Publications

Abstract: This paper reports on the strength and structure of recirculation gyres flanking the Kuroshio Extension. In the time-average, recirculation gyres are found flanking the Kuroshio Extension to the north and south of the jet. The observations were taken as part of the Kuroshio Extension System Study (KESS) which had a 2 year field program from June 2004 – June 2006. All of the combined observations from the KESS program; current meters, current and pressure recording inverted echo sounders, sub-surface floats, in addition to a high-resolution numerical model, support the presence of recirculation gyres to the north and south of the Kuroshio Extension jet.

At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyre transport approximately 121 Sv in the stream-coordinate average of the current meters. This represents an almost three-fold increase in the transport 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 existence of recirculation gyres to the north and south of the jet. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.

Abstract: As part of the Kuroshio Extension System Study (KESS), observations were made during April 2004 to July 2006 to identify and quantify the dynamic and thermodynamic processes governing the variability of and the interaction between the Kuroshio Extension and the recirculation gyre to the south. KESS is a collaborative project between U.S. and Japanese scientists. For KESS, a moored array was designed to measure the time-varying density and velocity fields with the 4-D mesoscale resolution required to determine dynamical balances and cross-frontal exchanges of heat, salt, momentum, and potential vorticity. This array was centered on the first quasi-stationary meander trough east of Japan and in the region of highest eddy kinetic energy. The array comprised inverted echo sounders equipped with bottom pressure gauges and current meters (CPIES) and McLane moored profilers (MMPs) equipped with upward looking acoustic Doppler current profilers (ADCPs) and deep current meters (CMs). CTD/shipboard acoustic Doppler current profiler (SADCP) surveys measured the broad-scale density and velocity structure. Argo profiling floats deployed within the recirculation gyre monitored the temporal evolution of the temperature and salinity in the near-surface mixed layer, the subtropical mode water, and the intermediate waters. Surface flux measurements from the Kuroshio Extension Observatory (KEO) buoy contributed to climate studies of the role of the Kuroshio jet and its recirculation gyre. To help understand the connection of the Kuroshio Extension and the recirculation gyre to the atmosphere and climate, surface air-sea flux measurements were conducted (atmospheric soundings). In this report, the CPIES data collected by URI researchers during the field experiment are presented. The collection, processing and calibration of the CPIES are documented. Time series plots of travel time, bottom pressure, temperature and currents are presented. Basic statistics of the hourly data are tabulated.

Abstract: 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. Spatiallyaveraged 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. Citation: Park, J.-H., D. R. Watts, K. A. Donohue, and S. R. Jayne (2008), A comparison of in situ bottom pressure array measurements with GRACE estimates in the Kuroshio Extension, Geophys. Res. Lett., 35, L17601, doi:10.1029/ 2008GL034778.

Abstract: Surface heat fluxes from the Kuroshio Extension Observatory (KEO) buoy are compared with surface heat fluxes from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research reanalysis (NRA1) and NCEP/ Department of Energy reanalysis (NRA2). KEO surface measurements include downward solar and longwave radiation, wind speed and direction, relative humidity, rain rate, and air and sea surface temperature. For solar radiation, NRA2 had better agreement with KEO than NRA1. Both reanalyses underestimated shortwave radiation in summer and slightly overestimated it in winter. Turbulent surface heat fluxes are estimated with the KEO surface data using the Coupled Ocean-Atmosphere Response Experiment (COARE) version 3.0 bulk algorithm. Both NRA1 and NRA2 latent heat flux (LHF) are larger than KEO LHF, consistent with previous studies. However, the comparison shows larger errors than previously thought. Indeed, the latent heat flux bias for NRA1 is 41 W m
2 and for NRA2 is 62 W m
2 (indicating that the bias between NRA1 and NRA2 is 21 W m
2). For latent heat flux, the large bias is caused primarily by the NRA bulk flux algorithm, while the root mean square (RMS) error is caused primarily by errors in the NRA meteorological variables. The combination of the biases for each heat flux is such that total NRA heat transfer from the ocean to the atmosphere is considerably larger than observed by KEO. These results highlight the importance of maintaining in situ observations for monitoring surface heat fluxes in the Kuroshio/Kuroshio Extension regions.

Abstract: As a contribution to the Global Earth Observation System of Systems, the National Oceanic and Atmospheric Administration (NOAA) is developing surface moorings that carry a suite of field-proven and cost-effective sensors to monitor air–sea heat, moisture, and momentum fluxes, carbon dioxide uptake, and upper ocean temperature, salinity, and currents. In June 2004, an NOAA surface mooring, referred to as theKuroshio Extension Observatory (KEO), was deployed in the Kuroshio Extension’s (KE) southern recirculation gyre, approximately 300 nautical miles east of Japan. In 2006, a partnership between NOAA and the Japan Agency for Marine-Earth Science and Technology was formed that deployed a second mooring (referred to as JKEO) north of the KE jet in February 2007. KE is a region of strong currents, typhoons, and winter storms. Designing and maintaining moorings in the KE is a challenging engineering task. All data are publicly available. A subset of the data are telemetered and made available in near real time through the Global Telecommunications System and web-based data distribution systems. Data from these time-series reference sites serve a wide research and operational community and are being used for assessing numerical weather prediction analyses and reanalyses and for quantifying the air–sea interaction in this dynamic region.

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.

Abstract: Mid-depth, time-mean circulation in the western North Pacifc Ocean (28°—45°N, 140°—165°E) is investigated using drift information from the profling 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 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, we find the eddy dissipation effect to be important as well.

Abstract: The weather patterns during periods of anomalous surface fluxes in the Kuroshio recirculation gyre of the western North Pacific are documented. Separate analyses are carried out for the cold season (October– March) when the net surface heat flux is controlled by the combination of the turbulent sensible and latent heat fluxes (Qturb), and for the warm season (May–August) when the net heating is dominated by the net radiative fluxes (Qrad). For analysis of high-frequency (daily to weekly) variations in the fluxes, direct measurements from the Kuroshio Extension Observatory (KEO) for the period June 2004–November 2005 are used to specify flux events. For analysis of interannual variations, these events are selected using NCEP–NCAR reanalysis estimates for Qturb in the cold season, and International Comprehensive Ocean– Atmosphere Data Set (ICOADS) data for cloud fraction, as a proxy for Qrad, in the warm season.

During the cold season, episodic high-frequency flux events are associated with significant anomalies in the east–west sea level pressure gradients, and hence meridional winds and lower-tropospheric air temperature, reflecting the dominance of the atmospheric forcing of the flux variability. On the other hand, interannual variations in Qturb are associated with relatively weak atmospheric circulation anomalies, implying a relatively important role for the ocean. During the warm season, high-frequency fluctuations in the net surface fluxes occur due to a mix of anomalies in Qturb and Qrad. Enhanced cloudiness in the vicinity of KEO, and hence reduced Qrad, tends to occur in association with weak cyclonic disturbances of extratropical origin. A regional atmospheric circulation favoring these types of events also was found for warm seasons that were cloudier on the whole. Results suggest that the ocean’s influence on air–sea fluxes at KEO is manifested mostly on interannual time scales during the cold season.

Abstract: During May to November 2004, 20 profiling floats were deployed in the recirculation gyre (RG) region south of the Kuroshio Extension (KE). With the KE and RG being in a stable state, most of these floats remained within the RG, resulting in a large number of satellite position fixes for estimating the velocities at the parking depth (1500 db) and a large number of temperature/salinity profiles for estimating the geostrophic velocities above the parking depth. The flows of the RG at different depths in the upper 1500 db are found to be approximately parallel to each other, with the speed at the surface (35 cm s^-1) being about three and a half times larger than that at the parking depth (10 cm s^-1). By analyzing the full-depth conductivity-temperature-depth profiles in the RG, in addition to the velocity estimates, we found that the vertical structure of the RG velocity field was dominated by the barotropic and first baroclinic modes. The two modes are combined in such a way that the resulting RG circulates anticyclonically in the same direction from the surface to the bottom and has an anticyclonic bottom velocity on the order of 5 cm s^-1. From the barotropic component (depth-averaged) of the RG velocity field, the RG transport with one standard error is estimated at 101 8 × 10⁶ m³ s^-1.

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 northernorigin, 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.

Abstract: A high-resolution numerical model is used to examine the formation and variability of the North Pacific Subtropical Mode Water (STMW) over a 3-year period. The STMW distribution is found to be highly variable in both space and time, a characteristic often unexplored because of sparse observations or the use of coarse resolution simulations. Its distribution is highly dependent on eddies, and where it was renewed during the previous winter. Although the potential vorticity fluxes associated with down-front winds can be of the same order of magnitude or even greater than the diabatic ones due to air-sea temperature differences, the latter dominate the potential vorticity budget on regional and larger scales. Air-sea fluxes, however, are dominated by a few strong wind events, emphasizing the importance of short time scales in the formation of mode waters. In the Kuroshio Extension region, both advection and mixing play important roles to remove the STMW from the formation region.

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.

Abstract: In situ temperature and altimetrically derived sea surface height data are used to investigate the lowfrequency variations in the formation of the North Pacific Ocean Subtropical Mode Water (STMW) over the past 12 yr. Inside the Kuroshio Extension (KE) recirculation gyre where STMW forms, the dominant signal is characterized by a gradual thinning in the late winter mixed layer depth and in the 16-18C thermostad layer from 1993 to 1999 and a subsequent steady thickening of these features after 2000. This same decadal signal is also seen in the low-potential-vorticity (PV) STMW layer in the interior subtropical gyre south of the recirculation gyre. By analyzing the air-sea flux data from the NCEP-NCAR reanalysis project, little correlation is found between the decadal STMW signal and the year-to-year changes in the cumulative wintertime surface cooling. In contrast, the decadal signal is found to be closely related to variability in the dynamic state of the KE system. Specifically, STMW formation is reduced when the KE path is in a variable state, during which time high regional eddy variability infuses high-PV KE water into the recirculation gyre, increasing the upper-ocean stratification and hindering the development of a deep winter mixed layer. A stable KE path, on the other hand, favors the maintenance of a weak stratification, leading to a deep winter mixed layer and formation of a thick STMW layer. The relative importance of the surface air-sea flux forcing versus the preconditioning stratification in controlling the variations in the late winter mixed layer depth is quantified using both a simple upper-ocean heat conservation model and a bulk mixed layer model. The majority of the variance (~80%) is found to be due to the stratification changes controlled by the dynamic state of the KE system.

Abstract: Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σ_θ ≅ 25.5 kg m^-3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of potential vorticity to be in the range of ~2-5 × 10^-4 m² s^-1. The latitudinal extent of the STMW formation was narrow, extending from 30N to the Kuroshio Extension jet near 35N. South of 30N, STMW did not form locally but was transported from the recirculation gyre by lateral induction.

Abstract: Shipboard radiosonde surveys were conducted during the 2003-04 winter east of Japan to study atmospheric boundary layer (ABL) structure over the Kuroshio Extension. ABL displayed large variations in vertical structure, most of which are attributable to changes in atmospheric surface stability. Where the surface atmosphere was unstable (neutral) as measured by the sea-air temperature difference, surface turbulent heat flux increased (decreased) and a mixed-layer developed (undeveloped) with weakened (intensified) vertical wind shear. A linear regression analysis indicates that ABL height tends to increase by 1km as the sea-air temperature difference increases by 7C or surface turbulent heat flux by 500 Wm^-2. While meridional thermal advection by weather disturbances seems to cause much of atmospheric stability variability during the 43-day surveys, the strong sensitivity of vertical mixing and wind shear to stability is consistent with the observed in-phase covariability of SST and surface wind from satellite on monthly and longer timescales.

Abstract: Twelve years of sea surface height (SSH) data from multiple satellite altimeters are used to investigate the low-frequency changes and the interconnections of the Kuroshio Extension (KE) jet, its southern recirculation gyre, and their mesoscale eddy field. The dominant signal is characterized by the steady weakening of the KE jet/recirculation gyre from 1993 to 1996, followed by a gradual strengthening after 1997. During the weakening period of 1993-96, the KE path migrated southward in general, and this path migration reversed in direction during the strengthening period of the KE jet and recirculation gyre after 1997. By hindcasting the SSH signals using linear vorticity dynamics, it was found that weakening (strengthening) in the KE jet and recirculation gyre is consistent with westward propagation of negative (positive) SSH anomalies generating in the eastern North Pacific and strengthening during their westward propagation. When the KE jet and recirculation gyre were in a weak mode during 1996-2001, the regional eddy kinetic energy level was observed to be higher than when the jet and recirculation gyre were in a strong mode. This negative correlation between the mean flow intensity and the level of regional eddy kinetic energy is found in both the SSH data and the linear vorticity model to result from the migration of the KE jet inflow over the Izu-Ogasawara Ridge. When it is forced southward by the impinging negative SSH anomalies, the KE jet inflow rides over the ridge through a shallow segment, leading to large-amplitude downstream meanders. Impinging of positive SSH anomalies, on the other hand, strengthens the recirculation gyre and forces the inflow northward where it passes through a deep channel, minimizing the path perturbations in the downstream region.

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 165E 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.

Abstract: Decade-long surface meteorological measurements from a Japan Meteorological Agency buoy at 29N, 135E are analyzed to elucidate the surface air-sea flux forcing in the western North Pacific Ocean. Besides the welldefined annual cycles, the observed heat and momentum fluxes are dominated by signals related to synopticscale weather disturbances. The synoptic-scale heat flux signals have a dominant time scale of 3-14 days, whereas the wind stress signals have a scale of 2-8 days. A comparison between the heat fluxes estimated using the buoy measurements and those from the NCEP reanalysis reveals that the daily NCEP product overestimates both the incoming solar radiation at sea surface and the turbulent heat flux amplitude associated with the individual weather events. The rms amplitude of the synoptic-scale net heat flux of the NCEP product is found to be positively biased by 23%. Despite this amplitude bias, the NCEP product captures the timing and relative strength of the synoptic-scale net heat flux forcing very well. A favorable comparison is also found between the daily surface wind stress forcing from the buoy and that from the NCEP product on the synoptic time scales. Using a bulk surface mixed layer model, we find that the synoptic-scale forcing can significantly change the SSTs in spring-summer seasons. The synoptic-scale heat flux-induced SST anomalies have a typical amplitude of 61C, whereas the wind-induced SST anomalies depend on the accumulation of large-amplitude wind events. Excessive accumulation, which occurred, for example, in 1997, can result in unseasonally cold summertime SST anomalies. From both the observations and the model, the frequency spectra for the synoptic-scale SST signals show a clear v22 dependency. While this dependency is consistent with the "white" surface heat flux forcing in the frequency band of 1/100-1/16 days, short-term mixed layer depth changes induced by the synoptic-scale atmospheric forcing are argued to be important in determining the SST spectral shape in the higher-frequency band.