This session aims at fostering discussions on the physical processes at work at the air-sea interface, including their observation and their representation in coupled numerical models. Examples of such processes are solar radiation-induced diurnal warming and rain-induced cool and fresh lenses, as well as gustiness associated with atmospheric boundary layer thermals or moist convection and cold pools induced by rain evaporation. Air-sea interaction related to surface temperature and salinity fronts, as well as oceanic meso- and sub-mesoscale dynamics, are also of great interest.

This session is thus intended for (i) contributions presenting observational or theoretical aspects of the processes described above and their impact on energy, water, momentum, gas and aerosols exchanges at the interface; and (ii) contributions focusing on the mathematical and algorithmic methods used to represent these processes in coupled ocean-atmosphere models.

This session seeks observational studies based on recent field campaigns or satellite remote sensing. This session also aims to gather studies using numerical models of any level of complexity (from highly idealized to realistic) and any resolution from Large Eddy Simulation (LES) to global circulation models. Studies describing the impact of the air-sea interaction physical processes on the mean global or regional climates and variability representation are also welcome.

Co-organized by OS4
Convener: Hugo Bellenger | Co-conveners: Kyla Drushka, Audrey Hasson, Brian Ward
| Attendance Fri, 08 May, 16:15–18:00 (CEST)

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Chat time: Friday, 8 May 2020, 16:15–18:00

Chairperson: Hugo Bellenger
D2813 |
| solicited
| Highlight
Aurélie Moulin, James Moum, and Emily Shroyer

Freshwater lenses (FWL) deposited by rain create surface salinity and temperature anomalies that can persist for extended periods of time (> 1 day). The resulting patchiness in near-surface density and sea surface temperature influence upper ocean dynamics and air-sea fluxes of heat. For these reasons, understanding lens formation and evolution has been a focus of recent observational and modeling efforts. The work presented here integrates near-surface ocean and atmosphere time series with remote sensing of sea surface roughness (X-band radar) and precipitation (C-band radar) to describe the formation and temporal evolution of lenses within the equatorial Indian Ocean. Twenty-six FWLs are observed at different stages of their evolution from freshly deposited and actively spreading to older, passively advected features. Salinity anomalies reached -1.2 psu near the surface, while temperature anomalies were observed to be both cool (down to -0.8°C) and warm (up to +0.4°C). The largest density anomaly reached -0.5 kg/m3. Remotely-sensed, ship-based radar imagery allows for quantification of the observed propagation speeds of ten lenses, which follow internal gravity wave theory. These results offer a novel perspective on the evolution of FWLs whose dynamics need to be properly accounted for to assess lens longevity, including persistence of salinity and temperature anomalies, as well as influences to air-sea interactions.

How to cite: Moulin, A., Moum, J., and Shroyer, E.: Propagation of Freshwater Lenses as Buoyant Gravity Currents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-713, https://doi.org/10.5194/egusphere-egu2020-713, 2020.

D2814 |
Shenjie Zhou, Xiaoming Zhai, and Ian Renfrew

High-frequency and small-scale processes in the atmosphere have an important influence on the evolution of the underlying ocean. They can not only introduce variability to the coupled systems but also have long-term ramification effects on the sea surface temperature, thermocline structure and large-scale ocean general circulation via nonlinear interactions.

Comparisons between the newly-released ECMWF fifth-generation global climate reanalyses (ERA5) wind product and satellite/in-situ observations show that the latest reanalyses winds still considerably underestimate wind variability at high-frequencies and small-scales. A novel approach, Cellular Automata (CA), is used here to stochastically perturb the ERA5 wind field. CA, originally introduced into the weather forecast model to mimic the near-grid-scale variability associated with convective cloud clustering, generates spatially and temporally coherent perturbation patterns. Results show that the CA-perturbed ERA5 wind field enjoys an improved wavenumber spectrum, especially over high wavenumber bands (scales <400 km), when compared to the QuikSCAT measurements. In addition, including CA patterns also brings the level of wind variability at high-frequencies (>1 cpd) closer to in-situ mooring measurements. The local response of the upper ocean properties is investigated by a Multi-Column K-Profile Parameterization (MC_KPP) ocean mixed layer model over Atlantic section. It is found that overall the sea surface temperature (SST) decreases and oceanic boundary layer (OBL) deepens to respond to the enhanced surface turbulent heat loss and shear instability generated at the base of surface OBL caused by the small-scale wind perturbations. In particular, SST tends to decrease the most over the summer hemisphere by up to 1°C locally and 0.1°C averaging across the basin, in correlation with shallower background OBL and smaller OBL heat capacity. 

The ocean states under the forcing of stochastic wind perturbation as expressed by the local response are found rectified by a non-negilible magnitude. We argued that although the missed small-scale and high-frequency wind variability may be represented differently than our approach, our results highlighted the fact that these variabilities should take a singificant part in driving the current generation of coupled climate model. Furhtermore, the temproal and spatial variabilities of the local signals can pose significant influence on the large-scale ocean circulation in terms of their pathways, strengths and variabilities. The dynamical response of the ocean circulation is to be further understood with an eddy-resolving Massachusette Institute of Technology General Circulation Model (MITgcm).

How to cite: Zhou, S., Zhai, X., and Renfrew, I.: Response of the North Atlantic circulation to the small-scale surface wind perturbations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-272, https://doi.org/10.5194/egusphere-egu2020-272, 2020.

D2815 |
Marta Wenta and Agnieszka Herman

In consequence of sea ice fragmentation in winter a range of physical processes take place between the sea/sea ice and the atmospheric boundary layer (ABL). Most of them occur on the level of individual ice floes and cracks and thus cannot be directly resolved by numerical weather prediction (NWP) models.  Parametrizations of those processes aim to describe their overall effect on grid scale values, given the grid scale variables. However, as many of the processes taking place during winter sea ice fragmentation remain largely unrecognized they cannot be incorporated into the NWP models. 

The aim of the presented study is to determine whether the floe size distribution (FSD) has an effect on the ABL. Our previous research (Wenta, Herman 2018 and 2019) indicates that FSD might determine the intensity and spatial arrangement of convection and heat fluxes. A coefficient has been proposed for the correction of moisture heat flux, which can be incorporated into the NWP models. However, this research is based entirely on idealized model simulations and requires further modelling and observations based studies.

In order to address this shortcoming, a field campaign is going to take place in the Bay of Bothnia in March 2020. Our goal is to create a 3D view of the atmosphere above fragmented sea and verify whether the processes and effects we found in the modeling results take similar form in real situations. Measurements results will be useful in the validation of our numerical modelling studies and will provide a unique dataset about the sea-ice-atmosphere interactions in the Bay of Bothnia area. Considering a significant decreasing trend in winter sea ice extent in the Baltic Sea it might contribute to our understanding of the role of ice in the local weather patterns. The field campaign is going to be complemented by numerical modelling with full version of Weather Research and Forecasting (WRF) model adjusted to the conditions over the Bay of Bothnia. 

Combined together - the results of our previous modelling studies and the results from the Bay of Bothnia field campaign, may considerably increase our knowledge about the surface-atmosphere coupling in the event of winter sea ice fragmentation.

How to cite: Wenta, M. and Herman, A.: Submesoscale physical processes in the atmospheric boundary layer above fragmented sea ice. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4980, https://doi.org/10.5194/egusphere-egu2020-4980, 2020.

D2816 |
Hye-young Lee and Joowan Kim

This study investigates synoptic characteristics of the cold surges over South Korea during winter season (December-February). A total of 61 cold events are selected by quantile regression analysis using daily mean temperature observations from 11 surface stations for 38 years(1981–2018). Composite analyses reveal that a synoptic-scale cyclone developing over the northern Japan is a key feature that significantly contribute to the enhancement of cold advection by increasing pressure gradient over the Korean peninsula. Enhanced sensible and latent heat fluxes are observed over the southern ocean of Korea and Japan during the cold surges due to increased temperature and humidity differences between the lower atmosphere and ocean surface. These fluxes are transported toward the center of the surface cyclone and help the development of the surface cyclone by inducing positive PV in the lower atmosphere. These processes make a positive feedback loop that amplifies strength of the cold surge. To examine how sea surface temperature (SST) affects the strength of cold surge, we categorize the cold surges into warm, normal and cold SST cases. As a result, stronger and more pronounced cyclones are observed in cases of warm SST. Thus, the positive feedback process particularly enhanced when SST is warmer in the early winter.

How to cite: Lee, H. and Kim, J.: Impact of SST on the development of the cold surges over East Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14066, https://doi.org/10.5194/egusphere-egu2020-14066, 2020.

D2817 |
Katherine Ackerman, Chung Taing, Alison Nugent, and Jorgen Jensen

Sea spray aerosol (SSA) play a significant role in the local climatology of coastal areas through direct radiative forcing and the indirect aerosol effect. Quantifying the size and concentrations of SSA is essential to understanding their influence in these coastal regions. Current observations of SSA in the surf zone come from coastline station measurements. These stations are often limited to observing only the mass of SSA produced and are unable to differentiate SSA concentrations of varying sizes. NCAR has developed an instrument known as a giant nucleus impactor (GNI) that allows deliquesced salt particles to impact onto polycarbonate slides exposed to a free airstream. These slides are then analyzed in a humidified environment under a microscope providing information about the SSA sizes and concentrations present. By modifying the NCAR GNI, we created a smaller, low-cost method known as a mini-GNI from 3D printing and Arduino microcontrollers. Using this new instrumentation, we attached the mini-GNI to a drone that sampled four locations over the ocean perpendicular to the coastline: the outer reef crest, inner reef crest, lagoon area, and shoreline. While sampling occurred, atmospheric and oceanographic measurements were recorded. This methodology provided us a baseline concentration or “open-ocean” concentration at the outer reef crest. It allowed us to compare how sizes and concentrations changed as the air parcel interacted with the surf zone. As the sampling locations moved closer to the shore, we’ve observed an increase in larger and more concentrated SSA that were contributed by the surf zone. We’ve also found that SSA concentrations have a stronger relationship to wave activity than to wind speed in our coastal environment. The ultimate goal is to quantify how the ocean environment and atmospheric conditions contribute to SSA production in the surf zone.

How to cite: Ackerman, K., Taing, C., Nugent, A., and Jensen, J.: Observing Sea Spray Aerosol Production in the Surf Zone of Hawaii, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20064, https://doi.org/10.5194/egusphere-egu2020-20064, 2020.

D2818 |
Thomas Spengler and Clemens Spensberger

Cold air outbreaks play a crucial role in the air-sea heat exchange in the higher latitudes. However, we still lack some basic understanding about the sensitivities of these phenomena to latent heating and the role of coupling to the ocean. Despite increasing model resolution, reliable forecasts of these events remain a challenge because of the vast range of scales and physical processes involved. To further explore these sensitivities and dependence on model representation, we couple a moist convective atmospheric boundary layer model with an ocean mixed layer model to investigate the response of moist convection as well as ocean mixing during cold air outbreaks. In addition, we perform sensitivity experiments based on the PolarWRF model in an idealised configuration to represent cold air outbreaks.

Varying sea ice concentration and resolution alters the distribution and intensity of the air-sea heat exchange with ramifications for mixed layer depths in both the atmosphere and ocean. Furthermore, integrated and local sensible and latent heat fluxes depend on the model resolution as well as the distribution of the sea-ice concentration in the marginal ice zone. While surface sensible heat fluxes appear to be rather consistent across different model resolutions, surface latent heat fluxes respond to the organisation of convection at higher resolutions, a feature that is absent for coarser model grids. Different geometries replacing a straight sea-ice edge with various simple geometrical shapes are also tested. Our results have implications for numerical weather prediction and climate models, in particular regarding model resolution and the degree of coupling for the representation of air-sea interaction during cold air outbreaks.

How to cite: Spengler, T. and Spensberger, C.: Sensitivity of Atmosphere-Ocean Interactions during Cold Air Outbreaks to Characteristics of the Sea-Ice Edge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5489, https://doi.org/10.5194/egusphere-egu2020-5489, 2020.

D2819 |
Olivier Marti, Sébastien Nguyen, Pascale Braconnot, Florian Lemarié, and Eric Blayo

For historical and practical reasons, present-day coupling algorithms implemented in ocean-atmosphere models are primarily driven by the necessity to conserve energy and water at the air-sea interface. However the asynchronous coupling algorithms currently used in ocean-atmosphere do not allow for a correct phasing between the ocean and the atmosphere.

In an asynchronous coupling algorithm, the total simulation time is split into smaller time intervals (a.k.a. coupling periods) over which averaged-in-time
boundary data are exchanged. For a particular coupling period, the average atmospheric fluxes are computed in the atmospheric model using the oceanic surface properties computed and averaged by the oceanic model over the previous coupling period. Therefore, for a given coupling period, the fluxes used by the oceanic model are not coherent with the oceanic surface properties considered by the atmospheric model. The mathematical consistency of the solution at the interface is not guaranteed.

The use of an iterative coupling algorithm, such as Schwarz methods, is a way to correct this inconsistency and to properly reproduce the diurnal cycle when the coupling period is less than one day. In Lemarié et al. (2014), preliminary numerical experiments using the Schwarz coupling method for the simulation of a tropical cyclone with a regional coupled model were carried out. In ensemble simulations, the Schwarz iterative coupling method leads to a significantly reduced spread in the ensemble results (in terms of cyclone trajectory and intensity), thus suggesting that a source of error is removed with respect to the asynchronous coupling case.

In the present work, the Schwarz iterative method is implemented in IPSLCM6, a state-of-the-art global ocean-atmosphere coupled model used to study past, present and future climates. We analyse the convergence speed and the quality of the convergence. A partial iterative method is also tested: in a first phase, only the atmosphere physics and the vertical diffusion terms are computed, until the convergence. This provide a first guess for the full model which is then iterated until convergence of the whole system. The impact on the diurnal cycle will also be presented.

How to cite: Marti, O., Nguyen, S., Braconnot, P., Lemarié, F., and Blayo, E.: Iterative Schwarz Method to Couple Ocean and Atmosphere in a Global Climate Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10095, https://doi.org/10.5194/egusphere-egu2020-10095, 2020.

D2820 |
Marcelo Dourado and Carlos Lentini

Recent studies suggest that the Tropical Atlantic Warm Pool (PQAT) contributes to modulate the variability of the ZCIT in the Atlantic Ocean basin and, consequently, the precipitation regime in Brazilian northeastern. Hourly surface meteorology observations from the PIRATA buoy at 19°S, 34°W from August 2010 to November 2018 was used to characterize and estimate the exchanges of heat, freshwater, and momentum between the ocean and the atmosphere over the Tropical. We focus here on recent efforts to observe the surface meteorology and air-sea fluxes using those data to gain insights into how atmospheric variability may govern the structure and variability of the upper ocean there at diurnal and seasonal time scales. The surface fluxes are calculated using the COARE 3.0 algorithm, positive values are to the ocean. Using the observations collected from the mooring deployments, we developed a good understanding of the annual march of the surface forcing of the ocean by the atmosphere. During spring (March, April) mean SST and air temperature are the hottest of the year, 28oC and 26.7oC, respectively; SST is greater than air temperature all over the year, 1oC on average. Wind speed is minimum, the air is drier and there is a peak of precipitation in April. During the autumn (August, September), mean SST and air temperature are the coldest of the year, 24.5oC and 23.6oC. Wind speed increases form 4.4m/s in March to 5.9 m/s in December. The monthly averaged incoming shortwave radiation in July was the lowest of the whole year and maximum in December. Net longwave radiation shows an inverse variability, i.e., maximum in the winter, minimum in the summer. This occurs because the winter air is drier than in the summer. Sensible heat flux is maximum in August due to the increase of the wind speed and an increase of the air-sea temperature difference. Latent flux is higher between April and August due to an increase in wind speed and a drier atmosphere. In the summer the humidity increases and, consequently, the latent heat flux diminishes. Finally, the net heat flux, positive between January and March, is negative between April and August (maximum -36W/m2 in July ) and, again, positive between September and December, maximum +116 W/m2 in December.

How to cite: Dourado, M. and Lentini, C.: Surface Meteorology and Air–Sea Fluxes in the Southwestern Tropical Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14749, https://doi.org/10.5194/egusphere-egu2020-14749, 2020.

D2821 |
Florian Lemarie, Charles Pelletier, Pierre-Etienne Brilouet, Eric Blayo, Jean-Luc Redelsperger, and Marie-Noëlle Bouin

Standard methods for determining air – sea fluxes typically rely on bulk algorithms derived from the Monin-Obukhov stability theory (MOST), using ocean surface fields and atmosphere near-surface fields. In the context of coupled ocean – atmosphere simulations, the shallowest ocean vertical level is usually assimilated to the surface, and the turbulent closure is one-sided: it aims at extrapolating atmosphere near-surface solution profiles (for wind speed, temperature and humidity) to the prescribed ocean surface values. Assimilating near-surface ocean fields as surface ones is equivalent to considering that in the ocean surface layer, solution profiles are constant instead of also being determined by a turbulent closure. Here we introduce a method for extending existing turbulent parameterizations to a two-sided context, by including the ocean surface layer and the viscous sublayers, which are also generally neglected in standard air – sea fluxes computation. The formalism we use for this method is derived from that of classical turbulent closure, so that our novelties can easily be implemented within existing formulations. Special care is taken to ensure the smoothness of resulting solution profiles. We investigate the impact of such two-sided bulk formulations on air - sea fluxes and discuss further implications such as resulting bulk formulation retuning. We also present leads on incorporating other mechanisms impacting air – sea fluxes within our framework, such as waves and radiation penetration.

How to cite: Lemarie, F., Pelletier, C., Brilouet, P.-E., Blayo, E., Redelsperger, J.-L., and Bouin, M.-N.: Two-sided turbulent boundary layer parameterizations for assessing ocean – atmosphere fluxes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14780, https://doi.org/10.5194/egusphere-egu2020-14780, 2020.

D2822 |
Hugo Bellenger, Xavier Perrot, Lionel Guez, Jean-Philippe Duvel, Alexandre Supply, Jacqueline Boutin, and Gilles Reverdin

Temperature and Salinity at the ocean interface can be substantially different than their bulk values in the ocean mixed layer at 5-10 meters depth. The main phenomena that account for these differences are (i) the interfacial millimeter scale diffusive microlayer usually cooler and saltier than below due to surface fluxes and (ii) diurnal warm layers of few tens of centimeters to few meters that form under weak wind condition due to solar absorption. Although characterized by small vertical scales, these tightly wind-related phenomena corresponds to coherent structures up to the large-scale where they can impact air-sea exchanges of heat, water and chemical species. Another phenomenon that can impact global air-sea exchanges is the freshwater lenses produced by rain. Rain freshens and cools the ocean surface, as raindrops temperature is usually lower than surface temperature. The induced negative salinity anomaly enables surface cold anomalies to be sustained and further cooled down by surface fluxes after rain has ceased. This study presents a first global estimate of basic statistics rain-induced ocean surface freshening and temperature changes and of their variations with seasons.

How to cite: Bellenger, H., Perrot, X., Guez, L., Duvel, J.-P., Supply, A., Boutin, J., and Reverdin, G.: Global ocean surface stratifications due to rain based on ERA5, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18373, https://doi.org/10.5194/egusphere-egu2020-18373, 2020.

D2823 |
Xavier Perrot, Jean-Philippe Duvel, and Lionel Guez

In coupled general circulation model, the accuracy of momentum and energy exchange at the air-sea interface is still a potential source of significant bias. In the framework of the COCOA project we investigate new methods (both mathematical and numerical) to have a more correct flux representation. One important source of error is the asynchronous coupling between oceanic and atmospheric model. Indeed, the time step of the coupling is generally longer than time steps used by either the atmospheric or the oceanic model. This introduces inconsistencies between the free evolution of the two models due to the exchange parameters that are held constant since the last coupling time step. In particular, non-synchronous exchange coefficients may lead to error in the diurnal evolution of the coupled system, or to bias in the ocean mixed layer temperature for period where surface fluxes increases or decrease linearly.

In order to evaluate the potential amplitude of this error, and its regional and sea- sonal distribution, we use the hourly fluxes that are available in the new ECMWF ERA5 re-analyses. The error due to asynchronous coupling is first evaluated by inspecting the flux difference between two successive time-steps. Results show more important differences over the western boundary currents and the circumpolar current for all the fluxes except for the solar flux. We also observe larger differences in summer compared to winter in the respective hemisphere. By taking in account the geometrical variation of the solar flux we show how we can reduce the error for the solar flux.

In a second time we are calculating the statistics for the linear increase and decrease of the flux for a fixed period (ig one day, two days...) over all the ocean for all the fluxes except the solar one. The results are showing coefficients that are decreasing as the period increase. We also use those coefficient in a simple mixed layer model to calculate the error made over the period of calcul. On the contrary we see the appearance of a plateau at two-three days on the impact of this linear bias. Finally, using the De Boyer de Montaigu climatology for the mixed layer height we show that the linear bias could lead to temperature change up to 0.1K.

How to cite: Perrot, X., Duvel, J.-P., and Guez, L.: Surface flux errors in asynchronous coupling of GCM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19820, https://doi.org/10.5194/egusphere-egu2020-19820, 2020.

D2824 |
| Highlight
Sabrina Speich, Johannes Karstensen, Chris Fairall, Paquita Zuidema, Chelle Gentemann, Dongxiao Zhang, Hugo Bellenger, Gilles Reverdin, Elizabeth Thompson, Sebastien Bigorre, Wiebke Mohr, and Stefan Kinne and the EUREC4A team

Ocean mesoscale eddies create specific air-sea interaction patterns that can have an integral effect on the large scale atmosphere and ocean dynamics. Recent advances in the state of the art of these processes has been predominately obtained from modeling efforts, but only very few observational studies exist, and there are all located in the extra-tropics. Adding a dedicated ocean mesoscale eddy air-sea interaction experiment to the EUREC4A campaign will enhance the objectives and success of the whole program as it sets a local, oceanic constrain to the atmospheric evolution (as been outlined in the overall EUREC4A design: www.eurec4a.eu; Bony et al. 2017). Temporal variability over a fixed mesoscale (500 km x 500 km) study area allows for sampling varying atmospheric states, on the time-scales of the EUREC4A field study. The oceanographic component that add on EUREC4A consists of four ships and many autonomous platforms (gliders, Saildrones, specific surface drifters, Argo floats) sampling the mesoscale ocean and air-sea exchanges at different edges of the Northwest Atlantic tropical region. This will enable the extended spatial sampling required to characterize ocean variability and gather enough air-sea observations at different locations to assess with accuracy the involved processes and impacts.

The western tropical Atlantic is an ideal laboratory for the proposed study. It hosts rich ocean mesoscale variability under an atmosphere characterized by a rather steady trade wind regime. In this region, eddies have a diameter of 200 to 300 km and lifetimes of several months up to years. In particular south of Barbados, very energetic and long-lived anticyclonic North Brazil Current rings are commonly found. These eddies are key for the northward transport of properties from the South to the North Atlantic within the Atlantic Meridional Ocean Circulation. Moreover, preliminary studies based on satellite observations have suggested that they play a crucial role in air-sea interactions and on the atmosphere. This region and eddies are the focus of EUREC4A-OA, the French oceanographic component of the larger EUREC4A field experiments.

In this talk we will present the preliminary lessons learned and results obtained by this unprecedented observing effort.

How to cite: Speich, S., Karstensen, J., Fairall, C., Zuidema, P., Gentemann, C., Zhang, D., Bellenger, H., Reverdin, G., Thompson, E., Bigorre, S., Mohr, W., and Kinne, S. and the EUREC4A team: A first glimpse from the EUREC4A-OA/ATOMIC air-sea experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21028, https://doi.org/10.5194/egusphere-egu2020-21028, 2020.

D2825 |
The impact of atmosphere-ocean coupling on the tropospheric response to stratospheric temperature perturbations
Natasha Trencham, Joanna Haigh, and Arnaud Czaja