Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Copernicus GmbH  

Abstract. The propagation of equatorial waves is essential for the onset of Atlantic Niño, but diagnosing waves with ocean reanalysis or in situ data remains a challenge. This study uses an ocean reanalysis to diagnose the wave energy transfer route during the 2019 event. The climatological values and the anomaly in 2019 at each grid point are decomposed into the first four baroclinic modes based on their local density profiles. The decomposed geopotential can well reproduce the displacement of the thermocline during the event. Wave energy flux is calculated by means of a group-velocity-based scheme. In addition to detecting wind-forced Kelvin waves and reflected Rossby waves, the wave energy flux reveals another possible energy transfer route along the western boundary, where some off-equatorial wave energy can excite coastally trapped Kelvin waves and transfer back to the equatorial Atlantic. Five transects are selected, across which the passing wave energy fluxes in 2019 are integrated. The results suggest that the Kelvin waves in the third and fourth mode are locally forced, while the wave energy in the second mode is more likely from the off-equatorial region. Therefore, in the autumn of 2019, the second-mode Kelvin waves can deepen the thermocline ahead of other modes from September, serving to precondition the Niño event.

DOI： 10.5194/os-19-1705-2023

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

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.

DOI： 10.1038/s41598-023-41159-5

Web of Science

PubMed

Other Link： https://www.nature.com/articles/s41598-023-41159-5

Publishing type：Research paper (scientific journal)  

DOI： 10.3389/fclim.2023.1253000

Web of Science

Scopus

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2023JC019972

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2022JC019272

Web of Science

Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

The hourly measurements of air-sea interaction parameters made at the Kuroshio Extension Observatory (KEO) buoy since 2004 have produced an extensive dataset that can be used to understand the dynamics of the oceanic mixed-layer under extreme weather conditions. Our analysis of the KEO-buoy measurements reveals the presence of sporadic low-salinity signals (SLSSs) with duration of 1–2 days in the mixed-layer when, for example, tropical cyclones (TCs) approach. Here SLSS events are identified within the buoy data by using a flag which is set to unity when the value of normalized salinity falls below −1 (minus one). Application of this definition to the hourly time series of salinity measured at 10 m depth by the KEO buoy has allowed us to identify 244 SLSS events over the 11-year analysis period. Both winter (January-March) and spring (April-June) seasons show a peak in the composite time series of wind velocity subdiurnal variation (WVDV) near the start time of SLSS events. SLSS events can be terminated by the entrainment of saltier subsurface water parcels to the mixed-layer in response to storms. The discovery of the WVDV peaks following SLSS commencement is the reason the present study has adopted a time scale of 1-day wind velocity analysis. This temporal resolution has enabled us to identify a TC-related delay in the development of the WVDV peaks, which may represent a fundamental characteristic of the influence of TCs on these events.

DOI： 10.3389/fclim.2022.820490

Web of Science

Publisher：Copernicus GmbH  

Abstract. This study develops an ensemble Kalman filter (EnKF)-based regional ocean data assimilation system, in which the local ensemble transform Kalman filter (LETKF) is implemented with the Stony Brook Parallel Ocean Model (sbPOM) version 1.0 to assimilate satellite and in-situ observations at a daily frequency. A series of sensitivity experiments are performed with various settings of the incremental analysis update (IAU) and covariance inflation methods, for which the relaxation-to-prior perturbations and spread (RTPP and RTPS, respectively) and multiplicative inflation (MULT) are considered. We evaluate the geostrophic balance and the analysis accuracy compared with the control experiment in which the IAU and covariance inflation are not applied. The results show that the IAU improves the geostrophic balance, degrades the accuracy, and reduces the ensemble spread, and that the RTPP and RTPS have the opposite effect. The experiment using the combination of the IAU and RTPP results in significant improvement for both balance and accuracy when the RTPP parameter is 0.8–0.9. The combination of the IAU and RTPS improves the balance when the RTPS parameter is ≤ 0.8 and increases the accuracy for the parameter values between 1.0 and 1.1, but the balance and accuracy are not improved significantly at the same time. The experiments with MULT do not demonstrate sufficient skill in maintaining the balance and reproducing the surface flow field regardless of whether the IAU is applied or not. Therefore, the combination of the IAU and RTPP with the parameter of 0.8–0.9 is found to be the best setting for the EnKF-based ocean data assimilation system.

DOI： 10.5194/gmd-2022-40

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：American Meteorological Society  

<title>Abstract</title>
The present study investigates the interannual variability of the tropical Indian Ocean (IO) based on the transfer routes of wave energy in a set of 61-year hindcast experiments using a linear ocean model. To understand the basic feature of the IO Dipole mode, this paper focuses on the 1994 pure positive event. Two sets of westward transfer episodes in the energy flux associated with Rossby waves (RWs) are identified along the equator during 1994. One set represents the same phase speed as the linear theory of equatorial RWs, while the other set is slightly slower than the theoretical phase speed. The first set originates from the reflection of equatorial Kelvin waves at the eastern boundary of the IO. On the other hand, the second set is found to be associated with off-equatorial RWs generated by southeasterly winds in the southeastern IO, which may account for the appearance of the slower group velocity. A combined empirical orthogonal function (EOF) analysis of energy-flux streamfunction and potential reveals the intense westward signals of energy flux are attributed to off-equatorial RWs associated with predominant wind input in the southeastern IO corresponding to the positive IO Dipole event.

DOI： 10.1175/JPO-D-21-0189.1

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Oceanographic Society of Japan  

DOI： 10.5928/kaiyou.30.5_199

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Meteorological Society  

<title>Abstract</title>A model diagnosis for the energy flux of off-equatorial Rossby waves in the atmosphere has previously been done using quasi-geostrophic equations and is singular at the equator. The energy flux of equatorial waves has been separately investigated in previous studies using a space-time spectral analysis or a ray theory. A recent analytical study has derived an exact universal expression for the energy flux which can indicate the direction of the group velocity for linear shallow water waves at all latitudes. This analytical result is extended in the present study to a height-dependent framework for three-dimensional waves in the atmosphere. This is achieved by investigating the classical analytical solution of both equatorial and off-equatorial waves in a Boussinesq fluid. For the horizontal component of the energy flux, the same expression has been obtained between equatorial waves and off-equatorial waves in the height-dependent framework, which is linked to a scalar quantity inverted from the isentropic perturbation of Ertel’s potential vorticity. The expression of the vertical component of the energy flux requires computation of another scalar quantity that may be obtained from the meridional integral of geopotential anomaly in a wavenumber-frequency space. The exact version of the universal expression is explored and illustrated for three-dimensional waves induced by an idealized Madden-Julian Oscillation forcing in a basic model experiment. The zonal and vertical fluxes manifest the energy transfer of both equatorial Kelvin waves and off-equatorial Rossby waves with a smooth transition at around 10°S and around 10°N. The meridional flux of wave energy represents connection between off-equatorial divergence regions and equatorial convergence regions.

DOI： 10.1175/JAS-D-20-0177.1

Web of Science

Scopus

Other Link： https://journals.ametsoc.org/downloadpdf/journals/atsc/aop/JAS-D-20-0177.1/JAS-D-20-0177.1.xml

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Meteorological Society  

<title>Abstract</title>Intraseasonal waves in the tropical Atlantic Ocean have been found to carry prominent energy that affects interannual variability of zonal currents. This study investigates energy transfer and interaction of wind-driven intraseasonal waves using single-layer model experiments. Three sets of wind stress forcing at intraseasonal periods of around 30 days, 50 days and 80 days with a realistic horizontal distribution are employed separately to excite the second baroclinic mode in the tropical Atlantic. A unified scheme for calculating the energy flux, previously approximated and used for the diagnosis of annual Kelvin and Rossby waves, is utilized in the present study in its original form for intraseasonal waves. Zonal velocity anomalies by Kelvin waves dominate the 80-day scenario. Meridional velocity anomalies by Yanai waves dominate the 30-day scenario. In the 50-day scenario, the two waves have comparable magnitudes. The horizontal distribution of wave energy flux is revealed. In the 30-day and 50-day scenarios, a zonally alternating distribution of cross-equatorial wave energy flux is found. By checking an analytical solution excluding Kelvin waves, we confirm that the cross-equatorial flux is caused by the meridional transport of geopotential at the equator. This is attributed to the combination of Kelvin and Yanai waves and leads to the asymmetric distribution of wave energy in the central basin. Coastally-trapped Kelvin waves along the African coast are identified by along-shore energy flux. In the north, the bend of the Guinea coast leads the flux back to the equatorial basin. In the south, the Kelvin waves strengthened by local wind transfer the energy from the equatorial to Angolan regions.

DOI： 10.1175/JPO-D-20-0262.1

Web of Science

Other Link： https://journals.ametsoc.org/downloadpdf/journals/phoc/aop/JPO-D-20-0262.1/JPO-D-20-0262.1.xml

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>A recently developed energy flux diagnosis scheme, which incorporates a smooth connection between the tropical and subtropical zones, is used in the present study to investigate vertically propagating waves in the tropical Indian Ocean (IO) based on the result of a linear, continuously stratified ocean model driven by climatological wind forcing. This extended diagnosis reveals deep-reaching eastward energy fluxes at the equator which develop four times per year and are associated with equatorial Kelvin waves (KWs) generated by semiannual winds. The authors find that the downward transfer of wave energy is particularly deep in the southern Bay of Bengal (SBoB). This downward flux is attributed to off-equatorial Rossby waves and appears four times per year, maximizing its amplitude during November–December. Southwesterly winds in the Arabian Sea intensify eastward energy flux of KWs at mid-depth, which maximizes in amplitude in August. This is contrastive to KW energy flux at the surface which peaks in May. These mid-depth equatorial KW packets subsequently arrive at the eastern boundary of the IO and are diffracted poleward to produce downward energy flux in November and December detected in the SBoB.

DOI： 10.1186/s40645-021-00432-9

Web of Science

Scopus

Other Link： https://link.springer.com/article/10.1186/s40645-021-00432-9/fulltext.html

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Meteorological Society  

<title>Abstract</title>A method is introduced for diagnosing the time evolution of wave energy associated with ENSO from an ocean reanalysis. In the diagnosis, time changes of kinetic and available potential energy are mainly represented by energy inputs caused by surface wind stress and horizontal energy fluxes for each vertically decomposed normal mode. The resulting time evolutions of the wave energy and vertical thermocline displacements in the 1997–1998 and 2014–2016 El Niño events are consistent with our previous knowledge of these events. Further, our result indicated that representation of several vertical modes is necessary to reproduce the broadly distributed downward thermocline displacements in the central to eastern equatorial Pacific, generated by a westerly wind event in the western equatorial Pacific (e.g., in March 1997), that are preconditioning for El Niño development. In addition, we investigated the wave energy budget, including the influence of data assimilation, on the complicated time evolution of equatorial thermocline displacements caused by repeated westerly and easterly wind events during the 2014–2016 El Niño event. Our result suggests that noise from a momentum imbalance near the equator associated with data assimilation, which possibly affected the El Niño prediction failure in 2014, was much reduced by our developed ocean data assimilation system and reanalysis. This study, which provides a new connection between the theoretical works and reanalysis products that use sophisticated systems for synthesizing OGCMs and observations, should be useful for climate research and operational communities interested in ENSO.

DOI： 10.1175/JCLI-D-20-0704.1

Web of Science

Other Link： https://journals.ametsoc.org/downloadpdf/journals/clim/aop/JCLI-D-20-0704.1/JCLI-D-20-0704.1.xml

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Meteorological Society of Japan  

DOI： 10.2151/sola.17A-003

Web of Science

Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

A quasi-global eddying ocean hindcast simulation using a new version of our model, called OFES2 (Ocean General Circulation Model for the Earth Simulator version 2), was conducted to overcome several issues with unrealistic properties in its previous version, OFES. This paper describes the model and the simulated oceanic fields in OFES2 compared with OFES and also observed data. OFES2 includes a sea-ice model and a tidal mixing scheme, is forced by a newly created surface atmospheric dataset called JRA55-do, and simulated the oceanic fields from 1958 to 2016. We found several improvements in OFES2 over OFES: smaller biases in the global sea surface temperature and sea surface salinity as well as the water mass properties in the Indonesian and Arabian seas. The time series of the Nino3.4 and Indian Ocean Dipole (IOD) indexes are somewhat better in OFES2 than in OFES. Unlike the previous version, OFES2 reproduces more realistic anomalously low sea surface temperatures during a positive IOD event. One possible cause of these improvements in El Nino and IOD events is the replacement of the atmospheric dataset. On the other hand, several issues remained unrealistic, such as the pathways of the Kuroshio and Gulf Stream and the unrealistic spreading of salty Mediterranean overflow. Given the worldwide use of the previous version and the improvements presented here, the output from OFES2 will be useful in studying various oceanic phenomena with broad spatiotemporal scales.

DOI： 10.5194/gmd-13-3319-2020

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

This study investigates the horizontal distribution of wave energy flux in the tropical Atlantic Ocean using shallow-water model experiments for three gravest baroclinic modes forced by climatological winds. This is the first attempt to apply a unified diagnostic scheme to the analysis of energy fluxes associated with both Rossby waves (RWs) and Kelvin waves (KWs) in the tropical Atlantic Ocean. Those analyses were difficult in previous studies owing to the difference of equatorial and quasigeostrophic dynamics. The scheme yields the transfer route (i.e., trajectories by group velocity vector) of wave energy originating from both local wind forcing and boundary reflection. The meridional flux of wave energy crossing two zonal sections (at 3 degrees N and at 2 degrees S) is examined for understanding where the equatorial and off-equatorial regions interconnect. Equatorward energy fluxes into the basin interior are found mainly on the zonal section in the Northern Hemisphere (at 3 degrees N), indicating the radiation of RWs from the Guinea coast. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both RWs and KWs. The energy flux of these RWs arrives at the western boundary in September, followed by reflection of KWs to yield eastward transbasin energy fluxes in October-December. The analysis also identifies an unexpected wind input in May at 4 degrees S which induces both RWs and inertial gravity waves.Plain Language Summary Propagation, diffraction, and reflection of large-scale waves play an important role in tropical climate variations. Horizontal energy fluxes associated with Rossby and Kelvin wave interaction have been little investigated owing to the lack of an appropriate diagnostic scheme in previous studies. A recent study has developed a diagnostic scheme which can reveal the trajectory of the group velocity of both Rossby and Kelvin waves with satisfying coastal boundary conditions. This scheme has been used in the present study to investigate energetics in the output of climatological model experiments. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both Rossby and Kelvin waves. The energy flux of these Rossby waves arrives at the western boundary in September, followed by reflection of Kelvin waves to yield eastward transbasin energy fluxes in October-December. The unified scheme may be used to understand the impact of tropical climate variations to off-equatorial regions and vice versa in a future study.

DOI： 10.1029/2019JC015407

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

In previous studies, the role of large-scale waves in tropical-extratropical interactions has been little investigated in terms of wave energy transfer owing to the lack of appropriate diagnostic techniques. Using a new seamless scheme, the present study identifies a series of well-defined horizontal circulation routes of wave energy in the Indian Ocean on an annual basis. One of these features represents a localized cyclonic circulation close to the western boundary in each hemisphere while the other manifests as a basin-scale cyclonic circulation in the eastern Indian Ocean. For the second baroclinic mode, eastward energy fluxes associated with equatorial Kelvin waves appear when zonal velocity anomaly exhibits eastward and westward peaks (four times per year) in response to the monsoon-wind cycle. Identification of these key pathways of wave energy circulation enables us to better understand how waves in the tropical and subtropical regions interconnect at the zonal boundaries of the Indian Ocean.

DOI： 10.1029/2019GL085670

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

This study makes a first attempt to apply the diagnostic scheme of Aiki et al. (2017) to the output of an ocean general circulation model (OGCM), in order to investigate the basin-wide pathway of equatorial and mid-latitude wave energy associated with intraseasonal variability in the Indian Ocean. The vertical mode decomposition shows that 90-day variability of the second baroclinic mode is dominant in a realistic OGCM experiment. For 90-day equatorial Kelvin wave (KW) and Rossby wave (RW), energy input by wind stress appears in the eastern equatorial Indian Ocean that is then transferred eastward by KWs along the eastern equatorial waveguide (while westward as RW off equator). For 30-day Mixed-Rossby Gravity waves (MRG), wave energy is transferred eastward while wave phase propagates westward that is consistent with the dispersion relationship of low-frequency MRG. The new diagnostic scheme is able to show, particularly for 30-day MRG, eastward energy fluxes along the equatorial waveguide, while the other schemes in previous studies (e.g. pressure flux, quasi-geostrophic flux) cannot show the direction of the group velocity of equatorial waves.

DOI： 10.2151/sola.2019-047

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

From 16 to 23 August 2016, typhoons T1607, T1609, and T1611 hit eastern Hokkaido in northern Japan and caused heavy rainfall that resulted in severe disasters. To understand future changes in typhoon-related precipitation (TRP) in midlatitude regions, climate change experiments on these three typhoons were conducted using a high-resolution three-dimensional atmosphere-ocean coupled regional model in current and pseudo-global warming (PGW) climates. All PGW simulations projected decreases in precipitation frequency with an increased frequency of strong TRP and decreased frequency of weak TRP in eastern Hokkaido. In the current climate, snow-dominant precipitation systems start to cause precipitation in eastern Hokkaido about 24 hours before landfall. In the PGW climate, increases in convective available potential energy (CAPE) developed tall and intense updrafts and the snow-dominant precipitation systems turned to have more convective property with less snow mixing ratio (QS). Decreased QS reduced precipitation area, although strong precipitation increased or remained almost the same. Only TRP of T1607 increased the amounts before landfall. In contrast, all typhoons projected to increase TRP amount associated with landfall, because in addition to increased CAPE, the PGW typhoon and thereby its circulations intensified, and a large amount of rain was produced in the core region.

DOI： 10.2151/sola.2019-044

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

Recent extreme weather events like the August 2016 flood disaster have significantly affected farmland in mid-latitude regions like the Tokachi River (TR) watershed, the most productive farmland in Japan. The August 2016 flood disaster was caused by multiple typhoons that occurred in the span of two weeks and dealt catastrophic damage to agricultural land. This disaster was the focus of our flood model simulations. For the hydrological model input, the rainfall data with 0.04 degrees grid space and an hourly interval were provided by a regional climate model (RCM) during the period of multiple typhoon occurrences. The high-resolution data can take account of the geographic effects, hardly reproduced by ordinary RCMs. The rainfall data drove a conceptual, distributed rainfall-runoff model, embedded in the integrated flood analysis system. The rainfall-runoff model provided discharges along rivers over the TR watershed. The RCM also provided future rainfall data with pseudo-global warming climate, assuming that the August 2016 disaster could reoccur again in the late 21st century. The future rainfall data were used to conduct a future flood simulation. With bias corrections, current and future flood simulations showed the potential inundated areas along riverbanks based on flood risk levels. The crop field-based agricultural losses in both simulations were estimated. The future cost may be two to three times higher as indicated by slightly higher simulated future discharge peaks in tributaries.

DOI： 10.3390/w11112421

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Detailed mechanisms of reinforcement/relaxation of a sea surface salinity front, i.e. frontogenesis/frontolysis, in the northeastern subtropical Pacific region are investigated using observational datasets. Throughout the year, meridional shears of zonal wind induce meridional ageostrophic convergence around the front. Saltier water is advected on the southern side than the northern side, and thus the convergence contributes to frontogenesis. The evaporation and precipitation gradients also strengthen the front, because higher sea surface temperatures on the southern side induce stronger evaporation through the formation of higher surface saturated specific humidity, and because the precipitation rate is low in the southern region in association with the subtropical high of the atmosphere. However, in summer-autumn when the mixed layer is seasonally thin and evaporation exceeds precipitation, the frontogenesis by the freshwater flux gradient is damped by the mixed layer depth (MLD) gradient. This is attributed to the thicker mixed layer south of the front being less sensitive to freshwater fluxes. During the mixed-layer deepening phase, the mixed layer on the southern side entrains lower salinity water from the lower layer, and thus the entrainment relaxes the front. Therefore, it is shown that the gradients of ageostrophic advection, evaporation, and precipitation result in frontogenesis, while those of MLD and entrainment cause frontolysis. Furthermore, a metric that quantifies the relative importance of the horizontal gradients of freshwater fluxes and MLD for salinity frontogenesis/frontolysis is proposed. Over the large domain, contribution from the horizontal gradient of freshwater fluxes dominates over that of MLD.

DOI： 10.1007/s00382-019-04907-w

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Frontolysis mechanisms by which surface heat flux relaxes the sea surface temperature (SST) front in the eastern Japan Sea (JS) are investigated in detail using observational datasets. On the warm southern side of the front, larger air-sea specific humidity and temperature differences induce stronger turbulent heat release compared to the cool northern side. As a result, stronger wintertime cooling and weaker summertime warming occur south of the front, and the meridional gradient in the surface net heat flux (NHF) tends to relax the SST front throughout the year. In the mixed-layer deepening phase (September-January), a higher entrainment velocity occurs on the warm southern side because of weaker stratification. Since the resulting thicker mixed layer on the southern side is less sensitive to surface cooling, the mixed layer depth (MLD) gradient damps the frontolysis by the NHF gradient. In the shoaling phase (April-June), a deeper mixed layer south of the front is caused by the weaker warming and reduced sensitivity of the thicker mixed layer to a shoaling effect by shortwave radiation. Owing to weaker sensitivity of the thicker mixed layer on the southern side to surface warming, the MLD gradient enhances the frontolysis by the NHF gradient. Therefore, it is shown that the mixed layer processes cause seasonality of weaker (stronger) frontolysisby surface heat fluxes, damping (enhancing) the frontolysis by the NHF gradient in winter (summer). This study reveals unique features of the frontolysis in the eastern JS compared with the Agulhas Return Current and Kuroshio Extension regions.

DOI： 10.1007/s10872-018-0502-0

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

©2018. American Geophysical Union. All Rights Reserved. A lower trophic ecosystem model coupled with a one-dimensional physical turbulence closure model was applied to study phytoplankton dynamics and spring bloom initiation in mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs). The model simulated ecosystem dynamics between nutrient-phytoplankton-zooplankton-detritus in AEs and CEs, while the physical model provided the seasonal cycle of convective turbulent mixing. The study was motivated by earlier work based on satellite and ship observations, which showed earlier initiation of the spring blooms in CEs with shallow mixed-layer depths than in AEs with deeper mixed-layer depths. The model results supported the hypothesis that mixed-layer depths in eddies play an important role in the dynamics of the spring bloom initiation. Model results revealed that in AEs convective mixing causes light limitation for phytoplankton growth due to deep winter mixing, and the bloom initiation is delayed until relaxation of turbulent convective mixing. Conversely, in the shallow mixed-layer CEs, blooms initiate before the end of convective mixing due to early improvement in light conditions following the increase in solar radiation. Furthermore, the model showed that the relaxation in zooplankton grazing for the deep mixing contributed to weak winter phytoplankton accumulation in AE, while winter phytoplankton accumulation was faster in the shallow mixed-layer CE. Overall, the initiation mechanism and the dynamics of the spring phytoplankton blooms are different for AEs and CEs. Therefore, we suggest that, in many parts of the global ocean, eddies play an important role regulating the dynamics of phytoplankton blooms.

DOI： 10.1029/2018JC014089

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGEROPEN  

In 2014, a sediment trap mooring was deployed adjacent to station Kuroshio Extension Observatory (KEO)'s National Ocean and Atmosphere Administration (NOAA) surface mooring. These data, from July 2014 to July 2016, are used here to investigate nutrient supply mechanisms that support ocean productivity in this oligotrophic region of the subtropical western North Pacific. Both years of sediment trap data show that biogenic material fluxes at similar to 5000 m increased between late winter (March) and late spring (June). Based on sea surface temperature and upper ocean water temperature profiles, from the NOAA surface buoy, and satellite-based surface chlorophyll-alpha, this increase was likely due to an increase of ocean productivity in early spring (March) that was supported by nutrients supplied by winter mixing. On the other hand, biogenic material fluxes also increased in October 2014, and between late December 2014 and January 2015 when concentrations of nutrients near the surface typically are low. Sea surface height anomalies and vertical profiles of water temperature in the upper 500 m showed cyclonic eddies passing station KEO and causing upwelling in late July-early August 2014 and November 2014. It appears that these events supplied nutrients to the upper layer, which then caused ocean productivity in the subsurface layer to temporally increase, resulting in increased deep biogenic material fluxes in autumn and winter. This interpretation of the data is consistent with a simple 3D physical- biological model simulation that shows meso-scale cyclonic eddies can supply nutrient to support new production at KEO. During the 2-year-long sediment trap deployments, several typhoons also passed near station KEO and near-inertial internal waves were observed near the nitracline depth after the typhoons passed. Although turbulent mixing caused by near-inertial internal wave could have possibly supplied nutrient to upper oligotrophic euphotic layer, numerical simulations of the turbulent nutrient supply indicate that enhanced turbulent diffusion across the nutrient concentration gradient did not supply enough nitrate to support the increase in biogenic material flux in autumn.

DOI： 10.1186/s40645-018-0196-3

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Typhoon Megi (2010), a very intense tropical cyclone with a minimum central pressure of 885 hPa, was characterized by especially rapid intensification. We investigated this intensification process by a simulation experiment using a high-resolution (0.02 degrees x0.02 degrees) three-dimensional atmosphere-ocean coupled regional model. We also performed a sensitivity experiment with a time-fixed sea surface temperature (SST). The coupled model successfully simulated the minimum central pressure of Typhoon Megi, whereas the fixed SST experiment simulated an excessively low minimum central pressure of 839 hPa. The simulation results also showed a close relationship between the radial SST profiles and the rapid intensification process. Because the warm sea increased near-surface water vapor and hence the convective available potential energy, the high SST in the eye region facilitated tall and intense updrafts inside the radius of maximum wind speed and led to the start of rapid intensification. In contrast, high SST outside this radius induced local secondary updrafts that inhibited rapid intensification even if the mean SST in the core region exceeded 29.0 degrees C. These secondary updrafts moved inward and eventually merged with the primary eyewall updrafts. Then the storm intensified rapidly when the high SST appeared in the eye region. Thus, the changes in the local SST pattern around the storm center strongly affected the rapid intensification process by modulating the radial structure of core convection. Our results also show that the use of a high-resolution three-dimensional atmosphere-ocean coupled model offers promise for improving intensity forecasts of tropical cyclones.

DOI： 10.1002/2017JD027252

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Satellite Chlorophyll a (CHL) data were used to investigate the influence of mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs) on the timing of spring phytoplankton bloom initiation around the Yamato Basin (133-139 degrees E and 35-39.5 degrees N) in the Japan Sea, for the period 2002-2011. The results showed significant differences between AEs and CEs in the timing and initiation mechanism of the spring phytoplankton bloom. Blooms were initiated earlier in CEs which were characterized by shallow mixed-layer depths (&lt; 100 m). The early blooming preceded the end of winter cooling (i.e., while net heat flux (Q(0)) is still negative) and is initiated by the increased average light within the shallow mixed-layer depth. Conversely, blooms appeared in the AEs despite deeper mixed-layer depth (&gt; 100 m) but close to the commencement of positive Q(0). This suggests that the relaxation of turbulent mixing is crucial for the bloom initiation in AEs.

DOI： 10.1002/2017GL074359

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER METEOROLOGICAL SOC  

Several equivalent equations for the evolution of the wave-averaged current momentum have been proposed, implemented, and used. In contrast, the equation for the total momentum, which is the sum of the current and wave momenta, has not been widely used because it requires a less practical wave forcing. In an update on previous derivations, Mellor proposed a new formulation of the wave forcing for the total momentum equation. Here, the authors show that this derivation misses a leading-order term that has a zero depth-integrated value. Corrected for this omission, the wave forcing is equivalent to that in the first paper by Mellor. When this wave forcing effect on the currents is approximated it leads to an inconsistency. This study finally repeats and clarifies that the vertical integration of several various forms of the current-only momentum equations are consistent with the known depth-integrated equations for the mean flow momentum obtained by subtracting the wave momentum equation from the total momentum equation. Several other claims in prior Mellor manuscripts are discussed.

DOI： 10.1175/JPO-D-17-0065.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

In August 2016, eastern Hokkaido in northern Japan had unusual typhoon landfalls and experienced heavy rainfall events that caused severe disasters. To understand the impact of global warming on typhoon-related rainfall in such midlatitude regions, numerical experiments on one of the typhoons in August 2016, Typhoon Chanthu, were conducted by using a high-resolution three-dimensional atmosphere-ocean coupled regional model in current and pseudo-global warming (PGW) climates. The amount, intensity, and duration of rainfall in eastern Hokkaido associated with the typhoon increased in the warming climate. The PGW typhoon traveled northward with relatively slower translation speed and resulted in a delay in the landfalls for 6 h. Furthermore, large amounts of near-surface water vapor > 22 g kg(-1) from the southern sea increased the convective instability around eastern Hokkaido and caused tall and intense updrafts. As a result, significant predecessor rainfall events with intense rainfall developed about 24 h before the typhoon landfall. Increased near-surface water vapor in the warming climate also enhanced rainfall associated with the typhoon passage over a widespread area. These results suggest that attention should be paid to future enhancement of heavy rainfall events in the midlatitude regions under global warming.

DOI： 10.2151/sola.2017-045

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/JAS-D-14-0172.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/JPO-D-12-0228.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s10236-013-0658-5

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1017/jfm.2013.369

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/JPO-D-12-059.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/JPO-D-11-095.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s10236-011-0382-y

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1175/2008JPO3820.1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.csr.2006.02.017

Web of Science