Cross-shelf variability of eddy polarity, Ekman forcing, and upwelling south of Java (2010–2022). Part II: cross-shelf coupled modes of eddy polarity, Ekman forcing, upwelling, and chlorophyll-a anomalies

Mesoscale eddies and wind-driven Ekman processes jointly regulate upwelling and biological productivity south of Java, but it remains unclear how these controls differ between coastal and offshore waters under monsoonal and El Nino Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) forcing. This study develops a regime-resolved baseline for 2010–2022 by comparing eddy polarity and occurrence, eddy kinetic energy (EKE), Ekman mass transport (EMT), Ekman pumping velocity (EPV), an SST-based upwelling index, and chlorophyll-a anomalies between inshore (≤100 km from the coast) and offshore (>100 km) regimes using fixed geographic masks and monthly, seasonal and interannual composites. The results reveal pronounced cross-shelf contrasts in how physical forcing translates into upwelling conditions and biological response. Inshore variability is strong tied to the monsoon cycle, with seasonal shifts in eddy polarity occurring alongside coherent changes in Ekman-based upwelling diagnostics and chlorophyll-a anomalies, consistent with upwelling-favourable conditions during the Southeast Monsoon and weaker or downwelling-favourable conditions during the Northwest Monsoon. By contrast, the offshore regime shows weaker seasonal modulation in eddy polarity and less consistent alignment with upwelling diagnostics, indicating a larger contribution from broader circulation and remote forcing. Interannually, El Nino and Positive IOD phases preferentially strengthen the coastal upwelling signal and associated chlorophyll-a anomalies, whereas offshore responses are generally smaller and less systematic. Overall, Part I establishes an interpretable cross-shelf baseline for eddy-Ekman-productivity variability south of Java and provides the foundation for the multivariate coupling analysis developed in Part II.

Arafura sea is located between the southern part of Papua Island and Aru Island. Previous studies on Sea Surface Temperature (SST) have described that the SST is strongly affected by the upwelling, but the effect of Ekman Mass Transport (EMT) and Ekman Pumping Velocity (EPV) has not yet been studied. In other areas, it has been shown that EMT and EPV generated by the winds could affect the SST. Thus, further research is needed to better understand the role of the winds on the variability of SST through the mechanism of EMT and EPV in the Arafura Sea. This study used SST data from a high-resolution satellite image (GHRSST) and wind data from a scatterometer satellite image (MetOp A ASCAT). The data were processed using the composite and time-series correlation. This study shows that the higher the wind speed, causes the colder the SST in the Arafura Sea. In contrast, when the wind speed is lower, the SST tends to be warmer. The variabilities of the SST are mostly related to the mixing process associated with the magnitude of EMT. In the shallow water where the calculated Ekman depth is deeper than the actual depth, EMT is more influencing than EPV. On the deeper water at the northeast of the Island of Aru, the negative EPV induces upwelling, bringing the colder water to the sea surface. Statistically, the correlation between EMT (EPV) and the SST in the shallow water of the Arafura Sea is considered strong (weak). On the other hand, at the deep water of the Arafura Sea (northeast of the Island of Aru) offers a strong correlation between the EPV and the SST, whereas the EMT and the SST correlation is considered weak.

The influence of mesoscale dynamics on variability of phytoplankton biomass in terms of chlorophyll-a (chl-a) concentration was studied in the coastal waters of the South Eastern Arabian Sea (SEAS) using long-term satellite data. Satellite-derived chl-a, sea level anomaly, sea surface temperature, and sea surface wind data for the period 1998–2016 were compiled from various sources and analysed to investigate the chl-a variability associated with coastal upwelling and mesoscale eddies. The Empirical Orthogonal Function and Morlet wavelet analyses were performed to estimate the quantitative variability and the result showed strong seasonal and interannual modulation in chl-a concentration and associated environmental variables. The Okubo–Weiss criterion was applied for the identification of mesoscale eddies. The results indicated the presence of cyclonic (cold core) eddies during the summer monsoon season (June–September). The wind-induced upwelling and the cyclonic eddies were most intense during the summer monsoon season, causing higher values of chl-a compared with other season. It is revealed that the variability of chl-a, which might be attributed to seasonal and interannual differences in the surface and sub-surface nutrients, is caused either by coastal upwelling or cyclonic eddies. In particular, the wind-induced upwelling strongly controls the spatial and temporal variability of chl-a compared with mesoscale eddies along the SEAS. The regression model we adopted points out the dominant role played by the wind and its forcing bring about variability in chl-a. The occurrence of extreme climatic events such as El Niño, La Niña and Indian Ocean Dipole (IOD) was noticed during the study period and particularly taken into account to understand the interannual fluctuations in chl-a and associated environmental variables. The relative variability in chl-a concentration was prominent during strong El Niño, La Niña, and IOD. We have attempted to determine the relationship between chl-a with coastal upwelling and mesoscale eddies, the overall importance of such physical forcings, and their influence on bio-production in the SEAS.

The waters south of Java to Nusa Tenggara are a critical upwelling region heavily influenced by monsoon dynamics. The interactions drive oceanographic variability, specifically Ekman Mass Transport (EMT) and Ekman Pumping Velocity (EPV), which regulate primary productivity. While previous studies have examined these dynamics, most have focused on short-term variations. To address this, this study analyzes the variability of EMT and EPV over a 22-year period (2003–2024) and their interaction with El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). EMT and EPV values were calculated using ECMWF wind data, SST, and chlorophyll-a data sourced from MODIS, and climate indices (Niño 3.4 and DMI), which were then analyzed using Pearson correlation. Results show that EMT and EPV peak during the Southeast Monsoon (JJA), reaching approximately 5.16 m²/s and -2.89 × 10⁻⁵ m/s, respectively. Notable anomalies occurred in 2010 and 2016; specifically, the 2010 interaction between La Niña and a negative IOD significantly suppressed upwelling. Correlation analysis reveals that SST is predominantly influenced by EPV, while chlorophyll a concentration is more closely linked to EMT. Although both ENSO and IOD modulate these dynamics, the IOD exerts a stronger influence due to the region’s proximity to the Indian Ocean. These findings provide critical insights into the oceanographic drivers of regional productivity, supporting sustainable fisheries management.

It has been widely known that the coastal upwelling along the southern coast of Java is generated by southeasterly wind which induces offshore Ekman Mass Transport (EMT) during southeast monsoon. However, the variability of EMT has not been fully described in previous studies. The present study investigated the variability of Ekman dynamics which consist of EMT and Ekman Pumping Velocity (EPV) along the southern coast of Java, based on remotely sensed data. We demonstrated the incongruity between the distribution of southeasterly wind speed and sea surface temperature (SST) during southeast monsoon which is related to the distribution of Ekman dynamics. Offshore EMT at the western region of the southern coast of Java is stronger than offshore EMT at the eastern region. However, stronger offshore EMT at the western part is inhibited by downwelling EPV while weaker offshore EMT at the eastern part is accelerated by upwelling EPV. Consequently, SSTs at the eastern parts are lower than those at the western parts. Thus, the changes of offshore EMT intensity from eastern to western parts are balanced by their EPV distributions which explain the incongruity between the distribution of wind speed and SST during southeast monsoon. On an interannual timescale, the combination of La Niña and negative Indian Ocean Dipole (IOD) events tends to weaken offshore EMT and EPV which reduce the intensity of Chl-a bloom and SST cooling during southeast monsoon season. Furthermore, ENSO has a less significant impact on the Ekman dynamics variability than IOD.

Patterns of variability of sea surface chlorophyll in the Mozambique Channel: A quantitative approach

This work compares the Haiyang-2 (HY-2) altimeter data with the well-recognized satellite oceanographic data (AVISO) product in the global ocean in their capability of detecting mesoscale eddy. A well-established method to detect mesoscale eddies from HY-2 and AVISO was applied to the sea level anomaly (SLA) data on a global scale in 2021−2022. It is found that the number of mesoscale eddies derived from the HY-2 dataset is three times more than that from the AVISO dataset and with larger eddy kinetic energy (EKE), shorter lifespan and smaller amplitude. Besides, the monthly-averaged number of generated and terminated mesoscale eddies is similar and the temporal variation of SLA exhibits strong lunar oscillation for both datasets. Most mesoscale eddies are generated in the mid-latitude oceans due to the Coriolis force and the strong El Niño. In the western boundary of the global ocean, equator, Antarctic Ocean and other areas with strong currents, the mesoscale eddies have stronger SLA amplitudes, EKEs and vorticity. Furthermore, EKE is low at the centre and high at the eddy meander. The spatial distribution characteristics of vorticity are the same as those of EKEs in the western boundary of oceans, but the vorticity value of HY-2 data near the equator is larger than that of AVISO data. However, considering the large detection error of eddy near the equator, no further exploration will be conducted in this paper. Furthermore, mesoscale eddies tend to move in the east–west direction due to Coriolis force and wind (Ekman transport) effects and large-scale ocean circulation or the pathway of drifters also have a certain impact. Overall, similar to AVISO data, the HY-2 data is also reliable for mesoscale identification in global oceans.

We study the two primary modes of variability associated with the East Asian summer monsoon, as identified using a multivariate Empirical Orthogonal Function (EOF) analysis. The second mode is shown to be related to changes in intensity of the South Asian High at 100 hPa while, consistent with previous work, the first mode is associated with an index for the shear vorticity of the 850 hPa zonal wind over the monsoon region. We show that a linear, dry dynamical model, when driven by the diabatic heating anomalies associated with each mode, can reproduce many of the anomalous circulation features, especially for the first EOF and in the lower troposphere. The model results indicate the importance of diabatic heating anomalies over the tropical Indian Ocean in the dynamics of both modes, especially EOF-1, and illustrate the role of local diabatic feedback for intensifying the circulation anomalies; in particular, the subtropical anticyclonic anomalies that are found in the positive phase of both modes, and the circulation anomaly associated with the Meiyu/Changma/Baiu rain band. A running cross-correlation analysis shows that the second EOF is consistently linked to both the decaying and the onset phase of El Nino/Southern Oscillation (ENSO) events throughout the study period (1958-2001). We attribute the connection in the onset phase to zonal wind anomalies along the Equator in the west Pacific associated with this mode. On the other hand, a link between the first EOF and ENSO is found only in the post-1979 period. We note also the role of sea-surface temperature anomalies in the tropical Indian Ocean in the dynamics of EOF-1, and a link to the variability of the Indian summer monsoon in the case of EOF-2. Copyright © 2010 Royal Meteorological Society

Coherent oceanic mesoscale eddies with unique dynamical structures have great impacts on ocean transports and global climate. Eddy kinetic energy (EKE), derived from time-dependent circulation, is commonly used to study mesoscale eddies. However, there are three deficiencies of EKE when focusing on the analysis of coherent mesoscale eddies. Here, we propose a comprehensive concept—Lagrangian EKE (LEKE) as an additional metric which is a combination of gridded EKE calculated in Eulerian framework and tracked coherent mesoscale eddies in Lagrangian framework. Evidences suggest that LEKE can make up these deficiencies as an effective supplement. In this study, regional application over Northwestern Pacific Ocean is taken as an example. It clearly demonstrates that LEKE reveals more accurate and detailed characteristics of both cyclonic and anticyclonic eddies than EKE when coherent mesoscale eddies are the specific focus, such as the variation rates of kinetic energy during the eddy propagation, spatial–temporal differences of kinetic energy between cyclonic and anticyclonic eddies. Overall, using LEKE to analyze coherent mesoscale eddies gives the rise to understand the spatial–temporal contrasts between eddies with different polarities, and provides a new perspective to recognize the crucial role played by coherent mesoscale eddies in the ocean.

Mesoscale eddies are highly active around the Kuroshio in the East China Sea (ECS), serving as a crucial component of the ECS’s complex dynamic environment. However, the long-term variation in mesoscale eddies in this region has not been systematically investigated. Based on daily satellite altimeter data spanning from January 1993 to December 2023, this study comprehensively investigates the trend characteristics of mesoscale eddies in the ECS during this period, using eddy metrics such as Eddy Kinetic Energy (EKE) and eddy polarity probability. EKE in the ECS is primarily high around the Kuroshio, exhibiting a significant increasing trend. This upward trend is more pronounced in summer, autumn, and winter, all of which pass the significance test. From the statistics of coherent mesoscale eddies, cyclonic and anticyclonic eddies show opposite trend characteristics: cyclonic eddies display trends of decreasing number and weakening intensity, while anticyclonic eddies exhibit trends of increasing number and strengthening intensity. Energy transfer from the background flow makes a certain contribution to the aforementioned trends, but is relatively complex. The opposing trend characteristics exhibited by eddies of different polarities are related to the northwestward shift of Kuroshio Current. The nonuniform changes in cyclonic and anticyclonic eddies could affect the regional patterns of ocean circulation and biogeochemical responses to future climate change.

The impacts of Kuroshio intrusion (KI) optimization on the simulation of meso-scale eddies (MEs) in the northern South China Sea (SCS) were investigated based on an eddy-resolving ocean general circulation model by comparing two numerical experiments with differences in their form and intensity of KI due to the optimizing topography at Luzon Strait (LS). We found that a reduced KI reduces ME activities in the northern SCS, which is similar to the observations. In this case, the biases of the model related to simulating the eddy kinetic energy (EKE) west of the LS and along the northern slope are remarkably attenuated. The reduced EKE modeling bias is associated with both the reduced number of anti-cyclonic eddies (AEs) and the reduced amplitude of cyclonic eddies (CEs). The EKE budget analysis further suggests that the optimization of the KI will change the EKE by changing the horizontal velocity shear and the slope of the thermocline, which are related to barotropic and baroclinic instabilities, respectively. The former plays the key role in regulating the EKE in the northern SCS due to the changing of the KI. The EKE advection caused by the KI is also important for the EKE budget to the west of the LS.

The majority of the eddy kinetic energy (EKE) in the ocean is found on scales of 50 km to 500 km, encompassed by mesoscale eddies and the meanders and rings of the boundary currents. Mesoscale eddies play a critical role in the dynamics of the ocean circulation with instabilities of the strong mean currents generating eddies in the upper ocean. Interactions between eddies transfer energy from the upper ocean to the deep ocean where eddies interact with bottom topography to generate abyssal mean flows and eddies transfer momentum back to the mean currents. The kinetic energy in a global Hybrid Coordinate Ocean Circulation Model (HYCOM) is compared with long‐term observations from surface drifters, geostrophic currents from satellite altimetry, subsurface floats and deep current meter moorings. HYCOM, configured at 1/12.5° (∼9 km, typical of the present generation of high resolution models), is deficient in EKE in both the upper and abyssal ocean (depths greater than 3000 m) by ∼21% and ∼24% respectively compared to surface drifting buoys and deep current meters. Increasing the model resolution to 1/25° (∼4.4 km) or injecting mesoscale eddies through the assimilation of surface observations in a 1/12.5° model increases the surface and the abyssal EKE to levels consistent with the observations. In these models, the surface (abyssal) EKE is increased by 23% (51%) and 15% (46%) for the higher resolution or data‐assimilative models, respectively, compared to the 1/12.5° non‐assimilative model. While data assimilation increases the EKE in both the upper and abyssal ocean, the kinetic energy of the mean flow in the upper ocean is decreased in the data‐assimilative hindcast.

Abstract. The ionospheric F2 peak height hmF2 is an important parameter that is much needed in ionospheric research and practical applications. In this paper, an attempt is made to develop a global model of hmF2. The hmF2 data, used to construct the global model, are converted from the monthly median hourly values of the ionospheric propagation factor M(3000)F2 observed by ionosondes/digisondes distributed globally, based on the strong anti-correlation existed between hmF2 and M(3000)F2. The empirical orthogonal function (EOF) analysis method, combined with harmonic function and regression analysis, is used to construct the model. The technique used in the global modelling involves two layers of EOF analysis of the dataset. The first layer EOF analysis is applied to the hmF2 dataset which decomposed the dataset into a series of orthogonal functions (EOF base functions) Ek and their associated EOF coefficients Pk. The base functions Ek represent the intrinsic characteristic variations of the dataset with the modified dip latitude and local time, the coefficients Pk represents the variations of the dataset with the universal time, season as well as solar cycle activity levels. The second layer EOF analysis is applied to the EOF coefficients Pk obtained in the first layer EOF analysis. The coefficients Ak, obtained in the second layer EOF analysis, are then modelled with the harmonic functions representing the seasonal (annual and semi-annual) and solar cycle variations, with their amplitudes changing with the F10.7 index, a proxy of the solar activity level. Thus, the constructed global model incorporates the geographical location, diurnal, seasonal as well as solar cycle variations of hmF2 through the combination of EOF analysis and the harmonic function expressions of the associated EOF coefficients. Comparisons between the model results and observational data were consistent, indicating that the modelling technique used is very promising when used to construct the global model of hmF2 and it has the potential of being used for the global modelling/mapping of other ionospheric parameters. Statistical analysis on model-data comparison showed that our constructed model of hmF2, based on the EOF expansion method, compares better with the observational data than the model currently used in the International Reference Ionosphere (IRI) model.

Low-frequency current variability observed at the shelfbreak in the northeastern Gulf of Mexico: May–October, 2004

The mesoscale eddy characteristics of the Mozambique Warm Current were investigated by detecting and tracking satellite altimetry data from 2010 to 2019. A total of 1,086 eddies were identified in the Mozambique Channel, comprising 509 cyclonic eddies and 577 anticyclonic eddies. The results revealed that the bay area on the northwest coast of Madagascar was the main hotspot of eddy generation, and the mean amplitude and radius of the anticyclonic eddies in the Mozambique Channel were 24.23 cm and 82.7 km, respectively, which are larger than those of the cyclonic eddies. Local wind forcing had a significant impact on the formation of mesoscale eddies in the Mozambique Channel. In winter, the wind stress in the northern and southern areas of the Mozambique Channel exhibited a strong correlation with the distribution of eddy kinetic energy (EKE), where both monsoonal winds in the north and trade winds in the south could facilitate mesoscale anticyclonic eddy formation. In addition, the variability in the number of anticyclonic and cyclonic eddies in the Mozambique Channel may have exerted a significant influence on the seasonal anomalous fluctuations in local sea surface temperatures (SSTs). This study presented a novel analysis of the mesoscale eddy characteristics in the Mozambique Channel.