divergent eddy heat flux (DEHF) from a high-resolution (0.1°) simulation of the Parallel Ocean Program (POP) are compared with estimates made during the Kuroshio Extension System Study (KESS). The results from POP are in good agreement with KESS observations. POP captures the lateral and vertical structure of mean-to-eddy energy conversion rates, which range from 2 to 10 cm2 s−3. The dynamical mechanism of vertical coupling between the deep and upper ocean is the process responsible for DEHFs in POP and is in accordance with baroclinic instability observed in the Gulf Stream and Kuroshio Extension. Meridional eddy heat transport values are ~14% larger in POP at its maximum value. This is likely due to the more zonal path configuration in POP. The results from this study suggest that HR POP is a useful tool for estimating eddy statistics in the Kuroshio Extension region, and thereby provide guidance in the formulation and testing of eddy mixing parameterization schemes.

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A Frequency-Dependent Description of Propagating Sea Level Signals in the Kuroshio Extension Region

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0185.1 ◽

2014 ◽

Vol 44 (6) ◽

pp. 1614-1635 ◽

Cited By ~ 2

Author(s):

Hongyang Lin ◽

Keith R. Thompson ◽

Jianyu Hu

Keyword(s):

Sea Level ◽

High Frequency ◽

Rossby Waves ◽

Kuroshio Extension ◽

Frequency Mode ◽

Global Ocean ◽

Western Boundary ◽

Frequency Dependent ◽

The Kuroshio ◽

Kuroshio Extension Region

Abstract Hilbert empirical orthogonal function analysis is used to provide a frequency-dependent description of observed sea level variability in the Kuroshio Extension region, 1993–2012 inclusive. The dominant high-frequency mode (periods between 140 and 350 days) describes signals that propagate westward with the largest amplitudes in the vicinity of the Shatsky Rise and Emperor Seamounts. Based on the close correspondence between the variance of the high-frequency variability and the underlying bathymetry, it is speculated that this mode is driven by jet–bathymetry interactions. The dominant low-frequency mode (periods longer than 350 days) is explained in terms of wind-forced, jet-trapped Rossby waves that propagate along the mean Kuroshio Extension jet. One of the most surprising findings of this study is that sea level changes north of the jet in the meander region anticipate changes south of the jet by about 3 yr. Based on correlations of observed sea level with the Pacific decadal oscillation, and western boundary transport variability estimated from the Global Ocean Reanalysis and Simulations (GLORYS), it is speculated that this anticipation is due to the differences in time taken for (i) Rossby waves to travel from the eastern North Pacific to the meander region and (ii) the much faster barotropic response of western boundary transport, and sea level north of the jet, to large-scale forcing by the wind stress curl.

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Interannual Modulations of Oceanic Imprints on the Wintertime Atmospheric Boundary Layer under the Changing Dynamical Regimes of the Kuroshio Extension

Journal of Climate ◽

10.1175/jcli-d-15-0545.1 ◽

2016 ◽

Vol 29 (9) ◽

pp. 3273-3296 ◽

Cited By ~ 26

Author(s):

Ryusuke Masunaga ◽

Hisashi Nakamura ◽

Takafumi Miyasaka ◽

Kazuaki Nishii ◽

Bo Qiu

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Atmospheric Boundary Layer ◽

Kuroshio Extension ◽

Surface Wind ◽

Vertical Wind Shear ◽

Heat Fluxes ◽

Convective Precipitation ◽

Positive Anomaly ◽

The Kuroshio

Abstract The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

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Separation of Climatological Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer: Their Sensitivity to SST Resolution Prescribed for Atmospheric Reanalysis

Journal of Climate ◽

10.1175/jcli-d-14-00314.1 ◽

2015 ◽

Vol 28 (5) ◽

pp. 1764-1787 ◽

Cited By ~ 43

Author(s):

Ryusuke Masunaga ◽

Hisashi Nakamura ◽

Takafumi Miyasaka ◽

Kazuaki Nishii ◽

Youichi Tanimoto

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Atmospheric Boundary Layer ◽

Kuroshio Extension ◽

Heat Fluxes ◽

Turbulent Heat ◽

Near Surface ◽

The Kuroshio ◽

Baroclinic Zone ◽

Sst Fronts

Abstract Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

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Recent Trends in Oceanic Conditions in the Western Part of East/Japan Sea: An Analysis of Climate Regime Shift That Occurred after the Late 1990s

Journal of Marine Science and Engineering ◽

10.3390/jmse9111225 ◽

2021 ◽

Vol 9 (11) ◽

pp. 1225

Author(s):

Hae-Kun Jung ◽

S. M. Mustafizur Rahman ◽

Hee-Chan Choi ◽

Joo-Myun Park ◽

Chung-Il Lee

Keyword(s):

Climate Change ◽

Regime Shift ◽

Japan Sea ◽

Climate Factors ◽

Atmospheric Conditions ◽

Climate Regime ◽

Climate Regime Shift ◽

Recent Climate ◽

Latitudinal Shift ◽

The Kuroshio

The western part of East/Japan Sea (WES) is an important area for understanding climate change processes and interactions between atmospheric and oceanic conditions. We analyzed the trends in recent oceanic conditions in the WES after the recent climate regime shift (CRS) that occurred in the late 1990s in the North Pacific. We explored the most important climate factors that affect oceanic conditions and determined their responses to changes in climate change. In the CRS that occurred in the late 1980s, changes in oceanic conditions in the WES were influenced by intensity changes in climate factors, and, in the late 1990s, it was by spatial changes in climate factors. The latitudinal shift of the Aleutian low (AL) pressure influences recent changes in oceanic and atmospheric conditions in the WES. The intensity of the Kuroshio Current and the sea level pressure in the Kuroshio extension region associated with the latitudinal shift of the AL pressure affects the volume of transport of the warm and saline water mass that flows into the WES and its atmospheric conditions. In addition, the fluctuations in the oceanic conditions of the WES affect various regions and depth layers differently, and these variations are evident even within the WES.

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