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Theoretical studies on oceans and large lakes have shown that submesoscale instabilities in frontal zones tend to reduce horizontal density gradients and enhance vertical density gradients, thereby re-stratifying the Surface Mixed Layer (SML). Submesoscale filament dynamics are primarily studied using numerical models and remote sensing imagery. However, in large lakes, this concept remains without substantial field validation, mainly due to the difficulty in conducting the necessary high-resolution water column measurements. Using a procedure we recently developed to predict the time and location of mesoscale and submesoscale features generated by strong wind fields, this work presents direct field evidence demonstrating the role of submesoscale cold filaments in re-stratifying the SML under weakly stratified conditions in a large lake (Lake Geneva). The dynamics of the observed filaments were further investigated with a high-resolution three-dimensional (3D) numerical model and Lagrangian particle-tracking. The numerical model accurately captured the formation of these filaments. The enhancement of thermal stratification strength, N², reached O(10-5) s-2 in areas adjacent to cold filaments under atmospheric cooling and heating conditions. In the pelagic zone (offshore), strong vertical velocities of O(100 m d-1) were associated with secondary circulation that rapidly transports and accumulates passive particles in the thermocline and hypolimnion layers, as confirmed by Acoustic Doppler Current Profiler (ADCP) backscattering intensity data. The field observations indicate that under weak stratification, Dissolved Oxygen (DO) variability reaches 0.5 mg l-1 near cold filaments. This documentation of strong vertical motions associated with submesoscale filaments is expected to contribute to the understanding of the vertical exchange of heat, contaminants and oxygen between the atmosphere and the pelagic zone of large lakes, as well as in oceans where carrying out such field measurements is very challenging. [ABSTRACT FROM AUTHOR]

Theoretical studies on oceans and large lakes have shown that submesoscale instabilities in frontal zones tend to reduce horizontal density gradients and enhance vertical density gradients, thereby re-stratifying the Surface Mixed Layer (SML). Submesoscale filament dynamics are primarily studied using numerical models and remote sensing imagery. However, in large lakes, this concept remains without substantial field validation, mainly due to the difficulty in conducting the necessary high-resolution water column measurements. Using a procedure we recently developed to predict the time and location of mesoscale and submesoscale features generated by strong wind fields, this work presents direct field evidence demonstrating the role of submesoscale cold filaments in re-stratifying the SML under weakly stratified conditions in a large lake (Lake Geneva). The dynamics of the observed filaments were further investigated with a high-resolution three-dimensional (3D) numerical model and Lagrangian particle-tracking. The numerical model accurately captured the formation of these filaments. The enhancement of thermal stratification strength, N2, reached O(10-5) s-2 in areas adjacent to cold filaments under atmospheric cooling and heating conditions. In the pelagic zone (offshore), strong vertical velocities of O(100 m d-1) were associated with secondary circulation that rapidly transports and accumulates passive particles in the thermocline and hypolimnion layers, as confirmed by Acoustic Doppler Current Profiler (ADCP) backscattering intensity data. The field observations indicate that under weak stratification, Dissolved Oxygen (DO) variability reaches 0.5 mg l-1 near cold filaments. This documentation of strong vertical motions associated with submesoscale filaments is expected to contribute to the understanding of the vertical exchange of heat, contaminants and oxygen between the atmosphere and the pelagic zone of large lakes, as well as in oceans where carrying out such field measurements is very challenging.

Deforestation, urbanization and construction of wind farms can change the land surface roughness, which can further influence surface heat fluxes and thus weather and climate. Land surface roughness anomalies can dynamically trigger convergence through changing mean wind speed. Here, we report a new mechanism, in which roughness anomalies cause thermally direct mesoscale circulations and anomalous precipitation. To study this mechanism, we conduct cloud‐permitting simulations over an idealized land surface with prescribed surface roughness anomalies. Anomalously high roughness increases turbulent mixing near the surface, which decreases land surface temperature and outgoing longwave radiation. The additional surface net radiation partly goes into greater sensible heat flux, which triggers mesoscale circulations driven by differential heating. As a result, precipitation over the high‐roughness anomaly is generally larger than that over the low‐roughness background. This new mechanism, not present in climate models, may be relevant to storm formation over wind farms, cities and forests. Plain Language Summary: The Earth's surface roughness determines the turbulent exchange efficiency of momentum, heat, water vapor, and carbon dioxide between the atmosphere and surface, and thus serves as a key boundary condition for predicting weather and climate. Surface roughness can be modified by human activities, such as deforestation, urbanization, and construction of wind farms. It is recognized that land surface roughness anomalies can dynamically trigger convergence and precipitation anomalies through changing mean wind speed. Compared to its dynamical effects, the thermal effects of roughness anomalies have largely been overlooked. In this study, we report that higher roughness can lead to thermally direct mesoscale circulations and more precipitation. This new mechanism, neglected in previous coarse‐resolution climate models, could be significant for predicting storms over wind farms, cities, and forests. Key Points: Land surface roughness anomalies caused by human activities can induce changes in climateAnomalously high roughness leads to greater sensible heat flux, triggering mesoscale circulations and precipitationThis new mechanism may be relevant to storm formation over wind farms, cities and forests [ABSTRACT FROM AUTHOR]

The winter climate of the northeastern Dominican Republic features steady rainfall, which sustains cash crop production. Using a representative season, December 2016–February 2017, the mesoscale climate is characterized by high-resolution reanalysis, satellite measurements and local observations, and statistical analyses of time series from an index area of 18.8–19.6° N, 70.4–69.6° W in the Cibao Valley, where cacao and coffee are grown. Winter rainfall depends on strong trade winds that push shallow stratiform convections over 100 km inland, where nocturnal drainage flows induce orographic uplift. Interannual climate variability is studied in the context of cacao and coffee production in the years 1976–2019. Lag correlations demonstrate that higher yields follow a wet autumn, a windy winter with cool sea temperatures, and a dry spring. Changes in high-value agricultural production in the northeastern Dominican Republic may be anticipated by the climatic determinants uncovered here. [ABSTRACT FROM AUTHOR]

The in-situ data gathered over two decades since mid-1990s in the South-Eastern Levantine Basin from 34 hydrographic campaigns and from 35 bi-weekly and monthly Voluntary Observing Ships (VOS) transects, after the pioneering POEM cruises, have provided insight on the dominant meso-scale coherent circulation features of this sub-basin. The most pronounced feature, the anticyclonic Cyprus Eddy, migrates over the broad region of the Eratosthenes seamount and exhibits significant seasonal and inter-annual spatial-temporal variability. Another prominent structure of the sub-basin is the anticyclonic Shikmona Eddy generated by instabilities of the strong northward flowing jet along the south-easternmost shelf and slope of the Levantine basin. Its evolution and co-existence with the Cyprus Eddy, cause the periodic re-establishment of the Shikmona Gyre, which was first observed during the POEM cruises in mid-1980s. The offshore, cross basin, eastward flowing Mid-Mediterranean Jet (MMJ) defines the northern and southern flanks of these sub-basin scale anticyclonic eddies and transports the lower salinity Modified Atlantic Water (MAW) in the subsurface layer, through the warmest and most saline waters of the Mediterranean. Periodically, the MMJ bifurcates and/or is diverted northward, along the western coast of Cyprus due to westward and northward spatial fluctuations of the Cyprus Eddy’s western boundaries. The current long term in-situ monitoring of the sub-basin provides an overview of the extent of the main water masses and characterizes their variability throughout the period considered. The analysis of the data indicates an increase of the temperature and salinity of the Levantine Surface Water (LSW) and of the subsurface MAW. The Eastern Mediterranean Transient Water (EMTW) is shown to occupy the deep cavities in the vicinity of the Eratosthenes seamount while its upper boundary was found to be lifted to shallower depths over a period of two decades.

In order to understand the impact of irrigation on weather and climate, the 2018 Great Plains Irrigation Experiment collected comprehensive observations straddling irrigated and non‐irrigated regions in southeast Nebraska. Using these observations, we examine how irrigation affects diurnal terrain‐generated slope circulations, specifically the slope wind. We find that irrigation applied to upslope regions of gently sloping terrain reduces terrain‐induced baroclinicity and the associated pressure gradient force by up to two‐thirds. This leads to the reduction in the afternoon and evening upslope wind and is supported through comparisons to the High‐Resolution Rapid Refresh operational model, which does not explicitly account for irrigation. Additionally, the presence of irrigation decreases daytime sensible heat flux (Bowen ratio reduced 40% compared to non‐irrigated regions), weakening turbulent transport of momentum. Modifications to the terrain‐forced circulation by irrigation has the potential to affect moisture transport and thus cloud and precipitation formation over the Great Plains. Plain Language Summary: Agricultural irrigation alters input of heat and moisture from the land surface to the atmosphere, which can affect weather and climate. Irrigation is expanding on all continents except Antarctica and is thus a major pathway through which humans impact the environment. However, the observations required to study the mechanisms though which irrigation affects weather and climate are lacking. The Great Plains Irrigation Experiment (GRAINEX) was conducted to collect such observations near the boundary between irrigated and non‐irrigated regions in southeast Nebraska. During the daytime, the slope of the Great Plains causes near‐surface upslope winds and downslope winds in the atmosphere above. At night this pattern reverses. This wind system influences storm formation by forcing upward motion and transporting moisture. Using the observations from GRAINEX and comparing to a weather model we find that irrigation weakens this wind system, potentially affecting cloud and rain formation in this region. Key Points: First extensive observational study of modification of slope wind circulations by irrigationPresence of irrigation in upslope regions weakens terrain‐induced baroclinicityIrrigation‐reduced baroclinicity weakens the afternoon slope wind circulation [ABSTRACT FROM AUTHOR]

Abstract The Atlantic Tradewind Ocean‐Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place in January–February, 2020. It was designed to understand the relationship between shallow convection and the large‐scale environment in the trade‐wind regime. A Lagrangian large eddy simulation, following the trajectory of a boundary‐layer airmass, can reproduce a transition of trade cumulus organization from “sugar” to “flower” clouds with cold pools, observed on February 2–3. The simulation is driven with reanalysis large‐scale meteorology, and in‐situ aerosol data from ATOMIC and its joint field study EUREC4A. During the transition, large‐scale upward motion deepens the cloud layer. The total water path and optical depth increase, especially in the moist regions where flowers aggregate. This is due to mesoscale circulation that renders a net convergence of total water in the already moist and cloudy regions, strengthening the organization. An additional simulation shows that stronger large scale upward motion reinforces the mesoscale circulation and accelerates the organization process by strengthening the cloud‐layer mesoscale buoyant turbulence kinetic energy production.

The Atlantic Tradewind Ocean‐Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place in January–February, 2020. It was designed to understand the relationship between shallow convection and the large‐scale environment in the trade‐wind regime. A Lagrangian large eddy simulation, following the trajectory of a boundary‐layer airmass, can reproduce a transition of trade cumulus organization from "sugar" to "flower" clouds with cold pools, observed on February 2–3. The simulation is driven with reanalysis large‐scale meteorology, and in‐situ aerosol data from ATOMIC and its joint field study EUREC4A. During the transition, large‐scale upward motion deepens the cloud layer. The total water path and optical depth increase, especially in the moist regions where flowers aggregate. This is due to mesoscale circulation that renders a net convergence of total water in the already moist and cloudy regions, strengthening the organization. An additional simulation shows that stronger large scale upward motion reinforces the mesoscale circulation and accelerates the organization process by strengthening the cloud‐layer mesoscale buoyant turbulence kinetic energy production. Plain Language Summary: Fair‐weather shallow clouds have different sizes and cloud properties. A field study called the Atlantic Tradewind Ocean‐Atmosphere Mesoscale Interaction Campaign (ATOMIC) and Elucidating the Role of Clouds‐Circulation Coupling in Climate (EUREC4A) was designed to further understand the properties of these clouds. On February 2–3, very small and shallow "sugar" clouds grow into wider and deeper "flower" cloud clusters, no more than 3 km high. The clear spaces between the clouds expand. This study finds that local air circulation is responsible for making the moist and cloudy areas moister, and dry and cloud‐free areas drier, enabling a process responsible for this transition. The large‐scale vertical winds modulate the rate and strength of this process, which occurs locally at smaller scales. Key Points: Lagrangian large eddy simulation can reproduce the transition of shallow cumulus organization from sugar to flowers observed on February 2–3, 2020 during Atlantic Tradewind Ocean‐Atmosphere Mesoscale Interaction CampaignWhile large‐scale upward vertical wind deepens the cloud layer, mesoscale wind renders moist areas moister assisting cloud organizationStronger large‐scale upward motion strengthens the mesoscale circulation and accelerates the sugar‐to‐flowers transition process [ABSTRACT FROM AUTHOR]

Helicity is inherent in many circulating motions and structures in the atmospheric boundary layer (ABL), where it is continuously reproduced due to the combined action of the Earth's rotation and friction, which also is closely related to the phenomena of the inverse cascade and large-scale restructuring of flows. The helicity factor should be correctly considered during the construction of atmosphere models, and, accordingly, the knowledge of the helicity distribution in the ABL and its relation to dynamic atmospheric processes is required. In this study, the helicity of the circulation structures of various spatial and temporal scales in the ABL is determined from an analysis of field measurement data. A qualitative and quantitative comparison with the observed values is carried out on the basis of results of numerical simulations using the quasi-two-dimensional model and a WRF-ARW mesoscale atmospheric nonhydrostatic model, in particular, WRF-LES. A good agreement with the observed spatial distributions of circulating motions is obtained. A link between the turbulent characteristics and helicity of the structures under study is shown. The helicity estimates for circulation structures of various scales in the ABL and in the free atmosphere are compared. [ABSTRACT FROM AUTHOR]

The mesoscale circulation of waters in the East Sakhalin Current region (the western Sea of Okhotsk) is studied for the period of 1993 to 2014 using satellite altimetry data. It is established that anticyclonic/cyclonic wind stress curl over the Sea of Okhotsk (and thus the winds of southern/northern rhumbs in the western Sea of Okhotsk) result in the mesoscale cyclonic/anticyclonic circulation of waters in July−August/October−December. It is shown that the origin of waters in mesoscale cyclones (with relatively high salinity and a low temperature) is associated with coastal upwelling and the tidal mixing. Waters in anticyclones (with relatively low salinities) form in the northwestern Sea of Okhotsk under the impact of the Amur River discharge. In October–December, there is an increasing inflow of low-salinity waters to the eastern coast of Sakhalin Island under the action of the winter monsoon, favoring the formation of mesoscale anticyclones. [ABSTRACT FROM AUTHOR]

Mesoscale features within the Gulf of Mexico (GOM) are known to influence zooplankton dynamics. Here we describe the composition of the zooplankton assemblage off shelf during summer in relation to environmental conditions, with emphasis on hyperiid amphipods and salps. Zooplankton samples were collected in summer of 2015 and 2016 in the central and southern GOM and in the Yucatan Channel in 2015. Two anticyclonic gyres were present in the north and less intense coupled cyclonic-anticyclonic gyres in the south. Zooplankton abundances differed temporally and spatially. Copepods were the dominant group (>55% of total abundance), while several less abundant taxa contributed to inter-annual and spatial differences. Amphipods and salps comprised <3% and their abundances were positively correlated. Fifty-six hyperiid and 10 salp species were identified. The dominant amphipod species were: Lestrigonus bengalensis (summer 2015), Anchylomera blossevillei and Primno spp. juveniles (summer 2016). Dominant salp species were Ihlea punctata, Iasis cylindrica and Thalia spp. Lower salp and amphipod species richness and abundance were associated with anticyclonic structures. Spatial and temporal differences were partly associated with symbiotic relationships between the groups. This study supports previous evidence of high spatial and temporal variability in zooplankton abundance in off-shelf waters of the GOM. [ABSTRACT FROM AUTHOR]

Aiming for an improved understanding of the different factors that determine the regional climate of the Lake Victoria Basin, the COSMO-CLM regional climate model is set up in a tropical, convection-permitting configuration and is directly nested in a recent reanalysis product (ERA5). The convection-permitting simulation outperforms state-of-the-art climate integrations that rely on convection parametrisations. Yet the domain-averaged model precipitation is larger than in the multi-observational ensemble, but the latter shows large spread. Overestimations of outgoing TOA shortwave and longwave radiation are much reduced compared to the COSMO-CLM CORDEX-Africa integration, but still suggest a general underestimation of the cloud fraction or frequency. Comparing the control with a no-lake simulation, the presence of Lake Victoria implies strong intensification of over-lake rainfall, but it only slightly increases the total domain-averaged precipitation. In addition, the easterly trade winds are shown to largely affect the mesoscale circulation and precipitation patterns. During daytime, fast trades and anabatic slope winds trigger convection at the lee-wind slopes, and subsidence over the basin. Slow trades allow the stationary air to produce spontaneous convective cells and to develop anabatic winds that result in orographic precipitation. During night-time, trade winds curl around the eastern branch of the East African Rift, generating a southerly and northerly evening convergence front entering the Lake Victoria Basin. Thus, our results highlight the key importance of the easterly trade winds and the complex orography in determining the total accumulation and location of precipitation in the Lake Victoria region. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O 3) plays predominant role in atmospheric chemistry, exacerbates the air quality, and contributes to the climate change. Near the Earth's surface, O 3 levels exhibit large heterogeneity due to diverse emissions of precursors and effects of meteorological conditions. Nevertheless, the understanding of key dynamical and photochemical processes governing O 3 remains limited in the Western Ghats, a tropical region of immense significance in context of the geosphere-biosphere processes. In this regard, we combined surface O 3 measurements at Ponmudi (8.758° N, 77.114° E, ∼1 km above mean sea level–amsl), a high-altitude site in the Western Ghats, with ground- and balloon-borne O 3 measurements at western coast of India (Thumba: 8.542° N, 76.858° E, ∼3 m amsl) and photochemical box model simulations. O 3 diurnal variation exhibits a small amplitude (3–9 ppbv) with lower values during the daytime, in contrast to substantial urban O 3 build-up (25–37 ppbv) at Thumba. The influence of regional pollution from the urban boundary layer, downdraft of O 3 -rich air during night, and dry deposition processes are suggested to govern the O 3 diurnal pattern over Ponmudi. Lower O 3 /CO and O 3 /NO 2 ratios indicating lower O 3 production efficiency, and dominant role of dynamics are consistently seen over the study region. O 3 variability between Ponmudi and Thumba are found to be well correlated (r = 0.7–0.8) during pre-monsoon and post-monsoon seasons as the sea breeze circulation transport the airmass from coast to the Western Ghats during the daytime. Mean O 3 levels at Ponmudi are typically lower than those above Thumba showing an effective O 3 loss towards Ponmudi. Chemistry involving strong natural emissions (e.g., isoprene) as simulated by photochemical box model, and greater dry deposition tend to supress O 3 build up over this densely vegetated tropical region. Unlike other high-altitude sites, O 3 seasonal cycle at Ponmudi is associated closely with synoptic wind changes, and peaks during the winter owing to the transport from Indo Gangetic Plain/northern India. Our study highlights the roles of regional pollution as well as natural processes including biogenic emissions in governing the surface O 3 variability over the Western Ghats. [Display omitted] • O 3 over the Western Ghats (Ponmudi) is well correlated (r = 0.7–0.8) with O 3 at western coastal station (Thumba). • Lower levels of O 3 is attributed to the higher dry deposition loss and biogenic emission of isoprene over the Western Ghats. • Unlike other high-altitude sites, Ponmudi-O 3 peaks during winter due to transport from Indo Gangetic Plain/northern India. [ABSTRACT FROM AUTHOR]

The blue shark (Prionace glauca), a highly migratory and wide-ranging shark, has been classified as Critically Endangered in the Mediterranean Sea by the International Union for Conservation of Nature since 2016 due to overfishing. Surprisingly, basic information on its biology and ecology, essential for its conservation, remains unknown, and its regional stock has not been assessed. A multi-year large-scale telemetry tagging programme was implemented to fill these gaps. A total of 39 blue sharks were equipped with satellite tags in three areas of the Western Mediterranean, and their movements and home ranges were examined by sex and size class. Their distributions and habitat preferences across seasons were identified using satellite-based environmental data. Tagged sharks were capable of long-range movement between distant regions, but they did not move neither towards the Eastern Mediterranean basins nor the North Atlantic. We identified the Gulf of Lions as the mating, parturition and nursery grounds. We demonstrated that sharks can use the general geostrophic current system for large movements. Our results support the current stock boundary in the Mediterranean based on international conventional tagging programmes and genetic studies, but the absence of connectivity between the two Mediterranean basins and the North Atlantic basin suggests that they have distinct stocks. Our findings provide critical input for developing effective conservation and management plans for this cosmopolitan species. For the first time in the Mediterranean, this study provides insights into the movement patterns of blue sharks and their habitat preferences. [Display omitted] • First large-scale telemetry tagging documenting the distribution and movements of blue sharks in the Mediterranean. • The Gulf of Lions (France) is a major mating, parturition and nursery ground in the Western Mediterranean. • Blue sharks use the general geostrophic circulation system to move long distances. • A tentative migration pattern is proposed. • No connectivity between the two Mediterranean basins and the North Atlantic basin was found. [ABSTRACT FROM AUTHOR]

An investigation into mesoscale roll circulation and its transport characteristics in the atmospheric boundary layer (ABL) is carried out. The case study of July 28, 2007, in Kalmykia, monitored during an expedition from the Obukhov Institute of Atmospheric Physics, is considered using the WRF-ARW model. The development of circulation with considerable asymmetry in the positive and negative components of the velocity field and helicity is recorded. The quasi-2D roll structures are characterized by an intensification of the dust capture and accumulation from the underlying terrain and, along with intensive vortices with a vertical axis, are a significant source of atmospheric impurities. Captured finely dispersed aerosols can be transported at long distances and form aerosol layers. [ABSTRACT FROM AUTHOR]

Convection-permitting simulations on an idealized land planet are performed to understand whether soil moisture acts to support or impede the organization of convection. Initially, shallow circulations driven by differential radiative cooling induce a self-aggregation of the convection into a single band, as has become familiar from simulations over idealized sea surfaces. With time, however, the drying of the nonprecipitating region induces a reversal of the shallow circulation, drawing the flow at low levels from the precipitating to the nonprecipitating region. This causes the precipitating convection to move over the dry soils and reverses the polarity of the circulation. The precipitation replenishes these soils with moisture at the expense of the formerly wet soils which dry, until the process repeats itself. On longer timescales, this acts to homogenize the precipitation field. By analyzing the strength of the shallow circulations, the surface budget with its effects on the boundary layer properties, and the shape of the soil moisture resistance function, we demonstrate that the soil has to dry out significantly, for the here-tested resistance formulations below 15% of its water availability, to be able to alter the precipitation distribution. We expect such a process to broaden the distribution of precipitation over tropical land. This expectation is supported by observations which show that in drier years the monsoon rains move farther inland over Africa. [ABSTRACT FROM AUTHOR]

Carbon export from the surface to the deep ocean is a primary control on global carbon budgets and is mediated by plankton that are sensitive to physical forcing. Earth system models generally do not resolve ocean mesoscale circulation ( O(10–100) km), scales that strongly affect transport of nutrients and plankton. The role of mesoscale circulation in modulating export is evaluated by comparing global ocean simulations conducted at 1° and 0.1° horizontal resolution. Mesoscale resolution produces a small reduction in globally integrated export production (<2%); however, the impact on local export production can be large (±50%), with compensating effects in different ocean basins. With mesoscale resolution, improved representation of coastal jets block off‐shelf transport, leading to lower export in regions where shelf‐derived nutrients fuel production. Export is further reduced in these regions by resolution of mesoscale turbulence, which restricts the spatial area of production. Maximum mixed layer depths are narrower and deeper across the Subantarctic at higher resolution, driving locally stronger nutrient entrainment and enhanced summer export production. In energetic regions with seasonal blooms, such as the Subantarctic and North Pacific, internally generated mesoscale variability drives substantial interannual variation in local export production. These results suggest that biogeochemical tracer dynamics show different sensitivities to transport biases than temperature and salinity, which should be considered in the formulation and validation of physical parameterizations. Efforts to compare estimates of export production from observations and models should account for large variability in space and time expected for regions strongly affected by mesoscale circulation. [ABSTRACT FROM AUTHOR]

The 'Effects of Lake Breezes on Weather in Manitoba' project was conducted during 6-24 July 2013 to better understand local lake-breeze characteristics. Data were collected using a variety of platforms including Doppler wind lidar, rawinsondes, Doppler radar, surface meteorological stations, and a mobile weather station. The spatial and temporal variability of thermodynamic and kinematic characteristics of lake-breeze fronts are presented for three cases. Lake-breeze frontal passages were characterized by an average increase in dew point of 2.5 $$^{\circ }$$ C and decrease in temperature of 0.5 $$^{\circ }$$ C. The lake-breeze front width varied significantly over multiple measurements and cases, ranging between 50 and 800 m. The depth of the lake-breeze circulation varied between 100 and 700 m. Vertical velocities were measured at the lake-breeze front using lidar, with upward velocities of 2-3 m s $$^{-1}$$ and small downward velocities of magnitude 0.5 m s $$^{-1}$$ behind the front. These observations of lake-breeze fronts in southern Manitoba contribute both to the local and broader understanding of the variability (temporally and spatially) of inland shallow lake breezes. [ABSTRACT FROM AUTHOR]

Fish larvae distribution and transport of dominant species were analyzed at the entrance to the Gulf of California in September 2016. Using the Sea Surface Temperature (SST) gradient, a frontal system was defined at the entrance to the Gulf. The most relevant gradient (∼0.06 °C km−1) in the region was transverse to the gulf axis (W-E), from south of Cabo San Lucas to the mainland, south of the Pescadero Basin. This SST gradient was associated with an anticyclonic flow (∼0.4 m s−1) that was connected with a cyclonic eddy outside the Gulf. Fish larvae of mesopelagic species such as Vincigueria lucetia and Benthosema panamense were widely distributed and increasing in abundance from north to south, while the mesopelagic Bregmaceros Bathymaster and the pelagic Auxis spp. showed an inverse abundance gradient. Most of these larvae were absent from the zone of strong SST gradient. A Lagrangian particle tracking model applied at the sampling stations where the fish larvae were collected, showed that during the first 5 days of trajectory, the transport was weak. But for the next 5 days, most of the particles were trapped by mesoscale flows that dominated the region. An unexpected result was that no particles crossed the transverse front, coinciding with the absence or low abundance of larvae there. The particles near to this front were transported west out the Gulf. We concluded that when the mesoscale flows are extended along the transverse axis of the Gulf entrance, these might trap inert particles and zooplankton organisms with little mobility, such as fish larvae in early phases of development, preventing their transport along the Gulf parallel axis (NW-SE), and therefore, affecting the species distribution. • A transverse thermal front in the mouth of the Gulf of California was detected in September 2016. • The front was associated with an anticyclonic flow that transported particles west out the Gulf. • Most of the fish larvae of dominant species had low abundance or were absent from the transverse front. • The high frequency of mesoscale flows in the Gulf mouth could prevent along-Gulf exchange fish larvae. [ABSTRACT FROM AUTHOR]

Within arid and semi-arid regions, deserts and oases generally act as the landscape matrix and mosaic, respectively. Oasis-desert interactions, i.e., the transport of mass and energy between the two, are very important for the stable co-existence of oasis and desert ecosystems. In recent decades, great progress has been made to advance our understanding of oasis-desert interactions. In this preface, we provide an overview of oasis-desert interaction studies available in the literature and our current understanding of the limitations and challenges of these studies. Future foci can be multiple-scale, high-accuracy observing matrices and seamless simulations from mesoscale circulations to large eddies, which are crucial for understanding small-scale structures of energy and water exchange and their connection with oasis-desert interaction. [ABSTRACT FROM AUTHOR]

A high resolution submesoscale resolving ocean model was implemented in a limited area north of Island of Elba where a maritime exercise, named Serious Game 1 (SG1), took place on May 2014 in the framework of the project MEDESS-4MS (Mediterranean Decision Support System for Marine Safety). During the exercise, CTD data have been collected responding to the necessity of a Rapid Environmental Assessment, i.e. to a rapid evaluation of the marine conditions able to provide sensible information for initialisation of modelling tools, in the scenario of possible maritime accidents. The aim of this paper is to evaluate the impact of such mesoscale-resolving CTD observations on short-term forecasts of the surface currents, within the framework of possible oil-spill related emergencies. For this reason, modelling outputs were compared with Lagrangian observations at sea: the high resolution modelled currents, together with the ones of the coarser sub-regional model WMED, are used to force the MEDSLIK-II oil-spill model to simulate drifter trajectories. Both ocean models have been assessed by comparing the prognostic scalar and vector fields as an independent CTD data set and with real drifter trajectories acquired during SG1. The diagnosed and prognosed circulation reveals that the area was characterised by water masses of Atlantic origin influenced by small mesoscale cyclonic and anti-cyclonic eddies, which govern the spatial and temporal evolution of the drifter trajectories and of the water masses distribution. The assimilation of CTD data into the initial conditions of the high resolution model highly improves the accuracy of the short-term forecast in terms of location and structure of the thermocline and positively influence the ability of the model in reproducing the observed paths of the surface drifters. [ABSTRACT FROM AUTHOR]

We describe the physical and dynamical characteristics of a mesoscale anticyclonic eddy observed in August 2013 over the shelf-break region of the northeastern sector of the Ligurian Sea, between the northeastern edge of the Northern Current (NC) and the coast. Results derive from a dense dataset of temperature, salinity and current measurements obtained from a multi-platform sampling strategy as well as from a diagnostic simulation with the Regional Ocean Modeling System (ROMS) at a horizontal resolution of 1.8 km. Model results are obtained from a strong nudging to observations and, as they are physically balanced, they allow a three-dimensional diagnosis of the dynamics and physical characteristics of the eddy. The eddy is centered around 9.5°E, 43.94°N, about 20 km from the coast, and has a radius of 16 km. It is characterized by low-density waters and penetrates the thermocline down to at least 300 m reflecting the main features of the NC. Horizontal velocities near the surface are around 0.4 m s −1 , while at 150 m are still significantly high and close to 0.2 m s −1 . Vertical velocities were estimated from model results; absolute values are below 4 m day −1 until depths shallower than 150 m and increase with depth to 15 m day −1 . The eddy's presence inverts the northwestwards flow traditionally portrayed in the region determining a southeastwards coastal circulation that replenishes coastal waters with those originating from the NC. We discuss several possible eddy formation mechanisms and suggest that its formation depends on the directionality of the NC when it enters the Ligurian Sea, as a result of the adjustment of the shear and orbital components of the current's relative vorticity. [ABSTRACT FROM AUTHOR]

Simple scaling arguments conclude that the dominant motions in the ocean are horizontal. However, the vertical velocity plays a crucial role, connecting the active upper layer with the deep ocean. Vertical velocities are mostly associated with the existence of non-transient atmospheric wind forcing or with the presence of mesoscale features. The former are the well known upwelling areas, usually found at the eastern side of the oceans and characterised by upward vertical velocities. The latter have been observed more recently in a number of areas of the world´s oceans, where the vertical velocity has been found to be of the order of several tens of meters per day, that is, an order of magnitude higher than the largest vertical velocity usually observed in upwelling areas. Nevertheless, at present, vertical velocities cannot be measured and indirect methods are therefore needed to estimate them. In this paper, the vertical velocity field is inferred via the quasi-geostrophic omega equation, using density data from a quasi-permanent upper ocean front located at the northern part of the western Alborán gyre.

A basic understanding of the mechanisms controlling the characteristics of thermally induced mesoscale circulations rests primarily on observations and model studies of dry convection, whereas the influence of moist convection on these characteristics is not well understood. Large-eddy simulations are used to investigate the effect of moist convection on an idealized mesoscale circulation. Sensitivity studies show that moist convection has a significant influence on the characteristics of the mesoscale circulation. We identify three distinct convective phases that influence the mesoscale circulation within the diurnal cycle: firstly, dry convective onset, with a weak circulation and a breeze front that propagates slowly from the cold region into the warmer fluid as a result of the surface discontinuity; secondly, a deep convective phase, where the circulation intensifies and the breeze front propagates faster; and finally a precipitating phase, where strong cold pools develop at the breeze front and accelerate the propagation speed further. Classical density-current theory fails to represent the second phase and is extended using the cloud-base mass flux to account for the observed effects of moist non-precipitating convection on the propagation speed. We demonstrate the applicability of this theory to the results from large-eddy simulations, identify the subtle role of cold pools on density-current propagation and highlight implications for numerical weather prediction. [ABSTRACT FROM AUTHOR]

During the summer, the relative influence of resident Atlantic Waters (AW) and new AW largely drives the mesoscale dynamics around the Balearic Islands (NW Mediterranean). Two principal summer hydrographic scenarios were identified in the region, differentiated by the relative position of the density front between new and resident AW within the archipelago and its associated mesoscale activity. In this study, we investigated how those early summer mesoscale scenarios influence larval fish assemblages, by analyzing data from two cruises representative of these two scenarios (2004 and 2005). Redundancy analysis was used to assess the variance in the larval fish assemblage that could be significantly explained by the most parsimonious combination of available environmental variables in both years. While depth was the most important variable in explaining the larval fish assemblage structure variability observed under both scenarios, indicators of mesoscale activity (dynamic height, geostrophic velocity) contributed significantly to understanding the dynamics of the larval fish community. Mesoscale activity was higher in summer 2004, leading to higher larval fish abundances and zooplankton biomass and lower larval fish diversity than in the unusually warm summer 2005, which showed lower mesoscale activity. The larval assemblage dynamics are discussed in terms of extrinsic and species-specific factors. [ABSTRACT FROM AUTHOR]

The frequency, the location and the characteristics of convective rainfall events induced by the convergence of different sea breeze systems on a Mediterranean peninsula (Salento, in southeastern Italy) are analyzed. Such events have been studied considering satellite/radar images and output fields from two Limited Area Models in the summer period of 2011–2013. A total of 20 days have been detected in which the precipitation due to sea-breeze convergence was clearly observed in satellite and radar images. The synoptic conditions associated with these events have been identified considering the averages of some relevant meteorological parameters in the selected days and the anomaly with respect to the climate. The presence of a cold trough in the central Mediterranean basin appears as a fundamental ingredient for the occurrence of sea breeze convergence and associated precipitation. High-resolution simulations with two state-of-art numerical models have revealed that both of them are generally able to simulate a convergence pattern correctly, apart from a couple of cases for each model. The higher rainfall amounts occur with weak synoptic wind, and weak-to-moderate values of Convective Available Potential Energy (CAPE). When the synoptic wind is of moderate intensity, the region of convergence moves toward the Adriatic coast for a prevailing southerly component, and toward the Ionian coast for a prevailing northerly component. On the opposite, the skin sea surface temperature is relatively uniform and the difference between the Ionian and the Adriatic Seas, surrounding the peninsula on the east and west side, is generally smaller than 1 K, having only a marginal effect on the sea breeze patterns. Similarly, the value of CAPE before the occurrence of rainfall has low prognostic value. The results shows that limited area models with a grid spacing of few km appear as appropriate tools for the simulation for such relatively small scale phenomena. [ABSTRACT FROM AUTHOR]

Using large-eddy simulation (LES), the effects of mesoscale local surface heterogeneity on the temporal evolution of low-level flows in the convective boundary layer driven by two-dimensional surface heat-flux variations are investigated at a height of about 100 m over flat terrain. The surface variations are prescribed with sinusoids of wavelength 32 km and varying amplitudes of 0, 50, 100, and 200 W m $$^{-2}$$ . The Weather Research and Forecasting numerical model is used as a mesoscale-domain LES model that has a grid spacing fine enough to explicitly resolve energy-containing turbulent eddies and a model domain large enough to include mesoscale circulations. Mesoscale circulations induced by the two-dimensional surface heterogeneity may undergo a flow transition and an associated spectral energy cascade, which has been found previously but only with one-dimensional surface heat-flux variations. Over a strongly heterogeneous surface prescribed with a two-dimensional sinusoid of amplitude 200 W m $$^{-2}$$ , the domain-averaged variance of the horizontal wind component initially grows rapidly, then undergoes a flow transition and subsequently rapidly decays. With a background wind, the induced mesoscale circulations are inhibited in the streamwise direction. However in the spanwise direction, somewhat stronger mesoscale circulations are induced, compared with those with no background wind. The background wind attenuates the significant reduction of the low-level temperature gradient by the fully-developed mesoscale horizontal flow. Spectral decomposition reveals that this rapid transition also exists in the mesoscale horizontal flows induced by the intermediate surface heterogeneity prescribed with a sinusoid of amplitude 100 W m $$^{-2}$$ . However the transition is masked by continuously growing turbulence. [ABSTRACT FROM AUTHOR]

The large-eddy simulation mode of the Weather Research and Forecasting model is employed to simulate the planetary boundary-layer characteristics and mesoscale circulations forced by an ideal urban heat island (UHI). In our simulations, the horizontal heterogeneity of the UHI intensity distribution in urban areas is considered and idealized as a cosine function. Results indicate that the UHI heating rate and the UHI intensity heterogeneity affect directly the spatial distribution of the wind field; a stronger UHI intensity produces a maximum horizontal wind speed closer to the urban centre. The strong advection of warm air from the urban area to the rural area in the upper part of the planetary boundary-layer causes a more stable atmospheric stratification over both the urban and rural areas. The mesoscale sensible heat flux caused by the UHI circulation increases with UHI intensity but vanishes when the background wind speed is sufficiently high $$(>$$ 3.0 $$\mathrm{{m\,s}}^{-1})$$ . [ABSTRACT FROM AUTHOR]

Abstract: The MESOBIO programme investigated mesoscale dynamics using an integrated ecosystem approach, linking physical and biogeochemical processes with different trophic levels. Observation and modeling were used in combination to explain the main processes occurring in the mesoscale eddy field. The particular shape of the Mozambique Channel, composed of two basins interconnected through a narrow zone, favours the generation of mesoscale eddies and increases the opportunity for eddy-shelf interactions. Phytoplankton abundance peaked in areas of nutrient enrichment that are often found in the core of cyclonic eddies, as well as on the continental shelf. Grazers in zooplankton communities exhibited high biovolume in cyclonic eddies, but their abundance was lower in fronts and divergence zones, with lowest biovolume in anticyclones. Biovolume was highest at shelf stations, but very variable and similar to phytoplankton. Age of eddies, their subsequent maturation stage and the dynamics of the eddy field played a major role effecting zooplankton abundance. Micronekton presented abundance patterns coherent with zooplankton distribution, however this was only demonstrated by acoustic methods, whereas mid-water trawl collection and predators stomach contents (predators being used as biological samplers) did not reveal significant relationships with mesoscale features. For upper trophic levels, the average density of foraging seabirds was lowest in anticyclones, highest in cyclones and at intermediate levels in divergence, shelf and frontal zones. However, multifaceted behavioral responses were observed in such a highly variable environment. Swordfish was clearly associated with divergence zones, and to a lesser extent with fronts, suggesting that the higher density in divergences was related to the presence of its main prey, essentially large squids. Although tunas tended to be more abundant in areas with weak geostrophic currents, their relationship to mesoscale features was not straightforward as adult tunas caught by longline have the ability to explore different foraging habitats over a broad range of depths. Several suggestions for advancing eddy-related research from the current state of knowledge are proposed in the second part of the paper. [Copyright &y& Elsevier]

Abstract: A multidisciplinary programme, MESOBIO (Influence of mesoscale dynamics on biological productivity at multiple trophic levels in the Mozambique Channel) was undertaken in the Mozambique Channel within the framework of a scientific partnership between France and South Africa. MESOBIO focused on the signature of the highly energetic eddy dynamics in the Mozambique Channel. The Channel, which is known to be one of the most turbulent areas in the world ocean, has a great diversity of marine organisms and is the site of active pelagic fisheries. MESOBIO was mostly based on observations at sea during 12 multidisciplinary cruises between 2002 and 2010. Hydrographic measurements, sampling of biological organisms ranging from phytoplankton to top predators, and experiments on primary production and energy transfer through the food web, were conducted onboard various research vessels. The data were analysed in relation to eddy field characteristics for the periods of the cruises, including seasonal or inter-annual variability in mesoscale activity. A modelling approach was also developed within MESOBIO for both the circulation in the Channel and the biogeochemical response to eddy forcing. This paper introduces the suite of articles on the MESOBIO investigations by summarizing background knowledge for the different disciplines and the key issues that were addressed within the programme. [Copyright &y& Elsevier]