Spring Sea Research Articles

Exploring the Spatial and Temporal Distribution of Frigate Tuna (Auxis thazard) Habitat in the South China Sea in Spring and Summer during 2015–2019 Using Fishery and Remote Sensing Data

Frigate tuna Auxis thazard fishery is considered a potential marine resource in the open South China Sea (SCS). However, little is known about the spatial and temporal distribution of its habitat, and how this may respond to environmental changes. Using fish survey and remote sensing data from 2015–2019, we applied generalized additive models to identify relationships between environmental factors and the distribution of A. thazard in the SCS. To examine seasonal patterns in the habitat of A. thazard in the SCS, we generated a habitat suitability index model using environmental factors screened by generalized additive models. Results showed that A. thazard migrates from south to north in the SCS, and its suitable habitat is patchily distributed. Significant environmental factors affecting the habitat of A. thazard distribution were different in different seasons; we demonstrated A. thazard to be sensitive to Chl-a in spring (optimum 0.155, optimal range ~0.1252–0.1840), and in summer to be sensitive to SST (optimum 30.405, optimal range ~29.789–31.021) and SSH (optimum 0.741, optimal range ~0.618–0.864). Suitable habitat in spring occurs mainly in northeastern areas, while in summer it occurs mainly around the southeastern Nansha Islands. Compared with that in spring, the area of suitable habitat increases in summer, as does the habitat suitability index of the entire sea area. These results improve our understanding of environmental factors that affect the distribution of A. thazard habitat in the SCS, and provide a scientific basis for the development and management of A. thazard resources.

Spatiotemporal patterns and influencing factors of dissolved heavy metals off the Yangtze River Estuary, East China Sea

Dissolved heavy metal pollution in the ocean is becoming an environmental concern. Their distribution patterns are complex and influenced by multiple factors in the coastal ocean. Therefore, more investigations are needed to understand their behavior in the seawater. This study systematically investigated the distribution of Cu, Pb, Cd, As, Zn and seawater properties in the surface and bottom water off the Yangtze River Estuary, East China Sea in spring, summer and autumn, 2019. The results showed significant spatiotemporal distribution that three-zone-pattern of estuary, nearshore, and offshore can be divided. While sources, hydrodynamics, biological uptake and sediment resuspension affected the overall distribution, dissolved oxygen and pH dominantly influenced the estuary and offshore respectively, with more complex factors in the nearshore. Low ecological risks were assessed during the study, but global warming, ocean acidification and hypoxia are essential concerns to understand the biogeochemistry of dissolved heavy metals in the ocean.

Inter-Annual Variability Of Spring And Summer Blooms In The Eastern Baltic Sea

Changes in environmental conditions may have an effect on the occurrence and intensity of phytoplankton blooms. However, few studies have been carried out on this subject, mainly due to the lack of long-term in situ observations. We study the inter-annual variability and phenology of spring and summer blooms in the eastern Baltic Sea using a physical-biological model. The one-dimensional NPZD model simulates the development of both blooms in the water column with realistic atmospheric forcing and initial conditions representative of the eastern Baltic Sea between 1990 and 2019. On average, the spring bloom started on day 85 ± 7, reached its maximum biomass on day 115 ± 6 and declined after day 144 ± 5. The summer bloom started on day 158 ± 5, had its maximum biomass on day 194 ± 9 and ended after day 237 ± 8. The results showed that the summer bloom occurs 9 days earlier and last 15 days longer over the 30-year simulation period, but changes in the phenology of the spring bloom were not statistically significant. There is strong evidence that warmer periods favor both blooms, but in different ways. Warmer periods caused spring blooms to peak earlier, while summer blooms reached higher abundance. Additionally, a higher energy gain by the ocean led to longer summer blooms of greater abundance and higher biomass maxima. Overall, summer blooms are more sensitive to changes in the environment than spring blooms, being therefore more vulnerable to changes generated by climate change in the Baltic Sea.

Comparative study of the variability and trends of phytoplankton biomass between spring and winter upwelling systems in the South China Sea

High phytoplankton biomass usually appears off the coastal waters southeast of Vietnam (SV) during summer and southwest of the Luzon Strait (SLS) in winter. Although this seasonal upwelling has been recognized in SV and SLS for more than half a century, there has been limited documentation comparing the characteristics and formation mechanisms of these two upwelling systems. To understand the dynamic features of phytoplankton growth in SV and SLS, seasonal and interannual variabilities of Chlorophyll-a (Chl-a) concentration were studied based on decadal-scale satellite-derived datasets (2003–2018). Bivariate wavelet coherence (BWC) and multiple wavelet coherence (MWC) were performed to evaluate the temporal variability of multiple controlling factors. The results demonstrated that there were different patterns of seasonal cycles and interannual variability of Chl-a concentrations in the two upwelling regions. In SV, high Chl-a concentrations during summer were primarily formed by the interactions of multiple factors, causing the mixed layer depth to increase during summer and sufficient nutrient concentrations on the surface (supplied by the mixed water column). In contrast, the high Chl-a concentrations in SLS are caused by a combination of deep mixing (induced by wind and buoyancy forcing) and enhanced upward advection (induced by strong eddy activities). These factors support intense levels of phytoplankton biomass during winter. The sea surface temperature (SST) was the best single factor to explain the Chl-a variance in both regions. The addition of more factors to the MWC analysis changed the main factors influencing the Chl-a variations. The two-factor combination of SST-Niño 3.4 in SV and the SST-mixed layer depth (MLD) in SLS controlled the Chl-a variations on an interannual scale. Moreover, the three-factor combination of SST-Niño 3.4-MLD was still meaningful for explaining the Chl-a variations. This indicated that the influencing the Chl-a variations differed in these two upwelling systems.

Sockeye salmon spawners in the basin of Nachikinskoye Lake (south-western Kamchatka): biology and abundance

State of the spawning part of the sockeye salmon population in Lake Nachikinskoye is characterized. Adults of the early race enter the lake from the sea in spring and early summer, mature at a depth and spawn from July to late August in three different loci — creeks, springs and lake, forming the corresponding ecotypes. The late race representatives enter the lake in July-August, mature, and spawn until December in the same loci. The races differ in age structure, size and fecundity. Males dominate in both races, with a growing proportion of small-sized individuals. According to the materials of aero surveys, the number of fish on the spawning grounds decreased gradually until 2016. Recent data are presented for the number of sockeye salmon belonged to early and late races on certain spawning grounds in the lake basin, based on results of quadrocopter surveys in 2018–2021. The number of the early race spawners was ~100–400 × 103 ind., late race ~300–500 × 103 ind.; the fluctuations were not synchronous between the races. These great spawning stocks require reliable protection and detailed annual monitoring to predict further changes.

Classification and properties of non-idealized coastal wind profiles – an observational study

Abstract. Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28–300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.

Distribution of phytoplankton in the East China Sea and the southern Yellow Sea in spring in relation to environmental variables and dimethylsulfide compounds

Distribution of phytoplankton in the East China Sea and the southern Yellow Sea in spring in relation to environmental variables and dimethylsulfide compounds

Match/Mismatch Between Phytoplankton and Crustacean Zooplankton Phenology in the Strait of Georgia, Canada

The Strait of Georgia, Canada, is an important region for numerous commercially and culturally important species (e.g., herring, salmon, and orcas), yet little is known about the links between lower trophic level (e.g. phytoplankton and zooplankton) phenology due to historical sampling gaps. Here, we present fourteen years (2003-2016) of data linking interannual variability in phytoplankton and zooplankton phenology in the Central Strait of Georgia, BC. Satellite-derived chlorophyll a (Chl a) data were used to calculate spring bloom dynamics (bloom initiation, bloom intensity, and bloom magnitude). Average spring bloom initiation occurred during the last week of March in the Central Strait of Georgia. Bloom initiation occurred in mid-to-late February/early March during “early” Chl a bloom years (2004, 2005, and 2015) whereas initiation did not occur until the end of April during “late” bloom years (2007, 2008). Spring Chl a bloom initiation was significantly correlated with the North Pacific Gyre Oscillation (NPGO; r = 0.75, p < 0.01) and spring sea surface temperature (SST; r = -0.70, p < 0.01); spring blooms occurred earlier during warm years. When all environmental variables were considered together, NPGO best explained variations in spring bloom initiation (Adj R2 = 0.53, p < 0.01) and bloom magnitude (Adj R2 = 0.57, p < 0.01), whereas stratification best explained variations in bloom intensity (Adj R2 = 0.38, p < 0.05). Early Chl a blooms were associated with high crustacean abundance (maximum of > 1000 ind m-3) but low biomass (37.5 mg m-3). Independent of the Chl a data, hierarchical cluster analysis revealed similar groupings of years for crustacean abundance data. Most notably, community composition in cluster Group 2 (2004, 2005, and 2015; early Chl a bloom years), was comprised of a higher proportion of small crustaceans (e.g. non-calanoid copepods) compared to the other cluster groups. To our knowledge, this study provides the first evidence linking early spring Chl a bloom timing to a shift in the crustacean community towards smaller taxa in response to multiple warm events in the Strait of Georgia. Our results show that early Chl a blooms may potentially result in a mismatch between phytoplankton and large energy-rich crustacean zooplankton, with lower abundances of the latter. In contrast, average Chl a bloom years were optimal for large-bodied euphausiids, whereas late Chl a blooms were a match for some crustaceans (e.g., medium calanoid copepods), but not others (e.g., large calanoid copepods and amphipods). We hypothesize that early bloom years may result in poorer feeding conditions for juvenile salmon and other predators in the region.

Impacts of Sea Bottom Temperature on CPUE of European Lobster Homarus gammarus (Linnaeus, 1758; Decapoda, Nephropidae) in the Eastern Adriatic Sea

The study describes recent decadal changes (2008–2017) in the landing biomass, fishing effort and CPUE (kg/day) data of European lobster Homarus gammarus in the eastern Adriatic Sea region, and relates these changes to increases of sea bottom temperatures detected at long-term in situ stations and modelled by an ocean numerical model (ROMS, Regional Ocean Modelling System). Modelling results were further used to quantify spatial and temporal differences of bottom temperature changes over different fishing zones. Trends of sea bottom temperature were positive and statistically significant between stations. Temporal trends of landing, effort and CPUE were also positive and significant for the northern Adriatic. Correlation analysis was used to test the relationship between winter and spring sea bottom temperatures and CPUE data of H. gammarus, separately for the northern and central Adriatic Sea, resulting in statistically significant correlations for both areas. Whether the increased CPUE in the northern Adriatic is due to increased abundance or catchability is discussed. The observed temperature changes likely reflect climate system changes recognised at the regional level and as such, lobster management measures will need to be revised and updated in the future.

Reconstruction of the main phytoplankton population off the Changjiang Estuary in the East China Sea and its assemblage shift in recent decades: From observations to simulation

Under eutrophication background, the increasing dinoflagellates blooms relative to diatoms blooms off the Changjiang Estuary has caused much concern. We have provided sediment evidence for the first time that the time window of diatoms-to-dinoflagellates shift off the Changjiang Estuary in the East China Sea is early 1990s. Investigations to the water column revealed different surface-bottom concentration matchup patterns between peridinin (dinoflagellates) and fucoxanthin (diatoms), which suggests that the diatoms-dinoflagellates shift recorded in the sediment may have come from more dinoflagellate blooms since 1990s. Physical-biogeochemical 3D numerical simulations for the past decades suggest that the effect of increasing spring sea surface temperature and increasing N/P ratio on the diatoms-dinoflagellates shift is dominant and recessive, respectively.

Self-Organizing Maps Identify Windows of Opportunity for Seasonal European Summer Predictions

We combine a machine learning method and ensemble climate predictions to investigate windows of opportunity for seasonal predictability of European summer climate associated with the North Atlantic jet stream. We particularly focus on the impact of North Atlantic spring sea surface temperatures (SST) on the four dominant atmospheric teleconnections associated with the jet stream: the summer North Atlantic Oscillation (NAO) in positive and negative phases, the Atlantic Ridge (At. Ridge), and Atlantic Low (At. Low). We go beyond standard forecast practices by not only identifying these atmospheric teleconnections and their SST precursors but by making use of these identified precursors in the analysis of a dynamical forecast ensemble. Specifically, we train the neural network-based classifier Self-Organizing Maps (SOM) with ERA-20C reanalysis and combine it with model simulations from the Max Planck Institute Earth System Model in mixed resolution (MPI-ESM-MR). We use two different sets of 30-member hindcast ensembles initialized every May, one for training and evaluation between 1902 and 2008, and one for verification between 1980–2016, respectively. Among the four summer atmospheric teleconnections analyzed here, we find that At. Ridge simulated by MPI-ESM-MR shows the best agreement with ERA-20C, thereby representing with its occurrence windows of opportunity for skillful summer predictions. Conversely, At. Low shows the lowest agreement, which might limit the model skill for early warning of warmer than average summers. In summary, we find that spring SST patterns identified with a SOM analysis can be used to guess the dominant summer atmospheric teleconnections at initialization and guide a sub-selection of potential skillful ensemble members. This holds especially true for At. Ridge and At. Low and is unclear for summer NAO. We show that predictive skill in the selected ensemble exceeds that of the full ensemble over regions in the Euro-Atlantic domain where spring SST significantly correlates with summer sea level pressure (SLP). In particular, we find a significant improvement in predictive skill for SLP, geopotential height at 500 hPa, and 2 m temperature at 3–4 months lead time over Scandinavia, which is robust among the two sets of hindcast ensembles.

HBIs and Sterols in Surface Sediments Across the East Siberian Sea: Implications for Palaeo Sea‐Ice Reconstructions

AbstractHighly branched isoprenoids (HBIs) in marine sediments have emerged as promising semi‐quantitative proxies to reconstruct seasonal sea ice in polar oceans. In this work, we examine the distribution of sympagic HBIs (IP25 and HBI‐II), pelagic phytoplankton biomarkers (brassicasterol, dinosterol and HBI‐III) as well as terrestrial sterols (campesterol and β‐sitosterol) in the surface sediments of the East Siberian Sea (ESS) to test their reliability as sea‐ice proxies under continental run‐off influence. Our data suggest that dinosterol performs better than brassicasterol to assess sea ice across the ESS shelf, yet the correlation between PDIP25 and spring sea ice is relatively weak but improves when removing sites with salinity <25. Strongest relationship is found between PIIIIP25 and summer sea ice in regions remote from riverine influence. Overall, our results show that semi‐quantitative estimates of sea ice based on biomarkers can be problematic in Arctic Ocean margins because of biases induced by continental run‐off on biological productivity and sea‐ice production.

Influence of migratory route on early maturation of swordtip squid, Uroteuthis edulis, caught off western Kyushu Island, Japan

Mature female Swordtip squid (Uroteuthis edulis) of ca. 100 mm mantle length are found in Tachibana Bay (western Kyushu, Japan) from June to August, although many mature females caught in the eastern Tsushima Straits (the entrance to the Japan Sea) are over ca. 150 mm. To reveal the cause for the difference, statolith analyses and Lagrangian tracer experiments were conducted. The results show that the small mature female squid would have hatched in the southern East China Sea in winter and migrated to the Sea of Amakusa via the Kuroshio Current, as also would many squid caught in the eastern Tsushima Straits. The two major differences between them were found to be the history of their empirical water temperature (estimated from the ratio of Strontium to Calcium in the statoliths) and the depth of the waters where they were caught. The small mature females would have experienced water temperatures rising from 18° to 20°C in the coastal waters off Kyushu, probably over 16 °C even at the bottom layer, for one month or more before being caught. Considering also the results from previous research, a hypothesis is proposed that, after reaching the minimum age of maturity, the squid could begin (and continue) to mature when the water temperature in the bottom layer (or in the deepest zone to which this species can dive) exceeds ca. 16 °C, and take about one month to reach full maturity. The method used here to explain the size differences detected among siblings born in the same locality but following migratory paths with very different temperature conditions is a useful and the results also suggest another small mature squid that appear in the southern East China Sea in spring may migrate from the warmer waters, which can be contributed to the stock management of this species.

Impact of Internal Climate Variability on the Relationship between Spring Northern Tropical Atlantic SST Anomalies and Succedent Winter ENSO: The Role of the North Pacific Oscillation

Abstract Previous studies suggested that spring sea surface temperature anomalies (SSTAs) in the northern tropical Atlantic (NTA) have a marked influence on the succedent winter El Niño–Southern Oscillation (ENSO). In this study, we examine the spring NTA SSTA–winter ENSO connection in a 50-member large-ensemble simulation conducted with the Canadian Centre for Climate Modelling and Analysis second-generation Canadian Earth System Model (CanESM2) and a 100-member ensemble simulation conducted with the Max Planck Institute Earth System Model (MPI-ESM). The observed out-of-phase relation of spring NTA SSTA with winter ENSO can be captured by the multimember ensemble means of the large-ensemble simulations from both models. However, the relation shows a large diversity among different ensemble members attributing to the internal climate variability. The preceding winter North Pacific Oscillation (NPO) is suggested to be an important source of the internal climate variability that modulates the spring NTA SSTA–ENSO connection. The modulation of the winter NPO on the subsequent spring NTA SSTA–winter ENSO relation is seen in both climate modeling and observational datasets. When winter NPO and spring NTA SSTA indices have the same (opposite) sign, the linkage between the spring NTA SSTA and the following winter ENSO tends to be weak (strong). The NPO modulates the spring NTA SSTA–winter ENSO relation mainly via changing the zonal wind anomalies over the tropical western-to-central Pacific induced by the spring NTA SSTA. In addition, our analysis indicates that winter NPO may have a marked effect on the predictability of winter ENSO based on the condition of spring NTA SSTA.

Net heterotrophy in High Arctic first-year and multi-year spring sea ice

The net productivity of sea ice is determined by the physical and geochemical characteristics of the ice–ocean system and the activity of organisms inhabiting the ice. Differences in habitat suitability between first-year and multi-year sea ice can affect the ice algal community composition and acclimation state, introducing considerable variability to primary production within each ice type. In this study, we characterized the biogeochemical variability between adjacent first-year and multi-year sea ice floes in the Lincoln Sea of the Canadian High Arctic, during the May 2018 Multidisciplinary Arctic Program—Last Ice sampling campaign. Combining measurements of transmitted irradiance from a remotely operated underwater vehicle with laboratory-based oxygen optode incubations, this work shows widespread heterotrophy (net oxygen uptake) in the bottom 10 cm of both ice types, particularly in thick multi-year ice (>2.4 m) and early morning of the 24-h day. Algal acclimation state and species composition varied between ice types despite similar net community production due to widespread light and nutrient limitation. The first-year ice algal community was increasingly dominated over spring by the potentially toxin-producing genus Pseudonitzschia that was acclimated to high and variable light conditions characteristic of a thinner ice habitat with mobile snow cover. In comparison, the multi-year ice harbored more shade-acclimated algae of mixed composition. This work highlights the potential for heterotrophy in sea ice habitats of the High Arctic, including first measurements of such O2-uptake in multi-year ice floes. Observed differences in photophysiology between algae of these sea ice types suggests that a shift toward higher light availability and a younger sea ice cover with climate change does not necessarily result in a more productive system. Instead, it may favor future sea ice algal communities of different species composition, with lower photosynthetic potential but greater resilience to stronger and more variable light conditions.

Interspecific differences in the response of coral trophic status to the decrease in symbiotic zooxanthellae in summer

Coral bleaching events are increasing in frequency and severity worldwide. From the perspective of energy supply, a decrease in symbiotic zooxanthellae density (ZD) during bleaching leads to a reduction in the amount of energy provided by photosynthesis to the coral host. However, there are still many unknowns about how corals maintain the stability of their energy supply when zooxanthellae decrease. In this study, 89 coral samples, including the relatively stress-tolerant massive Favia palauensis and Porites lutea and the relatively vulnerable branching Acropora millepora and Pocillopora damicornis, were collected from the Xisha Islands in the South China Sea in spring and summer 2020. The physiological parameters ZD, chl a, tissue biomass and lipid content were measured. The δ13C compositions of zooxanthellae and hosts were analyzed to explore the changes in trophic status in summer. Results show that the ZDs of A. millepora, P. damicornis, F. palauensis and P. lutea significantly decreased by 32, 29, 22 and 22%, respectively, in summer. P. lutea and F. palauensis maintained metabolic energy requirements depending largely on heterotrophy. A. millepora and P. damicornis were more dependent on consuming their own lipids. Our study shows that there are intergeneric differences in the energy maintenance mechanisms used to cope with decreases in ZD. We suggest that the higher heterotrophic ability of F. palauensis and P. lutea renders their trophic status more plastic than that of A. millepora and P. damicornis. This characteristic may potentially affect their bleaching resilience.

A Comparative Analysis of Chemical Properties and Factors Impacting Spring Sea Fog over the Northwestern South China Sea

A Comparative Analysis of Chemical Properties and Factors Impacting Spring Sea Fog over the Northwestern South China Sea

Oceanic and land relay effects linking spring tropical Indian Ocean sea surface temperature and summer Tibetan Plateau precipitation

The present study investigates the characteristics and factors of the variability of summer precipitation over the Tibetan Plateau (TP). It is found that interannual variations of summer precipitation tend to be opposite over the eastern and western TP, manifesting an east-west TP precipitation dipole pattern (TPPD). The summer TPPD is significantly correlated with the spring sea surface temperature (SST) in the tropical Indian Ocean, and the latter can be considered an important precursor for the former. The impacts of spring SST in the tropical Indian Ocean on summer TPPD are through oceanic and land relay effects. For the oceanic relay effect, the positive SST anomaly (SSTA) in the tropical Indian Ocean persists from spring to summer and induces the upper-tropospheric positive geopotential height anomaly and anomalous anticyclone to the south of the TP by stimulating a Rossby wave response. Winter El Niño and Indian Ocean Dipole (IOD) contribute to the persistence of the SSTA in the Indian Ocean from spring to summer through modulating a spring asymmetric mode in the Indian Ocean. The SSTAs in the Indian Ocean affect the western North Pacific anticyclone at lower troposphere through modulating the Indo-Western Pacific Ocean capacitor (IPOC) mode. These circulation anomalies adjust the variability of the summer TPPD. For the land relay effect, the SSTA in the tropical Indian Ocean induces the soil moisture (SM) anomaly in the Turan Plain through modulating atmospheric circulation around the Arabian Peninsula and the Mediterranean Sea during spring. The SM anomaly in the Turan Plain is maintained from spring to summer and induces the upper-tropospheric negative geopotential height anomaly and anomalous cyclone north of the TP during summer by stimulating an anomalous vertical motion. Through the above oceanic and land processes, a north-south opposite pattern of geopotential height anomalies around the TP appears and eventually modulates water vapor transport into the TP and hence the formation of the TPPD during summer. Further analyses reveal that the impact of the winter El Niño and IOD and spring tropical Indian Ocean SST on the summer TPPD becomes weaker when one of the abovementioned relay effects is absent.

Performance of helicopter-based biopsy darting of polar bears (Ursus maritimus) on the spring sea ice

Performance of helicopter-based biopsy darting of polar bears (Ursus maritimus) on the spring sea ice

Decadal shift in sea fog frequency over the northern South China Sea in spring: Interdecadal variation and impact of the Pacific Decadal Oscillation

Sea fog has an important impact on marine activities, coastal ecological environments, and regional radiation budgets. In the present study, we analyzed the characteristics of sea fog frequency (SFF) interdecadal variations over the northern South China Sea (NSCS) and the role of the Pacific Decadal Oscillation (PDO) in these changes, using independent datasets. The SFFs in the fog season derived from International Comprehensive Ocean–Atmosphere Data Set (ICOADS) observations and ERA-5 cross-validate a significant decadal transition from negative to positive phase in the late 1970s, which was further verified by a moving t-test. The composite fields after 1980s, including wind, low-atmospheric stability and moisture supply, were much more favorable for an increase in SFF than those before 1980s. These conditions were found to be closely associated with the PDO, with a significant correlation coefficient between the SFF derived from ICOADS (ERA-5) and the PDO at 0.53 (0.48), and even reaching 0.94 (0.92) after applying an 18-year moving average from 1950 to 1999. An anticyclonic circulation anomaly over the North western Pacific associated with positive PDO phase enhances the stability in the lower atmosphere and the water vapor supply due to a southerly wind anomaly at 850 hPa over the NSCS. With the strengthening of the winter monsoon in the positive PDO phase, northerly wind anomalies over the East Asian continent converge with southerly wind anomalies from the ocean, increasing the humidity in the NSCS, which helps to incease the SFF. In the negative PDO phase, the oceanic and atmospheric conditions slightly decrease the frequency of sea fog occurrence. This study improves the understanding of the mechanism on the interdecadal variation of SFF as modulated by the PDO, and has useful implications for further work on sea fog projection.