Cold Intrusion Research Articles

Impacts of an Upper-Tropospheric Cold Low on Tropical Cyclone Intensity

Abstract Following a previous study examining the influence of an upper-tropospheric cold low (CL) on the track of a nearby tropical cyclone (TC), this study investigates the impacts of a CL on TC intensity. The results suggest that the relative position and separation distance between the CL and the TC are the key factors affecting TC intensity. When located outside the CL’s radius of maximum winds (RMW) but within its circulation, TCs initially in the northwest quadrant of the CL intensify faster than those in the other quadrants. The β effect causes the CL to move northwestward toward the TC and enhances eddy angular momentum flux convergence. Meanwhile, the upper-level TC outflow erodes the CL and reduces the associated vertical wind shear, promoting TC intensification. In contrast, for TCs initially located southeast of the CL, the attraction of the Fujiwhara effect between the two entities counteracts the CL’s β drift and helps to maintain their separation distance. Moreover, Rossby wave energy dispersion induces an anticyclone southeast of the CL, which transports lower-θe air toward the TC and hinders the TC development. Furthermore, TCs within the CL’s RMW reach a similar intensity due to their PV superposition, irrespective of their relative positions to the CL. For TCs located outside the CL circulation, the CL’s impacts are largely negligible for TCs located northwest of the CL, but TCs located southeast of the CL may still be affected by the CL-induced anticyclone. Significance Statement This study examines the influence of an upper-tropospheric cold low on tropical cyclone intensity. The results illustrate that the relative position and separation distance between the cold low and tropical cyclone are crucial factors in determining tropical cyclone intensity. Tropical cyclones initially northwest of a cold low intensify faster than those to the southeast when located outside the cold low’s radius of maximum winds but still within its circulation. The main mechanisms are how the β steering and interactions between the two entities act together. The midlevel intrusion of cold, dry air and Rossby energy dispersion also contribute to their complex interaction. These insights provide a guide for forecasting the tropical cyclone intensity when influenced by a nearby upper-level cold low.

Cold Intermediate Water Formation in the Black Sea Triggered by March 2022 Cold Intrusions

In mid-March 2022, a Siberian High brought intense cold air masses, leading to severe weather conditions across southern Europe, including the Black Sea region. This study investigates the spatial and temporal evolution of cold intermediate water (CIW) masses in the Black Sea, with a particular focus on the successive anomalously cold episodes that occurred in March 2022. The research underscores the significance of the northwestern continental slope and cyclonic gyres, especially as the only cold-water mass observations during the warm winters of 2020 and 2021 were concentrated in these areas. Following two warm winters, the cold episodes of March 2022 revealed notable convection and simultaneous cooling, particularly in the cyclonic interior and the Rim Current periphery, excluding the northeastern periphery. Subsequently, cold waters spreading isopycnally throughout the summer months were transported laterally and reached these regions. Argo float measurements provided clear evidence of widespread replenishment of the CIW, indicating that it is not confined to specific areas. The study also highlights regional variability in the characteristics of CIW formation, which is influenced by local dynamics and preconditioning temperatures. The temperatures of CIW increased from west to east, in line with the sea surface temperature gradient. Notably, thicker and colder CIW was found in the western cyclonic gyre compared to the eastern cyclonic area. Furthermore, the study confirms that the warming trend in CIW, identified in previous research, not only continues but has intensified during the recent period analyzed. These findings, observed under the extreme conditions analyzed in this research, offer valuable insights into the widespread occurrence of CIW formation in the Black Sea. Additionally, the study confirms that the warming trend in CIW, identified in previous studies, continued in the region throughout the warm winter period and after the cold spell in 2022. These insights contribute to a deeper understanding of CIW dynamics and their response to extreme weather events in the Black Sea.

Out of oxygen: Stratification and loading drove hypoxia during a warm, wet, and productive year in a Great Lakes estuary

Hypolimnetic hypoxia, or low oxygen in bottom waters, impairs ecosystem services of freshwater lakes and estuaries globally. Both hypoxia incidence and intensity are increasing around the world due to eutrophication and climate change. As the hypolimnion becomes hypoxic and ultimately anoxic, sediment-bound legacy phosphorus is released. Water column mixing due to large storm events or fall turnover entrains these nutrients to the surface, causing harmful algal blooms. To assess the dynamics of hypoxia throughout the growing season, we evaluated Muskegon Lake, where hypoxia recurs annually, utilizing high-frequency time-series data from the Muskegon Lake Observatory (MLO) buoy (https://www.gvsu.edu/wri/buoy/), biweekly nutrient sampling, and seasonal respiration experiments during 2021. While water-column stratification set the stage for hypolimnetic hypoxia, frequent wind-mixing events, and episodic intrusions of cold, oxygenated, upwelled Lake Michigan waters intermittently reduced the thickness or intensity of the hypoxic zone. Respiration experiments revealed that riverine and surface organic matter inputs contributed most to hypolimnetic hypoxia in the spring, whereas surface inputs did so during summer, and riverine inputs during fall, indicating seasonally variable sources drive hypoxia. Biweekly measurements indicated increased soluble reactive phosphorus in the hypolimnion during anoxia via internal phosphorus loading from the sediment with the potential for fueling surface blooms with net export of soluble reactive phosphorus and total phosphorus to nearshore Lake Michigan. Our findings on the role of seasonally changing temperature, loading, phytoplankton production, hypolimnetic respiration, and internal phosphorus loading in shaping hypoxia dynamics have relevance to similarly afflicted ecosystems in the Great Lakes Basin.

On the Geomorphic, Meteorological, and Hydroclimatic Drivers of the Unusual 2018 Early Summer Salt Dust Storms in Central Asia

AbstractA series of synoptically forced dust events, including an extreme salt storm from the Aralkum desert, stuck large parts of Central Asia during early summer 2018, causing flight cancellation, widespread haze, and salt deposition on croplands. This manuscript explores the geomorphic, meteorological, and hydroclimatic drivers of the unusual dust activity based on the three prerequisite factors of dust outbreak: sediment supply, sediment availability, and dust‐uplifting winds. Dry lake beds and lower river floodplains are identified as the source area of frequent dust episodes likely due to the abundant supply of fine‐grain sediments. Whereas, minimal dust activity was detected at the region's major sandy deserts. The salt dust storms were triggered by cold intrusion and strong postfrontal winds at the southeast flank of a rapidly developing anticyclone over western Russia which was enhanced by the Scandinavian blocking upstream. The 2018 early summer dust events in Central Asia were identified as part of spatially compounding weather hazards linked to a slow‐moving synoptic wave pattern across the Northern Hemisphere midlatitudes. Finally, the aeolian sediment availability at the dust source areas was enhanced by below‐average soil moisture and vegetation during summer 2018 linked to reduced precipitation in the preceding winter. The 2017/18 winter precipitation deficit was driven by a double‐dip, Central Pacific La Niña episode associated with large‐scale precipitation‐suppressing subsidence and enhanced moisture flux divergence over Central Asia.

Hypolimnetic oxygen depletion in a deep oligomictic lake under climate change

Dissolved oxygen (DO) concentration is a fundamental metric to describe climate-related alterations in deep lakes. Increasing water temperatures enhance thermal stratification, leading in temperate basins to a growing isolation of deep waters. This leads to the depletion of hypolimnetic DO, which adds up to limited nutrient circulation and restricted replenishment of the trophogenic layers. With vanishing convective mixing, it is commonly believed that the only source of hypolimnetic DO replenishment will be represented by deep intrusions of cold oxygenated waters from the tributaries. In this study, we first analyse the 1993–2020 long-term observed trends of DO concentrations in the subalpine deep oligomictic Lake Maggiore (Italy/Switzerland). Then, through an algorithm calculating daily intrusion depths and mass discharges of DO for the major tributaries, we show that deep insertions are suppressed for increasing winter water temperatures and residual thermal stratification. Turbulent entrainment is proved fundamental for DO replenishment, leading to mass discharges of DO released into the deep hypolimnion up to more than two orders of magnitude larger than the original ones from the tributaries. Last, we discuss the results of simulations made through a one-dimensional coupled ecological–hydrodynamic model about the possible effects of a full turnover on DO concentrations in the deep hypolimnion. Two cases are displayed, with the turnover taking place either now or with an anoxic hypolimnion deriving from decades of isolation due to severe climate warming. Through this study, climate warming is shown to be a fundamental driver of DO in Lake Maggiore, its depletion harming both water quality and the ecosystem.

Comparative analyses of the heavy rainfall associated with landfalling tropical cyclones SOULIK (1307) and MARIA (1808) with similar routes

Different rainfall evolutions associated with landfalling tropical cyclones (TCs) SOULIK (1307) and MARIA (1808) are analyzed before and after they made landfalls in Fujian province based on reanalysis data of the ERA5 and CMORPH data. The two TCs shared similar tracks, motion speed as well as landing points, but showed different asymmetric features of rainfall intensities and locations relative to routes. The results primarily exhibited the discrepancy of the inland rainfall distributions between the two TCs and the related influence factors. The constant torrential rain over land with the maximum on the left of the storm track associated with SOULIK was attributed to favorable dynamical conditions. The northeasterly outflow of SOULIK was reinforced by the easterly wave in upper-level, which induced strong divergence along with the distinct ascending motions by suction effect. The strong ambient VWS broke the warm core structure of SOULIK, which pointed to south-southwest direction relevant to the strong northeasterly wind at 200 hPa, and further caused a more asymmetric rainfall pattern on the left of TC track. Besides, a cold intrusion was observed south of SOULIK in mid-levels, enhancing convective instability coincident with the alternating circumstance of cold and warm advections from the lower layer to the upper layer over south of SOULIK, which further intensify the precipitation on the left of TC track. Simultaneously, the meridional moisture convergence owing to the enhanced southerly wind at lower level played a key role in the maintenance of severe rainfall south of SOULIK. Comparatively, the 48-h accumulated rainfall over land associated with MARIA was much weaker than that associated with SOULIK, and the maximum is on the northern side. The strong convergence in the lower level induced by the predominant easterly winds interacting with coastal terrains, as well as the distinct divergence coincident with the upper level jet in the southern part of the South Asia High (SAH), provide favorable dynamical conditions for the maximum rainfall north of MARIA. Simultaneously, the strong westerlies over the South China Sea, together with the eastern cyclonic system over the Northwest Pacific, create favorable moisture conditions for the rainfall on the northern side of MARIA. Results presented in this study provides an improved predictive understanding of physical mechanism dictating storm rainfall evolution from landfalling TCs, especially those with similar tracks

Analysis of Extreme Meteorological Events in the Central Andes of Peru Using a Set of Specialized Instruments

A set of instruments to measure several physical, microphysical, and radiative properties of the atmosphere and clouds are essential to identify, understand and, subsequently, forecast and prevent the effects of extreme meteorological events, such as severe rainfall, hailstorms, frost events and high pollution events, that can occur with some regularity in the central Andes of Peru. However, like many other Latin American countries, Peru lacks an adequate network of meteorological stations to identify and analyze extreme meteorological events. To partially remedy this deficiency, the Geophysical Institute of Peru has installed a set of specialized sensors (LAMAR) on the Huancayo observatory (12.04° S, 75.32° W, 3350 m ASL), located in the Mantaro river basin, which is a part of the central Andes of Peru, especially in agricultural areas. LAMAR consists of a set of sensors that are used to measure the main atmosphere and soil variables located in a 30-meter-high tower. It also has a set of high-quality radiation sensors (BSRN station) that helps measure the components of short-wave (SW) (global, diffuse, direct and reflected) and long-wave (LW) (emitted and incident) irradiance mounted in a 6-meter-high tower. Moreover, to analyze the microphysics properties of clouds and rainfall, LAMAR includes a set of profiler radars: A Ka-band cloud profiler (MIRA-35c), a UHF wind profiler (CLAIRE), and a VHF wind profiler (BLTR), along with two disdrometers (PARSIVEL2) and two rain gauges pluviometers. The present study performs a detailed dynamic and energetic analysis of two extreme rainfall events, two intense frost events, and three high-pollution events occurring on the Huancayo observatory between 2018 and 2019. The results show that the rainfall events are similar to the 1965–2019 climatological 90th percentile of the daily accumulated rainfall. The results also highlighted the patterns of reflectivity in function of height for both events, which is measured by highlighting the presence of convective and stratiform rainfall types for both events. The first intense rainfall event was associated with strong easterly circulations at high levels of the atmosphere, and the second one was associated with the presence of strong westerly circulations and the absence of BH-NL system around the central Andes. The first frost event was mainly associated with continuous clear sky conditions in the few previous days, corresponding to a radiative frost event. The second one was mainly associated with the intrusion of cold surges from extra-tropical South America. For both events, the energy budget components were strong-lower in comparison to the mean monthly values during early morning hours. Finally, for the high pollution events, the study identified that the main source of aerosols were the forest fires that took place in Peru with certain contributions from the fires in the northern area of Bolivia.

Spring cold water intrusions as the beginningof the cold intermediate layer formation in the Baltic sea

Thermohaline intrusions are a typical feature of the Baltic Sea water body. We report observations of vivid colder/saltier intrusion activity in intermediate layers and upper pycnocline of the Gdansk Bay (the south-eastern part of the Baltic Sea) in early spring (March–April, 2013). Extremely low water temperature (down to 1.4–2 °C) and specific water salinity (SÃ7.5–7.8) of the intrusions point at a possible area of the intrusion waters formation: within the upper mixed layer in the Arkona/Bornholm basins at the end of March. Data on water temperature and salinity available from the International Council for the Exploration of the Sea (ICES) data repository, data of the Arkona Becken automated station, meteorological information, and remote sensing data for March–April 2013 are used to confirm the conclusions. It is confirmed once again that the transfer from the two-layered winter water stratification to the three-layered summer one in the Baltic Sea is a consequence of the sea-scale exchange process. The latter is manifested as simultaneous advection in upper and intermediate layers of waters with lower and higher salinity, correspondingly. The reported extremely cold intrusions indicate the beginning of the formation of the Cold Intermediate Layer (CIL), which thus contains waters of the Arkona/Bornholm basins. The main environmental factors driving the observed vivid intrusions in April 2013 are easterly winds and negative buoyancy fluxes due to seasonal solar heating of waters with the temperature below that of the density maximum. Substantial prolongation of the period of seasonal vertical mixing might be the most important process related to the transition of water temperature over the temperature of the density maximum (Tmd) in the Baltic Sea. With the climate warming, the cooling of surface waters below the Tmd becomes rare, limiting deep ventilation of Baltic waters in spring.

Recovery and variation of the coastal fish community following a cold intrusion event in the Penghu Islands, Taiwan.

Knowledge of community resilience aids the development of strategies to mitigate the impacts of a disturbance. An extreme low-seawater temperature event in late January and February 2008 resulted in high fish mortality in the coastal waters of the Penghu Islands, Taiwan. In this study, we used underwater diving visual censuses to analyze fish communities at eight sampling stations along the coast of the Penghu Islands for seven years after the 2008 event. We evaluated community metrics, including species richness, abundance-weighted diversity, average thermal affinity, and average trophic level, and described the temporal variation in select dominant species abundances. Species richness and diversity of the communities required 53 months to reach a steady-state at the sampling stations following the cold water intrusion. The cold event initially reduced community thermal affinity, which then increased throughout the study period, reflecting the recovery of the community to baseline thermal conditions. The increased average trophic level after the cold event implied that the temperature shock decreased the proportion of lower trophic-level fishes. Average trophic level declined as the communities recovered from the disturbance, reflecting the recovery of the community to baseline species composition in terms of feeding habit. Our results suggest that functional diversity may require longer to recover than taxonomic diversity for communities in the Penghu Islands.

The stratospheric ozone rich cold intrusion during El‐Niño over the Indian region: Implication during the Indian summer monsoon

AbstractOzone in the upper troposphere is a dominant radiative constituent. In this study, we investigate ozone variability due to stratospheric intrusions in the upper troposphere over India and associated radiative impacts during monsoon breaks co‐occurring with El Niño using the ECHAM5‐HAMMOZ, Global‐Chemistry‐climate model simulations, and ERA‐Interim reanalysis data. Our analysis shows that during El Niño, deep stratospheric intrusions occur at the North India‐Tibetan Plateau (NI‐TP) region and the eastern edge of the monsoon anticyclone. These intrusions lead to an enormous increase in ozone amounts (~100 ppb) in the upper troposphere over India. The intrusions frequently penetrate deep into the troposphere which enhances the surface ozone levels by ~10–20 ppb and augments radiative forcing by ~0.33 W m−2 at the top of the atmosphere. The stratospheric intrusions are associated with the eastward‐moving wave train (composed of cyclonic and anticyclonic circulations) in the upper troposphere emanating from the anomalously warm east Pacific, traversing towards NI‐TP locale. This wave train transport extra‐tropical cold air mass, producing an anomalous cooling of ~1.6–2 K in the upper troposphere over NI‐TP. The upper tropospheric anomalous cooling seems to act as a dominating factor opposing the heating due to enhanced ozone. The wave train likewise intensifies Rossby wave breaking, facilitating stratospheric intrusions, enhancing the anomalous subsidence over the NI‐TP region. Our analysis shows that break days with El Niño substantiate with simultaneous occurrence of (a) RWB, (b) transport of cold air mass via WT‐1 and (c) subsidence by El Niño circulation. These factors lead to exacerbation of deficit rainfall.

Dynamics and scale analysis of the transient convective flow induced by cooling a Pr

The convective boundary layer flow as well as the subsequent intrusion flow induced by cooling an initially isothermal and quiescent fluid at Pr<1 using a linear thermal forcing is investigated with a scaling analysis in the present study. It is demonstrated that the thickness of the convective boundary layer flow does not depend on the streamwise position in both the initial growth and the fully developed states, whilst the characteristic velocity is constantly streamwise location-dependant in these two states. It is also shown that the oscillatory behaviour of flow parameters in the transitional state of the boundary layer flow is dramatically weaker than the classical homogenously thermal forcing problems. Four possible flow regimes are identified for the cold intrusion flow and it is revealed that their occurrence is unconditional, which dramatically differentiates from the flow associated with Pr>1 fluids. It is further found that the first flow regime, i.e. the unsteady viscous-buoyancy dominance, is ephemeral in nature and can therefore be ignored. The intrusion flow can thus be practically divided into three flow regimes: an unsteady inertia-buoyancy regime; a steady inertia-buoyancy regime; and a steady viscous-buoyancy flow regime. The scaling relations quantifying the convective flow in different flow states are obtained. Selected scales are validated against the DNS (Direct Numerical Simulation) results and a good agreement is achieved.

Observations of Turbulence at a Near‐Surface Temperature Front in the Arctic Ocean

AbstractHigh‐resolution ocean temperature, salinity, current, and turbulence data were collected at an Arctic thermohaline front in the Nansen Basin. The front was close to the sea ice edge and separated the cold and fresh surface melt water from the warm and saline mixed layer. Measurements were made on 18 September 2018, in the upper 100 m, from a research vessel and an autonomous underwater vehicle. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross‐front Ekman transport by down‐front winds reduced the potential vorticity in the upper ocean. Turbulence structure in the mixed layer was generally consistent with turbulence production through convection by heat loss to atmosphere and mechanical forcing by moderate winds. Conditions at the front were favorable for forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. A clear signature of increased dissipation from symmetric instability could not be identified; however, this instability could potentially account for the increased dissipation rates at the front location down to 40 m depth that could not be explained by the atmospheric forcing. This turbulence was associated with turbulent heat fluxes of up to 10 W m−2, eroding the warm and cold intrusions observed between 30 and 60 m depth. A Seaglider sampled across a similar frontal structure in the same region 10 days after our survey. The submesoscale‐to‐turbulence‐scale transitions and resulting mixing can be widespread and important in the Atlantic sector of the Arctic Ocean.

Feature of formation of the Black Sea surface layer temperature during air cold intrusions

Application of the results of the drifter observations performed in the western Black Sea permitted to show that during air cold intrusions (CI) in winter accompanied by the wind (10 m/c and more), the upper mixed layer (UML) was cooled by 0,1-0,2 °C in course of a day. At that the seasonal thermocline (ST) and the cold intermediate water (CIW) sink deeper; after CI is over these layers rise to the depths smaller than their previous ones. It results in decrease of temperature in UML and its increase, as compared to the period preceding CI, in the layer below ST. The process of the sea upper layer mixing is explained by the fact that anti-cyclonic mesoscale vortices are involved in it.

Warm hole in Pacific Arctic sea ice cover forced mid-latitude Northern Hemisphere cooling during winter 2017\u201318

In North America and Asia, extreme cold weather characterized the winter of 2017–18. At the same time, the Pacific, the Bering Sea, and the Atlantic Arctic regions experienced anomalously low sea ice extent in the early winter. The jet stream dividing cold Arctic air from warm air deviated from normal zonal patterns northward into the ice-free areas north of the Bering Strait. Large southward jet stream pathways formed over Asia and America, allowing cold air to spread into Asia and the southern areas of North America. We hypothesise that the late autumn Bering Strait sea-ice anomaly and Pacific atmospheric rivers were partially responsible for the cold winter. We used data analyses and numerical experiments to test this hypothesis. We propose a positive feedback mechanism between the sea ice anomaly and atmospheric river activity, with anomalous south winds toward the sea ice anomaly potentially leading to more warm water injected by the wind-driven current through the Bering Strait. Our findings suggest that Poleward propagation of the atmospheric rivers made upper air warm, leading to their upgliding, which further heated the overlying air, causing poleward jet meanders. As a part of this response the jet stream meandered southward over Asia and North America, resulting in cold intrusions. We speculate that the positive feedback mechanism observed during the 2017–18 winter could recur in future years when the sea-ice reduction in the Pacific Arctic interacts with enhanced atmospheric river activity.

Hypoxia tolerance of giant axon-mediated escape jetting in California market squid (Doryteuthis opalescens).

Squids display a wide range of swimming behaviors, including powerful escape jets mediated by the giant axon system. For California market squid (Doryteuthis opalescens), maintaining essential behaviors like the escape response during environmental variations poses a major challenge as this species often encounters intrusions of cold, hypoxic offshore waters in its coastal spawning habitats. To explore the effects of hypoxia on locomotion and the underlying neural mechanisms, we made in vivo recordings of giant axon activity and simultaneous pressure inside the mantle cavity during escape jets in squid exposed to acute progressive hypoxia followed by return to normal dissolved oxygen (DO) concentration (normoxia). Compared with those in normoxia (>8 mg l-1 DO), escape jets were unchanged in moderate hypoxia (4 and 2 mg l-1 DO), but giant axon activity and associated mantle contractions significantly decreased while neuromuscular latency increased under severe hypoxia (0.5 mg l-1 DO). Animals that survived exposure to severe hypoxia reliably produced escape jets under such conditions and fully recovered as more oxygen became available. The reduction in neuromuscular output under hypoxia suggests that market squid may suppress metabolic activity to maintain sufficient behavioral output, a common strategy in many hypoxia-tolerant species. The ability to recover from the deleterious effects of hypoxia suggests that this species is well adapted to cope with coastal hypoxic events that commonly occur in Monterey Bay, unless these events become more severe in the future as climate change progresses.

Diffusive Convection under Rapidly Varying Conditions

AbstractIn most observations of diffusive convection in the ocean and in lakes, the characteristic diffusive staircases evolve over long time scales under quasi-stationary background conditions. In the Baltic Sea, however, diffusive staircases develop inside the flanks of intermittent intrusions that induce strong inverse temperature gradients over a vertical range of a few meters, varying on time scales of hours to days. Here, results are discussed from an extensive field campaign conducted in summer 2016 in the southern Baltic Sea, including temperature microstructure data from ocean gliders and an autonomous profiling platform. We find conditions favorable for diffusive instability in the vicinity of warm and cold intrusions with density ratios as small as Rρ = 1.3. The staircases evolving under these conditions are characterized by a small number of steps (typically 1–4) with order 0.1–1-m thickness, temperature differences exceeding 1 K across individual diffusive interfaces, and exceptionally large diffusive heat fluxes of order 10 W m−2. The standard heat flux parameterization of Kelley agrees within a factor of 2 with the directly observed interfacial heat fluxes, except for large fluxes at low Rρ, which are strongly overestimated. The glider surveys reveal a surprisingly small lateral coherency of order 100 m of the staircase patterns, and a spreading of the diffusively unstable intrusions across isopycnals.

RECENT CHANGES IN PRODUCING OF THE INTERMEDIATE WATER IN THE OKHOTSK SEA AND ITS PROPERTIES

Data of long-term observations on the cold intrusions in the warm intermediate layer of the Okhotsk Sea are overviewed, considering these intrusions as fresh portions of the Intermediate water of local origin. They are formed in the process of the high-density Bottom Shelf water cascading from the northwestern shelf, mainly in the submarine canyon at northwestern Sakhalin. Transport of these new-formed portions is traced in the flows of cyclonic circulation of the Okhotsk Sea waters from the area of their origin to the southern Kuril Straits, that takes from 2 to 6 years. Similar intrusions are found in the Pacific, mainly at southern Kuril Islands, where they penetrate presumably through the Vries and Bussol Straits. Year-to-year variability of the Bottom Shelf water and the new-formed Intermediate water properties are analyzed in dependence on external factors. There is determined that the volume and density of the Bottom Shelf water depend on winter severity that could be quantitatively explained by the ice cover; all these parameters decrease recently. Salinity of the new-formed Intermediate water has similar changes. Besides, tidal mixing influences on its salinity, in particularly at the Kuril Straits, so the tidal 18.6-year cycle is presented in the salinity variations. On the contrary, variations of the minimal temperature in the cold-water lenses of the fresh Intermediate water of local origin do not correlate with the changes in their production because of auto-compensatory effect for temperature in the process of its cascading to different depths. The same effect is obviously applied to dissolved oxygen, that’s why the oxygen content in the upper intermediate layer at the area of cascading is rather stable and doesn’t correlate with the ice cover changes. However, ventilation of the medium and deeper parts of the intermediate layer decreases on the background of recent lowering of the Intermediate water producing, in particularly the production of the water with σq ≥ 26.8. On a distance from the area of cascading, the tendency to deoxygenizing spreads over the whole intermediate layer of the Okhotsk Sea.

Influence of internal bores on larval fish abundance and community composition

A persistent semidiurnal internal tidal bore feature occurs at the head of the Monterey Bay Submarine Canyon and drives regular intrusions of cold, subthermocline waters onto the adjacent shelf. In this study, we examine the influence of this internal tidal bore feature on the larval fish community using over a year of periodic larval fish samples collected coincidently with physical measurements. Larval samples were categorized into one of two water mass periods: a “warm period” representative of shallow coastal shelf waters and a “cold period” characteristics of colder waters present during internal bore forcing. Using multivariate statistical methods, we show warm and cold periods, along with seasonality, are the primary drivers of larval fish community composition. A significantly different community composition was observed between warm and cold water mass periods. This difference was primarily due to decreased abundance in most taxa during the cold periods, and did not indicate an obvious shift in the assemblage of the taxa. However, our data do indicate that some taxa may show higher abundance during cold periods compared to warm periods, but further studies are warranted. Along with seasonality, the presence/absence of subthermocline waters driven by internal bores appears to be a key control on nearshore larval fish community composition at this location.

Organic matter in sediments of a tropical and upwelling influenced region of the Brazilian continental margin (Campos Basin, Rio de Janeiro)

Lipid biomarkers [fatty acids (FAs), sterols and alcohols] and carbon stable isotopes (δ13C values) in surface sediments from 9 cross shelf transects (25–3000 m water depth) from the Campos Basin, SE Brazilian continental margin were analyzed. The aim was to investigate the link between the prevailing regional specific oceanographic conditions (upwelling events, intrusion of cold and nutrient-rich water, low river input) and the nature and distribution of organic matter (OM) in the basin. A general predominance of OM from autochthonous processes, but with a relevant spatial gradient in the quality and quantity of the sedimentary OM, was observed. On the shelf (<150 m), concentrations of lipids were usually low, except in areas influenced by upwelling, but the presence of labile compounds suggested the occurrence of fresh OM in the sediment. The export of continental OM was observed only in shelf sediments near the Paraíba do Sul River. The upper and middle slope (400–1300 m) exhibited the highest concentrations of total organic carbon (TOC) and lipids, but lipid biomarkers suggested the presence of OM with a high degree of bacterial degradation. This may result from the export of material from shallow areas, possibly due to the action of eddies and meandering of the Brazil Current and bottom currents in the region. On the lower slope (1900–3000 m), only the more recalcitrant compounds were above detection limit. The presence of labile lipids in high amount in the shelf and slope suggests the presence of OM with a high potential for supplying the food requirements of heterotrophic organisms in the sediment, which may in turn have a major influence on the ecology of benthic communities.

Spring thermocline formation in the coastal zone of the southeastern Baltic Sea based on field data in 2010–2013

The transition from winter vertical mixing to the formation of the spring thermocline in the southeastern Baltic Sea is studied based on data from the hydrophysical measurements program (11 expeditions) in the Russian part of Gdansk Bay in March–June 2010, 2011, and 2013. CTD measurements were taken along the standard 18-km transect across the isobaths with a 500-m step abeam the city of Baltiysk. A set of frequently measured data was collected in a 1–2 week interval from the end of March to the beginning of May, which made it possible to analyze the transformation of the vertical thermal structure of water from inverse winter type to the summer stratification with the transition of temperature over the temperature of the density maximum. Series of repeated measurements at the deep and coastal stations as well as surface and subsurface towed measurements were carried out. The fact that lenses of freshened warmer water appear at the surface almost simultaneously with intensification of cold intrusions in intermediate (10–40 m) layers makes it possible not only to confirm the advective nature of the formation of the spring thermocline in the Baltic Sea, but also to hypothesize about the intensification of intrabasin exchange when winter-time vertical mixing ceases: the potential energy excess supported by vertical mixing in the 60-m upper quasi-homogeneous layer (UQL) of the Baltic Proper, in which the horizontal estuarine salinity gradient is significant, is converted to kinetic energy of exchange currents as the mixing process terminates. Such water dynamics makes it possible to explain the intensification of intrusions in the Baltic in spring and the formation of the cold intermediate layer due to the fast propagation of late-winter UQL water from the Bornholm Basin to the Baltic Proper. The results agree well with earlier published studies of other authors.