486 research outputs found
Ribbon Turbulence
We investigate the non-linear equilibration of a two-layer quasi-geostrophic
flow in a channel forced by an imposed unstable zonal mean flow, paying
particular attention to the role of bottom friction. In the limit of low bottom
friction, classical theory of geostrophic turbulence predicts an inverse
cascade of kinetic energy in the horizontal with condensation at the domain
scale and barotropization on the vertical. By contrast, in the limit of large
bottom friction, the flow is dominated by ribbons of high kinetic energy in the
upper layer. These ribbons correspond to meandering jets separating regions of
homogenized potential vorticity. We interpret these result by taking advantage
of the peculiar conservation laws satisfied by this system: the dynamics can be
recast in such a way that the imposed mean flow appears as an initial source of
potential vorticity levels in the upper layer. The initial baroclinic
instability leads to a turbulent flow that stirs this potential vorticity field
while conserving the global distribution of potential vorticity levels.
Statistical mechanical theory of the 1-1/2 layer quasi-geostrophic model
predict the formation of two regions of homogenized potential vorticity
separated by a minimal interface. We show that the dynamics of the ribbons
results from a competition between a tendency to reach this equilibrium state,
and baroclinic instability that induces meanders of the interface. These
meanders intermittently break and induce potential vorticity mixing, but the
interface remains sharp throughout the flow evolution. We show that for some
parameter regimes, the ribbons act as a mixing barrier which prevent relaxation
toward equilibrium, favouring the emergence of multiple zonal jets
Regime Change Behaviour During Asian Monsoon Onset
This is the final version of the article. Available from American Meteorological Society via the DOI in this record.As the ITCZ moves off the equator on an aquaplanet, the Hadley circulation
transitions from an equinoctial regime with two near symmetric, significantly
eddy-driven cells, to a monsoon-like regime with a strong, thermally direct
cross-equatorial cell, intense low-latitude precipitation, and a weak summer
hemisphere cell. Dynamical feedbacks appear to accelerate the transition.
This study investigates the relevance of this behavior to monsoon onset by using
primitive-equation model simulations ranging from aquaplanets to more
realistic configurations with Earthâs continents and topography.
A change in the relationship between ITCZ latitude and overturning strength
is identified once the ITCZ moves poleward of about ⌠7
âŠ
. Monsoon onset
is associated with off-equatorial ascent, in regions of non-negligible planetary
vorticity, and this is found to generate a vortex stretching tendency that reduces
upper level absolute vorticity. In an aquaplanet, this causes a transition
to the cross-equatorial, thermally direct regime, intensifying the overturning
circulation. Analysis of the zonal momentum budget suggests a stationary
wave, driven by topography and land-sea contrast, can trigger a similar transition
in the more realistic model configuration, with the wave extending the
ascent region of the Southern Hemisphere Hadley cell northward, and enhanced
overturning then developing to the south. These two elements of the
circulation resemble the East and South Asian monsoons.The work was supported by the UK-China Research & Innovation Partnership Fund, through the Met Office Climate Science for Service Partnership (CSSP) China, as part of the Newton Fund. GKV also acknowledges support from the Royal Society (Wolfson Foundation), the Leverhulme Trust, and NERC
Large-scale circulation with small diapycnal diffusion: The two-thermocline limit
The structure and dynamics of the large-scale circulation of a single-hemisphere, closed-basin ocean with small diapycnal diffusion are studied by numerical and analytical methods. The investigation is motivated in part by recent differing theoretical descriptions of the dynamics that control the stratification of the upper ocean, and in part by recent observational evidence that diapycnal diffusivities due to small-scale turbulence in the ocean thermocline are small (â0.1 cm2 sâ1). Numerical solutions of a computationally efficient, three-dimensional, planetary geostrophic ocean circulation model are obtained in a square basin on a mid-latitude ÎČ-plane. The forcing consists of a zonal wind stress (imposed meridional Ekman flow) and a surface heat flux proportional to the difference between surface temperature and an imposed air temperature. For small diapycnal diffusivities (vertical: Îșv â0.1 â 0.5 cm2 sâ1, horizontal: Îșh â105 â 5 Ă 106 cm2 sâ1), two distinct thermocline regimes occur. On isopycnals that outcrop in the subtropical gyre, in the region of Ekman downwelling, a ventilated thermocline forms. In this regime, advection dominates diapycnal diffusion, and the heat balance is closed by surface cooling and convection in the northwest part of the subtropical gyre. An âadvectiveâ vertical scale describes the depth to which the wind-driven motion penetrates, that is, the thickness of the ventilated thermocline. At the base of the wind-driven fluid layer, a second thermocline forms beneath a layer of vertically homogeneous fluid (âmode waterâ). This âinternalâ thermocline is intrinsically diffusive. An âinternal boundary layerâ vertical scale (proportional to Îșv1/2) describes the thickness of this internal thermocline. Two varieties of subtropical mode waters are distinguished. The temperature difference across the ventilated thermocline is determined to first order by the meridional air temperature difference across the subtropical gyre. The temperature difference across the internal thermocline is determined to first order by the temperature difference across the subpolar gyre. The diffusively-driven meridional overturning cell is effectively confined below the ventilated thermocline, and driven to first order by the temperature difference across the internal thermocline, not the basin-wide meridional air temperature difference. Consequently, for small diapycnal diffusion, the abyssal circulation depends to first order only on the wind-forcing and the subpolar gyre air temperatures. The numerical solutions have a qualitative resemblance to the observed structure of the North Atlantic in and above the main thermocline (that is, to a depth of roughly 1500 m). Below the main thermocline, the predicted stratification is much weaker than observed
Processes and Timescales in Onset and Withdrawal of 'Aquaplanet Monsoons'
This is the final version. Available from American Meteorological Society via the DOI in this record.Data availability: The research materials supporting this publication can be accessed by contacting Ruth Geen ([email protected]).Aquaplanets with low heat capacity slab ocean boundary conditions can exhibit rapid changes in the regime of the overturning circulation over the seasonal cycle, which have been connected to the onset of Earthâs monsoons. In
spring, as the ITCZ migrates off the Equator, it jumps poleward and a sudden transition occurs from an eddy-driven, equinoctial regime with two weak
Hadley cells, to a near angular momentum conserving, solstitial regime with a
strong, cross-equatorial winter hemisphere cell. Here, the controls on the transition latitude and rate are explored in idealised moist aquaplanet simulations.
It is found that the transition remains rapid relative to the solar forcing when
year length and slab ocean heat capacity are varied, and, at Earthâs rotation
rate, always occurs when the ITCZ reaches approximately 7°. This transition
latitude is, however, found to scale inversely with rotation rate. Interestingly,
the transition rate varies non-monotonically with rotation, with a maximum at
Earthâs rotation rate, suggesting that Earth may be particularly disposed to a
fast monsoon onset. The fast transition relates to feedbacks in both the atmosphere and the slab ocean. In particular, an evaporative feedback between the
lower-level branch of the overturning circulation and the surface temperature
is identified. This accelerates monsoon onset and slows withdrawal. Lastly,
comparing eddy-permitting and axisymmetric experiments shows that, in contrast with results from dry models, in this fully moist model the presence of
eddies slows the migration of the ITCZ between hemispheres.UK-China Research and Innovation Partnership FundRoyal SocietyLeverhulme Trus
Targeting telomerase with radiolabeled inhibitors
The expression of telomerase in approximately 85% of cancers and its absence in the majority of normal cells makes it an attractive target for cancer therapy. However the lag period between initiation of telomerase inhibition and growth arrest makes direct inhibition alone an insufficient method of treatment. However, telomerase inhibition has been shown to enhance cancer cell radiosensitivity. To investigate the strategy of simultaneously inhibiting telomerase while delivering targeted radionuclide therapy to cancer cells, 123I-radiolabeled inhibitors of telomerase were synthesized and their effects on cancer cell survival studied. An 123I-labeled analogue of the telomerase inhibitor MST-312 inhibited telomerase with an IC50 of 1.58Â ÎŒM (MST-312 IC50: 0.23Â ÎŒM). Clonogenic assays showed a dose dependant effect of 123I-MST-312 on cell survival in a telomerase positive cell line, MDA-MB-435
Optimally coherent sets in geophysical flows: A new approach to delimiting the stratospheric polar vortex
The "edge" of the Antarctic polar vortex is known to behave as a barrier to
the meridional (poleward) transport of ozone during the austral winter. This
chemical isolation of the polar vortex from the middle and low latitudes
produces an ozone minimum in the vortex region, intensifying the ozone hole
relative to that which would be produced by photochemical processes alone.
Observational determination of the vortex edge remains an active field of
research. In this letter, we obtain objective estimates of the structure of the
polar vortex by introducing a new technique based on transfer operators that
aims to find regions with minimal external transport. Applying this new
technique to European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40
three-dimensional velocity data we produce an improved three-dimensional
estimate of the vortex location in the upper stratosphere where the vortex is
most pronounced. This novel computational approach has wide potential
application in detecting and analysing mixing structures in a variety of
atmospheric, oceanographic, and general fluid dynamical settings
Importance of the pleckstrin homology domain of dynamin in clathrin-mediated endocytosis
AbstractThe GTPase dynamin plays an essential role in clathrin-mediated endocytosis [1â3]. Substantial evidence suggests that dynamin oligomerisation around the necks of endocytosing vesicles and subsequent dynamin-catalysed GTP hydrolysis is responsible for membrane fission [4,5]. The pleckstrin homology (PH) domain of dynamin has previously been shown to interact with phosphoinositides, but it has not been determined whether this interaction is essential for dynamin's function in endocytosis [6â9]. In this study, we address the in vivo function of the PH domain of dynamin by assaying the effects of deletions and point mutations in this region on transferrin uptake in COS-7 fibroblasts. Overexpression of a dynamin construct lacking its entire PH domain potently blocked transferrin uptake, as did overexpression of a dynamin construct containing a mutation in the first variable loop of the PH domain. Structural modelling of this latter mutant suggested that the lysine residue at position 535 (Lys535) may be critical in the coordination of phosphoinositides, and indeed, the purified mutant no longer interacted with lipid nanotubes. Interestingly, the inhibitory phenotype of cells expressing this dynamin mutant was partially relieved by a second mutation in the carboxy-terminal proline-rich domain (PRD), one that prevents dynamin from binding to the Src homology 3 (SH3) domain of amphiphysin. These data demonstrate that dynamin's interaction with phosphoinositides through its PH domain is essential for endocytosis. These findings also support our hypothesis that PRDâSH3 domain interactions are important in the recruitment of dynamin to sites of endocytosis
On variational principles for coherent vortex structures
Different approaches are discussed of variational principles characterizing coherent vortex structures in two-dimensional flows. Turbulent flows seem to form ordered structures in the large scales of the motion and the self-organization principle predicts asymptotic states realizing an extremal value of the energy or a minimum of enstrophy. On the other hand the small scales take care of the increase of entropy, and asymptotic results can be obtained by applying the theory of equilibrium statistical mechanics
Inferring directed climatic interactions with renormalized partial directed coherence and directed partial correlation
ACKNOWLEDGMENTS This work was supported in part by Spanish MINECO/FEDER (FIS2015-66503-C3-2-P) and ITN LINC (FP7 289447). C.M. also acknowledges partial support from ICREA ACADEMIAPeer reviewedPublisher PD
- âŠ