Kuroshio Extension System Study Research Articles

Penetration depth of diapycnal mixing generated by wind stress and flow over topography in the northwestern Pacific

AbstractThe role of turbulent diapycnal mixing in the northwestern Pacific was estimated by employing a fine‐scale parameterization method based on 6756 high‐resolution CTD profiles spanning a period of 8 years from the Japan Oceanography Data Center (JODC) and the Kuroshio Extension System Study (KESS). The rate of turbulent mixing in the upper ocean within 300–1800 m depth displayed a distinct seasonal cycle, bearing a statistically significant correlation to wind‐induced near‐inertial energy flux (hereafter denoted by WNEF). Enhanced turbulent mixing was also found near the rough seafloor relative to that over smooth topography. Enhanced dissipation at surface and bottom was found to be able to penetrate the ocean interior up to 1800 m and 3300 m, respectively, with penetration depths varying with the WNEF and topographic roughness. Our study here provides evidence for the important role of near‐inertial energy input from wind stress and the influence of bottom topography in maintaining mixing in the ocean interior.

Rapid Eddy-Induced Modification of Subtropical Mode Water during the Kuroshio Extension System Study

Abstract From 2004 to 2006 an observational array of current- and pressure-recording inverted echo sounders (CPIES) were deployed as part of the Kuroshio Extension (KEx) System Study (KESS). KESS observed a transition from a weakly meandering (“stable”) to strongly meandering (“unstable”) state (Qiu and Chen). As the KEx made this transition, potential vorticity (PV) observed within the southern recirculation gyre (SRG) rapidly increased from January to July 2005. In this study, the authors diagnose eddy PV fluxes (EPVFs) in isentropic coordinates within the subtropical mode water (STMW) layer from the CPIES data to determine the role of mesoscale eddies in this rapid increase of PV. The rapid increase in PV within the SRG coincided with enhanced cross-front EPVFs and eddy PV flux convergence upstream of a mean trough in the KEx path and adjacent to the SRG. The enhanced cross-front EPVFs were the result of the formation of a cold-core ring (CCR) and the interaction of the jet with a preexisting CCR. Eddy diffusivities are diagnosed for the unstable regime with values that range from 100 to 2000 m2 s−1. The high eddy diffusivities during the unstable regime reflect the nature of mesoscale CCR formation and CCR–jet interaction as efficient mechanisms for stirring and mixing high PV waters from the north side of the KEx into the low PV waters of the SRG where STMW resides. This mechanism for cross-frontal exchange can explain observed increases in the STMW PV in the SRG over the 16 months of KESS observations.

Variations of the North Pacific Subtropical Mode Water from Direct Observations

Abstract Mooring measurements from the Kuroshio Extension System Study (June 2004–June 2006) and from the ongoing Kuroshio Extension Observatory (June 2004–present) are combined with float measurements of the Argo network to study the variability of the North Pacific Subtropical Mode Water (STMW) across the entire gyre, on time scales from days, to seasons, to a decade. The top of the STMW follows a seasonal cycle, although observations reveal that it primarily varies in discrete steps associated with episodic wind events. The variations of the STMW bottom depth are tightly related to the sea surface height (SSH), reflecting mesoscale eddies and large-scale variations of the Kuroshio Extension and recirculation gyre systems. Using the observed relationship between SSH and STMW, gridded SSH products and in situ estimates from floats are used to construct weekly maps of STMW thickness, providing nonbiased estimates of STMW total volume, annual formation and erosion volumes, and seasonal and interannual variability for the past decade. Year-to-year variations are detected, particularly a significant decrease of STMW volume in 2007–10 primarily attributable to a smaller volume formed. Variability of the heat content in the mode water region is dominated by the seasonal cycle and mesoscale eddies; there is only a weak link to STMW on interannual time scales, and no long-term trends in heat content and STMW thickness between 2002 and 2011 are detected. Weak lagged correlations among air–sea fluxes, oceanic heat content, and STMW thickness are found when averaged over the northwestern Pacific recirculation gyre region.

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

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.

Divergent Eddy Heat Fluxes in the Kuroshio Extension at 144°–148°E. Part II: Spatiotemporal Variability

Abstract The Kuroshio Extension System Study (KESS) provided 16 months of observations to quantify divergent eddy heat flux (DEHF) from a mesoscale-resolving array of current- and pressure-equipped inverted echo sounders. KESS observations captured a regime shift from a stable to unstable state. There is a distinct difference in the spatial structure of DEHFs between the two regimes. The stable regime had weak downgradient DEHFs. The unstable regime exhibited asymmetry along the mean path with strong downgradient DEHFs upstream of a mean trough at ~147°E. The spatial structure of DEHFs resulted from episodic mesoscale processes. The first 6 months were during the stable regime in which fluxes were associated with eastward-propagating 10–15-day upper meanders. After 6 months, the Kuroshio Extension underwent a regime shift from a stable to unstable state. This regime shift corresponded with a red shift in mesoscale phenomena with the prevalence of ~40-day deep externally generated eddies. DEHF amplitudes more than quadrupled during the unstable regime. Cold-core ring (CCR) formation, CCR–jet interaction, and coupling between ~40-day deep eddies were responsible for asymmetry in downgradient fluxes in the mean maps not observed during the stable regime. The Kuroshio Extension has prominent deep energy associated with externally generated eddies that interact with the jet to drive some of the biggest DEHF events. These eddies play an important role in the variability of the jet through eddy–mean flow interactions. The DEHFs that result from vertical coupling act in accordance with baroclinic instability. The interaction is not growth from an infinitesimal perturbation, but from the start is a finite-amplitude interaction.

Divergent Eddy Heat Fluxes in the Kuroshio Extension at 144°–148°E. Part I: Mean Structure

AbstractThe Kuroshio Extension System Study (KESS) provided 16 months of observations to quantify eddy heat flux (EHF) from a mesoscale-resolving array of current- and pressure-equipped inverted echo sounders (CPIES). The mapped EHF estimates agreed well with point in situ measurements from subsurface current meter moorings. Geostrophic currents determined with the CPIES separate the vertical structure into an equivalent-barotropic internal mode and a nearly depth-independent external mode measured in the deep ocean. As a useful by-product of this decomposition, the divergent EHF (DEHF) arises entirely from the correlation between the external mode and the upper-ocean thermal front. EHFs associated with the internal mode are completely rotational. DEHFs were mostly downgradient and strongest just upstream of a mean trough at ~147°E. The downgradient DEHFs resulted in a mean-to-eddy potential energy conversion rate that peaked midthermocline with a magnitude of 10 × 10−3 cm2 s−3 and a depth-averaged value of 3 × 10−3 cm2 s−3. DEHFs were vertically coherent, with subsurface maxima exceeding 400 kW m−2 near 400-m depth. The subsurface maximum DEHFs occurred near the depth where the quasigeostrophic potential vorticity lateral gradient changes sign from one layer to the next below it. The steering level is deeper than this depth of maximum DEHFs. A downgradient parameterization could be fitted to the DEHF vertical structure with a constant eddy diffusivity κ that had values of 800–1400 m2 s−1 along the mean path. The resulting divergent meridional eddy heat transport across the KESS array was 0.05 PW near 35.25°N, which may account for ~⅓ of the total Pacific meridional heat transport at this latitude.

Near 13 day barotropic ocean response to the atmospheric forcing in the North Pacific

In the Kuroshio Extension System Study (KESS) east of Japan, bottom pressure observations over the 2 year study period exhibit strong high‐frequency variability near 13 days. The first cyclostationary empirical orthogonal function mode for the band‐pass‐filtered KESS bottom pressure explains about 57% of the near 13 day variance and exhibits almost in‐phase variability in space with a hint of westward propagation. The 13 day variability is strong during the winter and is driven by the large‐scale wind stress curl over a broad region of the North Pacific. Modeling results over the North Pacific closely follow the observations and indicate that topography confines the barotropic response to the west of Emperor Seamount Chain and slows the westward propagation of the near 13 day bottom pressure variability.

Evidence of Vertical Coupling between the Kuroshio Extension and Topographically Controlled Deep Eddies

Strong energy in the 30–60 day band was observed using 39 deep pressure and current records from the Kuroshio Extension System Study (KESS). Energy in this band accounted for 25–50% of the total deep-pressure variance and was strongest under the Kuroshio Extension jet axis. Often, deep-pressure anomalies propagated into the region from the north-northeast and locally intensified as they passed under and interacted with the Kuroshio Extension. The topographically controlled deeppressure anomalies translate nearly along lines of constant f/H. Statistically significant coherence between 30–60 day upper- and deep-ocean streamfunction anomalies demonstrated that there was strong vertical coupling in that time band. Twenty-five percent of the total upper-ocean streamfunction variance was contained within the 30–60 day band near the Kuroshio Extension. Joint CEOFs of the upper- and deep-ocean streamfunctions revealed that near the axis of the Kuroshio Extension the phases were laterally offset alongstream, with the deep ocean leading the upper ocean. This arrangement is attributed to producing joint development of upper-ocean meanders and deep-pressure anomalies. A numerical process model simulated the interaction of barotropic TRWs with an eastward-flowing baroclinic jet. When the TRWs, used as a surrogate for topographically steered deep-pressure anomalies, passed under the jet, they intensified and upper-ocean meanders steepened, much like the observed interactions. The model illustrates how the interaction between TRWs and an eastward-flowing jet, at its simplest level, can reproduce many of the major traits of our observations. The Ocean General Circulation Model for the Earth Simulator also showed similar processes in the 30–60 day band in the KESS region. The strongest variance in the deep fields occurred under the Kuroshio Extension. Upper and deep low- and high-pressure anomalies propagated south southwestward across the Kuroshio Extension, with model phase speeds and wavelengths matching the KESS observations.

Evidence of Bottom-Trapped Currents in the Kuroshio Extension Region

Abstract As part of the Kuroshio Extension System Study, observations from five current meter moorings reveal that the abyssal currents are weakly bottom intensified. In the framework of linear quasigeostrophic flow, the best fitted vertical trapping depths range from 8 to 15 km in the absence of steep topography, but one mooring near an isolated seamount exhibited vertical trapping that was more pronounced and energetic with a vertical trapping depth of 5 km. The ratios of current speeds and geostrophic pressure streamfunctions at the sea surface compared to the bottom are 88% in the absence of steep topography, 63% near an isolated seamount, and overall on average 83% of their value at a reference depth of 5300 m. It is hypothesized that weakly depth-dependent eddies impinging upon topographic features introduce to the flow the horizontal length scales of the topography, and these smaller lateral scales are subject to bottom intensification.

Eddy–Mean Flow Interaction in the Kuroshio Extension Region

Abstract The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

Western Boundary Currents and Frontal Air–Sea Interaction: Gulf Stream and Kuroshio Extension

Abstract In the Northern Hemisphere midlatitude western boundary current (WBC) systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean. Their potential contribution to the climate system motivated major parallel field programs in both the North Pacific [Kuroshio Extension System Study (KESS)] and the North Atlantic [Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE)], and preliminary observations and analyses from these programs highlight that complexity. The Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific have broad similarities, as subtropical gyre WBCs, but they also have significant differences, which affect the regional air–sea exchange processes and their larger-scale interactions. The 15-yr satellite altimeter data record, which provides a rich source of information, is combined here with the longer historical record from in situ data to describe and compare the current systems. While many important similarities have been noted on the dynamic and thermodynamic aspects of the time-varying GS and KE, some not-so-subtle differences exist in current variability, mode water properties, and recirculation gyre structure. This paper provides a comprehensive comparison of these two current systems from both dynamical and thermodynamical perspectives with the goal of developing and evaluating hypotheses about the physics underlying the observed differences, and exploring the WBC’s potential to influence midlatitude sea–air interaction. Differences between the GS and KE systems offer opportunities to compare the dominant processes and thereby to advance understanding of their role in the climate system.

Distribution of deep near-inertial waves observed in the Kuroshio Extension

The distribution of deep near-inertial waves (NIWs) is investigated using data mainly from an array of 46 near-bottom acoustic current meter sensors spanning a 600 km × 600 km region as part of the Kuroshio Extension System Study during 2004–2006. The deep NIW distribution is interpreted in the context of both upper-layer and near-bottom mapped circulations. The wintertime-mean mixed-layer NIW energy input, modeled from observed wind stress, has the same range of values north and south of the Kuroshio Extension in this region. Yet, the wintertime-mean deep NIW energy distribution reveals a sharp factor-of-5 decrease from north to south of the Kuroshio jet. This direct observational evidence shows that the Kuroshio Extension blocks the equatorward propagation of NIWs. The NIW energy that does reach the sea floor within the subset of wintertime observations in the subtropical gyre arrives with patchy spatial and temporal distribution. Elevated NIW energy in deep water is associated with anticyclones in the deep barotropic flow and unassociated with upper layer eddies.

Mapping Circulation in the Kuroshio Extension with an Array of Current and Pressure Recording Inverted Echo Sounders

AbstractThe Kuroshio Extension System Study (KESS) aimed to quantify processes governing the variability of and the interaction between the Kuroshio Extension and the recirculation gyre. To meet this goal, a suite of instrumentation, including 43 inverted echo sounders equipped with bottom pressure gauges and current meters [current and pressure recording inverted echo sounders (CPIES)], was deployed. The array was centered on the first quasi-stationary meander crest and trough east of Japan, which is also the region of highest eddy kinetic energy. KESS was the first experiment to deploy a large quantity of these new CPIES instruments, and it was unique in that the instruments were deployed in water depths (5300–6400 m) close to their limit of operation. A comprehensive narrative of the methodology to produce mesoscale-resolving four-dimensional circulation fields of temperature, specific volume anomaly, and velocity from the KESS CPIES array is provided. In addition, an improved technique for removing pressure drift is introduced. Methodology and error estimates were verified with several independent datasets. Temperature error was lowest on the equatorward side of the Kuroshio Extension core and decreased with depth (1.5°C at 300 m, 0.3°C at 600 m, and <0.1°C below 1200 m). Velocity errors were highest in regions of strong eddy kinetic energy, within and south of the jet core. Near the surface, the error in geostrophic velocity between adjacent CPIES was typically 10 cm s−1, decreasing downward to 6 cm s−1 at 500-m depth and 5 cm s−1 below 800 m. The rms differences from pointwise current measurements are nearly twice as large as the geostrophic errors, because the pointwise velocities include submesoscale and ageostrophic contributions.

The Kuroshio Extension and its recirculation gyres

This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of these recirculations gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program (June 2004–June 2006) and include current meters, pressure and current recording inverted echo sounders, and subsurface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114 Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio's transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current's transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.

Deep cyclogenesis by synoptic eddies interacting with a seamount

Strong deep eddies with cyclonic vorticity greater than 0.2f0 were detected using an array of bottom current and pressure measurements in the Kuroshio Extension System Study (KESS) in 2004–2006. Daily maps showed these deep eddies developed locally. As in the Gulf Stream, meandering of the upper baroclinic jet generates deep cyclones and anticyclones by stretching and squashing the lower water column. However, unlike the Gulf Stream, the smaller vertical stretching and greater water depth in the Kuroshio Extension limits the relative vorticity generated by this vertical coupling process to about 0.1f0. In the deep Kuroshio Extension the strong cases of vorticity generation and cyclone development are related to stretching driven when water columns are advected off isolated seamounts in the region. The large observed values of relative vorticity are consistent with a straightforward calculation of deep layer potential vorticity conservation. A barotropic model is used to illustrate the topographic generation of cyclones by ambient currents in synoptic eddies. Positive potential vorticity filaments also develop during the cyclogenetic process with width LR = O(20 km), where LR is the topographic Rhines scale, and travel anticyclonically around the seamount. Observational evidence lends support to the existence of submesoscale filaments, insomuch as current meter records near the flanks of seamounts exhibited bursts of eddy kinetic energy when bandpass-filtered between the inertial period and eight days.

Stream-coordinate structure and variability of the Kuroshio Extension

Fine horizontal-scale surveys performed during the Kuroshio Extension System Study in May 2004 provide near-synoptic ADCP and CTD data along cross-jet transects just up-stream of the first meander trough of the Kuroshio Extension. An array of current- and pressure-recording inverted echo sounders (CPIES) deployed over a ∼ 600 × 600 km region centered on the first meander trough provide time series of bottom pressure and currents as well as acoustic travel time ( τ ), which is converted via the gravest empirical mode method to profiles of temperature, salinity, and specific volume anomaly. These datasets are used here to determine the mean velocity, hydrographic, and potential vorticity structure of the Kuroshio Extension in its “weakly meandering” state in a stream-coordinate system, which avoids the lateral smearing of the jet structure that results from an Eulerian approach. Maximum surface down-stream velocities range from 1 to 2 m/s, averaging around 1.4 m/s. Down-stream velocities reach the bottom south of the core with average magnitudes of 1–5 cm/s but vary in magnitude and direction depending upon the presence of deep barotropic eddies. In the first meander crest, mean cross-stream flux ( O (0.5–2 cm/s)) is towards the cyclonic side, while entering the trough it is towards the anticyclonic side. However, cross-stream flows appear to be event-driven, with fluctuations in steepness of the meander pattern driven by jet instability. Transports above 5300 dbar decrease west to east from 138 Sv at 143 . 75 ∘ E in the first crest, to 124 Sv at the trough and 75 Sv entering the second crest at 148 . 5 ∘ E . The mean from the surveys shows relative vorticity ( ζ ) making contributions as high as 72% of f on the cyclonic side and - 41 % of f on the anticyclonic side, while the “twisting” term due to vertical shear and horizontal density gradients reaches a maximum in the mean of 45% of f north of the core. Comparison of the structure in crests and troughs from the CPIES dataset reveals differences in the distribution of relative vorticity across the core. Both datasets suggest the presence of four isopycnal potential vorticity gradient layers, where strong cross-stream gradients in shallower layers represent a barrier to cross-stream flow. Comparison of the Kuroshio Extension to the Gulf Stream suggests that mean velocities and gradients are generally about 30% weaker in the Kuroshio Extension, and the strong jet structure penetrates to about 25% greater depths in the Gulf Stream.

A comparison of in situ bottom pressure array measurements with GRACE estimates in the Kuroshio Extension

Ocean bottom pressure estimates from Gravity Recovery and Climate Experiment (GRACE) have been validated by comparisons with an array of in situ bottom pressure measurements. The 600 km by 600 km array comprised 46 bottom pressure sensors that were part of the Kuroshio Extension System Study (KESS). Validations in other ocean regions have been limited by available data to pointwise bottom pressure measurements. Spatially‐averaged monthly‐mean bottom pressure over the KESS array is highly correlated with GRACE bottom pressure estimated at the center of the array. The correlations are nearly equally high for three standard choices of spatial smoothing radius applied to GRACE estimates, 300, 500, and 750 km. In contrast, pointwise comparisons between GRACE and individual bottom pressures are high or low in sub‐regions of KESS, depending partially upon the local variance of deep mesoscale eddies whose energetic length scales are shorter than 300 km. KESS is a suitable validation experiment for the GRACE estimates at monthly scales with 300 to 750 km spatial radius of smoothing.

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.

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical‐subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1).The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

Effect of Mesoscale Eddies on Subtropical Mode Water Variability from the Kuroshio Extension System Study (KESS)

Abstract Forty-eight profiling floats have been deployed in the Kuroshio Extension (KE) region since May 2004 as part of the Kuroshio Extension System Study (KESS) project. By combining the float temperature–salinity measurements with satellite altimetry data, this study investigates the role played by mesoscale eddies in controlling the property changes in North Pacific Subtropical Mode Water (STMW). Following a 3-yr period of low eddy activity in 2002–04, the KE showed a transition to a high eddy kinetic energy state in 2005. This transition is the result of delayed oceanic response to the 2002 shift in the basin-scale surface wind forcing in connection with the Pacific decadal oscillation. The high eddy kinetic energy state of the KE is characterized by successive shedding of strong cold-core rings into the recirculation gyre, resulting from the interaction of the KE jet with the Shatsky Rise or the preexisting cutoff rings. By transporting northern-origin, high-potential-vorticity (PV) KE water into the recirculation gyre, the enhanced eddy activity affects STMW in two ways: first, it hinders the formation of deep winter mixed layer (hence the source for STMW) by modifying the upper-ocean stratification and, second, it provides a direct high-PV source to mix with the surrounding low-PV STMW. The eddies’ influence upon STMW is observed to be both significant in magnitude and efficient in time. Relative to 2004, the PV signal in the core of STMW was reduced by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than 7 months. This result supports recent findings by the authors based on historical temperature data that the variability in STMW formation depends more sensitively on the dynamic state of the KE than on the overlying atmospheric conditions.