Seasonal Thermocline Research Articles

Features of the vertical phytoplankton structure in the deep-sea parts of the Caspian Sea in summer

The new features of the vertical phytoplankton distribution in the central and southern deep-water parts of the Caspian Sea are identified on the basis of long-term observations (2004–2012, 419 samples). Systematic study of phytoplankton in the Middle Caspian for nine years has shown that the interannual variability in the dominant summer phytoplankton complex is due to traditional species of diatoms and dinoflagellates and also coccolithophores, a new systematic group for the Caspian Sea (June 2010). It was first determined that summer phytoplankton of the deep-water parts in the Middle and Southern Caspian is similar in species, quantitative, and spatial (vertical) structure. A zone of higher phytoplankton productivity was first found in the area of the Absheron Sill. Two types of communities and their boundary were first distinguished in the vertical structure of summer phytoplankton of the deep-sea parts: warm-water and cold-water (below the thermocline). The boundary between them corresponds to the lower boundary of the seasonal thermocline (maximum depths up to 50–60 m) with the highest wet total phytoplankton biomass and chlorophyll a concentrations. The intensity of stratification of the water column by temperature mainly causes the vertical phytoplankton structure. The anomalously large deep-sea accumulations of diatoms cells containing chlorophyll (remains of winter-spring blooms) were first found in the near-bottom layers of the northern slope of the Derbent Depression. Their presence at the depths of 300–400 m is probably caused by the slope cascading. The lower boundary (500 m) of phytoplankton abundance in the Caspian Sea with chlorophyll-containing cells of fresh water green algae were registered by the authors for the first time in the central areas of the Derbent and South Caspian depressions. This phenomenon was caused by the contribution of cold Caucasus rivers through a system of submarine canyons from the shelf to the deep sea areas.

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57°55′N, 1°38′E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the near-bottom temperature.The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front.

Structure and zonation of demersal and deep-water fish assemblages off the Cabo Verde archipelago (northeast-Atlantic) as sampled by baited longlines

The assemblages of the demersal and deep-water fish assemblages of the Cabo Verde archipelago are described for the first time from a total of 20 longline hauls that extends from coastline to 1200m water depth. A total of 98 species from 47 different families were caught, and despite the changes of biogeographic affinities with depth, the majority of the species are from a Subtropical and Tropical origin or have a broad geographic distribution (Wide and Amphiatlantic distributions) with small affinities with the Macaronesian islands and the Mediterranean. Depth distribution profiles show a clear turnover of species with depth, and an increase in the depth distribution range of the deeper living species. Furthermore, the indices of species abundance, biomass and the number of species caught, significantly decreased with increasing depth. Four main fish assemblages in a depth-aligned structure are proposed in the present work. A shelf assemblage mainly distributed above the 150m depth strata, a shelf-break/upper-slope assemblage mainly between the 150–350m depth strata, a deep/upper-slope assemblage between the 300–600m depth strata, and a mid- deep/mid-slope assemblage mainly distributed below the 600m. Species abundances and diversity found below 200m is relatively low compared with shallow waters. The observed depth zonation of the slope assemblages, roughly agrees with the expected occurrence or influence of different water masses: shelf and surface mixing and seasonal thermocline depth zone, the depth range of occurrence of the intermediate waters (North Atlantic Central Water—NACW), the South Atlantic Central Water—SACW), and Atlantic Equatorial Water—AEW), a mixing or overlapping zone of previous intermediate water masses with the Antarctic Intermediate Water (AAIW), and the Mediterranean Outflow Water (MOW), and a deeper zone where the AAIW predominate.

Along‐isopycnal variability of spice in the North Pacific

AbstractTwo hydrographic surveys in the Gulf of Alaska and the North Pacific subtropical gyre are presented. Both surveys are roughly perpendicular to lateral temperature gradients, and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline. Isopycnal displacements and horizontal velocities are dominated by internal waves. Spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms. The spectra of spice anomaly gradients along near‐surface isopycnals roughly follow power laws of (variance spectra power laws of ), and in most cases, the spectra become redder at depth. The near‐surface spectra are possibly consistent with the predictions of quasi‐geostrophic turbulence theory (when surface buoyancy effects are accounted for), but the spectra at depth are inconsistent with any quasi‐geostrophic theory. Probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients, symptomatic of fronts. Vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately . Lateral decorrelation length scales are 20–40 km, close to the baroclinic Rossby radius. Lateral stirring occurs over large scales, with average lateral displacements of about 200 km in the upper 75 m, decreasing to 100 km at greater depths. The depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important, or that there are unconsidered depth‐dependent stirring mechanisms.

Multiscale seismic reflectivity of shallow thermoclines

AbstractSeismic oceanography is coming of age as an established technique of observation of the thermohaline structure of the ocean. The present paper deals with the seismic reflectivity of the Armorican Shelf seasonal thermocline, west of France, based on two seismic experiments performed with a sparker source. The peak frequency was 500 Hz for the ASPEX experiment, where the thermocline was located at 27 m water depth, and reduced to 400 Hz associated to a higher source level for the IFOSISMO experiment, where the thermocline was 12 m deeper. Despite these settings, only the first experiment could clearly highlight the thermocline reflector, providing the first seismic observation of a shallow oceanic structure. To better understand the limitation of high‐resolution seismic devices in detecting weak oceanic features, we develop a wavelet‐based seismic analysis and consider, as a first approximation, a simple thermocline modeled by a Gauss error function, allowing an analytical expression for the associated seismic reflectivity. We show that the acoustic impedance profile of the thermocline is mainly controlled by a sound velocity proportional to the temperature. We show that the seismic reflectivity is controlled by the reflection coefficient of the large‐scale structure of the thermocline and by an attenuation factor which depends on the ratio between the seismic wavelength and the characteristic size of the thermocline. Depending on this ratio, the strength of the thermocline‐related reflection may be too weak to be detected by seismic measurement.

Impact of downward heat penetration below the shallow seasonal thermocline on the sea surface temperature

Observational data are used to investigate summer heat penetration into the subsurface ocean in order to quantify the heat capacity of the upper ocean with respect to surface heat exchange. Sea surface temperature is strongly modulated by the change in heat capacity, which could influence the overlying atmosphere and hence trigger climate variations, even during the warming season, when the ocean has been regarded as being rather passive. Few studies have focused on the heat exchange process in surface and subsurface layers because of the existence of a strong seasonal thermocline at the bottom of thin summer mixed layers (ML). By introducing the concept of the heat penetration depth (HPD), defined as the depth to which the downward net heat flux (Q net) distinctly penetrates, we here characterize the heat capacity in terms of the heat content above the HPD using a simple, one-dimensional vertical model during the warming season. Seasonal changes in the HPD indicate that the thermal effects of Q net gradually penetrate below the shallow seasonal thermocline due to vertical eddy diffusivity. Downward heat penetration into the layer below the shallow seasonal thermocline occurs widely throughout the North Pacific, and two-thirds of Q net penetrates below the ML. In a hypothetical analysis of the case where the observed Q net accumulates only within the ML, the change in SST is unrealistically larger than that of the observed SST. These results indicate that heat penetration plays a crucial role in climate variations during the warming season.

Structure and evolution of the cold intermediate layer in the southeastern part of the Baltic Sea by the field measurement data of 2004–2008

The cold intermediate layer (CIL) was distinguished in the series of vertical CTD profiles obtained in 2004–2008 in the Gdansk basin of the Baltic Sea. The CIL is determined as the layer between the maximum absolute values of the vertical gradient of the water temperature during its decrease in the seasonal thermocline and the growth in the ranges of the permanent halocline. The CIL contains both homogenous and high-gradient sublayers (by various parameters); in particular, a layer of abnormally cold water of the temperature below that of the maximum density might be registered right up to July. Three stages were distinguished in the seasonal cycle of the CIL development: the formation by local mechanisms (March–April), the geo-strophic adjustment within the general thermohaline structure of the sea and slow modification (May–August), and the decay (September–January). The annual maximum thickness of the upper quasi-homogeneous layer, as well as that of the CIL, is by no means owing to the winter vertical mixing but to the conditions during the time of the early spring warming (early March). The complicated structure and character of the temporal variability point to the significant role of advection in the formation and evolution of the Baltic CIL.

Variability of currents over the southern slope of the Gulf of Finland

Summary In our intraseasonal variability studies of currents in the coastal sea of the Gulf of Finland northeast of Pakri Peninsula, we compared the observation data from a bottom-mounted ADCP (March–June of 2009, 50 m depth) with the simulation data from High Resolution Operational Model of the Baltic (HIROMB). The structure of the current pattern appeared strongly dependent on the stratification conditions. The flow was quasi-barotropic during the periods of weak inverse thermal stratification at the end of winter season and at transition from the inverse thermal stratification to summer type stratification when the sea was thermally unstratified, but mostly two-layered (baroclinic) when the summer type thermal stratification had developed. The alternation of strong westward (eastward) currents (up to 30 cm s−1) in the upper layer is explained in terms of coastal upwelling (downwelling) due to favourable background winds. The measured and the modelled upper layers along isobath currents showed a noticeable correlation with the correlation coefficient of 0.52 and 0.82 during the periods of winter type and summer type stratifications, respectively, and the absence of a significant correlation during the transition period. The eastward (upwind) current episodes with speeds reaching 18 cm s−1 below the seasonal thermocline are likely to reflect the specific circulation response in the elongated basin caused by the easterly wind. The long-term mean (over 3.5 months) current vector (−2.0 cm s−1, −2.9 cm s−1) was westward in the upper sea and eastward, nearly along isobaths (1.1 cm s−1, −0.3 cm s−1) in the deeper layers.

Seasonal cycle of the mixed layer, the seasonal thermocline and the upper-ocean heat storage rate in the Mediterranean Sea derived from observations

We present a Mediterranean climatology (1°×1°×12months) of the mixed layer and of the seasonal thermocline, based on a comprehensive collection of temperature profiles spanning 44years (1969–2012). The database includes more than 190,000 profiles, merging CTD, MBT/XBT, profiling floats, and gliders observations. This data set is first used to describe the seasonal cycle of the mixed layer depth and temperature, together with the seasonal thermocline depth and averaged temperature, on the whole Mediterranean on a monthly climatological basis. Our analysis discriminates several regions with coherent behaviors, in particular the deep water formation sites, characterized by significant differences in the winter mixing intensity. Heat Storage Rate (HSR) is calculated as the time rate of change of the heat content due to variations in the temperature integrated from the surface down to the base of the seasonal thermocline. For the first time the quantification of heat storage rate in the upper-ocean, based only on in situ oceanographic data, is made for the whole Mediterranean. The spatial and temporal variability of the HSR in the Mediterranean Sea and its link with dynamic structures like oceanic gyres are also discussed.

Validation of Thorpe-scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

Abstract. The Thorpe scale is an energy-containing vertical overturning scale of large eddies associated with shear-generated turbulence. This study investigates indirect estimates of vertical diffusivities from the Thorpe scale method in the polar front region east of the Kerguelen Islands based on fine-scale density profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivities are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler. The results are sensitive to the choice of the diffusivity parameterization and the overturn ratio Ro, and the optimal results have been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion, rather than the parameterization by Osborn (1980) and the Ro = 0.2 criterion originally suggested by Gargett and Garner (2008). The Thorpe-scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5) m2 s−1 in the Winter Water layer and in the area immediately north of the polar front to a high value of O(10−4) m2 s−1 in the seasonal thermocline between the surface mixed layer and the Winter Water. The latter high diffusivities are found especially over the shallow plateau southeast of the Kerguelen Islands and along the polar front that is attached to the escarpment northeast of the islands. The interaction of strong frontal flow with prominent bottom topography likely causes the observed elevated mixing rates.

Oscillation of hydrophysical fields on the shelf and continental slope caused by nonstationary wind

Barotropic and baroclinic oscillations typical of the spring period have been studied based on the three-dimensional thermohydrodynamic model of the Black Sea. This season can be considered transient between the termination of the effect of strong winter winds and the onset of seasonal thermocline formation. The three-dimensional numerical model with a free surface and boundary conditions, corresponding to the real wind field for April 2006 and to the heat and salt flows on the surface, has been used. The horizontal and vertical spatial resolutions of the model are 1.5 km and 27 levels, respectively; the time discreteness is 1 h. This makes it possible to perform detailed modeling in the South Coastal Crimean sea sector. The spatial-time structure of the level, kinetic energy, and temperature fields has been studied using the spectral analysis method. The maximal energy-carrying frequency has been interpreted. Manifestations of Poincare and Kelvin waves, as well as captured shelf waves, have been distinguished in the submesoscale spectral region. A comparison with previous studies has been performed. It is indicated that the stratification variability caused by synoptic processes affects spectral characteristics of wave processes on smaller scales, making its contribution to the Black Sea water mixing.

Stratification and horizontal exchange in Lake Victoria, East Africa

We characterize stratification patterns over diel, seasonal, and annual time scales in inshore and offshore regions of Lake Victoria, East Africa; determine conditions leading to horizontal exchanges; and, using surface energy budgets derived from local meteorological stations and two reanalysis products, address whether stratification depends on advective as opposed to local processes. The largest change in the surface energy budget occurred when winds intensified at the end of the long rains, with the wind's intensification, duration, and spatial extent dependent on El Niño–Southern Oscillation cycles. These winds flush inshore waters and cause cross‐basin upwelling similar to that observed in the deep African Great Lakes. Wedderburn numbers indicated mixing and cross‐basin within‐thermocline transport. The internal wave‐induced mixing and enhanced latent heat fluxes of −300 to −400 W m−2 contributed to the loss of seasonal stratification. Advection of cool water was required to balance the heat budget of northern offshore waters in the latter half of the southeast monsoon except in an El Niño year. Northern waters became weakly stratified after the southeast monsoon, with nocturnal winds contributing to heat transport and ventilation of the lower water column. Following the rainy season, downwelling by sustained southerly albeit low winds is a likely cause of the seasonal thermocline. Inshore waters are 0.2–1.5°C warmer than those offshore, conditions conducive to horizontal convective circulation except during onshore winds. The seasonal cycle of stratification and inshore–offshore and cross‐basin exchanges are moderated by differential heating, cooling, and basin‐scale thermocline tilting.

Climatic controls on biophysical interactions in the Black Sea under present day conditions and a potential future (A1B) climate scenario

A dynamical downscaling approach has been applied to investigate climatic controls on biophysical interactions and lower trophic level dynamics in the Black Sea. Simulations were performed under present day conditions (1980–1999) and a potential future (2080–2099) climate scenario, based on the Intergovernmental Panel for Climate Change A1B greenhouse gas emission scenario. Simulations project a 3.7°C increase in SST, a 25% increase in the stability of the seasonal thermocline and a 37day increase in the duration of seasonal stratification. Increased winter temperatures inhibited the formation of Cold Intermediate Layer (CIL) waters resulting in near complete erosion of the CIL, with implications for the ventilation of intermediate water masses and the subduction of riverine nutrients. A 4% increase in nitrate availability within the upper 30m of the water column reflected an increase in the retention time of river water within the surface mixed-layer. Changes in thermohaline structure, combined with a 27% reduction in positive wind stress curl, forced a distinct change in the structure of the basin-scale circulation. The predominantly cyclonic circulation characteristic of contemporary conditions was reversed within the southern and eastern regions of the basin, where under A1B climatic conditions, anticyclonic circulation prevailed. The change in circulation structure significantly altered the horizontal advection and dispersion of high nutrient river waters originating on the NW self. Net primary production increased by 5% on average, with much spatial variability in the response, linked to advective processes. Phytoplankton biomass also increased by 5% and the higher nutrient environment of the future scenario caused a shift in species composition in favour of larger phytoplankton. No significant change in zooplankton biomass was projected. These results constitute one of many possible future scenarios for the Black Sea, being dependent on the modelling systems employed in addition to the choice of emission scenario. Our results emphasise in particular the sensitivity of dynamical downscaling studies to the regional wind forcing fields extracted from global models (these being typically model dependent). As atmospheric warming is projected with a high degree of confidence warming of the Black Sea upper layer, increased water column stability, and erosion of the CIL are believed to be robust results of this study.

Near‐surface temperature gradient in a coastal upwelling regime

AbstractIn oceanography, a near homogeneous mixed layer extending from the surface to a seasonal thermocline is a common conceptual basis in physics, chemistry, and biology. In a coastal upwelling region 3 km off the coast in the Mexican Pacific, we measured vertical density gradients with a free‐rising CTD and temperature gradients with thermographs at 1, 3, and 5 m depths logging every 5 min during more than a year. No significant salinity gradient was observed down to 10 m depth, and the CTD temperature and density gradients showed no pronounced discontinuity that would suggest a near‐surface mixed layer. Thermographs generally logged decreasing temperature with depth with gradients higher than 0.2 K m−1 more than half of the time in the summer between 1 and 3 m, 3 and 5 m and in the winter between 1 and 3 m. Some negative temperature gradients were present and gradients were generally highly variable in time with high peaks lasting fractions of hours to hours. These temporal changes were too rapid to be explained by local heating or cooling. The pattern of positive and negative peaks might be explained by vertical stacks of water layers of different temperatures and different horizontal drift vectors. The observed near‐surface gradient has implications for turbulent wind energy transfer, vertical exchange of dissolved and particulate water constituents, the interpretation of remotely sensed SST, and horizontal wind‐induced transport.

Responses of the circulation and water mass in the Beibu Gulf to the seasonal forcing regimes

In the past 20 a, the gulf-scale circulation in the Beibu Gulf has been commonly accepted to be driven by a wind stress or density gradient. However, using three sensitive experiments based on a three-dimensional baroclinic model that was verified by observations, the formation mechanisms were revealed: the circulation in the northern Beibu Gulf was triggered by the monsoon wind throughout a year; whereas the southern gulf circulation was driven by the monsoon wind and South China Sea (SCS) circulation in winter and summer, respectively. The force of heat flux and tidal harmonics had a strong effect on the circulation strength and range, as well as the local circulation structures, but these factors did not influence the major circulation structure in the Beibu Gulf. On the other hand, the Beibu Gulf Cold Water Mass (BGCWM) would disappear without the force of heat flux because the seasonal thermocline layer was generated by the input of heat so that the vertical mixing between the upper hot water and lower cold water was blocked. In addition, the wind-induced cyclonic gyre in the northern gulf was favorable to the existence of the BGCWM. However, the coverage area of the BGCWM was increased slightly without the force of the tidal harmonics. When the model was driven by the monthly averaged surface forcing, the circulation structure was changed to some extent, and the coverage area of the BGCWM almost extended outwards 100%, implying the circulation and water mass in the Beibu Gulf had strong responses to the temporal resolution of the surface forces.

Reconstructing the subsurface ocean decadal variability using surface nudging in a perfect model framework

Initialising the ocean internal variability for decadal predictability studies is a new area of research and a variety of ad hoc methods are currently proposed. In this study, we explore how nudging with sea surface temperature (SST) and salinity (SSS) can reconstruct the three-dimensional variability of the ocean in a perfect model framework. This approach builds on the hypothesis that oceanic processes themselves will transport the surface information into the ocean interior as seen in ocean-only simulations. Five nudged simulations are designed to reconstruct a 150 years “target” simulation, defined as a portion of a long control simulation. The nudged simulations differ by the variables restored to, SST or SST + SSS, and by the area where the nudging is applied. The strength of the heat flux feedback is diagnosed from observations and the restoring coefficients for SSS use the same time-scale. We observed that this choice prevents spurious convection at high latitudes and near sea-ice border when nudging both SST and SSS. In the tropics, nudging the SST is enough to reconstruct the tropical atmosphere circulation and the associated dynamical and thermodynamical impacts on the underlying ocean. In the tropical Pacific Ocean, the profiles for temperature show a significant correlation from the surface down to 2,000 m, due to dynamical adjustment of the isopycnals. At mid-to-high latitudes, SSS nudging is required to reconstruct both the temperature and the salinity below the seasonal thermocline. This is particularly true in the North Atlantic where adding SSS nudging enables to reconstruct the deep convection regions of the target. By initiating a previously documented 20-year cycle of the model, the SST + SSS nudging is also able to reproduce most of the AMOC variations, a key source of decadal predictability. Reconstruction at depth does not significantly improve with amount of time spent nudging and the efficiency of the surface nudging rather depends on the period/events considered. The joint SST + SSS nudging applied everywhere is the most efficient approach. It ensures that the right water masses are formed at the right surface density, the subsequent circulation, subduction and deep convection further transporting them at depth. The results of this study underline the potential key role of SSS for decadal predictability and further make the case for sustained large-scale observations of this field.

Impact of vertical mixing on sea surface pCO2 in temperate seasonally stratified shelf seas

AbstractA key parameter in determining the exchange of CO2 across the ocean‐atmosphere interface is the sea surface partial pressure of carbon dioxide (pCO2). Temperate seasonally stratified shelf seas represent a significant sink for atmospheric CO2. Here an analytical model is used to quantify the impact of vertical mixing across the seasonal thermocline on pCO2. The model includes the impacts of the resultant dissolved inorganic carbon, heat, salt, and alkalinity fluxes on the solubility of CO2 and the effect of the inorganic carbon sink created by the primary production fuelled by the flux of limiting nutrient. The results indicate that diapycnal mixing drives a modest but continuous change in pCO2 of order 1–10 µatm d−1. In quantifying the individual impacts of the fluxes of the different parameters, we find that the impact of the fluxes of DIC and nitrate fluxes dominate. In consequence, both the direction and magnitude of the change in pCO2 are strongly dependent on the C:N uptake ratio in primary production. While the smaller impacts of the heat and salt fluxes tend to compensate for each other at midshelf locations, the heat flux dominates close to the shelf break. The analysis highlights the importance of the accurate parameterization of the C:N uptake ratio, the surface‐mixed layer depth, and the TKE dissipation rate within the seasonal thermocline in models to be used to predict the air‐sea exchange of carbon dioxide in these regimes. The results implicate storms as key periods of pCO2 perturbation.

Changes in dominant mixing length scales as a driver of subpolar phytoplankton bloom initiation in the North Atlantic

Subpolar phytoplankton blooms have traditionally been attributed to changes in the depth of the ocean's seasonal thermocline: as the upper ocean warms and stratifies in the spring, phytoplankton reside within increasingly shallow depths where they experience higher light levels, and, as a result, begin to bloom. Recent studies have challenged this explanation, proposing instead that bloom initiation is driven either by the onset of positive heat fluxes, decreases in wind strength, decreases in grazing pressure, or by eddy-induced stratification. We compare traditional and recent ideas of bloom initiation and present a new argument that attributes the initiation to a decrease in the dominant mixing length scales in the upper ocean. From an examination of data across the subpolar North Atlantic, we find that decreases in this length scale are a better predictor of bloom initiation than current theories, thus providing a new explanation of bloom dynamics in a one-dimensional framework.

Seasonal variability and north–south asymmetry of internal tides in the deep basin west of the Luzon Strait

Temporal and spatial variability of internal tides (ITs) in the deep basin west of the Luzon Strait were examined, based on two 9-month mooring current observations from autumn 2008 to summer 2009. The baroclinic current measurements exhibited north–south asymmetry and temporal variation. At the southern site (19.5°N), the kinetic energy of diurnal IT was dominant over that of semidiurnal IT by a factor of 3 to 4, whereas at the northern site (~21°N), the diurnal energy was comparable to or a little larger than semidiurnal energy. At both sites, the motions of semidiurnal IT were recognized as seasonally invariant, while diurnal IT showed notable seasonal variation, namely, stronger in summer and winter but weaker in spring and autumn. The seasonal variation was mainly modulated by the astronomical tides in the Luzon Strait rather than by the seasonal thermocline. Both diurnal and semidiurnal ITs contained stronger coherent signals than incoherent counterparts at two sites, but the IT at the southern site was more coherent than that at the northern site. Diurnal IT is more coherent than the semidiurnal IT at each site. The spatial–temporal variations were probably due to IT interferences from various sources within the Luzon Strait and modulation by varying background conditions.

Paradox reconsidered: Methane oversaturation in well‐oxygenated lake waters

The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid‐water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro‐anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen‐rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m−2 d−1. Mid‐water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0–20% dissolved oxygen (DO) in the bottom water, and a positive gradient within ≥ 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle.