The sea-surface microlayer (SML) is located at the air-sea interface and experiences instantaneous meteorological forcing by e.g. solar radiation, wind, and precipitation. Although solar radiation and wind-driven turbulence are known drivers of SML biogeochemical and physical properties, surprisingly little is known about the SML response to solar radiation. The latter is, however, important given that the SML is involved in all air–sea exchanges of mass and energy, especially in relation to how it regulates the air–sea exchange of climate-relevant gases and aerosols.

The international and multidisciplinary campaign MILAN (Sea Surface Microlayer at Night) was designed to characterize the SML during full diel cycles. MILAN addressed the scientific fields of marine (micro)biology, biogeochemistry, marine chemistry, atmospheric chemistry and physics, and physical oceanography using diverse approaches in the field and in the laboratory to study the diel properties of the SML and their effects on the air–sea exchange of climate-relevant gases and aerosols.

In spring 2017, the radio-controlled catamaran Sea Surface Scanner (S³) and research vessels followed a passively drifting CO2 buoy during diel cycles in the coastal North Sea. Meteorological conditions and water currents were recorded continuously, supported by observations from land-based weather stations. Water column physical properties were profiled every hour. S³ continuously measured physicochemical properties of the SML and from 1 m water depth, and collected large-volume water samples for subsequent analyses in the laboratory, for laboratory experiments using a gas-exchange tank, a solar simulator, and a sea spray simulation chamber, and for microsensor experiments. A land-based aerosol sampler collected aerosol samples continuously throughout the campaign.

This presentation will highlight initial results of the MILAN campaign, which point to a radiation dependence of several SML processes, such as increasing lipid degradation in the SML during the night, or the dose-dependent enrichment of specific phytoplankton groups in the SML. Other, seemingly contradictors results will be discussed, such as the finding of highest surfactant concentrations in the field during night, while experiments with the solar simulator clearly implied daytime surfactant production. The diel dynamics of SML organisms and organic material will be put into the context of air–sea exchange processes, as one important finding shows different day and night CO₂ fluxes under low wind speed conditions (<2.5 m s^–1). Taken together, MILAN underlines the value and the need of multidisciplinary campaigns for integrating SML complexity into the context of air–sea processes that have important implications for biogeochemical cycles and climate regulation.

How to cite: Ribas-Ribas, M., Stolle, C., and Wurl, O. and the MILAN team: Studying Diel Light Effects on the Air–Sea Interface - The MILAN Campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3956, https://doi.org/10.5194/egusphere-egu21-3956, 2021.

Introduction:
Breaking waves on the ocean surface lead to sea spray aerosol emission to the atmosphere. Sea spray aerosols are a major source of uncertainty in climate models. The physical processes governing sea spray aerosol production play an important part in determining sea spray aerosol emission, size distribution, and chemical composition. Sea spray often contains organic material, but it is unclear how this material affects the ability of particles to act as cloud condensation nuclei (CCN).

Methods:
We have measured the CCN-derived hygroscopicity of different types of aerosol particles generated from the following seawater proxies and real seawater using a sea spray simulation tank (Christiansen et al., 2019), AEGOR, or an atomizer in a laboratory setup (Christiansen et al., 2020): 

• Artificial seawater
• Artificial seawater spiked with diatoms cultured in the laboratory
• Samples of sea surface microlayer (SML) collected during field campaigns in the North Atlantic and Arctic Ocean.
• A continuous supply of fresh seawater during a three-week field campaign (June 2019) on the Faroe Islands, while following oceanic biogeochemical parameters.   

Large-eddy simulation (LES) has been used to evaluate the general role of aerosol hygroscopicity in governing mixed-phase low-level cloud properties in the high Arctic.

Conclusions: 

• We show that sea spray aerosols generated using diatom cultures and surface microlayer water exhibit CCN activity similar to that of inorganic sea salt (κ value of ∼1.0), independent of dry particle size (50, 75, and 100 nm).
• The critical supersaturation of dry 80 nm SSA was relatively invariable (0.158±0.04%), corresponding to the overall hygroscopicity parameter κ of 1.08±0.05% derived from CCN during the phytoplankton bloom. This is despite indications that the chemical composition of both the seawater and the SSA were impacted by the presence of the phytoplankton.
• For accumulation mode aerosol, the simulated mixed-phase cloud properties do not depend strongly on κ, unless κ < 0.4. In addition, the cloud is sustained for all simulated cases.
• For Aitken mode aerosol, the hygroscopicity is more important changing the microphysical structure of the cloud and its radiative properties; here the particles can sustain the cloud only when κ ≥ 0.4. 

The experimental and model results combined suggest that the internal mixing of biogenic organic components in SSA does not have a substantial impact on the cloud droplet activation process and the cloud lifetime in Arctic mixed-phase clouds.

References:
Christiansen et al. (2020). J. Geophys. Res. Atm. https://doi.org/10.1029/2020JD032808
Christiansen et al. (2019). Environ. Sci. Technol. https://doi.org/10.1021/acs.est.9b04078 

How to cite: Christiansen, S., Ickes, L., Bulatovic, I., Leck, C., Murray, B., Bertram, A., Wagner, R., Gorokhova, E., Salter, M., Ekman, A., and Bilde, M.: Influence of sea surface microlayers and phytoplankton blooms on sea spray aerosol hygroscopicity and the possible implications for mixed-phase clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9765, https://doi.org/10.5194/egusphere-egu21-9765, 2021.

During the last couple of decades the sea-ice cover on Isfjorden at the west coast of Spitsbergen in Svalbard has seen a dramatic reduction, which has been linked to, amongst others, regional and local changes in weather patterns. As Isfjorden is the most heavily trafficked fjord in Svalbard, these changes directly impact all kinds of operations at sea. Therefore, good information about the atmospheric state over the fjord system does not only enhance our scientific understanding of air-ice-sea interactions and the local processes leading to the formation of sea ice, but furthermore contribute to planning and conducting field activities in a safer manner.

With a horizontal resolution of 2.5 km, the current operational version of the AROME-Arctic weather forecasting model of the Norwegian Meteorological Institute can provide a good overall representation of the atmospheric state over the Isfjorden fjord system. However, the complex topography, as well as fine-scale variations in the surface cover and the sea surface temperature due to the oceanographic circulation within the fjord, lead to local variabilities of atmospheric variables, which are only poorly resolved by the model. Amongst others, high-wind events and associated phenomena like channeling effects are suspected to have a large effect on both the air-ice-sea interactions and the formation of sea ice within the fjord as well as the safety at sea.

Therefore, we aim at establishing an automatic meteorological measurement network across Isfjorden. The network will consist of several all-in-one weather stations deployed at lighthouse stations all around the fjord. Additionally, mobile stations will be installed onboard small tourist fjord cruise ships. In that way, small-scale local variations in near-surface atmospheric wind and temperature fields can be resolved and their changes can be monitored throughout the year. By making use of already existing infrastructure as platforms for the instrumentation, the high-resolution measurements can be performed in remote areas at low costs and with a minimal environmental impact. In the end, a real-time transfer of the measured data via the cellular network will additionally provide very valuable information for planning and execution of field activities performed by e.g. UNIS, tourist companies, private individuals or the Governor of Svalbard.

We will in detail present the measurement network, the status of its setup and first results. A special focus will be put on the comparison of the measurements with the AROME-Arctic model data.

How to cite: Frank, L., Opsanger Jonassen, M., Claes, S., and Schalamon, F.: Establishing an automatic atmospheric measurement network across an Arctic fjord system in Svalbard, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12481, https://doi.org/10.5194/egusphere-egu21-12481, 2021.

Remoteness and tough conditions have made the Arctic Ocean historically difficult to access; until recently this has resulted in an undersampling of trace gas and gas exchange measurements. The seasonal cycle of sea ice completely transforms the air sea interface and the dynamics of gas exchange. To make estimates of gas exchange in the presence of sea ice, sea ice fraction is frequently used to scale open water gas transfer parametrisations. It remains unclear whether this scaling is appropriate for all sea ice regions. Ship based eddy covariance measurements were made in Hudson Bay during the summer of 2018 from the icebreaker CCGS Amundsen. We will present fluxes of carbon dioxide (CO₂), heat and momentum and will show how they change around the Hudson Bay polynya under varying sea ice conditions. We will explore how these fluxes change with wind speed and sea ice fraction. As freshwater stratification was encountered during the cruise, we will compare our measurements with other recent eddy covariance flux measurements made from icebreakers and also will compare our turbulent CO₂ fluxes with bulk fluxes calculated using underway and surface bottle pCO₂ data. 

How to cite: Sims, R., Butterworth, B., Papakyriakou, T., Ahmed, M., and Else, B.: Eddy covariance flux measurements of momentum, heat and carbon dioxide in Hudson Bay at the onset of sea ice melt , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15711, https://doi.org/10.5194/egusphere-egu21-15711, 2021.

Stable water isotopes in marine boundary layer water vapour are strongly influenced by the strength of air-sea moisture fluxes and are thus tracers of air-sea interaction. Air-sea moisture fluxes in the extratropics are modulated by large-scale air advection, for instance the advection of warm and moist air masses in the warm sector of extratropical cyclones. A distinct isotopic composition of water vapour in the latter environment has been observed in near-surface water vapour over the Southern Ocean during the 2016/17 Antarctic Circumnavigation coordinated by the Swiss Polar Institute. Most prominently, the second-order isotope variable d-excess shows negative values in the cyclones’ warm sector. Here, we present three single-process air parcel models, which simulate the evolution of d-excess and specific humidity in an air parcel induced by dew deposition, decreasing ocean evaporation or upstream cloud formation, respectively. The air-parcel models are combined with simulations with the isotope-enabled numerical weather prediction model COSMO_iso (i) to validate the air parcel models, (ii) to study the extent of non-linear interactions between the different processes, and (iii) to quantify the relevance of the three processes for stable water isotopes in the warm sector of the investigated extratropical cyclone. This analysis reveals that dew deposition and decreasing ocean evaporation lead to the strongest d-excess decrease in near-surface water vapour in the warm sector. Furthermore, COSMO_iso air parcel trajectories show that the persistent low d-excess observed in the warm sector of extratropical cyclones is not a result of material conservation of low d-excess. Instead the latter feature is sustained by the continuous production of low d-excess values in new air parcels entering the warm sector. We show that with the mechanistic approach of using single-process air parcel models we are able to simulate the evolution of d-excess during the air parcel’s transport. This improves our understanding of the effect of air-sea interaction and boundary layer cloud formation on the stable water isotope variability of marine boundary layer water vapour.

How to cite: Thurnherr, I., Wernli, H., and Aemisegger, F.: Disentangling the impact of air-sea interaction and boundary layer cloud formation on stable water isotope signals in the warm sector of a Southern Ocean cyclone , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4277, https://doi.org/10.5194/egusphere-egu21-4277, 2021.

The study deals with the thermodynamic characterization of marine atmospheric boundary layer (MABL) prevailing over regions of Indian Ocean and Indian Ocean sector of Southern Ocean from 29 high-resolution radiosondes launched during the International Indian Ocean Expedition (IIOE-2) and Southern Ocean Expedition (SOE-9). IIOE-2 was conducted during December 2015 onboard ORV Sagar Nidhi during which 11 radiosondes were launched, whereas SOE-9 was conducted during January-March 2017 onboard MV SA Agulhas which had 18 radiosonde ascents. These observations spanned latitudes from ~15^oN to 70^oS having crossed three major atmospheric circulation cells: Hadley cell, Ferrell cell and Polar cell. In addition, crucial atmospheric mesoscale phenomena such as inter-tropical convergence zone (ITCZ), sub-tropical jet (STJ) and polar jet (PJ) were encountered along with several oceanic fronts. Analysis of thermodynamic structure of MABL showed large variability in the formation of atmospheric sub-layers such as surface layer, mixed layer, cloud layer and trade wind inversion layer within MABL. MABL height varied spatially from tropics and mid-latitudes (12^oN to 50^oS) to polar latitudes (60^oS to 68^oS). Deep mixed layer were found over the tropics and mid-latitudes (~700 m) while shallow mixed layer was observed over the polar latitudes (~200 m). Deep mixed layer over the tropics were attributed to intense convective mixing while shallow mixed layer over polar regions was attributed to limited convective overturning associated with negative radiation balance at the surface. Convection was negligible over mid-latitudes (43^oS to 55^oS) where most of the atmospheric mixing were forced by frontal systems where lifting of air mass was mechanically driven by high speed winds rather than by convection. The enhanced convection over the tropics was confirmed from higher values of convective available potential energy (CAPE > 1000 J/kg) and large negative values of convective inhibition energy (CINE < -50 J/kg). Over the mid-latitude region (43^oS to 50^oS), enhanced advection and detrainment of convection was evident with maximum values of BRN shear (~65 knots) and lowest CAPE (~4 J/kg). Over polar latitudes (~60^oS to 68^oS), minimum CAPE (~17 J/kg) and low BRN shear (~5 knots) was noticed, which indicated presence of stable boundary layer conditions. A mesoscale phenomenon (i.e., ITCZ) was witnessed at ~5.92^oS with highest CAPE ~2535.17 J/kg which signifies large convective instability resulting in strong convective updraft aiding thunderstorm activity and moderate precipitation over ITCZ. Analysis of conserved variables (CVA) revealed formation of second mixed layer (SML) structure between 12^oN and 40^oS. However, south of 40^oS this structure ceases. The characteristics of SML structure and the plausible causes for its existence are also investigated.  

How to cite: Salim, N., Menon, H. B., and Kiran Kumar, N. V. P.: Spatio-temporal variability of the thermodynamic characteristics of the marine atmospheric boundary layer (MABL) over the Indian and Southern Ocean (15oN to 70oS), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13969, https://doi.org/10.5194/egusphere-egu21-13969, 2021.

Many formulations to determine the sea surface roughness length (z₀) have been proposed in the past. The well-known Charnock’s equation is applied in most of the previous research. In this study, a different point of view is adopted to develop a new formulation. The starting point is an alternative method for surface roughness length calculation, i.e., the Lettau’s method. This method has already been validated onshore in the presence of obstacles over a domain; for obstacles with a defined cross-section perpendicular to the wind direction plane. Over deep waters, it is expected to find only one type of obstacle, i.e., consecutive waves forming straight lines. Different wave systems and the presence of swell add complexity to determine the sea surface profile. Hence, the adaptation of Lettau’s method seems reasonable, but the demonstrated dependency of z₀ to wave age cannot be neglected.

Wind-generated waves result from a kinetic energy transfer between the atmosphere and the sea surface. However this physical process is not represented in the well-known logarithmic law. While this effect can be neglected onshore, in offshore environments it can be significant, as 20% of the time z₀ is found to be over the expected range. Therefore, a kinetic energy transfer correction is included into an offshore logarithmic law. With an aerodynamic z₀, achieved by the adaptation of the Lettau’s equation, and the new offshore logarithmic law, an empirical method for the kinetic energy transfer correction is proposed.

How to cite: Rabaneda, A.: Development of surface roughness length and drag parameterizations over deep seas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12069, https://doi.org/10.5194/egusphere-egu21-12069, 2021.

The polar ocean regions are characterised by a large variety of interactions between sea ice surfaces, open water, and the atmosphere. Especially between late autumn and spring, leads (open-water channels in sea ice) may play a crucial role within this system: Due to large temperature differences between the surface of leads and the near-surface atmosphere, strong turbulent convective plumes are generated with an enhanced turbulent transport of heat, moisture, and momentum. In consequence, lead-generated convection has a strong impact on the characteristics of the polar atmospheric boundary layer (ABL).

We apply a plume- but non-eddy-resolving, microscale model to study the convection over three different leads, which had been observed during the aircraft campaign STABLE over the Arctic Marginal Sea Ice Zone in March 2013. Model simulations are performed using a local and a non-local turbulence closure. The latter represents a lead-width-dependent approach for the turbulent fluxes based on large eddy simulation and it is designed for an idealised, lead-perpendicular, and near-neutral inflow in an ABL of 300m thickness. The observed cases from STABLE are also characterised by lead-perpendicular inflow conditions, but the ABL is much shallower than in the idealised cases and the inflow stratification is partly (slightly) stable. Our main goal is to study the quality of both parametrizations and to evaluate, if the non-local parametrization shows advantages as compared to the local closure.

We show that the basic observed features of the lead-generated convection are represented with both closures despite some minor differences that will be explained. However, the advantages of the non-local closure become clearly obvious by the physically more realistic representation of regions with observed vertical entrainment or where the observations hint at counter-gradient transport. Moreover, we also show that some weaknesses of the simulations can be almost overcome by introducing two further modifications of the non-local closure. We consider our results as another important step in the development of atmospheric turbulence parametrizations for non-eddy-resolving, microscale simulations of strongly inhomogeneous convective boundary layers.

How to cite: Michaelis, J., Lüpkes, C., Schmitt, A., and Hartmann, J.: Non-eddy-resolving modelling and parametrization of turbulent convection over sea ice leads and evaluation with airborne observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6661, https://doi.org/10.5194/egusphere-egu21-6661, 2021.

Traditional estimates of convection/water mass formation at the sea surface rely on measurements of air-sea fluxes of heat and freshwater
(evaporation minus precipitation), that are estimated by combining in-situ data with meteorological modelisation. Satellite-based estimates of ocean convection are thus largely impacted by the relatively high uncertainties and low space-time resolution of those fluxes. However, direct satellite measurements of the ocean surface offer a unique opportunity to study convection (upwelling, downwelling) events with unprecedented spatio-temporal resolution compared to in-situ measurements. In this work, we propose an alternative approach to the traditional framework for estimating ocean convection using satellites. Instead of combining high-resolution ocean data of sea surface temperature and salinity with the much less precise, less resolved air-sea interaction data, we estimate the air-sea fluxes by computing the material derivatives (using satellite ocean currents) of the satellite sea surface variables. We therefore obtain estimates at the same resolution of the satellite products, and with much better accuracy than what was estimated before. We present some examples of application in the Atlantic ocean and in the Mediterranean sea. Future directions of this work is the study of the seasonal and interannual variability of ocean convection, and the potential changes on deep convection associated to climate variability at different time scales.

How to cite: Piracha, A., Turiel, A., Olmedo, E., and Portabella, M.: An update to the traditional water mass (trans)formation framework., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12354, https://doi.org/10.5194/egusphere-egu21-12354, 2021.

Air-sea turbulent heat fluxes and their spatial gradients are important to the ocean, climate, weather, and their interactions. Satellite-based estimation of air-sea latent and sensible fluxes, providing broad coverage, require measurements of sea surface temperature, ocean-surface wind speed, and air temperature and humidity above sea surface. Because no single satellite has been able to provide simultaneous measurements of these input variables, they typically come from various satellites with different spatial resolutions and sampling times that can be offset by hours. These factors introduce errors in the estimated heat fluxes and their gradients that are not well documented. As a model-based assessment of these errors, we performed a simulation using a Weather Research and Forecasting (WRF) model forced by high-resolution blended satellite SST for the Gulf Stream extension region with a 3-km resolution and with 30-minute output. Latent and sensible heat fluxes were first computed from input variables with the original model resolutions and at coincident times. We then computed the heat fluxes by (1) decimating the input variables to various resolutions from 12.5 to 50 km, and (2) offsetting the “sampling” times of some input variables from others by 3 hours. The resultant estimations of heat fluxes and their gradients from (1) and (2) were compared with the counterparts without reducing resolution and without temporal offset of the input variables. The results show that reducing input-variable resolutions from 12.5 to 50 km weakened the magnitudes of the time-mean and instantaneous heat fluxes and their gradients substantially, for example, by a factor of two for the time-mean gradients. The temporal offset of input variables substantially impacted the instantaneous fluxes and their gradients, although not their time-mean values. The implications of these effects on scientific and operational applications of heat flux products will be discussed. Finally, we highlight a mission concept for providing simultaneous, high-resolution measurements of boundary-layer variables from a single satellite to improve air-sea turbulent heat flux estimation.

How to cite: Lee, T., Centemann, C., Clayson, C. A., Bourassa, M., Brown, S., Farrar, T., Lombardo, K., Gille, S., Parfitt, R., Seo, H., Subramanian, A., and Zlotnicki, V.: Effects of spatial resolution and temporal offset of air-sea boundary-layer variables on turbulent heat flux estimates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3813, https://doi.org/10.5194/egusphere-egu21-3813, 2021.

The exchanges, or fluxes, of heat, moisture and momentum between the atmosphere and the Earth's surface play a crucial role in the Earth's climate system, but the quantification of these fluxes remains challenging due to the non-linearity of surface dynamics and turbulent processes. Bulk formulae are frequently used to estimate surface turbulent fluxes from observed mean (or bulk) meteorological quantities. Uncertainties are inherent in the parameterisations as they rely on observations that are themselves uncertain and may not sample the full range of important conditions or include all of the variables that may affect the fluxes. As a consequence, different parameterisations of the bulk formulae may give different estimates of the fluxes.

AirSeaFluxCode is an open-source software package implemented in Python 3.6 for the computation of surface turbulent fluxes of heat (latent and sensible) and momentum. It includes ten different parameterisations, each based on published algorithms. The parameterisations implemented in the new package can be used to caclulate the fluxes using a small number of input parameters that are typically observed or available as model output: wind speed; air temperature; sea surface temperature; atmospheric pressure; and humidity. Some parameterisations require the input of a "skin" sea surface temperature so code to relate skin temperature to temperature at depth is included. In addition the code can be used to do height corrections of mean parameters.

This open-source software package is intended to be accessible, easy to use in its default implementation, and to provide a more informed choice of suitable parameterisations for particular applications.

How to cite: Biri, S., Kent, E., Berry, D., Cornes, R., and Yelland, M.: AirSeaFluxCode: Open-source software for calculating turbulent air-sea fluxes from meteorological parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2947, https://doi.org/10.5194/egusphere-egu21-2947, 2021.

Large oceanic eddies are formed by the retroflection of the North Brazil Current (NBC) near 8°N in the western tropical Atlantic. The EUREC⁴A-OA/Atomic cruise took place in January - February 2020, and extensively documented two NBC rings. The NBC flows northward across the Equator and pass the mouth of the Amazon River, entraining fresh and nutrient-rich water along its nearshore edge. From December to March, the Amazon river discharge is low but a freshwater filament stirred by a NBC ring was nevertheless observed. The strong salinity gradient can be used to delineate the NBC ring during its initial phase and its westward propagation. Using satellite sea surface salinity and ocean color associated to in-situ measurements of salinity, temperature, dissolved inorganic carbon, alkalinity and fugacity of CO₂ we characterize the salinity and biogeochemical signature of NBC rings.

How to cite: Olivier, L., Boutin, J., Lefèvre, N., Reverdin, G., Landschützer, P., Speich, S., and Karstensen, J.: Impact of mesoscale eddies on salinity and CO2 ocean parameters in the western tropical Atlantic in February 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1253, https://doi.org/10.5194/egusphere-egu21-1253, 2021.

Relative wind stress (calculated by including the surface current terms) is known to remove energy from mesoscale eddies, but how they respond to this damping mechanism over their lifetime is poorly understood. A method for predicting eddy energy is made by time stepping forward the energy equation of a linear two-layer model using an analytical relative wind stress damping term. Results of this prediction are then compared with numerical experiments of an idealised two-layer anticyclonic eddy in a high-resolution general circulation model. The energy in both experiments displays a quantitative agreement in relative wind stress damping, though this is not the case when the eddy in the numerical experiment becomes baroclinically unstable. In addition to this well-known relative wind stress damping mechanism, we found that relative wind stress can trigger eddy instabilities sooner, leading to quicker decay. The earlier onset of these instabilities by relative wind stress is observed in a Lorenz energy cycle.

How to cite: Wilder, T., Zhai, X., Joshi, M., and Munday, D.: Role Of Relative Wind Stress In Generating Eddy Instabilities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10862, https://doi.org/10.5194/egusphere-egu21-10862, 2021.

The ongoing development of NWP (Numerical Weather Prediction) models and their increasing horizontal resolution have significantly improved forecasting capabilities. However, in the polar regions models struggle with the representation of near-surface atmospheric properties and the vertical structure of the atmospheric boundary layer (ABL) over sea ice. Particularly difficult to resolve are near-surface temperature, wind speed, and humidity, along with diurnal changes of those properties. Many of the complex processes happening at the interface of sea ice and atmosphere, i.e. vertical fluxes, turbulence, atmosphere - surface coupling are poorly parameterized or not represented in the models at all. Limited data coverage and our poor understanding of the complex processes taking place in the polar ABL limit the development of suitable parametrizations. We try to contribute to the ongoing effort to improve the forecast skill in polar regions through the analysis of unmanned aerial vehicles (UAVs) and automatic weather station (AWS) atmospheric measurements from the coastal area of Bothnia Bay (Wenta et. al., 2021), and the application of those datasets for the analysis of regional NWP models' forecasts. 

Data collected during HAOS (Hailuoto Atmospheric Observations over Sea ice) campaign (Wenta et. al., 2021) is used for the evaluation of regional NWP models results from AROME (Applications of Research to Operations at Mesoscale) - Arctic, HIRLAM (High Resolution Limited Area Model) and WRF (Weather Research and Forecasting). The presented analysis focuses on 27 Feb. 2020 - 2 Mar. 2020, the time of the HAOS campaign, shortly after the formation of new, thin sea ice off the westernmost point of Hailuoto island.  Throughout the studied period weather conditions changed from very cold (-14℃), dry and cloud-free to warmer (~ -5℃), more humid and opaquely cloudy. We evaluate models’ ability to correctly resolve near-surface temperature, humidity, and wind speed, along with vertical changes of temperature and humidity over the sea ice. It is found that generally, models struggle with an accurate representation of surface-based temperature inversions, vertical variations of humidity, and temporal wind speed changes. Furthermore, a WRF Single Columng Model (SCM) is launched to study whether specific WRF planetary boundary layer parameterizations (MYJ, YSU, MYNN, QNSE), vertical resolution, and more accurate representation of surface conditions increase the WRF model’s ability to resolve the ABL above sea ice in the Bay of Bothnia. Experiments with WRF SCM are also used to determine the possible reasons behind model’s biases. Preliminary results show that accurate representation of sea ice conditions, including thickness, surface temperature, albedo, and snow coverage is crucial for increasing the quality of NWP models forecasts. We emphasize the importance of further development of parametrizations focusing on the processes at the sea ice-atmosphere interface.

Reference:

Wenta, M., Brus, D., Doulgeris, K., Vakkari, V., and Herman, A.: Winter atmospheric boundary layer observations over sea ice in the coastal zone of the Bay of Bothnia (Baltic Sea), Earth Syst. Sci. Data, 13, 33–42, https://doi.org/10.5194/essd-13-33-2021, 2021. 

How to cite: Wenta, M. and Herman, A.: Evaluation of regional NWP results for sea ice covered Bothnia Bay (Baltic Sea) in winter 2020., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14612, https://doi.org/10.5194/egusphere-egu21-14612, 2021.

The Northwest Tropical Atlantic is characterized by the strong North Brazilian Current (NBC), its rings, and numerous mesoscale eddies, which ceaselessly interact with the persistent trade winds and trade cumuli. Near the coast, the ocean stratification is maintained by the Amazon and Orinoco river discharges, which control the vertical mixing and the near-shore circulation dynamics. Breaking waves and swells are ubiquitous under the trade winds, and hence, the wave-induced mixing and wave-mediated air-sea fluxes are expected to modulate the eddy variability and low-level clouds. Our study aims to enhance understanding of the air-sea fluxes mediated by the mesoscale ocean currents and surface waves and evaluate their impacts on the ocean and atmosphere.

High-resolution ocean model (ROMS) and wave model (WW3) simulations are conducted for the period of the ATOMIC/EUREC4A experiments. The model surface state variables are used to compute offline the air-sea heat and momentum fluxes using the latest COARE v3.6 bulk flux algorithm under various sea state conditions induced by surface waves, ocean currents, and their interaction. The results demonstrate that considering the spatial variability in sea states via wave slope and wave age (e.g., swells and wind-seas) leads to enhanced spatial variability in drag coefficient and wind stress. Comparison to wind stress estimated using the wind-speed dependent formulation, meaning that COARE makes sea state assumptions under given wind, indicates that, at any given time, wind and wave in fact, rarely match those assumptions. The swells (wind-seas) decreases (increases) the sea surface roughness length, drag coefficient, and wind stress by 10-15%. However, we find that the sea state impact on turbulent heat flux is negligible.

More importantly, we also show that considering the ocean currents in the COARE algorithm yields much stronger spatio-temporal variations in not just the wind stress but also turbulent heat fluxes. The intense and small-scale current fields in this region are associated with the NBC and its rings, smaller mesoscale eddies, and filamentary density fronts associated with the freshwater plumes. The surface currents associated with these small-scale energetic features alter the relative wind speed and thus the air-sea fluxes depending on the directional alignment between the wind and current; the increase (decrease) in both the wind and heat fluxes by ~20% is found with the current and wind are in the opposite (same) direction wind. Moreover, this relative wind effect appears to be reinforced by wave direction as well, also via the directional alignment between waves and currents, since the waves are mainly aligned with the trade wind in this region.

Further analyses are underway to examining the seasonality of the modulation by the wave-current interaction, quantifying the role of the freshwater distribution, and exploring the time-mean influence on the low-level clouds. The results from the ocean and wave modeling efforts will guide our ongoing fully coupled ocean-atmosphere (and wave) model simulations to quantify their impacts on the atmosphere, including low-level clouds.

How to cite: Sauvage, C., Seo, H., Clayson, C. A., and Edson, J.: Effects of Sea State and Small-Scale Currents on Air-Sea Fluxes in the Northwest Tropical Atlantic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13820, https://doi.org/10.5194/egusphere-egu21-13820, 2021.

Sea Surface Temperature (SST) is known to affect the marine atmospheric boundary layer (MABL) at scales smaller than O(1000 km) via different mechanisms. In particular, the oceanic thermal forcing induces modification in the wind speed, its divergence and its curl by the action of the Downward Momentum Mixing (DMM) mechanism and the Pressure Adjustment (PA) one. 

By analyzing 25 years of observations of surface wind speed and SST in the Mediterranean, it is found that the probability of observing surface wind convergence is significantly higher over a thermal oceanic front crossed from the warm to the cold side, in agreement with the DMM mechanism. Physically, this is due to a deceleration of the surface wind over the cold side of the SST front because of the increased atmospheric stability over the cold water. The strongest response in terms of surface convergence is found when atmospheric fronts (already characterized by strong surface convergence) cross SST gradients from the warm to the cold side.

Using 25 years of ERA5 reanalysis data, it is also found that the wind divergence variability within the MABL (until about 925 hPa) is partially driven by mesoscale SST patterns via their effect on the boundary layer stability. This results in a cloud cover and rainfall response: when a wind blows from warm-to-cold (cold-to-warm) ocean patterns, a converging (diverging) cell is enhanced, increasing (decreasing) low-cloud cover and favouring rainfall. Specifically, strong warm-to-cold fronts (the upper 25th percentile) 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 than in the rest of the year, probably due to the stronger SST gradients present at the end of the summer season. The effects on cloud cover, in particular, are a preferential way through which mesoscale SST structures can impact the radiation budget and, thus, the Earth climate.

How to cite: Desbiolles, F., Meroni, A., Alberi, M., Hamouda, M. E., Giurato, M., Ragone, F., and Pasquero, C.: Climatic effects of mesoscale sea frontal structures in the Mediterranean Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5208, https://doi.org/10.5194/egusphere-egu21-5208, 2021.

Regional high resolution biogeochemical modeling studies generaly use an oceanic model forced by prescribed atmospheric conditions. The computational cost of such approach is far lower than using an high resolution ocean-atmosphere coupled model. However, forced oceanic models cannot represent adequately the atmospheric reponse to the oceanic mesoscale (~10-100km) structures and the impact on the oceanic dynamics.

To assess the bias introduce by the use of a forced model, we compare here a regional high resolution (1/12º) ocean-atmosphere coupled model with oceanic simulations forced by the outputs of the coupled simulation. Several classical forcing strategies are compared : bulk formulae, prescribed stress, prescribed heat fluxes with or without Sea Surface Temperature (SST) restoring term, .... We study the Chile Eastern Boundary Upwelling System, and the oceanic model includes a biogeochemical component,

The coupled model oceanic mesoscale impacts the atmosphere through surface current and SST anomalies. Surface currents mainly affect the wind stress while SST impacts both the wind stress and the heat fluxes. In the forced simulations, mesoscale structures generated by the model internal variability does not correspond to those of the coupled simulation. According to the forcing strategy, the atmospheric conditions are not modified by the forced model mesoscale, or the modifications are not realistic. The regional dynamics (coastal upwelling, mesoscale activity, …) is affected, with impact on the biogeochemical activity.

This work was supported by the FONDECYT project 3180472 (Chile), with computational support of the NLHPC from the Universidad de Chile, the HPC from the Pontificia Universidad Catolica de Valparaiso and the Irene HPC from the GENCI at the CEA (France).

How to cite: Oerder, V., Auger, P.-A., Bento, J., and Hormazabal, S.: Regional oceanic and biogeochemical modeling strategy :forced or coupled model ?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9240, https://doi.org/10.5194/egusphere-egu21-9240, 2021.

Air-sea interactions have been found to substantially affect and drive marine extreme events. Such extreme events comprise, among others, highly anomalous conditions in ocean temperature, pH, and oxygen content - all of which are crucial parameters directly impacting marine ecosystem. Nevertheless, our understanding of the role of such events in the marine environment remains limited. In addition, the extent to which the interplay between atmospheric and oceanic forcings impacts the spatial and temporal scales of extreme events and affects the marine ecosystem and ocean biogeochemistry remains largely unknown.

Given these complex interactions between the atmosphere, the ocean, and marine biogeochemistry, we developed a coupled regional high-resolution Earth System Model (ROMSOC). ROMSOC comprises the latest officially released GPU-accelerated Consortium for Small-Scale Modeling (COSMO) version as the atmospheric model, coupled to the Regional Oceanic Modeling System (ROMS). ROMS in turn includes the Biogeochemical Elemental Cycling (BEC) model that describes the functioning of the lower trophic ecosystem in the ocean and the associated biogeochemical cycle. Our current model setup includes thermodynamical coupling and will be extended further to include mechanical coupling between the atmosphere and the ocean. Here, we present first simulations of our coupled model system for the California Current System (CalCS) at the US west coast at kilometer-scale resolution. We will test the hypothesis if the strong mesoscale coupling of the atmosphere and the ocean as represented in our model impacts the spatial and temporal scales of marine heatwaves and can potentially act to shorten their duration.

How to cite: Eirund, G., Münnich, M., Leclair, M., and Gruber, N.: Marine extreme events in high-resolution coupled model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14193, https://doi.org/10.5194/egusphere-egu21-14193, 2021.