Sea Surface Height Research Articles

Local Gravity and Geoid Improvements around the Gavdos Satellite Altimetry Cal/Val Site

The isle of Gavdos, and its wider area, is one of the few places worldwide where the calibration and validation of altimetric satellites has been carried out during the last, more than, two decades using dedicated techniques at sea and on land. The sea-surface calibration employed for the determination of the bias in the satellite altimeter’s sea-surface height relies on the use of a gravimetric geoid in collocation with data from tide gauges, permanent global navigation satellite system (GNSS) receivers, as well as meteorological and oceanographic sensors. Hence, a high-accuracy and high-resolution gravimetric geoid model in the vicinity of Gavdos and its surrounding area is of vital importance. The existence of such a geoid model resides in the availability of reliable, in terms of accuracy, and dense, in terms of spatial resolution, gravity data. The isle of Gavdos presents varying topographic characteristics with heights larger than 400 m within small spatial distances of ~7 km. The small size of the island and the significant bathymetric variations in its surrounding marine regions make the determination of the gravity field and the geoid a challenging task. Given the above, the objective of the present work was two-fold. First, to collect new land gravity data over the isle of Gavdos in order to complete the existing database and cover parts of the island where voids existed. Relative gravity campaigns have been designed to cover as homogenously as possible the entire island of Gavdos and especially areas where the topographic gradient is large. The second focus was on the determination of a high-resolution, 1′×1′, and high-accuracy gravimetric geoid for the wider Gavdos area, which will support activities on the determination of the absolute altimetric bias. The relative gravity campaigns have been designed and carried out employing a CG5 relative gravity meter along with geodetic grade GNSS receivers to determine the geodetic position of the acquired observations. Geoid determination has been based on the newly acquired and historical gravity data, GNSS/Leveling observations, and topography and bathymetry databases for the region. The modeling was based on the well-known remove–compute–restore (RCR) method, employing least-squares collocation (LSC) and fast Fourier transform (FFT) methods for the evaluation of the Stokes’ integral. Modeling of the long wavelength contribution has been based on EIGEN6c4 and XGM2019e global geopotential models (GGMs), while for the contribution of the topography, the residual terrain model correction has been employed using both the classical, space domain, and spectral approaches. From the results achieved, the final geoid model accuracy reached the ±1–3 cm level, while in terms of the absolute differences to the GNSS/Leveling data per baseline length, 28.4% of the differences were below the 1cmSij [km] level and 55.2% below the 2cmSij [km]. The latter improved drastically to 52.8% and 81.1%, respectively, after deterministic fit to GNSS/Leveling data, while in terms of the relative differences, the final geoid reaches relative uncertainties of 11.58 ppm (±1.2 cm) for baselines as short as 0–10 km, which improves to 10.63 ppm (±1.1 cm) after the fit.

Satellite observed oceanographic drivers of Mobulidae fisheries catch in the Southeast Indian Ocean

Indonesian coastal waters include several marine megafauna biodiversity hotspots. Several fish populations of ecological and socio-economic importance, such as elasmobranchs (sharks and rays), have experienced rapid decline due to unsustainable human activities, primarily overfishing. Small-scale fisheries (SSF) are currently exempt from governmental fisheries management measures despite contributing a significant proportion of a total catch. The Generalised Additive Models were used to investigate the effect of variations in oceanographic parameters of the Teluk Penyu fishing ground, south of central Java, on the magnitude of Mobulidae (Mobula spp.) catch based on its landings data over ten years (2009–2018) from one of Indonesia's largest ports, Cilacap, Central Java, Indonesia. Mobulidae catch from Teluk Penyu fishing ground was generally higher from June to November when the water exhibited relatively high sea surface salinity (sal >34.1 ‰), chlorophyll (0.32–0.45 mg/m3), and nitrate (nit >0.0045 mg NO3/m3), water speed (>0.29 m/s) and eddy kinetic energy (>0.04 m3/s2) levels, and relatively low sea surface temperature (<28 °C), oxygen (<0.182 mg O2/m3) and sea surface height (<0.9 m) levels than the other months of the year. This study reveals that satellite Earth Observation (EO) data provided a preliminary relationship between oceanographic conditions and the amount of catch for developing more effective management and conservation measures for endangered species like Mobulidae. Utilizing EO data may also be applied to help inform much-needed ecosystem-based management measures, including habitat protection and bycatch reduction for conserving endangered Mobulidae species in the Southeast Indian Ocean. The in-situ onboard ocean observation and temporal species-specific catch data will greatly complement the current work.

Examining fishing activities based on in-situ tracking and oceanographic characteristics in Aru Sea and surroundings

Fisheries activites and oceanographic conditions have a strong relationship. Understanding the complex interplay between fisheries and oceanographic conditions is essential for effective fisheries management. The aim of this research is to analyze the in-situ fishing activities with oceanographic conditions in the Fisheries Management Area (FMA) or WPP-NRI 718 located in the Aru Sea and its surroundings. The main data source is from open global data ship tracking and oceanographic conditions from satellite data. In general, fishing is conducted around the waters of Aru Island. The fishing grounds are strongly influenced by a combination of environmental factors, including sea surface temperatures, chlorophyll-a (Chlor-a) concentrations, sea surface height, and current velocities. A decrease in fishing patterns around Aru Island waters occurs in the eastern season, where fishing tends to occur in the western region (near Timor-Leste). Based on the oceanographic conditions, fishing tends to occur in regions with warmer conditions ranging from 27 to 29 °C, moderate Chlor-a (1.02–3.01 mg/m3), a relatively high surface height (0.17–0.32 m), and slow surface currents.

An integrated dataset of near-surface Eulerian fields and Lagrangian trajectories from an ocean model

A dataset consisting of numerically simulated oceanic velocities and sea surface height changes, provided conjointly from Eulerian and Lagrangian points of view, is made available as cloud-optimized archives on a cloud storage platform for unrestricted access. The Eulerian component of the dataset comprises oceanic velocity components at 0 m and 15 m depth, as well as total and steric sea surface height changes, obtained at hourly time steps for one year, with an approximate horizontal resolution of 1/25 degree on an irregular global geographical spatial grid, from the HYbrid Coordinate Ocean Model. The Lagrangian component of the dataset comprises the trajectories of particles advected in the Eulerian velocity field of the model. The particles were advected forward and backward for 30 days from a regular 1/4 degree grid in order to achieve 60-day long trajectories at 0 m and 15 m depths, with start times separated by 30 days, in 11 releases. This integrated dataset may help to link Eulerian and Lagrangian observational perspectives.

Aggregation and transport of microplastics by a cold-core ring in the southern recirculation of the Kuroshio Extension: the role of mesoscale eddies on plastic debris distribution

Mesoscale eddies – ocean vortices with spatial scales of tens to hundreds of kilometers and time scales of months to years – are among the most energetic forms of flow in the ocean, and may act as significant transporters of floating microplastics. Yet, shipboard observation has thus far not clarified the abundance and transport of microplastics in mesoscale eddies. We conducted floating microplastic surveys in an intense cyclonic mesoscale eddy with a large sea surface height depression (approximately 0.8 m), a so-called cold-core ring, in the Kuroshio Extension recirculation gyre (KERG) southeast of Japan. The concentration of microplastics within the eddy (460 × 104 pieces/km2) was one or two orders of magnitude higher than in the adjacent oceanic waters, likely due to the acquisition of microplastics from the microplastic-rich Kuroshio Extension (KE) when detaching, as well as the horizontal entrainment of particles by the eddy. Our examination by using an assimilation product showed that most particles captured by the eddy remained within for several months while the eddy moved clockwise in the KERG. These results suggest that mesoscale eddies are of importance in the transportation and redistribution of microplastics on the ocean surface.

Enhanced regional ocean ensemble data assimilation through atmospheric coupling in the SKRIPS model

We investigate the impact of ocean data assimilation using the Ensemble Adjustment Kalman Filter (EAKF) from the Data Assimilation Research Testbed (DART) on the oceanic and atmospheric states of the Red Sea. Our study extends the ocean data assimilation experiment performed by Sanikommu et al. (2020) by utilizing the SKRIPS model coupling the MITgcm ocean model and the Weather Research and Forecasting (WRF) atmosphere model. Using a 50-member ensemble, we assimilate satellite-derived sea surface temperature and height and in situ temperature and salinity profiles every three days for one year, starting January 01 2011. Atmospheric data are not assimilated in the experiments. To improve the ensemble realism, perturbations are added to the WRF model using several physics options and the stochastic kinetic energy backscatter (SKEB) scheme. Compared with the control experiments using uncoupled MITgcm with ECMWF ensemble forcing, the EAKF ensemble mean oceanic states from the coupled model are better or insignificantly worse (root-mean-square errors are 23% to −1.3% smaller), especially when the atmospheric model uncertainties are accounted for with stochastic perturbations. We hypothesize that the ensemble spreads of the air–sea fluxes are better represented in the downscaled WRF ensembles when uncertainties are well accounted for, leading to improved representation of the ensemble oceanic states from the new experiments with the coupled model. This indicates the ocean model assimilation will be improved with coupled models and may relax the need for operational centers to provide atmospheric ensembles to drive ocean forecasts. Although the feedback from ocean to atmosphere is included in this two-way regional coupled configuration, we find no significant effect of ocean data assimilation on the ensemble mean latent heat flux and 10-m wind speed over the Red Sea. This suggests that the improved skill using the coupled model is not from the two-way coupling, but from downscaling the ensemble atmospheric forcings (one-way coupled) to drive the ocean model.

Interannual Variability and Trend of the Bering Strait Throughflow Gleaned from Satellite Altimeters

Abstract Beginning with the TOPEX/Poseidon altimetry mission in 1992, along-track satellite sea surface height (SSH) estimates have been taken every 6-7 km at 10-day intervals at a cluster of satellite tracks across the Bering Strait near 66°N where the satellite tracks turn. Year-to-year geostrophic SSH estimates of the Bering Strait flow show that the flow in the warmer months is almost entirely confined to a 50m depth channel rather than the whole cross-sectional area, and that the peak-to-peak transport change is comparable to the approximate 0.8 Sverdrup mean. Energy flux calculations using the high-resolution SSH across Bering Strait, as well as an analysis of historical coastal Russian sea levels and wind stress along the Russian Arctic shelf and Bering Sea shelves, suggest that the interannual Bering Strait transport in the warmer water months can be described using wind-forced Arrested Topographic Waves (ATWs) driven by along-shelf wind stress on both the Russian Arctic and Bering Sea shelves. Most of the Bering Strait transport is barotropic in the 50m-depth channel, but analysis of SSH and in situ Alaskan coastal sea level shows that interannual transport variations in the narrow baroclinic Alaskan Coastal Current are not negligible. In disagreement with previous transport estimates, the satellite SSH analysis and almost all of the in situ current measurements do not indicate a significant long-term trend in barotropic Bering Strait northward transport in the warmer water late-summer/fall months.

Spatiotemporal variations in vertical profiles of Fukushima-derived 137Cs in the Kuroshio-Oyashio confluence region from 2011 to 2018: Implications for local water mass dynamics and basin-scale circulations

Tracking the processes of the spread of Fukushima-derived 137Cs (137CsF) contributes to a better understanding of North Pacific water dynamics. In this study, the vertical distributions of 137Cs and 90Sr in the Kuroshio-Oyashio confluence region were investigated in May 2018, and 137CsF was separated from the background 137Cs by exploiting the constant global fallout 137Cs/90Sr ratio. To the north of 35°N, 137CsF peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), 137CsF exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the 137CsF maxima were distributed mainly within the range of lighter central mode water (L-CMW) during May 2018, even in and just south of the KE. We found that anticyclonic (cyclonic) eddies can promote (prevent) the intrusion of 137CsF into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of 137CsF-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the 137CsF vertical profiles and inventories revealed that 137CsF in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of 137CsF after July 2014, whereas 137CsF in and just south of the KE has remained constant since July 2014, indicating that the 137CsF entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface 137CsF concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of 137CsF to our study area via the western or whole subarctic gyre by May 2018. In contrast, the sea surface height distributions from 2016 to 2017 provide clear evidence that the warm-core rings and quasistationary Isoguchi western jet generated from the Kuroshio Current and KE intruded into the transitional region and even into the subarctic region. Therefore, we concluded that a portion of the 137CsF that subducted into the subtropical western North Pacific during 2011–2012 have entered the transition zone and even the subarctic region since 2016. These results not only enhance our understanding of the protracted spread and fate of 137CsF in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.

Internal-tide vertical structure and steric sea surface height signature south of New Caledonia revealed by glider observations

Abstract. In this study, we exploit autonomous underwater glider data to infer internal-tide dynamics south of New Caledonia, an internal-tide-generation hot spot in the southwestern tropical Pacific. By fitting a sinusoidal function to vertical displacements at each depth using a least-squares method, we simultaneously estimate diurnal and semidiurnal tides. Our analysis reveals regions of enhanced tidal activity, strongly dominated by the semidiurnal tide. To validate our findings, we compare the glider observations to a regional numerical simulation that includes tidal forcing. This comparison assesses the simulation's realism in representing tidal dynamics and evaluates the glider's ability to infer internal-tide signals and their signature in sea surface height (SSH). The glider observations and a pseudo glider, simulated using hourly numerical model output with identical sampling, exhibit similar amplitude and phase characteristics along the glider track. Existing discrepancies are in large part explained by tidal incoherence induced by eddy–internal-tide interactions. We infer the semidiurnal internal-tide signature in steric SSH by the integration of vertical displacements. Within the upper 1000 m, the pseudo glider captures roughly 78 % of the steric SSH total variance explained by the full water column signal. This value increases to over 90 % when projecting the pseudo glider's vertical displacements onto climatological baroclinic modes and extrapolating to full depth. Notably, the steric SSH from glider observations aligns closely with empirical estimates derived from satellite altimetry, highlighting the internal tide's predominant coherent nature during the glider's sampling.

Downscaling sea surface height and currents in coastal regions using convolutional neural network

Downscaling is the process to obtain high-resolution data from low-resolution data. Recently statistical models using convolutional neural networks have gained popularity for fast downscaling of environmental fields, while their application to the coastal sea surface height and currents is lacking. This research aims to downscale sea surface height and depth-averaged current to a resolution of hundreds of meters in coastal regions with dynamic shorelines using convolutional neural networks. Hourly outputs over one year from a physical numerical model for a coastal region in the German Bight are used as the low-resolution input and high-resolution ground truth for the network. The results show that the network effectively reconstructs sea surface height and current in the region to a resolution of hundreds of meters with a scale factor of 16 or even 64, and accurately traces the moving sea surface and shorelines. The global mean absolute error and root mean square error for the sea surface height are found to be less than 0.03 m and 0.07 m, respectively, and for the current less than 0.03 m/s and 0.05 m/s, respectively. These values are around ten times smaller than those obtained from interpolation methods including nearest neighbor, bilinear and bicubic. The network also effectively replicates the distribution of high-resolution data. The errors in the reconstructed time average, 1st percentile and 99th percentile are significantly smaller than those from interpolation methods, especially for the current. These results highlight the ability of the network to downscale sea surface height and currents in regions with complex shorelines, and have implications for downscaling other coastal fields and shoreline tracking.

A matter of scale: Identifying the best spatial and temporal scale of environmental variables to model the distribution of a small cetacean.

The importance of scale when investigating ecological patterns and processes is recognised across many species. In marine ecosystems, the processes that drive species distribution have a hierarchical structure over multiple nested spatial and temporal scales. Hence, multi-scale approaches should be considered when developing accurate distribution models to identify key habitats, particularly for populations of conservation concern. Here, we propose a modelling procedure to identify the best spatial and temporal scale for each modelled and remotely sensed oceanographic variable to model harbour porpoise (Phocoena phocoena) distribution within the Irish Exclusive Economic Zone. Harbour porpoise sightings were recorded during dedicated line-transect aerial surveys conducted in the summers of 2016, 2021 and 2022. Binary generalised additive models were used to assess the relationships between porpoise presence and oceanographic variables at different spatial (5-40 km) and temporal (daily, monthly and across survey period) scales. Selected variables included sea surface temperature, thermal fronts, chlorophyll-a, sea surface height, mixed layer depth and salinity. A total of 30,514 km was covered on-effort with 216 harbour porpoise sightings recorded. Overall, the best spatial scale corresponded to the coarsest resolution considered in this study (40 km), while porpoise presence showed stronger association with oceanographic variables summarised at a longer temporal scale. Habitat models including covariates at coarse spatial and temporal scales may better reflect the processes driving availability and abundance of resources at these large scales. These findings support the hypothesis that a multi-scale approach should be applied when investigating species distribution. Identifying suitable spatial and temporal scale would improve the functional interpretation of the underlying relationships, particularly when studying how a small marine predator interacts with its environment and responds to climate and ecosystem changes.

Oceanic Mesoscale Eddy Fitting Using Legendre Polynomial Surface Fitting Model Based on Along-Track Sea Level Anomaly Data

Exploring the spatial distribution of sea surface height involves two primary methodologies: utilizing gridded reanalysis data post-secondary processing or conducting direct fitting along-track data. While processing gridded reanalysis data may entail information loss, existing direct fitting methods have limitations. Therefore, there is a pressing need for novel direct fitting approaches to enhance efficiency and accuracy in sea surface height fitting. This study demonstrates the viability of Legendre polynomial surface fitting, benchmarked against bicubic quasi-uniform B-spline surface fitting, which has been proven to be a well-established direct fitting method. Despite slightly superior accuracy exhibited by bicubic quasi-uniform B-spline surface fitting under identical order combinations, Legendre polynomial surface fitting offers a simpler structure and enhanced controllability. However, it is pertinent to note that significant expansion of the spatial scope of fitting often results in decreased fitting efficacy. To address this, the current research achieves the precise fitting of sea surface height across expansive spatial ranges through a regional stitching methodology.

Response of Cyclonic Eddies to Typhoon Surigae and Their Weakening Effect on the Kuroshio Current in the Western North Pacific Ocean

This study investigated the dynamic and thermal responses of cyclonic eddies (CEs) to Typhoon Surigae in the western North Pacific Ocean using satellite data and a coupled ocean–atmosphere model. Observations and simulations revealed that the typhoon enhanced the two preexisting CEs (C1 and C2). After the typhoon passed the two eddies, the sea surface height (SSH) lowered and the eddy velocity increased above 200 m. C1 was stretched with elliptical deformation accompanied by an SSH trough and jets on the sides of the typhoon track at the eddy edge. The comparative experiments indicated that the typhoon caused the SSH of C1 and C2 to lower by 53.52% and 25.14% compared to conditions without the typhoon, respectively, and the kinetic energy of C1 and C2 to increase by 12 times and 65.76%, respectively. The positive vorticity anomaly input from the typhoon to the CEs was the main mechanism for the enhancement of the CEs. The enhanced CEs modulated the typhoon-induced sea surface temperature (SST) cooling, causing the temperature within the eddies to decrease by upwelling and mixing, and the SST cooling became significant at the center of the CEs and propagated westward with the eddies. This study also revealed that typhoons can significantly perturb eddy dynamic structures by enhancing or generating cyclonic cold eddies and eradicating anticyclonic eddies, thereby weakening the Kuroshio Current transport via eddy–Kuroshio interactions.

The habitat preference of commercial tuna species based on a daily environmental database approach in the tropical region of the Eastern Indian Ocean off Java-Bali waters

This paper explores the habitat characteristics of commercial tuna species based on daily oceanography parameters in the Eastern Indian Ocean Off Java-Bali Waters. Moreover, more research is needed combining the daily spatial distribution of oceanographic variables of surface and sub-surface data to analyse the habitat characteristics of large pelagic fish, including tuna species. In this study, we used five main daily oceanography parameters: sea surface temperature (SST), sea surface chlorophyll (CHLa), sea surface height (SSH), dissolved oxygen at 100m (DO100), the temperature at 100m (temp100) and the combination of the catch of yellowfin (YFT), albacore (ALT) and bigeye (BET) tuna that use long lines. To analyse the relationship between the environmental database and tuna catch, we utilized Generalized Additive Models (GAMs) from univariate variables until the combination of all variables. The result stated that all the variables influence the existence of all tuna species with P-values <0.001. Temperature is the most critical predictor variable, SST is the most vital predictor for BET and YFT tuna, and temp100 is the most critical for ALT. The second most essential variables were DO100 for BET, Temp 100 for YFT, and SSH for ALT. Moreover, BET and YFT prefer to stay at a lower temp100, and ALT tuna remains at a higher temp100. However, all of them avoid an SST higher than 29 °C. Further assessment of the long-term SST trend specific to tuna species is required to fully account for the effects of global warming on the oceans.

Upper Ocean Responses to Tropical Cyclone Mekunu (2018) in the Arabian Sea

Based on Argo observations and a coupled atmosphere–ocean–wave model, the upper ocean responses to the tropical cyclone (TC) Mekunu (2018) were investigated, and the role of a pre-existing cold eddy in modulating the temperature response to TC Mekunu was quantified by employing numerical experiments. With TC Mekunu’s passage, the mixed layer depth (MLD) on both sides of its track significantly deepened. Moreover, two cold patches (&lt;26 °C) occurred, where the maximum cooling of the mixed layer temperature (MLT) reached 6.62 °C and 6.44 °C. Both the MLD and MLT changes exhibited a notable rightward bias. However, the changes in the mixed layer salinity (MLS) were more complex. At the early stage, the MLS on both sides of the track increased by approximately 0.5 psu. When TC Mekunu made landfall, the MLS change around the track was asymmetric. Significantly, a cold eddy pre-existed where the second cold patch emerged, and this eddy was intensified after TC Mekunu’s passage, with an average sea surface height reduction of approximately 2.7 cm. By employing the stand-alone ocean model, the numerical experimental results demonstrated that the pre-existing cold eddy enhanced TC-induced MLT cooling by an average of approximately 0.41 °C due to steeper temperature stratification at the base of mixed layer. Moreover, heat budget analysis indicated that the pre-existing cold eddy also enhanced subsurface temperature cooling mainly through zonal advection.

Exploring Long-Term Persistence in Sea Surface Temperature and Ocean Parameters via Detrended Cross-Correlation Approach

Long-term cross-correlational structures are examined for pairs of sea surface temperature anomalies (SSTAs) and advective forcing parameters and sea surface height anomalies (SSHAs) and current velocity anomalies (CVAs) in the East/Japan Sea (EJS); all these satellite datasets were collected between 1993 and 2023. By utilizing newly modified detrended cross-correlation analysis algorithms, incorporating local linear trend and local fluctuation level of an SSTA, the analyses were performed on timescales of 400–3000 days. Long-term cross-correlations between SSTAs and SSHAs are strongly persistent over nearly the entire EJS; the strength of persistence is stronger during rising trends and low fluctuations of SSTAs, while anti-persistent behavior appears during high fluctuations of SSTAs. SSTA-CVA pairs show high long-term persistence only along main current pathways: the zonal currents for the Subpolar Front and the meridional currents for the east coast of Korea. SSTA-CVA pairs also show negative long-term persistent behaviors in some spots located near the coasts of Korea and Japan: the zonal currents for the eastern coast of Korea and the meridional currents for the western coast of Japan; these behaviors seem to be related to the coastal upwelling phenomena. Further, these persistent characteristics are more conspicuous in the recent decades (2008~2023) rather than in the past (1993~2008).

Variations in the Central Mode Water in the North Pacific as a manifestation of the Pacific Decadal Oscillation

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than 150m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150 m$$\\end{document} is used as an index of CMW variations, which successfully represents the year-to-year and decadal variations in the CMW volume. The CMW index shows the variation close to that of the Pacific Decadal Oscillation (PDO). The CMW variation is strongly tied with large-scale, dominant variations in sea surface temperature (SST), surface dynamic height (SDH), and sea surface height (SSH) anomalies in the North Pacific, with a significant correlation with the PDO. Year-to-year and decadal variations of CMW volume in May to July are significantly correlated with the wintertime Aleutian Low and 500hPa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$hPa$$\\end{document} geopotential height variations, which indicates that the Aleutian Low induces the eastward extension/retreat of the strong winter westerlies and resultant net surface heat flux anomaly over the CMW distribution region. Thus, the CMW volume variation can be regarded as a significant manifestation of the PDO. The SST, SDH, and SSH anomalies are associated with the surface cooling in the northern sector of the CMW distribution region. On the other hand, the SDH and SSH anomalies throughout a year and the SST anomaly in the cold season in the southeastern sector of the CMW region are formed due to the heaving of isopycnal surfaces in the subsurface layer above the CMW in response to its volume variations.

Applications of the Fourier neural operator in a regional ocean modeling and prediction

In this paper, we apply the Fourier neural operator (FNO) paradigm to ocean circulation and prediction problems. We aim to show that the complicated non-linear dynamics of an ocean circulation can be captured by a flexible, efficient, and expressive structure of the FNO networks. The machine learning model (FNO3D and the recurrent FNO2D networks) trained by simulated data as well as real data takes spatiotemporal input and predicts future ocean states (sea surface current and sea surface height). For this, the double gyre ocean circulation model driven by stochastic wind stress is considered to represent an ideal ocean circulation. In order to generate the training and test data that exhibits rich spatiotemporal variability, the initial states are perturbed by Gaussian random fields. Experimental results confirm that the trained models yield satisfactory prediction accuracy for both types of FNO models in this case. Second, as the training set, we used the HYCOM reanalysis data in a regional ocean. FNO2D experiments demonstrated that the 5-day input to 5-day prediction yields the averaged root mean square errors (RMSEs) of 5.0 cm/s, 6.7 cm/s, 7.9 cm/s, 8.9 cm/s, and 9.4 cm/s in surface current, calculated consecutively for each day, in a regional ocean circulation of the East/Japan Sea. Similarly, the RMSEs for sea surface height were 2.3 cm, 3.5 cm, 4.2 cm, 4.6 cm, and 4.9 cm, for each day. We also trained the model with 15-day input and 10-day prediction, resulting in comparable performance. Extensive numerical tests show that, once learned, the resolution-free FNO model instantly forecasts the ocean states and can be used as an alternative fast solver in various inference algorithms.

Seasonal and Fortnight Variations in Internal Solitary Waves in the Indonesian Seas From the SWOT Measurements

AbstractUsing the wide‐swath sea surface height (SSH) data from the 1‐day repeat calibration/validation phase of the Surface Water and Ocean Topography (SWOT) mission, we explored fine‐scale evolution of nonlinear internal solitary waves (ISWs) in the Banda/Molucca Seas in the Indonesian Archipelagos. Generated in the Ombai Strait and Lifamatola Passage through semidiurnal tide‐topography interaction, respectively, ISWs in the Banda/Molucca Seas have a SSH amplitude of 10–20 cm and exhibit multiple wave packets with the leading wave crest followed by a series of rank‐ordered secondary wave crests. Due to nonlinearity, the ISWs are observed to propagate northward at a speed faster than the regional mode‐1 internal gravity waves and their amplitudes are modulated fortnightly following the regional spring‐neap tidal cycle. On longer timescale, the observed ISW amplitudes are controlled by seasonal upper ocean stratification changes with a weaker stratification favoring a larger amplitude. By converting the SWOT‐measured SSH data to the interior ocean pressure signals, we quantified the depth‐integrated energy flux associated with the northward‐propagating ISWs to be 5 and 2 kW m−1 in the central Banda and Molucca Seas, respectively. Comparisons with past studies indicate that the ISWs contribute to close to 100% and 40% of the tidally‐induced, northward energy fluxes, respectively, across the Banda and Molucca Seas.

Spatiotemporal Variation and Predictors of the Purpleback Flying Squid (Sthenoteuthis oualaniensis) Distribution Surrounding the Xisha and Zhongsha Islands during a Fishing Moratorium

As an economic species widely distributed in the South China Sea (SCS), the purpleback flying squid (Sthenoteuthis oualaniensis) still has a large potential for exploitation, and the variations in its use as a resource are highly correlated with environmental and other factors. In this study, using a generalized additive model (GAM) and gradient forest analysis (GFA), in conjunction with environmental factors, the distribution of purpleback flying squid surrounding the Xisha and Zhongsha islands during the fishing moratorium period was investigated. The results indicated that catch per unit effort (CPUE) had a gradual increase from May to July 2023 in the primary fishing area surrounded the Xisha Islands during May to June, then moved southward towards 13–15° N after July. CPUE is used as an important indicator to reflect the abundance of the fishery, while the GFA results show that CPUE has a better fit than catch in this study. Therefore, the subsequent analysis focused on CPUE. Longitude and sea surface temperature (SST) were of relative higher importance, followed by sea surface salinity (SSS), latitude, chlorophyll a concentration (Chla), sea surface height (SSH), and mixed layer depth (MLD). Longitude and CPUE had a significant, positive correlation. The CPUE gradually increased with latitude within 14–16° N. The CPUE increased slowly as SST increased from 29.5 to 30.5 °C in the primary fishing area. The Chla in this fishing zone was 0–0.2 mg/m3 and displayed a significant positive association with CPUE. Conversely, SSS, SSH, and MLD had negative correlations with CPUE. These findings will promote the sustainable utilization of purpleback flying squid in the SCS.