Climate dynamics

Abstract:

It has been demonstrated that decadal variations in the North Atlantic Oscillation (NAO) can be predicted by current forecast models. While Atlantic Multidecadal Variability (AMV) in sea surface temperatures (SSTs) has been hypothesised as the source of this skill, the validity of this hypothesis and the pathways involved remain unclear. We show, using reanalysis and data from two forecast models, that the decadal predictability of the NAO can be entirely accounted for by the predictability of decadal variations in the speed of the North Atlantic eddy-driven jet, with no predictability of decadal variations in the jet latitude. The sub-polar North Atlantic (SPNA) is identified as the only obvious common source of an SST-based signal across the models and reanalysis, and the predictability of the jet speed is shown to be consistent with a forcing from the SPNA visible already within a single season. The pathway is argued to be tropospheric in nature, with the SPNA-associated heating extending up to the mid-troposphere, which alters the meridional temperature gradient around the climatological jet core. The relative roles of anthropogenic aerosol emissions and the Atlantic Meridional Overturning Circulation (AMOC) at generating predictable SPNA variability are also discussed. The analysis is extensively supported by the novel use of a set of seasonal hindcasts spanning the 20th century and forced with prescribed SSTs.

Abstract:

The discovery of over 5700 exoplanets has led to a boom in the field of exoplanet demographics over the past decade. Led by swaths of exoplanet discoveries from NASA\u27s Kepler space mission, astronomers have been conducting statistical studies of the exoplanet population in search of trends in various planetary and host stellar parameters. These investigations are informing our understanding of how planets form and evolve, thus putting our solar system into a galactic context. In this chapter, we review many of the major features uncovered in the distributions of physical and orbital parameters of known exoplanets including the Radius Valley, the Neptunian Desert, the Peas in a Pod pattern, dynamical properties that point toward likely formation/migration mechanisms, as well as trends with host stellar parameters such as the time-evolution of exoplanetary systems and the search for planets within the Habitable Zone. The overarching theme is that exoplanetary systems exhibit an incredible diversity of planet properties and system architectures that do not exist within our own solar system. A promising future awaits the field of exoplanet demographics with increasingly deep investigations planned following the launch of numerous dedicated space telescopes over the coming years and decades.Preprint of a chapter for the \u27Encyclopedia of Astrophysics\u27 (Editor-in-Chief Ilya Mandel, Section Editor Dimitri Veras) to be published by Elsevier as a Reference Module. 18 pages, 10 figure

Abstract:

The mean state of the MLT (mesosphere – lower thermosphere) is key in the exchange of energy, momentum, and trace species between the middle and upper atmosphere. Knowledge of the mean state wind and temperature is endangered by an upcoming gap in measurements. Needed actions include continued operation of existing space-borne instruments and rapid development of replacement options.

Abstract:

The impact of the quasi-biennial oscillation (QBO) on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Center Unified Model. The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The convection and precipitation in the atmosphere-only simulations do not differ between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and sea-surface temperature relationships with the QBO phase disappear when nudging is applied. Imposing a realistic QBO-driven static stability anomaly in the upper-troposphere lower-stratosphere is not sufficient to simulate tropical surface impacts. The nudging reduced the influence of the lower troposphere on the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper tropospheric zonal circulation has been decoupled from the surface by the nudging. These results suggest that grid-point nudging mutes relevant feedback processes occurring at the tropopause level, including high cloud radiative effects and wave mean flow interactions, which may play a key role in stratospheric-tropospheric coupling.