OS4.3

Atmospheric rivers (ARs) dominate moisture transport globally, accounting for 90% of poleward atmospheric freshwater transport in the mid-to-high latitudes while only covering 10% of the surface. Yet, it is unknown what impact ARs have on the surface ocean buoyancy in the high latitudes. This is explored using high-resolution surface observations from a Wave glider deployed at a site in the Southern Ocean (54°S, 0°E) during austral summer. During this time (19 December 2018 - 12 February 2019, 55 days) we show that when ARs combine with storms over this area, the associated precipitation is enhanced significantly (162%). AR-induced precipitation events provided a major source of surface ocean buoyancy equivalent to the input of surface heat fluxes on a daily timescale. Cumulatively, ARs account for 44% of the summer precipitation equating to 9% of surface buoyancy gain. These results show that AR variability is a previously unaccounted driver of Southern Ocean surface buoyancy that may ultimately impact upper ocean water mass transformation and the dynamics of the ocean surface boundary layer.

How to cite: Edholm, J., Swart, S., du Plessis, M., and Nicholson, S.-A.: Glider observed surface buoyancy forcing from an atmospheric river in the Weddell Sea during austral summer 2019, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-115, https://doi.org/10.5194/egusphere-egu22-115, 2022.

Including the ocean surface current in relative wind stress is known to damp mesoscale eddies through a negative wind power input. This is thought to have potential ramifications for eddy longevity. Here, we study the spin-down of a baroclinic anticyclonic eddy subject to absolute and relative wind stress forcing by employing an idealised high-resolution numerical model. To assess the effect of relative wind stress on the eddy, we examine wind-induced vertical motions and energetics. Results from this study show that relative wind stress damps eddy kinetic energy (EKE) at the eddy’s surface. However, relative wind stress also induces additional vertical motions, in the form of Ekman pumping, that increases baroclinic conversion i.e., a conversion of potential to kinetic energy. When horizontally integrated, this additional baroclinic conversion by relative wind stress is positive throughout the eddy water column. The positive baroclinic conversion in the lower depths of the eddy leads to an increase in deep EKE, relative to the absolute wind stress case. In fact, over the eddy volume, the damping of EKE by relative wind stress is offset by this conversion of energy. Moreover, this conversion turns out to dominate any damping by wind during the later stages of eddy lifetime. A scaling analysis of relative wind stress-induced baroclinic conversion and relative wind stress damping also confirms these numerical findings, showing that energy conversion is greater than wind damping. Overall, this highlights the complexities of ocean-atmosphere interactions at the mesoscale, and points to the need for further study in this area.

How to cite: Wilder, T., Zhai, X., Joshi, M., and Munday, D.: The Response of a Baroclinic Anticyclonic Mesoscale Eddy to Relative Wind Stress Forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1161, https://doi.org/10.5194/egusphere-egu22-1161, 2022.

How to cite: Yang, M., Smyth, T., Kitidis, V., Brown, I., Wohl, C., Yelland, M., and Bell, T.: Natural variability in air–sea gas transfer efficiency of CO2, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2527, https://doi.org/10.5194/egusphere-egu22-2527, 2022.

A mixing length theory which considers the impact of TC characters and upper ocean stratification, is used to estimate the tropical cyclone (TC) induced diapycnal diffusivity, and investigate the trend, interannual and interdecadal variability of TC-induced diapycnal diffusivity in the globe and each basin. The annual mean climatology of the TC-induced diapycnal diffusivity is consistent with previous research, with maximum values in the Western North Pacific (WP) ranging from 0.05 cm²/s up to 1 cm²/s. The trends of TC-induced diapycnal diffusivity exhibit great inter-basin differences, which are not only related with TC itself, but also the ocean stratification. On the interannual timescales, El Niño and Southern Oscillation (ENSO) can modulate the variability of TC-induced diapycnal diffusivity in the globe by regulating the ocean stratification rather than TC intensity, because the impacts of ENSO on TC intensity in each basin cancel out each other. As for each basin, ENSO can affect TC-induced diapycnal diffusivity mainly by regulating the variability of TC intensity. In addition, the relationship of TC-induced diapycnal diffusivity with dominant climate modes such as Pacific Decadal Oscillation (PDO) and North Atlantic Oscillation (NAO) may be interactive on the interdecadal timescales, especially in the areas which are significantly influenced by PDO and NAO, such as WP, Eastern North Pacific and North Atlantic. We anticipate that these results can provide insights into the variability and physical mechanisms of TC-induced diapycnal mixing.

How to cite: Cao, Y., Wang, X., and Shao, C.: Global Estimate of Tropical Cyclone-Induced Diapycnal Mixing and Its Links to Climate Variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3266, https://doi.org/10.5194/egusphere-egu22-3266, 2022.

Tropical Cyclones (TC) are strongly coupled systems as the underlying warm ocean serves as an energy source for the TC while the strong cyclonic winds modify the ocean state. Good predictions of the TC development are dependant on our knowledge of the ocean heat content which may favor or inhibit the TC. Understanding how the ocean stratification evolves at the same time the TC does is thus crucial to improve TC forecasts.

The 2018-2019 cyclonic season of the South Western Indian Ocean was active and saw the development of nine intense TCs. These cyclones went through regions with different oceanic properties in terms of stratification and heat content. The aim of this study is to understand how such ocean properties affect TC evolution.

To this end, we conducted several idealized simulations of TC using the same atmospheric state but with different oceanic profiles (temperature, salinity) derived from 5-month MERCATOR analysis data (from November 2018 to March 2019). The experiments were conducted using a state of the art coupled modelling system with CROCO (for the ocean) and Meso-NH (for the atmosphere) models with a grid spacing of 4 km.

The TC lifecycle (i.e intensity, structure) as well as the ocean response (i.e. sea surface cooling, advection and mixing processes) are investigated with a particular emphasis on the heat budget analysis. We found that a rapid TC intensification phase occurred due to the warm oceanic surface layers (the first 40 meters) and a strong decaying phase occurred due to the cooler underlying ocean. Moreover we highlight the chronology of the cooling processes in the oceanic mixed layer and the importance of the advection processes within it, which are then relayed by vertical mixing.

How to cite: Kaouah, M., Lee, K.-O., Bielli, S., and Lapeyre, G.: Oceanic restratification processes associated with tropical cyclone intensification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4142, https://doi.org/10.5194/egusphere-egu22-4142, 2022.

Modeling air-sea interactions during cold air outbreaks poses a major challenge because of the vast range of scales and physical processes involved. Using the WRF model, we investigate the sensitivity of air mass transformation in an idealised cold air outbreak across a lead-fractured sea ice to (a) lead width, (b) lead orientation relative to the atmospheric flow, and (c) model resolution.

The extent to which leads are resolved in WRF strongly affects the overall air-sea heat exchange. In fact, even the direction of the heat exchange is dependent on model resolution. Further, the dependence of the overall heat exchange on model resolution is strongly non-linear, with the worst representation of the heat exchange through leads occuring when they are just about to become resolved by the model grid. In addition, the orientation of the leads relative to the atmospheric flow affects the air-sea heat exchange. Heat exchange is least effective when the leads are oriented perpendicular to the atmospheric flow.

How to cite: Spengler, T. and Spensberger, C.: Sensitivity of air-sea heat exchange to lead width and orientation as well as model resolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4167, https://doi.org/10.5194/egusphere-egu22-4167, 2022.

On January 8, 2020, an extreme storm event took place in the Eastern Mediterranean Sea, during which 100-130mm of rain fell in the northern part of Israel in one day. The heavy precipitation event resulted in seven deaths and damages to homes, vehicles, and infrastructure. At the same time, about 100km to the west of northern Israel, the sea was characterized by a mesoscale eddy with a warm core. In recent years, it was established that small-scale sea features affect the atmosphere above and synoptic-scale circulation patterns, including long-term rainfall. However, it is still unclear how these features may affect the propagation and intensity of individual storms, such as the January 8, 2020 storm event.

Recently, the WRF (The Weather Research and Forecasting) atmospheric model was coupled with the ocean model MITgcm (MIT general circulation model). The coupled model was named the SKRIPS (Scripps–KAUST Regional Integrated Prediction System) model. The two SKRIPS model components (WRF and MITgcm) are well tested at high resolutions, and the regionality of the coupled model allows us to isolate local features while maintaining the large-scale circulation as observed.

In this talk, I will present results from a high-resolution (~5km) coupled atmosphere-ocean regional simulation using the SKRIPS model performed during the January 8, 2020 event. The importance of the sea eddy in determining the storm intensity and propagation will be discussed, elaborating on the role of air-sea coupling and the model resolution. Understanding the effect of such small-scale sea features on extreme atmospheric events may improve their representation in weather and climate models, extending models prediction skill.

How to cite: Strobach, E., Klein, P., and Ziv, B.: The role of a Mediterranean Sea eddy in the January 2020 flooding in Israel, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6257, https://doi.org/10.5194/egusphere-egu22-6257, 2022.

The impact of oceanic mesoscale eddies on sensible heat fluxes and related air-sea variables in the South China Sea, an eddy-active area, is investigated by using 20 years (2000–2019) of remotely sensed sea surface temperature, mesoscale eddy trajectories atlas with satellite altimetry and a high-resolution air-sea heat flux product. Composite analyses based on 623 cyclonic eddies (CEs) and 508 anticyclonic eddies (AEs) revealed that CEs (AEs) eddies tend to decrease (increase) the surface sensible heat fluxes over the eddies with maximum mean anomalies of -5.79W/m2 (4.36 W/m2), cool (warm) the sea surface and cause surface winds to decelerate (accelerate). The composite results of fluxes and variables anomalies are stronger near the eddies centres, but the extrema of anomalies locate westward relative to the CEs (AEs) cores due to the dominant moving direction of eddies in this region. The dynamic analysis of multiple mesoscale eddies tracks demonstrates the sustained and delayed response of the marine atmospheric boundary layer to oceanic eddies. The reduction (increase) of sensible heat flux over CEs (AEs) tracks reaches the maximum after CEs (AEs) pass 2 (3) days and averagely last for more than one week. In addition, the effect of mesoscale eddies on sensible heat fluxes increases with eddy amplitude and radius and negatively correlates with their moving speed. The results also show remarkable seasonal variations of CEs (AEs) influence on fluxes and variables anomalies, stronger in winter and weaker in summer.

How to cite: Huang, Y.: The signature of air-sea sensible heat fluxes associated with mesoscale eddies in the South China Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6772, https://doi.org/10.5194/egusphere-egu22-6772, 2022.

Air-sea gas exchange has up-scale ramifications for global climate and ocean biogeochemistry that are of paramount relevance. Gas transfer velocity (k) measurements or appropriate parameterizations for them are required to quantify the fluxes and budgets of the important trace gases (e.g., CO₂, DMS, and CH₄). Where gas flux and concentration gradients are not explicitly measured, k is subdivided into diffusive and bubble-mediated components – each parameterized. Although diffusive transfer velocity, k_s , has been well-described by power-law relationships involving the Schmidt number Sc, large variability exists in parameterizations for bubble-mediated gas transfer velocity, k_b. Since k_b is driven primarily by entrainment of gases through wave breaking, the uncertainty is acutely problematic at high winds where gas flux measurements are scarce. To address the paucity of such data, the High Wind Gas Exchange Study (HiWinGS) directly calculated gas transfer velocity of CO2 (k_CO2) from flux and concentration gradient measurements taken in the Labrador Sea from October 9 – November 13, 2013, where 10-meter neutral wind speeds ranged between 1.8 – 25.2 m s^-1. We use these data to validate a novel gas transfer velocity parameterization constructed using output from a wave hindcast obtained with the spectral wave model (ecWAM) forced with the European Centre for Medium-Range Weather Forecasts (ECMWF) 5th Generation Reanalysis (ERA5). Our parameterisation combines a diffusive term based on wind speed and Sc, and a bubble-mediated term based on gas solubility, wave age, and wave breaking energy dissipation rate to capture gas transfer velocity. We compare our results to common wind-speed-only parameterisations and more recent sea-state based relationships.

How to cite: Smith, A., Callaghan, A., and Bidlot, J.-R.: Parameterising CO2 air-sea gas transfer with wave breaking energy dissipation rate, sea state, and wind speed, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7473, https://doi.org/10.5194/egusphere-egu22-7473, 2022.

Remote sensing measurements of sea surface salinity (SSS) and sea surface temperature (SST) have been used to generate satellite-derived surface T-S diagrams [1], and to compute surface density flux, spiciness and water masses (WM) formation rates and extension [2].

More recently [3], this framework has been expanded in several directions, ranging from the extension of the studied basins and their temporal span, to the inclusion of a wider pool of source datasets. Satellite uncertainties have also been propagated to the final estimates (including also heat and freshwater fluxes uncertainties) of water masses formation rates and location. Several water masses have been characterized, showing a remarkable consistency with literature estimates.

The current efforts are devoted to additional investigation pathways. Firstly, it has been studied the impact on the actual estimates of water masses formation of satellite inputs at variable spatial (0,5  to 1 ) and temporal (weekly to monthly) scales. Secondly, the temporal evolution of the estimates over a 10-yr-long timespan has been studied, both in the T/S and geographical domains, detecting possible linear trends and anomalies. Lastly, investigation on additional water masses in the Pacific Ocean under the influence of ENSO variability is ongoing.

[1] Sabia R., et al. (2014), A first estimation of SMOS‐based ocean surface T‐S diagrams, J. Geophys. Res. Oceans, 119, 7357–7371.

[2] Sabia R., et al., Variability and Uncertainties in Water Masses Formation Estimation from Space, Ocean Sciences 2016, New Orleans, LA, USA, February 2016.

[3] Piracha A., et al., Satellite-driven estimates of water mass formation and their spatio-temporal evolution, Frontiers in Marine Science, 2019.

How to cite: Sabia, R., Caughtry, J., Fernandez-Prieto, D., and Piracha, A.: Spaceborne Water Mass formation detectability and temporal evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12821, https://doi.org/10.5194/egusphere-egu22-12821, 2022.