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Sea surface temperature (SST) is characterized by abundant warm and cold structures that influence the overlying atmospheric boundary layer dynamics through two different mechanisms. First, turbulence and large eddies in the lower troposphere are affected by atmospheric stability, which can be modified by local SST, resulting in enhanced vertical mixing and larger surface winds over warmer waters. Second, the thermodynamic adjustment of air density to the underlying SST structures and the subsequent changes in atmospheric pressure drive secondary circulations. This paper aims to disentangle the effects of these processes and explore the environmental conditions that favor them. Two main environmental variables are considered: the large-scale air–sea temperature difference (proxy for stability) and wind speed. Using 5 years of daily reanalyses data, we investigate the 10-m wind response to SST structures. Based on linear regression between wind divergence and SST derivatives, we show that both mechanisms operate over a large spectrum of conditions. Ten-meter wind divergence is strongly impacted by the local SST via its effect on vertical mixing for midwind regimes in slightly unstable to near-neutral conditions, whereas the secondary circulation is important in two distinct regimes: low wind speed with a slightly unstable air column and high background wind speed with a very unstable air column. The first regime is explained by the prolonged Lagrangian time that the air parcel stays over an SST structure while the second one is related to strong heat fluxes at the air–sea interface, which greatly modify the marine atmospheric boundary layer properties. Location and frequency of the environmentally favorable conditions are discussed, as well as the response in low-cloud cover and rainfall. Significance Statement The main objective of this study is to explore the wind response to thermal structures at the sea surface under different environmental conditions using the latest atmospheric reanalysis. Recent literature suggests that fine-scale air–sea interactions affect a large spectrum of atmospheric dynamics, from seasonal to weather-type regimes. It is thus important to characterize the atmospheric response to ocean surface variability. Our findings describe the environmental conditions for which the two main physical processes through which the atmosphere responds to sea surface temperature structures are active the most and can guide the development of high-resolution observing missions and campaigns in specific geographical locations and seasons to retrieve data that can be used to improve parameterization in models.

Even though winter land–sea thermal contrast (LSC) is expected to undergo a strong weakening in the future warmer climate, its effects have been poorly investigated. Here we run a set of idealized winter simulations featuring reduced LSC in the Northern Hemisphere (NH) extratropics, or in individual extratropical sectors of the NH (Atlantic and Pacific), using an intermediate-complexity atmospheric general circulation model. Reduced LSC is obtained by imposing a warming of surface land temperatures in East Asia and North America. For similar warming intensities over the two regions, the response of the model to East Asia forcing is significantly stronger and dominates the response to the sum of the two forcing patterns. We find that the LSC reduction causes a weakening and poleward shift of the midlatitude jet streams, and a strong interference with zonal wavenumbers 1 and 2. In particular, East Asian warming reduces the amplitude of waves 1 and 2, producing a strengthening of the stratospheric vortex, while a weaker vortex due to a moderate amplification of wave 1 is detected when warming North America. Eventually, stratospheric signals propagate downward in the troposphere affecting the midlatitude winter NH even remotely from the forcing. In this work we pinpoint some mechanisms by which weakened winter LSC influences the NH extratropical circulation: the results may become useful to interpret the response to long-term projections displaying reduced LSC along with other climate change forcing patterns.

Wind Induced Surface Heat Exchange (WISHE) mechanism is considered very important for tropical cyclone intensification in a large part of the scientific literature([1], [2], [3] ): heat flux from the ocean increase with increasing wind speed, building up a positive feedback on the intensification.Simple WISHE-based models of tropical intensification predict that tropical cyclones intensify up to a steady state at the Potential Intensity (PI), obtained from the balance of heat supply rate from the ocean and dissipation rate in the boundary layer and dependent on boundary conditions only ([1]). The main problem of such models is the fact that they typically drastically simplify the convective motion within the cyclone, assuming a troposphere neutral to moist convection. ([4]).In our work we tested these predictions in idealized numerical experiments performed using the non-hydrostatic, high-resolution model System for Atmospheric Modelling (SAM). The results showed a significantly different intensity evolution, with the cyclone undergoing a oscillation in surface wind speed with peak intensity significantly lower than the PI. This intensity evolution was related to that of the environmental conditions along the whole air column: convective heating exports latent and sensible heat in the middle-upper troposphere, increasing environmental air buoyancy and so reducing CAPE. Radiative heating from the clouds further stabilizes the upper troposphere, weakening convection and thus cyclone intensity. After the intensity decay phase the upper level air surrounding the cyclone cools down through radiation emission: entrainment of cold air by the cyclone itself rebuilts CAPE and triggers a new intensification. Despite this work showed some limits in the predictivity of WISHE theory, WISHE feedback itself was proved to be fundamental for tropical cyclone intensification with a sensitivity numerical experiment. [1] K. Emanuel et al., “Tropical cyclones,” Annual review of earth and planetary sciences, vol. 31,no. 1, pp. 75–104, 2003[2] K. A. Emanuel, “An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-StateMaintenance.,” Journal of Atmospheric Sciences, vol. 43, pp. 585–605, Mar. 1986.[3] C. J. Muller and D. M. Romps, “Acceleration of tropical cyclogenesis by self-aggregationfeedbacks,” Proceedings of the National Academy of Sciences, vol. 115, no. 12, pp. 2930–2935, 2018.[4] K. A. Emanuel, “The behavior of a simple hurricane model using a convective scheme basedon subcloud-layer entropy equilibrium,” Journal of Atmospheric Sciences, vol. 52, no. 22,pp. 3960 – 3968, 1995.

EUREC4A-OA is a large international project, connecting experts of ocean and atmosphere observations and modelling to enhance the understanding of key ocean and air-sea processes at the and to improve the skill of forecasts and future projections.The core of EUREC4A-OA has been a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data have been collected using cutting-edge technology on ships, airplanes and autonomous vehicles. EUREC4A-OA investigates heat, momentum, water and CO2 transport within the ocean and exchanges across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. EUREC4A-OA focuses on ocean dynamics at the small-scale (0.1–100 km) and related atmospheric boundary layer processes. EUREC4A-OA is centered on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. However, the internal ocean and atmosphere dynamics convolute the diurnal, synoptic, seasonal and longer time scales to climate variability.The talk will present some of the results we obtained so far from the observations collected during the field experiment and from numerical simulations. The analyses carried out revealed with unprecedented detail the particular characteristics of the ocean small-scale dynamics, enlightening that such scales are also very active in the tropical regions and not only over the mid and higher latitudes ocean. Observations and models also unveil that the ocean small scales is important in contributing to the exchanges of heat, freshwater and CO2 between the ocean and the atmosphere. Moreover, the evaluation of the intensity of the coupling between the ocean and the atmosphere assessed from data and high-resolution simulations show that they are very important and intimately linked with the 3D structure of the small-scale ocean dynamics. The project has also provided preliminary results in terms of parametrization of different processes influencing the ocean and atmosphere exchanges that have been uncovered by the EUREC4A-OA field experiment. Notably a better representation of the small-scale freshwater patches due to precipitation has been introduced in the French Earth-System model that improves the overall simulations of air-sea interactions and clouds. A similar parametrization is now been introduced to take into account these physical processes in air-sea fluxes of CO2.

This study addresses the role of the ocean small scale in the north-west tropical Atlantic air-sea interactions using satellite observations, ERA5, a set of regional WRF simulations and in-situ data. First, we focus on the coupling between SST and the atmospheric boundary layer, evaluating latent heat flux (LHF) sensitivity to SST. When smaller scales are considered, U10m-SST are positively correlated and the q2m-SST regression significantly differs from the Clausius-Clapeyron scaling. This is interpreted in terms of an active ocean modifying the near-surface atmospheric state. The small-scale coupling information is used to develop a downscaling method aiming to include the impact of these fine-scale SST features into an available low-resolution LHF data set. The results show that they induce a significant increase of LHF (30% - 40% per °C of SST), partitioned between a dynamic (~28%) and a thermodynamic (~5%) mode. To validate our results, the downscaling is applied to a set of high-resolution simulations, reducing by a factor of 2 our LHF estimate biases.The north-west tropical Atlantic is also subject to the Amazon water discharge. This generates large salinity contrasts and affects LHF, which is significantly reduced over the freshwater plume (20% – 30% less than in saltier waters). To elucidate the causes of this behaviour, we use several in-situ datasets collected during the EUREC4A-OA/ATOMIC campaigns. The high spatio-temporal resolution of these data sources, is likely to provide better estimates of the air-sea interface processes involved and their connections with the upper ocean heat budget and/or the concomitant atmospheric conditions., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Increased Net Primary Productivity (NPP) around small islands have been documented worldwide. Despite having been known for decades, the interactions between physical and biogeochemical processes behind this phenomenon – that takes the name of Island Mass Effect –remain unclear. In this paper we review the physical processes involved while proposing a method to identify the prevailing mechanisms by analyzing their imprint on NPP and Sea Surface Temperature (SST). These mechanisms can be quite different, but all enhance vertical exchanges, increasing the input of nutrients in the euphotic layer and favoring biological productivity. Nutrient-rich deeper waters are brought up to the surface through upwelling and mixing, leaving a cold imprint on the SST as well. Here we analyze satellite data of SST and NPP around small islands and archipelagos to catalog the physical mechanisms that favor the Island Mass Effect, with the aid of oceanic and atmospheric reanalysis. The multiplicity of these processes and the convolution of their interactions highlight the complexity of the physical forcing on the biomass production and the uniqueness of each island. However, analysis from 19 small islands throughout the tropics shows that two kinds of SST patterns emerge, depending on the size and altitude of the island. Around islands with considerable elevation and greatest diameters, cold/warm anomalies, most likely corresponding to upwelling/downwelling zones, emerge. This signal can be mainly ascribed to oceanic and atmospheric forcing. Around small islands, on the other hand, warm anomalies do not appear and only local cooling, associated with current-island interactions, is found. In the vicinity of a single island, more than one process responsible for the increased nutrient input into the euphotic layer might coexist, the prevailing one varying along the year and depending on the strength and direction of the incoming atmospheric and oceanic flow. publishedVersion

Aerosols play a crucial role in climate through different feedback mechanisms, affecting radiation, clouds, and air column stability. This study focuses on the altitude dependence of the cloud-mediated indirect effects of aerosols in the Great Alpine Region (GAR), an area characterized by high pollution levels from anthropic activities in the Po Valley and a complex orography with some of the highest mountains in Europe. Using a regional atmospheric model, 5-year-long convective-permitting sensitivity experiments have been run with different surface aerosol fluxes. The results show that seasonal mean cloud cover, temperature, and precipitations are affected by the aerosol concentrations in the air column and that the response to pollution is both elevation- and season-dependent. The overall cloud cover increase with aerosol levels leads to either surface cooling or warming depending on the surface albedo (snow covered or not). Furthermore, different types of clouds have a different response: while the lifetime of low-pressure-system clouds and orographic clouds is generally increased at high levels of aerosols, convective clouds (typical of the summer season) can decrease at high levels of pollution due to the reduction in strong updrafts associated with an increased air column stability.

Thermal structures at the sea surface are known to affect the overlying atmospheric dynamics over various spatio-temporal scales, from hourly and sub-kilometric to annual and O(1,000 km). The relevant mechanisms at play are generally identified by means of correlation coefficients (in space or time) or by linear regression analysis using appropriate couples of variables. For fine spatial scales, where sea surface temperature (SST) gradients get stronger, the advection might disrupt these correlations and, thus, mask the action of such mechanisms, just because of the chosen metrics. For example, at the oceanic sub-mesoscale, around 1–10 km and hourly time scales, the standard metrics used to identify the pressure adjustment mechanism (that involves the Laplacian of sea surface temperature, SST, and the wind divergence) may suffer from this issue, even for weak wind conditions. By exploiting high-resolution realistic numerical simulations with ad hoc SST forcing fields, we introduce some new metrics to evaluate the action of the pressure adjustment atmospheric response to the surface oceanic thermal structures. It is found that the most skillful metrics is based on the wind divergence and the SST second spatial derivative evaluated in the across direction of a locally defined background wind field.

Using 25years of ERA5 reanalysis data, this study shows that wind divergence is partially driven by small-scale sea surface temperature (SST) patterns via their effect on the boundary layer stability. Moreover, strong warm-to-cold fronts (the upper quartile) are associated with a mean increase of cloud cover of 10%±5% and a mean increase in the probability of a rain event of 15%±6%, with respect to the average values. The cloud and rainfall dependence on SST fronts is more pronounced in Fall, probably due to the stronger SST gradients present at the end of the summer season.

International audience; The ocean fine scale (from the mesoscale to the submesoscale) is susceptible to impact air-sea exchange and has an integral effect on the large scale atmosphere and ocean dynamics. Many recent advances in understanding underlying processes have been obtained from modeling efforts and only few in-situ observational studies exist one of them being the EUREC4A-OA/ATOMIC campaign that was added to the EUREC4A atmospheric campaign. This experiment took place in January-February 2020 in the Northwest Tropical Atlantic Ocean with the aim to collect synchronized ocean and atmosphere data to improve our understanding of the role of fine scale processes in the internal ocean dynamics and air-sea interaction.Four oceanographic vessels, coordinated with air-borne observations and autonomous ocean platforms (underwater gliders, Saildrones, drifters), simultaneously acquired ocean and atmosphere data east of the island of Barbados and further south, up to the border of French Giuana. This way, ocean and atmospheric data was acquired in two contrasting regions: (1) the Trade wind region and (2) a region filled with mesoscale eddies. Operations allowed investigating upper ocean processes from small to mesoscale and from sub-diurnal to monthly.A variety of mesoscale eddies were crossed with diverse characteristics, ranging from shallow cyclonic and anticyclonic eddies to the deep reaching structures. Some of these eddies, and in particular North Brazil Rings, have been previously observed and described in dedicated oceanographic experiments. Nonetheless, the EUREC4A-OA/ATOMIC campaign brings in new details about the vertical structure, the dynamics and the potential impact on air-sea interactions of these mesoscale features.With the various observing platforms it was possible to sample the upper-ocean in great detail, resolving frontal scales and stratification. For example, the remnants of the Amazon plume, flowing northward along the shelf-break and being advected far offshore though NBC rings, create a rich variety of submesoscale fronts and a strong barrier layer impacting air-sea exchange of heat and momentum. The ongoing analyses on the ocean dynamics regional and local structures and specifics of air-sea interaction will be highlighted in this presentation.

European wintertime precipitation is known to be skilfully estimated in reanalysis data and model simulations since it is highly correlated with large scale, low frequency modes of variability, namely the North Atlantic Oscillation (NAO). Since the NAO is mainly a wintertime mode of variability, the skill of estimating precipitation becomes more limited in other seasons, most importantly in summer, when precipitation is mainly a result of mesoscale convection. In this study, we use the Weather Research and Forecast (WRF) model, to show the added value of using a high resolution, convection-permitting model to estimate precipitation extremes. The results show that WRF succeeds to correct the failure of ERA-Interim reanalysis to capture the positive trends over the last decades of European extreme precipitation in summer and transition seasons, that are indicated by observational data (E-OBS) and previous literature. Partial improvements are evident using ERA5 reanalysis, specifically in Spring and in Autumn. In winter, changes in European extreme precipitation over the last decades are dominated by variations in the NAO index, and are well reproduced both in reanalysis data and in the high resolution WRF downscaling.

International audience; Using a combination of multi-directional SAR and TIR measurements, the Harmony Earth Explorer 10 mission candidate will provide high resolution simultaneous measurements of surface stress, surface currents SST and wave spectra over oceans, 3-D deformation vectors over solid Earth, and time-series of surface elevation changes over volcanic areas and land ice masses. This will serve a series of science objectives aimed at better understating multi-scale processed and feedbacks in the Earth System.

The frequency of coral bleaching events has been increasing in recent decades due to the temperature rise registered in most regions near the ocean. Their occurrence in the Maldivian Archipelago has been observed in the months following the peak of strong El Niño events. Bleaching has not been uniform, and some reefs have been only marginally impacted. Here, we use satellite observations and a regional ocean model to explore the spatial and temporal variability of sea surface temperatures (SSTs), and quantify the relative magnitude of ENSO-related episodes with respect to the recent warming. In line with other studies, it is confirmed that the long-term trend in SST significantly increases the frequency of stress conditions for the Maldivian corals. It is also found that the interaction between currents and the steep bathymetry is responsible for a local cooling of about 0.2°C in the Archipelago during the warmest season, with respect to the surrounding waters. This cooling largely reduces the frequency of mortality conditions.

Established theories ascribe much of the observed long‐term Cenozoic climate cooling to atmospheric carbon consumption by erosion and weathering of tectonically uplifted terrains, but climatic effects due to changes in magmatism and carbon degassing are also involved. At timescales comparable to those of Milankovitch cycles, late Cenozoic building/melting of continental ice sheets, erosion, and sea level changes can affect magmatism, which provides an opportunity to explore possible feedbacks between climate and volcanic changes. Existing data show that extinction of Neo‐Tethyan volcanic arcs is largely synchronous with phases of atmospheric carbon reduction, suggesting waning degassing as a possible contribution to climate cooling throughout the early to middle Cenozoic. In addition, the increase in atmospheric CO2 concentrations during the last deglaciation may be ascribed to enhanced volcanism and carbon emissions due to unloading of active magmatic provinces on continents. The deglacial rise in atmospheric CO2 points to a mutual feedback between climate and volcanism mediated by the redistribution of surface masses and carbon emissions. This may explain the progression to higher amplitude and increasingly asymmetric cycles of late Cenozoic climate oscillations. Unifying theories relating tectonic, erosional, climatic, and magmatic changes across timescales via the carbon cycle offer an opportunity for future research into the coupling between surface and deep Earth processes.

Air and sea interact on a wide range of scales, shaping climate and influencing weather. The direct effect of sea surface temperature (SST) structures on the extratropical atmosphere at the daily time-scale is generally masked by the large variability associated with atmospheric dynamics. With 25 years of daily SST and surface wind observational products, obtained with data from buoys, satellite and atmospheric analysis in the Mediterranean, we show that strong surface wind convergence preferentially occurs when the air encounters a cold SST front. The mechanism responsible for the influence of ocean fronts on surface winds is rooted in the thermal disequilibrium that emerges at the air–sea interface, where cold water enhances the stability of the boundary layer, decoupling surface winds from the stronger winds aloft. Surface convergence drives upward motion which, under appropriate conditions, favours cloud formation. Thus, these results suggest that weather forecast models need to properly represent the small-scale ocean thermal structures, which could affect rainfall. © 2020 Royal Meteorological Society

The importance of the upper ocean thermal vertical structure (mixed-layer depth and stratification) in the control of the precipitation during a heavy-rain-producing mesoscale convective system is investigated by means of numerical simulations. In particular, the fully compressible, nonhydrostatic Euler equations for the atmosphere and the hydrostatic Boussinesq equations for the ocean are numerically integrated to study the effect of the ocean-atmosphere coupling both with realistic initial and boundary conditions and with simpler, analytical vertical temperature profile forcing. It is found that the action of the winds associated with the synoptic system, in which the heavy precipitation event is embedded, can entrain deep and cold water in the oceanic mixed layer, generating surface cooling. In the case of a shallow mixed layer and strongly stratified water column, this decrease in sea surface temperature can significantly reduce the air column instability and, thus, the total amount of precipitation produced.

The mechanisms controlling the influence of the sea surface temperature (SST) structure on the surface winds are studied by means of numerical simulations run with a nonhydrostatic fully compressible state-of-the-art numerical model in a realistic midlatitudes setup, leading to the 9 October 2014 Genoa heavy rainfall event. Starting from a simulation with high-resolution submesoscale eddy-permitting SST field, the surface temperature boundary conditions are changed to enhance, reduce, or smooth the SST gradient. It is found that the marine atmospheric boundary layer responds to the submesoscale SST forcing structures over time scales of the order of hours. In particular, through the downward momentum mixing mechanism, the presence of SST horizontal gradients impacts the spatial structure of the surface wind convergence, which can displace the convective heavy rain bands that develop over the sea.

Climate change impacts are non uniformly distributed over the globe. Mountains have a peculiar response to large scale variations, documented by elevation gradients of surface temperature increase observed over many mountain ranges in the last decades. Significant changes of precipitation are expected in the changing climate and orographic effects are important in determining the amount of rainfall at a given location. It thus becomes particularly important to understand how orographic precipitation responds to global warming and to anthropogenic forcing. Here, using a large rain gauge dataset over the European Alpine region, we show that the distribution of annual precipitation among the lowlands and the mountains has varied over time, with an increase of the precipitation at the high elevations compared to the low elevations starting in the mid 20 century and peaking in the 1980s. The simultaneous increase and peak of anthropogenic aerosol load is discussed as a possible source for this interdecadal change. These results provide new insights to further our understanding and improve predictions of anthropic effects on mountain precipitations, which are fundamental for water security and management.

The Western Mediterranean Sea is often subject to intense winds, especially during the winter season. Intense winds induce surface cooling associated with anomalous ocean heat loss, upwelling and diapycnal mixing. In this study we investigate the overall impact of extreme wind events on the upper ocean in the Western Mediterranean sea using sea surface temperature and sea surface height observational data products over the period 1993&ndash, 2014. We show that the largest thermal anomaly is observed a couple of days after the intense wind event and that it is dependent on the wind intensity. During winter, when deep water formation occurs, it persists for over a month. During summer, when the thermocline is very shallow, the recovery time scale is typically less than 10 days. The sea surface height signal reaches a minimum in correspondence to the intense wind, and normal conditions recover in about six weeks. Unlike for intense winds in the tropics associated to the passage of tropical cyclones, no long term sea surface height anomaly is observed, indicating that the water column heat content is not significantly modified. The observed recovery times suggest instead the possibility of feedbacks on the dynamics of intense cyclones at sub-monthly time scales.

The Western Mediterranean Sea is often subject to intense winds, especially during the winter season. The effects of the enhanced enthalpy and momentum fluxes on the upper ocean is investigated using sea surface temperature and sea surface height observational data products in the period 1993–2014. The maximum surface cooling associated with the anomalous ocean heat loss, with upwelling, and with diapycnal mixing is shown to occur a couple of days after the intense wind event, to be dependent on the wind intensity and to persist for over a month during winter, when deep water is formed, and for about 10 days during summer, when the thermocline is very shallow. The sea surface height signal reaches a minimum in correspondence of the intense wind, and normal conditions recover in about six weeks. Unlike for intense winds in the tropics, associated to tropical cyclones, no long term sea surface height anomaly is observed, indicating that the water column heat content is not significantly modified.

The semi-enclosed Mediterranean basin, surrounded by high mountains, is placed in a favorable location for cyclonic storms development. Most of these are extratropical cyclones of baroclinic and orographic origin, but occasionally, some low pressure systems may develop to assume features characteristic of tropical cyclones. Medicanes (MEDIterranean hurriCANES) are infrequent and small-sized tropical-like cyclones. They originate and develop over sea, and are associated with strong winds and heavy precipitations. Proper definitions and classifications for Medicanes are still partially lacking, and systematic climatic studies have appeared only in recent years. In this work, we provide climatologies of Medicanes in the Western Mediterranean basin based on multidecadal runs performed with the Weather Research and Forecasting regional model with different resolutions and setups. The detection of Medicanes is based on a cyclone tracking algorithm and on the methodology of Hart cyclone phase space diagrams. We compare the statistics of Medicanes in the historical period 1979&ndash, 1998 between runs at a resolution of 11 km with different convective parameterizations and microphysics schemes and one run at a resolution of 4 km with explicitly resolved convection. We show how different convective parameterization schemes lead to different statistics of Medicanes, while the use of different microphysical schemes impacts the length of the cyclone trajectories.

The nonlinear dynamics leading to the generation of superinertial internal waves in the ocean, in the wake of a cyclonic storm, is investigated by means of theoretical arguments and of numerical integration of the hydrostatic Boussinesq equations in a simplified, realistic, open-ocean setting. The velocity fields are first decomposed into internal baroclinic modes, and then the energy transfer across modes and at different frequencies is calculated analytically. The energy transfer across modes is dominated by the advection of high-mode m waves by the second- and third-mode waves (n = 2 or 3), which are the most energetic, and this results in the excitation of the l = m − 2 or m − 3 mode wave at the double-inertial frequency. The analyzed nonlinear interactions lead to a transfer of energy from near-inertial waves, directly excited by the storm, to superinertial waves, which typically propagate faster and farther than their lower-frequency parents and can lead to internal mixing even at large distances from the region of large air–sea momentum fluxes. Energy is found to flow from large to small scales as well. Thus, the double-inertial peak formation is thought to represent a small but fundamental intermediate step in the energy cascade toward dissipation.

The spatial structure and temporal evolution of the sea surface temperature (SST) anomaly (SSTA) associated with the passage of tropical cyclones (TCs), as well as their sensitivity to TC characteristics (including TC intensity and translation speed) and oceanic climatological conditions (represented here by latitude), are thoroughly examined by means of composite analysis using satellite-derived SST data. The magnitude of the TC-generated SSTA is larger for more intense, slower-moving, and higher-latitude TCs, and it occurs earlier in time for faster-moving and higher-latitude storms. The location of maximum SSTA is farther off the TC track for faster-moving storms, and it moves toward the track with time after the TC passage. The spatial extension of the cold wake is greater for more intense and for slower-moving storms, but its shape is quite independent of TC characteristics. Consistent with previous studies, the calculations show that the mean SSTA over a TC-centered box nearly linearly correlates with the wind speed for TCs below category 3 intensity while for stronger TCs the SSTA levels off, both for tropical and subtropical regions. While the linear behavior is expected on the basis of the more vigorous mixing induced by stronger winds and is derived from a simple mixed-layer model, the level-off for intense TCs is discussed in terms of the dependence of the maximum amplitude of the area-mean SSTA on TC translation speed and depth of the prestorm mixed layer. Finally, the decay time scale of the TC-induced SSTA is shown to be dominated by environmental conditions and has no clear dependence on its initial magnitude and on TC characteristics.

The thermodynamical and dynamical bases for understanding the static stability and the conditional instability of atmospheric and oceanic fluid columns are here presented. The theoretical treatment is done in parallel for air and seawater, to highlight similarities and differences between the two. Vertical kinetic energy, as a measure of the intensity of convection, is discussed in terms of instability metrics.

Mathematical models used to represent plankton dynamics often display limit-cycle behavior in a range of realistic parameter values. However, experimental data do not show evidence of plankton oscillations besides externally driven seasonal blooms, casting doubts on the validity of the models themselves. In this work we show that spatial-temporal variability, coupled with advection by mesoscale turbulence, can disguise limit-cycle behavior to the point that it cannot be detected in fixed-point measurements of plankton abundance. The results presented here have more general implications as they indicate that the behavior of ecosystem models in the presence of advection can be very different from that occurring for homogeneous conditions. Care should thus be exercised in drawing general conclusions from the analysis of homogeneous ecosystem models. © 2007 Elsevier Inc. All rights reserved.

A statistical convective adjustment scheme is proposed that attempts to account for the effects of mesoscale and submesoscale variability of temperature and salinity typically observed in the oceanic convective regions. Temperature and salinity in each model grid box are defined in terms of their mean, variance, and mutual correlations. Subgrid-scale instabilities lead to partial mixing between different layers in the water column. This allows for a smooth transition between the only two states (convection on and convection off) allowed in standard convective adjustment schemes. The advantage of the statistical parameterization is that possible instabilities associated with the sharp transition between the two states, which are known to occasionally affect the large-scale model solution, are eliminated. The procedure also predicts the generation of correlations between temperature and salinity and the presence of convectively induced upgradient fluxes that have been obtained in numerical simulations of heterogeneous convection and that cannot be represented by standard convective adjustment schemes.

Ocean warming is a predicting factor for typhoon intensity., Dominant climatic factors controlling the lifetime peak intensity of typhoons are determined from six decades of Pacific typhoon data. We find that upper ocean temperatures in the low-latitude northwestern Pacific (LLNWP) and sea surface temperatures in the central equatorial Pacific control the seasonal average lifetime peak intensity by setting the rate and duration of typhoon intensification, respectively. An anomalously strong LLNWP upper ocean warming has favored increased intensification rates and led to unprecedentedly high average typhoon intensity during the recent global warming hiatus period, despite a reduction in intensification duration tied to the central equatorial Pacific surface cooling. Continued LLNWP upper ocean warming as predicted under a moderate [that is, Representative Concentration Pathway (RCP) 4.5] climate change scenario is expected to further increase the average typhoon intensity by an additional 14% by 2100.

A simplified model is used to study the possible effects of the horizontal upper-ocean wind-driven circulation (WDC) on the variability of the overturning meridional circulation driven by buoyancy fluxes. It is found that the added interaction with the WDC adds interesting new classes of variability. First, self-sustained variability of the thermohaline circulation (THC) becomes possible, on time scales of interdecades to a few centuries. Furthermore, these oscillations may be either small amplitude or large amplitude and either periodic or chaotic, depending on the amplitude of the freshwater forcing and on the strength of the WDC. Even a relatively weak WDC changes the well-known stability properties of the THC that are seen in numerous models of the THC alone. The variability modes found here may account for similar modes of variability observed in GCM studies. © 2004 American Meteorological Society.

We investigate the performance of standard stochastic models of single-particle dispersion in two-dimensional turbulence. Owing to the presence of coherent vortices, particle dispersion in two-dimensional turbulence is characterized by a non-Gaussian velocity distribution and a non-exponential velocity autocorrelation, and it cannot be properly captured by either linear or nonlinear stochastic models with a single component process. Based on physical and dynamical considerations, we introduce a family of two-process stochastic models that provide a better parameterization of turbulent dispersion in rotating barotropic flows.

[1] Plankton patchiness is ubiquitous in the oceans, and various physical and biological processes have been proposed as its generating mechanisms. However, a coherent statement on the problem is missing, because of both a small number of suitable observations and an incomplete understanding of the properties of reactive tracers in turbulent media. It has been suggested that horizontal advection may be the dominant process behind the observed distributions of phytoplankton and zooplankton, acting to mix tracers with longer reaction times (Rt) down to smaller scales. Conversely, the relative distributions of sea surface temperature and phytoplankton has been attributed to small-scale upwelling, where tracers with longer Rt are able to homogenize more than those with shorter reaction times. Neither of the above mechanisms can explain simultaneously the (relative) spectral slopes of temperature, phytoplankton, and zooplankton. Here, with a simple advection model and a large suite of numerical experiments, we concentrate on some of the physical processes influencing the relative distributions of tracers at the ocean surface, and we investigate (1) the impact of the spatial scale of tracer supply, (2) the role played by coherent eddies on the distribution of tracers with different Rt, and (3) the role of diffusion (so far neglected). We show that diffusion determines the distribution of temperature, regardless of the nature of the forcing. We also find that coherent structures together with differential diffusion of tracers with different Rt impact the tracer distributions. This may help in understanding the highly variable nature of observed plankton spectra.

Strong winds affect mixing and heat distribution in the upper ocean. In turn, upper-ocean heat content affects the evolution of tropical cyclones. Here the authors explore the global effects of the interplay between tropical cyclones and upper-ocean heat content. The modeling study suggests that, for given atmospheric thermodynamic conditions, regimes characterized by intense (with deep mixing and large upper-ocean heat content) and by weak (with shallow mixing and small heat content) tropical cyclone activity can be sustained. A global general circulation ocean model is used to study the transient evolution of a heat anomaly that develops following the strong mixing induced by the passage of a tropical cyclone. The results suggest that at least one-third of the anomaly remains in the tropical region for more than one year. A simple atmosphere–ocean model is then used to study the sensitivity of maximum wind speed in a cyclone to the oceanic vertical temperature profile. The feedback between cyclone activity and upper-ocean heat content amplifies the sensitivity of modeled cyclone power dissipation to atmospheric thermodynamic conditions.

Geophysical flows are characterized by the presence of coherent vortices, localized concentrations of energy and vorticity that have a lifetime much longer than the local turbulence time (sometimes called the eddy turnover time). In the ocean, coherent vortices, or eddies, are ubiquitous features whose size varies between several to a few hundred kilometers, and that account for a large portion of the ocean turbulent kinetic energy [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. The presence of vortices can be revealed in various ways. Vortices at the ocean surface imprint their signature on the sea surface height and can be tracked by satellite, while floats with looping trajectories can help revealing the presence of vortices at depth. Coherent vortices significantly affect the dynamics and the statistical properties of ocean flows, with important consequences on transport processes. In this contribution, we shall briefly review some of these issues, focusing on the simplified conceptual model provided by two-dimensional turbulence. © 2008 Springer-Verlag Berlin Heidelberg.

As more high-resolution observations become available, our view of ocean mesoscale turbulence more closely becomes that of a “sea of eddies.” The presence of the coherent vortices significantly affects the dynamics and the statistical properties of mesoscale flows, with important consequences on tracer dispersion and ocean stirring and mixing processes. Here we review some of the properties of particle transport in vortex-dominated flows, concentrating on the statistical properties induced by the presence of an ensemble of vortices. We discuss a possible parameterization of particle dispersion in vortex-dominated flows, adopting the view that ocean mesoscale turbulence is a two-component fluid which includes intense, localized vortical structures with non-local effects immersed in a Kolmogorovian, low-energy turbulent background which has mostly local effects. Finally, we report on some recent results regarding the role of coherent mesoscale eddies in marine ecosystem functioning, which is related to the effects that vortices have on nutrient supply. Introduction The ocean transports heat, salt, momentum and vorticity, nutrients and pollutants, and many other material and dynamical quantities across its vast spaces. Some of these transport processes are at the heart of the mechanisms of climate variability and of marine ecosystem functioning. In addition, a large portion of the available data on ocean dynamics are in the form of float and drifter trajectories. These provide a Lagrangian view of the ocean circulation which is not always easy to disentangle.

We calculate the coupling constants of D*DSK and D*SDK vertices using the QCD sum rules technique. We compare our results with results obtained in the limit of SU(4) symmetry and we found that the symmetry is broken at the order of 40%.

Previous results from deep-sea pore fluid data demonstrate that the glacial deep ocean was filled with salty, cold water from the South. This salinity stratification of the ocean allows for the possible accumulation of geothermal heat in the deep-sea and could result in a water column with cold fresh water on top of warm salty water and with a corresponding increase in potential energy. For an idealized 4000 dbar two-layer water column, we calculate that there are ~106 J/m2 (~0.2 J/kg) of potential energy available when a 0.4 psu salinity contrast is balanced by a ~2°C temperature difference. This salt-based storage of heat at depth is analogous to Convectively Available Potential Energy (CAPE) in the atmosphere. The ‘‘thermobaric effect’’ in the seawater equation of state can cause this potential energy to be released catastrophically. Because deep ocean stratification was dominated by salinity at the Last Glacial Maximum (LGM), the glacial climate is more sensitive to charging this ‘‘thermobaric capacitor’’ and can plausibly explain many aspects of the record of rapid climate change. Our mechanism could account for the grouping of Dansgaard/Oeschger events into Bond Cycles and for the different patterns of warming observed in ice cores from separate hemispheres.

[1] Oceanic carbon uptake depends on the productivity of the marine ecosystem. Here we study the dependence of primary productivity on the spatial and temporal variability of the nutrient flux and the functional form used to parameterize it. We show that primary productivity is significantly affected by the form of the nutrient input. For a restoring flux, used to parameterize nutrient input by upwelling, primary productivity depends on the size and/or temporal duration of the upwelling events. For a fixed-flux nutrient input, we show that high-nutrient, low-chlorophyll (HNLC) regions can easily appear, without necessarily implying the lack of some micronutrient. These results have interesting implications on the interpretation of primary productivity estimates from observational data and ocean circulation models, and indicate a way to obtain upper and lower bounds to primary productivity in coarse-resolution models.

The marine ecosystem dynamics in the subtropical regions is controlled by the availability of nutrients. Their supply depends on lateral transport from higher latitudes, which is affected by the characteristics of the oceanic mesoscale turbulence. In many cases turbulent transport is parameterized in terms of eddy diffusion. We here test the validity of this approach in a mesoscale turbulence model coupled to a simple ecosystem model. We show that the parameterization can be profitably used, provided that an effective eddy diffusion coefficient that depends on the reaction time scale of the transported tracers is used. If the diffusion coefficient valid for conservative tracers is used to express diffusion of nutrients and plankton, turbulent transport and biological productivity can be significantly overestimated. Copyright 2005 by the American Geophysical Union.

The mechanism of banded iron formation (BIF) deposition is controversial, but classically has been interpreted to reflect ferrous iron [Fe(II)] oxidation by molecular oxygen after cyanobacteria evolved on Earth. Anoxygenic photoautotrophic bacteria can also catalyze Fe(II) oxidation under anoxic conditions. Calculations based on experimentally determined Fe(II) oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that, even in the presence of cyanobacteria, anoxygenic phototrophs living beneath a wind- mixed surface layer provide the most likely explanation for BIF deposition in a stratified ancient ocean and the absence of Fe in Precambrian surface waters.