Sea Surface Research Articles

Mapping phenoregions and phytoplankton seasonality in Northeast Pacific marine coastal ecosystems via a satellite-based approach

Phytoplankton phenology describes yearly algal growth cycles and characterises the timing, duration, and magnitude of bloom occurrences. This study used satellite chlorophyll-a data from 1998 to 2020 and the Hierarchical Agglomerative Clustering method to define phenoregions based on phytoplankton phenology spatial patterns over the British Columbia and Southeast Alaska coastal oceans. The defined phenoregions were used to simplify the spatial complexity of the heterogenous study region and thus better describe phytoplankton seasonality across the target area. The cluster analysis allowed the delineation of four coherent regions: two coastal regions and northern and southern shelf/offshore regions. Results showed that each phenoregion had distinguishable phytoplankton phenological characteristics, likely due to different physical forcings acting in these areas. Moreover, the interannual variability of the spring bloom initiation was evaluated considering interactions between sea surface temperature (SST) anomalies and the El Niño Southern Oscillation Index (ENSO). Early spring blooms were associated with positive SST anomalies and El Niño conditions; conversely, average or late spring blooms occurred in years with negative SST anomalies and La Niña conditions, with the strongest relationship occurring in the southern shelf/offshore phenoregion. This study provided new insights into the regionalisation of the British Columbia and Southeast Alaska coastal oceans based on phytoplankton phenology patterns. Given the critical role of phytoplankton as the base of the marine food web, such phenoregions have implications for regional zooplankton biomass and fish production. The link between phytoplankton phenology and climate drivers points to the importance of environmental change in phytoplankton bloom dynamics. Further research into the connection between phytoplankton bloom indices and zooplankton community structure and production would be an important step towards using these indices for ecosystem monitoring and fisheries management.

West African Monsoon System’s Responses to Global Ocean–Regional Atmosphere Coupling

Abstract This study explores the added value (AV) of a regional Earth system model (ESM) compared to an atmosphere-only regional climate model (RCM) in simulating West African monsoon (WAM) rainfall. The primary goals are to foster discussions on the suitability of coupled RCMs for WAM projections and deepen our understanding of ocean–atmosphere coupling’s influence on the WAM system. The study employs results from dynamical downscaling of the ERA-Interim reanalysis and Max Plank Institute ESM, low resolution (MPI-ESM-LR), by two RCMs, atmosphere only (REMO) and REMO coupled with Max Planck Institute Ocean Model (MPIOM) (ROM), at ∼25-km horizontal resolution. Results show that in regions distant from coupling domain boundaries such as West Africa (WA), constraint conditions from ERA-Interim are more beneficial than coupling effects. REMO, reliant on oceanic sea surface temperatures (SSTs) from observations and influenced by ERA-Interim, is biased under coupling conditions, although coupling offers potential advantages in representing heat and mass fluxes. Contrastingly, as intended, coupling improves SSTs and monsoon fluxes’ relationships under ESM-forced conditions. In this latter case, the coupling features a dipole-like spatial structure of AV, improving precipitation over the Guinea Coast but degrading precipitation over half of the Sahel. Our extensive examination of physical processes and mechanisms underpinning the WAM system supports the plausibility of AV. Additionally, we found that the monsoonal dynamics over the ocean respond to convective activity, with the Sahara–Sahel surface temperature gradient serving as the maintenance mechanism. While further efforts are needed to enhance the coupled RCM, we advocate for its use in the context of WAM rainfall forecasts and projections.

Heat stress mediates toxicity of rutile titanium dioxide nanoparticles on fertilisation capacity in the broadcast spawning mussel Mytilus galloprovincialis

Titanium dioxide nanoparticle (nTiO2) pollution of marine environments is rapidly increasing with potentially deleterious effects on wildlife. Yet, the impacts of nTiO2 on reproduction remain poorly understood. This is especially the case for broadcast spawners, who are likely to be more severely impacted by environmental disturbances because their gametes are directly exposed to the environment during fertilisation. In addition, it is unclear whether rising water temperatures will further exacerbate the impact of nTiO2 toxicity. Here, in a series of fertilisation trials, we systematically examine the main and interactive effects of nTiO2 exposure and seawater temperature on fertilisation success in the Mediterranean mussel Mytilus galloprovincialis. Specifically, our fertilisation trials explored whether nTiO2 exposure influences fertilisation rates when (i) eggs alone are exposed, (ii) both sperm and eggs are exposed simultaneously, and (iii) whether increases in seawater temperature interact with nTiO2 exposure to influence fertilisation rates. We also ask whether changes in nTiO2 concentrations influence key sperm motility traits using computer-assisted sperm analysis (CASA). In fertilisation trials for treatment groups (i) and (ii), we found no main effects of nTiO2 at environmentally relevant concentrations of 5, 10 and 50 μg L−1 on fertilisation capacity relative to the control. Consistent with these findings, we found no effect of nTiO2 exposure on sperm motility. However, in treatment group (iii), when fertilisation trials were conducted at higher temperatures (+6 °C), exposure of gametes from both sexes to 10 μg L−1 nTiO2 led to a reduction in fertilisation rates that was significantly greater than when gametes were exposed to elevated temperature alone. These interacting effects of nTiO2 exposure and seawater temperature demonstrate the toxic potential of nTiO2 for fertilisation processes in a system that is likely to be impacted heavily by predicted future increases in sea surface temperatures.

Diverse Responses of Strong Positive SST and Rainfall Indian Ocean Dipole Events under Greenhouse Warming

Abstract Recent research has shown that there is a projected increase in the frequency of strong positive Indian Ocean dipole (spIOD) events in terms of both sea surface temperature (SST) and precipitation. However, it is not clear whether the spIOD events defined by SST and precipitation, hereafter referred to as SST-spIOD and Pr-spIOD events, respectively, will increase at the same pace under greenhouse warming. Using climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and CMIP6 that can reasonably simulate the spIOD events, here we find that the occurrence of Pr-spIOD events increases much more than that of SST-spIOD events (+217% vs +77%) under the future high-emission scenario. The diverse response stems from a notably large increase in the spIOD events with strong rainfall but moderate SST anomalies (Pr-only spIOD), which is facilitated by a change in the local meridional Hadley circulation induced by land–sea thermal contrast in addition to the background mean-state SST warming. For the spIOD events with prominent anomalies in both rainfall and SST (concurrent spIOD), their occurrences are projected to increase, but their average SST amplitude is projected to weaken. The increased occurrence is associated with a westward-extended structural change induced by mean linear zonal advection feedbacks, while the weakened event-average SST amplitude results from a more stabilized atmosphere and coincides with the projected reduction in IOD SST skewness. Our results suggest that IOD-related mitigation strategies should consider the diverse responses of different kinds of spIOD events to greenhouse warming.

A geospatial web service for small pelagic fish spatial distribution modeling and mapping with remote sensing

Small pelagic fish are an essential resource for coastal countries worldwide and their assessment and monitoring are a key part of successful fisheries management. Advances in marine satellite remote sensing can contribute to the creation of methodologies for continual small pelagic fish spatial distribution monitoring that can act as supplementary tools for fisheries management decision making, enhancing traditional field practices. In this work a comprehensive Geospatial Web Service (GWS) is proposed that utilizes Sentinel-3 data to publish Spatial Distribution Modeling (SDM) maps for anchovy (Engraulis encrasiculous) and sardine (Sardina pilchardus). The proposed GWS is developed through the sole use of open-source programming languages and software and provides fishery management related data through various parameters: A) Sea Surface Temperature (SST) and Chlorophyll-a Concentration (CHL), B) mesoscale oceanic fronts and C) the SDM maps for the target species. The SDM results are produced through a Random Forest algorithm and utilized oceanographic parameters relevant to the ecological needs of the target species (CHL, SST, oceanic fronts and bathymetry). All data are processed and gap-free through a spatiotemporal DINEOF interpolation, allowing the continuous provision of information independently of the weather conditions. Furthermore, the service integrates auxiliary information, such as weather and sea state forecasts, that aim to contribute to maritime safety for effective decision-making. The resulting GWS offers an easy to use and interactive tool that bridges the gap between the scientific community and the decision makers. The utilization of satellite remote sensing enhances the scalability of the proposed service for future improvements and continuous monitoring.

Impact of Monsoon Variability on the Northern Hemisphere Terrestrial Carbon Cycle on an Interannual Time Scale

Abstract A monsoon climate covers a large portion of the global land and exhibits notable interannual variations. However, the effect of monsoon variability on the terrestrial carbon cycle has been investigated less due to the gap between physical and biogeochemical climate research communities. Here, using the FLUXCOM dataset, we show that the Northern Hemisphere land monsoon (NHLM) regions account for 40.24% (±3.40%) [±one standard deviation (SD)] of the climatological summer mean net ecosystem production (NEP) over the NH land from 1981 to 2010 and contribute 21.35% (±7.57%) to the interannual variability (IAV) in NH NEP. Using singular value decomposition (SVD), we find that the leading modes of NEP anomalies are associated with El Niño development and decay phases. During the El Niño–developing summer, a higher sea surface temperature in the tropical central-eastern Pacific results in reduced photosynthetic productivity and weaker carbon uptake because of the reduced precipitation associated with monsoon circulation changes. Precipitation is the predominant driver of NEP anomalies relative to temperature and radiation, contributing 65.20% in the NHLM regions. During the El Niño–decaying summer, abundant rainfall along the Yangtze River valley in China results in increased photosynthetic productivity and ecosystem respiration, leading to less change in carbon uptake. In other monsoon regions, photosynthesis anomalies are driven by higher temperatures, and respiration anomalies are driven by enhanced precipitation. Both of these conditions are favorable for the release of CO2 into the atmosphere. Our study highlights the impact of monsoon variability on the carbon cycle in monsoon regions during different phases of El Niño–Southern Oscillation (ENSO) evolution.

Disagreement on the North Atlantic Cold Blob Formation Mechanisms among Climate Models

Abstract Despite global warming, the sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob, signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and their constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from phase 6 of Coupled Model Intercomparison Project, we assess the primary processes driving the North Atlantic cold blob within individual models and whether the mechanisms are consistent across models. We show that 11 out of 32 models, which we call “Cold Blob” models, simulate the subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of the subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. While 4 of the 11 Cold Blob models indicate decreased oceanic heat transport convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes is comparable in the remaining three models. Such a model disagreement on the mechanism of cold blob formation may be associated with simulated base-state Atlantic meridional overturning circulation (AMOC) strength, which explains 39% of the intermodel spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate that radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single model should be interpreted with caution. Significance Statement The mechanisms driving sea surface temperatures over the subpolar North Atlantic to cool since the 1900s remain uncertain due to the lack of direct observations. Here, we use a temperature change decomposition framework to dissect the historical trend of surface temperature simulated in multiple global climate models. The models diverge on whether the subpolar North Atlantic cooling is induced by reduced ocean heat transport convergence or altered radiative processes. Notably, the importance of ocean heat transport convergence is influenced by the simulated base-state strength of Atlantic meridional overturning circulation and the Irminger Sea’s mixed layer depth. This finding cautions against concluding the cooling mechanism from a single model and highlights a need for ongoing observations to constrain AMOC-related climate projection in the subpolar North Atlantic.

Increasing WNP tropical cyclone-related extreme precipitation over East Asia during boreal summer associated with PDO shift

Over the past two decades, there has been a significant shift in tropical cyclone (TC) activity in the western North Pacific (WNP) basin during the boreal summer. Our analysis of data spanning from 1979 to 2021 reveals significant shifts in the WNP TC characteristics and rainfall pattern variation. To deepen our understanding of TC-related precipitation dynamics, we expressly address the difference between TC-related core precipitation (TCP) and remote precipitation (TRP). The results show that TRP significantly impacts the East Asian (EA) continent, especially on the Korean Peninsula. Notably, TCP exhibits decadal variability, with a pronounced negative correlation identified between it and the Pacific decadal oscillation (PDO) following a strong climate shift. This pivotal shift was marked by the PDO first transitioning to its negative phase in 1997, a notable change since 1979, resulting in a marked increase in TC-related extreme rainfall over the EA area. Concurrently, the rising sea surface temperatures (SSTs) over the WNP have intensified the western Pacific subtropical high (WPSH) circulation. The easterly steering flow associated with the WPSH then strengthened, leading to the continental migration of TC trajectories, thereby intensifying TC-related extreme precipitation.

Variability of SST through Koopman Modes

Abstract The majority of dynamical systems arising from applications show a chaotic character. This is especially true for climate and weather applications. We present here an application of Koopman operator theory to tropical and global sea surface temperature (SST) that yields an approximation to the continuous spectrum typical of these situations. We also show that the Koopman modes yield a decomposition of the datasets that can be used to categorize the variability. Most relevant modes emerge naturally, and they can be identified easily. A difference with other analysis methods such as empirical orthogonal function (EOF) or Fourier expansion is that the Koopman modes have a dynamical interpretation, thanks to their connection to the Koopman operator, and they are not constrained in their shape by special requirements such as orthogonality (as it is the case for EOF) or pure periodicity (as in the case of Fourier expansions). The pure periodic modes emerge naturally, and they form a subspace that can be interpreted as the limiting subspace for the variability. The stationary states therefore are the scaffolding around which the dynamics takes place. The modes can also be traced to the Niño variability and in the case of the global SST to the Pacific decadal oscillation (PDO). Significance Statement We compute the Koopman modes for the tropical and global SST, demonstrating that significant dynamical modes can be identified also in complex high-dimensional datasets with continuous spectra. The Koopman modes are then used to identify the stationary subspace, namely, the limit subspace for the invariant evolution of the system.

A high-resolution coupled circulation-wave model for regional dynamic downscaling of water levels and wind waves in the western North Atlantic ocean

Reanalysis datasets provide temporally/spatially continuous data for sea level and wind wave climate studies but often lack the resolution to resolve the complex coastal physical processes. To address this issue, a high-resolution coupled circulation-wave model (ADCIRC + SWAN) is applied to dynamically downscale storm tides (tide + surge + wave setup) and waves in the western North Atlantic Ocean. The model is forced at its sea surface boundary with hourly surface pressure and wind fields, and its open boundary with water levels and direction-frequency wave spectrum from the ERA5 reanalysis dataset. The sensitivity of the model is evaluated for different combinations of internal tide energy conversion, quadratic friction coefficients, and wave physics packages. For the best model setup, the results show an RMSE range of 0.114 m–0.295 m for storm tide, 0.176 m–0.298 m for non-tidal residual, 0.143 m–0.39 m for significant wave height, and 0.42 s–1.04 s for mean wave period. Incorporating the direction-frequency wave spectrum as an open boundary condition improves the model's accuracy, reducing the RMSE for significant wave heights by up to 7% and for the mean wave period by up to 30% over a one-month simulation period.

Monopile-induced turbulence and sediment redistribution form visible wakes in offshore wind farms

Offshore wind farms are becoming an increasingly common feature in the marine environment as a renewable energy source. There is a growing body of evidence on the effects of wind farms on the seabed and its organisms. However, an important and understudied aspect of site development is the interaction of turbine foundations on the surrounding marine environment. Structures exert significant disturbance on tides, waves and currents; these are visible as optically-distinct, elongate wakes at the sea surface with elevated suspended particulate matter. Despite this, there is uncertainty on the mechanisms that lead to the visible manifestation of wakes at turbine foundations, primarily due to a lack of direct measurements. Here, in situ measurements along with a 15-year time series of satellite images of the Thanet offshore wind farm, located within the Thames Estuary, were used to investigate the formation of visible monopile wakes, and the effects these have on the surrounding water column. We show the optically distinct wakes are near-constant at Thanet; visible in >90% of all satellite images, yet no regional change in sea surface turbidity could be attributed to wind farm construction or operation. Monopile wake in situ water samples and acoustic Doppler current profiler (ADCP) backscatter measurements demonstrated colour change related to elevated sea surface sediment concentration. However, averaged water column measurements of suspended sediment within wakes, and upstream of monopiles, remained consistent. These measurements demonstrate that sediment was redistributed towards surface waters, rather than additional sediment becoming suspended in the wake. ADCP velocity measurements supported a mechanism of sediment lofting towards the surface, with enhanced vertically upwards flow recorded in wakes.

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.

Influence of sea surface waves on numerical modeling of an oil spill: Revisit of symphony wheel accident

The greatest purpose of this study is to analyze the importance of surface waves on the hindcasting of the oil spill through the Symphony wheel accident in the Qingdao coastal waters. During the accident period, a total of four synthetic aperture radar (SAR) images by Gaofen-3 (GF-3) were acquired from 2 to 19 May 2021. The hindcasting of two sea surface dynamics, namely currents and waves, is carried out using a coupled marine numeric model. This model, known as the finite-volume community ocean model-simulating waves nearshore (FVCOM-SWAVE), employs a triangular grid. Simulated significant wave height (SWH) is validated against remotely sensed product by the Haiyang-2B (HY-2B) altimeter on April 2021 yields a root mean square error (RMSE) of 0.38, a correlation coefficient (COR) of 0.78, and a scatter index (SI) of 0.34. Subsequently, Stokes drift estimated by waves are included to hindcasting oil spills using the oil particle-tracing method. The bias of the spatial coverage (SAR minus simulations) of an algorithm called the constant false alarm rate (CFAR) is −73.92 km2 with Stokes drift, which is significantly less than the 55.45 km2 coverage without Stokes drift. Moreover, compared with model-simulated oil spills, the bias of the geographic location at the center point with Stokes drift is 8.18 km, which is less than the 12.95 km bias without Stokes drift. These results demonstrate that Stokes drift needs to be included in the prediction of oil spills.

The variability of particulate organic carbon in the northern South China Sea during the 2009–2010 El Niño

The influences of El Niño on the changes of sea surface temperature (SST) and chlorophyll have been well studied, but its impact on changes of particulate organic carbon (POC) remains unclear. In this study, we analyzed the time series of POC and found that from January 2003 to December 2011, the spatially-averaged POC increased at a rate of 0.24 mg/m3 per year. However, during the 2009–2010 El Niño, the spatially-averaged POC increased at a rate of 1.78 mg/m3 per month. This indicated that the increase of POC during 2009–2010 may be related to El Niño. To investigate the relationship between POC and El Niño, we introduced the phase shift time between the POC and the Multivariate ENSO Index (MEI) to characterize the response of POC to El Niño, and calculated the correlation between the time series of the MEI and POC anomalies during El Niño to determine the phase shift time. The results of cross-correlation analysis indicated that changes of POC were delayed from the onset of El Niño by 4 months. Furthermore, a detailed analysis was presented based on a range of in situ observations and satellite-derived datasets available during the 2009–2010 El Niño. The correlation coefficients between POC and chlorophyll, as well as between POC and SST, are 0.9263 and −0.8315, respectively, and the constructed model indicated that chlorophyll and SST had a significant impact on POC. The spatial distribution of POC anomaly maximum occurred in the spring of 2010, because in the winter of 2009, the Rossby-type anomalies were reflected as upwelling Kelvin wave. However, the vertical POC anomaly maximum occurred in the spring of 2011, because the 2009–2010 El Niño experienced a fast phase transition from warm El Niño to cold La Niña, which was associated with oceanic upwelling.

Applying satellite sea surface temperature as Dirichlet-type surface thermal boundary condition in an ocean model

Following the Dirichlet-type boundary condition that specifies value at boundary of a system in mathematics and physics, this study suggests applying satellite sea surface temperature (SST) as Dirichlet-type surface thermal boundary condition (STBC) in ocean models. Numerical experiments with Dirichlet-type and Combined-type STBC with different configurations of satellite SST, for the period of January to April 2019 in the Northwest Pacific Ocean, were carried out based on the Princeton Ocean Model. The experiments were assessed using satellite and in situ observations of temperature, GOFS3.1 analysis and GLORYS12v1 reanalysis. Results show that applying satellite SST as Dirichlet-type STBC could constrain the modeled SST and near-surface temperature in upper 50 m depth well, which is better than the scheme that uses satellite SST as the relaxation terms of Combined-type STBC. The temperature section along the 137°E resulted from the Dirichlet-type STBC is comparable with GOFS3.1 analysis and better than GLORYS12v1 reanalysis in upper 50 m depth. These results suggest that applying high-accuracy satellite SST as Dirichlet-type STBC in ocean models has a promising prospect in numerical simulation.

On the association of Pacific Decadal Oscillation with the interdecadal variability in Asian-Pacific Oscillation

The Asian-Pacific Oscillation (APO), featuring a seesaw pattern in the upper-tropospheric temperatures between Asia and the North Pacific, plays profound roles in the Northern Hemispheric climate anomalies. So, its variability and associated physical mechanisms are of great interest. Although progress has been achieved for the APO variability at the interannual time scale, the understanding of its interdecadal variability remains relatively poor. Based on the twentieth-century reanalysis and the simulations of the preindustrial control, Pacific pacemaker, and large ensemble experiments, this study reports a salient contribution of the Pacific Decadal Oscillation (PDO) to the interdecadal variability of the APO during summer, featured as a warm (cold) PDO phase accompanying a negative (positive) APO polarity. During the warm PDO phase, the sea surface temperature (SST) warming over the eastern Pacific is conducive to local ascending anomalies, which teleconnect with descending anomalies over the Tibetan Plateau through zonal vertical circulations. The anomalous ascent (descent) over the eastern Pacific (Tibetan Plateau) tends to strengthen (weaken) the atmospheric heating, beneficial for a warming (cooling) of the troposphere in situ. In addition, the PDO-related meridional SST gradient anomalies favor a southward shift of the subtropical jet stream, weakening the South Asian high and the North Pacific trough in the upper troposphere, which corresponds to the atmospheric situation of a negative APO phase. Quantitative analysis from the large ensemble simulations indicates that the PDO may contribute to approximately 48% of recent APO interdecadal variation from the negative phase to the positive phase in the late 1990s.

Intensification mechanisms and moisture dynamics of super cyclonic storm ‘Amphan’ over the Bay of Bengal: Implications for aerosol re-distribution

The present research investigates the dynamics and underlying causes contributing to the exceptional intensity of Super Cyclonic Storm (SuCS) Amphan (16th to 21st May 2020) over the Bay of Bengal (BoB), as well as its impact on aerosol redistribution along the four cities of eastern coast and north-eastern India. Notably, the SuCS was formed during the first phase of the COVID-19 lockdown in India, giving it a unique aspect of study and analysis. Our analysis based on 30 years of climatology data from Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis reveals ‘positive’ monthly anomalous winds (0.8 to 1.6 m/s) prevailed over the central BoB for May 2020. The present study further found the evolution of ‘barrier layer thickness’(BLT) leading up to landfall, noting a thickening trend from 8 to 3 days before landfall, contributing to maintaining warmer sea surface temperatures near the coast. Additionally, utilizing European Centre for Medium-Range Weather Forecasts (ECMWF), reanalysis version-5 (ERA-5) data, a mean positive sea surface temperature (SST) anomaly of 0.8 to 1 °C was observed ‘before’ cyclone period (10–15 May 2020) near the cyclogenesis point. A detailed examination of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) vertical cross-section plots during the cyclone's intensification stage reveals the presence of high-altitude clouds composed primarily of ice crystals. Further, analysis also indicates that the cyclone transported Sea-salt PM2.5 aerosols from the ocean, dispersing them in the landfall region.The aerosol optical Depth (AOD) data obtained from the National Aeronautics and Space Administration's (NASA) ‘Clouds and the Earth's Radiant Energy System (CERES)’ mission and MERRA-2 were also analysed, revealing that the cyclone redistributed aerosols over the Bengal basin region (mainly over ‘Kolkata') and three other nearby cities along the track of the cyclone (i.e., Bhubaneswar (Odisha) Agartala (Tripura) and Shillong (Meghalaya) respectively).

Latest Pleistocene-Holocene sedimentary evolution from sediment records of the Tam Giang-Cau Hai coastal lagoon system, central Vietnam

The latest Pleistocene-Holocene sedimentary evolution was reconstructed from two boreholes collected in the Tam Giang-Cau Hai coastal lagoon region of central Vietnam. This research was based on analyzing radiocarbon dating, grain size, sedimentary structure, petrological components, and diatom assemblages. Besides, some high-resolution seismic data from the lagoon water body were interpreted to estimate Holocene sediment thickness. Sedimentary records of two boreholes revealed various sedimentary facies under the sea-level change since the Last Glacial Maximum (LGM): Fluvial channel facies, fluvial-lagoonal transition facies, lagoon facies, lagoonal-to-fluvial facies, and flood plain facies. Radiocarbon dating indicated that the coastal lagoon system possibly initiated around 8.8–8.5 cal kyr BP, corresponding to the meltwater pulse 1 C when saltwater intruded into the lagoon system. The borehole-sediment data combined with the high-resolution seismic data shows the thickness of Holocene sediments is approximately 20–35 m in the lagoon region. The study also suggested that the sandy barrier complex along the coast between the lagoon system and the open sea has been likely built during the transgression since 10–8.8 cal kyr BP. Additionally, five ecological zones were identified based on diatom assemblages in borehole HK02 indicating variations in marine and fluvial influence in the coastal lagoon system during the past 8.8 kyr: Dia-HK02–1 indicates a low presence of diatoms, suggesting a weak marine influence in the coastal lagoon system; Dia-HK02–02 shows the strongest marine influence and stable ecological conditions in the coastal lagoon system; Dia-HK02–03 demonstrates a partial decrease in marine influence; Dia-HK02–4 shows an increasing fluvial influence; and Dia-HK02–5 developed under completely dominated fluvial conditions on land.

Warming of mid-latitude North Atlantic Sea surface temperature strengthens the daily variability of winter PM2.5 in eastern China

Abstract Climate factors, in addition to human activities, are acknowledged to exert a notable influence on the synoptic PM2.5 variations over eastern China in extensive case studies. Based on observed daily PM2.5 concentrations data, this study reveals the enhanced daily variability (DV) of PM2.5 concentrations over eastern China and identifies its association with mid-latitude sea surface temperature anomalies over the North Atlantic. The dominant daily mode of PM2.5 concentrations identified through empirical orthogonal function analysis accounts for 43.75% of the total variance, with its DV experienced a significant enhancement from 1979 to 2019. The identified enhancement is attributed to the intensified wave train propagation along the mid-latitudes on a time scale of 10–30 d. The eastward propagation of the identified wave train can expose eastern China to recurrent influences of cyclonic and anticyclonic anomalies, resulting in an initial increase and subsequent decrease in PM2.5 concentrations. Statistical analysis and dynamic diagnostics show that the warming of the mid-latitude North Atlantic Ocean enhances the wave train at its source through the local energy exchange, and ultimately leads to an increased DV of PM2.5 concentrations.

An atmosphere–ocean coupled model for simulating physical and biogeochemical state of north Indian Ocean: Customisation and validation

An atmosphere-ocean biogeochemistry coupled model is configured, customised and validated to decipher the role of atmospheric and oceanic processes in the north Indian Ocean (NIO, 3–30° N, 40–100° E), Arabian Sea (AS, 4–25° N, 50–75° E) and Bay of Bengal (BoB, 4–25° N, 76–100° E). The validation of model results with measurements shows that Sea Surface Temperature is better simulated by the coupled model than the standalone ocean model, with an average bias of ±0.2 °C in NIO. The simulated Sea Surface Salinity has a smaller bias with respect to reanalysis in AS (0.2 psu), but slightly higher in BoB (0.5 psu). The vertical distribution of temperature and salinity is also better represented in the coupled model. The atmospheric forcing, such as Long Wave Radiation, wind stress, and net surface heat and salt fluxes are better simulated by the coupled model with comparable variability and seasonality to the reanalysis data. The winter and summer Chlorophyll-a (Chl-a) blooms in AS are also well reproduced by the coupled model. The model also well simulates NO3 within top 100 m in AS and BoB. The nitrate budget analysis indicates that vertical advection and entrainment play big roles in governing the overall budget in upwelling regions of NIO. Nitrate uptake by phytoplankton is the dominant biological process, except in the northern AS, where denitrification is dominant. New production contributes to Net Primary Productivity (NPP) in most upwelling regions in AS except in the northern AS, where regenerated production is higher than the new production. In BoB, the regenerated production dominates except in East Coast of India during monsoon season. Therefore, our study provides new insights on the capability of the coupled ocean-atmosphere models in simulating the physical and biogeochemical processes, and air-sea interactions in NIO.