The sea surface microlayer (SML) is the uppermost thin oceanic surface layer in the range of 100µm with properties that are distinct from the water below. With an ocean coverage of up to 70% it is supposed to have a significant impact on air-sea gas exchange rates. In global studies, the SML is often supposed to be a missing source of trace gases, when oceanic production and the subsequent emissions alone cannot explain observed atmospheric mixing ratios. Despite the attention in the past 20 years, also in the SOLAS science plan, it remains difficult to sample volatile trace gases from the SML with existing sampling techniques. Consequently, an incomplete process understanding of trace gas cycling within the SML inhibits its effect on air-sea gas exchange.

In this study, we focus on existing and common SML sampling methods (glass plate, Garrett screen) in order to ensure that trace gas samples are comparable to other parameters sampled with the same method. A series of laboratory experiments was set up to determine a correction factor which quantifies the loss of trace gases due to the sampling method itself. Dimethyl sulfide, isoprene and carbon disulfide were sampled with a glass plate and with a Garrett screen under varying surfactant concentrations and environmental conditions (salinity, temperature). Based on physiochemical properties of the examined trace gases, we extended the correction factor to nitrous oxide and methane. Losses are high, but not as variable as expected. Around 90% are lost due to sampling with small variations between different gases. The presence of surfactants has a small effect on the losses.

The lab-based correction factors are applied to in-field SML samples from a mesocosm study in May/June 2023 conducted within the DFG research unit BASS. Those results clearly indicate that the composition of the SML highly influences the correction factor for each trace gas individually. Comparing corrected SML concentrations with underlying bulk water concentrations reveal the accumulation of specific traces gases in the SML which highly influence the magnitude of trace gas emissions to the atmosphere.

How to cite: Lange, L., Booge, D., Karnatz, J., Bange, H., and Marandino, C.: Sampling the SML for traces gases: a case study of sampling technique and resulting correction factors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1139, https://doi.org/10.5194/egusphere-egu24-1139, 2024.

Carbohydrates, amino acids, and lipids are important contributors to organic carbon (OC) in the marine environment. To study their sea-to-air transfer, including their enrichment in the sea surface microlayer (SML), potential atmospheric in situ formation or degradation, and their oceanic contribution to the ambient marine aerosol particles, we provide measurements from the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO) where the above compounds were investigated in both surface seawater and in ambient submicron aerosol particles.

In bulk seawater and the SML, similar distributions among species were found for the lipids and carbohydrates with moderate SML enrichments (enrichment factor EF_SML = 1.3 ± 0.2 and 1.1 ± 0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an average EF_SML of 2.3 ± 0.4 although they are less surface-active than lipids. The same compounds studied in the seawater were found on the ambient submicron aerosol particles whereas the lipids were more pronounced enriched (EF_aer. = 1.6x10⁵) compared to the amino acids and carbohydrates (EF_aer. = 1.5x10³ and 1.3x10³ respectively), likely due to their high surface activity and/or the lipophilic character. Detailed molecular analysis of the seawater and aerosol particles revealed changes in the relative abundance of the individual organic compounds. They were most pronounced for the amino acids and are likely related to an in situ atmospheric processing by biotic and/or abiotic reactions.

On average 49% of the OC on the aerosol particles (≙ 97 ng m^-3) could be attributed to the specific components or component groups investigated in this study. The majority (43%) was composed of lipids. Amines, oxalic acid, and carbonyls, comprised an OC fraction of around 6%. Carbohydrates and amino acids made up less than 1% of the OC. This shows that carbohydrates, at least when resolved via molecular measurements of single sugars, do not comprise a very large fraction of OC on marine aerosol particles, in contrast to other studies. However, carbohydrate-like compounds are also present in the high lipid fraction (e.g., as glycolipids), but their chemical composition could not be revealed by the measurements performed here.

Since the identified compounds constituted about 50% of the OC and belong to the rather short-lived biogenic material probably originating from the surface ocean, a pronounced coupling between ocean and atmosphere was indicated for this oligotrophic region. The remaining, non-identified OC fraction might in part contain recalcitrant OC, however, this fraction does not constitute the vast majority of OC in the aerosol particles here investigated.

The study contributes to the international SOLAS program.

Ref: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation, Atmos. Chem. Phys., 23, 6571–6590, https://doi.org/10.5194/acp-23-6571-2023, 2023.

How to cite: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6195, https://doi.org/10.5194/egusphere-egu24-6195, 2024.

Sea spray aerosol (SSA) formed after wave breaking at the ocean surface influences our climate by scattering incoming solar radiation and acting as cloud condensation nuclei. Furthermore, they provide a microenvironment for aqueous phase chemistry, selective uptake of surfactants, and gas-to-particle partitioning of compounds by providing an acidic pH at the air-water interface (Angle et al., 2022). Despite these known effects, a crucial question remains unanswered: which volatile organic compounds (VOCs) are emitted from SSA, and how do they change over time via atmospheric aging?

To address this, we designed a novel experimental setup during the AGENA* Campaign 2022 at Graciosa Island, Portugal. For the first time, we connected a sea spray simulation chamber to a chemical ionization mass spectrometer (CIMS) to measure the freshly emitted gases from both seawater and SSA. Additionally, we aged the samples for an equivalent period of about 3-3.5 days in an oxidation flow reactor to investigate compositional changes after ageing.

Surprisingly, our findings reveal that nearly half of the mass-spectrometer signal from the fresh samples constituted fluorinated compounds, specifically short-chain perfluoroalkyl carboxylic acids - a class of perfluoroalkyl substances (PFAS). While, previous studies have shown that SSA can release and play a key role in the long-range transport of PFAS, these studies have primarily focused on particle-phase emissions (Johansson et al., 2019, Sha et al, et al., 2022). In contrast, our study provides new insights into oceanic PFAS emissions and transport to the atmosphere by examining gas-phase emissions.  

Furthermore, we observed that the gas-phase PFAS almost completely disappears after ageing. Our hypothesis is that these compounds partition into the particle phase. We plan to test this hypothesis by analyzing the particle filters collected during the campaign.

*Aerosol Growth in the Eastern North Atlantic (AGENA) https://www.arm.gov/research/campaigns/ena2022agena

Angle, K. J., Crocker, D. R., Simpson, R. M., Mayer, K. J., Garofalo, L. A., Moore, A. N., ... & Grassian, V. H. (2021). Acidity across the interface from the ocean surface to sea spray aerosol. Proceedings of the National Academy of Sciences, 118(2), e2018397118.

Johansson, J. H., Salter, M. E., Navarro, J. A., Leck, C., Nilsson, E. D., & Cousins, I. T. (2019). Global transport of perfluoroalkyl acids via sea spray aerosol. Environmental Science: Processes & Impacts, 21(4), 635-649.

Sha, B., Johansson, J. H., Tunved, P., Bohlin-Nizzetto, P., Cousins, I. T., & Salter, M. E. (2021). Sea spray aerosol (SSA) as a source of perfluoroalkyl acids (PFAAs) to the atmosphere: field evidence from long-term air monitoring. Environmental Science & Technology, 56(1), 228-238.

How to cite: Aggarwal, S., Garmash, O., Kilgour, D., Jernigan, C., Zinke, J., Gong, X., Zhou, S., Zhang, J., Wang, J., Bertram, T., Thornton, J., Salter, M., Zieger, P., and Mohr, C.: Can sea spray aerosol be a source of gas-phase perfluoroalkyl substances (PFAS)? A study in the Eastern North Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11282, https://doi.org/10.5194/egusphere-egu24-11282, 2024.

Mesoscale structures are key dynamical features of the ocean. They are associated with a variety of short lived and small-scale dynamics linked to physical, biological, and chemical processes at the submesoscale, such as cascading energy, impacting ocean stratification, and guiding ocean carbon and oxygen uptake. In the high latitudes, the spatial extent of the mesoscale is only tens of kilometres, making it challenging to observe the submesoscale processes. In August-September 2022, an extensive submesoscale-resolving multiplatform experiment was conducted across an Irminger Ring in the Labrador Sea. The experiment leveraged two underwater electric gliders equipped with nitrate, microstructure shear, chlorophyll fluorescence, oxygen, and turbidity sensors, operated in concert with a variety of ship operated instruments including underway-CTD’s, a moving vessel profiler, Thermosalinograph, ADCPs and a X-band radar system. Observations were acquired both, along the peripheries and within the core of the eddy, and offered insight into submesoscale dynamics of the ring. Making use of nearly concurrent turbulence and nutrients observations, we estimated the vertical flux pattern across the eddy’s frontal and interior regions. From the recorded and expected glider vehicle motion a vertical water velocity could be inferred and compared with the nutrient flux pattern. The stability of the ring was tracked with surface drifters, for weeks after the ship and glider survey ended, and a link between the disintegration of the ring and an atmospheric event was investigated

How to cite: Dilmahamod, A. F., Karstensen, J., Horstmann, J., and Krahmann, G.: Vertical fluxes in subpolar eddies from a high-resolution, multiplatform experiment in the Labrador Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16489, https://doi.org/10.5194/egusphere-egu24-16489, 2024.

The Arctic Ocean covers only 3 % of the Earth’s surface but contributes 5 - 14 % of the global ocean carbon sink. Sparse and unevenly distributed observations of the partial pressure of CO₂ (pCO₂) hinder our understanding of the magnitude and the controlling mechanisms of this carbon sink. In order to constrain the magnitude of this flux, we adapt the Self-Organising Map – Feed-Forward neural Network (SOM-FFN) method of Landschützer et al. (2016) to interpolate existing observations and construct a monthly 1 x 1 degree pCO₂ product for the Arctic Ocean from 1991 - 2022. We first divide the Arctic Ocean (i.e., the region ≥ 55° N) into five biogeochemical provinces; four obtained from using the SOM method and a fifth for all grid cells with greater than 85 % ice cover. For each province, we then derive non-linear relationships between pCO₂ and predictor variables (i.e., biogeochemical drivers) using the FFN method. The monthly reconstructed Arctic pCO₂ product is then evaluated against existing observations of surface ocean pCO₂, chiefly from SOCATv2023 and from independent timeseries stations. Our study shows that biogeochemical properties previously selected as predictor variables at the global scale are not well suited to the Arctic Ocean. Limiting the spatial domain from which relationships are derived also improves performance, with less biased p(CO₂) values predicted when excluding the Baltic Sea. 

How to cite: Dutch, V., Bakker, D., Landschützer, P., Roobaert, A., and Kaiser, J.: Improving Estimates of Arctic Ocean CO2 Uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-152, https://doi.org/10.5194/egusphere-egu24-152, 2024.

Quantification of air-sea CO₂ exchange over coral reefs has relied primarily on measurements of the CO₂ partial pressure (pCO₂) gradient between the water overlying a reef and the lower atmosphere. A gas transfer velocity based on wind speed is then used to estimate the air-sea CO₂ mass exchange. While this approach may be suitable over the oceans or where instrumented buoys have been deployed for long-term monitoring, the method overlooks many factors that influence turbulent transport and air-sea CO₂ exchange. These include surfactants, bubble exchange, atmospheric turbulence, and wave breaking, which may be particularly important over near shore fringing coral reefs.

Using eddy covariance (EC) systems deployed at the shoreline adjacent to coral reefs and on pontoons we show through direct measurements these ecosystems may be net sinks of atmospheric CO₂. Results show sequestration of atmospheric CO₂ by healthy coral reefs and adjacent lagoons at time scales of several days to several months exceed published CO₂ sequestration rates of mature pine plantations measured by EC by an order of magnitude. These findings highlight the importance of coral reefs in carbon budgets in addition to their widely known ecosystem services and societal benefits. Conserving coral reef ecosystems and ensuring they remain healthy and resilient to the threats of climate change, pollution, overfishing, tourism, and mining should be a priority. Future research will aim to track the CO₂ influx through coral reef ecosystems.        

How to cite: McGowan, H., Lensky, N., Abair, S., and Saunders, M.: Coral Reefs: Sinks of Atmospheric CO2 ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-232, https://doi.org/10.5194/egusphere-egu24-232, 2024.

Precipitation alters sea surface physical and biogeochemical properties locally. However, due to its high temporal and spatial variations, it has largely been overlooked in studies assessing global ocean carbon uptake. Air-sea CO₂ flux is mainly due to the interfacial exchange of CO₂ molecules between the liquid and gaseous phases media. Rain may impact this interfacial air-sea CO₂ flux by (i) enhancing the turbulence at the air-sea interface and (ii) diluting the CO₂ concentration near the ocean surface. At the same time, rain directly injects into the ocean CO₂ absorbed by the raindrops during their fall. This latter component, known as wet deposition, contributes to the CO₂ flux into the ocean. This study provides the first comprehensive global estimate of these effects and their combined influence on the global ocean carbon uptake during the period 2008-2018. We use different representations of the ocean surface response to rain and different rain products with different rain rate distributions (ERA5 and IMERG) to quantify the uncertainty of the global impact of rain on CO₂ sink. We show that rain increases the global ocean carbon sink by +0.14 to +0.19 PgC yr^-1 over 2008-2018, representing an increase of 5 to 7% of the global carbon uptake (2.66 PgC yr^-1). Both interfacial flux and wet deposition have comparable orders of magnitude. Rain mainly increases the CO₂ sink in the tropics, where strong rain rates and weak winds induce noticeable dilution at the ocean surface, in the storm track regions, and in the Southern ocean.

How to cite: Parc, L., Bellenger, H., Bopp, L., Perrot, X., and Ho, D.: The impact of rain on the global ocean carbon uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7477, https://doi.org/10.5194/egusphere-egu24-7477, 2024.

Modeling the air-sea flux of CO₂ is a key factor in understanding climate change and predicting its effects. The contribution of sea spray to this flux is highly uncertain yet important for reducing error margins in global estimates. In this work, a modified CO2SYS routine is used to quantify the effect of evaporation on aqueous carbonate reactions in sea spray in order to assess this flux. Factors that affect these reactions are the increasing salinity and temperature changes of the droplet as it evaporates. The size of the droplet is also a determining factor as it affects the time aloft and thus the amount of evaporation and gas exchange that can occur. Using these factors and a number of simplifying assumptions, we model the change in DIC, TA, pCO₂ and pH in an evaporating sea spray droplet.

How to cite: Hendrickson, L., Vlahos, P., and Romero, L.: Carbonate System Changes Within an Evaporating Sea Spray Droplet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1372, https://doi.org/10.5194/egusphere-egu24-1372, 2024.

How to cite: Renault, L.: Submesoscale air-sea interactions: atmospheric response and impacts on the ocean dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20391, https://doi.org/10.5194/egusphere-egu24-20391, 2024.

The Mediterranean basin is well recognized as one of the main climate change hotspots; besides, this region is one of the most active cyclogenetic area of the Northern Hemisphere with a large number of intense cyclones occurring every year. Intense Mediterranean cyclones are often responsible for extreme precipitation and strong wind events leading to severe socio-economic and environmental impacts especially over densely populated coastal areas. Complex feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the variability in and extremes of the regional climate system.

This study aims to investigate the impact of the ocean-atmosphere coupling on the regional climate during intense Mediterranean cyclones. To this end, two simulations are performed using the ENEA-REG regional earth system model at 12 km atmospheric horizontal resolution over the Med-CORDEX domain, both driven by ERA5 reanalysis. The first experiment uses the mesoscale WRF model with prescribed ERA5 Sea Surface Temperature (SST), while the second is coupled to the MITgcm ocean model at horizontal resolution of 1/12°. Cyclones are tracked by applying a Lagrangian algorithm to the mean sea level pressure field. The 500 most intense cyclones mainly occur in winter over the Thyrrenian, Adriatic, Ionian and Aegean Sea. They are similarly reproduced between WRFs and ERA5 in terms of seasonal and spatial distribution, due to the same large-scale atmospheric conditions. The coupled simulation is compared with the standalone WRF in terms of sub-daily fields, such as evaporation, precipitation and wind speed, during the mature stage of the cyclones. The different SST distribution between the models appears to be the main controlling factor for the differences in the atmospheric properties affecting not only the surface, but also the entire atmospheric boundary layer (ABL) and its height, due to the mixing of the turbulent processes, enhanced during intense cyclones. A statistically significant higher specific humidity and wind speed are found in the coupled model from the surface to the top of the ABL, as well as higher precipitation over sea and coastal areas. These results are consequences of higher turbulent heat and moisture fluxes in the coupled model that destabilize the ABL and provide higher moisture content available for convection.

We conclude that the use of the coupled model is crucial for a more realistic representation of the energy redistribution in both the ocean mixed layer and the ABL during intense Mediterranean cyclones. This highlights the importance of the coupled model to study the influence of climate change on intense Mediterranean cyclones and associated impacts under different future scenarios.

How to cite: Chericoni, M., Fosser, G., and Anav, A.: Impact of the ocean-atmosphere coupling on Mediterranean cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2187, https://doi.org/10.5194/egusphere-egu24-2187, 2024.

The prototype of a short-term forecasting system of the ocean dynamics of Southern European Seas (SEAS), was developed. It is based on a regional coupled ocean-atmosphere model, with NEMO and WRF codes implemented on the same computational grid, with 1/24° resolution, which encompasses Mediterranean Sea, Marmara Sea and Black Sea. The domain extends also westward and northward in the Atlantic Ocean to downscale properly the mid-latitudes atmospheric perturbations from the parent ECMWF HRES model.

The forecasting uncertainty of the atmospheric regimes in such a complex Euro-Mediterranean region must be considered along with the uncertainties of the parametrizations of the surface processes at the ocean-atmosphere interface. Therefore, the goal of coupling oceanic and atmospheric models is to reduce these uncertainties and exploit the second type predictability to increase the forecast skills of the ocean dynamics.

The uncoupled ocean model has been validated against Sea Surface Temperature (SST) satellite observed data, and the skills compared to those of the Copernicus Mediterranean Forecasting System (MedFS hereafter) both in the short-term forecast over two seasonal periods and in the simulation of the medicane Ianos.

Various physical schemes, domain extensions, boundary, and initial conditions were initially tested using the uncoupled atmospheric model to obtain the best representation of the medicane Ianos. Furthermore, these experiments were also useful to determine the coupling strategy more appropriate to reduce the heat fluxes imbalance between the two components.

The SST differences between coupled and uncoupled experiments are determined by the heat fluxes computation in the atmospheric component rather than using the MedFS bulk formulae implemented in the ocean model. These differences are largely dependent on the surface boundary layer scheme used in WRF, therefore, several coupled experiments were conducted.

In terms of SST, the coupled model replicates the skills of the MedFS in the winter period while in the summer period the skills are worsened due to the larger heat fluxes. Numerical experiments focused on the parametrizations of the atmospheric boundary layer are still ongoing work.

The skill of the coupled model in reproducing the observed SST during the medicane Ianos is comparable with the one of the uncoupled oceanic model in the Ionian Sea. In terms of heat fluxes, the coupling changes significantly the heat budget locally in the Ionian Sea, mainly through the latent heat flux and the shortwave radiation. The coupling is not that relevant for the intensification of the cyclone, whereas it enhances the representation of its path and the time of the landfall on the Ionian Islands.

How to cite: Maicu, F., Pinardi, N., Guadi, S., Clementi, E., Trotta, F., and Coppini, G.: SEAS: a simulation system for forecasting the atmosphere-coupled ocean dynamics in the Southern EuropeAn Seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8197, https://doi.org/10.5194/egusphere-egu24-8197, 2024.

The oceans are steadily warming, which affects global weather and climate and leads to an increase in extreme events such as storms, hurricanes and marine heatwaves (MHWs). Future warming scenarios predict an increase in the frequency and intensity of such events. In the Mediterranean region, both extratropical cyclones and occasional Mediterranean hurricanes (medicanes) occur, causing considerable damage to infrastructure and major socio-economic losses in coastal regions. Using ERA-5 atmospheric reanalysis data and satellite-derived sea surface temperatures (SST), this study looks at medicanes that occurred between 2011 and 2023 and examines their characteristics and impacts on the water column. The interaction with simultaneous MHWs in the Mediterranean is also investigated. A total of 15 medicanes occurred during the study period. Of these, 5 occurred in the western Mediterranean (WMed), mainly in November; 9 in the central Mediterranean and Ionian (CMed) between September and December, two of which terminated in the eastern basin; and 1 event was localised entirely in the eastern Mediterranean (EMed) in October. During the study period, 2014 recorded the highest number of medicanes with three events. One event, Ilona, occurred in January in the WMed, while the other two events, Qendresa and Xandra, occurred in November in the CMed and WMed respectively. Two events took place in both 2020 and 2021. In 2020, both Ianos in September and Elaina in December were in the CMed. In 2021, the CMed and WMed witnessed the passage of Apollo in October and Blas in November respectively. In the 15 medicanes, the mean sea level pressure (MSLP) was between 988 and 1005 hPa, while the wind speeds (Ws) were between 17 and 23 m/s. Among the events, Ilona in January 2014 had the lowest MSLP and highest Ws and the lowest associated MSLP anomaly. Of the 15 events, 11 (73%) were associated with anomalously high sea surface temperatures (SSTA) and five of these SSTAs were defined as MHW events. Moreover, the high SST anomalies were observed three or more days before the onset of these medicanes, which may have contributed to the intensification of the passing storms and amplified their impact through air-sea heat exchange. In turn, the medicanes were also observed to influence the MHWs, as the heat released from the ocean during the medicanes prevented the MHWs from deepening beyond the surface layer, demonstrating a dynamic interplay between these events. In summary, as the oceans warm, medicanes and MHWs in the Mediterranean increase, with complex interactions determining their behavior and impacts. Understanding these dynamics is crucial for predicting and mitigating the impacts of these events on marine ecosystems and coastal regions. 

How to cite: Uba, K., Hamdeno, M., Barth, A., and Alvera-Azcárate, A.: Two extremes: Investigating the impact of the co-occurrence of medicanes and marine heatwaves in the Mediterranean Sea., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13779, https://doi.org/10.5194/egusphere-egu24-13779, 2024.

The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model has been employed to simulate the anomalous post-monsoon tropical cyclone (TC) Jawad that originated over the Bay of Bengal (BoB) in December 2021. The atmospheric initial and boundary conditions (IC and BC) have been obtained from the Global Forecasting System (GFS) Analyses and Forecasts and two contrasting ocean IC and BCs, viz., HYCOM (experiment name GFS-HYCOM) and INCOIS (experiment name GFS-INCOIS), are implemented in two separate coupled experiments to evaluate the influence of TC Jawad on the surface and sub-surface characteristics of BoB. The track of the TC, including its recurvature, was well captured by both experiments with significant accuracy. A proper contrast in temperature between the two sides of the TC track was noted in the surface and sub-surface temperatures observed by two buoys, i.e., (1) BD11 (west of the TC track) and (2) BD13 (east of the TC track), and the simulated temperatures were validated with these observations. Contrary to the usual scenario, the higher sub-surface warming on the eastern side of the TC track was captured by GFS-HYCOM, but with a significant overestimation. The lower temperature on the western side of the TC track can be attributed to the weak upwelling associated with the cyclonic circulation caused by the interaction of the TC with the southward coastal currents. An unusually higher downwelling on the eastern side of the TC track was observed in the vertical distribution of the temperature across the longitudes, which suggested the existence of a strong clockwise circulation near the location of BD13. GFS-HYCOM, which simulated a higher current magnitude in the sub-surface than GFS-INCOIS on the eastern side of the TC track, captured the circulation near BD13 more rigorously. From further analysis, it was inferred that the interaction of the cyclonic wind flow of TC Jawad (westerly) near the surface with the easterly flow caused the generation of the clockwise circulation over the ocean surface on the eastern side of the TC track, leading to intense downwelling and warming of the sub-surface temperature. This scenario was further corroborated by the simulated Ekman transport and higher convective activity in the eastern quadrants of the TC. The present study not only emphasizes the capability of the coupled ocean-atmosphere models to simulate TCs but also highlights the necessity of investigating the air-sea interaction processes and their responses to the passage of an anomalous TC like Jawad.

How to cite: Chakraborty, T., Pattnaik, S., and Joseph, S.: Modulation of surface and sub-surface circulation in the Bay of Bengal by the passage of tropical cyclone Jawad: coupled ocean-atmosphere feedback , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11375, https://doi.org/10.5194/egusphere-egu24-11375, 2024.

Regional climate models (RCMs) over Europe often exhibit wet precipitation biases, primarily attributed to excess oceanic evaporation across time scales. One likely source for such wet biases are therefore imperfections in the models’ lower boundary condition (LBC) over the ocean, as realized by time-evolving sea surface temperature (SST) fields. SST data from atmospheric reanalyses (e.g., ERA5) are commonly adopted in RCMs, but ambiguity exists about the exact SST variable in these products (e.g., foundation or skin temperature) and the manner with which they represent the diurnal cycle and spatial gradients. Here we explore these questions with a ~12-km setup of ICON-CLM (Icosahedral Nonhydrostatic Model in Limited-Area Mode) over the EURO-CORDEX domain, run repeatedly for 6 years with various SST datasets. We use ERA5-based daily SST and skin temperature and hourly upper-layer SST drawn from our own global ocean simulations with FESOM2 (Finite Element Sea-Ice Ocean Model) at ~10-km node spacing in the eastern North Atlantic. Specifically, prescribing the FESOM2 SST fields in ICON-CLM both with and without spatial smoothing allows us to examine the effects of oceanic eddies and fronts on precipitation characteristics onshore. Preliminary results from 7 months of integration with ICON-CLM suggest that the choice of the SST data appreciably impacts latent heat fluxes, moisture transport onto land, and cumulative continental precipitation, generally in areas of pronounced moisture recycling. “Mind your SST” is therefore the advice we can give to ongoing dynamical downscaling efforts aimed at modeling future precipitation changes over land.

How to cite: Da Silva Lopes, F. and Schindelegger, M.: How the treatment of sea surface temperature affects the water cycle in EURO-CORDEX simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-290, https://doi.org/10.5194/egusphere-egu24-290, 2024.

The Kuroshio Extension (KE) bimodality has important effects on the ocean environment, ecosystem and climate. Previous studies have revealed that the Kuroshio Extension (KE) bimodality is mainly determined by the westward-propagating Rossby wave triggered by the North Pacific decadal variability such as PDO or NPGO: the positive (negative) phase of NPGO corresponds to the stable (unstable) KE state. However, the KE state and the NPGO seem to be decoupled since 2017, during which the NPGO takes a negative phase but the KE is in a stable state. This study employs the Convergent Cross Mapping (CCM) method to investigate the causality between the KE bimodality and NPGO. Simultaneously, we divide the KE region into the upstream (west of 146°E) and downstream regions. It is found that the NPGO has a significant causal impact on the downstream KE state. But the effect on the upstream KE state significantly weakens around 2017. Further analysis indicates that the upstream KE state is mainly caused by eddy activity in the Kuroshio large meander region south of Japan. In particular, the changes in the eddy activity affect the downstream advection of eddies and induce changes in the Kuroshio position over the Izu ridge, which cause different states in the KE upstream region. Therefore, we should not only consider the NPGO change, but also the eddy activity change in the Kuroshio region south of Japan when understanding and predicting the KE low-frequency variability.

How to cite: Wang, Q.: Revisiting the relationship between the North Pacific decadal variability and the Kuroshio Extension bimodality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7259, https://doi.org/10.5194/egusphere-egu24-7259, 2024.

The ocean's upper layer is inherently turbulent and constantly forced by momentum and buoyancy fluxes, and their interplay operates to mix and/or stratify the first meters of the water column. Incoming solar short-wave radiation acts to stabilize the upper layer, whereas the wind transfers momentum to the ocean and acts to vertically mix the water column. However, if the wind is not strong enough to trigger mixing, stratification in the near-surface is immediately formed in a layer of O(10) m thickness, called the diurnal warm layer (DWL). Above the bottom boundary of the DWL, shear production can be enhanced leading to large dissipation of turbulent kinetic energy (TKE) rates, i.e. high turbulence. Based on observational data from an ocean glider with a mounted microstructure package and drifters, we show the evolution of three DWLs sampled on the rim of a mesoscale eddy in the South Atlantic ocean (32^oS, 4^oE) with respect to temperature and buoyancy anomalies, potential energy and dissipation of TKE. In the near-surface, temperature and buoyancy anomalies increase with the evolution of the DWL, and the latter has the same magnitude as the time-integrated surface buoyancy flux. We also show the development of a diurnal jet with magnitude of O(10) cm/s that veers with the wind. Late in the afternoon, when the diurnal jet is fully developed, the bulk Richardson number (Ri_b) indicates that the stratified layer related to the DWL becomes marginally unstable (Ri_b ~ 0.25). During this period, the potential energy also decays, suggesting that the enhanced turbulence within the DWL acts to destroy stratification through turbulent mixing. We further assess whether a one-dimensional turbulence model is able to reproduce the observed DWL’s characteristics and the change in stability throughout the day.

How to cite: Miracca Lage, M., Ménesguen, C., Merckelbach, L., Dräger-Dietel, J., Griesel, A., and Carpenter, J.: The evolution of turbulence, stratification, and the surface jet in Diurnal Warm Layers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8431, https://doi.org/10.5194/egusphere-egu24-8431, 2024.

An improved understanding of wave breaking is still a hot topic owing to its relevance in the coupling of ocean and atmosphere. Multiple communities are focusing on numerical simulations of the fully coupled two-phase flow, the validation of such models remains challenging. Herein, we demonstrate how coherent marine radars can help to shed light on how different wave and wind parameters influence the evolution of waves towards breaking in the nearshore. The interpretation of the results is undermined by SWASH simulations of shoaling waves for different wave spectra and two beaches and simulations of radvarimages of these waves. Data of three independent measurement campaigns shows that the shoaling characteristics are strongly influenced by the wave steepness, relative depth the Ursell number and the wind. The influence of individual parameters cannot be isolated, but must be understood in its entirety.

How to cite: Støle-Hentschel, S., Catalán, P., Streßer, M., Horstmann, J., and Dias, F.: On the influence of hydrodynamic and environmental conditions on wave breaking in the nearshore, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20158, https://doi.org/10.5194/egusphere-egu24-20158, 2024.