Kuroshio-Oyashio Confluence Region Research Articles

Spatiotemporal variations in vertical profiles of Fukushima-derived 137Cs in the Kuroshio-Oyashio confluence region from 2011 to 2018: Implications for local water mass dynamics and basin-scale circulations

Tracking the processes of the spread of Fukushima-derived 137Cs (137CsF) contributes to a better understanding of North Pacific water dynamics. In this study, the vertical distributions of 137Cs and 90Sr in the Kuroshio-Oyashio confluence region were investigated in May 2018, and 137CsF was separated from the background 137Cs by exploiting the constant global fallout 137Cs/90Sr ratio. To the north of 35°N, 137CsF peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), 137CsF exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the 137CsF maxima were distributed mainly within the range of lighter central mode water (L-CMW) during May 2018, even in and just south of the KE. We found that anticyclonic (cyclonic) eddies can promote (prevent) the intrusion of 137CsF into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of 137CsF-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the 137CsF vertical profiles and inventories revealed that 137CsF in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of 137CsF after July 2014, whereas 137CsF in and just south of the KE has remained constant since July 2014, indicating that the 137CsF entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface 137CsF concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of 137CsF to our study area via the western or whole subarctic gyre by May 2018. In contrast, the sea surface height distributions from 2016 to 2017 provide clear evidence that the warm-core rings and quasistationary Isoguchi western jet generated from the Kuroshio Current and KE intruded into the transitional region and even into the subarctic region. Therefore, we concluded that a portion of the 137CsF that subducted into the subtropical western North Pacific during 2011–2012 have entered the transition zone and even the subarctic region since 2016. These results not only enhance our understanding of the protracted spread and fate of 137CsF in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.

Experimental construction of eddy real-time structure based on gradient-dependent OI in the Kuroshio-Oyashio confluence region

The internal structures of mesoscale eddies are variable due to different generation mechanism, spatiotemporal scale, and movement characteristics. Based on the principle of data assimilation, this study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. Three eddy cases with different types and locations were selected to validate the construction results. The reliability of construction was evaluated by satellite observation, in-situ comparisons, and contrast with the numerical output. The results showed that the real-time geostrophic current characters were basically consistent with the surface and deep current (observed by satellite and acoustic doppler current profiler, ADCP). The density features exhibited by constructed results (eddy centers, mean radii, and variation tendency) were similar to the expendable conductivity-temperature-depth (XCTD) survey data and numerical output. Compared to the composite structure, the real-time structures represented more small or mesoscale fluctuations. During eddy cases evolution, the mean radii at the same depth first increased and then decreased. The horizontal scale, influence depth, and internal stability of an anticyclonic eddy with low-latitude were more powerful than the other two eddy cases owing to the domination of warm water in the Kuroshio extension region. The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution.

Application of data-driven mixing parameterization scheme in a regional ocean model

In this study, based on the data-driven parameterization proposed in previous studies, we implemented a data-driven vertical turbulence parameterization scheme (backpropagation neural network, BPNN) into a regional ocean model and compared the simulation results with those obtained using the traditional physics-driven scheme (K-profile parameterization, KPP). The Kuroshio-Oyashio Confluence Region (KOCR) with rich ocean dynamics was selected as the study region. Comparisons of modeled temperature, salinity, and velocity outputs show that the two parameterization schemes produce similar spatial and temporal distribution characteristics. For the sea temperature, the maximum difference appeared at the subsurface layer (3.6 °C) while the maximum mean difference appeared at the surface layer (0.1 °C). Overall, sea temperature in the surface layer simulated by the model adopting the BPNN parameterization scheme was warmer. For the salinity, the maximum difference (0.28 psu) and maximum mean difference (1 × 10−3 psu) appeared in the subsurface layer. In the surface layer, when the model adopting BPNN parameterization scheme overestimated sea temperature compared to the KPP scheme, the result was usually accompanied by overestimated salinity. In the bottom layer, when the model adopting the BPNN parameterization scheme simulated temperature overestimation, the result corresponded to underestimated salinity. For the velocity, the maximum difference (0.31 m/s) and maximum mean difference (-3 × 10−3 m/s) between the two parameterization schemes appeared in the surface layer, and the high values of velocity difference generally occurred in sea areas with energetic dynamic processes. The KPP scheme and the model adopting BPNN parameterization produced similar mixed layer depth (MLD) spatial distribution and ocean fronts spatial distribution, but the latter parameterization scheme simulated an overall shallower mixed layer. For the ocean fronts, the differences between the two parameterization schemes (O∼10−5) is smaller than the intensity of the fronts (O∼10−4). This study presents a new attempt to apply the data-driven parameterization scheme in ocean numerical models.

Application of Gradient-Dependent Optimal Interpolation in Fishery Analysis of Neon Flying Squid (Ommastrephes bartramii) in the Kuroshio–Oyashio Confluence Region

A key issue in fishery forecasting is the collection of high-precision subsurface environmental data. A data assimilation method, named gradient-dependent optimal interpolation, was used to construct the near-real-time vertical temperature and salinity structure of a squid fishery ground based on Argo observations. The results were verified by truth-finding comparisons and applied to analyze the relationship between neon flying squid and the subsurface environment in the Kuroshio–Oyashio Confluence Region. The temperature and salinity differences between the constructed results and survey data were less than ±0.5 °C and ±0.02, respectively. Most of the relative analysis errors were less than the observational errors. Statistical analysis revealed that the most suitable temperature for squid was 18–24 °C at the near-surface (<5 m), although the squid can endure a temperature range from 11 to 12 °C at a depth of 300 m. There was an obvious thermocline in the fishery ground, with a thermocline depth of 65 m and a mean strength of approximately 0.10 °C/m. The regressive relationship between vertical temperature (thermocline parameters) and squid catch per unit effort (CPUE) followed the exponential (Gaussian) function. The most suitable salinity was 33.0–34.2 at depths shallower than 300 m.

Water mass mixing contributes dynamics of dissolved organic matter in Kuroshio-Oyashio confluence region

The Kuroshio-Oyashio confluence region (K/O region) plays a significant role in the global carbon cycle and climate change. In this study, we examined the concentration and composition of dissolved organic matter (DOM) in water samples collected from depths of 0 to 1000 meters between 2019 and 2021. The concentration of dissolved organic carbon (DOC) ranged from 30.54 to 113.21 μmol L-1, with higher concentrations observed in surface waters. The biological index (BIX) decreases with increasing depth, indicating decreasing in-situ production of plankton downwards. Additionally, the humification index (HIX) increased with depth, suggesting the transformation of labile to refractory DOC by bacteria. Using parallel factor analysis, we identified two humic-like (C1, C3) and one protein-like (C2) fluorescent components in 467 water samples. Water samples collected in 2020 exhibited significantly higher specific ultraviolet absorbance at 254 nm (SUVA254) (0.5 ± 0.2 L mg-1 m-1) and higher intensities of C1 (8.2 ± 2.69 *10-3 R.U.) and C3 (9.22 ± 4.39 *10-3 R.U.) compared to samples from 2019 and 2021. We found that water temperature had a positive correlation with C2 intensity (r = 0.33; p< 0.01), but negative correlations with C1 (r = −0.72; p< 0.01) and C3 intensity (r = −0.55; p< 0.01). Comparison of measured and theoretical values using a conservative physical mixing model revealed active biogeochemical processes of DOM during water mixing of the Kuroshio, Oyashio, and North Pacific Intermediate Water that change concentration and composition of DOM. Since the hydrology in the K/O region is sensitive for the ongoing climate warming, further investigation is needed to understand water mass mixing and associated DOM dynamics.

Seasonal modulation of mixed-layer temperature anomaly in Kuroshio–Oyashio confluence region by bimodal Kuroshio extension

Seasonal modulation of mixed-layer temperature anomaly in Kuroshio–Oyashio confluence region by bimodal Kuroshio extension

Optical characterization of dissolved organic matter in Kuroshio-Oyashio confluence region: Implication for water mass mixing

Chromophoric dissolved organic matter (CDOM) and fluorescent DOM (FDOM) are important components of DOM and play many important roles in biogeochemical processes. Here, we examined the concentration, composition, and distribution of DOM in waters (0–1000 m depth) from the Kuroshio-Oyashio confluence region (Northwestern Pacific), a key area for ocean carbon cycle and an important fishery. Dissolved organic carbon concentration varied between 30.5 and 78.2 μmol L−1 with a higher level in surface waters. Two humic-like components (C1, C3) and one protein-like component (C2) were identified by fluorescence excitation emission matrices-parallel factor analysis. The C1 and C3 intensity generally increased with depth, and positively correlated with apparent oxygen utilization (p < 0.01), whereas the C2 intensity decreased downward, and negatively correlated with apparent oxygen utilization (p < 0.01), suggesting in-situ production of bio-refractory C1 and C3 during the microbial transformation of labile organic matter (like C2). The C1 intensity was significantly different among three water masses (p < 0.01) that was 11.7 ± 2.5 × 10−3 R.U. for Kuroshio Current, 20.1 ± 1.9 × 10−3 R.U. for Oyashio Current, and 26.1 ± 5.8 × 10−3 R.U. for North Pacific Intermediate Water. The application of three-endmember mixing model based on the C1 intensity and salinity revealed variable contributions of Kuroshio, Oyashio, and North Pacific Intermediate Water that were consistent with the predication from water temperature and salinity. Our study suggests that the optical property of DOM is a useful tracer for water masses in the Kuroshio-Oyashio confluence region, consisting with previous reports for other marine environments.

Broadband Characteristics of Zooplankton Sound Scattering Layer in the Kuroshio–Oyashio Confluence Region of the Northwest Pacific Ocean in Summer of 2019

Acoustic technology, as an important investigation method for fishery resources, has been widely used in zooplankton surveys. Since the Kuroshio–Oyashio confluence region has an extensive distribution of zooplankton, describing and analyzing the characteristic of the zooplankton sound scattering layer (SSL) in this area is essential for marine ecology research. To understand its spatial–temporal distribution, acoustic data of the Kuroshio–Oyashio confluence region at the Northwest Pacific Ocean, obtained by a Simrad EK80 broadband scientific echosounder in 2019, were used on board the research vessel (RV) Songhang. After noise removal, the volume backscattering strength (SV) was measured to plot the broadband scattering spectrogram of each water layer and to exhibit zooplankton distribution. The results show that the main sound scattering within 0–200 m originate from the zooplankton, and the SV of each layer increases with the rise of the transducer frequency. The magnitude of SV was closely synchronized with the solar altitude angle, which gets smaller when the angle is positive, then larger when the angle is negative. It means that the SSL has a diel vertical migration (DVM) behavior with the variation of solar height. Meanwhile, scattering strength was positively correlated with temperature in the vertical direction and showed a maximum of −54.31 dB at 20–40 m under the influence of the thermocline. The Kuroshio and Oyashio currents had an obvious influence on the scattering strengths in this study, indicating a low value when next to the Oyashio side and a high value on the Kuroshio side. The scattering strength near the warm vortex center was higher than that at the vortex edge. The results of this study could provide references for a long-term study on ecological environment variation and its impacts on zooplankton distribution.

Impacts of Salinity Variation on the Mixed‐Layer Processes and Sea Surface Temperature in the Kuroshio‐Oyashio Confluence Region

AbstractIn this study, salinity variations in the Kuroshio–Oyashio confluence region (KOCR) are examined through analyses of observational datasets and an ocean reanalysis product, and their potential impacts on sea surface temperature are assessed by sensitivity experiments using a one‐dimensional mixed layer model (1‐D ML model). We have detected prominent covariations in near surface temperature and salinity in the KOCR during the boreal winter to spring. Further investigation revealed that such covariations are closely related to the dynamical stability of the Kuroshio Extension (KE), and anomalous warming and salinification (cooling and freshening) are observed in the KOCR when the upstream of the KE is in an unstable (a stable) state. It is found that modulation heat and freshwater transport by mesoscale eddies and large‐scale current anomalies are closely related to such observed variation. Then, we have quantitatively estimated the impacts of these salinity variations on local density by a detailed decomposition of total anomaly fields. Although the total density anomalies are dominated by contributions from temperature, the salinity contribution has sizable magnitude especially in the northern part of the KOCR, where the background temperature is low and the dependence of density on temperature variations is weak. To further quantify the impact of salinity anomalies, we conducted a series of sensitivity experiments utilizing the 1‐D ML model. The results from these experiments revealed that salinity anomalies significantly alter the strength of vertical mixing and eventually lead to differences in sea surface temperature of approximately 1.0°C.

A Case Study of the Thermohaline Fine-scale Structure of the Sub-Arctic Front

ABSTRACT This paper analyzes the characteristics of the thermohaline fine-scale structure of the sub-Arctic front (SAF) in the Kuroshio–Oyashio confluence region using temperature, salinity, and current data obtained from surveys in June 2016. Thermohaline intrusion is found to be prevalent in the sea area of the SAF, occurring predominantly in frontal zones with depths less than 200 m and in North Pacific intermediate water (NPIW) layers with depths greater than 350 m. The intrusion in the SAF zones with depths less than 200 m has a higher intensity but smaller dimensions in the NPIW. According to our analysis, the thermohaline intrusion in the SAF sea zone is mainly attributable to the joint effect of double diffusion and flow instability. At depths of 0–350 m, the thermohaline intrusion is vulnerable to the influences of salt fingering, diffusive convection, and symmetric instability. At depths deeper than 350 m, diffusive convection is significant. In addition, shear instability might be a contributor. Considering that the effect of turbulent mixing is primarily governed by symmetric instability in the SAF sea areas, the flow instability characterized by a dominant contribution from symmetric instabilities might exert a more significant influence on thermohaline intrusion. The more distinct thermohaline staircases at layers deeper than 350 m, which are mainly produced by diffusive convection, have larger vertical scales than the staircases in other layers.

An Index for Depicting the Long-Term Variability of Mesoscale Eddy Activity over the Kuroshio Extension Region

Using high-resolution satellite-derived sea surface temperature (SST) data from September 1981 to December 2015, the present study develops a new index to detect the long-term variation in mesoscale eddy activity over the Kuroshio Extension (KE) region. This eddy activity index (EAI) highlights the strength of eddy-induced poleward heat transport and has obvious advantages over the other existing KE indices in depicting the low-frequency changes in KE eddy activity. An analysis of the EAI shows that over the long term, the KE eddy activity variability presents a significant spectral peak of about 8 years and is not directly modulated by wind-driven oceanic Rossby waves generated in the central North Pacific. When the EAI is positive, the strengthened KE eddy activity significantly enhances the heat release from ocean to atmosphere over the Kuroshio–Oyashio confluence region (KOCR). This induces an anomalous dipole pattern of near-surface baroclinicity over this region that can persist for up to 6 months, favoring a weakened and northward-moving East Asian jet, and vice versa. It is believed that the new EAI will facilitate future studies focusing on the climatic effects of the KE eddy activity variation.

Temperature profiling measurements by sea turtles improve ocean state estimation in the Kuroshio-Oyashio Confluence region

We demonstrate that assimilation of water temperature measurements by sea turtles into an operational ocean nowcast/forecast system improves representation of mesoscale eddies and front variations in the Kuroshio-Oyashio Confluence region. For a period from August to December in 2009, vertical ranges of the measurements by six turtles covered from surface to maximum 150 m depth, and the number of the measurements in the region was comparable to that of usual sampling by research vehicles and profiling floats. Comparison of the turtle measurements and products of an operational ocean nowcast/forecast system JCOPE2M elucidated low-temperature bias in JCOPE2M due to assimilation of high-resolution satellite sea surface temperature data. Additional assimilation of the turtle data into a modified JCOPE2M system with correction of the low-temperature bias led to reasonable modification of horizontal temperature/salinity gradient associated with northward moving warm core rings separating from the Kuroshio Extension and southward moving the Oyashio water intrusion branches. Effects of the turtle data assimilation propagated to far east of the Kuroshio-Oyashio confluence region along the subarctic boundary and front through advection. Another experiment assimilating the in situ profile data excluding the traditional temperature/salinity profiles data but including only the turtle data showed that the turtle measurements captured the warm core rings better than the Oyashio intrusion branches, suggesting possible optimum design of future ocean observing systems composed of different kinds of animals and/or autonomous underwater vehicles.

Mixing characteristics of the subarctic front in the Kuroshio-Oyashio confluence region

Summary This paper analyzes the mixing characteristics of the Subarctic Front (SAF) in the Kuroshio-Oyashio Confluence Region based on temperature, salinity, and current data obtained from surveys and remote sensing in June 2016. The frontal zone of the observed area is at 145°–151°E, 38°–41°N. The front is distributed between 25.5–26.7 σθ in a band pattern inclined from north to south and is deeper in the south. The region shallower than 200 m and distributed along the isopycnal of 25.9–26.1 σθ has the strongest horizontal temperature and salinity gradients, and the largest of the former can reach over 0.7°C/km. Diapycnal mixing of the SAF is mainly turbulent; it is stronger in the north than in the south. The region with stronger turbulence (Kρ > 10−3.5 m2/s) is distributed mainly in water layers within and under the front (26.1–26.7 σθ), showing that the SAF is shallower in the north and deeper in the south along the front. Symmetric instability may be the main factor causing strong turbulent mixing in the frontal zone. Double diffusion mixing is stronger in the south than in the north; the region with stronger double diffusion (Kθ > 10−4.5 m2/s) is distributed mainly in water layers within and above the front (25–26.5 σθ) on the southern side of the SAF. These water layers are dominated mainly by ``salt-fingering’’ double diffusion, with only a few water layers dominated by ``diffusive layering’’ double diffusion mixing in middle and lower waters deeper than 300 m.

Bacterial Diversity and Nitrogen Utilization Strategies in the Upper Layer of the Northwestern Pacific Ocean.

Nitrogen (N) is a primary limiting nutrient for bacterial growth and productivity in the ocean. To better understand bacterial community and their N utilization strategy in different N regimes of the ocean, we examined bacterial diversity, diazotrophic diversity, and N utilization gene expressions in the northwestern Pacific Ocean (NWPO) using a combination of high-throughput sequencing and real-time qPCR methods. 521 and 204 different operational taxonomic units (OTUs) were identified in the 16s rRNA and nifH libraries from nine surface samples. Of the 16s rRNA gene OTUs, 11.9% were observed in all samples while 3.5 and 15.9% were detected only in N-sufficient and N-deficient samples. Proteobacteria, Cyanobacteria and Bacteroidetes dominated the bacterial community. Prochlorococcus and Pseudoalteromonas were the most abundant at the genus level in N-deficient regimes, while SAR86, Synechococcus and SAR92 were predominant in the Kuroshio-Oyashio confluence region. The distribution of the nifH gene presented great divergence among sampling stations: Cyanobacterium_UCYN-A dominated the N-deficient stations, while clusters related to the Alpha-, Beta-, and Gamma-Proteobacteria were abundant in other stations. Temperature was the main factor that determined bacterial community structure and diversity while concentration of NOX-N was significantly correlated with structure and distribution of N2-fixing microorganisms. Expression of the ammonium transporter was much higher than that of urea transporter subunit A (urtA) and ferredoxin-nitrate reductase, while urtA had an increased expression in N-deficient surface water. The predicted ammonium transporter and ammonium assimilation enzymes were most abundant in surface samples while urease and nitrogenase were more abundant in the N-deficient regions. These findings underscore the fact that marine bacteria have evolved diverse N utilization strategies to adapt to different N habitats, and that urea metabolism is of vital ecological importance in N-deficient regimes.

Local atmospheric response to warm mesoscale ocean eddies in the Kuroshio\u2013Oyashio Confluence region

In the extratropical regions, surface winds enhance upward heat release from the ocean to atmosphere, resulting in cold surface ocean: surface ocean temperature is negatively correlated with upward heat flux. However, in the western boundary currents and eddy-rich regions, the warmer surface waters compared to surrounding waters enhance upward heat release–a positive correlation between upward heat release and surface ocean temperature, implying that the ocean drives the atmosphere. The atmospheric response to warm mesoscale ocean eddies with a horizontal extent of a few hundred kilometers remains unclear because of a lack of observations. By conducting regional atmospheric model experiments, we show that, in the Kuroshio–Oyashio Confluence region, wintertime warm eddies heat the marine atmospheric boundary layer (MABL), and accelerate westerly winds in the near-surface atmosphere via the vertical mixing effect, leading to wind convergence around the eastern edge of eddies. The warm-eddy-induced convergence forms local ascending motion where convective precipitation is enhanced, providing diabatic heating to the atmosphere above MABL. Our results indicate that warm eddies affect not only near-surface atmosphere but also free atmosphere, and possibly synoptic atmospheric variability. A detailed understanding of warm eddy–atmosphere interaction is necessary to improve in weather and climate projections.

Response of rapidly developing extratropical cyclones to sea surface temperature variations over the western Kuroshio–Oyashio confluence region

AbstractThe dynamical response of rapidly developing extratropical cyclones to sea surface temperature (SST) variations over the western Kuroshio–Oyashio confluence (WKOC) region was examined by using regional cloud‐resolving simulations. This study specifically highlights an explosive cyclone that occurred in early February 2014 and includes a real SST experiment (CNTL run) and two sensitivity experiments with warm and cool SST anomalies over the WKOC region (warm and cool runs). The results derived from the CNTL run indicated that moisture supply from the ocean was enhanced when the dry air associated with the cold conveyor belt (CCB) overlapped with warm currents. Further, the evaporated moisture contributed substantially to latent heat release over the bent‐back front with the aid of the CCB, leading to cyclone intensification and strengthening of the asymmetric structure around the cyclone's center. Such successive processes were more active in the warm run than in the cool run. The dominance of the zonally asymmetric structure resulted in a difference in sea level pressure around the bent‐back front between the two runs. The WKOC SST variations have the potential to affect strong wind distributions along the CCB through modification of the cyclone's inner system. Additional experiments with two other cyclones showed that the cyclone response to the WKOC SST variations became evident when the CCB north of the cyclone's center overlapped with that region, confirming that the dry nature of the CCB plays an important role in latent heat release by allowing for larger moisture supply from the ocean.

Distant Influence of Kuroshio Eddies on North Pacific Weather Patterns?

High-resolution satellite measurements of surface winds and sea-surface temperature (SST) reveal strong coupling between meso-scale ocean eddies and near-surface atmospheric flow over eddy-rich oceanic regions, such as the Kuroshio and Gulf Stream, highlighting the importance of meso-scale oceanic features in forcing the atmospheric planetary boundary layer (PBL). Here, we present high-resolution regional climate modeling results, supported by observational analyses, demonstrating that meso-scale SST variability, largely confined in the Kuroshio-Oyashio confluence region (KOCR), can further exert a significant distant influence on winter rainfall variability along the U.S. Northern Pacific coast. The presence of meso-scale SST anomalies enhances the diabatic conversion of latent heat energy to transient eddy energy, intensifying winter cyclogenesis via moist baroclinic instability, which in turn leads to an equivalent barotropic downstream anticyclone anomaly with reduced rainfall. The finding points to the potential of improving forecasts of extratropical winter cyclones and storm systems and projections of their response to future climate change, which are known to have major social and economic impacts, by improving the representation of ocean eddy–atmosphere interaction in forecast and climate models.

Influence of SST Anomalies on Winter Turbulent Heat Fluxes in the Eastern Kuroshio–Oyashio Confluence Region

Abstract Variations in the turbulent heat flux (THF; the sum of the sensible and latent heat fluxes) in the eastern Kuroshio–Oyashio confluence region (EKOCR; 36°–40°N, 155°–160°E) were investigated over a period of 27 consecutive winters (December–February) from 1985/86 to 2011/12. The THF was calculated from a 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 fluxes (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 THF over the EKOCR showed low-frequency variations, with larger THF values in the early 2000s and smaller values in the late 1990s and late 2000s. The heat release in the early 2000s was up to ~40% greater than that in the late 1990s and late 2000s. By performing experiments using combinations of daily raw data values and daily climatological data, the relative contributions of SST, surface air specific humidity, surface air temperature, and surface wind speed were quantitatively assessed in determining the THF over the EKOCR. Results showed that SST predominantly determines the THF: large amounts of heat are released during times of positive SST anomalies. By using Argo float (temperature–salinity) profiles of 2003–12 and a satellite altimetry dataset of 1992–2012, it was found that the warm–salty water transported by an occurrence of the Kuroshio bifurcation was responsible for the generation of positive SST anomalies in the EKOCR.

Quasi-Decadal Variation in Intensity of the Western Part of the Winter Subarctic SST Front in the Western North Pacific: The Influence of Kuroshio Extension Path State

Abstract Variations of the subarctic front (SAF), which is extracted as a strong sea surface temperature (SST) gradient in the longitudinal band of 146°–152°E, were investigated over 30 winters (January–March) from 1982 to 2011, using high-spatial-resolution satellite-derived SST dataset and hydrographic observations. The SAF had little meridional movement in this longitudinal band, lying between 40° and 41°N throughout the analysis period. The SAF intensity had a dominant quasi-decadal time scale (about 8 yr); its intensity in the strong phase was up to about 50% higher than in the weak phase. The SAF intensity strongly reflected upper-ocean conditions in the Kuroshio–Oyashio Confluence (KOC) region just south of the SAF. In the strong SAF phase, large positive anomalies in both temperature and salinity were found in the KOC region, not only at the sea surface but also below 300 m, and the water properties in the KOC region were those of the Kuroshio water. The warm, salty water in the KOC region was supplied by mesoscale eddies, which detached northward from the Kuroshio Extension during the unstable state of the Kuroshio Extension path.

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

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.