Analysis of the atmospheric and oceanic circulations of the southern ocean with the help of numerical models

Bu çalışmada amacımız, Güney Okyanusu üzerindeki rüzgar dinamiklerini ve bunun okyanus devinim sirkülasyonu üzerindeki etkisini incelemektir. Bu amaçla, atmosfer ve bileşik okyanus-deniz buzu yüksek çözünürlüklü bölgesel modelleri ayrı ayrı koşturulmuştur. 2007 ve 2013 yılları arasında eşzamanlı olarak üç benzetim gerçekleştirilmiştir. İlk benzetim, gözlemlenen deniz yüzeyi sıcaklığı ve deniz buzu konsantrasyonu tarafından zorlanan sadece atmosfer bölgesel modelidir. Model, ortalama deniz seviyesi basıncı, 2 metre hava sıcaklığı, yukarı atmosfer jetleri ve Stratosferik Polar Vortex gibi önemli atmosferik özelliklerin mevsimselliğini başarıyla yakalamıştır. Model, Antarktika'daki gözlem istasyonlarıyla uyumluluk göstermektedir. İkinci benzetim, reanaliz atmosferik veri seti ile zorlanan kontrol okyanus-deniz buzu bileşik bölgesel modeldir. Okyanus modeli, deniz yüzeyi sıcaklık gradyanını doğru şekilde yakalamayı başarmıştır. Drake Geçidi'ndeki taşınım değerleri gözlemler dahilinde yaklaşık 152 Sv'dir. Son olarak, Güney Okyanusu üzerindeki bölgesel rüzgar gerilmesinin 1,5 kat arttığı bir duyarlılık benzetimi de yapılmış ve daha güçlü Drake Geçidi taşınımı ve Deacon Hücresi sirkülasyonu gözlemlenmiştir. Bu çalışma ileride gerçekleştirilebilecek Güney Okyanusu tamamen bütünleşik atmosfer-okyanus modeli geliştirilmesi için kapasite ve kabiliyetlerin ortaya konmasını sağlamıştır.In this study, our aim is to investigate Southern Ocean wind dynamics and its impact on the ocean overturning circulation. To this end, we performed atmosphere and ocean-sea ice coupled regional high-resolution models separately. We conduct three concurrent simulations spanning between 2007 and 2013. The first simulation is atmosphere only regional model forced by observed sea surface temperature and sea ice concentration. The model successfully captures important atmospheric properties such as mean and seasonality of the sea level pressure, 2 meter air temperature, upper level jet, Stratospheric Polar vortex. The model compares well against the observation stations throughout the Antarctica. The second simulation is the control ocean-sea ice coupled regional model forced with reanalysis atmospheric dataset. In the ocean model, we capture the sea surface temperature gradient. The transport at the Drake Passage is around 152 Sv which is within the observation values. Finally, we conduct a sensitivity simulation where the zonal wind stress over the Southern Ocean is increased 1.5 times. This leads to stronger Drake Passage transport and Deacon Cell overturning circulation in the model. This study has provided to demonstrate the capacity and capabilities to develop a Southern Ocean integrated fully coupled atmosphere-ocean model that can be carried out in the future

An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part III: Hydrography and fluxes

In this paper we compare the simulated Arctic Ocean in 15 global ocean–sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves

How does the Red Sea outflow water interact with Gulf of Aden Eddies?

► In this study, we model the interaction between the RSOW and Gulf of Aden eddies. ► The Red Sea overflow becomes a western boundary undercurrent in the absence of eddies. ► The modeled Red Sea overflow’s pathways rely on Gulf of Aden eddies. ► We find that the modeled RSOW exits western part of the GOA in short episodic bursts. ► The transport and mixing of the RSOW within GOA depend on mesoscale eddies. As the Red Sea overflow water (RSOW) enters the Gulf of Aden (GOA), it interacts with a sequence of nearly barotropic, mesoscale eddies originating in the Indian Ocean. To investigate how these eddies impact the dispersal and eastward transport of the RSOW toward the Indian Ocean, a high resolution 3D regional model is employed to explore systematically the interaction between the RSOW and mesoscale eddies. Two types of experiments are conducted. In the first set, we simulate the behavior of RSOW in the presence of an idealized cyclone and an idealized anticyclone. The second type of simulation involves nesting of the regional model (ROMS) within a data-assimilating global model (HYCOM), in which a sequence of mesoscale eddies entering the Gulf of Aden is realistically captured. This simulation is integrated for one year, and includes a simple representation of the seasonality of the RSOW. Bower et al. (2002) suggest that the Red Sea overflow might be a western boundary undercurrent. Consistent with these expectations, the idealized simulations show that the preferred pathway of the RSOW in the absence of eddies is along the coast of Somalia (southern continental shelf) as a western boundary undercurrent. Simultaneously, a cyclonic circulation is generated in the far western GOA due to vortex stretching by the descending outflow. The presence of a cyclone in the western GOA increases the peak RSOW transport, but the cyclone itself rapidly loses its coherence after interacting with the rough topography in the western GOA. The presence of an anticyclone tends to block the preferred boundary pathway and inhibits the eastward transport of the RSOW. The eddies also result in substantially increased mixing of the RSOW in the western GOA. On the basis of the more realistic ROMS experiment, it is found that the modeled RSOW leaves the western part of the Gulf of Aden in short episodic bursts with transports that are an order of magnitude greater than that associated with the quasi-steady RSOW inflow into GOA. Such enhancement in RSOW transport is shown to be induced by cyclonic eddies that cause a rapid discharge of RSOW from the western part of the GOA. We conclude that mesoscale eddies play a key role in the transport and mixing of the RSOW within GOA

Very large eddy simulation of the Red Sea overflow

Mixing between overflows and ambient water masses is a critical problem of deep-water mass formation in the downwelling branch of the meridional overturning circulation of the ocean. Modeling approaches that have been tested so far rely either on algebraic parameterizations in hydrostatic ocean circulation models, or on large eddy simulations that resolve most of the mixing using nonhydrostatic models. In this study, we examine the performance of a set of turbulence closures, that have not been tested in comparison to observational data for overflows before. We employ the so-called very large eddy simulation (VLES) technique, which allows the use of k – ε models in nonhydrostatic models. This is done by applying a dynamic spatial filtering to the k – ε equations. To our knowledge, this is the first time that the VLES approach is adopted for an ocean modeling problem. The performance of k – ε and VLES models are evaluated by conducting numerical simulations of the Red Sea overflow and comparing them to observations from the Red Sea Outflow Experiment (REDSOX). The computations are constrained to one of the main channels transporting the overflow, which is narrow enough to permit the use of a two-dimensional (and nonhydrostatic) model. A large set of experiments are conducted using different closure models, Reynolds numbers and spatial resolutions. It is found that, when no turbulence closure is used, the basic structure of the overflow, consisting of a well-mixed bottom layer (BL) and entraining interfacial layer (IL), cannot be reproduced. The k – ε model leads to unrealistic thicknesses for both BL and IL, while VLES results in the most realistic reproduction of the REDSOX observations

Performance of two-equation turbulence closures in three-dimensional simulations of the Red Sea overflow

Mixing of overflows released from polar and marginal seas is a key process shaping the structure of the meridional overturning circulation. Ocean general circulation models have difficulty in resolving the overflows, and therefore they must rely on parameterizations. In this study, the performance of a set of turbulence closures in reproducing mixing of an overflow is quantified. We simulate the Red Sea overflow by employing standard k– ε, k– ω and Mellor–Yamada schemes with various stability functions, as well as a modified k– ε model that relies on the prescription of the turbulent Prandtl number rather than on stability functions. The simpler KPP mixing scheme and experiments without turbulent fluxes serve as useful references. To our knowledge, this is the first time that the performance of two-equation turbulence models has been examined so closely using data from an overflow. It is found that without turbulence closures, the hydrodynamic model has difficulty in reproducing the correct three-dimensional pathway of the Red Sea overflow, consisting of a distinct bifurcation into two diverging channels. All turbulence models capture the vertical structure of this overflow consisting of an interfacial layer, characterized by the density gradient, and a well-mixed bottom layer. Mean eddy diffusivity values from most closures are comparable those from observations. But we find that KPP leads to eddy diffusivity values that are too small while those from Mellor–Yamada with Galperin [Galperin, B., Kantha, L.H., Hassid, S., Rosati, A., 1988. A quasi-equilibrium turbulent energy model for geophysical flows. J. Atmos. Sci. 45, 55–62] stability functions are too large. Such high diffusivities lead to excessive mixing in the bottom layer of the overflow, ultimately resulting in a salinity deficit of approximately 1 psu in the product water mass. Salinity deviations between the models and observations are quantified using both data taken along the channels and two sections across the overflow. KPP and Mellor–Yamada with Galperin (1988) stability functions produce the largest deviations from the observations, while the modified k– ε exhibits the smallest deviations. The other four closures fall in between, showing results similar to one another. The performance of the Mellor–Yamada turbulence closure is improved considerably by using the stability functions by Kantha and Clayson [Kantha, L.H., Clayson, C.A., 1994. An improved mixed layer model for geophysical applications. J. Geophys. Res. 99 (December), 25235–25266], which allow for a stationary Richardson number of 0.21. In conclusion, we find that most turbulence closures lead to a satisfactory reproduction of the Red Sea overflow, within the temporal and spatial sampling uncertainties of the REDSOX data, provided that fairly high-resolution regional models are used