Western Boundary Current System Research Articles

Mixing Dynamics Within the Kuroshio Area Are Reflected in Dissolved Inorganic Radiocarbon Values

AbstractKuroshio is an important western boundary current system in the North Pacific subtropical gyre. Mesoscale eddies play an important role in Kuroshio path variations, which significantly affect fisheries, marine navigation, and climate in regions along the Kuroshio path. However, the direct physical impacts of the Kuroshio variabilities on mixing dynamics within the water columns off the southern coast of Japan remain unclear due to the lack of observational records. The radiocarbon (Δ14C) in dissolved inorganic carbon (DIC) of seawater has been used as a reliable tracer of water circulation and mixing processes. Here, we present the high‐resolution dissolved inorganic radiocarbon (DIC ∆14C) measurements for water samples collected during the Kuroshio large‐meander (LM) period in March 2022 to observe the mixing dynamics within the Kuroshio area and elucidate the controls of mixing processes. In the present study, horizontal variations (12–175‰) in DIC Δ14C values were observed between 400 and 1,000 m and were attributed mainly to changes in the depth of isopycnal surfaces associated with mesoscale eddies and the position of the Kuroshio axis. Furthermore, fluctuations in DIC Δ14C values (10–25‰) were observed on the same isopycnal surfaces ( 25.5–27.0), suggesting the occurrence of diapycnal mixing. By comparing this newly obtained data with values previously reported at similar locations, variations in penetration depths of high bomb 14C signals from past observations are found to be mainly caused by changes in the depth of isopycnal surfaces associated with mesoscale eddies and Kuroshio path variations.

Scale-to-scale energy flux in the oceanic global circulation models

Lewis Fry Richardson proposed his famous picture of turbulent flows in 1922, where the kinetic energy is transferred from large-scale to small-scale structures until the viscosity converts it into heat. This cascade idea, also known as the forward energy cascade, is now widely accepted and is treated as the cornerstone of not only turbulent modeling, but also global circulation models of the ocean and atmosphere. In this work, the Filter-Space-Technique is applied to the oceanic flow field provided by the CMEMS reanalysis model to quantify the scale-to-scale energy flux. A rich dynamical pattern associated with different scales is observed. More precisely, either positive or negative fluxes are observed, indicating the direction of the energy cascade, where the energy is transferred from large-scale structures to small-scale ones or vice versa. High-intensity energy exchange is found mainly in the Western Boundary Current Systems and Equatorial Counter Currents. For the latter case, a wavelike pattern is observed on the westward travel. Moreover, strong seasonal variation is evident for some scales and regions. These results confirm the existence of forward and inverse cascades and rich regional dynamics.

An ARGO and XBT Observing System for the Atlantic Meridional Overturning Circulation and Meridional Heat Transport (AXMOC) at 22.5°S

AbstractChanges in the Atlantic Meridional Overturning Circulation (AMOC) and associated Meridional Heat Transport (MHT) can affect climate and weather patterns, regional sea levels, and ecosystems. Direct observations of the AMOC are still limited, particularly in the South Atlantic. This study establishes a cost‐effective trans‐basin section to estimate for the first time the AMOC and MHT at 22.5°S, using only sustained ocean observations. For this, an optimal mapping method that minimizes the difference between surface in situ dynamic height and satellite altimetry was developed to retrieve monthly temperature and salinity profiles from Argo and XBT data along the 22.5°S section. The mean states, as well as the interannual and seasonal changes of the obtained AMOC and MHT were compared with other products. The mean AMOC and MHT for 22.5°S are 16.3 ± 3.2 Sv and 0.7 ± 0.2 PW, respectively, showing stronger transports during austral fall/winter and weaker in spring. The high‐density XBT data available at the western boundary were vital for capturing the highly variable Brazil Current (BC), whose mean and variability was improved compared to other products. At 22.5°S, the North Atlantic Deep Water is divided into two cores that flow along both the western and the eastern boundaries near 2,500 m depth. Our results (a) suggest a greater influence of the western boundary current system on the AMOC variability at 22.5°S, (b) highlight the importance of high‐density in situ data for AMOC estimates, and (c) contribute to a better understanding of the AMOC and MHT variability in the South Atlantic.

Interactions between cold cyclonic eddies and a western boundary current modulate marine heatwaves

Marine heatwaves are known to cause severe ecosystem damage and therefore have received attention in recent years. However, the focus has tended to be on global (surface) studies, but not coastal waters. Cyclonic eddies are important and underappreciated components in the eddy-dominated western boundary current system, but their impacts on the path of the western boundary currents have largely been unexplored. Here we show that cold cyclonic eddies can modulate the most intense coastal marine heatwaves on record inshore of the East Australian Current. We show that the marine heatwave was driven and modulated by the lateral movement of the western boundary current jet and cyclonic eddies. This study reveals that the interplay of cyclonic eddies and a western boundary current can drive coastal ocean warming, paving the way for future investigations into eddy interactions and the evolution of coastal marine heatwaves in other western boundary current regions.

Exceptional multi-year prediction skill of the Kuroshio Extension in the CESM high-resolution decadal prediction system

The Kuroshio Extension (KE) has far-reaching influences on climate as well as on local marine ecosystems. Thus, skillful multi-year to decadal prediction of the KE state and understanding sources of skill are valuable. Retrospective forecasts using the high-resolution Community Earth System Model (CESM) show exceptional skill in predicting KE variability up to lead year 4, substantially higher than the skill found in a similarly configured low-resolution CESM. The higher skill is attained because the high-resolution system can more realistically simulate the westward Rossby wave propagation of initialized ocean anomalies in the central North Pacific and their expression within the sharp KE front, and does not suffer from spurious variability near Japan present in the low-resolution CESM that interferes with the incoming wave propagation. These results argue for the use of high-resolution models for future studies that aim to predict changes in western boundary current systems and associated biological fields.

Sensitivity of gyre-scale marine connectivity estimates to fine-scale circulation

Abstract. We investigated the connectivity properties of an idealized western boundary current system separating two ocean gyres, where the flow is characterized by a well-defined mean circulation as well as energetic fine-scale features (i.e., mesoscale and submesoscale currents). We used a time-evolving 3D flow field from a high-resolution (HR-3D) ocean model of this system. In order to evaluate the role of the fine scales in connectivity estimates, we computed Lagrangian trajectories in three different ways: using the HR-3D flow, using the same flow but filtered on a coarse-resolution grid (CR-3D), and using the surface layer flow only (HR-SL). We examined connectivity between the two gyres along the western boundary current and across it by using and comparing different metrics, such as minimum and averaged values of transit time between 16 key sites, arrival depths, and probability density functions of transit times. We find that when the fine-scale flow is resolved, the numerical particles connect pairs of sites faster (between 100 to 300 d) than when it is absent. This is particularly true for sites that are along and near the jets separating the two gyres. Moreover, the connectivity is facilitated when 3D instead of surface currents are resolved. Finally, our results emphasize that ocean connectivity is 3D and not 2D and that assessing connectivity properties using climatologies or low-resolution velocity fields yields strongly biased estimates.

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.

Obliquity-driven subtropical forcing of the thermocline after 240 ka in the southern sector of the Western Pacific Warm Pool

Changes in vertical geochemical gradients within the Western Pacific Warm Pool (WPWP) are important indicators of the region's upper ocean response to climate changes and to the oceanographic coupling between the thermocline and surface mixed layer. Here, we reconstruct temperature and δ18Osw at International Ocean Discovery Program Site U1486 (1332 m water depth, 2°22′S, 144°36′E) located in the Bismarck Sea in the southern sector of the WPWP. A 670-kyr record of Δδ18O between the surface-dwelling foraminifera Globigerinoides ruber sensu stricto and the thermocline-dwelling foraminifera Pulleniatina obliquiloculata and Globorotalia tumida (when combined with Mg/Ca-based temperature and δ18Osw estimates) suggests long-term thermocline shoaling and a progressively increasing vertical salinity gradient commencing near 240 ka. Through a detailed comparison to other Pacific records, it becomes clear this is not solely a local phenomenon, as we identify widespread cooling of the thermocline in the low-latitude Pacific after ∼240 ka. After examining our temperature reconstructions alongside new δ18Osw and constant flux proxy-derived focusing factor records, we potentially also validate previous models which find obliquity-induced strengthening of low-latitude Pacific currents. We extend this to support periods of increased transport of high-salinity thermocline water masses through the South Pacific low-latitude western boundary current system. These results indicate greater variability in thermocline circulation given amplified obliquity and strengthen previous evidence that variability in the WPWP thermocline is independent from the drivers of WPWP surface variability.

Characterizing the Variability of Boundary Currents and Ocean Heat Content Around New Zealand Using a Multi‐Decadal High‐Resolution Regional Ocean Model

AbstractNew Zealand is located in the southwest Pacific Ocean and is surrounded by a complex system of boundary currents that vary on a variety of time scales, with important impacts on weather, primary productivity, and fisheries. While various observational and modeling studies have shed light on many of the characteristics of New Zealand's ocean circulation, this study provides a comprehensive, quantitative, seamless, 3‐dimensional characterization of the region's boundary current circulation and ocean heat content. We use a 28 yr long hydrodynamic model simulation that accurately represents the mean and variability of the ocean circulation to characterize and quantify the temporal and spatial variability across the region. We show that low‐frequency variability in boundary current transport and upper ocean heat content are correlated over the entire New Zealand oceanic region. Oceanic eddies dominate heat content variability at intra‐annual scales in the northeast of the region, while on the west and southeast coasts heat content varies predominantly at interannual scales controlled by large‐scale changes in the subtropical and sub‐Antarctic fronts. The model shows a significant downstream strengthening and deepening of the boundary currents off northeast New Zealand which, if confirmed by observations, could suggest a significant new understanding of this Western Boundary Current system. This study presents a region‐wide characterization of the temporal and spatial variability of ocean currents and heat content, and their interconnections, providing a vital basis for understanding climate change impacts and the increasing occurrence of marine heatwaves.

Impact of assimilating repeated subsurface temperature transects on state estimates of a western boundary current

Western Boundary Currents and the eddies they shed are high priorities for numerical estimation and forecasting due to their economic, ecological and dynamical importance. However, the rapid evolution, complex dynamics and baroclinic structure that is typical of eddies and the relatively sparse sampling in western boundary currents leads to significant challenges in understanding the 3-dimensional structure of these boundary currents and mesoscale eddies. Here, we use Observing System Simulation Experiments (OSSEs) to explore the impact of assimilating synthetic subsurface temperature observations at a range of temporal resolutions, to emulate expendable bathythermograph transects with different repeat frequencies (weekly to quarterly). We explore the improvement in the representation of mesoscale eddies and subsurface conditions in a dynamic western boundary current system, the East Australian Current, with a data-assimilating regional ocean model. A characterisation of the spatial and temporal ocean variability spectrum demonstrates the potential for undersampling and aliasing by a lower sampling frequency. We find that assimilating subsurface temperature data with at least a weekly repeat time best improves subsurface representation of this dynamic, eddy-rich region. However, systemic biases introduced by the data assimilation system hinder the ability of the model to produce more accurate subsurface representation with fortnightly or monthly sampling. Removal of this bias may improve subsurface representation in eddy-rich regions with fortnightly or even less frequent observations. These results highlight the value of both increased subsurface observation density in regions of dynamic oceanography as well as continued development of data assimilation techniques in order to optimise the impact of existing observations.

The Response of the Subtropical Front to Changes in the Southern Hemisphere Westerly Winds—Evidence From Models and Observations

AbstractThe location of the Subtropical Front (STF), the boundary between Subtropical and Subantarctic Water in the Southern Ocean is proposed to be influenced by the strength and location of the Southern Hemisphere westerly winds. We use a hydrodynamic hindcast model and recent observations to test if changes in the westerly winds can cause meridional shifts in the STF over interannual to decadal timescales by modulating local Ekman transport. We find that increased, or northward, shifted westerly winds lead to an enhanced northward Ekman transport over large parts of the Southern Ocean, resulting in a northward shift in the STF. Conversely for weaker or southward shifted westerly winds where the STF migrates south. However, this relationship breaks down in regions with strong eddy variability and western boundary current systems. In these regions an increase in westerly winds lead to a southward shift in the STF. Observation data from 2004 to 2019 display a southward shift of STF associated with the positive Southern Annular Mode. However, the shift is smaller than the latitudinal shifts in the location of the zero wind stress curl and maximum westerly winds (−0.4° latitude/decade). This discrepancy is due to positive Ekman trends resulting from the intensification of the westerly winds, which oppose the southward migration. Changes in the Ekman transport and the overall southward shift of the STF have also resulted in an observed positive trend in chlorophyll‐a concentrations south of the STF, which could have ramifications for marine ecosystems and the biological pump in the Southern Ocean.

Ocean Modeling with Adaptive REsolution (OMARE; version 1.0) – refactoring the NEMO model (version 4.0.1) with the parallel computing framework of JASMIN – Part 1: Adaptive grid refinement in an idealized double-gyre case

Abstract. High-resolution models have become widely available for the study of the ocean's small-scale processes. Although these models simulate more turbulent ocean dynamics and reduce uncertainties of parameterizations, they are not practical for long-term simulations, especially for climate studies. Besides scientific research, there are also growing needs from key applications for multi-resolution, flexible modeling capabilities. In this study we introduce the Ocean Modeling with Adaptive REsolution (OMARE), which is based on refactoring Nucleus for European Modelling of the Ocean (NEMO) with the parallel computing framework of JASMIN (J parallel Adaptive Structured Mesh applications INfrastructure). OMARE supports adaptive mesh refinement (AMR) for the simulation of the multi-scale ocean processes with improved computability. We construct an idealized, double-gyre test case, which simulates a western-boundary current system with seasonally changing atmospheric forcings. This paper (Part 1) focuses on the ocean physics simulated by OMARE at two refinement scenarios: (1) 0.5–0.1∘ static refinement and the transition from laminar to turbulent, eddy-rich ocean, and (2) the short-term 0.1–0.02∘ AMR experiments, which focus on submesoscale processes. Specifically, for the first scenario, we show that the ocean dynamics on the refined, 0.1∘ region is sensitive to the choice of refinement region within the low-resolution, 0.5∘ basin. Furthermore, for the refinement to 0.02∘, we adopt refinement criteria for AMR based on surface velocity and vorticity. Results show that temporally changing features at the ocean's mesoscale, as well as submesoscale process and its seasonality, are captured well through AMR. Related topics and future plans of OMARE, including the upscaling of small-scale processes with AMR, are further discussed for further oceanography studies and applications.

Planktonic cnidarian responses to contrasting thermohaline and circulation seasonal scenarios in a tropical western boundary current system

Abstract. In western boundary current systems (WBCSs), strong currents flow coastward carrying oceanic water masses and their associated planktonic fauna. Variation in the intensity of these currents and in the continental runoff may affect the dynamic interplay between oceanic and coastal communities. In addition, changes in the continental runoff and the thermohaline structure modulate the primary production, adding complexity to the dynamics of these oligotrophic systems. These dynamics likely shape the planktonic cnidarian communities. To further understand such relationships, we used a comprehensive dataset encompassing samples collected above the shelf and slope and around oceanic seamounts and islands of the Fernando de Noronha Ridge in the western tropical South Atlantic, in two seasons characterised by distinct thermohaline structure and circulation patterns. Results show that in the tropical South Atlantic and, likely, other western boundary systems with narrow continental shelves, coastward currents spread oceanic waters and their associated cnidarian species over the continental shelf. However, while both coastal and oceanic communities co-occur when the continental runoff is notable, oceanic species dominate almost the entire shelf during the dry season characterised by a stronger boundary current intensity. We also conclude that when the mixed-layer depth and associated nutricline are shallower, the enhanced primary productivity supports larger populations of planktonic cnidarian species through a bottom–up control.

Mesoscale eddies in the southwestern tropical Atlantic

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. However, origin, occurrence and characteristics of mesoscale eddies were still not described in the region. Using satellite altimetry data from 1993 to 2018 off Northeast Brazil (37-25°W; 13-1°S), we reconstruct eddy trajectories and analyse the main eddy surface characteristics (e.g., size, amplitude, polarity) and their spatiotemporal variations. The study reveals two distinct dynamic regions before quantifying mesoscale eddies characteristics. Approximately 2000 mesoscale eddies crossed the region during the study period, among which 76% were generated inside the region, with amplitudes and radii ranging between 1 and 2 cm and 25 and 205 km, respectively. Eddies are preferentially formed between August and September and propagate westward. In the region around the FN Archipelago (36-26°W; 6-1°S), the formation of cyclonic eddies is likely favoured by barotropic instabilities of surface currents and the wind stress curl. On the other hand, in the south of the region (36-26°W; 12-8°S), eddies formation is likely associated with the barotropic instabilities, wind stress curl and the meandering of surface currents. Based on vertical temperature and salinity profiles from Argo floats’ data, we determined that in average, the core of cyclonic eddies is centred at ~ 130 m (140 m) in the northern (southern) region while the core of anticyclonic eddies is centred at ~90 m (125 m) in the northern (southern) region. Moreover, mesoscale eddies formed in the tropical Atlantic do not connect the eastern tropical Atlantic and northeast Brazil.

Contrasting life-history responses to climate variability in eastern and western North Pacific sardine populations

Massive populations of sardines inhabit both the western and eastern boundaries of the world’s subtropical ocean basins, supporting both commercial fisheries and populations of marine predators. Sardine populations in western and eastern boundary current systems have responded oppositely to decadal scale anomalies in ocean temperature, but the mechanism for differing variability has remained unclear. Here, based on otolith microstructure and high-resolution stable isotope analyses, we show that habitat temperature, early life growth rates, energy expenditure, metabolically optimal temperature, and, most importantly, the relationship between growth rate and temperature are remarkably different between the two subpopulations in the western and eastern North Pacific. Varying metabolic responses to environmental changes partly explain the contrasting growth responses. Consistent differences in the life-history traits are observed between subpopulations in the western and eastern boundary current systems around South Africa. These growth and survival characteristics can facilitate the contrasting responses of sardine populations to climate change.

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.

Upwelling phenomenon in the marine regions of Southern Central of Vietnam: a review

Upwelling is an oceanographic phenomenon that involves the physical process and contributes to changes in chemistry, biology, and natural resources. So, systematically, it is the particular ecosystems of whole marine regions with the upwelling. The strong upwelling waters in South Central Regions of Vietnam have uncertain features of the East Vietnam Sea (Bien Dong) and special characteristics of a coastal upwelling area, recorded in international scientific papers in the twentieth century. Their first signals were discovered in the early 1930s through conceptual ideas. The upwelling phenomenon is officially confirmed by scientific results of marine investigations of the NAGA Expedition (1959–1961). The paper aims to review and discuss the physical from Vietnamese investigation and results since 1990s. The following factors are the most contributing to forming and developing the strong upwelling in Southern Central Waters: (1) Influence scale (Mezo- and micro-scale); (2) Forming causes and developing mechanism of upwelling phenomenon, such as monsoon, morphography, shoreline, and western boundary current system of the East Vietnam Sea; (3) Influence of the water-mass from Mekong River on the upwelling area; (4) Ecological environmental consequences; (5) Impacts of the atmospheric-oceanic interaction processes on the western EVS on upwelling. Additionally, the review has targeted findings of upwelling phenomenon mainly in Vietnamese waters based on remote sensing analysis and reanalysis data series to simulate their forming, mechanizing, fluctuating models and the impacts of upwelling in the EVS on resources and ecosystems. The coupled atmosphere-ocean models resulted the upwelling mechanisms and formation. The long-time series of upwelling phenomenon (Macroscale) were evaluated by remote sensing and reanalyzed data series. It is also providing the supplementing and detailing causes and mechanisms of upwelling formation; impacts and interactions of upwelling on marine physics and hydrodynamics (ocean vortexes, seawater temperature), biochemical (nutrients, plankton organisms), and resources (fish, seafood). Within the framework of strong upwelling waters in the Southern Central Regions (Vietnam), the review has not only mentioned partly clarified scientific results but also indicates the limitations and challenges which were faced and encountered in the forecasters of upwelling phenomena in the future.

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.

Synchronized decadal variabilities in the Kuroshio and Kuroshio Extension system

Using satellite altimetry and wind stress data, the spatially coherent decadal variabilities in current intensity and current path during 1993–2018 were investigated for a western boundary current system, namely, the Kuroshio from the east of Luzon Island to the south of Japan and the Kuroshio Extension (KE). Analyses based on Hovmöller diagrams showing decadal variabilities along the entire current demonstrated three types of synchronic relationship between different regions: 1) an out-of-phase relationship in current intensity between the Kuroshio south of Japan and the KE during 1993–2018; 2) an out-of-phase relationship in current intensity between the Kuroshio from the east of Taiwan (ETW) to the East China Sea (ECS) and the KE during 2006–2014; 3) an out-of-phase relationship in current position between the Kuroshio paths in the ETW to ECS and the Luzon Strait during 1993–2018. These patterns were dynamically regulated by the Pacific Decadal Oscillation (PDO). The synchronized current-intensity variability in 2006–2014, which governed almost the entire current system, was caused by coherent phenomena comprising two kinds of baroclinic Rossby wave propagations along the KE and the subtropical countercurrent (STCC), and a regional sea surface height (SSH) anomaly advection from the South China Sea (SCS) to the ECS via the Taiwan Strait. However, the synchronized current-path variability between 1993 and 2018 was caused by an SSH anomaly migration advected by the Kuroshio from the western North Equatorial Current zone to the ECS and an SSH variability localized in the SCS.In this study, we paid particular attention to that the current intensity of the Kuroshio in the ETW to ECS had a positive no-lag correlation with the SSH-based PDO index in 1996–2005 while a positive 3–4 years lag correlation with that index during 2006–2014. It was emphasized that this transition was due to the following mechanism: the current-intensity variability in 1996–2005 was mainly caused by the variability of the eddy activity in the western STCC zone, while that variability in 2006–2014 was mainly caused by the amplification of baroclinic Rossby waves propagating along the STCC zone; the shift from the former to the latter was generated by a rapid phase shift with a relatively large amplitude for the wind stress curl anomaly and a timescale change of the eddy kinetic energy over the STCC zone around 2006.

An Intercomparison of Global Reanalysis Products for Southern Africa’s Major Oceanographic Features

A regional assessment of three global ocean reanalysis products is presented for southern Africa’s major oceanographic features. The reanalyses include Mercator Ocean’s Global Reanalysis (GLORYS), the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) Bluelink Reanalysis (BRAN) and the Fleet Numerical Meteorology and Oceanography Center’s (FNMOC) global Hybrid Coordinate Ocean Model (HYCOM) reanalysis. The aim is to provide modelers with sufficient information for selecting the appropriate product for use as boundary conditions to force their regional ocean models, as well as to provide marine industries, relevant government agencies and academics with insight into the optimal reanalysis product for their purposes. The reanalyses are compared to both assimilated and independent observational datasets spanning various regions within the southern African marine environment. While all reanalysis products reproduce the eastern and western boundary current systems surrounding southern Africa, limitations exist. BRAN outperforms the other reanalyses in its representation of the Mixed Layer Depth, contributing to its good representation of coastal SSTs in the Benguela upwelling system, whereas GLORYS and HYCOM’s misrepresented MLD result in significant warm biases in this region. The Angola-Benguela Frontal Zone and it’s variability is best reproduced by BRAN and HYCOM. The Agulhas Current system’s major components are well reproduced by both GLORYS and BRAN. HYCOM, however, simulates considerably more early retroflections than are observed which have resulted in its mean eastward location. While all the reanalyses overestimate the occurrence of Agulhas meanders, GLORYS and BRAN resolve the associated variability best. Agulhas Current transport is best resolved by GLORYS, unlike BRAN and HYCOM which largely overestimate the magnitude of its south-westward flow, linked to their misrepresentation of the Current’s vertical structure. The bay-scale and nearshore evaluations highlighted issues pertaining to the resolution of the reanalyses and their use at such a small scale. The reanalyses are limited by their resolution, as well as by their misrepresentation of submesoscale processes or lack thereof, prompting the need for the development of regional downscaled models in and around the southern African oceans based on the global ocean reanalysis products.