The Kuroshio-Oyashio Extension (KOE) region has experienced increasingly frequent extensive summer marine heatwaves (MHWs), yet the governing physical mechanisms remain unclear, particularly regarding the role of mixed layer depth (MLD). This study investigates seven major MHW events between 2001 and 2024 using a composite heat budget analysis, providing a generalized basin-scale assessment of dominant drivers beyond single-event perspectives. Anomalies in the surface heat flux term, calculated as the net surface heat flux divided by the MLD, explain approximately 65% of the warming during the development phase, while the same term (34%) and vertical processes (39%) contributed comparably to cooling during the decay phase. MLD variability modulates upper ocean warming through two distinct processes. Shallow MLD increases the anomalous surface heat flux term, and MLD shoaling induces the detrainment effect. These MLD-related contributions (47%) are comparable in magnitude to the surface heat flux anomaly effect (36%). In the decay phase, the surface heat flux anomaly effect (40%) and entrainment associated with MLD deepening (36%) both contribute comparably to the total cooling. Reduced low cloud cover and intensified wind speed played key roles in driving these processes during the development and decay phases, respectively. This study highlights not only the multiple atmospheric and oceanic processes that shape the evolution of extensive summer MHWs in the KOE region, but also the distinct and quantitatively assessed role of MLD, which can exert an influence comparable to that of surface heat flux.

An accurate representation of tidal dynamics is critical for simulating physical and biogeochemical processes in marginal seas, such as the Yellow Sea, where energetic tides and strong seasonal stratification coexist. This study assessed the impact of vertical coordinate systems on the simulation of barotropic and baroclinic tides using Modular Ocean Model version 6, evaluating two configurations: the z* coordinate system (ZSTAR) and hybrid z*-isopycnal coordinate system (HYBRID). The model outputs were validated against satellite-derived sea surface temperatures, in situ temperature profiles, and TPXO tidal harmonics, with a focus on contrasting winter and summer conditions. HYBRID more accurately reproduced sea surface temperatures and vertical thermal structures, particularly during strongly stratified summers, and maintained a sharper and deeper thermocline, as confirmed by long-term temperature diagnostics and age tracer experiments, indicating reduced vertical mixing and improved stratification. For barotropic tides, HYBRID showed better agreement with TPXO for the dominant M2 constituent (RMSE: 24.19 cm; correlation: 0.76) than ZSTAR (RMSE: 32.55 cm; correlation: 0.67). A similar improvement is found for the K1 constituent, further confirming the superior barotropic tidal performance of HYBRID across multiple tidal frequencies. HYBRID also produced stronger barotropic tidal energy fluxes across key regions, yielding 13.5 %–27.6 % larger fluxes in winter and 17.2 %–51.1 % larger fluxes in summer relative to ZSTAR. Baroclinic tidal dynamics exhibited more contrasting model behavior. In winter, ZSTAR produced 19.5 %–36.8 % greater baroclinic kinetic energy (KE). However, in summer, HYBRID simulated consistently stronger baroclinic KE, with 9.2 %–33.6 % larger magnitudes across all regions, reflecting more realistic baroclinic tide generation under strong stratification. Analysis of the baroclinic energy budget further revealed that, although ZSTAR often yielded greater barotropic-to-baroclinic conversion, a substantial portion of this converted energy was locally dissipated rather than radiated, resulting in a much larger residual dissipation term. This indicates that spurious diapycnal mixing in ZSTAR rapidly removes internal-tide energy, degrading propagation and vertical energy transfer. In contrast, HYBRID preserved baroclinic tidal energy more effectively, enabling more coherent energy radiation away from generation hotspots. These results highlight that vertical coordinate design critically influences tidal energetics and stratification-dependent processes in high-resolution regional models. The improved stratification maintenance enabled by HYBRID offers substantial advantages for accurately representing internal-tide dynamics and associated vertical energy pathways in the Yellow Sea.

The Northwest Pacific (NWP) has a complex ocean circulation system and is among the regions most affected by climate change. To facilitate rapid responses to marine incidents and effectively address climate variability impacts, the Korea Institute of Ocean Science and Technology (KIOST) developed the Korea Operational Oceanographic System-Ocean Predictability Experiment for Marine Environment (KOOS-OPEM), a high-resolution regional ocean prediction system based on Modular Ocean Model version 5 (MOM5). In this study, the base model of KOOS-OPEM was upgraded to MOM6 to enhance its regional ocean modeling capabilities. A key advancement of MOM6 is its flexible vertical coordinate system enabled by a Lagrangian remapping system. Taking advantage of this feature, we evaluated the impact of vertical coordinate choices on model performance by comparing the HYBRID (z & lowast;-isopycnal) and ZSTAR (z & lowast;) configurations. Model outputs from the 2003-2012 period were assessed against multiple observational datasets and reanalysis products to determine their ability to reproduce key oceanographic features. The results indicated that HYBRID better preserved stratification and reduced spurious diapycnal mixing, significantly improving the representation of North Pacific Intermediate Water (NPIW). In contrast, ZSTAR exhibited excessive diapycnal mixing, resulting in a thicker isopycnal layer associated with NPIW and a salinity bias of approximately 0.2 psu. An idealized age tracer experiment further confirmed that ZSTAR facilitates excessive downward diffusion of younger surface waters, eroding the minimum salinity layer of the NPIW. In tidal simulations, HYBRID outperformed ZSTAR in reproducing M2 tidal amplitudes in the Yellow Sea, where stratification plays a key role. Conversely, ZSTAR underestimated these amplitudes due to its limitations in representing stratification. Despite its advantages, HYBRID underperformed in high-latitude regions, exhibiting larger temperature and salinity biases between 100 and 600 m depth, with temperature biases reaching approximately -1 degrees C. This discrepancy arose because HYBRID maintained fewer active layers in weakly stratified regions, reducing vertical resolution and leading to errors in water mass representation. To mitigate these issues and improve HYBRID's performance in high-latitude regions, adjustments to target density profiles are necessary. In addition, both configurations showed limitations in simulating winter SST, largely due to insufficient vertical resolution in the surface mixed layer. To address these issues, adopting a finer surface layer resolution (e.g., 1 m instead of 2 m) will further enhance the model's representation of mixed-layer processes.

This study conducted various sensitivity experiments to assess and improve the performance of low-resolution global ocean circulation models. The MOM6 (Modular Ocean Model Version 6), developed by the Geophysical Fluid Dynamics Laboratory, was utilized. We focused on analyzing the effects of implementing the ePBL (energetics based planetary boundary layer) mixed layer scheme, including tidal simulation, and applying hybrid vertical coordinate system on the simulation accuracy of ocean circulation. The results revealed that the ePBL scheme effectively mitigated excessive mixed layer thickness and high temperature biases in the equatorial Pacific, while tidal simulations contributed to improving the oceanic structures in the Yellow Sea and the East Sea. Additionally, the hybrid vertical coordinate system enabled more accurate simulations of the vertical structure of temperature and salinity, enhancing model performance. This study proposes specific approaches to enhance the accuracy of ocean circulation models, contributing to global ocean and climate modeling efforts.

The East/Japan Sea (EJS), a marginal sea of the Northwestern Pacific, is one of the ocean regions showing the most rapid warming and greatest increases in ocean heatwaves over the last several decades. Predictability and skillful prediction of the summer season EJS variability are crucial, given the increasing severity of ocean temperature events impacting fisheries and reinforcing climate conditions like the East Asian rainy season, which in turn affects adjacent high-population density areas over East Asia. We use observations and the Geophysical Fluid Dynamics Laboratory (GFDL) Seamless System for Prediction and Earth System Research (SPEAR) seasonal forecast system to investigate the summertime EJS Sea Surface Temperature (SST) predictability and prediction skill. The observations and seasonal prediction system show that the summer season EJS SST can be closely linked to the previous winter air-sea coupling and predictable 8-9 months in advance. The SPEAR seasonal prediction system demonstrates skillful forecast of EJS SST events from summer to late fall, with added skill for long-lead forecasts initialized in winter. We find that winter large-scale atmospheric circulations linked to Barents Sea variability can induce persistent surface wind anomalies and corresponding northward Ekman heat transport over the East China Sea. The ocean advection anomalies that enter the EJS in prior seasons appear to play a role in developing anomalous SST during summer, along with instantaneous atmospheric forcing, as the source of long-lead predictability. Our findings provide potential applications of large-scale ocean-atmosphere interactions in understanding and predicting seasonal variability of East Asian marginal seas.

The Korea Institute of Ocean Science and Technology developed the Korea Operational Oceanographic System-Ocean Predictability Experiment for Marine Environment (KOOS-OPEM), a high-resolution (1/24°, 51 vertical levels) ocean prediction model for the Northwest Pacific Ocean that incorporates ensemble optimal interpolation. In this study, we present KOOS-OPEM ReAnalysis version 2022 (K-ORA22), which covers the period from 2011 to 2022. We conducted a comparative analysis between K-ORA22 and other high-resolution (1/10°–1/12°) global reanalyses, including the Hybrid Coordinate Ocean Model, Global Ocean Reanalysis and Simulation (GLORYS), and Bluelink ReAnalysis (BRAN), to demonstrate the reproducibility and reliability of regional characteristics. Statistical comparisons revealed that while K-ORA22 exhibited some warm biases, its sea surface temperature (SST) anomaly correlation after removing the seasonal cycle was approximately 0.87, comparable to other reanalyses. Additionally, K-ORA22 effectively reproduced coastal upwelling, which is characterized by a sharp decrease in SST, as observed by marine meteorological buoys in the Southwest of the East/Japan Sea. K-ORA22 exhibits a warm bias of approximately 0.50 °C around 200 m, slightly higher than those of GLORYS and BRAN, while maintaining a low salinity bias in the subsurface. Notably, K-ORA22 outperformed the other reanalyses in accurately reproducing the unique characteristics of North Pacific and East Sea intermediate waters, characterized by a salinity minimum layer. In addition, K-ORA22 stands out in its ability to accurately reproduce the Yellow Sea Cold Water Mass with a low-temperature root-mean-square error (RMSE) of 0.76 °C in the Yellow Sea (YS) region. However, it exhibited the highest RMSE for salinity in the YS region and Korea/Tsushima Strait, indicating a potential overestimation of river discharge from Korea and China. While the sea surface height (SSH) anomaly correlation of K-ORA22 did not surpass 0.80 in the entire region because of limitations in the background error covariance used, its ability to reproduce the Kuroshio path was comparable to those of other reanalysis datasets. In conclusion, K-ORA22 excels in reproducing the unique characteristics of Korean marginal seas. Still, it exhibits weaknesses, such as the overestimation of river discharge and a somewhat limited ability to simulate SSH variability, compared with other global reanalyses. We plan to enhance K-ORA22 by updating background error covariance, addressing biases related to river discharge and assimilating the best available in situ observations and satellite data.

Monitoring and understanding marine heatwaves (MHWs) are essential for assessing the impacts of climate change on oceans and implementing strategies to mitigate their consequences. This study examined the spatial distribution, long-term trends, and temporal variability of MHW characteristics (frequency, duration, intensity, and total number of MHW days) using four satellite-based sea surface temperature (SST) datasets from 1982 to 2021. Despite slight variations, the spatial patterns, trends, and temporal variabilities in the MHW characteristics were consistent across the observational SST datasets. Discrepancies in high-frequency variability among SST datasets may lead to differences in MHW characteristics. This study introduces new proxy indices for annual MHW characteristics, derived from the annual counts of months exceeding specified SST percentiles and the sum of positive SST anomalies beyond these thresholds. The evaluation, conducted in terms of temporal correlation and spatial root-mean-square errors, suggests superior performance in capturing the interannual variability of MHW characteristics over previously proposed proxy indices. The robustness of the new proxy indices is further assessed using an extended analysis period. These findings underscore the potential of these indices for the multi-decadal to centennial scale estimation of MHW characteristics.

This study investigates the variability of coastal currents off the east coast of Korea, specifically in the Wangdolcho area of the East/Japan Sea, from June 2021 to October 2022. The observations revealed significant fluctuations in depth-averaged current velocity, with a peak of 0.81 m/s in August 2021 and a minimum of −0.05 m/s in August 2022. These year-to-year variations highlight the complex dynamics of coastal currents influenced by regional wind patterns and mesoscale eddies. In the summer of 2021, the development of offshore eddies, coupled with variable alongshore wind stress, led to increased current velocities and alternating upwelling and downwelling conditions, resulting in abrupt changes in current intensity. Conversely, in the summer of 2022, the negative vorticity in the Korea Strait and negative wind stress curl along the coast likely caused the East Korean Warm Current to shift further offshore, resulting in a weakened southward flow along the coast. This study emphasizes the need for long-term monitoring to better understand the coastal current dynamics and their environmental impacts. The anticipated completion of the Wangdolcho Ocean Research Station is expected to greatly improve monitoring capabilities, providing continuous and comprehensive data that will enhance our understanding of coastal currents and their broader impacts on the marine environment.

The Ocean Predictability Experiment for Marine environment (OPEM), operated by the Korea Institute of Ocean Science and Technology, provides weekly predictions of ocean conditions as part of the Korea Operational Oceanographic System project. Since March 2017, it has produced three-dimensional analysis fields and 10-day prediction data for the western North Pacific. The OPEM utilizes a data assimilation process based on a highly cost-effective scheme, ensemble optimal interpolation, to integrate ocean observations. Three-dimensional ocean analysis fields from the OPEM were validated using mean biases and root-mean-square differences of ocean temperature, salinity, sea surface height, along with mean and eddy kinetic energies and volume transports. The validation results indicated that the OPEM was comparable to other high-resolution reanalysis products for ocean temperature, salinity, and ocean circulation, though it showed less accuracy in the complex dynamics of the Kuroshio Extension region. The OPEM accurately reproduced the subsurface layer ocean temperature and the surface salinity in the marginal seas around Korea; this was possibly attributed to the local implementation of the OPEM, including the adjustment of the Changjiang freshwater discharge to 2 psu. In addition to validation, the 10-day prediction performance of the OPEM was assessed using case studies. The assessment revealed that the OPEM could simulate the intense air–sea interaction caused by Typhoon Hinnamnor in 2022 for at least three days in advance, while the upwelling driven by coastal winds was predictable more than four days ahead. The assessment results highlight the critical importance of accurate atmospheric forecasts in improving ocean prediction accuracy.

<p>Due to climate change, the occurrence of extreme events such as typhoons, marine heat waves, storminess, and cold waves is increasing in many regions, and these events could dramatically change with significant impacts on the marine environment (e.g., ocean circulation). The East Korea Warm Current (EKWC) has been recognized to flow along the western boundary current of the East Sea (Japan Sea). To examine the variations of EKWC, six bottom-mounted current profiler moorings were operated off the east coast of Korea (the Hupo Bank and Wangdolcho) since June 2021. The observed mean current speed and their principal axis were 0.40 m/s and 58<sup>o </sup>(counterclockwise from the east) implying northeastward EKWC over the region. In August 2021, an unprecedentedly strong surface current was observed with a maximum of 1.89 m/s and observed currents showed similar variability at mooring sites and several depths. This strong current lasted for about a month, and then rapidly disappeared within a few days. At that time, the geostrophic currents based on satellite-altimetry has a strong current pattern with the Inertial Boundary Current pattern, which is one of the EKWC patterns that flows strongly northward currents closer coast. In addition, the highest mean speed of the EKWC near the mooring sites from 1993 was found in August 2021. The high-speed period was similar to the period of the North Pacific marine heat waves that were already reported, and the low-speed period was related to typhoon passage. This study reported the results of observed EKWC for two years from 2021 and the unprecedentedly enhanced EKWC in August 2021. In particular, it can be a case in which the rapid changes of western boundary currents interact with extreme events such as marine heatwaves and typhoons.</p>

This study analyzes the influence of the Pacific–Japan (PJ) atmospheric teleconnection pattern and its interaction with oceanic processes on sea surface warming over the Northwestern Pacific. The PJ pattern is a thermally driven Rossby wave that originates over the tropical western Pacific through deep convection and propagates toward high latitudes. It plays a significant role in sea surface warming by inducing anticyclonic circulation and the corresponding northwestward extension of the subtropical high over the Northwestern Pacific. This study revealed that the key processes responsible for sea surface warming were an increase in insolation and a decrease in the ocean-to-atmosphere latent heat flux under the anticyclonic conditions driven by the PJ. This finding provides valuable insights into the role of atmospheric processes, we refer to it as the “atmospheric pathway”, in the development of East Asian marine heatwaves (MHWs). A detailed understanding of this process will contribute to the prediction and mitigation of MHWs in East Asian countries.

The impacts of observation data sets on the high-resolution (1/24°) Northwest Pacific prediction system were investigated with the model sensitivity tests. We compared the model experiments assimilating the different combinations of the observation data sets, such as the sea surface height derived from satellite altimetry, sea surface temperature, and in-situ profiles, based on the Ensemble Optimal Interpolation. Pseudo-profiles constructed by the method of Cooper and Haines (1996, CH96) were assimilated into the model to assimilate sea surface height data. CH96 applied a conservation principle to derive pseudo-profiles by rearranging preexisting profiles. The comparison of the model experiments suggests that each observation data set enhances the model performance. Especially, the assimilation of the sea surface height reduces the model error by more than 9.81% and 6.44%, respectively, in terms of the root-mean-square error of the ocean temperature and salinity in the subsurface layer. It is interesting that the assimilation of the in-situ temperature profiles in the Korean marginal seas contributes to improving the reproducibility of the subsurface temperature and salinity in the East/Japan Sea (EJS) as well as Kuroshio-Kuroshio Extension (K-KE) regions. The improvement in the K-KE region seems to be related to the reproducibility of the Kuroshio axis. As the water mass in the EJS flows into the Pacific Ocean through the Tsugaru Strait, it affects the front of the Sanriku confluence, and it seems to eventually control the Oyashio Current and Kuroshio axis. In conclusion, this study evaluated the contribution of each observation component to ocean analysis in the KOOS-OPEM and confirmed the role of the existing observation networks. This study also suggests that greater attention should be paid to the role of regional ocean observation networks to improve the forecast skill of the ocean prediction system not only in the region but also in the open ocean, such as the Pacific Ocean.

In this study, we investigate the record-breaking intensification and abrupt weakening of the East Korea Warm Current (EKWC) in the summer of 2021. We analyzed the ocean data assimilation products resolving this event to examine the association between the abrupt changes in the EKWC and various oceanic/atmospheric factors. The results indicate that during the summer of 2021, the EKWC extended northward beyond its climatology, reaching up to 40°N with the maximum speed of 1.16 m s-1 on August 1. In mid-August, the EKWC underwent a rapid weakening, returning to its climatological level. We could attribute the temporal variability in the anomalous EKWC in 2021 to the distinct temporal variability in the dynamic height anomalies between coastal and offshore regions. The offshore variability in the dynamic height anomaly, which is related to warm eddy variability, led to an anomalously increased EKWC velocity (up to 0.59 m s-1) during the EKWC peak velocity period in 2021. However, anomalous coastal downwelling induced by a weak northerly wind anomaly decelerated the EKWC by -0.06 m s-1 in the same period. In mid-August, a typhoon-related northerly wind induced a sudden rise in the coastal dynamic height anomaly, resulting in a rapid weakening of the EKWC. Our findings suggest that changes in geostrophic current related to warm eddies and typhoons have substantially contributed to the temporal variability in the EKWC, improving our understanding of the temporal variability in the western boundary currents.

During July of 2021, the sea surface temperature of the mid-latitude western North Pacific had increased by five degrees over 10 days. This high temperature was maintained for approximately a month before it disappeared rapidly in approximately five days. The underlying mechanisms of this unprecedented marine heatwave event have not yet been researched through a quantitative approach. The development and decay processes of the marine heatwave event were investigated using heat budget analysis and one-dimensional modeling. In mid-July, an anomalous high-pressure atmospheric circulation, affecting to the reduced cloud coverage and increased solar radiation, anchored where the marine heatwave occurred. The increased solar radiation accompanied by the weakened wind reduced the vertical mixing and resulted in a thinner mixed-layer, which accelerated the sea surface warming. The impact of reduced mixing is as important as the increase in solar radiation. In mid-August, typhoon-induced entrainment mainly caused sea surface cooling. The wind-driven mechanical mixing between warm surface water and cooler subsurface water lowered the SST. Additionally, evaporative cooling by strong winds, which drives buoyancy-driven vertical mixing, contributed to the decay of the MHW. The effect of mechanical mixing on cooling is comparable to that of buoyancy-driven mixing.

The East Sea Intermediate Water (ESIW) is the subsurface salinity minimum layer in the depth range 200-400 m in the East/Japan Sea. The origin, variability, and pathways of the ESIW were investigated using a 4-year-long (2015-2018) high-resolution (1/24 degrees) ocean reanalysis and Lagrangian particle modeling. A backward particle tracking analysis revealed that the ESIW was formed south of Vladivostok and shows strong interannual variability. In 2018, owing to strong northwesterly winds, heat loss to the air intensified, and freshwater transport from the northern East/Japan Sea was simultaneously decreased by about 18% compared with the previous year. Furthermore, there was increased formation of water masses heavier than that of the typical ESIW, and the volume of the ESIW was reduced. A forward particle tracking analysis showed that the contribution of the particles originated from the northern end of the East/Japan Sea was about two times greater than those from the southern end. The major portion of the ESIW's freshwater came from the northern East/Japan Sea, where sea ice melted and the Amur River discharged. The remaining portion was from the Changjiang River, which came through the Korea Strait. The ESIW reached the Ulleung Basin over the course of a year mainly through the deep channel, the Ulleung Trough, located to the north of the basin. The diapycnal diffusivity of 1.0 x 10(-4) m(2) s(-1) could erode away the low salinity core in about 27 years.

In our presentation, we will show the performance of a new earth system model developed at the Korea Institute of Ocean Science and Technology (KIOST), called the KIOST-ESM. The KIOST-ESM is based on a low-resolution version of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.5. The main changes made to the base model include using new cumulus convection and ocean mixed layer parameterization schemes, which improve the model fidelity significantly. In addition, the KIOST-ESM adopts dynamic vegetation and new soil respiration schemes in its land model component. The performance of the KIOST-ESM was assessed in pre-industrial and historical simulations that are made as part of its participation into Climate Model Intercomparison Project phase 6. The response of the earth system to increases in greenhouse gas concentrations were analyzed in the ScenarioMIP simulations. The KIOST-ESM exhibited superior performance compared to the base model in terms of the mean sea surface temperature over the Southern Ocean and over the cold tongue in the tropical Pacific. The KIOST-ESM can also simulate the dominant tropical variability in the intraseasonal (Madden-Julian Oscillation) and interannual (El Niño-Southern Oscillation) timescales more realistically than the base model. On the other hand, like many other contemporary ESMs, the KIOST-ESM showed notable cold bias in the Northern Hemisphere, and the so-called double-Intertropical Convergence Zone bias remains. The ScenarioMIP results confirm the global average surface atmospheric temperature responds to the CO2 concentration.

Tropical cyclones (TC), which are among the most destructive natural phenomena on Earth, have significant impacts on the western boundary current (WBC). We quantify the direct impact of TCs on the surface geostrophic velocity of the Kuroshio, the WBC in the western North Pacific, by analyzing satellite‐derived geostrophic current data and using results from theoretical and numerical models. The study reveals that TCs decrease the surface geostrophic velocity by 14 cm s −1 which is 16% of the mean velocity of the Kuroshio (85 cm s −1 ), with the reduced velocity maintained for a month. The decrease in velocity of the Kuroshio is mainly (87%) due to the effect of TC‐driven upwelling (“Cooling effect”), the effect of ocean heat uptake after the TC (“Warming effect”) being minor (13%); these are basically enabled by the strong thermal gradient around the Kuroshio.

The formation mechanism as well as its temporal change of the North Pacific subtropical mode water (NPSTMW) is investigated using a 50-year (1960-2009) ocean general circulation model hindcast. The volume budget analysis suggests that the formation of the NPSTMW is mainly controlled by the air-sea interaction and ocean dynamics, but there is a regime shift of the relative importance between the two around late-1980s. While the local air-sea interaction process is a main driver of the NPSTMW formation prior to late-1980s, ocean dynamics including the vertical entrainment become dominant since then. The NPSTMW formation is affected by the North Pacific Oscillation simultaneously in the early period, but with a few years lag in the later period. The interdecadal change of the driving mechanism of the interannual variability of the NPSTMW is probably due to the stronger (weaker) influence of local atmospheric forcing in the western North Pacific and unfavorable (favorable) wind stress curl condition for the remote oceanic forcing from the central North Pacific during the former (later) period. This regime shift may be related to the change of centers of the actions of the wind stress curl since the late-1980s. Plain Language Summary Using a three-dimensional ocean model, we conduct a numerical simulation for 1960-2009. The ocean model reproduces well the feature of the North Pacific subtropical mode water (NPSTMW), which is characterized by a thick subsurface layer of the relatively uniform temperature and potential density, to the east and south of Japan. During cooling seasons (from December to following March), the NPSTMW is formed by the atmospheric cooling and the oceanic processes, such as the deepening of the mixed layer depth and advective flux. Our study found that the relative contributions from the two factors differ in two different epochs, before and after late-1980s. Before late-1980s, the atmospheric cooling mainly controls the NPSTMW formation, while the oceanic processes driven by remote forcing significantly contribute the NPSTMW formation after late-1980s. The variability of the NPSTMW formation is related to the basin-scale climate variability, particularly the North Pacific Oscillation.

This study focuses on the intercomparisons of 22 net heat flux (NHF) data sets in terms of mean, linear trend, and interannual variability during 1993–2007 over the western North Pacific and its surrounding waters. The data sets can be categorized into the dominant group that shares common interannual variabilities and the outcasts that exhibit exceptional variability. Based on our analysis, we provide guidelines for users to choose reliable NHF data sets in the study regions. In addition, many NHF data sets tend to represent similar interannual variabilities in the marginal sea regions, whereas the opposite is true in open oceans where interannual variability is weak. Also, we find an evidence showing the mean biases of the NHF are mainly due to the difference in calculation of the latent heat flux. The aforementioned information will be useful for a wide community of scientists who are interested in using heat flux data.