The northern Indian Ocean serves as an ideal space to study the interaction between Ocean and atmosphere as it accommodates unique and versatile oceanic conditions and the largest monsoonal circulation in the world. The South Asian Monsoon System, the largest of its kind, directly impacts the lives and livelihoods of billions of people living in the Indian Subcontinent. Various factors that influence its strength and characteristics have been studied extensively throughout the years. But, owing to its complex and dynamic nature, a comprehensive understanding and accurate monsoon prediction remain a work in progress. Several Oceanic components that play a part in monsoon processes have been identified. Our study focuses on the Bay of Bengal, distinguished from other oceans due to its highly stratified upper layers. Through this study, we aim to understand the Impact of mesoscale eddies in the Bay of Bengal on the Indian Summer monsoon.

The influence of oceanic mesoscale eddies on the circulation and precipitation directly over them has been addressed through different studies after the advent of high-resolution satellite data. The current research focuses on the large-scale influence of the eddies in the Bay of Bengal on the seasonal rainfall during the Indian summer monsoon(ISM). Indices were created using the Okubo Weiss parameter to understand the inter-annual variation of eddies (classified according to polarities and regions of occurrence). These indices correlated with the ISM system suggested that Anticyclonic eddies in the  Western Bay of Bengal strongly influenced wind and rainfall patterns over the monsoon region. The Anticyclonic Eddy activity that peaked during the El Nino years countered the suppression of rainfall by El Nino through enhanced synoptic activity in BoB. The low-pressure system formation and propagation in BoB were found to be stronger in the years having more anticyclonic eddies. The warming created by the warm-core Anticyclonic eddies initiates an Anticyclonic(Clockwise) circulation around the region, which feeds back into the existing oceanic conditions. This coupled Ocean-Atmospheric system mediated through the mesoscale eddies needs to be further analyzed through stand-alone and coupled modeling experiments. Improving the representation of the mesoscale processes in the Northern Indian Ocean can serve as a crucial step in improving the monsoon prediction systems.

How to cite: Vg, K., A Rao, S., and A Pillai, P.: Impact of Bay of Bengal mesoscale eddies on Indian Summer Monsoon Rainfall, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-514, https://doi.org/10.5194/egusphere-egu23-514, 2023.

The so-called frontal-scale air-sea interaction describing the atmospheric responses to oceanic fronts has been mainly discussed in the context of interactions between sea surface temperature and surface winds. The ocean current also influences the surface winds, which can significantly affect the atmosphere, especially in regions of energetic ocean currents and mesoscale activities as in the western boundary current systems. This study uses an atmosphere-ocean coupled model to analyze how the Kuroshio Current affects the momentum and turbulent heat fluxes and the atmospheric boundary layer and how these responses feed back to the ocean and atmosphere in this region. The ocean current coupling influences the path of Kuroshio extension and the eddy activities by mechanical and thermal current feedbacks. Mechanical current feedback reduces momentum flux and damps eddy kinetic energy (EKE) by reducing wind work as expected. On the other hand, the thermal current feedback associated with turbulent heat fluxes injects EKE by baroclinic energy conversion. Overall, the shift of Kuroshio Current and the change of eddy activities impact the region of strong turbulent heat release to the atmosphere, which can eventually trigger changes in weather systems.

How to cite: Cho, A., Song, H., and Seo, H.: Atmospheric and Oceanic Responses to Surface Current Coupling near the Kuroshio Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11795, https://doi.org/10.5194/egusphere-egu23-11795, 2023.

Coral bleaching events are more frequent and severe as global temperatures rise and marine heat waves are more frequent. However, quantifying the surface energy fluxes in coral reefs at various geoclimatic regions and the mechanisms by which the air-water interactions regulate water temperature is rare. We measure surface energy fluxes over coral reefs using Eddy Covariance towers in two contrasting geo-climatic regions: The typical setup of humid/tropical coral reefs (Heron Reef, Great Barrier Reef, Australia) and the rarer desert coral reef (Golf of Eilat, Israel). We analyze how the surface heat fluxes regulate the temperature of shallow coral reef environments. We show that in the desert reefs, the dry air overlying the shallow coral reef results in extremely high evaporation rates which in turn results in extensive cooling of the water. In humid/tropical reefs, evaporation is suppressed by humidity and is limited in the ability to offset the heating of the water. The extreme difference in evaporative cooling in desert versus tropical reefs is key in the response to marine heat waves. Marine heat waves which impose thermal stress on corals are a result of synoptic-scale circulation variations which suppress evaporation and increase heating. The most severe marine heatwave ever measured at the Gulf of Eilat (August 2021) was found to be caused by low evaporation rates, which resulted from synoptic circulation with weak winds and high humidity. After the onset of water temperature rise and the return of the dry winds, evaporation instantly cooled the water overlying the corals- relieving potential stress. Whereas, at the humid/tropical Heron Reef (Great Barrier Reef, Australia) evaporation solely is unable to reduce the high water temperature and therefore coral heat induce stress events are inevitably longer and more frequent. We conclude that evaporative cooling is a key mechanism protecting coral reefs located in deserts from extreme high-water temperatures, thereby representing possible thermal refugium for corals against background global warming.

How to cite: Abir, S., McGowan, H. A., Shaked, Y., and Lensky, N.: Identifying an Evaporative Thermal Refugium for the Preservation of Coral Reefs in a Warming World—The Gulf of Eilat (Aqaba), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16883, https://doi.org/10.5194/egusphere-egu23-16883, 2023.

Hitherto unresolved oceanic sub-mesoscale variability may retard air-sea exchange of heat and carbon in contemporary IPCC-type model projections. Here, we set out to put this hypothesis to the test in the Atlantic in a region off Mauretania that is renown for its complex coastal dynamics. Results from a suite of coupled ocean-circulation biogeochemical models suggest that the oceanic bottleneck between the atmosphere and the vast abyssal stocks of heat and carbon is relatively insensitive in the range from mesoscale and to sub-mesoscale resolution. More specifically we find that the sensitivity of effective vertical mixing to changes in spatial resolution is comparable to infamous uncertainties associated with contemporary numerical algorithms.

How to cite: Dietze, H., Löptien, U., Hordoir, R., and Renz, M.: Beyond Mesoscale off Mauretania, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8293, https://doi.org/10.5194/egusphere-egu23-8293, 2023.

In the past decades, numerous studies have shown that oceanic mesoscale activity, over scales of O(50–250) km, has a strong influence on the atmosphere through both the Thermal FeedBack (TFB) and the Current FeedBack (CFB). However, at the submesoscale, over scales of O(1–10) km, both TFB and CFB are not well understood, mainly due to technical barriers (observation and simulation). Here, a realistic high-resolution coupled oceanic model (dx = 1 km), including tidal forcing and river discharge, and atmospheric (dx = 2 km) model in the lower North Atlantic trades region over a period of 1-year (from July 2019 to June 2020) is used to assess the atmospheric response to submesoscale processes. We used classic coupling coefficients between the ocean and the atmosphere to quantify spatial and temporal variabilities of the TFB and CFB coupling.

Our results show that, similar to oceanic mesoscale activity, at the submesoscale, both TFB and CFB have an imprint on the low-level wind, surface stress and turbulent heat fluxes. On the one hand, the linear relationship between surface wind (stress) curl and surface current vorticity existing at the mesoscale regime is also supported at the submesoscale. At submesoscale, CFB, as at the mesoscale, is acting as a sink of energy from the ocean to the atmosphere, acting as an submesoscale eddy killer. Furthermore, the magnitude of surface stress curl introduced by submesoscale processes is greater by ~17 % than that presented by mesoscale processes, which is explained by a reduction of the wind response by ~55 %. On the other hand, the linear relationship between wind stress magnitude, or latent heat flux, and sea surface temperature (SST) anomalies, widely present at the mesoscale, is also found at the submesoscale. Similar results are found when considering wind stress curl/divergence and crosswind/downwind SST gradients coupling coefficients. However, the magnitude of the corresponding TFB coupling at submesoscale is reduced by ~30 % with respect to those at mesoscale.

Overall, our results emphasize the significant impact of both oceanic currents and strong SST fronts on the local wind/stress and latent heat flux response at the submesoscale regime.

How to cite: Conejero, C., Renault, L., and Giordani, H.: Mesoscale and Submesoscale air-sea interactions in the lower North Atlantic trades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13039, https://doi.org/10.5194/egusphere-egu23-13039, 2023.

Full understanding of air-sea interaction requires observations not only at the air-sea interface but also through the entire air-sea transition zone (the upper ocean, air-sea interface, and marine atmospheric boundary layer as a single identity). Our capabilities of observing collocated and simultaneous profiles through the entire column of the air-sea transition zone are currently limited. This study explores the feasibility of using combined uncrewed robotic systems in conjunction with conventional platforms to provide such collocated and simultaneous profiles of the air-sea transition zone. An introduction is given to experimental deployments of uncrewed surface vehicles (saildrones), underwater vehicles (gliders), aerial vehicles in coordination with profiling floats, surface drifters, airborne dropsondes, and moored buoys to observe the air-sea transition zone. Based on the preliminary results and lessons learned, a vision of potential capabilities of observing the air-sea transition zone in the future using combined uncrewed systems is offered.

How to cite: Zhang, C. and the Saildrone Hurricane Observations Mission Team: Observing the Air-Sea Transition Zone using Combined Uncrewed Systems and Other Conventional Platforms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8166, https://doi.org/10.5194/egusphere-egu23-8166, 2023.

Extreme surface turbulent heat fluxes affecting convective processes in subpolar ocean regions may amount to 1000-3000 W/m². Their quantitative estimation is critically important for many oceanographic and meteorological applications. Extreme turbulent fluxes are largely responsible for the vertical mixing in the ocean, especially in the subpolar latitudes where deep convection forms intermediate waters. Over western boundary currents and their extension regions very strong turbulent heat fluxes may result in local responses in the lower atmosphere on time scales from several hours to days and spatial scales from several kilometers to several tens of kilometers. Accurate estimation of extreme turbulent fluxes also strongly relates to the sampling problem especially for the poorly and irregularly sampled regions. Estimation of extreme turbulent fluxes is thus requires knowledge of probability distribution of fluxes. We suggest a concept for determination of extreme surface turbulent heat fluxes based upon theoretical probability distributions, which allow for accurate estimation of extreme fluxes. In this concept the absolute extremeness of surface turbulent fluxes is quantified from the Modified Fisher-Tippett (MFT) distribution. Further we extend MFT distribution to a fractional distribution, quantifying the so-called relative extremeness, representing the fraction of surface flux accumulated during continuous time (e.g. months, season) due to most intense surface fluxes (e.g. the strongest 1% of flux events). We provide explicit form of the fractional distribution and effective algorithms for parameter estimation.

Further we demonstrate applications of the concept for the global ocean using reanalyses and Voluntary Observing Ship (VOS) data for the period 1979 onwards. Global climatologies reveal that the regions with the strongest relative extremeness are not collocated with the strongest mean fluxes. Moreover, interannual variability of the absolute and relative flux extremes is not necessarily correlated with variability of mean fluxes. Growing mean flux may result in both increase and decrease of absolute and relative extremes that has implications for estimates of linear trends which may have different signs for mean fluxes, absolute and relative flux extremes – the situations found for the western boundary currents and major convections sites. Suggested concept has also profound implications for comparative assessments of surface turbulent fluxes from reanalyses (ERA5, ERA-Interim, CFSR, MERRA2, NCEP-DOE, JRA55) and satellite (IFREMER, J-OFURO, HOAPS, SEAFLUX) products. High mean fluxes in some products are not necessarily associated with the strongest absolute and relative extremeness in the same products and vice versa. Also a new concept allows for accurate estimation and minimization of sampling biases in VOS and satellite flux products. Finally, we analyzed the mechanisms responsible for forming extreme surface turbulent fluxes associated with cold air outbreaks in the rear parts of midlatitudinal cyclones under locally high winds and strong air-sea temperature gradients.

How to cite: Gulev, S., Belyaev, K., and Tilinina, N.: Extreme air-sea turbulent heat fluxes over the global oceans: determination, implications and mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2957, https://doi.org/10.5194/egusphere-egu23-2957, 2023.

Ocean heat stored in the Western Pacific Warm Pool (WPWP) helps drive some of the foremost modes of weather and climate variability including ENSO, Intraseasonal Oscillations, and tropical cyclones. To understand the associated coupled mechanisms that regulate ocean temperature, we use reanalysis and a novel moored microstructure dataset yielding estimates of the turbulent ocean heat flux (J_q(z)) to describe how WPWP mixing is regulated by seasonal, intraseasonal, and synoptic-scale atmospheric disturbances. It is argued that observed variations in J_q(z) impact seasonal-to-synoptic trends in SST and mixed-layer depth. J_q(z) is weakest during Spring (dry season), when heat fluxes into the ocean (Q_net > 0) create a shallow mixed layer (ML) of warm water that lays undisturbed atop the seasonal thermocline. In the Summer, westerly winds associated with the Asian Monsoon create favorable conditions for tropical depression-like (TD-like) storms, which in turn cause episodic spikes in J_q(z) that gradually deepen the ML and momentarily cool sea surface temperature (SST) while the background SST continues to rise. Towards the end of the summer, SST at our mooring was greater than 30.7 °C but rapidly dropped and stabilized below 29.5 °C after a strong pulse of the Madden-Julian Oscillation (MJO) cooled the upper ocean and deepened the ML for almost 15 days in a row. Enhanced upper ocean mixing continued to be episodic throughout the Fall as TD-like storms and intraseasonal disturbances moved over the mooring site. Mixing remained high throughout the Winter, when cold outbreaks forced the upper ocean at high frequencies and mean convective cooling (Q_net < 0) further contributed to mixing. A similar seasonality is observed at the thermocline level, where J_q(z) is enhanced by storm-driven near-inertial internal waves (NIWs) whose power peaks between Fall and Winter. While intraseasonal wind bursts had the greatest impact on near-surface mixing, synoptic disturbances generated greater-amplitude NIWs and thus had a greater potential to mix temperature gradients in the permanent thermocline. Lastly, we use reanalysis data to argue that storm-driven mixing shapes interannual relations between SST, storm activity, and the Asian Monsoon. 

How to cite: Gutierrez Brizuela, N., Xia, Y., Xie, S.-P., Alford, M., and Moum, J.: Seasonal buildup and downward transfer of Warm Pool heat content by wind-driven ocean mixing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3693, https://doi.org/10.5194/egusphere-egu23-3693, 2023.

How to cite: Putrasahan, D. and von Storch, J.-S.: Geographical distribution and spatio-temporal scale dependence of air-sea coupling via the vertical mixing mechanism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4685, https://doi.org/10.5194/egusphere-egu23-4685, 2023.

Tropical cyclones (TCs) are severe weather marvels, occur in warm tropical waters. These phenomena are among the most influential atmospheric-oceanic events in subtropics regions as the northern part of the Indian Ocean, affecting the Arabian Sea and the Oman Sea, which often cause severe damage to the coastal areas. The interaction between the atmosphere and the upper ocean plays an important role in the structure of TCs, in which successful connections between ocean circulations and the intensity and track of TCs have been identified. TCs derive their energy primarily from the release of latent heat through evaporation and sea spraying in the atmosphere boundary layer. This implies that the presence of a moisture source, with sufficiently warm sea surface temperature (frequently above 26°C) is required to sustain the flux of moisture from the ocean to the atmosphere. The most visible effect of TC passage is the cooling of the sea surface temperature (SST) as the response of the ocean mixed layer (OML) temperature. This decrease in SST has a negative feedback on the intensity of TCs, as it suppresses the heat exchange flux between the atmosphere and the ocean, consequently it can affect the TC track.

To investigate the effect of the temperature field on TCs structure, TC Shaheen (2021) with unusual track and entry into the Gulf of Oman is studied. In this regard, Weather Research and Forecasting (WRF) model was used with two different configurations. First, WRF was ran standalone and SST field was only adopted from global models as initial conditions and were not updated during the simulation. Then, as the second configuration, WRF model was coupled with an ocean finite volume model and the SST field was updated online during the simulation. The initial conditions of the ocean temperature, salinity and velocity field were taken from GOFS 3.1 global reanalysis product from the HYCOM Consortium. Primary result for the selected event implies that SST correction during TC simulation with WRF improves air-sea heat flux and has a pronounced effect on the TCs’ intensity and track predictions. Cold wake underside of the TC led to a remarkable heat flux loss from ocean surface into the storm. Hence, the TC size is reduced and the maximum wind speed of the storm is intensified.

How to cite: Farhangmehr, A., Ghader, S., Ali Akbar Bidokhti, A. A., and Ranji, Z.: A Preliminary Forecast Study of the North Indian Ocean Tropical Cyclones Using a Coupled Atmosphere-Ocean Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7362, https://doi.org/10.5194/egusphere-egu23-7362, 2023.

A new ocean-atmosphere-wave regional coupled model named Windwave version 1.0 for simulating and predicting winds and waves has been developed for the Northwest Pacific Ocean. In particular, the global-to-regional nesting technique is adopted for the ocean component to alleviate the bias due to the inconsistency in the lateral boundary. This paper is devoted to describing the coupling details of Windwave and the initialization scheme and assessing its basic performance, especially in predicting surface winds and significant wave heights (SWH) on the weather timescale. The control experiment set contains 31 experiments for August 2020, with seven typhoons passing through the Northwest Pacific. Each experiment starts at 0:00 am UTC of each day and runs for three days. Experiment results show that the new coupled model performs well in predicting surface winds, SWH, surface air temperature, and sea surface temperature on the weather timescale. In particular, the Root Mean Square Errors (RMSEs) of surface winds at 10 m height over the Northwest Pacific of the control experiment are 1.82 m s^-1, 2.22 m s^-1, and 2.59 m s^-1, respectively, at lead times of 24 h, 48 h, and 72 h. Meanwhile, the RMSEs of SWH at lead times of 24 h, 48 h, and 72 h are 0.39 m, 0.43 m, and 0.51 m. In addition, we have explored the impacts of the different sea surface aerodynamic roughness parameterization schemes on predicting surface winds and SWH. In total, five different sea surface aerodynamic roughness parameterization schemes are adopted, corresponding to one control set and four sensitivity sets of experiments. Under normal conditions, the sea surface aerodynamic roughness parameterization scheme considering the effects of wind-wave direction tends to perform better for winds and waves, while that depending on wave age and SWH tends to perform worse. Under extreme wind and wave conditions, the schemes considering the effects of wind-wave direction and that considering wave age and peak wave length have better performance. These findings can provide new insights for developing a more advanced sea surface aerodynamic roughness parameterization scheme.

How to cite: Fu, S., Huang, W., Lv, R., Yang, Z., Qiu, T., Sun, D., Luo, J., Huang, X., Fu, H., Luo, Y., and Wang, B.: Develop an ocean-atmosphere-wave regional coupled model Windwave version 1.0 for predicting wind and wave conditions of the Northwest Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3127, https://doi.org/10.5194/egusphere-egu23-3127, 2023.