On the Reset of the Wind-Forced Decadal Kuroshio Extension Variability in Late 2017

2020 ◽  
Vol 33 (24) ◽  
pp. 10813-10828
Author(s):  
Bo Qiu ◽  
Shuiming Chen ◽  
Niklas Schneider ◽  
Eitarou Oka ◽  
Shusaku Sugimoto
Keyword(s):  
Wind Field ◽  
Kuroshio Extension ◽  
Surface Wind ◽  
Stable State ◽  
Storm Tracks ◽  
Dynamic State ◽  
Large Meander ◽  
Basin Scale ◽  
Kuroshio Large Meander ◽  
The Kuroshio

AbstractDecadal modulations of the Kuroshio Extension (KE) system between a stable and an unstable dynamic state in the western North Pacific have prevailed in the past three decades. This dominance of decadal variations is controlled by the negative feedback loop involving the wind-forced KE variability and its feedback onto the overlying extratropical storm tracks and the basin-scale surface wind field. The wind-forced decadal KE modulations were disrupted in August 2017 due to the development of the Kuroshio large meander south of Japan. By forcing the inflow KE paths northward and by avoiding overriding the shallow Izu Ridge, the Kuroshio large meander was able to compel the KE to change rapidly from the wind-forced, pre-existing, unstable state to a stable state. Following the large meander occurrence in late 2017, the stabilized KE change is found to affect the overlying storm tracks and the basin-scale wind field the same way as those generated by the wind-forced KE change prior to 2017. Given the consistent atmospheric response to both the large-meander-induced and wind-forced KE variability, we expect that the KE dynamic state will resume its decadal modulation after the phase reset relating to the 2017 large meander event.

Revisit of the Occurrence of the Kuroshio Large Meander South of Japan

2021 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen
Keyword(s):  
Kuroshio Extension ◽  
Surface Wind ◽  
Wind Forcing ◽  
Dynamic State ◽  
Large Meander ◽  
Subtropical Countercurrent ◽  
Eddy Generation ◽  
Anticyclonic Eddies ◽  
The Kuroshio

AbstractA unique characteristic by the Kuroshio off the southern coast of Japan is its bimodal path variations. In contrast to its straight path that follows coastline, the Kuroshio takes a large meander (LM) path when its axis detours southward by as much as 300 km. Since 1950, eight Kuroshio LM events took place and their occurrences appeared random. By synthesizing available in-situ/satellite observations and atmospheric reanalysis product, this study seeks to elucidate processes conducive for the LM occurrence. We find both changes in the inflow Kuroshio transport from the East China Sea and in the downstream Kuroshio Extension dynamic state are not determinant factors. Instead, intense anticyclonic eddies with transport > 20 Sv emanated from the Subtropical Countercurrent (STCC) are found to play critical roles in interacting with Kuroshio path perturbations southeast of Kyushu that generate positive relative vorticities along the coast and lead the nascent path perturbation to form a LM. Occurrence of this intense cyclonic{anticyclonic eddy interaction is favored when surface wind forcing over the STCC is anticyclonic during the positive phasing of Pacific decadal oscillations (PDOs). Such wind forcing strengthens the meridional Ekman flux convergence and enhances eddy generation by the STCC, and seven of the past eight LM events are found to be preceded by 1 ~ 2 years by the persistent anticyclonic wind forcings over the STCC. Rather than a fully random phenomenon, we posit that the LM occurrence is regulated by regional wind forcing with a positive PDO imprint.

scholarly journals A Coupled Decadal Prediction of the Dynamic State of the Kuroshio Extension System

2014 ◽  
Vol 27 (4) ◽  
pp. 1751-1764 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen ◽  
Niklas Schneider ◽  
Bunmei Taguchi
Keyword(s):  
Wind Stress ◽  
Large Scale ◽  
Current System ◽  
Kuroshio Extension ◽  
Surface Wind ◽  
Storm Tracks ◽  
Dynamic State ◽  
Western Boundary ◽  
Wind Stress Curl ◽  
The Kuroshio

Abstract Being the extension of a wind-driven western boundary current, the Kuroshio Extension (KE) has long been recognized as a turbulent current system rich in large-amplitude meanders and energetic pinched-off eddies. An important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits well-defined decadal modulations between a stable and an unstable dynamic state. Here the authors show that the decadally modulating KE dynamic state can be effectively defined by the sea surface height (SSH) anomalies in the 31°–36°N, 140°–165°E region. By utilizing the SSH-based KE index from 1977 to 2012, they demonstrate that the time-varying KE dynamic state can be predicted at lead times of up to ~6 yr. This long-term predictability rests on two dynamic processes: 1) the oceanic adjustment is via baroclinic Rossby waves that carry interior wind-forced anomalies westward into the KE region and 2) the low-frequency KE variability influences the extratropical storm tracks and surface wind stress curl field across the North Pacific basin. By shifting poleward (equatorward) the storm tracks and the large-scale wind stress curl pattern during its stable (unstable) dynamic state, the KE variability induces a delayed negative feedback that can enhance the predictable SSH variance on the decadal time scales.

scholarly journals Atmospheric Effects of the Kuroshio Large Meander during 2004–05*

2010 ◽  
Vol 23 (17) ◽  
pp. 4704-4715 ◽  
Author(s):  
Haiming Xu ◽  
Hiroki Tokinaga ◽  
Shang-Ping Xie
Keyword(s):  
Wind Speed ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Atmospheric Effects ◽  
Water Pool ◽  
Large Meander ◽  
Cool Water ◽  
Japanese Coast ◽  
Kuroshio Large Meander ◽  
The Kuroshio

Abstract In the summer of 2004, the Kuroshio took a large meander path south of Japan for the first time since 1991, and this large meander event persisted until the next summer. Satellite observations and numerical model simulations are used to study the effect of this large meander event on the atmosphere. The large meander leaves a cool water pool between the Kuroshio and Japanese coast. Sea surface temperature (SST) in the cool water pool is about 2°–3°C colder than the surroundings during winter and spring, whereas the SST signature substantially weakens in summer. A local reduction of wind speed is found over the cool water pool, and the positive SST–wind speed correlation is indicative of ocean forcing of the atmosphere. Cloud liquid water (CLW) content and precipitation also decrease over the cool SST pool. A regional atmospheric model successfully simulates atmospheric response to the Kuroshio large meander. The model experiments suggest that the reduced surface wind speed and precipitation are due to the large meander-induced SST cooling. Analysis of the surface perturbation momentum budgets shows the importance of the pressure adjustment mechanism in surface wind response to the cold SST anomalies.

scholarly journals Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I—formation and advection over the entire distribution region

2021 ◽  
Author(s):  
Eitarou Oka ◽  
Hatsumi Nishikawa ◽  
Shusaku Sugimoto ◽  
Bo Qiu ◽  
Niklas Schneider
Keyword(s):  
North Pacific ◽  
Decadal Variability ◽  
Kuroshio Extension ◽  
Formation Rate ◽  
Stable State ◽  
Large Meander ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
The Kuroshio ◽  
Central North Pacific

AbstractSince August 2017, the Kuroshio has taken a large-meander (LM) path, which has forced the Kuroshio extension (KE) to be in its stable state against its wind-forced decadal variability. How such current conditions have impacted the formation and advection of North Pacific subtropical mode water (STMW) over its distribution region was examined using Argo float data during 2005–2020. Out of the whole STMW defined as a low-potential vorticity layer of 16–19.5 ºC, a relatively cold variety of 16–18 ºC, which was formed south of the KE and advected westward and southward, occupied more than 80% of the total volume. The formation rate of the 16–18 ºC variety was low during 2006–2009 in an unstable-KE period and high during 2010–2015 in a stable-KE period, and then dropped drastically in 2016 despite the KE still being in the stable state. After a short unstable-KE period in 2016–2017, the LM-forced, stable-KE period began, but the formation rate of the 16–18 ºC variety has not restored, possibly due to stronger background stratification propagated from the central North Pacific. In addition, the 16–18 ºC variety has had to make a southern detour around the LM, and its westward advection from the formation region south of the KE to the region south of Japan has been significantly decreased, possibly because it is dissipated more strongly over a southern part of the Izu–Ogasawara Ridge. Due to such decline in the formation and advection, the volume of the 16–18 ºC variety and hence that of the whole STMW have gradually decreased since 2016.

scholarly journals Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

2020 ◽  
Vol 33 (1) ◽  
pp. 3-25
Author(s):  
Ryusuke Masunaga ◽  
Hisashi Nakamura ◽  
Bunmei Taguchi ◽  
Takafumi Miyasaka
Keyword(s):  
Kuroshio Extension ◽  
Surface Wind ◽  
Convective Precipitation ◽  
Western Boundary ◽  
Boundary Currents ◽  
Sst Front ◽  
Atmospheric Structure ◽  
Mean Convergence ◽  
Daily Scale ◽  
The Kuroshio

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

The Kuroshio large meander formation in 2004 analyzed by an eddy-resolving ocean forecast system

2008 ◽  
Vol 113 (C10) ◽  
Author(s):  
Yasumasa Miyazawa ◽  
Takashi Kagimoto ◽  
Xinyu Guo ◽  
Hirofumi Sakuma
Keyword(s):  
Large Meander ◽  
Forecast System ◽  
Kuroshio Large Meander ◽  
The Kuroshio

scholarly journals A case study of Kuroshio Extension Front: evolution, structure, diapycnal mixing and instability

2020 ◽  
Author(s):  
Jiahao Wang ◽  
Xi Chen ◽  
Kefeng Mao ◽  
Kelan Zhu
Keyword(s):  
Kuroshio Extension ◽  
Stable State ◽  
Shear Instability ◽  
Intermediate Water ◽  
North Pacific Intermediate Water ◽  
Intermediate Layers ◽  
Turbulence Mixing ◽  
Minimum Zone ◽  
The Kuroshio ◽  
Kuroshio Extension Front

Abstract. Satellite measurements during April to June in 2019 and direct observations from 28th to 30th May in 2019 about the Kuroshio Extension Front are conducted. The former shows the front experience a process of stable-unstable-stable state caused by the movement of the Kuroshio Extension’s second meander and a pinched-off eddy. The latter indicates the steep upward slopes of the isopycnals tilt northward in the strong frontal zone as well as several over 100 m thick blobs of cold and fresh water in the salinity minimum zone of North Pacific Intermediate Water. Using isopycnal anomaly method and diapycnal spiciness curvature method, characteristic interleaving layers are shown primarily in σθ = 26.3–26.9 kg/m3, which corresponds to large variations of potential spiciness in intermediate layers. Further analysis indicates the development of thermohaline intrusions may be driven by the double diffusive instability and the velocity anomalies. Besides, we find the turbulence mixing attributed to symmetric instability and shear instability is very strong in intermediate layer.

scholarly journals Interactions between Typhoon Choi-wan (2009) and the Kuroshio Extension System

2013 ◽  
Vol 2013 ◽  
pp. 1-17 ◽  
Author(s):  
Akiyoshi Wada ◽  
Norihisa Usui ◽  
Masaru Kunii
Keyword(s):  
Surface Temperature ◽  
Kuroshio Extension ◽  
Surface Wind ◽  
Specific Humidity ◽  
Central Pressure ◽  
Extension System ◽  
Stationary Front ◽  
Axisymmetric Structure ◽  
The Impact ◽  
The Kuroshio

We investigated interactions between Typhoon Choi-wan (2009) and the Kuroshio Extension system with a coupled atmosphere-wave-ocean model in 14 numerical simulations performed with initial conditions obtained from daily oceanic reanalysis data for the northwestern Pacific Ocean from September 12 to September 25, 2009. Preexisting oceanic conditions affected the simulated central pressure and the inner-core axisymmetric structure of the simulated typhoon differently during the intensification, mature, and decaying phases, but they had little impact on the simulated track. Among the simulations, the simulated central pressure range during the mature phase was ~10 hPa. Simulated central pressure was highly correlated with the axisymmetric mean horizontal specific humidity flux between radii of 50 and 150 km and altitudes of 20 to ~2000 m, suggesting that this flux has potential as a new metric for predicting tropical cyclone intensity. Variations in preexisting oceanic conditions in the Kuroshio Extension region on the simulated atmospheric and oceanic horizontal fields affected the typhoon and a nearby stationary front differently. Around the stationary front, the impact on hourly precipitation was closely related to that on surface wind speed, whereas the impact on surface temperature was greatly affected by that on sea surface temperature through the latent heat flux.

scholarly journals Roles of SST Anomalies on the Wintertime Turbulent Heat Fluxes in the Kuroshio–Oyashio Confluence Region: Influences of Warm Eddies Detached from the Kuroshio Extension

2011 ◽  
Vol 24 (24) ◽  
pp. 6551-6561 ◽  
Author(s):  
Shusaku Sugimoto ◽  
Kimio Hanawa
Keyword(s):  
Kuroshio Extension ◽  
Surface Wind ◽  
Surface Air Temperature ◽  
Surface Wind Speed ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
The North ◽  
Turbulent Heat Fluxes ◽  
The Kuroshio

Abstract Variations of turbulent heat fluxes (sum of sensible and latent heat fluxes) in the North Pacific during 16 winters from December 1992/February 1993 to December 2007/February 2008 are investigated because the months from December to February correspond to the period having peak winter conditions in the atmosphere field. Turbulent heat fluxes are calculated from the bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea flux (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter turbulent heat fluxes over the Kuroshio–Oyashio Confluence Region (KOCR; 142°–150°E, 35°–40°N) have the largest temporal variances in the North Pacific. The relative contributions among observed variables in SST, surface air temperature, and surface wind speed causing turbulent heat flux variations in the KOCR are assessed quantitatively by performing simple experiments using combinations of two types of variables: raw daily data and daily climatological data. Results show that SST is primarily responsible for the turbulent heat flux variations—a huge amount of heat is released in the state of the positive SST anomaly. Using the datasets of satellite-derived SST and sea surface height with high spatial and temporal resolutions, it is found that the SST anomalies in the KOCR are formed through activities of the anticyclonic (warm) eddies detached northward from the Kuroshio Extension; SSTs take positive (negative) anomalies when more (less) anticyclonic eddies are distributed there, associated with a more convoluted (straight) Kuroshio Extension path.

Sign in / Sign up

Export Citation Format

Share Document