Dense Water Formation Research Articles

Levantine Intermediate and Levantine Deep Water Formation: An Argo Float Study from 2001 to 2017

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea.

Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea

Southern Adriatic (Eastern Mediterranean Sea) is a region strongly dominated by large-scale oceanographic processes and local open-ocean dense water formation. In this study, picoplankton biomass, distribution, and activity were examined during two oceanographic cruises and analyzed in relation to environmental parameters and hydrographic conditions comparing pre and post-winter phases (December 2015, April 2016). Picoplankton density with the domination of autotrophic biomasses was higher in the pre-winter phase when significant amounts of picoaoutotrophs were also found in the meso-and bathy-pelagic layers, while Synechococcus dominated the picoautotrophic group. Higher values of bacterial production and domination of High Nucleic Acid content bacteria (HNA bacteria) were found in deep waters, especially during the post-winter phase, suggesting that bacteria can have an active role in the deep-sea environment. Aerobic anoxygenic phototrophic bacteria accounted for a small proportion of total heterotrophic bacteria but contributed up to 4% of bacterial carbon content. Changes in the picoplankton community were mainly driven by nutrient availability, heterotrophic nanoflagellates abundance, and water mass movements and mixing. Our results suggest that autotrophic and heterotrophic members of the picoplankton community are an important carbon source in the food web in the deep-sea, as well as in the epipelagic layer. Besides, viral lysis may affect the activity of the picoplankton community and enrich the water column with dissolved organic carbon.

Warm Deep Water Variability During the Last Millennium in the CESM–LME: Pre-Industrial Scenario versus Late 20th Century Changes

Water transformation around Antarctica is recognized to significantly impact the climate. It is where the linkage between the upper and lower limbs of the Meridional Overturning Circulation (MOC) takes place by means of dense water formation, which may be affected by rapid climate change. Simulation results from the Community Earth System Model Last Millennium Ensemble (CESM–LME) are used to investigate the Weddell Sea Warm Deep Water (WDW) evolution during the Last Millennium (LM). The WDW is the primary heat source for the Weddell Sea (WS) and accounts for 71% of the Weddell Sea Bottom Water (WSBW), which is the regional variety of the Antarctic Bottom Water (AABW)—one of the densest water masses in the ocean bearing directly on the cold deep limb of the MOC. Earth System Models (ESMs) are known to misrepresent the deep layers of the ocean (below 2000 m), hence we aim at the upper component of the deep meridional overturning cell, i.e., the WDW. Salinity and temperature results from the CESM–LME from a transect crossing the WS are evaluated with the Optimum Multiparameter Analysis (OMP) water masses decomposition scheme. It is shown that, after a long–term cooling over the LM, a warming trend takes place at the surface waters in the WS during the 20th century, which is coherent with a global expression. The subsurface layers and. mainly. the WDW domain are subject to the same long–term cooling trend, which is decelerated after 1850 (instead of becoming warmer like the surface waters), probably due interactions with sea ice–insulated ambient waters. The evolution of this anomalous temperature pattern for the WS is clear throughout the three major LM climatic episodes: the Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and late 20th century warming. Along with the continuous decline of WDW core temperatures, heat content in the water mass also decreases by 18.86%. OMP results indicate shoaling and shrinking of the WDW during the LM, with a ~6% decrease in its cross–sectional area. Although the AABW cannot be directly assessed from CESM–LME results, changes in the WDW structure and WS dynamics have the potential to influence the deep/bottom water formation processes and the global MOC.

On the salinity structure in the South Adriatic as derived from float and glider observations in 2013–2016

Between 2013 and 2016 a drastic change in the vertical salinity structure in the South Adriatic Pit was recorded by Argo floats and ocean gliders. The analysis revealed that high salinity waters characterised 2013 at the surface and intermediate layers, creating the conditions for dense water formation. The year 2014 was instead marked by less saline water, dominant in the first 100 m, that spread in the entire Pit. Heat gain, negative E-P anomalies and abundant river runoff, during that period, maintained this saline stratification for more than a year. This water mass contributed to the dilution of the underlying layer, during the convection event of 2015. The spring transition period, later in 2015, as defined by new saline water placed over the winter formatted water mass and the unperturbed part of the existent salinity profile, in the deeper layers, resulted in a particular vertical structure, characterised by a double salinity maximum. High salinity waters continued to be present at the surface and intermediate layers until the end of 2016, when the strong convection led to a deep mixing, eliminating that structure. The analysis of the available datasets in the South Adriatic Pit, since 1984, linked the presence of these high salinity waters, with the cyclonic phase of the North Ionian Gyre, additionally revealing, also, years of a decreasing trend, during the anticyclonic mode of the Gyre.

The Mediterranean Sea Overturning Circulation

AbstractThe time-mean zonal and meridional overturning circulations of the entire Mediterranean Sea are studied in both the Eulerian and residual frameworks. The overturning is characterized by cells in the vertical and either zonal or meridional planes with clockwise circulations in the upper water column and counterclockwise circulations in the deep and abyssal regions. The zonal overturning is composed of an upper clockwise cell in the top 600 m of the water column related to the classical Wüst cell and two additional deep clockwise cells, one corresponding to the outflow of the dense Aegean water during the Eastern Mediterranean Transient (EMT) and the other associated with dense water formation in the Rhodes Gyre. The variability of the zonal overturning before, during, and after the EMT is discussed. The meridional basinwide overturning is composed of clockwise, multicentered cells connected with the four northern deep ocean formation areas, located in the Eastern and Western Mediterranean basins. The connection between the Wüst cell and the meridional overturning is visualized through the horizontal velocities vertically integrated across two layers above 600 m. The component of the horizontal velocity associated with the overturning is isolated by computing the divergent components of the vertically integrated velocities forced by the inflow/outflow at the Strait of Gibraltar.

Impact of North Atlantic Freshwater Forcing on the Pacific Meridional Overturning Circulation under Glacial and Interglacial Conditions

Abstract The Pacific meridional overturning circulation (PMOC) is not well known compared to the Atlantic meridional overturning circulation (AMOC), due to its absence today. However, considering PMOC development under different climate conditions shown by proxy and modeling studies, a better understanding of PMOC is appropriate to properly assess the past and future climate change associated with global ocean circulation. Here, the PMOC response to freshwater forcing in the North Atlantic (NA) is investigated using an Earth system model of intermediate complexity under glacial (i.e., Last Glacial Maximum) and interglacial [i.e., preindustrial with/without inflow through Bering Strait (BS)] conditions. The water hosing over NA led to the shutdown of the AMOC, which accompanied an active PMOC except for the preindustrial condition with the opening BS, indicating that the emergence of the PMOC is constrained by the freshwater inflow through the BS, which hinders its destabilization through enhancing ocean stratification. However, the closure of the BS itself could not explain how the sinking motion is maintained in the North Pacific. Here we found that various atmospheric and oceanic processes are involved to sustain the active PMOC. First, an atmospheric teleconnection associated with the collapsed AMOC encouraged the evaporation in the sinking region, causing buoyancy loss at the surface of the North Pacific. Second, the strengthened subpolar gyre transported saltier water northward, enhancing dense water formation. Finally, the vigorous upwelling in the Southern Ocean enabled a consistent mass supply to the sinking region, with the aid of enhanced westerlies.

Effect of an extreme cold event on the metabolism of planktonic microbes in the northernmost basin of the Mediterranean Sea

In the northern Adriatic Sea (the northernmost basin of the Mediterranean Sea) wind forcing plays a key role in shaping the water column structure, circulation patterns and dense water formation. The latter process might be enhanced during winter by local north-easterly wind, the Bora, responsible for intense and long-lasting episodic outbreaks. Although outbreaks of Bora are extensively studied in the context of thermohaline circulation, little is known about their effects on planktonic microbial communities. From late February to early March 2018, a persistent and strong Bora event caused an unusual drop of seawater temperature (down to 5.7 °C) in the Gulf of Trieste. To investigate the effect of this extreme meteorological event on the coastal planktonic microbes, we evaluated changes in their abundance and cytometric fingerprinting, heterotrophic carbon production rates and the activity of six exoenzymes (β-glucosidase, β-galactosidase, chitinase, lipase, alkaline-phosphatase and leucine aminopeptidase). Additionally, we set up temperature manipulation experiments to investigate the potential limiting effect of the temperature drop on microbial degradation and uptake of organic matter. The dataset clustered in two distinct groups: a cold-affected period (26th February - 8th March), characterised by a reduced particulate organic matter availability and slow metabolic rates, and a recovery one, (12th - 16th March) defined by an overall increase of substrate availability and enhanced heterotrophic carbon production and hydrolysis rates. Our findings revealed that the exoenzymatic activities were constrained at their lower thermal limit during the cold-affected period and that, deviating from the Arrhenius’ linear response to temperature, the prokaryotic growth was limited by the interactive effect of temperature and substrate availability. By limiting the microbial processing of organic matter, extreme meteorological events could alter the functioning of the entire planktonic compartment functioning for several days during and after the onset of the phenomenon, affecting the structure of local trophic webs.

Structure and Variability of the North Icelandic Jet From Two Years of Mooring Data

AbstractMooring data from September 2011 to July 2013 on the Iceland slope north of Denmark Strait are analyzed to better understand the structure and variability of the North Icelandic Jet (NIJ). Three basic configurations of the flow were identified: (1) a strong separated East Greenland Current (EGC) on the mid‐Iceland slope coincident with a weak NIJ on the upper slope, (2) a merged separated EGC and NIJ, and (3) a strong NIJ located at its climatological mean position, coincident with a weak signature of the separated EGC at the base of the Iceland slope. Our study reveals that the NIJ‐dominant scenario was present during different times of the year for the two successive mooring deployments—appearing mainly from September to February the first year and from January to July the second year. Furthermore, when this scenario was active it varied on short timescales. An energetics analysis demonstrates that the high‐frequency variability is driven by mean‐to‐eddy baroclinic conversion at the shoreward edge of the NIJ, consistent with previous modeling work. The seasonal timing of the NIJ dominant scenario is investigated in relation to the atmospheric forcing upstream of Denmark Strait. The resulting lagged correlations imply that strong turbulent heat fluxes in a localized region on the continental slope of Iceland, south of the Spar Fracture zone, lead to a stronger NIJ dominant state with a two‐month lag. This can be explained dynamically in terms of previous modeling work addressing the circulation response to dense water formation near an island.

Reduced Sea Ice Production Due to Upwelled Oceanic Heat Flux in Prydz Bay, East Antarctica

AbstractThe coastal shelf region of East Antarctica is hypothesized to be shielded from the offshore heat of Circumpolar Deep Water (CDW) due to the dynamic barrier of the Antarctic Slope Front. Yet modified CDW (mCDW) intrudes into the coastal environment in key locations, with impacts on dense shelf water formation and ocean/ice shelf interaction that remain largely unquantified. Using moored measurements and conductivity‐temperature‐depth‐instrumented seal hydrographic data collected in Prydz Bay, East Antarctica, we find buoyancy‐driven upwelling of mCDW into the subsurface (~50 m) layer of the southeastern embayment. Wintertime convection extends as deep as 300 m, entraining heat of the upwelled mCDW to the surface. Accumulated sensible heat supply to the surface through deep convection during June–July reduces the potential sea ice production by 45% in the Davis Polynya, demonstrating that stronger/warmer mCDW intrusions onto the shelf will likely reduce the shelf water density and threaten Antarctic Bottom Water formation.

Compensating Biases and a Noteworthy Success in the CMIP5 Representation of Antarctic Sea Ice Processes

AbstractCoupled Model Intercomparison Project phase 5 (CMIP5) climate models simulate a wide range of historical sea ice areas. Even models with areas close to observed values may contain compensating errors, affecting reliability of their projections. This study focuses on the seasonal cycle of sea ice, including analysis of model concentration budgets. Many models have insufficient autumn ice growth, leading to large winter biases. A subset of models accurately represent sea ice evolution year‐round. However, comparing their winter ice concentration budget to observations reveals a range of behaviors. At least one model has an accurate ice budget, which is only possible due to realistic ice drifts. The CMIP5 generation of model physics and resolution is therefore structurally capable of accurately representing processes in Antarctic sea ice. This implies that substantially improved projections of Antarctic dense ocean water formation and ice sheet melting are possible with appropriate subsetting of existing climate models.

Deep-water formation variability in the north-western Mediterranean Sea during the last 2500 yr: A proxy validation with present-day data

Abstract Here we investigate the sensitivity of deep-water formation in the north-western Mediterranean Sea to climate variability during the last 2500 yr. With this purpose, the grain-size parameter UP10 (fraction > 10 μm) is used as a proxy for intensity of deep-water circulation. Such a proxy is first validated through the analysis of oceanographic data collected from October 2012 to October 2014 by means of two deep-water mooring lines equipped with sediment traps and currentmeters in the Gulf of Lion and north of Minorca Island. Enhancements of deep current speed resulted from dense shelf water cascading and open-sea deep convection in February 2013 leading to dense-water formation. The grain-size distribution of settling particles from sediment traps collected during 2012–2013 shows a distinctive particle mode and high UP10 values correlated to deep-water formation. These data are consistent with grain-size values in sediment cores from the north of Minorca, thus supporting the validity of the UP10 parameter to reconstruct changes of intensity in deep-water formation and associated near-bottom currents. The deep-water sediment record of the north of Minorca for the last 2.5 kyr shows that the strongest deep-water formation events occurred during relatively warm intervals, such as the Roman Period (123 BCE-470 CE 2 ), the end of the Medieval Climate Anomaly (900–1275 CE) and the first part of the Little Ice Age (1275–1850 CE). By contrast, our data indicate a progressive reduction in the overturning conditions during the Early Middle Ages (470–900 CE) resulting in weaker deep-water formation events during most of the Medieval Climate Anomaly. Intense deep-water formation events appear to be mostly associated with periods of enhanced Evaporation-Precipitation balance rather than to buoyancy loss due to winter cooling only. Our results suggest that warm sea surface temperature during spring months could have played an important role by increasing the Evaporation–Precipitation balance and favouring buoyany loss by increased of salinity. The comparison our data with other oceanographic and climatic records indicates a change in the proxy relation before and after the Early Middle Ages. Western Mediterranean Deep Water and Levantine Intermediate Water behave in opposite way after the Early Middle Ages, thus indicating that the previously proposed Mediterranean see-saw pattern in the Evaporation-Precipitation balance also influenced convection patterns in the basins during the last 1500 yr. These changes are discussed in the frame of different configurations of the North Atlantic Oscillation and East Atlantic/ West Russian modes of atmospheric variation.

Trace and minor element records in aragonitic bivalve shells as environmental proxies

Investigation of the geochemical composition of bivalve shells can provide information on changes in the marine environment occurring during the lifespan of an organism. Three species, locally abundant in the Adriatic Sea, were chosen in this study, namely Glycymeris pilosa, Callista chione, and Venus verrucosa. Of these, G. pilosa has the longest lifespan, exceeding 50 years, and therefore presents a potential archive of decadal climate variability. The other two species, C. chione and V. verrucosa, are commercially important. Samples were collected alive by SCUBA diving in the North Adriatic Sea, near Barbariga, Istria. Major growth increments in these shells form on an annual basis and are clearly visible in shell cross-sections enabling correct temporal positioning of geochemical data obtained from different shell parts. In this study, we applied laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) in line scan mode to determine the trace and minor element composition along the major growth axis in the hinge area of the shells. Objectives of this study were to compare signals within and between species, and to relate results to available environmental parameters. High resolution time-series of Na/Cashell, Mg/Cashell and Sr/Cashell display characteristic cyclic variations. Na/Cashell minima and Mg/Cashell and Sr/Cashell maxima occur near shell annual growth lines in all three studied species. Results indicate that seawater temperature is not a main driver for incorporation of these elements. Ba/Cashell data vary between years and species and have noncyclic sharp peaks. Callista chione and V. verrucosa specimens show higher peaks and background Ba/Cashell values than G. pilosa. Most pronounced peaks in Ba/Cashell occurred at the beginning of the growing season corresponding to the end of year 2011 and beginning of year 2012 and coincided with unusual dense water formation and subsequent changes in the plankton communities in the Adriatic Sea. Implications of our study suggest that synchronous peaks in studied specimens are caused by external forcing which could occur on larger, regional scales, encouraging future studies covering wider area in the Adriatic Sea and other parts of the Mediterranean.

Coastal oceanographic signatures of heat waves and extreme events of dense water formation during the period 2002-2012 (Barcelona, NW Mediterranean)

In a global context of climate change affecting the marine environment, it is important to consider the effect of extreme events in driving ecological change and to gain a better understanding of conditions to be expected under future scenarios. In this study we focus on monthly oceanographic data collected off Barcelona city during the period 2002-2012, in which extreme air temperatures and exceptional oceanographic events were reported in the western Mediterranean basin. These included two extreme heat waves and major episodes of dense water formation that produced unusually large deep-water contributions, induced oceanographic changes in the coastal zone and caused significant alterations to the marine ecosystem. To determine whether routine monitoring of oceanographic variables in a coastal zone can provide information for recognizing such large-scale events, temperature, salinity, turbidity and fluorescence were analysed to identify their signatures. The results provide an additional tool for monitoring oceanographic events and improving forecasts and future projections.

Razmjena guste vode između otvorenog Sjevernog Jadrana i hrvatskog obalnog mora

t has been known for a while that there are two sites of wintertime dense water formation in the North Adriatic - one in the open sea and the other in the Croatian coastal sea. Recently, it has been established that dense water is transported between the two basins, with both directions of the transport being possible. Here, a simple two-box model is developed in order to interpret the finding. The model allows for surface heat loss from the two basins and for an advective exchange of heat between the basins. Explicit solution is obtained, not only for the original, nonlinear problem but also for a simplified, linearized problem, when the initial temperature difference between the two basins vanishes. Moreover, the effect of the initial temperature difference is explored with the linearized model. The solutions point to a continuous temperature decrease in the two basins, with the temperature differences tending to limiting values. The temporal variability is controlled by the initial temperature differences, surface heat fluxes and basin dimensions and it suggests that the sum of surface heat loss and advective heat gain in one basin tends to become equal to the sum of surface and advective heat losses in the other basin. The solutions also indicate that the sign of the temperature difference between the two basins could be positive or negative, implying that the cold, dense water could be transported either way. Finally, an index, incorporating the initial temperatures, the surface heat fluxes and the basin depths, is proposed with the aim of quantifying relative importance of the two North Adriatic sites of dense water formation for each particular winter.

Performance of multi-decadal ocean simulations in the Adriatic Sea

A performance study of seven regional ocean configurations based on NEMO has been carried out for the Adriatic Sea over a common period (1980–2012). Assessed models differ in resolution, model physics, atmospheric forcing (forced vs. coupled models) and river discharges imposed within the Adriatic Sea. Models have been evaluated on the long-term temperature and salinity measurements in all of the Adriatic sub-basins, in particular within dense water collectors (Jabuka Pit and South Adriatic Pit) and dense water formation sites (northern and southern Adriatic). Adriatic-wide salinity content is mostly linked to the proper introduction of the overall water budget, rather than to the local river forcing. Forced models mostly overestimate temperature and salinity values. On average, coupled models better reproduce the thermohaline properties and processes, in particular the Adriatic-Ionian Bimodal Oscillating System (BiOS) reversals and its decadal variability. Wintertime heat losses are playing major role in defining the Adriatic Deep Water (ADW) transport rates in coupled models, while in forced models preconditioning in salinity is the most important factor. Further on, increase of resolution of the atmospheric forcing results in more realistic ocean behaviour, including dense water formation (DWF) in the complex coastal northern Adriatic. However, all models have large temperature biases and lower deep stratification at the dense water collector sites, indicating overall underrepresentation of the Adriatic DWF. Consequently, mixed-layer depth in the southern Adriatic is overestimated, reaching the bottom during some years. Ocean model resolution and river forcing seem to play a second-order role in defining the overall Adriatic-Ionian thermohaline properties, while inclusion of aerosol trend only slightly modified the BiOS reproduction.

The Density‐Driven Winter Intensification of the Ross Sea Circulation

AbstractThe circulation over the Ross Sea continental shelf facilitates the exchange between the Southern Ocean and the Ross Ice Shelf cavity. Here transport and mixing processes control the access of oceanic heat from the Southern Ocean to the ice shelf base, the formation of sea ice, and the production of High Salinity Shelf Water (HSSW) in polynyas and hence the subsequent formation of Antarctic Bottom Water. A climatological ocean‐ice shelf coupled model of the Ross Sea Sector including the cavity, with prescribed sea ice fluxes, was used to examine the details of currents and their seasonal variability over the continental shelf. A system of two cyclonic and three anticyclonic persistent circulation features has been identified. Transports steadily increase throughout winter, with individual currents carrying up to 2 Sv, nearly doubling their minimum. The seasonal modulation is driven by lateral differences in density and subsequent baroclinic pressure gradients, induced through dense shelf water formation in the Ross Sea and the Terra Nova Bay Polynas. Wind plays a minor role in ocean momentum variability. Sensitivity experiments suggest a weakening of transports with increasing wind stress. Horizontal density variations at the ocean surface are smoothed by the wind. The source of momentum in the cavity is the gravity‐driven bottom flow of HSSW, produced in the Ross Sea Polynya as part of the thermohaline overturning circulation. Tracer experiments suggest that HSSW forming in the Terra Nova Bay Polynya has no cavity access, but instead is the main contributor to Antarctic Bottom Water formed in the northwest slope.

Effects of Wind Stress and Surface Cooling on Cross-Shore Exchange

AbstractThe formation of coastal dense shelf water in winter provides the available potential energy (APE) to fuel baroclinic instability. The combined effects of baroclinic instability and wind forcing in driving cross-shelf exchange are investigated using idealized numerical simulations with varied bottom slope, wind stress, and heat loss rate. The results show that under upwelling-favorable winds, the intensity of the instability decreases as the wind stress increases. This is caused primarily by enhanced turbulence frictional dissipation. Under downwelling-favorable winds, an increase in wind stress and/or a decrease in heat loss rate tends to constrain the baroclinic instability, leading to a circulation resembling that driven purely by wind forcing. In the latter case, once a critical value of cross-shore density gradient is reached, isopycnal slumping is initiated, leading to increased vertical stratification and narrowing of the inner shelf. The change in depth of the inner-shelf outer boundary, defined as the location corresponding to the maximum cross-shore gradient of the surface Ekman transport, is proportional to an empirically derived multiparametric quantity , where a2 is a dimensional constant, B0 is a constant heat loss rate, γ = 0.43, f is the Coriolis parameter, α is the shelf slope, B is the heat loss rate, and τ is the wind stress. This relationship is found to hold for cases when instabilities are present.

Meteorology and Climatology of the Mediterranean and Black Seas: Introduction

The topical issue includes 22 papers on different aspects of meteorology and climatology of the Mediterranean and Black Seas. The papers are grouped into three categories, based on scale analysis of processes, and whether the study is dominantly related to the atmospheric or oceanic phenomenon. The category ‘‘Storms, Extremes and Mesoscale Processes’’ encompasses seven papers, ranging from observation and modelling data analysis of extreme atmospheric (severe winds, hailstorms, waterspout, cyclones) and oceanic (meteotsunamis, surface waves) events, occurring locally, but being connected to wider patterns and processes. The six papers in category ‘‘Atmospheric Climate, Variability and Climate Change’’ contain analyses of long-term observations and climate projection outputs, but also assess atmosphere–ocean–land interactions and cycles, that are important in a climate perspective. The category ‘‘Ocean Climate and Variability’’ includes nine papers, which map ocean processes (sea level variability, circulation, sea surface temperatures, vertical thermal structure, water masses and dense water formation) on a climate timescale, both, in the Mediterranean and Black Sea sub-basins, or taking them as a whole.

Observing Winter Mixing and Spring Bloom in the Mediterranean

A new special issue of JGR: Oceans and JGR: Atmospheres presents new insights into the dynamics of dense water formation in the western Mediterranean Sea and its biogeochemical consequences.

Preface to the Special Section: Dense Water Formations in the Northwestern Mediterranean: From the Physical Forcings to the Biogeochemical Consequences

AbstractThe northwestern Mediterranean Sea is one of the few sites of open‐sea deep convection and dense water formation. The area is characterized by intense air‐sea exchanges favored by the succession of strong northerly and northwesterly winds during autumn and winter that eventually induce deep convection episodes and the formation of the Western Mediterranean Deep Water. This region exhibits a significant spring phytoplankton bloom, which appears to be largely influenced in intensity and diversity by the winter mixing. To understand and resolve the interplay between the atmosphere and ocean, and the impact of ocean circulation and mixing on biogeochemistry, we carried out an unprecedented observational effort during a large experiment between July 2012 and July 2013. A multiplatform approach—combining aircraft, balloons, ships, moorings, floats, and gliders ‐ aimed both at characterizing the dynamics of the atmosphere and at quantifying the physical, biogeochemical, and biological properties of the water masses. Beyond a better understanding of the wind dynamics, the interannual variability of the deep convection, and the seasonal variability of the nutrient distribution and plankton structure in the pelagic ecosystem, the experiment provided a reference data set that was used as a benchmark for advancing the modeling of the surface fluxes, convective processes, dense water formation rates, and physical‐biogeochemical coupling processes. It also represented an opportunity for complementary investigations such as evaluating model parameterizations or studying the role of submesoscale eddies in dense water spreading and biogeochemistry.