Kuroshio Extension Region Research Articles

Search and tracking strategy of autonomous surface underwater vehicle in oceanic eddies based on deep reinforcement learning

Due to dynamic changes and instability of oceanic eddies, continuous tracking and sampling using mobile platforms is a challenging field. Aiming at the requirements of accurate observation of mesoscale eddies, this paper studies the problem of searching and tracking the eddy center in a mapless environment with an underactuated autonomous surface underwater vehicle (ASUV) and proposes a path planning method based on the deep reinforcement learning (DRL). Firstly, the existing observation methods are summarized, and the dynamic tracking framework of mesoscale eddies is established. Then, the DRL and long short-term memory (LSTM) are combined to train end-to-end and real-time planning strategies in an eddy environment. Finally, a high-fidelity simulation platform in the eddy environment is built, and actual data from the Kuroshio Extension region is used to verify the effectiveness and feasibility of the strategies. The experimental results show that the ASUV with the proposed strategies can realize the autonomous search and tracking of the eddy center, have good stability and real-time performance, and the tracking error is always within an acceptable range.

The Three-Dimensional Structure of the Mesoscale Eddy in the Kuroshio Extension Region Obtained from Three Datasets

The high-resolution observation data from June 2019, Argo data from 1997 to 2019, and the multi-observation ARMOR 3D dataset from 1993 to 2019 were used to study the three-dimensional (3D) structural characteristics of the mesoscale cyclonic eddy (CE) in the Kuroshio Extension region (KER). The observed eddy has a typical 3D structure of the KER CE, which was a longer lifespan eddy in this KER. The maximum anomalies of temperature and salinity were −7.69 °C and −0.71 PSU, which were located at the 350 m depth. In the vertical, the observed and composite eddy had a dipole structure, while ARMOR 3D had a monopole. The study of the velocity fields indicate that ARMOR 3D underestimates the velocity below 500 m. The 3D structures of the CE composite eddy of Argo were comparable to the observations, whereas the temperature and salinity anomalies were weaker than the observation. The surface of the Argo composite eddy shows a positive temperature anomaly within 50 m, which used to be opposite to the observation. This phenomenon was due to the limited Argo data of the composite eddy, and most of them were the observed profiles of winter CE in the weak years of EKE in the KER. We tried using ARMOR 3D to explore the reliability of ARMOR 3D composite eddy and compared the seasonal variations of temperature/salinity anomalies of the cyclonic and anticyclonic eddy. The anomalies of temperature and salinity caused by CE have seasonal variations: the anomalies have been strong in summer and weak in winter. This is consistent with the variant of eddy kinetic energy (EKE), but AE has no seasonal variation.

Responses of East Asian Climate to SST Anomalies in the Kuroshio Extension Region during Boreal Autumn

Abstract Sea surface temperature anomalies (SSTAs) in the Kuroshio Extension (KE) region play a key role in influencing midlatitude climate variations. This study investigates the impacts of SSTAs in the KE region (KE-SSTA) on East Asian climate during boreal autumn. The results reveal that positive KE-SSTA, changing the meridional temperature gradient in the lower troposphere, contributes to the formation of an anomalous quasi-barotropic anticyclonic circulation (AC) over the KE region. Such a configuration tends to be affected by wind through thermal wind adjustment. Simultaneously, it is also impacted by eddy activity related to the strengthened atmospheric baroclinicity through transient eddy feedback. By both means, obvious descending motion and warm advection are generated, which increases tropospheric temperature through adiabatic heating and further maintains the abnormal AC over the KE region. This anomalous circulation changes wind in the east of the East Asian trough, which favors the transportation of warm and wet air from the midlatitude northwest Pacific and further causes the increase of temperature in northeastern East Asia. Moreover, it also leads to the convergence and divergence of water vapor fluxes in Northeast Asia and eastern North China, favoring the increase and decrease of precipitation in the two regions, respectively. The above responsive characteristics of the midlatitude atmospheric circulation to positive KE-SSTA can be confirmed by numerical experiments. These results suggest that anomalous SST in the KE region can be used as a potentially effective predictability source for autumn climate variations in the mid- to high latitudes of East Asia. Significance Statement Anomalous variations of sea surface temperature (SST) in the Kuroshio Extension (KE) region play an essential role in influencing midlatitude climate variations. Considering the importance of autumn climate in East Asia to agricultural production, this study aims to explore the relationship between the East Asian climate and autumn SST anomaly in the KE region (KE-SSTA), and to understand how KE-SSTA effects East Asian climate during boreal autumn. Our results suggest that KE-SSTA can be used as one of the effective predictors of East Asian climate, which will contribute to the improvement of short-term climate prediction levels over East Asia.

Benthic nitrogen cycling in the deep ocean of the Kuroshio Extension region

Benthic nitrogen cycling, including nitrification, N-loss, and other nitrogen transformations, plays a crucial role in the marine nitrogen budget. However, studies on benthic nitrogen cycling mainly focus on marginal seas, while attention to the deep ocean, which occupies the largest area of the seafloor, is severely lacking. In this study, we investigate the benthic nitrogen cycling in the Kuroshio Extension region (KE) of the northwest Pacific Ocean at water depths greater than 5,000 m through 15N enrichment slurry incubation and pore-water dissolved oxygen and inorganic nitrogen profiles. The slurry incubation indicates nitrification is the predominant process in benthic nitrogen cycling. The potential nitrification rates are nearly an order of magnitude higher than dissimilatory nitrate reduction. Nitrification and total N-loss flux estimated from pore-water nitrate and ammonium profiles are 6–42 and 5–30 μmol N m−2 d−1, respectively. Generally, anammox is the predominant N-loss process in KE sediment. The temperature gradient experiment indicates that the optimum temperature for anammox and denitrification is 13 and 41°C, respectively, partially explaining anammox as the dominant process for deep-ocean benthic N-loss. Both the low concentration of ammonium in pore-water and the discrepant results between anoxic incubation amended with 15NO3− and 15NH4++14NO3− suggest that ammonium is another limiting factor for benthic anammox. N-loss activity gradually declines with the distance from the Oyashio–Kuroshio transition zone. However, nitrification has the opposite trend roughly. This reveals that the sediment in KE transfers from nitrate sink to source from north to south. This trend is mainly caused by the variation of primary production and the supplement of active organic matter, which is the energy source for microbes and the potential source for ammonium through remineralization. Overall, our results highlight temperature and ammonium as two limiting factors for deep-ocean benthic N-loss and also exhibit a tight coupling relationship between pelagic primary production and the benthic nitrogen cycle in KE.

Stable carbon isotopes of dissolved inorganic carbon in the Western North Pacific Ocean: Proxy for water mixing and dynamics

The uptake of atmospheric CO2 and the cycle of dissolved inorganic carbon (DIC) in the ocean are the major mechanisms and pathways controlling global climate change and carbon cycling. The stable carbon isotope (δ13C) of DIC, therefore, provides an important tracer for processes such as air-sea exchange, photosynthesis, and water dynamics in the ocean. Here, we present new δ13C-DIC data on water samples collected from a north-south transect (13°N–40°N, 150°E) in the western North Pacific (NP) Ocean in November 2019 and compare the results with those previously reported for similar transects (149.3°E) during WOCE and CLIVAR projects over the past three decades. The values of δ13C-DIC, ranging from -0.83‰ to 0.86‰, were higher in the surface waters and decreased with depth. The high δ13C-DIC values in the surface waters were influenced primarily by isotopic fractionation during air-sea exchange and photosynthesis. With depth, the movement of different water masses and mixing, as well as bathypelagic respiration in the dark water of the ocean, all play important roles in influencing the distribution and isotopic signatures of δ13C-DIC in the western NP Ocean. The δ13C-DIC values of the 0–200 m water layer varied from -0.17‰ to 0.86‰, with lower values at high latitudes, affected by the low δ13C-DIC values carried by the Oyashio Current to the Kuroshio Extension (KE) region. A downward trend was present in the δ13C-DIC signature from north to south in the North Pacific Intermediate Water (NPIW) and Pacific Deep Water (PDW) in the western NP, which reflected the remineralization of organic matter with a horizontal transport of NPIW and PDW. We found a strong 13C Suess Effect in the upper 2,000 m in the western NP Ocean, and δ13C-DIC at the surface (<50 m) has decreased by 0.60‰-0.85‰ since 1993. The mean δ13C-DIC change in the surface ocean was estimated at 0.28‰ per decade between 1993 and 2019. The air-sea exchange and water mixing in the study area may have accelerated the absorption of anthropogenic CO2 in recent years, which likely caused a slightly faster rate of decrease in the δ13C-DIC from 2005–2019 than that observed from 1993–2005.

Seasonal variability of upper ocean primary production along the Kuroshio off Japan: Roles of eddy-driven nutrient transport

We assessed spatial and seasonal variabilities of eddy-driven vertical nutrient fluxes, which are essential for maintaining primary production in the upper ocean. A climatological model based on a Regional Oceanic Modeling System (Regional Oceanic Modeling System) coupled with a Nutrient Phytoplankton Zooplankton and Detritus (NPZD) biogeochemical model at a submesoscale eddy-permitting resolution was used to investigate the mechanisms driving such variabilities around the Kuroshio, off the coast of Japan. The model realistically reproduced the spatial segmentations in primary production on both sides of the Kuroshio path with a higher chlorophyll-a concentration on the northern side than the southern side. In winter, downward eddy-induced nitrate flux is predominantly provoked in the upstream Kuroshio region (KR), while upward nitrate fluxes prevail in the downstream Kuroshio Extension (KE) region, due to both shear and baroclinic instabilities. Baroclinic instability plays a crucial role in inducing seasonal variability, leading to enhancement (reduction) of the eddy flux in winter (summer), particularly in regions away from the Kuroshio axis. Furthermore, we found that the influence of the Izu-Ogasawara Ridge, located in the KR, on regional dynamics and resultant spatial variability of the biogeochemical response are mostly confined in the KR. The Kuroshio is less turbulent in the upstream of the ridge, while it becomes unstable to shed mesoscale eddies in laterally wider and vertically deeper regions downstream. Consequently, although the near-surface nitrate concentration is lower downstream, the upward eddy-driven nitrate flux is more effective in maintaining active primary production due to the shear and baroclinic instabilities in winter.

Stronger decadal variability of the Kuroshio Extension under simulated future climate change

Understanding the behavior of western boundary current systems is crucial for predictions of biogeochemical cycles, fisheries, and basin-scale climate modes over the midlatitude oceans. Studies indicate that anthropogenic climate change induces structural changes in the Kuroshio Extension (KE) system, including a northward migration of its oceanic jet. However, changes in the KE temporal variability remain unclear. Using large ensembles of a global coupled climate model, we show that in response to increasing greenhouse gases, the time scale of KE sea surface height (SSH) shifts from interannual scales toward decadal and longer scales. We attribute this increased low-frequency KE variability to enhanced mid-latitude oceanic Rossby wave activity induced by regional and remote atmospheric forcing, due to a poleward shift of midlatitude surface westerly with climatology and an increase in the tropical precipitation activity, which lead to stronger atmospheric teleconnections from El Niño to the midlatitude Pacific and the KE region. Greenhouse warming leads to both a positive (elongated) KE state that restricts ocean perturbations (e.g., eddy activity) and stronger wind-driven KE fluctuations, which enhances the contributions of decadal KE modulations relative to short-time scale intrinsic oceanic KE variations. Our spectral analyses suggest that anthropogenic forcing may alter the future predictability of the KE system.

Enhanced Near-Inertial Waves and Turbulent Diapycnal Mixing Observed in a Cold- and Warm-Core Eddy in the Kuroshio Extension Region

Abstract Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models.

Mesoscale eddies induce variability in the sea surface temperature gradient in the Kuroshio Extension

The Kuroshio Extension (KE) region is one of the most energetic regions in the global ocean where prominent mesoscale dynamics persistently occur. The spatial distribution and temporal evolution of the sea surface temperature (SST) gradient and mesoscale eddies in the KE are investigated. The SST gradient can be applied for identifying the fronts, and the SST gradient within two times the radii of the eddies is composited to quantify the impact of eddies on frontal activities. Depressed SST gradients are identified for eddies with both polarities, but prominent spatial variance in the SST gradient reveals that a large SST gradient is located to the north of anticyclones and along the south periphery for cyclones. The eddies are further separated into two groups depending on their location relative to the main path of the KE, as the background fields to the north and south of the KE are largely different. The spatial pattern, e.g., monopole and dipole features, and temporal variation in the SST gradient are fully studied over the lifespans of eddies. The results show that most eddies can significantly weaken the internal SST gradient and induce the horizontal redistribution of the SST gradient in surrounding regions. Cyclonic eddies north of the KE elevate the fronts along the periphery of eddies. The temporal variability in the SST gradient is prominent and largely varies for each group of eddies. This study offers quantitative analyses of the spatial and temporal relationships between eddies and fronts that are important for understanding the mesoscale dynamics in the world’s oceans.

Functional diversity of copepod assemblages along a basin-scale latitudinal gradient in the North Pacific Ocean

The northern Pacific Ocean is one of the most sensitive areas globally to climate change. Copepods typically account for between 60% and 90% of mesozooplankton in the open ocean. Because copepods are a key link in marine food webs, their response to environmental changes is an important topic in marine biodiversity and ecosystem functioning relationships. The relationship between copepod assemblages and the marine environment in the northern Pacific Ocean from a traits-based perspective has been largely unknown until now. In this study, we used the functional traits and geographic distribution of 177 copepod species along a latitudinal gradient, ranging from 4°S to 46°N in the northern Pacific Ocean, to evaluate the latitudinal variation of functional diversity and assembly rules of copepod assemblages. Based on a cluster analysis of four key functional traits, seven functional groups were identified. Redundancy analysis revealed environmental preferences for different functional groups. Large carnivores showed a stronger preference for higher temperatures than small carnivores, Omnivores and herbivores showed a stronger preference for higher chlorophyll a concentrations. The distribution of detritivores was nearly independent of temperature and chlorophyll a concentration. Most functional diversity indices showed non-linear decreasing trends along the latitudinal gradient. These trends remained stable in the tropic and subtropic regions (WARM and NPSG), but decreased sharply in the Kuroshio extension and Pacific subarctic gyres regions (KURO and PSAG). A null model revealed the assembly rules of copepod assemblage significantly varied with latitude: environmental filtering was dominant in the KURO and PSAG, whereas both environmental filtering and limited similarity played important roles in the WARM and NPSG, in addition to the neutral process. Our results suggested that with ocean warming, a northward shift in the distribution range of specific functional groups (such as large carnivores) might significantly alter the biodiversity and ecosystem function of zooplankton communities in the Kuroshio extension region. This study highlights the importance of a traits-based approach in the study of biodiversity and ecosystem function in pelagic ecosystems at large scales.

Effects of symmetric instability in the Kuroshio Extension region in winter

Submesoscale symmetric instability (SI) in the ocean surface mixed layer (SML) plays a significant role in the forward energy cascade and vertical material transports. Due to its small spatial scales of typically less than 1 km, SI is not resolved by current climate ocean models and most regional models. This work investigates the SI effects in the Kuroshio Extension (KE) region by using a SI parameterization scheme implemented in the Coastal and Regional Ocean Community Model (CROCO). Compared to a SI-lacking model at the same spatial resolution of 500 m, a SI-parameterized model more closely resembles an ultra-high resolution (20 m) non-hydrostatic model that is intended to resolve SI directly on the basis of evaluating the energy source of SI: geostrophic shear production. As SI restratifies the SML, the SI-parameterized model simulates a shallower SML depth in the KE region compared to the SI-lacking one, which is closer to the observed state. The potential vorticity is also modulated and a reduction of negative PV is found due to the parameterized SI. The results here demonstrate the capability of the SI scheme in the representation of the SI impacts and an improvement in the simulation of the KE.

High-efficient built-in wave energy harvesting technology: From laboratory to open ocean test

The ocean contains a huge amount of renewable energy. There is a tremendous need to develop new ocean energy harvesting technology for long-term and self-sustained global ocean observation. Herein, an omnidirectional and high-efficient built-in wave energy harvesting (WEH) system fully integrated in ocean observing platform is introduced, realizing energy harvesting and self-powered ocean-wave sensing, simultaneously. Based on the chaotic pendulum design and high-efficient electromagnetic coupling effect, the high output power of 520 mW and power density of 0.66 mW/cm3 have been achieved under ultra-low-frequency wave excitation with wave height of 20 cm and period of 1 s, which is extremely higher than most of the reported ones. More significantly, the built-in WEH system was fully integrated with the buoy and successfully completed the offshore test in the Yellow Sea for one month and open ocean test in the Kuroshio Extension (KE) region of Northwestern Pacific for four months. During the offshore test, the working time of an autonomous positioning sensor of the buoy was significant extended from 10 to 25 days, which is 2.5 times longer. During the open ocean test, the built-in WEH system was able to long-term survive in the Kuroshio Extension, which is one of the most dynamically-complex regions in the global ocean. The maximum and averaged output power of 210 and 24.5 mW, respectively, have been achieved, under the ocean wave heights and periods varying from 0.4 to 2.2 m and from 4.2 to 7.2 s. Meanwhile, the generated voltage data can be transmitted via Iridium Satellite and utilized for evaluating and sensing the real-time wave conditions, i.e., wave height and period. It comes to an encouraging conclusion that the built-in WEH system cannot only harvest sufficient energy to extend the service life of ocean observing platform, but also as a self-powered wave sensor to assist ocean monitoring. This work shows a promising milestone in WEH technology from laboratory prototype to practical open ocean application.

Submesoscale processes-induced vertical heat transport modulated by oceanic mesoscale eddies

Oceanic submesoscale processes mainly arise from phenomena such as jets, density fronts, and mesoscale eddies. Previous studies suggest that the overall submesoscale processes transport heat vertically upgradient, i.e. from cold to warm. However, it is not clear whether the submesoscale processes-induced vertical heat transport (VHT) in mesoscale eddies remains upgradient. Therefore, the present study focuses on submesoscale-induced VHT modulated by mesoscale eddies. A high-resolution oceanic numerical model product is applied to examine the VHT induced by submesoscale processes associated with a pair of cyclonic and anticyclonic mesoscale eddies in the Kuroshio Extension (KE) region. Eddies at different time steps are collocated in terms of the eddy centers to reconstruct a time series of a new eddy dataset that presents eddy-modulated submesoscale processes. The frequency-wavenumber spectra, Rossby numbers, and strain rates in the pair of eddies reveal the existence of submesoscale motions surrounding mesoscale eddies. The variables are decomposed into monthly mean mesoscale and submesoscale components to calculate submesoscale VHT at low-frequency (LF) and high-frequency (HF). The results indicate that submesoscale VHT is modulated by mesoscale eddies mainly along the eddy boundaries. The LF submesoscale VHT (upward in both cyclonic and anticyclonic eddies) dominates the monthly-averaged submesoscale VHT. The magnitude of the hourly HF submesoscale VHT is larger than the LF component, while its monthly average becomes negligible because it is caused by periodical internal waves. The submesoscale VHT along the eddy edges is related to strong frontogenesis which develops the vertical flux.

Dissolved Organic Carbon Along a Meridional Transect in the Western North Pacific Ocean: Distribution, Variation and Controlling Processes

Dissolved organic carbon (DOC) in the ocean is one of the largest reduced and exchangeable organic carbon pools on Earth and plays important roles in carbon cycling and biogeochemical processes in the ocean. Here, we report the concentrations and distributions of DOC in water samples collected along a meridional transect in the western North Pacific (NP) Ocean in November 2019. Concentrations of DOC ranged from 33-102 μM, were higher in surface water, decreased rapidly with depth to 1,000 m, and then remained relatively constant. The labile fraction of DOC accounted for 20-40% of the surface bulk DOC and was respired very rapidly in the upper 200 m depth. The semi-labile fraction of DOC accounted for 15-20% of the surface bulk DOC and was exported downward and turned over at water depths of 200-2,000 m. The formation of NP Intermediate Water (NPIW) in the Kuroshio Extension (KE) region is a major process carrying some surface semi-labile DOC down. The Low concentrations of DOC (33-44 μM) were present in the entire water column below 1,000 m along the transect in the NP. Primary production and microbial consumption played major roles in the concentration and distribution of DOC in the euphotic zone, and hydrodynamic mixing and circulation of different water masses appear to be dominant factors controlling the distribution and dynamics of DOC in the deep water of the western NP.

Surface Wind Responses to Mesoscale Sea Surface Temperature over Western Boundary Current Regions Assessed by Spectral Transfer Functions

Abstract Satellite observations have revealed that mesoscale sea surface temperature (SST) perturbations can exert distinct influence on sea surface wind by modifying the overlying atmospheric boundary layer. Recently, spectral transfer functions have been shown to be useful to elucidate the wind response features. Spectral transfer functions can represent spatially lagged responses, their horizontal scale dependence, and background wind speed dependence. By adopting the transfer function analysis, the present study explores seasonality and regional differences in the wind response over the major western boundary current regions. Transfer functions estimated from satellite observations are found to be largely consistent among seasons and regions, suggesting that the underlying dominant dynamics are ubiquitous. Nevertheless, the wind response exhibits statistically significant seasonal and regional differences depending on background wind speed. When background wind is stronger (weaker) than 8.5 m s−1, the wind response is stronger (weaker) in winter than in summer. The Agulhas Retroflection region exhibits stronger wind response typically by 30% than the Gulf Stream and Kuroshio Extension regions. Although observed wind distributions are reasonably reconstructed from the transfer functions and observed SST, surface wind convergence zones along the Gulf Stream and Kuroshio Extension are underrepresented. The state-of-the-art atmospheric reanalysis and regional model represent well the structure of the transfer functions in the wavenumber space. The amplitude is, however, underestimated by typically 30%. The transfer function analysis can be adapted to many other atmospheric responses besides sea surface wind, and thus provide new insights into the climatic role of the mesoscale air–sea coupling.

Seasonal-to-Decadal Variability and Prediction of the Kuroshio Extension in the GFDL Coupled Ensemble Reanalysis and Forecasting System

Abstract The Kuroshio Extension (KE), an eastward-flowing jet located in the Pacific western boundary current system, exhibits prominent seasonal-to-decadal variability, which is crucial for understanding climate variations in the northern midlatitudes. We explore the representation and prediction skill for the KE in the GFDL SPEAR (Seamless System for Prediction and Earth System Research) coupled model. Two different approaches are used to generate coupled reanalyses and forecasts: 1) restoring the coupled model’s SST and atmospheric variables toward existing reanalyses, or 2) assimilating SST and subsurface observations into the coupled model without atmospheric assimilation. Both systems use an ocean model with 1° resolution and capture the largest sea surface height (SSH) variability over the KE region. Assimilating subsurface observations appears to be essential to reproduce the narrow front and related oceanic variability of the KE jet in the coupled reanalysis. We demonstrate skillful retrospective predictions of KE SSH variability in monthly (up to 1 year) and annual-mean (up to 5 years) KE forecasts in the seasonal and decadal prediction systems, respectively. The prediction skill varies seasonally, peaking for forecasts initialized in January and verifying in September due to the winter intensification of North Pacific atmospheric forcing. We show that strong large-scale atmospheric anomalies generate deterministic oceanic forcing (i.e., Rossby waves), leading to skillful long-lead KE forecasts. These atmospheric anomalies also drive Ekman convergence and divergence, which forms ocean memory, by sequestering thermal anomalies deep into the winter mixed layer that re-emerge in the subsequent autumn. The SPEAR forecasts capture the recent negative-to-positive transition of the KE phase in 2017, projecting a continued positive phase through 2022.

Seasonal Surface Eddy Mixing in the Kuroshio Extension: Estimation and Machine Learning Prediction

AbstractResults of coarse‐resolution climate models are sensitive to the specification of ocean eddy mixing coefficients. Therefore, it is important to estimate, rationalize and predict eddy diffusivities. Here, we estimate the seasonal variability of surface eddy diffusivities in the Kuroshio Extension region using numerical particles advected by a submesoscale‐permitting model solution. We find that both the spatial structure and the domain‐averaged value of the particle‐based eddy diffusivities have a significant seasonal cycle. We also assess the predictability of cross‐stream mixing lengths in this region using the methods of machine learning, suppressed mixing length theory (SMLT), and multiple linear regression (LR). The predictors we choose are all variables from SMLT that represent eddy‐ and mean‐flow properties, and these predictors correlate well with the particle‐based cross‐stream mixing lengths. We demonstrate that, compared to SMLT and LR, machine learning methods, in particular the random forest (RF) and convolutional neural network (CNN), can better represent both the spatial structure and the domain‐averaged value of cross‐stream mixing lengths. The skill in predicting the mixing lengths with CNN has much less seasonal variability than that with RF. Our results indicate that the machine learning approach may be useful in future development of eddy parameterization schemes.

Submesoscale Ageostrophic Motions Within and Below the Mixed Layer of the Northwestern Pacific Ocean

AbstractSubmesoscale dynamics below the mixed layer (ML) and their mechanisms are still unclear. By a series of nested simulations in the Pacific Northwest with high horizontal resolution of ∼500 m, this study reveals that there exist strong submesoscale ageostrophic motions in the upper pycnocline of the Kuroshio Extension region. These motions exhibit enhanced lateral buoyancy gradient and vigorous vertical velocity but with weak vertical vorticity distinct from the ML submesoscale activities. The vertical velocity in the high‐resolution simulation reaches tens of meters per day, consistent with recent observations (e.g., SubMESI and OSMOSIS). Our analysis shows that the enhanced vertical velocity is mostly attributed to the along‐isopycnal motions at the Kuroshio front, but in the region nearby the large vertical velocity mostly arises from the wave‐like vertical movement of isopycnals. To understand the mechanisms for the large vertical velocity, this paper further examined the instability of the flow and the frequency‐wavenumber spectra of vertical vorticity, lateral divergence, lateral buoyancy gradient, and vertical velocity. A criterion based on the ratio between divergence and vorticity variance in spectral space is used to roughly identify the upper bound of unbalanced submesoscales. The results suggest that the high‐frequency, high‐wavenumber processes dominate the vertical motions within and below the ML and significantly enhance the net vertical heat transport between the ML and the ocean interior. This study seeks to provide comprehension of the submesoscale ageostrophic motions below the ML and their impacts on the upper ocean.

A robust mode of the winter oceanic mixed layer depth in the North Pacific

Based on a modified mixed layer depth (MLD) calculation method and ocean reanalysis datasets, the second leading empirical orthogonal function (EOF) mode (EOF-2) of the North Pacific winter MLD anomalies is investigated, which shows a robust spatial pattern in three reanalysis datasets with opposite phases in the Kuroshio and the Kuroshio Extension region and the central and east of the North Pacific region. The results presented in this study are from SODA 2.2.4. The explained variance of the EOF-2 mode is comparable to the variance of the leading EOF (EOF-1). We reveal that on the interannual time scale, the corresponding Principal Components of EOF-2 (PC-2) are significantly correlated with Niño 3.4 Index and the MLD EOF-2 mode is dynamically driven by the entrainment processes, which are linked to the anomalies of surface heat fluxes and wind velocity induced by ENSO through atmospheric bridge processes. On decadal time scales, EOF-2 is primarily modulated by the variability of wind stress curl and net heat flux, but the influence of net heat flux is only limited in the Kuroshio Extension region.

北西太平洋の台風通過に伴って生じる気象庁準リアルタイム版全球海面水温解析のバイアス

The near-real-time merged satellite and in-situ data global daily sea surface temperature (SST) of the Japan Meteorological Agency (hereinafter abbreviated as R-MGD) is subjected to filtering out short-time-scale fluctuations from observations prior to the analysis time. Therefore, the rapid SST change due to the passage of tropical cyclones (TCs) is thought to cause biases. Here, the biases in the R-MGD with respect to in-situ observations were quantified along the passage of TCs in the western North Pacific. First, we examined a case study on the approach of three successive TCs in August–September 2020. The R-MGD had positive biases of > 2°C just after the passage of three TCs, and negative biases were observed after one week of the last TC's passage. The comparison of the R-MGD with a moored buoy indicates that the biases can be explained by short-term fluctuations filtered out and the SST prior to the analysis time in R-MGD analysis. Second, the composite analysis from May 2015–October 2020 indicates that the statistically significant biases at the observation points ranged between −1 days and +4 days for positive biases and between +7 days and +14 days for negative biases relative to the time of the closest approach of a TC within 500 km. The positive SST bias is largely associated with cold subsurface water and intense TCs, being pronounced in the mid-latitude, except around the Kuroshio and Kuroshio extension regions. The assimilation of in-situ observations recorded within 72 h prior to the R-MGD analysis time through additional optimal interpolation alleviates these biases because this process redeems short-time-scale fluctuations. The impact on TC forecasts and the validity of the optimal interpolation experiment against the independent observations were also investigated.