Hydrographic Changes Research Articles

A 70-year perspective on water-mass transformation in the Greenland Sea: From thermobaric to thermal convection

The hydrography of the central Greenland Sea was reconstructed from observations including bottle measurements, Conductivity/ Temperature/ Depth (CTD) measurements, and Argo floats for the period 1950 to 2020. Greenland Sea Deep Water was renewed during bottom-reaching convection prior to the mid-1980s, facilitated by the thermobaric effect. During a period of shallow convection in the late 1980s and early 1990s, a stratification maximum formed and isolated the deep from the intermediate Greenland Sea. As a consequence, convection was limited to depths shallower than 2000m during the past decades and a new class of intermediate water formed instead of deep water. The initial cause for the formation of the stratification maximum was a near-surface freshwater anomaly. A subsequent, rapid temperature and salinity increase in the upper 2000m resulted in an overall density reduction of the intermediate water which strengthened the stratification maximum. Along with the transition from formation of deep water to formation of intermediate water, the Greenland Sea became temperature-stratified at intermediate depths. This regime-shift in stratification can be traced to increased temperature and salinity in the inflowing Atlantic-origin Water. Below the stratification maximum, the Greenland Sea Deep Water became warmer and more saline, predominantly caused by lateral mixing with deep water masses from adjacent basins. The hydrographic changes in the Greenland Sea were investigated in the context of a reduction of the sea-ice extent and associated changes in winter heat loss. While interannual variability of convection depth may depend on atmospheric forcing, we found that the decadal variability of water-mass transformation in the Greenland Sea was largely determined by the hydrographic structure of the water column.

Nature-based solutions for effective flood mitigation: potential design criteria

Few studies attempt to measure changes to discharge hydrographs during floods resulting from nature-based Solutions (NbS) for risk mitigation. The Q-NFM project in the UK has sought to measure and compare such changes for a wide range of NbS pilots applied to managed grasslands and woodlands. Also measured were underlying shifts in key hydrological processes leading to flood hydrograph changes of enhanced evaporation, hillslope-, channel- and floodplain-storage, and infiltration. How well particular NbS pilots changed these processes to reduce flood hydrographs was found to depend on the attributes of the NbS features and scheme. This learning is presented for the first time to highlight, with supporting evidence, seven potential criteria to help practitioners of flood risk management to improve existing and future designs of NbS for more effective flood mitigation within temperate grassland and woodland environments.

Revisiting circulation and water masses over the East Antarctic margin (80–150°E)

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m3) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.

Structure of the eastern Arabian Sea upper water column in the middle Miocene: Implications for the development of the South Asian monsoon

Global atmospheric circulation experienced drastic changes during the Middle Miocene climate transition (MMCT∼14.7–13.0 Ma) possibly related to the glaciation on West-Antarctica. Palaeoceanographic reconstructions have found that upwelling in the western Arabian Sea due to summer South Asian monsoon (SAM) winds likely occurred since ∼14.7 Ma, with fully modern-like monsoonal wind patterns after the end of the MMCT at around ∼13 Ma. Whether the changes in monsoonal circulation since ∼14.7 Ma are also associated with upper ocean hydrographic changes in the eastern Arabian Sea (EAS) is currently not known for the middle Miocene. To this end, the difference in Mg/Ca-based temperatures (ΔT°C) of surface-dwelling with lower mixed layer/thermocline-dwelling and sub-thermocline-dwelling planktic foraminifera was reconstructed to estimate the upper ocean thermal gradient at Site NGHP-01-01 A in the EAS and thus changes in the upper surface water column structure, i.e., mixed layer depth (MLD) and the depth of thermocline (DOT). The Mg/Ca-temperature estimates from the upper mixed layer down to the sub-thermocline show a prominent cooling trend between ∼14.2 Ma and 13.2 Ma. The upper water column reconstructions reveal weaker mixing, a shallower thermocline, and therefore a well stratified upper water column in the EAS after ∼14.2 Ma. We suggest that the weaker mixing and shallower thermocline in the EAS after ∼14.2 Ma are most likely due to an intensification of the summer SAM. A strong salinity stratification and/or the formation of a barrier layer (BL) because of increased SAM rainfall and advection of low salinity water may also have contributed to a weaker mixing and shallower thermocline in the EAS during the investigated time period. The change in the upper ocean hydrography in the EAS after ∼14.2 Ma fits well into the emerging picture of monsoonal-driven upper ocean hydrographic changes in the equatorial and northern Indian Ocean due to an intensification of the summer SAM since ∼14.7 Ma.

Planktonic habitats in the Amazon Plume region of the Western Tropical North Atlantic

The Western Tropical North Atlantic is a highly dynamic marine system where the Amazon River Plume (ARP) generates a patchwork of environmental conditions that favor different phytoplankton groups. To study phytoplanktonic community structure in such heterogeneous conditions, we used a set of five standard ship-based measurements taken from oceanographic surveys between 2010 and 2021 to characterize different habitat types. We then utilized a variety of multiparametric approaches to examine phytoplankton biodiversity in the different habitats to assess the biological relevance of our delineated habitats. Our approach generated a consistent set of habitat types across cruises carried out in multiple different years and the Amazon’s two predominant (wet and dry) seasons. Our phytoplankton community analyses revealed strong distinctions among all habitats along the plume gradient using in-vivo fluorescence and diagnostic pigments, and clear contrasts of diazotroph community along the mesohaline waters using direct cell-count, a pattern consistent with niche partitioning among similar species. The few apparent mismatches we found between phytoplankton community composition and habitat may reflect recent hydrographic changes driven by mixing and/or upwelling and thus may be a useful index to biologically-relevant temporal variation. Our habitat classification approach is straightforward and broadly applicable in identifying biologically distinct areas within heterogeneous and dynamic regions of the ocean.

Depositional and circulation changes at the Chukchi margin, Arctic Ocean, during the last two glacial cycles

The shallow Chukchi-East Siberian margin of the Arctic Ocean was repeatedly impacted by Pleistocene glaciations and related changes in sea level and circulation. The depositional history across the last two Arctic glacial cycles is investigated in sediment core ARC6-C15 from the foot of the Chukchi Rise, an extension of the Chukchi Sea shelf that was impacted by the East Siberian Ice Sheet (ESIS). Our proxy data indicate a large extent of the ESIS, likely blocking the westward sediment transport from the Laurentide Ice Sheet (LIS) during the penultimate glaciation estimated to have occurred within an age span from Marine Isotope Stage (MIS) 4 to 6. The deglacial environments are characterized by enhanced sediment inputs from the East Siberian and Chukchi shelves, while overlying interstadial/interglacial sediments have a signature of the Chukchi Sea and the Beaufort Gyre. During the last glaciation, the ESIS had a smaller impact on the Chukchi margin, and the deglaciation was dominated by the LIS sediment sources. Glacial discharge from the Mackenzie area is identified in the Bølling/Allerød interval, while later deglacial pulses can be tracked to the Canadian Arctic Archipelago. In addition to glacigenic sediment inputs, both deglacial intervals contain FeMn micronodules, possibly formed under the influence of meltwater pulses during sea level rise. An increase in the chlorite content that likely marks the flooding of the Bering Strait is identified at the start of the Holocene. A pronounced oxygen and carbon isotope excursion in the early Holocene may indicate hydrographic changes in the Arctic Ocean related to the 8.2 ka meltwater discharge event.

Seasonal enhancement of phytoplankton biomass in the southern tropical Indian Ocean: Significance of meteorological and oceanography parameters

The present study focused on understanding the seasonality of the phytoplankton biomass (chlorophyll a) distribution in the oligotrophic, Equatorial, and Southern Tropical Indian Ocean (ESTIO; 0–30°S and 60–90°E). The long-term satellite data analyses (2003–2020) showed a strong seasonality in sea surface temperature (SST), wind, currents, mean sea level anomaly (MSLA), photosynthetically available radiation (PAR), euphotic depth (ZEU) and mixed layer depth (MLD). As a response to the hydrographical changes, the phytoplankton biomass showed noticeable seasonal variation with the highest biomass during the Austral Winter (AW; June–September; avg. 0.11 ± 0.03 mg/m³) and lowest during the Austral Summer (AS; November–February; avg. 0.07 ± 0.03 mg/m). High chlorophyll patches (>0.1 mg/m³) were found between 0°–8°S during the AS and expanded over 0°–18°S during the AW. As multi-year mean chlorophyll a was higher (>0.1 mg/m³) in the northern part of the ESTIO (north of ∼13°S; HCD: high chlorophyll a domain) than the southern side (LCD: low chlorophyll a domain), the study area was divided into two domains and all the variables were analysed. In the HCD, enhancement of chlorophyll a was positively correlated with variables such as wind speed, wind stress, Ekman pumping, stronger northward and westward winds, as well as the presence of cyclonic eddies. These features are likely to stimulate primary production by uplifting the thermocline and enhancing nutrient supply. In the LCD, mixed layer depth also showed a strong positive correlation with elevated chlorophyll a, apparently because it is deep throughout the year (thereby keeping lower biomass) and deepens more strongly in winter than in the HCD. Another contrast with the HCD is that the cyclonic eddies appear to be insufficiently abundant to influence its chlorophyll a. Pearson's multivariable correlation analysis and principle component analysis confirmed the statistical significance of the above parameters on the enhancement of chlorophyll a in the ESTIO.

Large Scale Salinity Anomaly Has Triggered the Recent Decline of Winter Convection in the Greenland Sea

AbstractThe Greenland Sea is a key region for open ocean convection and ventilation, which exhibit a large variability with periods of strong convection and shutdowns. After a long period of weak winter convection (from the 1970s to the early 1990s), a recovery has been reported, beginning in the 1990s and intensifying in the early 2000s until 2013. Using ISAS, an optimal interpolation product based on Argo observations, we document a recent significant weakening of deep convection between 2014 and 2020, accompanied by a continuous warming of the mixed layer but also a freshening after 2014. These hydrographic changes likely increase the ocean stratification and precondition the shutdown of winter convection. We suggest that these property changes result from a shift of the large scale atmospheric circulation, affecting the source of Atlantic Water to the Nordic seas, causing a freshening of about −0.1 g kg−1 that spreads into the Greenland Sea.

Astrochronology of the late Oligocene-middle Miocene in the Maldives, central Indian Ocean

Substantial work based on biostratigraphy has been done to constrain the chronology of the late Oligocene-middle Miocene in the Maldives (Indian Ocean), but a high-resolution temporal framework for the sedimentary record of this region remains elusive. This hampers a fuller understanding of the evolution of the carbonate platform, which is the second largest modern isolated carbonate platform. Moreover, the sedimentary record of the Maldives platform has the potential to provide insights on the onset of the ‘modern-like’ South Asian Monsoon (SAM) system. Here we present a detailed astrochronology for International Ocean Discovery Program (IODP) Site U1468 through the late Oligocene-middle Miocene in the Maldives. Our cyclostratigraphy of the site is integrated with existing biostratigraphy and statistical evaluation, providing a robust ∼12-Myr absolute astronomical time scale (ATS). Using this time scale, we assign numerical ages of 12.229 ± 0.46 Ma and 11.561 ± 0.46 Ma for the core depths of 450 mbsf and 430 mbsf, respectively. These depths, constrained by biostratigraphy, have previously been suggested to mark the onset of the SAM in the Maldives. Our ATS provides time constraints on the evolution of the platform sequence stratigraphy, and we find evidence of a ∼1.2-Myr obliquity amplitude modulation (AM) control on sea level changes and sequence development. In addition, our results indicate that the deposition of organic-rich sediments towards the base of the record in this area was driven by obliquity cycles and the combined effect of glacio-eustasy and hydrographic changes.

Toward assimilation of acoustic travel times into an ocean state estimate

Direct ocean observing systems are a scarce set of data that build the backbone for understanding the current state of the ocean. These systems are limited in areas of high variability in temperature and salinity. Pioneering work of Wunsch (1977) proposed efforts to extract detailed hydrographic information using sound propagation through the ocean, providing the scaffolding to improve our understanding of the ocean's interior. The integrated nature of such measurements makes acoustic thermometry a powerful application for monitoring regional to basin-averaged hydrographic changes in the ocean, a measurement that is difficult to achieve by individual “point” measurements (moorings, ship-borne CTD casts, or autonomous floats) alone. This work models acoustic travel times corresponding to eigenrays between a fixed source and receiver from an evolving ocean state. Inclusive computation of acoustic travel times within a dynamically evolving modeled ocean state provide a novel approach to model data comparison within a general ocean circulation model. An adjoint assimilation framework combines observations with numerical models to estimate and update oceanic state variables Forget et al. (2015). This process provides the necessary operators for model data comparison of ocean acoustic observable quantities within a consistent state estimation framework allowing for data assimilation from acoustic tomography measurements.

Two centuries of changes - revision of the hydrography of the Biebrza Valley, its transformation and probable ecohydrological challenges

Human-induced changes in hydrography have led to serious changes in the hydrological and ecological condition of ecosystems, including habitat fragmentation and reduction of water in the landscape. The aim of this study was to assess the transformation of the hydrographic network in the Biebrza National Park (BbNP) over the XIX-XXI centuries and to analyze its probable ecohydrological challenges. The analysis of changes in the hydrographic network was based on a spatial comparison of hydrographic network elements on the basis of spatial sources. As a result of both natural and human-induced processes, there have been significant changes to elements of the hydrographic network of the BbNP. The length, area and sinuosity of the rivers in the BbNP area have decreased significantly in the XIX-XXI centuries, in contrast to the length of the canals dug in the XIX century - their length increased. On average, river channelization reduced channel water storage capacity by 49%. In other cases the average decrease of storage capacity of the channel was about 14%. As a result of drainage works in the XIX century, the network of drainage ditches was established in the BbNP, which was significantly extended (more than 6 times) in the XXI century. A comprehensive analysis of hydrographic changes, their causes and possible consequences is necessary to understand the dynamics of river networks. It also makes it possible to predict the future ecohydrological response of rivers to anthropogenic influences and climate change, thus helping to establish appropriate management and ecohydrological restoration of aquatic ecosystems.

Dense water formation in the eastern Mediterranean under a global warming scenario

Abstract. Dense water formation in the eastern Mediterranean (EMed) is essential in sustaining the Mediterranean overturning circulation. Changes in the sources of dense water in the EMed point to changes in the circulation and water properties of the Mediterranean Sea. Here we examine with a regional climate system model the changes in the dense water formation in the EMed through the 21st century under the RCP8.5 emission scenario. Our results show a shift in the dominant source of Eastern Mediterranean Deep Water (EMDW) from the Adriatic Sea to the Aegean Sea in the first half of the 21st century. The projected dense water formation is reduced by 75 % for the Adriatic Sea, 84 % for the Aegean Sea, and 83 % for the Levantine Sea by the end of the century. The reduction in the intensity of deep water formation is related to hydrographic changes in surface and intermediate water that strengthen the vertical stratification, hampering vertical mixing and thus convection. Those changes have an impact on the water that flows through the Strait of Sicily to the western Mediterranean and therefore on the whole Mediterranean system.

Recent Convection Decline in the Greenland Sea: Insights From the Mercator Ocean System Over 2008–2020

AbstractWe investigated wintertime convection evolution in recent years over the Greenland Sea. This area is a major location regarding dense water production and supply of the lower limb of the Atlantic Meridional Overturning Circulation, a key component of the global climate. Previous studies mentioned an increase in Greenland Sea wintertime convection intensity during the 2000s in comparison with the previous decade till 2015/2016. Here, we further document the ongoing oceanic changes within the Greenland Sea through the Mercator Ocean Physical System, an operational ocean model with data‐assimilation. The model has shown a large variability, a later start and a decline of convection in the Greenland Sea in recent years. In particular, the depth of the annual maximum mixed layer diminished by 52% between 2008–2014 and 2015–2020, from 1,168 to 559 m, over the convective area. This decline of the convection depth is corroborated with Argo float observations. Within the Greenland Sea, hydrographic changes especially the increasing temperature are associated with isopycnal deepening and stratification strengthening. The stratification is building up at a larger rate in the Boreas Basin compared to the Greenland Basin. The changes of the Greenland Sea hydrography in the model are in part related to Atlantic Water spreading over the Boreas Basin and the eastern part of the Greenland Basin. The model also indicates a decrease in the intensity of the gyre in accordance with the isopycnal deepening while local surface winds and fluxes do not exhibit significant trends nor significant interannual variations.

Disentangling multiproxy temperature reconstructions from the subtropical North Atlantic

Reliable reconstruction of past sea surface temperature (SST) is of prime importance for understanding the Earth’s sensitivity to external forcing. Yet, it remains a major challenge in paleoceanography because comparison between SST estimates from different proxies reveals mismatches and raise the question as to what the contrasting proxies actually record. A better understanding of these mismatches in the light of seasonal occurrence of the proxy bearing organisms (archives) and water mass changes help to assess climate models. Here, we analyze data from the last deglaciation using a sediment core site situated at the northern boundary of the North Atlantic subtropical gyre influenced by fast latitudinal migrations of the subtropical Azores Front (AF) and resulting changes in water masses that may affect the SST records. Differences between the SST estimates from different deglacial SST reconstructions obtained from (1) Mg/Ca in planktic foraminifer tests, (2) alkenone UK′37, and (3) planktic foraminifer assemblages (SIMMAX), are assumed to result from the ecology of the proxy bearing organisms, and are assessed for the impact on different SST reconstructions from local seawater δ18O (δ18Ow) reconstructions. The general trends of SSTs from all four proxies confirm the well-known deglacial succession of warm and cold events. Mismatches between amplitudes of temperature changes are explained by differences in the phenology of the proxy-bearing organisms and local changes in hydrography. The combination of δ18O SST from the three different archives of δ18Ow reconstructions may cause offsets that exceed the climate driven signals.

Centennial Channel Response to Climate Change in an Engineered River

AbstractHuman intervention makes river channels adjust their slope and bed surface grain size as they transition to a new equilibrium state in response to engineering measures. Climate change alters the river controls through hydrograph changes and sea level rise. We assess how channel response to climate change compares to channel response to human intervention over this century (2000–2100), focusing on a 300‐km reach of the Rhine River. We set up a schematized numerical model representative of the current (1990–2020), non‐graded state of the river, and subject it to scenarios for the hydrograph, sediment flux, and sea level rise. We conclude that the lower Rhine River will continue to adjust to past channelization measures in 2100 through channel bed incision. This response slows down as the river approaches its new equilibrium state. Channel response to climate change is dominated by hydrograph changes, which increasingly enhance incision, rather than sea level rise.

3D morphological variability in foraminifera unravel environmental changes in the Baltic Sea entrance over the last 200 years

Human activities in coastal areas have intensified over the last 200 years, impacting also high-latitude regions such as the Baltic Sea. Benthic foraminifera, protists often with calcite shells (tests), are typically well preserved in marine sediments and known to record past bottom-water conditions. Morphological analyses of marine shells acquired by microcomputed tomography (µCT) have made significant progress toward a better understanding of recent environmental changes. However, limited access to data processing and a lack of guidelines persist when using open-source software adaptable to different microfossil shapes. This study provides a post-data routine to analyze the entire test parameters: average thickness, calcite volume, calcite surface area, number of pores, pore density, and calcite surface area/volume ratio. A case study was used to illustrate this method: 3D time series (i.e., 4D) of Elphidium clavatum specimens recording environmental conditions in the Baltic Sea entrance from the period early industrial (the 1800s) to present-day (the 2010 s). Long-term morphological trends in the foraminiferal record revealed that modern specimens have ∼28% thinner tests and ∼91% more pores than their historic counterparts. However, morphological variability between specimens and the BFAR (specimens cm−2 yr−1) in E. clavatum were not always synchronous. While the BFAR remained unchanged, morphological variability was linked to natural environmental fluctuations in the early industrial period and the consequences of anthropogenic climate change in the 21st century. During the period 1940–2000 s, the variations in BFAR were synchronous with morphological variability, revealing both the effects of the increase in human activities and major hydrographic changes. Finally, our interpretations, based on E. clavatum morphological variations, highlight environmental changes in the Baltic Sea area, supporting those documented by the foraminiferal assemblages.

Influence of glacier type on bloom phenology in two Southwest Greenland fjords

Along Greenland's coastline, the magnitude and timing of primary production in fjords is influenced by meltwater release from marine-terminating glaciers. How local ecosystems will adapt as these glaciers retreat onto land, forcing fundamental changes in hydrography, remains an open question. To further our understanding of this transition, we examine how marine- and land-terminating glaciers respectively influence fjord bloom phenology. Between spring and autumn 2019, we conducted along-fjord transects of hydrographic variables, biogeochemical properties and pico- and nanophytoplankton counts to illustrate the contrasting seasonal bloom dynamics in the fjords Nuup Kangerlua and Ameralik. These fjords are in the same climatic region of west Greenland but influenced by different glacial structures. Nuup Kangerlua, a predominantly marine-terminating system, was differentiated by its sustained second summer bloom and high Chl a fluorescence in summer and autumn. In Ameralik, influenced by a land-terminating glacier, we found higher abundances of pico- and nanophytoplankton, and high cyanobacteria growth in autumn. The summer bloom in Nuup Kangerlua is known to be coincident with subglacial freshwater discharge sustaining renewed nutrient supply to the fjord. We observe here that the intermediate baroclinic circulation, which creates an inflow at subsurface depths, also plays an important role in increasing nutrient availability at shallower depths and potentially explains the distribution of primary producers. Our observations suggest that the retreat of marine-terminating glaciers onto land, with consequent increases in surface water temperature and stratification, and reduced light availability, may alter the magnitude, composition, and distribution of summer productivity.

Response of planktonic foraminifera to seasonal and interannual hydrographic changes: Sediment trap record from the northern South China Sea

Foraminifera play an important role in the organic and inorganic carbon pumps of the ocean. Understanding their responses in seasonal and interannual time scales to the climate and seawater chemistry changes is important for studying carbon cycles and paleoclimatology. This study examined about 5.1-year continuous mass fluxes in 2013/9-2019/8 and 1.8-year planktonic foraminiferal shell fluxes in 2016/8-2019/8 census from ten and four sets of sediment traps, respectively, moored at the South East Asia Time-series Study (SEATS) site in the northern South China Sea (SCS), the world’s largest marginal sea. A total of four sets of sediment trap mooring with 11 common species were identified, spanning from August 2016 to August 2019 with 8-day and 16-day collecting periods, which provided access to evaluate the impact of the prevailing monsoon system and interannual climatic conditions. Our results show that the winter monsoon enhances the nutrient inventories in the euphotic zone, supporting the productivity and the mass and shell fluxes. In addition to seasonal cycles, the variation of chlorophyll-a concentration in the SCS shows a strong response to wind speed under the influence of El Niño–Southern Oscillation (ENSO). The reduction in wind speed and the intensification of Kuroshio intrusion during the ENSO warm phases reduces the productivity and mass and shell fluxes, and vice versa in the case of the ENSO cold phases. The imprint of an ENSO cold phase (La Niña event) in 2017 was signified by 2–3-fold higher values than the 3-year average of total mass and foraminiferal shell fluxes. Instead of the common dominance of Trilobatus sacculifer and Globigerinoides ruber among species composition, Neogloboquadrina dutertrei was the predominant species comprising over 40%–60% of total shells greater than 212 μm. Furthermore, the interval with an elevated abundance of N. dutertrei lasted throughout January 2018 (four collecting intervals). Foraminifera shell fluxes were the lowest during warm months (March-August) in 2019, which was coeval with the increase in proportions of Orbulina universa and Globigerinella calida. The unusual species composition might signify a weak ENSO warm phase (A weak El Niño event) between September 2018 and August 2019.

Rapid 20th century warming reverses 900-year cooling in the Gulf of Maine

The Gulf of Maine, located in the western North Atlantic, has undergone recent, rapid ocean warming but the lack of long-term, instrumental records hampers the ability to put these significant hydrographic changes into context. Here we present multiple 300-year long geochemical records (oxygen, nitrogen, and previously published radiocarbon isotopes) measured in absolutely-dated Arctica islandica shells from the western Gulf of Maine. These records, in combination with climate model simulations, suggest that the Gulf of Maine underwent a long-term cooling over most of the last 1000 years, driven primarily by volcanic forcing and North Atlantic ocean dynamics. This cooling trend was reversed by warming beginning in the late 1800s, likely due to increased atmospheric greenhouse gas concentrations and changes in western North Atlantic circulation. The climate model simulations suggest that the warming over the last century was more rapid than almost any other 100-year period in the last 1000 years in the region.

Community structure of zooplankton during the southwest-northeast monsoon transition period in the southeastern East China Sea

Abstract The assessment of zooplankton spatial structure is critical to quantifying plankton production and biochemical cycling. Here, we show the effect of monsoon-driven hydrographic changes on the spatial patterns of zooplankton at regional scales. Based on a field survey conducted during the monsoon transition, we assess the community structure and ash-free dry weight above 100 m depth in the Kuroshio edge area, southeastern East China Sea. The zooplankton community was dominated by copepods, with calanoids as the most abundant taxon followed by poecilostomatoids. Copepod assemblages comprised 81 species belonging to 41 families, and 23 genera, with Undinula vulgaris as the most abundant species. The richness of zooplankton taxa showed a mounting trend from the coast to offshore, which was opposite to the spatial variation of total zooplankton biomass. Our results suggest a leading influence of the Kuroshio Current during the southwest-northeast monsoon transition period in the southeastern East China Sea.