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Yizhak Feliks, Hezi Gildor, and Nadav Mantel

Abstract

The intraseasonal oscillations (ISOs) in sea currents in the eastern Mediterranean Sea near the central coast of Israel were analyzed by examining the velocity components of the sea currents at different depths as measured by acoustic Doppler current profilers located at various depths between 0 and 675 m. The total period covered by the observations was from December 2016 to May 2018. Prominent intraseasonal oscillations, much stronger than tidal velocity components, were observed in the upper part of the sea, at 30–70 m. The amplitudes of these oscillations are between 4 and 10 cm s−1 and their periods are 7, 11, 22, and 34–36 days. The strongest oscillations are found in the boreal winter. The ISOs in the sea currents were apparently induced by corresponding oscillations found in atmospheric wind velocity over the eastern Mediterranean at the surface and at 500 and 250 hPa, as suggested by the high correlations, 0.6–0.9, between the wind velocity components of the oscillatory modes in the atmosphere and the velocity component of the oscillatory modes in the sea currents with similar periods. We propose that the source of the ISOs in the atmosphere over the eastern Mediterranean is the South Asian jet wave train. The track of this wave train passes over the eastern Mediterranean, and the periods of the ISOs in the wave train are in the same band as the oscillations found here. The wave train is equivalently barotropic and strongest in the upper troposphere. This property of the wave train can explain the high correlation found between the oscillatory modes of wind velocity at 250 or 500 hPa and those in the sea currents. In all the cases besides the 7-day oscillatory mode, the significant oscillatory modes found at 250 or 500 hPa are also significant in the velocity components of the surface wind.

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Leonel Romero and Kabir Lubana

Abstract

We present an investigation of the azimuthal bimodality of the wind-wave spectrum for waves shorter than the dominant scale comparing numerical model solutions of developing waves from idealized experiments using WAVEWATCH III (WW3). The wave solutions were forced with the “exact” Webb–Resio–Tracy (WRT) nonlinear energy fluxes and the direct interaction approximation (DIA) with three different combinations of wind input and breaking dissipation parameterizations. The WRT gives larger azimuthal bimodal amplitudes compared to the DIA regardless of wind input/dissipation. The widely used wind input/dissipation parameterizations (i.e., ST4 and ST6) generally give narrow directional distributions with relatively small bimodal amplitudes and lobe separations compared to field measurements. These biases are significantly improved by the breaking dissipation of Romero (R2019). Moreover, the ratio of the resolved cross- to downwind mean square slope is significantly lower for ST4 and ST6 compared to R2019. The overlap integral relevant for the prediction of microseisms is several orders of magnitude smaller for ST4 and ST6 compared to R2019, which nearly agrees with a semiempirical model.

Significance Statement

Spectral gravity wave models generally cannot accurately predict the directional distribution which impacts their ability to predict the resolved down- and crosswind mean square slopes and the generation of microseisms. Our analysis shows that a directionally narrow spectral energy dissipation, accounting for long-wave–short-wave modulation, can significantly improve the directional distribution of the wind-wave spectrum by coupling to the nonlinear energy fluxes due to wave–wave interactions, which has important implications for improved predictions of the mean square slopes and the generation of microseisms.

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Lorine Behr, Niklas Luther, Simon A. Josey, Jürg Luterbacher, Sebastian Wagner, and Elena Xoplaki

Abstract

Accurate representation of the Atlantic–Mediterranean exchange in climate models is important for a reliable simulation of the circulation in the North Atlantic Ocean. We evaluate the performance of 10 global climate models in representing Mediterranean Overflow Water (MOW) over the recent period 1986–2005 by using various performance metrics. The metrics are based on the representation of the climatological mean state and the spatiotemporal variability of temperature, salinity, and volume transports. On the basis of analyses and observations, we perform a model ranking by calculating absolute, relative, and total relative errors Ej over each performance metric and model. The majority of models simulate at least six metrics well. The equilibrium depth of the MOW, the mean Atlantic–Mediterranean exchange flow, and the dominant pattern of the MOW are represented reasonably well by most of the models. Of those models considered, MPI-ESM-MR, MPI-ESM-LR, CSIRO Mk3.6.0, and MRI-CGCM3 provide the best MOW representation (Ej = 0.14, 0.19, 0.19, and 0.25, respectively). They are thus likely to be the most suitable choices for studies of MOW-dependent processes. However, the models experience salinity, temperature, and transport biases and do not represent temporal variability accurately. The implications of our results for future model analysis of the Mediterranean Sea overflow are discussed.

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Qing Qin, Zhaomin Wang, Chengyan Liu, and Chen Cheng

Abstract

Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. Here, we show that more persistent open-ocean polynyas occur in the Cooperation Sea (CS) (60°–90°E), a sector of the Southern Ocean off the Prydz Bay continental shelf, between 2002 and 2019. Polynyas are formed annually mainly within the 62°–65°S band, as identified by sea ice concentrations less than 0.7. The polynyas usually began to emerge in April and expanded to large sizes during July–October, with sizes often larger than those of the Maud Rise polynya in 2017. The annual maximum size of polynyas ranged from 115.3 × 103 km2 in 2013 to 312.4 × 103 km2 in 2010, with an average value of 188.9 × 103 km2. The Antarctic Circumpolar Current (ACC) travels closer to the continental shelf and brings the upper circumpolar deep water to much higher latitudes in the CS than in most other sectors; cyclonic ocean circulations often develop between the ACC and the Antarctic Slope Current, with many of them being associated with local topographic features and dense water cascading. These oceanic preconditions, along with cyclonic wind forcing in the Antarctic Divergence zone, generated polynyas in the CS. These findings offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean. Future efforts should be particularly devoted to more extensively observing the ocean circulation to understand the variability of open-ocean polynyas in the CS.

Significance Statement

An open-ocean polynya is an offshore area of open water or low sea ice cover surrounded by pack ice. Open-ocean polynyas are important for driving the physical, biogeochemical, and biological processes in the Southern Ocean. Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. The purpose of this study is to document the existence of more persistent open-ocean polynyas in the Cooperation Sea (60°–90°E) and explore the atmospheric and oceanic forcing mechanisms responsible for the formation of the open-ocean polynyas. Our results would offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean.

Open access
Lichuan Wu, Øyvind Breivik, and Fangli Qiao

Abstract

The momentum flux to the ocean interior is commonly assumed to be identical to the momentum flux lost from the atmosphere in traditional atmosphere, ocean, and coupled models. However, ocean surface gravity waves (hereafter waves) can alter the magnitude and direction of the ocean-side stress (τ oc) from the air-side stress (τ a). This is rarely considered in coupled climate and forecast models. Based on a 30-yr wave hindcast, the redistribution of the global wind stress and turbulent kinetic energy (TKE) flux by waves was investigated. Waves play a more important role in the windy oceans in middle and high latitudes than that in the oceans in the tropics (i.e., the central portion of the Pacific and Atlantic Oceans). On average, the relative difference between τ oc and τ a, γ τ, can be up to 6% in middle and high latitudes. The frequency of occurrence of γ τ > 9% can be up to 10% in the windy extratropics. The directional difference between τ oc and τ a exceeds 3.5° in the middle and high latitudes 10% of the time. The difference between τ oc and τ a becomes more significant closer to the coasts of the continents due to strong wind gradients. The friction velocity-based approach overestimates (underestimates) the breaking-induced TKE flux in the tropics (middle and high latitudes). The findings presented in the current study show that coupled climate and Earth system models would clearly benefit from the inclusion of a wave model.

Significance Statement

The purpose of this study is to investigate the redistribution of the global wind stress and turbulent kinetic energy flux due to surface waves based on a 30-yr wave hindcast. The mean relative difference of the magnitude between the air-side and ocean-side stress is up to 6% with a 90th percentile of more than 9% in the windy extratropics. Due to strong wind gradients, the redistributive role of waves in the stress becomes more significant closer to coasts. The results indicate that we should consider the redistributive role of waves in the momentum and energy fluxes in climate and Earth system models since they are the key elements in the predictability of weather forecasting models and climate models.

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Manita Chouksey, Alexa Griesel, Carsten Eden, and Reiner Steinfeldt

Abstract

Transit time distributions (TTDs) for the Antarctic Intermediate Water (AAIW) in the South Atlantic Ocean are estimated from an eddying ocean model with a twofold scope: validation of the TTD method and identifying pathways of the AAIW. The TTDs are inferred both from Lagrangian particle backtracking and the modeled CFC-11 concentrations, under the assumption that the TTDs can be described with an inverse Gaussian function. A bimodal distribution is obtained for the Lagrangian TTDs with four major subduction regions identified: near the Agulhas retroflection, south of New Zealand, west of the Drake Passage (smallest mean age Γ = 13 years), and in the Argentine basin (largest mean age Γ = 25 years). With the Southern Ocean as source region, the inverse Gaussian is a reasonable representation for the TTDs in the eastern Atlantic basin (40°–35°S, 0°–20°E), whereas the fit for region west (40°–35°S, 60°–40°W) of the mid-Atlantic ridge is not as good and overestimates the TTDs for transit times < 15 years. Mean ages from the modeled CFC-11 are mostly larger (up to 12 years) in the eastern Atlantic basin, and they are mostly smaller than the Lagrangian mean ages in the west. Both methods yield mean ages smaller in the western than in the eastern Atlantic basin and an aging of AAIW from the 1990s to the 2000s that is consistent with reduced flow velocities. The Antarctic Circumpolar Current appears to be the prime determinant of the transit times. The results suggest that the inverse Gaussian, despite assuming 1D advection–diffusion with constant mean flow and diffusivity, is a surprisingly good fit.

Significance Statement

In this article, we assess the transit time distribution method, often used to estimate anthropogenic carbon uptake in the ocean from observations, thereby exploring particle pathways from the surface into the ocean interior in the South Atlantic Ocean. We track thousands of particles in a model from their point of origin near the surface to the ocean interior. The tracking reveals multiple routes and gives the actual travel time of these particles, which we compare with the travel times predicted by theory. Thus, this research deepens our understanding of the routes and travel times of the water particles, which is important for the ocean circulation, and provides insights to improve the methods to infer anthropogenic carbon uptake, storage, and transport.

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Swantje Bastin, Martin Claus, Peter Brandt, and Richard J. Greatbatch

Abstract

Equatorial deep jets (EDJ) are zonal currents along the equator in all three ocean basins that alternate in direction with depth and time. In the Atlantic Ocean below the thermocline, they are the dominant variability on interannual time scales. Observations of equatorial deep jets are available but scarce, given the EDJs’ location at depth, their small vertical scale, and their long periodicity of several years. In the last few years, Argo floats have added a significant number of measurements at intermediate depth. In this study we therefore revise estimates of the EDJ scales based on Argo float data. Mostly, we use velocity data at 1000-m depth calculated from float displacement, which yield robust estimates of the Atlantic EDJ period (4.6 yr), amplitude distribution, phase distribution, zonal wavelength (146.7°), and meridional structure. We also show that the equatorial amplitude of the EDJs’ first meridional mode Rossby wave component (9.8 cm s−1) is larger than that of their Kelvin wave component (2.8 cm s−1). In addition, we present a new estimation of the EDJs’ vertical structure throughout the Atlantic basin, based on an equatorial geostrophic velocity reconstruction from hydrographic Argo float measurements from depths between 400 and 2000 m. Our new estimates from Argo float data provide the first basinwide assessment of the Atlantic EDJ scales, as well as having smaller uncertainties than estimates from earlier studies.

Open access
Changlong Liu, Xinyu Li, Jinbao Song, Zhongshui Zou, Jian Huang, Jun A. Zhang, Ganxin Jie, and Jun Wang

Abstract

The deviation of the mean wind profile from Monin–Obukhov similarity theory (MOST) within the wave boundary layer (WBL) is investigated by combining four levels of turbulence data measured on a fixed platform with wave measurements. The data suggest that the mean wind profile follows MOST under wind-sea conditions because the turbulence statistics and structure are consistent with the attached eddy model. However, pronounced swell-related peaks appeared in the velocity spectra and uw cospectra under swell conditions. The upward wave-induced stress resulted in a large wind gradient within the WBL when light winds traveled with the swell, while the opposite result was found for the wind-opposite swell. These phenomena were analyzed based on the velocity spectra and turbulence variances. We found that the deviation of the wind profile was due to the longer or shorter length of the f   −1 scaling region appearing in the velocity spectra.

Significance Statement

The interactions between ocean waves and adjacent airflow affect the functioning of Earth’s climate and weather systems. However, our physical understanding of air–wave interactions remains incomplete, for example, the deviation of the mean wind profile from Monin–Obukhov similarity theory within the wave boundary layer. In this study, we aim to provide new insights by analyzing four-level turbulence data and wave measurements. We found that when the local wind travels faster than the wave, the airflow over the wavy surface is similar to that over land because the effects of waves do not reach the measurement heights. In conditions with wind-following or -opposing fast-propagating waves, the overlying airflow is greatly modified as the turbulence is affected by wave-induced perturbations.

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Zoé Caspar-Cohen, Aurélien Ponte, Noé Lahaye, Xavier Carton, Xiaolong Yu, and Sylvie Le Gentil

Abstract

The Lagrangian and Eulerian surface current signatures of a low-mode internal tide propagating through a turbulent balanced flow are compared in idealized numerical simulations. Lagrangian and Eulerian total (i.e., coherent plus incoherent) tidal amplitudes are found to be similar. Compared to Eulerian diagnostics, the Lagrangian tidal signal is more incoherent with comparable or smaller incoherence time scales and larger incoherent amplitudes. The larger level of incoherence in Lagrangian data is proposed to result from the deformation of an Eulerian internal tide signal induced by drifter displacements. Based on the latter hypothesis, a theoretical model successfully predicts Lagrangian autocovariances by relating Lagrangian and Eulerian autocovariances and the properties of the internal tides and jet. These results have implications for the separation of balanced flow and internal tides signals in the sea level data collected by the future Surface Water and Ocean Topography (SWOT) satellite mission.

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Takaya Uchida, Bruno Deremble, and Stephane Popinet

Abstract

Mesoscale eddies, although being on scales of O(20–100) km, have a disproportionate role in shaping the mean stratification, which varies on the scale of O(1000) km. With the increase in computational power, we are now able to partially resolve the eddies in basin-scale and global ocean simulations, a model resolution often referred to as mesoscale permitting. It is well known, however, that due to gridscale numerical viscosity, mesoscale-permitting simulations have less energetic eddies and consequently weaker eddy feedback onto the mean flow. In this study, we run a quasigeostrophic model at mesoscale-resolving resolution in a double gyre configuration and formulate a deterministic closure for the eddy rectification term of potential vorticity (PV), namely, the eddy PV flux divergence. Our closure successfully reproduces the spatial patterns and magnitude of eddy kinetic and potential energy diagnosed from the mesoscale-resolving model. One novel point about our approach is that we account for nonlocal eddy feedbacks onto the mean flow by solving the “subgrid” eddy PV equation prognostically in addition to the mean PV.

Open access