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Cusp-shaped fluctuations of the sea surface temperature (SST) front in the tropical Pacific, now known as tropical instability waves (TIWs), were discovered by remote sensing in the 1970s. Their discovery was followed by both theoretical and analytical studies, which, along with in situ observations, identified several possible generation mechanisms. Although modeling studies have shown that TIWs strongly influence the heat budget, their influence on local variations of realistically initialized predictions is not yet understood. We here evaluate a series of medium-range (up to ~ 10 days) coupled atmosphere–ocean predictions by a coupled model with different horizontal resolutions. Observational SST, surface wind stress, heat flux, and pressure data showed that representation of temporally and spatially local variations was improved by resolving fine-scale SST variations around the initialized coarse-scale SST front fluctuations of TIWs. Our study thus demonstrates the advantage of using high-resolution coupled models for medium-range predictions. In addition, analysis of TIW energetics showed two dominant sources of energy to anticyclonic eddies: barotropic instability between equatorial zonal currents and baroclinic instability due to intense density fronts. In turn, the eddy circulation strengthened both instabilities in the resolved simulations. This revealed feedback process refines our understanding of the generation mechanisms of TIWs. [ABSTRACT FROM AUTHOR]

This study aims to identify patterns of surface heat fluxes, and corresponding surface ocean responses, associated with synoptic-scale atmospheric events and their modulation on seasonal time scales. In particular, northerly and southerly wind events associated with atmospheric disturbances were analyzed using high-temporal resolution time-series data from two moored buoys (JKEO: 2007–2010 and KEO: 2004–2019) north and south of the Kuroshio Extension current. Although each synoptic-scale wind event generally impacted both sites, the composite surface heat flux was larger at the northern site, especially for northerly events. Both types of wind events were observed throughout the year, with a minimum during June-July–August. Northerly wind events tended to be accompanied by lowered air-temperature, while southerly events tended to have elevated air-temperature relative to the previous three days. The resulting anomalous surface heat loss was asymmetric, with larger changes in northerly events compared to the southerly events. A large and significant ocean response of − 0.28 to − 0.46 K (p-value < 0.05) in SST was confirmed only for northerly events in spring–summer at the northern site, while smaller changes were found at the southern site. The results of this study suggest that sub-monthly air-sea interactions may affect seasonal variability and potentially climate change over longer timescales. [ABSTRACT FROM AUTHOR]

Insufficient in situ observations in high winds make it difficult to verify climatological data sets and the results of tropical cyclone (TC) simulations. Reliable data sets are necessary for developing numerical models that predict TCs more accurately. This study attempted to compare the third-generation Japanese Ocean Flux Data Sets with Use of Remote-Sensing Observations (J-OFURO3) data, with TC simulations conducted by a 2 km mesh coupled atmosphere-wave-ocean model. This is a case study of Typhoon Dujuan (2015) and the area of approximately 20̊N, 130̊E, south of Okinawa, was selected. The comparison reveals that J-OFURO3 data are reliable for verifying the atmospheric and oceanic components of TC simulations with two different initial sea surface temperature (SST) conditions, although the blank area remains within the inner core area for air temperature, specific humidity, and latent heat flux owing to issues with the construction method. Simulated maximum surface wind speeds (MSWs) are significantly correlated with J-OFURO3 MSWs. The asymmetrical distribution of simulated surface wind speeds within the inner core area can be reproduced well in the J-OFURO3 data set. In terms of the oceanic response to the TC, TC-induced sea surface cooling was reproduced well in the J-OFURO3 data set and is consistent with the simulation results. Unlike simulated SST, simulated surface wind speeds, surface air temperature, and surface specific humidity are still inconsistent with the J-OFURO3 data, even when the J-OFURO3 SST is used as the initial condition. New algorithms, more satellite data used, and model improvement are expected in the future. [ABSTRACT FROM AUTHOR]

In order to evaluate the air-sea sensible and latent heat fluxes (SHF and LHF, respectively) over the Japan Sea obtained from satellite-derived, atmospheric reanalysis, and objective analysis products, we conducted an intercomparison of climatological monthly mean fields and a validation of the daily means with in situ observations. As a result, we found significant differences in the climatological mean fields of SHF in products during the winter. The SHF values from the Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations version 2 (J-OFURO2) showed finer-scale spatial structures, large values near the coastal regions, and small values over the center of the Japan Sea. From a comparison with in situ observations, we concluded that J-OFURO2 SHF has better performance compared with the other global products. The differences in LHF in the products are not so large compared with the results for SHF. However, in a comparison between the satellite-derived products and the others, the representations of the frontal features associated with the sub-polar front and the Tsushima warm current were different. A comparison in the near-coastal region showed that application of the creeping-sea-fill technique used in J-OFURO2 provided appropriate data in the region and succeeded in reducing the adverse effects from land contamination encountered when using coarse atmospheric reanalysis data. [ABSTRACT FROM AUTHOR]

Abstract: Data from the Kuroshio Extension Observatory (KEO) surface mooring are used to analyze the balance of processes affecting the upper ocean heat content and surface mixed layer temperature variations in the Recirculation Gyre (RG) south of the Kuroshio Extension (KE). Cold and dry air blowing across the KE and its warm RG during winter cause very large heat fluxes out of the ocean that result in the erosion of the seasonal thermocline in the RG. Some of this heat is replenished through horizontal heat advection, which may enable the seasonal thermocline to begin restratifying while the net surface heat flux is still acting to cool the upper ocean. Once the surface heat flux begins warming the ocean, restratification occurs rapidly due to the low thermal inertia of the shallow mixed layer depth. Enhanced diffusive mixing below the mixed layer tends to transfer some of the mixed layer heat downward, eroding and potentially modifying sequestered subtropical mode water and even the deeper waters of the main thermocline during winter. Diffusivity at the base of the mixed layer, estimated from the residual of the mixed layer temperature balance, is roughly 3×10−4 m2/s during the summer and up to two orders of magnitude larger during winter. The enhanced diffusivities appear to be due to large inertial shear generated by wind events associated with winter storms and summer tropical cyclones. The diffusivity''s seasonality is likely due to seasonal variations in stratification just below the mixed layer depth, which is large during the summer when the seasonal thermocline is fully developed and low during the winter when the mixed layer extends to the top of the thermocline. [Copyright &y& Elsevier]