Confluence Region Research Articles

Riverbed morphology and hydrodynamics in the confluence of complex mega rivers - A study in the Branco and Negro rivers, Amazon basin

The study of the morphology and hydrodynamics of large rivers has advanced in recent decades. Nonetheless, there are a few studies with surveyed data carried out at the confluences of the mega rivers (Q > 17,000 m³s−1), especially those from the Amazon basin. The Negro River is the main river in the northwestern Amazon and is considered the sixth-largest in the world in terms of water discharge, has a complex multichannel pattern and a peculiar geological configuration. This article aims at analyzing the bed morphology and hydrological processes in the confluence region between the Branco and Negro rivers. Field surveys with echo-sounding and Acoustic Doppler Current Profiler were carried out over an area of 31 km2 during the low water period of 2021 year. The results of this survey made it possible to identify the main morphological characteristics of the riverbed in the confluence hydrodynamic zone between Branco and Negro River, as well as the dynamics of the velocity, streamflow, and transport of suspended sediment of these important northwestern Amazonian rivers. Analysis of bed morphology and their neotectonic control are also discussed. These results presented in this work contribute to expanding the knowledge about the confluence of large tropical rivers with significant bed discordance and differences in the density of flows.

Seasonal Cycle of the Confluence of the Tsugaru Warm, Oyashio, and Kuroshio Currents East of Japan

AbstractThe Sanriku Confluence east of Japan is the region of confluence of the Tsugaru Warm Current from the Sea of Japan, the Oyashio Current from the western subarctic gyre and the Sea of Okhotsk, and Kuroshio water that has detached from the subtropical gyre. It is a field of vigorous stirring driven by variability in current systems, but transition processes between water masses have yet to be clarified. High‐resolution underwater conductivity‐temperature‐depth observations were undertaken during each of the four seasons to test a hypothesis that the water‐mass gradient in the Sanriku Confluence is set without full stirring. Analyses in isopycnal coordinates indicate the seasonal occurrence of prominent fronts of both salinity and displacement, related to the emergence of various water masses. After fitting error functions to frontal features, the seasonally emerging isopycnal salinity fronts often became extremely sharp with widths typically narrower than 3 km, supporting the hypothesis. Disturbances distinguished from frontal structures indicate variance peaks with ranges similar to or slightly greater than the baroclinic Rossby radius, likely stirring the large‐scale tracer gradient at this scale. Elevated variances in isopycnal salinity were often observed around the fronts, and variances caused by isolated patches with salinity anomalies were also large. By determining the horizontal scales at which the effects of internal wave heaving becomes dominant, we were able to calculate the potential vorticity (PV) of the balanced flow field. As the PV becomes negative or near zero around the fronts, symmetric instability might have developed.

Holocene palaeoceanographic history of the western South Atlantic

Although the circulation of the western South Atlantic tightly controls the meridional heat and salt transport, our knowledge about its long-term past changes is still fragmented and sparse. While many studies focused their attention on the last glacial and deglaciation periods, Holocene studies mostly lack of an integrated analysis from the subtropical and midlatitude regions. The analysis of selected regionally distributed palaeoceanographic records along the western South Atlantic between 27° and 39°S reveals that, although weaker in amplitude than the dramatic shifts of the last glacial cycle, the Brazil-Malvinas Confluence region showed substantial changes throughout the Holocene. The shift in the location of both southern and northern source waters in the Atlantic implied a strengthening of the Southern Atlantic Meridional Overturning Circulation since the Mid-Holocene. This, together with the progressive southward displacement of the Brazil-Malvinas Confluence confirms the progressive weakening of the Atlantic Meridional Overturning Circulation towards the preindustrial times.

Optical characterization of dissolved organic matter in Kuroshio-Oyashio confluence region: Implication for water mass mixing

Chromophoric dissolved organic matter (CDOM) and fluorescent DOM (FDOM) are important components of DOM and play many important roles in biogeochemical processes. Here, we examined the concentration, composition, and distribution of DOM in waters (0–1000 m depth) from the Kuroshio-Oyashio confluence region (Northwestern Pacific), a key area for ocean carbon cycle and an important fishery. Dissolved organic carbon concentration varied between 30.5 and 78.2 μmol L−1 with a higher level in surface waters. Two humic-like components (C1, C3) and one protein-like component (C2) were identified by fluorescence excitation emission matrices-parallel factor analysis. The C1 and C3 intensity generally increased with depth, and positively correlated with apparent oxygen utilization (p < 0.01), whereas the C2 intensity decreased downward, and negatively correlated with apparent oxygen utilization (p < 0.01), suggesting in-situ production of bio-refractory C1 and C3 during the microbial transformation of labile organic matter (like C2). The C1 intensity was significantly different among three water masses (p < 0.01) that was 11.7 ± 2.5 × 10−3 R.U. for Kuroshio Current, 20.1 ± 1.9 × 10−3 R.U. for Oyashio Current, and 26.1 ± 5.8 × 10−3 R.U. for North Pacific Intermediate Water. The application of three-endmember mixing model based on the C1 intensity and salinity revealed variable contributions of Kuroshio, Oyashio, and North Pacific Intermediate Water that were consistent with the predication from water temperature and salinity. Our study suggests that the optical property of DOM is a useful tracer for water masses in the Kuroshio-Oyashio confluence region, consisting with previous reports for other marine environments.

Part I: Hydrological properties within the eastern Indonesian throughflow region during the INDOMIX experiment

The Indonesian Mixing (INDOMIX) cruise of July 2010, resolves the Pacific Ocean water masses spreading from the Halmahera Sea through the Seram Sea, Manipa Strait, Banda Sea, to the Ombai Strait, and along the southern margin of Lesser Sunda Arc across the northern Savu Sea, Sumba Strait, and south of Lombok Strait. This paper focuses on the characteristics and stratification of the water masses, along this path and Part II of this study discusses the biogeochemical aspects. Two companion papers on quantification of turbulent mixing have been published elsewhere. We find: a) A marked transformation of South Pacific (SP) thermocline water within a sharp salinity front between the Halmahera Seram Seas. Tidally induced vertical mixing is the main process weakening the SP stratification, as suggested by a simple 1-dimensional diffusion model forced by vertical diffusivity of INDOMIX vertical microstructure profiler dataset. The transformation of SP water by vertical mixing can occur after 3 days, which is in good agreement with previously reported water mass residence times. Lateral advection plays a minor role; b) Interleaving salinity structure within the thermocline of the central Halmahera Sea and two deeper inflow channels. The interleaving salinity features within the central Halmahera Sea thermocline occurs where vertical diffusivity is relatively weak, compared to the vertical mixing at entry/exit portal that removes the interleaving salinity features. Two deeper inflow channels (about 950 m and 740 m depth) in the entry portal of the Halmahera Sea are mapped from the multi-beam echo sounder measurement, where both channels allow the renewal of SP thermocline water and salinity interleaving processes; c) Northward flow of thermocline water (100–200 m depth) in the Manipa Strait that injects Banda homogeneous salinity water into Seram Sea that erode Halmahera salty water via strong diapycnal mixing; d) Cyclonic upper layer circulation in Banda where eastward (westward) flow occurs in the northern (southern) Banda, confirmed by recent modeling study; and e) Relative salty Indian Ocean intermediate water flowing along southern margin of the Lesser Sunda Arc, within the South Java Undercurrent. Two distinct upper thermocline water, due to different source of Indonesian Throughflow (ITF) water, are found in the Banda Sea: fresher and colder water contrasting to saltier and warmer water that converge in this confluence region. Beneath it, Banda intermediate homogeneous salinity water and low dissolved oxygen water is dominant.

Flow regimes, mixing and reaction yield of a mixture in an X-microreactor

In the present work, experimental flow visualizations and numerical simulations are carried out jointly to investigate how flow regimes affect the reaction yield in an X-microreactor. The X-microreactor is fed with an aqueous solution of ascorbic acid into one of the inlet channels and an aqueous solution of methylene blue and hydrochloric acid into the other which, upon reacting, produce a colorless reaction product that can be directly measured in experiments. Different kinetic constants are considered.At low Reynolds number, Re, the two reactant streams remain stratified in the mixing channels due to their different densities and mixing is promoted only by diffusion. By increasing the Reynolds number, the engulfment regime occurs, which is characterized by the presence of a single vortical structure in the confluence region and in the outlet channels, enhancing mixing thanks to convection. The reaction yield is studied both numerically and experimentally showing a remarkable agreement between the results. In the stratified regime, the reaction yield decreases with the Reynolds number due to a decrease in residence time. Instead, with the onset of the engulfment regime, the reaction yield starts to increase with Re because enhancement in the mixing degree between the streams overcomes the reduction of the residence time. However, due to the opposite behaviors of mixing efficiency and residence time with Re, the reaction yield does not monotonically increase with Reynolds number within the engulfment regime, but it reaches a maximum and then slightly decreases. The reaction yield obtained in the X-microreactor is significantly larger than that of a T-microreactor having the same cross-section geometry and operating in the same conditions.

Mixing Improvement in a T-Shaped Micro-Junction through Small Rectangular Cavities.

The T-shaped micro-junction is among the most used geometry in microfluidic applications, and many design modifications of the channel walls have been proposed to enhance mixing. In this work, we investigate through numerical simulations the introduction of one pair of small rectangular cavities in the lateral walls of the mixing channel just downstream of the confluence region. The aim is to preserve the simple geometry that has contributed to spread the practical use of the T-shaped micro-junction while suggesting a modification that should, in principle, work jointly with the vortical structures present in the mixing channel, further enhancing their efficiency in mixing without significant additional pressure drops. The performance is analyzed in the different flow regimes occurring by increasing the Reynolds number. The cavities are effective in the two highly-mixed flow regimes, viz., the steady engulfment and the periodic asymmetric regimes. This presence does not interfere with the formation of the vortical structures that promote mixing by convection in these two regimes, but it further enhances the mixing of the inlet streams in the near-wall region of the mixing channel without any additional cost, leading to better performance than the classical configuration.

The Migration Patterns and Identity of the Okun-Yoruba People of Central Nigeria

Migration is a global phenomenon. From time immemorial, man has been moving from one place to another for temporary or permanent settlements. This is largely facilitated by natural or artificial factors. The former include nat­ural disasters such as ecological change and draught, while the latter can be in­stigated by wars and search for better fortunes. This study examines the nature of the movements of the Okun-Yoruba people from the western region of Ni­geria to the confluence of the Niger and Benue rivers in the Central Geograph­ical Zone of Nigeria. It reveals how their long years of interactions with peoples of the confluence region were aided by the European delimitation of the area as part of the North, which has been the basis of sociopolltical agitation. The study reveals that Okun-Yoruba people, despite their geographical delimitation as northerners, see themselves more as southerners.

Characteristics of water contamination in abandoned coal mines: a case study on Yudong River area, Kaili, Guizhou Province, China

The seasonal and spatial changes in the chemical composition of the water in abandoned mine drainages and rivers in Yudong River area in the years of 2017–2018 were analyzed. The effects of mine water drainage on the seasonality and physicochemical properties of the river water after mine closure were evaluated, and the feasibility of irrigation using river water and the degree of pollution to farmland were assessed using the Water Quality Standard for Farmland Irrigation. The results show that the mine water has low pH value (< 3.5–4) and high levels of total hardness, SO42−, Fe, Al, and Zn. In addition, the pH of the mine water is negatively correlated with the presence of other metal ions. The correlation coefficient between the chemical oxygen demand (COD) and Fe reached 0.989. While the pollution levels of Pinglu and Baishui rivers were low, the confluence region of the two rivers was seriously polluted. However, only the levels of Fe and Cd slightly exceeded the Surface Water Environmental Quality Standard after the confluence of Yudong and Chongan rivers. Overall, the heavy pollution type of the confluent river is consistent with mine water pollution. The water quality is slightly better in the dry season compared than in the high-water period. Sulfate and Fe content decreased by 39 and 16 mg/L, respectively, and Cd content decreased two-fold. Despite these findings, this study shows that from 2017 to 2018, the pH and Cd content of the rivers at the confluence exceeded the irrigation limit and the water quality continued to deteriorate, which may pose a soil contamination risk. Long-term use of the river for irrigation water may cause toxic elements such as Cd, Fe, Mn, SO42−, Al, and F- to enter the food chain, thereby endangering the life and health of villagers in Yudong River area.

Broadband Characteristics of Zooplankton Sound Scattering Layer in the Kuroshio–Oyashio Confluence Region of the Northwest Pacific Ocean in Summer of 2019

Acoustic technology, as an important investigation method for fishery resources, has been widely used in zooplankton surveys. Since the Kuroshio–Oyashio confluence region has an extensive distribution of zooplankton, describing and analyzing the characteristic of the zooplankton sound scattering layer (SSL) in this area is essential for marine ecology research. To understand its spatial–temporal distribution, acoustic data of the Kuroshio–Oyashio confluence region at the Northwest Pacific Ocean, obtained by a Simrad EK80 broadband scientific echosounder in 2019, were used on board the research vessel (RV) Songhang. After noise removal, the volume backscattering strength (SV) was measured to plot the broadband scattering spectrogram of each water layer and to exhibit zooplankton distribution. The results show that the main sound scattering within 0–200 m originate from the zooplankton, and the SV of each layer increases with the rise of the transducer frequency. The magnitude of SV was closely synchronized with the solar altitude angle, which gets smaller when the angle is positive, then larger when the angle is negative. It means that the SSL has a diel vertical migration (DVM) behavior with the variation of solar height. Meanwhile, scattering strength was positively correlated with temperature in the vertical direction and showed a maximum of −54.31 dB at 20–40 m under the influence of the thermocline. The Kuroshio and Oyashio currents had an obvious influence on the scattering strengths in this study, indicating a low value when next to the Oyashio side and a high value on the Kuroshio side. The scattering strength near the warm vortex center was higher than that at the vortex edge. The results of this study could provide references for a long-term study on ecological environment variation and its impacts on zooplankton distribution.

Impacts of Salinity Variation on the Mixed‐Layer Processes and Sea Surface Temperature in the Kuroshio‐Oyashio Confluence Region

AbstractIn this study, salinity variations in the Kuroshio–Oyashio confluence region (KOCR) are examined through analyses of observational datasets and an ocean reanalysis product, and their potential impacts on sea surface temperature are assessed by sensitivity experiments using a one‐dimensional mixed layer model (1‐D ML model). We have detected prominent covariations in near surface temperature and salinity in the KOCR during the boreal winter to spring. Further investigation revealed that such covariations are closely related to the dynamical stability of the Kuroshio Extension (KE), and anomalous warming and salinification (cooling and freshening) are observed in the KOCR when the upstream of the KE is in an unstable (a stable) state. It is found that modulation heat and freshwater transport by mesoscale eddies and large‐scale current anomalies are closely related to such observed variation. Then, we have quantitatively estimated the impacts of these salinity variations on local density by a detailed decomposition of total anomaly fields. Although the total density anomalies are dominated by contributions from temperature, the salinity contribution has sizable magnitude especially in the northern part of the KOCR, where the background temperature is low and the dependence of density on temperature variations is weak. To further quantify the impact of salinity anomalies, we conducted a series of sensitivity experiments utilizing the 1‐D ML model. The results from these experiments revealed that salinity anomalies significantly alter the strength of vertical mixing and eventually lead to differences in sea surface temperature of approximately 1.0°C.

Assessing the distribution and abundance of larval Longfin Smelt: What can a larval monitoring program tell us about the distribution of a rare species?

Following its listing as threatened under the California Endangered Species Act in 2009, Longfin Smelt (Spirinchus thaleichthys) became a focus of resource managers in the San Francisco Estuary. Water exports were identified as one of the factors affecting Longfin Smelt abundance, and managers were challenged with balancing freshwater flows through the Sacramento-San Joaquin River Delta between human and ecosystem needs. This balance becomes especially challenging during the winter and spring when Longfin Smelt are spawning. Resource managers identified that the impact associated with entrainment of larval Longfin Smelt in the winter was uncertain, and to understand and manage this risk, new data was needed. In 2009 the Smelt Larva Survey was implemented and has since sampled newly hatched larvae from January–March. Here, I analyze this data and ask specific questions regarding distribution and densities of the larvae throughout five regions of the Upper Estuary – Napa River, Suisun, Confluence, Northern Delta, and Southern Delta – with the goal of understanding the spatial and temporal patterns of larval distribution since 2009. I found that larvae were most prevalent in the Suisun, Confluence, and Northern Delta regions, and less common in the Southern Delta and Napa River regions. Larval Longfin Smelt densities changed following a recent drought and record low population abundances. Median per-station averaged densities ranged from 154 to 274 fish per 1,000 m3 between 2009 and 2013 but declined to 1 to 65 fish per 1,000 m3 from 2014 to 2019. This survey data demonstrates that Longfin Smelt reproductive output has declined since their listing in 2009 and that their distribution into the Southern Delta is low relative to the rest of the Upper Estuary. These results reaffirm the species’ continued decline since its listing, and that improving the abundance of spawning adults is one of the many important steps needed for long-term recovery and resilience.

A Case Study of the Thermohaline Fine-scale Structure of the Sub-Arctic Front

ABSTRACT This paper analyzes the characteristics of the thermohaline fine-scale structure of the sub-Arctic front (SAF) in the Kuroshio–Oyashio confluence region using temperature, salinity, and current data obtained from surveys in June 2016. Thermohaline intrusion is found to be prevalent in the sea area of the SAF, occurring predominantly in frontal zones with depths less than 200 m and in North Pacific intermediate water (NPIW) layers with depths greater than 350 m. The intrusion in the SAF zones with depths less than 200 m has a higher intensity but smaller dimensions in the NPIW. According to our analysis, the thermohaline intrusion in the SAF sea zone is mainly attributable to the joint effect of double diffusion and flow instability. At depths of 0–350 m, the thermohaline intrusion is vulnerable to the influences of salt fingering, diffusive convection, and symmetric instability. At depths deeper than 350 m, diffusive convection is significant. In addition, shear instability might be a contributor. Considering that the effect of turbulent mixing is primarily governed by symmetric instability in the SAF sea areas, the flow instability characterized by a dominant contribution from symmetric instabilities might exert a more significant influence on thermohaline intrusion. The more distinct thermohaline staircases at layers deeper than 350 m, which are mainly produced by diffusive convection, have larger vertical scales than the staircases in other layers.

Projecting ocean acidification impacts for the Gulf of Maine to 2050

Ocean acidification (OA) is increasing predictably in the global ocean as rising levels of atmospheric carbon dioxide lead to higher oceanic concentrations of inorganic carbon. The Gulf of Maine (GOM) is a seasonally varying region of confluence for many processes that further affect the carbonate system including freshwater influences and high productivity, particularly near the coast where local processes impart a strong influence. Two main regions within the GOM currently experience carbonate conditions that are suboptimal for many organisms—the nearshore and subsurface deep shelf. OA trends over the past 15 years have been masked in the GOM by recent warming and changes to the regional circulation that locally supply more Gulf Stream waters. The region is home to many commercially important shellfish that are vulnerable to OA conditions, as well as to the human populations whose dependence on shellfish species in the fishery has continued to increase over the past decade. Through a review of the sensitivity of the regional marine ecosystem inhabitants, we identified a critical threshold of 1.5 for the aragonite saturation state (Ωa). A combination of regional high-resolution simulations that include coastal processes were used to project OA conditions for the GOM into 2050. By 2050, the Ωa declines everywhere in the GOM with most pronounced impacts near the coast, in subsurface waters, and associated with freshening. Under the RCP 8.5 projected climate scenario, the entire GOM will experience conditions below the critical Ωa threshold of 1.5 for most of the year by 2050. Despite these declines, the projected warming in the GOM imparts a partial compensatory effect to Ωa by elevating saturation states considerably above what would result from acidification alone and preserving some important fisheries locations, including much of Georges Bank, above the critical threshold.

Wind speed and mesoscale features drive net autotrophy in the South Atlantic Ocean

A comprehensive in situ dataset of chlorophyll a (Chl a; N = 18,001), net primary production (NPP; N = 165) and net community production (NCP; N = 95), were used to evaluate the performance of Moderate Resolution Imaging Spectroradiometer on Aqua (MODIS-A) algorithms for these parameters, in the South Atlantic Ocean, to facilitate the accurate generation of satellite NCP time series. For Chl a, five algorithms were tested using MODIS-A data, and OC3-CI performed best, which was subsequently used to compute NPP. Of three NPP algorithms tested, a Wavelength Resolved Model (WRM) was the most accurate, and was therefore used to estimate NCP with an empirical relationship between NCP with NPP and sea surface temperature (SST). A perturbation analysis was deployed to quantify the range of uncertainties introduced in satellite NCP from input parameters. The largest reductions in the uncertainty of satellite NCP came from MODIS-A derived NPP using the WRM (40%) and MODIS-A Chl a using OC3-CI (22%).The most accurate NCP algorithm, was used to generate a 16 year time series (2002 to 2018) from MODIS-A to assess climate and environmental drivers of NCP across the South Atlantic basin. Positive correlations between wind speed anomalies and NCP anomalies were observed in the central South Atlantic Gyre (SATL), and the Benguela Upwelling (BENG), indicating that autotrophic conditions may be fuelled by local wind-induced nutrient inputs to the mixed layer. Sea Level Height Anomalies (SLHA), used as an indicator of mesoscale eddies, were negatively correlated with NCP anomalies offshore of the BENG upwelling fronts into the SATL, suggesting autotrophic conditions are driven by mesoscale features. The Agulhas bank and Brazil-Malvinas confluence regions also had a strong negative correlation between SLHA and NCP anomalies, similarly indicating that NCP is forced by mesoscale eddy generation in this region. Positive correlations between SST anomalies and the Multivariate ENSO Index (MEI) in the SATL, indicated the influence of El Niño events on the South Atlantic Ocean, however the plankton community response was less clear.

The Regional Ice Ocean Prediction System v2: a pan-Canadian ocean analysis system using an online tidal harmonic analysis

Abstract. Canada has the longest coastline in the world and includes diverse ocean environments, from the frozen waters of the Canadian Arctic Archipelago to the confluence region of Labrador and Gulf Stream waters on the east coast. There is a strong need for a pan-Canadian operational regional ocean prediction capacity covering all Canadian coastal areas in support of marine activities including emergency response, search and rescue, and safe navigation in ice-infested waters. Here we present the first pan-Canadian operational regional ocean analysis system developed as part of the Regional Ice Ocean Prediction System version 2 (RIOPSv2) running in operations at the Canadian Centre for Meteorological and Environmental Prediction (CCMEP). The RIOPSv2 domain extends from 26∘ N in the Atlantic Ocean through the Arctic Ocean to 44∘ N in the Pacific Ocean, with a model grid resolution that varies between 3 and 8 km. RIOPSv2 includes a multivariate data assimilation system based on a reduced-order extended Kalman filter together with a 3D-Var bias correction system for water mass properties. The analysis system assimilates satellite observations of sea level anomaly and sea surface temperature, as well as in situ temperature and salinity measurements. Background model error is specified in terms of seasonally varying model anomalies from a 10-year forced model integration, allowing inhomogeneous anisotropic multivariate error covariances. A novel online tidal harmonic analysis method is introduced that uses a sliding-window approach to reduce numerical costs and allow for the time-varying harmonic constants necessary in seasonally ice-infested waters. Compared to the Global Ice Ocean Prediction System (GIOPS) running at CCMEP, RIOPSv2 also includes a spatial filtering of model fields as part of the observation operator for sea surface temperature (SST). In addition to the tidal harmonic analysis, the observation operator for sea level anomaly (SLA) is also modified to remove the inverse barometer effect due to the application of atmospheric pressure forcing fields. RIOPSv2 is compared to GIOPS and shown to provide similar innovation statistics over a 3-year evaluation period. Specific improvements are found near the Gulf Stream for all model fields due to the higher model grid resolution, with smaller root mean squared (rms) innovations for RIOPSv2 of about 5 cm for SLA and 0.5 ∘C for SST. Verification against along-track satellite observations demonstrates the improved representation of mesoscale features in RIOPSv2 compared to GIOPS, with increased correlations of SLA (0.83 compared to 0.73) and reduced rms differences (12 cm compared to 14 cm). While the RIOPSv2 grid resolution is 3 times higher than GIOPS, the power spectral density of surface kinetic energy provides an indication that the effective resolution of RIOPSv2 is roughly double that of the global system (35 km compared to 66 km). Observations made as part of the Year of Polar Prediction (2017–2019) provide a rare glimpse at errors in Arctic water mass properties and show average salinity biases over the upper 500 m of 0.3–0.4 psu in the eastern Beaufort Sea in RIOPSv2.

Numerical prediction of tropical cyclogenesis part I: Evaluation of model performance

AbstractThis is a two‐part study investigating the numerical prediction of tropical cyclogenesis. This paper (Part I) presents a comprehensive statistical assessment of the performance of the high‐resolution European Centre for Medium‐Range Weather Forecasts (ECMWF) deterministic forecast in predicting tropical cyclone (TC) genesis over the western North Pacific (WNP) between 2007 and 2018 using The International Grand Global Ensemble (TIGGE) data. Each genesis forecast at each lead time is classified into one of these categories: well predicted (WP), early formation (EF), late formation (LF), or failed prediction (FP). Based on the synoptic patterns at the actual genesis time, the genesis of a particular TC is grouped into one of five flow patterns: monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), easterly wave (EW), and preexisting tropical cyclone (PTC). Any case that does not fall in one of these patterns is labeled as unclassified flow pattern (UCF). Overall, the prediction skill of SL cases is the highest, followed by CR, GY, and PTC, while the EW and UCF patterns have the lowest skill. No significant improvement in the model performance is found during the 12 years, probably partly because of the low percentage of cases with the SL pattern in the later years. The prediction skills of TC genesis in May, June, September, and October are the highest, while those in July, August, and November are the lowest. TCs that form further to the south are easier to predict in the SL pattern. In Part II, we will examine the model predictions of the dynamic and thermodynamic variables in the WP cases to identify the physical processes responsible for TC genesis.

Planktonic foraminifera assemblages from the Brazil–Malvinas Confluence: palaeoceanographic implications of sub-surface temperature reconstructions in the western South Atlantic

Planktonic foraminifera census data have been widely used to reconstruct changes in ocean ecosystems as well as ocean chemistry and circulation through calibration functions. Here we analyse new core-top census data from 22 sites in the western South Atlantic, improving the geographical coverage and spatial resolution of the environmental gradients from the region covered by the Brazil–Malvinas Confluence (˜38°S–53°W). We combine them with previous data to provide an up-to-date compilation of the western South Atlantic planktonic foraminifera–calibration data set. We study the relationship between the assemblages present in the core-top samples and the most dynamic environmental variables from the region to establish the environmental variable(s) more resolved by the assemblages. Therefore, we develop and assess a new calibration function using the data set and testing several statistical models at different water depths. Our results reveal that the distribution and abundance of the species in the Brazil–Malvinas Confluence region mainly reflect the influence of two environmental variables: the mixed layer temperature and mixed layer depth (57% of total variance). The most precise reconstructions were obtained when using sub-surface temperatures between 40 and 50 m water depth. The application of the calibration function to a Holocene sediment core at ˜37°S–53°W revealed mean annual sub-surface temperature reconstructions between 8°C and 11°C, confirming the northward displacement of the Brazil–Malvinas Confluence during the onset of the Holocene and suggesting a major influence of the Malvinas Current during the entire Holocene at the studied site.

Future Changes in Western North Pacific Tropical Cyclone Genesis Environment in High-Resolution Large-Ensemble Simulations

This study applied the database for Policy Decision making for Future climate change (d4PDF) and tropical cyclone (TC) genesis (TCG) environment factors to project future changes in the frequency and characteristics of TCs over the western North Pacific. We examined current and future TCG environmental conditions in terms of the contribution of five factors: shear line (SL), confluence region (CR), monsoon gyre, easterly wave (EW), and Rossby wave energy dispersion from a preexisting TC (PTC). Among summer and autumn TCs, the contributions of SL and EW to future TCG increased by about 4% and 1%, respectively, whereas those of CR and PTC decreased by the same amounts. In future climate projections, the average lifetime maximum intensity (LMI) of TCs associated with EW (EW-TCs) was significantly higher than those of TCs associated with other factors except PTC. At higher sea surface temperatures and wetter conditions, higher lower-tropospheric relative vorticity was related to increases in the development rate of EW-TCs. Findings of this study suggest that increases in the average LMI of all future TCs were caused by large contributions from the average LMI of future EW-TCs.

INTERPRETATION OF MAJOR STRUCTURES WITHIN THE BASEMENT REGION OF BENUE-NIGER CONFLUENCE FROM AEROMAGNETIC AND RADIOMETRIC DATA KOGI STATE NIGERIA

Interpretation of Aeromagnetic and Radiometric Data covering the basement region of Benue-Niger confluence was executed to delineate major structures and other geologic frame works of mineral interest. The study area which hosts the Benue-Niger confluence also encloses two major geologic units which are basement complex and sedimentary basins. The Aeromagnetic data set comprising sheet 227 (Koton-Karfe), 247 (Lokoja) and 267 (Idah) was enhanced to reveal geologic structures while radiometric data was analysed to map lithology and zones affected by hydrothermal alterations. A set of mathematical algorithms was used to enhance the data for interpretation. First Vertical derivatives, Analytical Signal and Euler deconvolution filters were applied to the Aeromagnetic data while Ratio and Ternary images of the three radiogenic elements were obtained for the radiometric data. Magnetic signatures from the TMI showed a mixture of high and low susceptibility below koton-karfe due to intrusion of oolitic iron ore within the sedimentary formation. Lokoja regions recorded highest susceptibility of 165 nT due to magnetic signatures emanating from exposed basement rocks. The southern Idah regions recorded relatively low susceptibility. Result of First Vertical Derivative revealed near surface mineral potent structures labelled F1 – F8, cringing surface features B1, B2 and B3. Analytical signal revealed high amplitudes range of 0.174 to 0.579 cycles for magnetic sources majorly at the basement regions, while low amplitude range of 0.021 to 0.157 cycles were recorded around the sedimentary regions. Euler depth analysis revealed shallower depth to sources in the basement and deeper depth to sources in the sedimentary regions due to thick overburden. Radiometric signatures from the K/Th ratio map revealed portions around Latitude 8°00’ NW and 7°30’ SW shaded in pink colour and having values above known threshold of 0.2 %/ppm to be hydrothermally altered. Mapping of lithology from Ternary map revealed K-Feldspar mineral bearing rocks dominated the NW and SW regions, while sandstones, ironstones, mudstones, shale, alluvium and other fluvial sedimentary lithologies dominated the sedimentary North-east and South-Eastern regions. The western regions (NW and SW) hosted the major structures in form of magnetic lineaments trending NE-SW and E-W which also coincided with regions delineated to be hydrothermally altered and apparently represents the most prospective regions of mineralisation in the study area.