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1. Factors Controlling Superrotation in a Terrestrial Aquaplanet

   https://doi.org/10.1175/JAS-D-24-0080.1  

   Zurita-Gotor, Pablo; Held, Isaac M (November 2024, Journal of the Atmospheric Sciences)

   Extending previous work with a dry model, this study investigates the sensitivity of superrotation to the location/strength of baroclinic eddies in an idealized moist aquaplanet GCM with terrestrial rotation rate and planetary radius. A suite of fixed-SST experiments is performed in which the extratropical SST gradient is flattened poleward of some specified latitude. Consistent with the dry simulations, transition to superrotation is found as this reference latitude moves near the subtropics. The superrotation is dependent on the equatorial acceleration due to interactions between equatorial Kelvin waves and subtropical Rossby waves, but is strongly enhanced by a reduction in drag by the baroclinic eddies on the subtropical upper troposphere. The reduction in the extratropical drag and the strength of superrotation depend on the strength and structure of the Hadley cell, and hence on convective closure. The transition to strong superrotation is aided by a positive feedback that cannot occur when a strong Hadley cell drag limits the equatorial vertical shear and upper-troposphere equatorial westerlies. 

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2. Eddy Vertical Structure and Variability: Deepglider Observations in the North Atlantic

   https://doi.org/10.1175/JPO-D-21-0068.1  

   Steinberg, Jacob M.; Eriksen, Charles C. (June 2022, Journal of Physical Oceanography)

   Abstract Hundreds of full-depth temperature and salinity profiles collected by Deepglider autonomous underwater vehicles (AUVs) in the North Atlantic reveal robust signals in eddy isopycnal vertical displacement and horizontal current throughout the entire water column. In separate glider missions southeast of Bermuda, subsurface-intensified cold, fresh coherent vortices were observed with velocities exceeding 20 cm s −1 at depths greater than 1000 m. With vertical resolution on the order of 20 m or less, these full-depth glider slant profiles newly permit estimation of scaled vertical wavenumber spectra from the barotropic through the 40th baroclinic mode. Geostrophic turbulence theory predictions of spectral slopes associated with the forward enstrophy cascade and proportional to inverse wavenumber cubed generally agree with glider-derived quasi-universal spectra of potential and kinetic energy found at a variety of locations distinguished by a wide range of mean surface eddy kinetic energy. Water-column average spectral estimates merge at high vertical mode number to established descriptions of internal wave spectra. Among glider mission sites, geographic and seasonal variability implicate bottom drag as a mechanism for dissipation, but also the need for more persistent sampling of the deep ocean. Significance Statement Relative to upper-ocean measurements of temperature, salinity, and velocity, deep ocean measurements (below 2000 m) are fewer in number and more difficult to collect. Deep measurements are needed, however, to explore the nature of deep ocean circulation contributing to the global redistribution of heat and to determine how upper-ocean behavior impacts or drives deep motions. Understanding of geographic and temporal variability in vertical structures of currents and eddies enables improved description of energy pathways in the ocean driven by turbulent interactions. In this study, we use newly developed autonomous underwater vehicles, capable of diving to the seafloor and back on a near daily basis, to collect high-resolution full ocean depth measurements at various locations in the North Atlantic. These measurements reveal connections between surface and deep motions, and importantly show their time evolution. Results of analyzing these vertical structures reveal the deep ocean to regularly “feel” events in the upper ocean and permit new comparisons to deep motions in climate models. 

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3. A Chlorophyll Biomass Time-Series for the Distributed Biological Observatory in the Context of Seasonal Sea Ice Declines in the Pacific Arctic Region

   https://doi.org/10.3390/geosciences12080307  

   Cooper, Lee W.; Grebmeier, Jacqueline M. (August 2022, Geosciences)

   Declines in seasonal sea ice in polar regions have stimulated projections of how primary production has shifted in response to greater light penetration over a longer open water season. Despite the limitations of remotely sensed observations in an often cloudy environment, remote sensing data provide strong indications that surface chlorophyll biomass has increased (since 2000) as sea ice has declined in the Pacific Arctic region. We present here shipboard measurements of chlorophyll-a that have been made annually in July since 2000 from the Distributed Biological Observatory (DBO) stations in the Bering Strait region. This time series as well as shipboard observations made in other months since the late 1980s implicate complexities that intrude on a simple expectation that, as open water periods increase, the production and biomass of phytoplankton will increase predictably. These shipboard observations indicate that there have not been sharp increases in chlorophyll-a, for either maxima observed in the water column or integrated over the whole water column, at the DBO stations over a time-series extending for as long as 20 years coinciding with seasonal sea ice declines. On the other hand, biomass may be increasing in other months: we provide a shipboard confirmation of a fall bloom in October as wind mixing introduced nutrients back into the upper water column. The productive DBO stations may be at a high enough production already that additional enhancements in chlorophyll-a biomass should not be expected, but our time-series record does not exclude the possibility that additional enhanced production may be present in other areas outside the DBO station grid. These findings may also reflect limitations imposed by nutrient cycling and water column structure. The increasing freshwater component of waters flowing through the Bering Strait is likely associated with increased stratification that limits the potential change in biological production associated with decreases in seasonal sea ice persistence. 

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4. Structure and dynamics of mesoscale eddies over the Laptev Sea continental slope in the Arctic Ocean

   https://doi.org/10.5194/os-14-1329-2018  

   Pnyushkov, Andrey; Polyakov, Igor V.; Padman, Laurie; Nguyen, An T. (January 2018, Ocean Science)

   Abstract. Heat fluxes steered by mesoscale eddies may be a significant, but still notquantified, source of heat to the surface mixed layer and sea ice cover inthe Arctic Ocean, as well as a source of nutrients for enhancing seasonalproductivity in the near-surface layers. Here we use 4 years (2007–2011)of velocity and hydrography records from a moored profiler over the LaptevSea slope and 15 months (2008–2009) of acoustic Doppler current profilerdata from a nearby mooring to investigate the structure and dynamics ofeddies at the continental margin of the eastern Eurasian Basin. Typical eddyscales are radii of the order of 10 km, heights of 600 m, andmaximum velocities of ∼0.1 m s−1. Eddies areapproximately equally divided between cyclonic and anticyclonicpolarizations, contrary to prior observations from the deep basins and alongthe Lomonosov Ridge. Eddies are present in the mooring records about 20 %–25 % of the time,taking about 1 week to pass through the mooring at anaverage frequency of about one eddy per month. We found that the eddies observed are formed in two distinct regions – near FramStrait, where the western branch of Atlantic Water (AW) enters the ArcticOcean, and near Severnaya Zemlya, where the Fram Strait and Barents Seabranches of the AW inflow merge. These eddies, embedded in the ArcticCircumpolar Boundary Current, carry anomalous water properties along theeastern Arctic continental slope. The enhanced diapycnal mixing that wefound within EB eddies suggests a potentially important role for eddies inthe vertical redistribution of heat in the Arctic Ocean interior. 

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5. Dynamic Mechanisms Associated with the Structure and Evolution of Roll Vortices and Coherent Turbulence in the Hurricane Boundary Layer: A Large Eddy Simulation During the Landfall of Hurricane Harvey

   https://doi.org/10.1007/s10546-022-00775-w  

   Li, Xin; Pu, Zhaoxia (January 2023, Boundary-Layer Meteorology)

   Abstract Roll vortices are a series of large-scale turbulent eddies that nearly align with the mean wind direction and prevail in the hurricane boundary layer. In this study, the one-way nested WRF-LES model simulation results from Li et al. (J Atmos Sci 78(6):1847–1867,https://doi.org/10.1175/JAS-D-20-0270.1, 2021) are used to examine the structure and generation mechanism of roll vortices and associated coherent turbulence in the hurricane boundary layer during the landfall of Hurricane Harvey from 00 UTC 25 to 18 UTC 27 August 2017. Results indicate that roll vortices prevail in the hurricane boundary layer. The intense roll vortices and associated large turbulent eddies above them (at a height of ~ 200 to 3000 m) accumulate within a hurricane radius of 20–40 km. Their intensity is proportional to hurricane intensity during the simulation period. Before and during hurricane landfall, strong inflow convergence leads to horizontal advection of roll vortices throughout the entire hurricane boundary layer. Combined with the strong wind shear, the strongest roll vortices and associated large turbulent eddies are generated near the eyewall with suitable thermodynamic (Richardson number at around − 0.2 to 0.2) and dynamic conditions (strong negative inflow wind shear). After landfall, the decayed inflow weakens the inflow convergence and quickly reduces the strong roll vortices and associated large turbulent eddies. Diagnosis of vertical turbulent kinetic energy indicates that atmospheric pressure perturbation, caused by horizontal convergence, transfers the horizontal component of turbulence to the vertical component with a mean wavelength of about 1 km. The buoyancy term is weak and negative, and the large turbulent eddies are suppressed. 

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