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Mixing along sloping isopycnals plays a key role in the transport and uptake of heat and carbon by the ocean. This mixing is quantified by a lateral diffusivity, which can be measured by tracking the lateral spreading of point release tracer patches. We present a definition for the area of a tracer patch, the time derivative of which provides the lateral diffusivity. To accurately estimate the diffusivity, an ensemble mean concentration field of many tracer release experiments is required. We use numerical experiments to quantify how accurately the “true” lateral diffusivity (obtained from the ensemble mean concentration field) can be estimated from a single tracer release experiment (one ensemble member). To simulate observational campaigns, we also estimate the diffusivity from a single tracer release that is spatially and/or temporally subsampled, quantifying how the error between the estimated diffusivity and the true diffusivity grows as this sampling resolution worsens. We perform these numerical experiments in a two-layer quasigeostrophic model of turbulent flow on a β plane, using an ensemble of 50 passive tracer release experiments, each initialized as a 2D Gaussian but with differing realizations of the turbulent flow. We find that the diffusivity estimates from the single tracer releases have a relative root-mean-square error (RMSE) of 1.43% from the true diffusivity. Subsampling a single tracer release experiment every 956 km increases the relative RMSE from the true diffusivity to 3.1%; also subsampling every 277 days raises this figure to 6.5%.

Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this northward excursion, the mean potential vorticity (PV) increases dramatically (decreases in magnitude) by up to a factor of 2 along mean geostrophic streamlines on middepth buoyancy surfaces. This increase is driven by drag near the continental slope, or by breaking eddies further offshore, and is balanced by a remarkably steady, eddy-driven decrease of mean PV along these northern circumpolar streamlines in the open ocean. We show how two related eddy processes that are fundamental to ACC dynamics—poleward buoyancy fluxes and downward fluxes of eastward momentum—are also concomitant with materially forcing PV to increase on the northern flank of a jet at middepth, and decrease on the southern flank. For eddies to drive the required mean PV decrease along northern streamlines, the ACC merges with the subtropical gyres to the north, so these streamlines inhabit the southern flanks of the combined ACC–gyre jets. We support these ideas by analyzing the time-mean PV and its budget along time-mean geostrophic streamlines in the Southern Ocean State Estimate. Our averaging formalism is Eulerian, to match the model’s numerics. The thickness-weighted average is preferable, but its PV budget cannot be balanced using Eulerian 5-day averaged diagnostics, primarily because the z-level buoyancy and continuity equations’ delicate balances are destroyed upon transformation into the buoyancy-coordinate thickness equation. Significance Statement The Antarctic Circumpolar Current is the world’s largest ocean current and a key controller of Earth’s climate. As the westerly winds that drive this current shift poleward under global warming, it is vital to know whether the current will follow. To begin addressing this, we study the current’s fundamental dynamics, and constraints, under present-day conditions. By analyzing angular momentum and stratification together, we show that the current is weakened near boundaries and strengthened by eddies elsewhere. The strengthening effects of eddies are isolated to the current by merging the current with oceanic gyres to the north. This gives a new perspective on why the current travels so far northward alongside South America, and may provide dynamical constraints on future changes.

The striatum is a large brain region containing two major cell types: D1 (dopamine receptor 1) and D2 (dopamine receptor 2) expressing spiny projection neurons (SPNs). We generated a cell type atlas of the adult murine striatum using single-cell RNA-seq of SPNs combined with quantitative RNAin situhybridization (ISH). We developed a novel computational pipeline that distinguishes discrete versus continuous cell identities in scRNA-seq data, and used it to show that SPNs in the striatum can be classified into four discrete types that reside in discrete anatomical clusters or are spatially intermingled. Within each discrete type, we find multiple independent axes of continuous cell identity that map to spatial gradients and whose genes are conserved between discrete types. These gradients correlate well to previously-mapped gradients of connectivity. Using these insights, we discovered multiple novel spatially localized region of the striatum, one of which contains patch-D2 SPNs that expressTac1, Htr7, andTh. Intriguingly, we found one subtype that strongly co-expresses both D1 and D2 dopamine receptors, and uniquely expresses a rare D2 receptor splice variant. These results collectively suggest an organizational principal of neuron identity in which major neuron types can be separated into discrete classes with little overlap and no implied spatial relationship. However these discrete classes are then continuously subdivided by multiple spatial gradients of expression defining anatomical location via a combinatorial mechanism. Finally, they suggest that neuronal circuitry has a substructure at far higher resolution than is typically interrogated which is defined by the precise identity and location of a neuron.

SummaryThe striatum contributes to many cognitive processes and disorders, but its cell types are incompletely characterized. We show that microfluidic and FACS-based single-cell RNA sequencing of mouse striatum provides a well-resolved classification of striatal cell type diversity. Transcriptome analysis revealed ten differentiated, distinct cell types, including neurons, astrocytes, oligodendrocytes, ependymal, immune, and vascular cells, and enabled the discovery of numerous marker genes. Furthermore, we identified two discrete subtypes of medium spiny neurons (MSNs) that have specific markers and that overexpress genes linked to cognitive disorders and addiction. We also describe continuous cellular identities, which increase heterogeneity within discrete cell types. Finally, we identified cell type-specific transcription and splicing factors that shape cellular identities by regulating splicing and expression patterns. Our findings suggest that functional diversity within a complex tissue arises from a small number of discrete cell types, which can exist in a continuous spectrum of functional states.

The mammalian striatum is involved in many complex behaviors and yet is composed largely of a single neuron class: the spiny projection neuron (SPN). It is unclear to what extent the functional specialization of the striatum is due to the molecular specialization of SPN subtypes. We sought to define the molecular and anatomical diversity of adult SPNs using single-cell RNA sequencing (scRNA-seq) and quantitative RNA in situ hybridization (ISH). We computationally distinguished discrete versus continuous heterogeneity in scRNA-seq data and found that SPNs in the striatum can be classified into four major discrete types with no implied spatial relationship between them. Within these discrete types, we find continuous heterogeneity encoding spatial gradients of gene expression and defining anatomical location in a combinatorial mechanism. Our results suggest that neuronal circuitry has a substructure at far higher resolution than is typically interrogated, which is defined by the precise identity and location of a neuron.

The role of engineering geology in the final design and construction of a road crossing the Blue Nile (Abay) gorge in Ethiopia is described. This new road between Mekhane Selam and Gundewein is a strategic link in the Ethiopian road network and encounters very steep and difficult terrain that poses significant engineering geological challenges. Remote sensing, landscape modelling, reference condition classification, and conventional field mapping have been applied to assist in the finalisation of the alignment, the prediction of ground conditions for earthworks design, and the implementation of slope and drainage protection works. The approach adopted serves as a blueprint for other road construction and improvement projects in the region though limitations exist with respect to the prediction of variable volcanic rock sequences at depth without the benefit of ground investigation boreholes. Although ground conditions different from those predicted have been exposed along 15 % of the alignment, these have been of relatively minor significance and have necessitated unanticipated remedial action affecting less than 2 % of the alignment in the gorge. Thus far, the main damage to the road from geohazards has been caused by road-related effects, including the concentration of road drainage below culverts and seepages from broken and blocked side drains. The susceptibility of the terrain to uncontrolled runoff is such that preventative and reactive maintenance will be required throughout the lifetime of the road.

Microinfarctions are present in the aged and injured human brain. Their clinical relevance is controversial, with postulated sequelae ranging from cognitive sparing to vascular dementia. To address the consequences of microinfarcts, we used controlled optical methods to create occlusions of individual penetrating arterioles or venules in rat cortex. Single microinfarcts, targeted to encompass all or part of a cortical column, impaired performance in a macrovibrissa-based behavioral task. Furthermore, the targeting of multiple vessels resulted in tissue damage that coalesced across cortex, even though the intervening penetrating vessels were acutely patent. Post-occlusion administration of memantine, a glutamate receptor antagonist that reduces cognitive decline in Alzheimer's disease, ameliorated tissue damage and perceptual deficits. Collectively, these data imply that microinfarcts likely contribute to cognitive decline. Strategies that have received limited success in the treatment of ischemic injury, which include therapeutics against excitotoxicity, may be successful against the progressive nature of vascular dementia.

The effects of the dopamine receptor antagonist flupenthixol were examined on the development of conditioned partner preference induced by paced copulation in female rats. In Experiment 1, ovariectomized, hormone-primed rats were conditioned to associate scented and unscented males with paced and nonpaced copulation, respectively. Females in Experiment 2 associated albino or pigmented males with paced or nonpaced copulation. Flupenthixol or saline was administered before each conditioning trial. During a final drug-free preference test, females could choose to copulate with either a pacing-related or nonpacing-related male. Saline-trained females copulated preferentially with the pacing-related male, whereas flupenthixol disrupted odor but not strain conditioning. The role of dopamine in conditioned partner preference depends on the type of stimuli to be learned.