Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Explosive cyclones(ECs)occur frequently over the Kuroshio/Kuroshio Extension region.The most rapidly intensified EC over the Kuroshio/Kuroshio Extension region during the 42 years(1979-2020)of cold seasons(October-April)was studied to reveal the variations of the key factors at different explosive-developing stages.This EC had weak low-level baroclinicity,mid-level cyclonic-vorticity advection,and strong low-level water vapor convergence at the initial explosive-developing stage.The low-level baroclinicity and mid-level cyclonic-vorticity advection increased substantially during the maximum-deepening-rate stage.The diagnostic analyses using the Zwack-Okossi equation showed that diabatic heating was the main contributor to the initial rapid intensification of this EC.The cyclonic-vorticity advection and warm-air advection enhanced rapidly in the middle and upper troposphere and contributed to the maximum rapid intensification,whereas the diabatic heating weakened slightly in the mid-low troposphere.The relative contribution of the diabatic heating decreased from the initial explosive-developing stage to the maximum-deepening-rate stage due to the enhancement of other factors(the cyclonic-vorticity advection and warm-air advection).Furthermore,the physical factors contributing to this EC varied with the explosive-developing stage.The non-key factors at the initial explosive-developing stage need attention to forecast the rapid intensification.

Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment

In this paper,a statistical method called Generalized Equilibrium Feedback Analysis(GEFA)is used to investigate the responses of the North Pacific Storm Track(NPST)in the cold season to the multi-scale oceanic variations of the Kuroshio Extension(KE)system,including its large-scale variation,oceanic front meridional shift,and mesoscale eddy activity.Results show that in the cold season from the lower to the upper troposphere,the KE large-scale variation significantly weakens the storm track activity over the central North Pacific south of 30°N.The northward shift of the KE front significantly strengthens the storm track activity over the western and central North Pacific south of 40°N,resulting in a southward shift of the NPST.In contrast,the NPST response to KE mesoscale eddy activity is not so significant and relatively shallow,which only shows some significant positive signals near the dateline in the lower and middle troposphere.Furthermore,it is found that baroclinicity and baroclinic energy conversion play an important role in the formation of the NPST response to the KE multi-scale oceanic variations.

Position variability of the Kuroshio Extension sea surface temperature front

High spatial resolution sea surface temperature （SST） data from 1993 to 2013 are used to detect the position of the Kuroshio Extension sea surface temperature front （KEF） from 141°E to 158°E, and the seasonal, monthly and interannual-to-decadal variations of the KEF position are investigated. The latitudinal position of the KEF varies with longitudes： the westernmost part of the KEF from 141°E to 144°E is relatively stable, whereas the easternmost part from 153°E to 158°E exhibits the largest amplitude of its north-south displacement. In the light of the magnitudes of the standard deviations at longitudes, then the KEF is divided into three sections： western part of the KEF （KEFw, 141°-144°E）, central part of the KEF （KEFc, 144°-153°E） and eastern part of the KEF （KEFe, 153°-158°E）. Further analysis reveals that the KEFw position is dominated by the decadal variability, while the KEFc and KEFe positions change significantly both on interannual and decadal time scales. In addition, the KEFw position is well correlated with the KEF path length. The possible mode leading to the decadal oscillation of the KEFw is further discussed. The KEFw position exhibits significant connections with the Pacific decadal oscillation （PDO） index and the north Pacific gyre oscillation （NPGO） index with a time lag of 40 and 33 months, respectively.

Optimal Precursors Triggering the Kuroshio Extension State Transition Obtained by the Conditional Nonlinear Optimal Perturbation Approach

In this study, the initial perturbations that are the easiest to trigger the Kuroshio Extension （KE） transition connecting a basic weak jet state and a strong, fairly stable meandering state, are investigated using a reduced-gravity shallow water ocean model and the CNOP （Conditional Nonlinear Optimal Perturbation） approach. This kind of initial perturbation is called an optimal precursor （OPR）. The spatial structures and evolutionary processes of the OPRs are analyzed in detail. The results show that most of the OPRs are in the form of negative sea surface height （SSH） anomalies mainly located in a narrow band region south of the KE jet, in basic agreement with altimetric observations. These negative SSH anomalies reduce the merid- ional SSH gradient within the KE, thus weakening the strength of the jet. The KE jet then becomes more convoluted, with a high-frequency and large-amplitude variability corresponding to a high eddy kinetic energy level; this gradually strengthens the KE jet through an inverse energy cascade. Eventually, the KE reaches a high-energy state characterized by two well defined and fairly stable anticyclonic meanders. Moreover, sensitivity experiments indicate that the spatial structures of the OPRs are not sensitive to the model parameters and to the optimization times used in the analysis.

A comparison of the strength and position variability of the Kuroshio Extension SST front

This study compares the seasonal and interannual-to-decadal variability in the strength and position of the Kuroshio Extension front(KEF)using high-resolution satellite-derived sea surface temperature(SST)and sea surface height(SSH)data.Results show that the KEF strength has an obvious seasonal variation that is similar at different longitudes,with a stronger(weaker)KEF during the cold(warm)season.However,the seasonal variation in the KEF position is relatively weak and varies with longitude.In contrast,the low-frequency variation of the KEF position is more distinct than that of the KEF strength even though they are well correlated.On both seasonal and interannual-to-decadal time scales,the western part of the KEF(142°–144°E)has the greatest variability in strength,while the eastern part of the KEF(149°–155°E)has the greatest variability in position.In addition,the relationships between wind-forced Rossby waves and the low-frequency variability in the KEF strength and position are also discussed by using the statistical analysis methods and a wind-driven hindcast model.A positive(negative)North Pacific Oscillation(NPO)-like atmospheric forcing generates positive(negative)SSH anomalies over the central North Pacific.These oceanic signals then propagate westward as Rossby waves,reaching the KE region about three years later,favoring a strengthened(weakened)and northward(southward)-moving KEF.

An oceanic eddy statistical comparison using multiple observational data in the Kuroshio Extension region

Eddy characteristics derived from different data resources are compared： sea surface temperature（SST）, sea surface height anomaly（SSHA） and surface drifter trajectories. The comparison suggests that the eddy statistical characteristics are different using different variables to delineate eddies, but they show the similar trend. Based on the comparison, abnormal eddies with warm（cold） cores but counter-clockwise（clockwise） rotation are found in the Kuroshio Extension region.

A dual-period response of the Kuroshio Extension SST to Aleutian Low activity in the winter season

Based on our previous work, the winter sea surface temperature（SST） in the Kuroshio Extension（KE） region showed significant variability over the past century with periods of ~6 a between 1930 and 1950 and ~10 a between1980 and 2009. How the activity of the Aleutian Low（AL） induces this dual-period variability over the two different timespans is further investigated here. For the ~6 a periodicity during 1930–1950, negative wind stress curl（WSC）anomalies in the central subtropical Pacific associated with an intensified AL generate positive sea surface height（SSH） anomalies. When these wind-induced SSH anomalies propagate westwards to the east of Taiwan, China two years later, positive velocity anomalies appear around the Kuroshio to the east of Taiwan and then the mean advection via this current of velocity anomalies leads to a strengthened KE jet and thus an increase in the KE SST one year later. For the ~10 a periodicity during 1980–2009, a negative North Pacific Oscillation-like dipole takes2–3 a to develop into a significant positive North Pacific Oscillation-like dipole, and this process corresponds to the northward shift of the AL. Negative WSC anomalies associated with this AL activity in the central North Pacific are able to induce the positive SSH anomalies. These oceanic signals then propagate westward into the KE region after 2–3 a, favoring a northward shift of the KE jet, thus leading to the warming of the KE SST. The feedbacks of the KE SST anomaly on the AL forcing are both negative for these two periodicities. These results suggest that the dual-period KE SST variability can be generated by the two-way KE-SST-AL coupling.

Interdecadal change of winter SST variability in the Kuroshio Extension region and its linkage with Aleutian atmospheric low pressure system

By utilizing multiple datasets from various sources available for the last 100 years, the existence for the interdecadal change of the winter sea surface temperature（SST） variability in the Kuroshio Extension（KE） region is investigated. And its linkage with the Aleutian Low（AL） activity changes is also discussed. The results find that the KE SST variability exhibits the significant ~6 a and ~10 a oscillations with obvious interdecadal change. The ~6 a oscillation is mainly detected during 1930–1950, which is largely impacted by the anomalous surface heat flux forcing and Ekman heat transport associated with the AL intensity variation. The ~10 a oscillation is most evident after the 1980s, which is predominantly triggered by the AL north-south shift through the bridge of oceanic Rossby waves.

Impact of Kuroshio Extension dipole mode variability on the North Pacific storm track

An index of a large-scale Kuroshio Extension（KE） sea surface height dipole（KED） mode is constructed using satellite altimeter sea level anomaly observations from January 1993 to December 2015 based on previous work of the second author. It is found that the KED mode that undergoes a decadal variation from a negative phase（a positive-over-negative dipole, KED.） to a positive phase（a negative-over-positive dipole, KED＋） can affect the variability of the oceanic SST front and the North Pacific storm track. The results show that the oceanic SST fronts in the north of the KE region and in the KE region — referred to as the NSST and KSST fronts, respectively — are closely correlated with the KED mode. In the NSST front region, the SST front is stronger for KED. than for KED＋, and the opposite is the case in the KSST region. It is further revealed that the decadal phase transition of the KED mode can change the location and strength of the North Pacific storm track, with the North Pacific storm track being slightly weaker and moving more northwards as a whole during the KED. mode than during the KED＋ mode. The westerly wind associated with the storm track on the downstream side of the KE region intensifies and shifts northwards under KED. compared to KED＋.Furthermore, the transition of the KED mode gives rise to changes in the North Pacific storm track by changing the NSST and KSST fronts and meridional heat flux.

Oceanic mesoscale eddy in the Kuroshio extension:Comparison of four datasets

Some relatively mature mesoscale eddy products have been released for scientific purposes in recent decades.However,the metrics used to identify eddies,the tracking methods,and the definition of the physical parameters are all different across the different datasets,so intercomparisons and validation of these datasets are badly needed.Here,the authors intercompare the basic features of ocean mesoscale eddies in the Kuroshio extension region from four eddy datasets—namely,Chelton,GEM-M,Faghmous,and Dong.In the case study,eddy numbers and locations as well as the eddy tracks identified by the four datasets are compared for a specific date.The authors find that all the datasets have different eddy numbers,but more than 50%of identified eddies coincide.GEM-M,with the so-called"segmentation"algorithm,can identify considerably more eddies than others,while Chelton identifies fewer eddies due to tracking errors,which also lead to a long lifespan.From the analysis of the probability distribution function of eddy features,GEM-M eddies tend to have a larger amplitude and radius and Chelton tends to have long-life eddies.It is further found that the geographic distributions and temporal variation of normalized eddy features are highly similar among the four datasets—particularly among Chelton,Fahgmous,and Dong.In addition,the mean trajectories of the four datasets are generally overlapped initially,and then spread after 245 days.The findings help toward better understanding the uncertainties of eddy features in the Kuroshio extension region.

The decadally modulating eddy field in the upstream Kuroshio Extension and its related mechanisms

Both the level of the high-frequency eddy kinetic energy（HF-EKE） and the energy-containing scale in the upstream Kuroshio Extension（KE） undergo a well-defined decadal modulation, which correlates well with the decadal KE path variability. The HF-EKE level and the energy-containing scales will increase with unstable KE path and decrease with stable KE path. Also the mesoscale eddies are a little meridionally elongated in the stable state, while they are much zonally elongated in the unstable state. The local baroclinic instability and the barotropic instability associated with the decadal modulation of HF-EKE have been investigated. The results show that the baroclinic instability is stronger in the stable state than that in the unstable state, with a shorter characteristic temporal scale and a larger characteristic spatial scale. Meanwhile, the regional-averaged barotropic conversion rate is larger in the unstable state than that in the stable state. The results also demonstrate that the baroclinic instability is not the dominant mechanism influencing the decadal modulation of the mesoscale eddy field, while the barotropic instability makes a positive contribution to the decadal modulation.

Variability of the Kuroshio extension system in 1992-2013 from satellite altimetry data

The Kuroshio Extension （KE） plays an important role in climate and environmental change in the North Pacific. In this paper, more than 20 years of merged absolute dynamic topography and merged sea level anomaly products from satellite altimetry are used to analyze the stability of the KE system. By analyzing the annually averaged sea surface topography, the variations of inter-annual path and annually averaged eddy kinetic energy at the KE region, the KE＇s two dynamic states are given as： the relatively stable state during 1993 1995, 2002-2005, and 2010-2012, and the unstable dynamic state among 1996-2001 and 2006-2009. During the stable state, the KE spindle had a shorter path length and smaller time-varying amplitude, as well as a trend to move northward. While during the unstable state, the KE spindle had a longer path length and an integral southward transport trend, and was observed to oscillate significantly over time. The analysis on the KE＇s upstream and downstream region gives the same variations, indi- cating that they are significantly affected by the El Nino events. The power spectrum of the mean latitudinal position variation of the KE＇s upstream and downstream shows significant quasi-decadal oscillation characteristics and strong annual signals. Furthermore, the correlation of the strength vari- ation between the southern RG and the KE＇s upstream is calculated to be 0.50 after low-pass filtering, and that of the mean latitudinal position variation between the southern RG and the KE＇s upstream/ downstream are 0.75/0.69 after low-pass filtering, respectively. The strong correlations demonstrated that the southern RG and the KE are closely linked.

Effect of mesoscale wind stress-SST coupling on the Kuroshio extension jet

Effect of mesoscale wind stress-SST coupling on the Kuroshio extension jet is studied using the Regional Ocean Modeling System. The mesoscale wind stress perturbation( τ_(MS)) is diagnostically determined from modelled mesoscale SST perturbation(SST_(MS)) by using their empirical relationship derived from corresponding observation. From comparing two experiments with and without the τ_(MS) feedback, it is found that the interactively represented τ_(MS)-SST_(MS) coupling can modulate the kinetic energy along the Kuroshio extension jet, with little effect on the Kuroshio pathway. Similar results are also obtained in three additional sensitivity experiments, which consider half strength of the τ_(MS), and the momentum flux and heat flux effect induced by τ_(MS), respectively. That means simply taking into account the τ_(MS)-SST_(MS) coupling has little effect on improving the simulation of the Kuroshio Current system.

Multisatellite observations of smaller mesoscale eddy generation in the Kuroshio Extension

Smaller mesoscale eddies(SMEs)have an important effect on the transmission of ocean temperatures,salinity,energy,and marine biochemical processes.However,traditional altimeters,the dominant sensors used to identify and track eddies,have made it challenging to observe SMEs accurately due to resolution limitations.Eddies drive local upwelling or downwelling,leaving signatures on sea surface temperatures(SSTs)and chlorophyll concentrations(Chls).SST can be observed by spaceborne infrared sensors,and Chl can be measured by ocean color remote sensing.Therefore,multisatellite observations provide an opportunity to obtain information to characterize SMEs.In this paper,an eddy detection algorithm based on SST and Chl images is proposed,which identifies eddies by characterizing the spatial and temporal distribution of SST and Chl data.The algorithm is applied to characterize and analyze SMEs in the Kuroshio Extension.Statistical results on their distribution and seasonal variability are shown,and the formation processes are preliminarily discussed.SMEs generation may be contributed by horizontal strain instability,the interaction of topographic obstacles and currents,and wind stress curl.

NUMERICAL EXPERIMENT ON IMPACT OF ANOMALOUS SST WARMING IN KUROSHIO EXTENSION IN PREVIOUS WINTER ON EAST ASIAN SUMMER MONSOON

The impact of anomalous sea surface temperature (SST) warming in the Kuroshio Extension in the previous winter on the East Asian summer monsoon (EASM) was investigated by performing simulation tests using NCAR CAM3.The results show that anomalous SST warming in the Kuroshio Extension in winter causes the enhancement and northward movement of the EASM.The monsoon indexes for East Asian summer monsoon and land-sea thermal difference,which characterize the intensity of the EASM,show an obvious increase during the onset period of the EASM.Moreover,the land-sea thermal difference is more sensitive to warmer SST.Low-level southwesterly monsoon is clearly strengthened meanwhile westerly flows north (south) of the subtropical westerly jet axis are strengthened (weakened) in northern China,South China Sea,and the Western Pacific Ocean to the east of the Philippines.While there is an obvious decrease in precipitation over the Japanese archipelago and adjacent oceans and over the area from the south of the Yangtze River in eastern China to the Qinling Mountains in southern China,precipitation increases notably in northern China,the South China Sea,the East China Sea,the Yellow Sea,and the Western Pacific to the east of the Philippines.North China is the key area where the response of the EASM to the SST anomalous warming in the Kuroshio Extension is prominent.The surface air temperature shows a warming trend.The warming in the entire troposphere between 30oN and 50oN increases the land-sea thermal contrast,which plays an important role in the enhancement of the EASM.Atmospheric circulation and precipitation anomalies in China and its adjacent regions have a close relationship with the enhancement of the Western Pacific subtropical high and its northward extension.

Seasonal Variability in the Kuroshio Extension Current System

Based on the GDEM hydrographic data with a resolution of 0.5°× 0.5°, the current system (Kuroshio south of Japan and Kuroshio Extension east of Japan) is determined by using the P-Vector Method, and its seasonal variability is investigated. The Kuroshio Meander south of Japan, the two lee-wave meanders in the Kuroshio Extension and the bifurcation of the Kuroshio Extension are properly presented. The path of the Kuroshio Meander, the position of the second (east) meander in the Kuroshio Extension and the bifurcation of the Kuroshio Extension display evident seasonal variation.

THE INFLUENCES OF SSTA OVER KUROSHIO AND ITS EXTENSION ON RAINFALL IN NORTHEAST CHINA UNDER THE BACKGROUND OF TWO DIFFERENT EL NIO CASES

By using the gauged rainfall in 160 stations within China's Mainland and the NCEP/NCAR reanalysis data, the impacts of anomalous SST in Kuroshio and its extension on precipitation in Northeast China were investigated. The results show that a difference in the meridional circulation such as the East Asia/Pacific teleconnection pattern(EAP)may be responsible for the difference in rainfall between 1998 and 2010. In comparison with 1998, the anomalous meridional circulation pattern in 2010 shifted northeastward, and then the western subtropical high, the mid-latitudinal trough and the northeastern Asia blocking high also shifted northeastward, causing intensified convergence of the cold and warm air masses at the southern region and thus more rainfall in the southwestern region and less in the northwestern region. In 1998, the anomalous cyclone, one component of the meridional pattern, located at the Songhuajiang-Nengjiang River basin, resulted in more rainfall in the majority of the area. The results of observation and the model show that the difference in SSTA in Kuroshio and its extension under the background of different El Ni觡o events is the key point:(1) The anomalous warmth moved westward from the mid-Pacific to the east of the Philippine Sea during the central event, which led the heat resources shifting to the northeast in 2010; subsequently, a shift occurred to the north of the anomalous ascent and decent, followed by a warm SSTA in the region of Kuroshio's extension in 2010 and Kuroshio in 1998.(2) The warm SSTA in the Kuroshio extension causing the Rossby wave activity flux strengthened in 2010, and then the westerly jet shifted northward and extended eastward. A warm SSTA in Kuroshio and cold SSTA in its extension in 1998 caused the westerly jet to shift southward and weaken. As a result,the anomalous anticyclone and cyclone shifted northward in 2010, and the blocking high also shifted northward.

Response of atmospheric circulation to multiscale SST anomaly associated with Kuroshio Extension decadal variability warming in winter

The response of atmospheric circulation to sea surface temperature anomaly(SSTA)of different scales in extratropical oceans has always been a popular issue in air-sea interactions,especially regarding the influence of widely active,small-scale ocean eddies on the atmosphere.Based on the regional climate model RegCM4.6,three sets of ensemble experiments with different initial values were designed,and the response of atmospheric circulation and possible mechanisms to Kuroshio Extension Decadal Variability SSTA with different scales(KEDV-induced SSTA)during winter were discussed.The response of atmospheric circulation to the KEDV-induced mesoscale SSTA presents a broadly tripolar pattern,while the response to the KEDV-induced large-scale SSTA presents a baroclinic structure in the central Pacific and a dipole-type response with a barotropic structure in the eastern Pacific.Further diagnostic analysis shows that under the influence of mesoscale SSTA,transient eddy activity is strengthened,and feedback of transient eddy plays a major role in the large-scale circulation anomaly in the central-eastern Pacific.The associated barotropic energy conversion also plays an important role in maintaining the largescale circulation anomaly in the northwestern Pacific Ocean.Under the influence of large-scale SSTA,diabatic heating is stronger,and the feedback of diabatic heating plays a major role in the large-scale circulation anomaly.

Seismic Images of Shallow Waters over the Shatsky Rise in the Northwest Pacific Ocean

Recent studies have demonstrated the ability of seismic oceanography to reveal finescale vertical structures of water column in the oceans based on multichannel seismic(MCS)reflection data.Such information can clarify the dynamic processes of mixing,exchange,and translation of water mass and energy.In this study,we present four MCS lines and satellite data to show high-resolution seismic images of shallow waters over the Shatsky Rise in the Northwest Pacific Ocean,where the Kuroshio Exten-sion passes and bifurcates.One of our MCS transects crossed the center of an anticyclonic warm eddy on August 28,2010,confirmed by satellite data such as sea level anomaly(SLA),geostrophic current anomaly(GCA),and sea surface temperature anomaly(SSTa).The seismic image showed that the eddy vertical structure featured a bowl-like shape and onion-like internal layering.The slightly tilted(<0.5°)surface of the eddy was 400m below the sea surface,indicating a subsurface eddy.The eddy was inferred to have a radius of 50 km and a maximum thickness of 500m.Other MCS sections demonstrated the submesoscale structure of oceanfronts,characterized by the dipping reflectors(>2°-3°)at the boundaries between water masses with differing properties.In addition,the discrepancies in SLA,GCA,and SSTa between water masses resulted in different seismic reflectivities.The water masses with high SLA,anticyclonic GCA and positive SSTa featured high-amplitude,continuous,clear-layered,and non-linear reflections,whereas those with low SLA,cyclonic GCA,and negative SSTa were associated with weak,fragmented,less stratification,and more linear reflectors.

Persistence of Summer Sea Surface Temperature Anomalies in the Midlatitude North Pacific and Its Interdecadal Variability

The present study investigates the persistence of summer sea surface temperature anomalies（SSTAs） in the midlatitude North Pacific and its interdecadal variability. Summer SSTAs can persist for a long time（approximately 8–14 months）around the Kuroshio Extension（KE） region. This long persistence may be strongly related to atmospheric forcing because the mixed layer is too shallow in the summer to be influenced by the anomalies at depths in the ocean. Changes in atmospheric circulation, latent heat flux, and longwave radiation flux all contribute to the long persistence of summer SSTAs. Among these factors, the longwave radiation flux has a dominant influence. The effects of sensible heat flux and shortwave radiation flux anomalies are not significant. The persistence of summer SSTAs displays pronounced interdecadal variability around the KE region, and the variability is very weak during 1950–82 but becomes stronger during 1983–2016. The changes in atmospheric circulation, latent heat flux, and longwave radiation flux are also responsible for this interdecadal variability because their forcings on the summer SSTAs are sustained for much longer after 1982.