Intense Vertical Mixing Research Articles

Oceanic eddy with submesoscale edge drives intense air-sea exchanges and beyond

Oceanic mesoscale eddies influence air-sea interaction and atmosphere dynamics through ventilating heat and moisture upward. However, whether the sea surface temperature (SST) gradient on the eddy edge could affect the heat and moisture release is still unknown because of the limited observations and coarse-resolution climate models. Using high-resolution atmospheric simulations, this study compares the atmospheric response to the mesoscale (~ 40 km) and submesoscale (~ 4 km) SST gradients at the edge of an eddy. Results show that submesoscale SST gradient drives stronger surface heat and moisture fluxes, enhancing the vertical mixing intensity by 2–3 times within and above the marine atmospheric boundary layer. As a result, one local precipitation event is found to be an order of magnitude larger overlying the eddy. Our findings highlight the importance of resolving oceanic submesoscale features for accurately predicting atmosphere dynamics and precipitation over the ocean.

Secondary estuarine circulation and the related vertical mixing at the sill of Ambon Bay, eastern Indonesia

Secondary estuarine circulation at the sill of Ambon Bay during wet season (July 2019) was investigated to add important insights into the existing knowledge on primary estuarine circulation done by a recent study. The datasets from the previous study were employed to calculate tidal-mean vertical advection as secondary estuarine circulation at the sill. Vertical mixing at the sill was also quantified using the continuity equation. The vertical profiles of vertical advection at the sill of Ambon Bay formed bow-like shapes with zero value at the surface that increased to reach maximum at the mid-depth of the sill (∼6 m depth), subsequently decreasing to zero at the seabed. Vertical advection at the sill was found to be larger during spring tide (∼2.5 × 10−3 m/s) than during neap tide (∼1 × 10−3 m/s). Vertical diffusivity, indicating the magnitude of vertical mixing at the sill of Ambon Bay, showed similar characteristics to vertical advection in terms of vertical profiles (bow-like shapes) and spring/neap tidal variation (spring tide value: 8.5 × 10−3 m2/s; neap tide value: ∼5 × 10−3 m2/s). The intense vertical mixing at the sill of Ambon Bay has the potential to create turbid water linked to resuspension of seabed sediment in the location.

Diapycnal Mixing and Double Diffusion over the Continental Slope in the Northern Sea of Japan in the Warm Half-Year

This study investigates double diffusion processes responsible for diapycnal mixing in the Primorye current in the northern Sea of Japan. The results of processing and analysis of a long time series of data from regular Aqualog-profiler measurements of thermohaline characteristics and current velocity over the continental slope between the horizons of 60 and 420 m from April to October 2015. Using the Osborne-Cox parameterization (1972) and Gregg’s (1989) parameterization, we estimated heat and salt diffusivities, as well as the buoyancy flux. To reveal the processes of double diffusion, the Turner angle was estimated. We estimated contributions of diffusive convection and salt fingers into the buoyancy flux. Due to double diffusion, the intense vertical mixing covered the layer from 80 to 170 m. From mid-spring to early May, diffusive convection penetrated to a horizon of 250 m, and then a process like salt fingers played a leading role. Shear instability caused by the weakly nonlinear interaction of internal waves prevailed in the underlying layers.

Impact of vertical stratification on the 2020 spring bloom in the Yellow Sea

The Yellow Sea is one of the world’s most abundant marine resources, providing food and economic benefits to the Korean and Chinese populations. In spring 2020, a decrease in the intensity of phytoplankton bloom was observed. While one study attributed this decline to a decrease in nutrient associated with the COVID-19 pandemic, our previous research proposed weakened thermal stratification accompanied by a surface cooling anomaly as the cause. However, the relationship between the marine environment and ecosystem has not been fully elucidated. Using observations and marine physical-biogeochemical model data, we identified the weakened stratification as a critical factor for suppressing the 2020 spring bloom. Intense vertical mixing hindered the accumulation of nutrient and chlorophyll-a concentrations within the euphotic zone, resulting in a diminished phytoplankton bloom. In contrast, reduced nitrate and phosphate concentrations in 2020 were insignificant compared to those in 2017–2019, despite the notable decline in PM2.5 in March 2020 due to COVID-19. In April 2020, nutrient levels fell within the range of interannual variability based on long-term observations, reflecting a negligible effect on the spring phytoplankton bloom. Our findings provide insight into the importance of marine physical factors on the phytoplankton biomass in the Yellow Sea.

Spatiotemporal Variability of Tropical Cyclone–Induced Ocean Heat Uptake and Its Effect on Ocean Heat Content

Abstract Tropical cyclones (TCs) can pump heat downward into the ocean through inducing intense vertical mixing. Many efforts have been made to estimate the TC-induced ocean heat uptake (OHU), but spatiotemporal variability of TC-induced OHU remains unclear. This study estimates the TC-induced OHU, which takes into account the heat loss at the air–sea interface during TC passage compared to previous studies and investigates the spatiotemporal variability of TC-induced OHU and its potential impacts on ocean heat content (OHC) during the period 1985–2018. It is found that the spatial distribution of OHU is inhomogeneous, with the largest OHU occurring in the northwest Pacific, and category 3–5 TCs contribute approximately 51% of the total global OHU per year. The annually accumulated TC-induced OHUs in the regional basins exhibit pronounced interannual variability, which is closely related to the TC power dissipation index (PDI). By decomposing PDI into TC intensity, frequency, and duration, we find that the TC characteristics influencing OHU variability vary by basin. Correlation analyses suggest that the interannual variations of OHUs are linked to El Niño–Southern Oscillation (ENSO). In addition, the OHU might have the potential to influence OHC variability, especially in the equatorial eastern Pacific where there are significant positive correlations between the OHU and OHC with lags of 2–6 months. This has an important implication that TC-induced OHU might have potential effects on ENSO evolution.

Understanding tidal mixing at the Strait of Gibraltar: A high-resolution model approach

The Strait of Gibraltar is a narrow and shallow channel that controls the Mediterranean Sea thermohaline and biogeochemical balances. Strong tidal currents significantly modulate exchanges across this strait and induce an intense vertical mixing, impacting both the Mediterranean Sea and the Atlantic Ocean on a climatic scale. However, the turbulent processes controlling the tidal mixing location, timing, and magnitude remain unclear. To fill this gap, we investigate tidal mixing at the Strait of Gibraltar in yearly twin tidal and non-tidal simulations from a regional configuration of the three-dimensional numerical model MITgcm, using a high spatial resolution around the Strait of Gibraltar (1/200°, 100 vertical levels). More specifically, we investigate the model turbulence closure scheme, based on a turbulent kinetic energy budget, and illustrate that vertical buoyancy fluxes should be preferred to diapycnal diffusivities as mixing indicators. In agreement with previous literature, we find that tides strongly intensify vertical mixing and motions within the Strait of Gibraltar. We then demonstrate that tidal mixing relies on two main ingredients: a sustained vertical shear of horizontal velocities and a local weakening of stratification. In the Mediterranean layer, the former drives diapycnal mixing near the seafloor and the latter in shallower areas above the prominent sills of the strait. We also evidence the frequent but irregular occurrences of static instabilities in the vicinity of these sills. In the Atlantic layer, both vertical shear and stratification are involved in diapycnal mixing that develops along the trail of the eastward internal bore released at the Camarinal sill. At high frequency, the local weakening of stratification results from convergence and divergence patterns in the vicinity of the Camarinal and Espartel sills, feeding recirculation cells between the Atlantic and Mediterranean layers. In addition, we highlight that diapycnal mixing mainly develops during the westward tidal phase in the Mediterranean layer and the eastward tidal phase in the Atlantic layer. We conclude by proposing a revised conceptual view of tidal mixing at the Strait of Gibraltar, where tidally-induced recirculation cells play an instrumental role in transforming the exchanged water masses. Overall, this study emphasizes the relevance of a realistic representation of both tides and abrupt topography to simulate the exchanges through the Strait of Gibraltar and argues for the use of a specific tidal mixing parameterization otherwise.

7Be, 210Pb, airborne particulate matter and PM10 concentrations in relation to meteorological conditions in southern Poland in 1998–2016

An analysis of the concentration of 7Be in aerosol samples collected in one of the most polluted areas in Europe (Katowice and Krakow in southern Poland) indicated seasonal variability, with a maximum in the summer months. The average concentrations of 7Be were 4616.1 μBq m−3 in Katowice and 3259.4 μBq m−3 in Krakow, respectively, and they are among the highest values recorded in Poland in the studied period (1998–2016). These cities are also characterised by Poland's highest concentrations of 210Pb (547.8 μBq m−3 and 513.2 μBq m−3). The highest radioactive concentrations of 210Pb were observed in the winter and autumn, since in the case of these industrial areas, the combustion processes related to heating in the cold season of the year are an additional source of this isotope, next to its natural origin. The airborne particulate matter concentrations at both locations correlate with the concentrations of 210Pb. The average values of PM10 concentrations (71.1 μg m−3 in Krakow to 45.0 μg m−3 in Katowice), were 2–3 times higher than the average ones recorded in northern Poland. It has been proven that air temperature is the key parameter affecting the transport of isotopes, especially in the warm season of the year, when its increase causes increased thermal convection, leading to intense vertical mixing and exchange in the troposphere. Analyses using the machine learning method allowed for an indication of the correlation between relative humidity and atmospheric precipitation, as well as higher wind speed and concentrations of 7Be which is inversely proportional. Geographical factors (the latitude of the station and the land elevation) have no impact on near-surface concentrations of 7Be in Poland.

Three‐Dimensional Temperature Responses to Northward‐Moving Typhoons in the Shallow Stratified Yellow Sea in Summer

AbstractCharacterized by a sharp thermocline and large cold‐water mass below, the Yellow Sea (YS) in summer is a shelf sea that is one of the most vulnerable to typhoons in the world. With observations and high‐resolution numerical simulations, we investigated the three‐dimensional temperature changes and underlying dynamics during the passage of Typhoon Bavi in August 2020, which was representative of northward‐moving typhoons in the eastern YS. Bavi's cyclonic strong winds caused intense turbulent mixing and Ekman divergence. The intense vertical mixing generated spatially coherent temperature cooling in the surface layer and partly caused temperature warming in the subsurface layer on both sides of the typhoon track. The largest surface cooling and subsurface warming both occurred on the right side of the typhoon track. Wind‐induced Ekman divergence generated two overturning circulation systems zonally across the YS, consisting of upper layer shoreward currents, coastal downwelling, lower layer seaward currents and upwelling on the typhoon track. Downwelling and seaward currents caused temperature warming below the thermocline, especially in the bottom layer. Upwelling shoaled the thermocline, resulting in subsurface cooling on the typhoon track. Additionally, Ekman divergence produced sea surface height minima on the typhoon track, causing strong southward barotropic currents to the left, which subsequently drove strong temperature cooling above the southern bottom slope of the South YS and fast southward movement of the YS Cold Water Mass. Further numerical experiments indicated that the magnitudes of the above changes increased with increasing maximum wind speeds and/or decreasing moving speeds of typhoons.

Numerical Study of Effects of Winds and Tides on Monthly-Mean Circulation and Hydrography over the Southwestern Scotian Shelf

A nested-grid modelling system is used to quantify effects of winds and tides on the three-dimensional (3D) circulation and hydrography over the southwestern Scotian Shelf (swScS) and surrounding areas in 2018. The performance of the nested-grid modelling system is assessed by comparing model results with observations and reanalysis data. Analysis of model results demonstrates that both winds and tides enhance the vertical mixing and modify the 3D circulation over the swScS. In winter (summer), the wind-induced vertical mixing warms (significantly cools) the sea surface temperature (SST) over the Scotian Shelf (ScS). In addition to intense vertical mixing associated with winter convection, the wind-induced mixing raises the sea surface salinity (SSS) by entraining the relatively salty sub-surface waters with the surface waters. The effect of wind-induced vertical mixing is evident in the upper water columns of ~40 m (~15 m) in February (August) 2018 over the swScS, reflecting the typically stronger wind forcing in winter than in summer. The wind forcing also enhances the seaward spreading of river runoff. Strong tidal mixing and advection also play an important role in affecting the hydrography and density-driven currents over the Bay of Fundy (BoF), Georges Bank (GeB), and swScS. In summer, tides significantly reduce the SST, increase the SSS, and affect large density-driven currents over the BoF, GeB, and swScS. Winds and tides also modify the large-scale ocean circulation, eddies, meanders, and frontal structures in the deep waters off the swScS through the modulation of baroclinic hydrodynamics.

Air-sea interaction processes during hurricane Sandy: Coupled WRF-FVCOM model simulations

A fully-coupled atmospheric-ocean model was developed by coupling WRF (Weather Research and Forecasting Model) with FVCOM (the unstructured-grid, Finite-Volume Community Ocean Model) through the Earth System Model Framework (ESMF). The coupled WRF-FVCOM is configured with either hydrostatic or non-hydrostatic oceanic dynamics and can run with wave-current interactions. We applied this model to simulate the 2012 Hurricane Sandy in the western Atlantic Ocean. The experiments examined the impact of air-sea interactions on Sandy’s intensity/path and oceanic responses under hydrostatic and non-hydrostatic conditions. The results showed that the increased storm wind rapidly deepened the mixed layer depth when ocean processes were included. Intense vertical mixing brought cold water in the deep ocean towards the surface, producing a cold wake within the maximum wind zone underneath the storm. This process led to a sizeable latent heat loss from the ocean within the storm, and hence rapid air temperature and vapor mixing ratio drop above the sea surface. The storm intensified as the central sea-level pressure dropped. Improving air pressure simulation with ocean processes tended to reduce the storm size and strengthen its intensity, providing a better simulation of hurricane path and landfall. Turning on the non-hydrostatic process slightly improved the hurricane central sea-level pressure simulation and intensified the winds on the right side of the hurricane center. Hydrostatic and non-hydrostatic coupled WRF-FVCOMs captured Sandy-induced rapidly-varying flow over the shelf and the wind-induced surge level at the coast. The coupled models predicted a higher water elevation around the coastal areas where Sandy made landfall than the uncoupled model. The uncoupled and coupled models both showed more significant oceanic responses on the right side of the hurricane center, with a maximum during the Sandy crossing period when the clockwise-rotating frequency of Sandy wind was close to the local inertial frequency. The area with a maximum response varied with Sandy’s translation speed, more prominent in the deep region than over the slope, and more substantial under the non-hydrostatic condition. The simulated ocean responses agreed with the theoretical work of Price (1981). The nonhydrostatic experiments suggest that to resolve a fully storm-induced convection process, the oceanic model grid configuration should meet the O(1) criterion for the ratio of local water depth to the model horizontal resolution.

Severe winter conditions improve recruitment success of blue whiting (Micromesistius poutassou), a temperate water fish species, in the NW Mediterranean sea

Fish recruitment variability results from a complex mix of biological and physical processes, and their interactions, acting in the early life stages. In this study, we aim to investigate the environmental factors driving the recruitment success of blue whiting (Micromesistius poutassou) in the NW Mediterranean through reconstruction of early life history traits derived from otoliths microstructure analysis. To this purpose, the study characterizes the oceanographic conditions in winter-spring of two contrasted years, 2017 and 2018, and relates them to the onset and duration of the spawning, growth performance and condition of blue whiting recruits. Winter 2017 was mild, with a short period of cold temperatures and limited winter vertical mixing. In winter 2018,temperatures reached lower values and extended for a longer period with intense vertical mixing and dense water cascading down the continental slope at the end of February. These different conditions were mirrored in the subsequent phytoplanktonic bloom, which in 2018 occurred later and was more intense than in 2017, extending over a longer period and occupying a wider area. The reproductive period of blue whiting was linked to the duration of low winter temperatures (∼13 °C), shorter in 2017 than in 2018 (∼40 and ∼ 60 days respectively). While in 2017 all individuals were born in a relatively short period of time, and under similar environmental conditions, in 2018 the hatching period presented two differentiated peaks, separated by a period of relatively low hatchings at the end of February. This gap corresponded to dense water cascading and intense vertical mixing events, suggesting that these phenomena limited the survival of eggs and larvae. The few individuals hatched in this period showed the lowest growth rates (maximum values ∼ 1.5 mm TL day−1), which would be related with the poor trophic environment during these events of intense mixing. Conversely, in both years, higher growth rates corresponded to the individuals born just before the phytoplankton bloom (attaining ∼ 2 mm TL day−1). Recruits of 2018 showed better condition than those born the previous year which would be associated with the higher primary production detected in spring 2018. The recruitment strength in 2018, estimated from landings, was also much higher indicating that severe winter conditions translate into improved recruitment of a temperate water fish in the NW Mediterranean Sea.

Response of Moist Convection to the Spectral Feature of the Surface Flux Field at Model Resolution Across the Gray Zone

AbstractThe Weather Research and Forecasting model is run at the effective resolution Δf across the gray zone, or the “terra incognita”, at which Δf ∼ l, where l is the scale of energy‐containing turbulent eddies. We examine the developments of afternoon moist convection over different multiscale features of the surface flux field as a function of the model grid spacing Δ that varies from 100 m to 1 km. Numerical experiments demonstrate that the model performs better under heterogeneous than homogeneous surface conditions in the gray zone in terms of the spatial distribution of moist convection. This improved performance is due to the explicit resolvability of the mesoscale temperature and humidity perturbations, which are critical for the growth of deep moist convection over the heterogeneous surface condition. However, under both heterogeneous and homogeneous surface conditions, the timing and intensity of moist convection vary with the model resolution. Compared with that in the large eddy simulation at Δ = 100 m, the earlier onset of moist convection at coarser resolutions appears to be related to the overestimated total buoyancy fluxes and total kinetic energy near the surface. That is, the more intense vertical mixing near the surface might induce a warmer surface layer, which yields individual cell updrafts strong enough to penetrate the capping inversion. In addition, misrepresentation of the diluting effects of entrainment on the penetrating updrafts in coarser‐resolution simulations could be responsible for the rapid development of deep convection instead of a gradual transition from shallow to deep moist convection.

Influence of non-stationary river discharge on the quality of water withdrawal in the presence of density stratification

The paper presents the results of field measurements and numerical modeling of different aspects of water quality in the Kama River (Kama reservoir), which is the source of technical water supply of industrial enterprises of the city of Berezniki. During the summer season, there are significant risks related to sustainability of water supply. It is traditionally believed that, over the winter period of low water, these risks increase manyfold. However, the results of departmental monitoring and field studies performed in winter provide evidence of the performance stability of technical water intakes from the Kama river. It was found that, in summer, when this part of the Kama river is located in the backwater zone of the Kama hydroelectric power station and, respectively, the flow velocity decreases, vertical stratification of water masses is formed, which significantly reduces the stability of water use from the near-bottom area. In winter, with a decrease in the water level in the Kama reservoir, the typical river conditions characterized by increased flow velocities occur. Under these conditions, the intense vertical mixing of water masses is implemented, which improves the consumer properties of water for sustainable water supply. The numerical simulations carried out within the framework of a three-dimensional model confirmed this conclusion. It is shown that if the amplitude of the flow velocity at a point changes with time according to a harmonic law, then the value of the salt concentration at this point also changes harmonically, but in antiphase with the velocity.

Model and Experimental Estimates of Vertical Mixing Intensity in the Sea Upper Homogeneous Layer

Purpose. The study is aimed at qualitative and quantitative analysis (based on the updated previously proposed multiscale model) of the experimental data on turbulence intensity and their comparison with theoretical and semi-empirical relationships for the purpose of describing the contributions of various turbulence sources. Methods and Results. A comparative analysis of experimental data and model calculations of turbulence characteristics near the sea surface was performed. The methods of theoretical assessing generation of turbulence in the near-surface sea layer by various physical processes are considered. The results of calculations by the well-known models of turbulent exchange were compared with the experimental data collected by the scientists of the Turbulence Department of MHI, RAS, using the specialized equipment. The analysis results made it possible to determine the possibility of applying the considered models for calculating turbulence intensity under different hydrometeorological conditions. At light winds, none of the models yielded the results which matched the measurement data. At moderate winds, the simulation results showed quite satisfactory agreement with the experiment data; and for strong winds, the multiscale model results were the best. This model was modified to assess the contributions of two other mechanisms of turbulence generation: the Stokes drift and the Langmuir circulations. Conclusions. Objective assessment of the turbulent exchange intensity requires taking into account of three main mechanisms of turbulence generation, namely flow velocity shear, wave motions and wave breaking. Depending on the hydrometeorological situation, each of these mechanisms can dominate in a certain depth range. The calculations performed using the updated model showed that the Stokes drift added 2–17 % to the total dissipation in the upper 30-meter layer, whereas the contribution of the Langmuir circulations calculated through dependence of the vertical velocity of kinetic energy transfer upon the Langmuir number, can reach 15 % for small Langmuir numbers.

Seasonal variations in salinity of the North Pacific Intermediate Water and vertical mixing intensity over the Okinawa Trough

Seasonal variations in vertical mixing intensity in the intermediate layer of the northern Okinawa Trough was deduced using the seasonal variations in salinity in the North Pacific Intermediate Water (NPIW) using two distinct datasets: the World Ocean Atlas 2013 and historical conductivity, temperature, and depth data in the Okinawa Trough. Water mass analysis of the NPIW with a simple advection model showed that the vertical mixing intensity in the intermediate layer is enhanced in the winter at a rate of 20–50% of the annual mean amplitude value. To determine the driving force behind the seasonal variations in vertical mixing, we examined moored current-meter data in the northern Okinawa Trough during 2004–2011 and showed that the near-inertial internal wave energy in the intermediate layer was enhanced around January beneath the Kuroshio on the continental slope; this was due to strong southwestward flows associated with deep cyclonic eddies developing in the winter. We used density and velocity data from a realistic high-resolution ocean general circulation model simulation to hypothesize that the large-amplitude Kuroshio meander and associated deep cyclonic eddies in the winter generate near-inertial internal waves through centrifugal/inertial instability in the area of negative potential vorticity that appears not only beneath the Kuroshio near the western slope but also near the eastern slope of the trough. In addition, the symmetric instability along the offshore Kuroshio front and the wind energy input in the winter are suggested as possible sources of winter-enhanced near-inertial internal wave energy in the intermediate layer.

Impacts of Oceanic Mixed Layer on Hurricanes: A Simulation Experiment With Hurricane Sandy

AbstractInfluences of the ocean mixed layer (OML) dynamics on intensity, pathway, and landfall of October 2012 Hurricane Sandy were examined through an experiment using the Weather Research and Forecasting (WRF) model. The WRF model was run for two cases with or without coupling to the OML. The OML in the WRF was calculated by an oceanic mixed layer submodel. The initial conditions of the depth and mean water temperature of the OML were specified using Global‐FVCOM and Global‐HYCOM fields. The comparison results between these two cases clearly show that including the OML dynamics enhanced the contribution of vertical mixing to the air‐sea heat flux. When the hurricane moved toward the coast, the local OML rapidly deepened with an increase of storm wind. Intense vertical mixing brought cold water in the deep ocean toward the surface to produce a cold wake underneath the storm, with the lowest sea temperature at the maximum wind zone. This process led to a significant latent heat loss from the ocean within the storm and hence rapid drops of the air temperature and vapor mixing ratio above the sea surface. As a result, the storm was intensified as the central sea level pressure dropped. Improving air pressure simulation with OML tended to reduce the storm size and strengthened the storm intensity and hence provided a better simulation of hurricane pathway and landfall.

Vertical distribution of brittle star larvae in two contrasting coastal embayments: implications for larval transport

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. In species characterized by large variations in population density, like many echinoderm species, larval dispersal may contribute to outbreak and die-off phenomena. A proliferation of the ophiuroid Ophiocomina nigra was observed for several years in western Brittany (France), inducing drastic changes on the benthic communities. We here studied the larval vertical distribution in this species and two co-occurring ophiuroid species, Ophiothrix fragilis and Amphiura filiformis, in two contrasting hydrodynamic environments: stratified in the bay of Douarnenez and well-mixed in the bay of Brest. Larvae were collected at 3 depths during 25 h within each bay. In the bay of Brest, all larvae were evenly distributed in the water column due to the intense vertical mixing. Conversely, in the bay of Douarnenez, a diel vertical migration was observed for O. nigra, with a night ascent of young larvae, and ontogenetic differences. These different patterns in the two bays mediate the effects of tidal currents on larval fluxes. O. fragilis larvae were mainly distributed above the thermocline which may favour larval retention within the bay, while A. filiformis larvae, mostly concentrated near the bottom, were preferentially exported. This study highlighted the complex interactions between coastal hydrodynamics and specific larval traits, e.g. larval morphology, in the control of larval vertical distribution and larval dispersal.

The KPP Trigger of Rapid AMOC Intensification in the Nonlinear Dansgaard‐Oeschger Relaxation Oscillation

AbstractMillennium time scale Dansgaard‐Oeschger oscillations of glacial climate, clear evidence for the occurrence of which was first provided on the basis of oxygen isotopic data from a Greenland ice core 25 years ago, have recently been shown to arise naturally (without explicit freshwater forcing) in a fully coupled modern climate model description of the interactions between the overturning circulation of the oceans, the atmosphere, and sea ice under maximum glacial conditions. The fast transitions from cold stadial to warm interstadial conditions in a typical D‐O oscillation are characterized by the appearance of an extensive polynya in the stadial sea ice cover of the Irminger Sea south of Denmark Strait that opens due to the onset of intense vertical mixing below the sea ice lid. Through detailed stability analysis of the water column in the region where the polynya first forms, together with analysis of the action of the KPP (Kappa Profile Parameterization) of stratified turbulence employed to represent water column diapycnal diffusivity, the authors show that the opening of the polynya is controlled by this turbulence parameterization. The relative contributions of the different components of the parameterization to polynya opening are investigated in order to better understand the rapid climate change that ensues. The authors furthermore show that the characteristic period of the model predicted Dansgaard‐Oeschger oscillation is also controlled by the detailed nature of this parameterization, a characteristic of the oscillation not previously explained.

The Southern Caribbean upwelling system off Colombia: Water masses and mixing processes

The upwelling system off the southern Caribbean coast is probably the main source of the nutrients that support biological productivity in the oligotrophic Caribbean Sea. Subtropical underwater (SUW) that forms the subsurface salinity maximum in the Caribbean Sea is the main source of upwelled waters in this system. Profiles of salinity and temperature with depth derived from four oceanographic cruises and Argo floats showed that upwelled waters have a salinity that is ~0.11 g kg−1 lower than the SUW in the central Caribbean Sea and a seasonal variation of approximately 0.09 g kg−1 that reflects the rainy/dry seasons. In addition, the SUW is ~50 m shallower on the continental shelf slope (~100 m) compared to the depth of the SUW in the central Caribbean Sea. The origin of these modified SUW was analyzed using the Mercator numerical model, which reproduces the main vertical characteristics of the subsurface salinity maximum. The modeled data showed that SUW upwelling off of the La Guajira Peninsula and Venezuela arrive into the system via an intense Caribbean Coastal Undercurrent (CaCU, mean speed ~0.28 m s−1). This current is formed in front of the Nicaragua platform from the divergence of subsurface water flow at the salinity maximum depth. The lower salinity observed in the upwelled waters may be the result of intense vertical mixing processes that could occur when the SUW are transported by the CaCU below the Panama-Colombia Gyre (PCG) region before reaching the upwelling zones. The mixing processes—involving double diffusion and mechanical turbulence driven by vertical shear of horizontal currents—were analyzed using the Turner Angle and the Thorpe scale, respectively. Below a depth of 200 m, double diffusion by salt fingers (diffusivities > 5 x 10−5 m2 s−1) was the main process of salt diffusion, generating a downward salt flux of >2 x 10−2 g kg−1 m d−1 between the SUW and the North Atlantic Central Waters (NACW). Above a depth of 100 m, mechanical turbulent diffusion generates a salt flux towards the surface ranging 0.5–4 x 10−2 g kg−1 m d−1, where double diffusion by salt finger is not possible. The diluted SUW is subsequently transported by the CaCU, connecting—at the subsurface level—the PCG region with the upwelling zones off of Colombia and Venezuela. As well as modifying the salt content of the coastal SUW, these mixing processes may also alter the nutrient content of upwelling waters, the ecosystem effects of which are still unknown.

Cyclone Enhanced Chlorophyll in the Bay of Bengal as Evidenced from Satellite and BGC-Argo Float Observations

Temporal evolution of biophysical properties of Bay of Bengal (BoB) was measured using an Argo float equipped with fluorescence sensor (BGC-float) during the winter monsoon of 2013–2014. This season was subjected to intense vertical mixing due to the passage of several cyclonic storms beginning with ‘Phailin’ in October to ‘Madi’ in December 2013. These events had resulted in anomalous chlorophyll bloom (> 0.9 mg/l) together with an increase of ~ 1 PSU in the surface salinity and a drop-in temperature (~ 1 °C) in the southwestern BoB. The event was well captured by a BGC-float (WMO ID 2902086) present in the region close to the tracks of cyclones ‘Leher’ and ‘Madi’. The presence of a strong cold-core eddy as observed from the sea level anomaly data played a vital role in spreading the bloom over a wider area and maintaining it for nearly two months from November 2013 to January 2014. This was also evident from the satellite observations of surface chlorophyll-a concentration and Ekman pumping velocity (~ 2 × 10–3 m/s). The intense mixing caused by the cyclonic storms has resulted in eroding the stratification and assisting the entrainment of nutrients into the euphotic layer, which was subsequently sustained by the mesoscale eddy. The present study reveals the strong synergistic application potential of BGC-float data together with satellite observations in monitoring the changes in the vertical structure of chlorophyll during the cyclonic storms and other mesoscale processes.