Deep-sea Research Part Ii-topical Studies in Oceanography最新文献

The formation of Arabian Sea High Salinity Water (ASHSW) during the winter monsoon in the northern Arabian Sea is driven by surface buoyancy loss, which increases surface density and triggers convective mixing. The depth of convective mixing is influenced by the interplay between surface cooling (buoyancy loss) and upper-ocean stratification. Mesoscale eddies present during winter can alter this stratification and modulate convective mixing. We investigated the impact of these eddies on convective mixing and ASHSW formation utilizing a combination of observations, data assimilative model results, and 1-D and 3-D model experiments. Our analyses consistently demonstrate that the depth of winter convective mixing is influenced by the stratification imposed by mesoscale features, resulting in distinct mixing characteristics. When subjected to identical buoyancy forcing, convective mixing associated with cyclonic eddies occurs at shallower depths compared to anticyclonic eddies. This difference is particularly pronounced during the peak period of convective mixing (January–February), exceeding 50 m, compared to the early stages (November–December). The combined effect of cyclonic and anticyclonic eddies generates spatially inhomogeneous convective mixing, which cannot be solely explained by buoyancy flux. These conclusions are supported by Argo observations and analyses of 1-D and 3-D model experiments. Overall, our study highlights the significant role of mesoscale eddies in modulating convective mixing and ASHSW formation in the northern Arabian Sea.

Moving weather systems determine the history of wind variations with patterns as the systems transit through the ocean. These weather systems are integrated entities that can provide a system level perspective. A vessel-based survey repeatedly occupying a 30-km transect 12 times in 36 h provided non-aliased measurements of velocity field that showed how the along-shelf transport varied by more than three-fold in response to a transiting high-pressure weather system following an atmospheric cold front. To further illustrate the impact of different weather systems, we analyzed time series data from moored ADCPs, which showed influence on the velocity field from infrequent summertime cold fronts and remote hurricanes moving through the region, one on the west and the other on the east of the study site. Rotary spectrum analysis showed that the flow field rotated mostly clockwise with a smaller but non-negligible counterclockwise component, consistent with near inertial oscillations mixed with weak tidal currents. A theoretical model is presented by a Laplace transform and a general relationship between the velocity field and forcing functions is obtained, which shows that the contributions to the rotary velocity field from various forcing functions are through mathematical convolutions between the forcing functions and the complex frictional-rotary inertial function (CFRIF). These convolutions include an integrated effect of history of the forcing. CFRIF is effectively a frictional rotary filter that favors inertial oscillations. The wind-stress induced velocity field over a few days is computed and it shows significant variations after the passage of a cold front, with a magnitude consistent with observations. The wind-stress induced velocity is a few times greater than the density driven flow during the ship-based observations. The weather systems passing through the region can impact coastal currents causing a great variability over short time scales.

Theories of ocean–atmosphere interaction during a Madden–Julian Oscillation (MJO) are generally based on a thermodynamic model with surface fluxes dictating changes in sea surface temperature. Evidence from a two month ocean glider deployment in early 2019 in the southeast Indian Ocean suggests the impact of mesoscale dynamics on upper-ocean stratification likely affects ocean–atmosphere interaction at MJO scales. Until mid-February, local surface fluxes consistent with a convectively suppressed MJO phase drove near-surface ocean evolution. With the advection of a fresh-core eddy to the glider location in late February, ocean dynamics then becomes an additional driver of this evolution by modulating local stratification and generating a barrier layer of ≈12 m thickness for 10 days. One-dimensional modelling experiments based on the ocean and atmospheric conditions experienced during our sampling period show that the ocean subsurface structure within the eddy induce changes in SST of physical significance for ocean-atmosphere interaction. Moreover, results also suggest that the presence of a thick eddy-induced barrier layer during the MJO suppressed phase modulates the magnitude of temperature anomalies forced by surface fluxes during the following enhanced MJO phase. As eddies are abundant in this area, their dynamics must be considered to correctly represent SST variability for MJO modelling.

The Antarctic environment offers a unique opportunity to study the interactions between Porifera and their microbial symbionts. Reports on the association between prokaryotes and Antarctic sponges are increasing. However, a comparison of the bacterial communities associated to the same sponge species but inhabiting different Antarctic areas has seldom been addressed. This study explored the prokaryotes associated with the sponge species Mycale (Oxymycale) acerata (Kirkpatrick, 1907) and Dendrilla antarctica (Topsent, 1905) collected from South Cove at Rothera Point (Antarctic Peninsula) and Thetys Bay (Ross Sea). In D. antarctica, some groups were equally represented at both sites (e.g., Amylibacter, Cutibacterium, Yoonia-Loktanella), whereas members in the genera Polaribacter and Kistimonas were more abundant in Rothera. Similarly, M. acerata individuals collected from Rothera showed a higher relative abundance of some bacterial genera, such as Polaribacter, Sulfitobacter, and Ulvibacter. The results allowed us to identify some taxa common to sponges belonging to the same species and highlighted the possible influence of site-specific environmental conditions in shaping symbionts.

Studies on the bathyal fauna of northern Pacific waters suggested that a transition or boundary between the North Pacific Province and Central Pacific Provinces would be found somewhere along the Emperor Seamount Chain. Strong currents flow west to east across the seamount chain in a region known as the Main Gap and it was proposed that any larvae produced either north or south of the Main Gap would not be capable of crossing the gap. An expedition to test the hypothesis that a faunal change would be found in the vicinity of the Main Gap was conducted in 2019. Eleven ROV dives were conducted, one on an unnamed seamount at the southern edge of Hess Rise, and 10 dives on seven seamounts along the Emperor Seamount Chain. Six dives were on seamounts north of the Main Gap, while four (including the dive on Hess Rise) were on the southern side. Of the six northern dives, three were at deeper depths (∼2000–1800 m) and three were shallower (∼1500–1100 m); of the southern dives two were at the deeper depths and two were shallower. One shallower dive occurred on Jingu Seamount, situated on the southern edge of the Main Gap. Analysis of the fauna from both collected specimens and annotations of the dive video produced four clusters: a, the four dives south of the Main Gap; b, the three deeper dives north of the Main Gap; c, the shallower dive at Jingu Seamount; and d, the four shallower bathyal dives north of the Main Gap. It was concluded that the bathyal fauna underwent a significant change from north to south across the area of the Main Gap and the adjacent Small Gap, in the area of 37–39 °N, covering distances as small as 75 km or as much as 400 km.

The Ross Sea is characterized by a series of subsystems with different characteristics making it an extremely productive area. To understand whether species composition and functional traits of the plankton community can be used as biological tracers, we have analyzed the composition of phytoplankton and microzooplankton, and their potential relationships, in two different polynyas of the Ross Sea during the austral summer 2017. Sampling activities were carried out near Terra Nova Bay, between Cape Washington and the northern shore of the Drygalski Ice Tongue, and in the South-Central Ross Sea. We investigated the phytoplankton and microzooplankton structure using the phytoplankton body size classes and the tintinnids lorica oral diameter as functional traits, speculating on the relationship between the two plankton communities and their use as biological indicators in a changing Southern Ocean. Our data showed significant differences in terms of plankton composition and related functional traits between the two areas, suggesting the existence of distinct ecological dynamics despite the similar total carbon content. In Terra Nova Bay, heterotrophic dinoflagellates were the most abundant microzooplankton, in association with a large phytoplankton biomass mainly represented by diatoms and nano- and micro-phytoplankton. Tintinnids with large lorica oral diameters were abundant in Central Ross Sea, where phytoplankton was dominated by Phaeocystis antarctica and by the micro size class. Among microzooplankton, Protoperidinium defectum, P. applanatum and P. incertum were the most abundant dinoflagellates species, while Codonellopsis gaussi, C. gaussi forma cylindroconica, Laackmanniella prolongata and Cymatocylis drygalskii were the most abundant tintinnids. The phytoplankton was dominated by diatoms Pseudo-nitzschia subcurvata, Fragilariopsis cylindrus, F. curta and by the haptophyte P. antarctica. Our data indicate that beyond physical and chemical features defining distinct sectors of the Ross Sea, both species composition and functional traits of phytoplankton and microzooplankton represent a valid monitoring tool, especially with the ongoing global warming and its effects on Antarctic food webs.

The marine environment is a crucial component of global biogeochemical cycles. Recent BGC-Argo observations provide new opportunities to study the profiles of biogeochemical parameters. The study analyzed the diurnal variations of temperature, salinity, chlorophyll-a and dissolved oxygen using a high-frequency (∼5 h) cycle BGC-Argo float in the Bay of Bengal. The hydrography showed the existence of a strong barrier layer with a thickness of around 30 m, with fresh water on top and an inversion layer within it. Analysis showed that the Mixed Layer Depth (MLD) was dominated by diffuse convection, while the Isothermal Layer Depth (ILD) exhibited salt-fingering regimes. In the upper layer (0–60 m), temperature showed significant variation on a daily scale; however, notable changes were not observed for salinity. Additionally, temperature and chlorophyll-a were found to be strongly linked to solar insolation. The mean chlorophyll-a in the upper layer increased from 0600 h and peaked around 1800 h local time. However, surface chlorophyll-a increased only from 1100 h to 1800 h. It is suggested that this difference between surface and mean chlorophyll-a during high availability of sunlight was due to the process of photoacclimation. The dissolved oxygen cycle closely followed the variability of biomass production. The similarity between dissolved oxygen and the difference between the surface and mean chlorophyll-a further indicated photoacclimation variations on a diurnal scale. The Sverdrup model was used to indicate luminosity and an accumulation time of 14 h was used to show a strong correlation with diel chlorophyll-a variation. The work highlights the importance of having high-frequency BGC-Argo floats with finer vertical resolution and the need for time-series observations of biological parameters in the Bay of Bengal.

To date, a total of 30 nemertean species have been described from depths greater than 1000 m. All deep-sea species of the infraorder Oerstediina belong to the genera found at great depths only. A new species of the genus Oerstedia Quatrefages, 1846 (O. sashae sp. nov.), whose described species inhabit shallow waters, has been collected on the Emperor Seamounts from a depth of 1407 m. Oerstedia sashae sp. nov. differs from the other species of the genus by the lack of eyes. Another new species, Oerstedia sofiae sp. nov., which is very close to the symbiotic Cryptonemertes actinophila (Bürger, 1904), has been described from the Sea of Okhotsk, from a depth of 250–490 m. A phylogenetic analysis based on five gene markers (COI, 16 S, 18 S, 28 S, and histone H3) has shown that O. sashae sp. nov., O. sofiae sp. nov., and C. actinophila belong to clade Paroerstediella of the genus Oerstedia. Cryptonemertes actinophila is here proposed to be transferred to the genus Oerstedia. Oerstedia sofiae sp. nov. and O. actinophila comb. nov. differ from the other species of the genus by the red blood and a very short proboscis and rhynchocoel.

During the summer monsoon, the Somali region undergoes a significant upwelling phenomenon that enhances plankton productivity, thereby benefiting fisheries. Wind and coastal dynamics initially drive this upwelling, but eventually, eddy flows influence it. Our study explores the interplay between ocean currents, eddies, and chlorophyll-a concentrations using the backward Finite-Size Lyapunov Exponents (bFSLEs) technique. We also delve into the specific role of Ekman transport in distributing chlorophyll-a across the region. The Great Whirl (GW), an anticyclonic eddy, predominantly causes strong downwelling, interrupting the summer monsoon upwelling along the Somali coast longitudinally. Despite the GW's significant impact on moving upwelled water offshore, the influence of downwelling diminishes northward. As a result, the northern Somali coast, especially around 9°N and 10°N, showcases the most extensive offshore upwelling, reaching as far as 55°E. Our findings highlight a robust connection between chlorophyll-a levels and oceanic dynamics, influenced by both currents and eddies, as evidenced by bFSLEs, and by cross-shore Ekman transport, particularly within chlorophyll-a concentrations ranging from 0.2 to 0.6 mg m⁻³. The data suggests that Ekman transport-induced upwelling primarily drives coastal phytoplankton biomass. Furthermore, bFSLEs analysis underlines the supportive role of ocean currents and eddies in the offshore distribution of chlorophyll-a, especially near the coast. Further examination of lagged correlations reveals a temporal lag between peak concentrations of chlorophyll-a and Ekman transport; the lag increases offshore and is at least 9 days near the coast.

The Gulf of Mexico (GoM) is characterized by strong mesoscale eddy activities that have been studied extensively, yet the comprehensive three-dimensional (3-D) kinematic properties of GoM eddies are still not well documented. In this study, the 3-D mesoscale eddy activities in the upper layer (0–800 m) of the GoM are characterized using 14-year (1997–2010) global Hybrid Coordinate Ocean Model (HYCOM) outputs. Most eddies in the upper layer (both cyclonic and anticyclonic) have radii of ∼30–60 km and lifespans shorter than 30 days. The spatial distributions of GoM eddies do not vary much with depth, while their intensity decreases with depth. The size of cyclonic eddies does not vary much with depth, while the size of anticyclonic eddies decreases slightly with depth. Cyclonic eddies are often found to be generated in the eastern GoM (especially in the Loop Current region), the Bay of Campeche, and on the continental slope of the Campeche Bank, while anticyclonic eddies are often generated on the northeastern and northwestern GoM continental slopes, and in the central GoM (near 24°N) and the Bay of Campeche (92–94°W). In addition, long-lived GoM eddies (e.g., lifespan >150 days) tend to have intermediate eddy intensity (e.g., 0.13–0.32 for cyclonic eddies at the 10 m level). Both cyclonic and anticyclonic eddies are found to play an important role in the horizontal and vertical transport of heat and salt, and eddy-induced anomalies of water temperature and salinity at both surface and subsurface are generally more pronounced in the eastern GoM than in the western GoM.