ALBERT

Description: Despite the well-known limitations of linear stability theory in describing nonlinear and turbulent flows, it has been found to accurately capture the transitions between certain nonlinear flow behavior. Specifically, the transition in heat flux scaling in rotating convective flows can be well predicted by applying a linear stability analysis to simple profiles of a convective boundary layer. This fact motivates the present study of the linear mechanisms involved in the stability properties of simple convective setups subject to rotation. We look at an idealized two-layer setup and gradually add complexity by including rotation, a bounded domain, and viscosity. The two-layer setup has the advantage of allowing for the use of wave interaction theory, traditionally applied to understand stratified and homogeneous shear flow instabilities, in order to quantify the various physical mechanisms leading to the growth of convective instabilities. We quantitatively show that the physical mechanisms involved in the stabilization of convection by rotation take two different forms acting within the stratified interfacial region, and in the homogeneous mixed layers. The latter of these we associate with the tendency of a rotating flow to develop Taylor columns (TCs). This TC mechanism can lead to both a stabilization or destabilization of the instability and varies depending on the parameters of the problem. A simple criterion is found for classifying the influence of these physical mechanisms.

Description: Earth's climate can be understood as a dynamical system that changes due to external forcing and internal couplings. Essential climate variables, such as surface air temperature, describe this dynamics. Our current interglacial, the Holocene (11 700 yr ago to today), has been characterized by small variations in global mean temperature prior to anthropogenic warming. However, the mechanisms and spatiotemporal patterns of fluctuations around this mean, called temperature variability, are poorly understood despite their socioeconomic relevance for climate change mitigation and adaptation. Here we examine discrepancies between temperature variability from model simulations and paleoclimate reconstructions by categorizing the scaling behavior of local and global surface air temperature on the timescale of years to centuries. To this end, we contrast power spectral densities (PSD) and their power-law scaling using simulated and observation-based temperature series of the last 6000 yr. We further introduce the spectral gain to disentangle the externally forced and internally generated variability as a function of timescale. It is based on our estimate of the joint PSD of radiative forcing, which exhibits a scale break around the period of 7 yr. We find that local temperature series from paleoclimate reconstructions show a different scaling behavior than simulated ones, with a tendency towards stronger persistence (i.e., correlation between successive values within a time series) on periods of 10 to 200 yr. Conversely, the PSD and spectral gain of global mean temperature are consistent across data sets. Our results point to the limitation of climate models to fully represent local temperature statistics over decades to centuries. By highlighting the key characteristics of temperature variability, we pave a way to better constrain possible changes in temperature variability with global warming and assess future climate risks.

Description: Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.

Description: Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like β-grasp fold, containing two α helices and a mixed five-stranded β sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first β strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40phox and p67phox (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40phox regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.