Dynamical Influences of Sudden Stratospheric Warmings on Surface Climate

Abstract The stratospheric winter polar vortex consists of strong westerly winds; this winter flow regime can undergo a complete breakdown during sudden stratospheric warming events. In the Northern Hemisphere these events are often accompanied by the descent of anomalous flow regimes which can result in extreme surface weather. The focus of this thesis is to assess sudden stratospheric warmings and their place in the coupled climate system. Portions of this work are dedicated to algorithm development with the aim of quickly and accurately isolating and subcategorising events. A method is successfully created that is computationally cheap, easy to implement, based on dynamically relevant criteria and has error rates clearly outlined. Impacts on the surface and ocean are assessed with focus on specific subclasses of sudden stratospheric warmings. It is found that there is, on average, stronger surface and oceanic impacts following events that split the polar vortex. The ocean system is impacted via modifications to the implied Ekman heat transport and the net atmosphere-surface heat flux. Furthermore, there is a relationship between the initial location of the disturbed polar vortex and the strength of anomalous flow regime at the surface. Analysis is conducted predominantly using general circulation model output, with direct comparison between an atmosphere-only model and a coupled atmosphere-ocean model. For the coupled model there is a reduction in the number of simulated sudden stratospheric warmings, a result of altered atmospheric wave dynamics. This is partially attributed to a cold bias over the equatorial Pacific. The frequency of sudden stratospheric warmings is found to be insensitive to North Atlantic sea surface temperature anomalies

IGCM4: a fast, parallel and flexible intermediate climate model

The IGCM4 (Intermediate Global Circulation Model version 4) is a global spectral primitive equation climate model whose predecessors have extensively been used in areas such as climate research, process modelling and atmospheric dynamics. The IGCM4’s niche and utility lies in its speed and flexibility allied with the complexity of a primitive equation climate model. Moist processes such as clouds, evaporation, atmospheric radiation and soil moisture are simulated in the model, though in a simplified manner compared to state-of-the-art global circulation models (GCMs). IGCM4 is a parallelised model, enabling both very long integrations to be conducted and the effects of higher resolutions to be explored. It has also undergone changes such as alterations to the cloud and surface processes and the addition of gravity wave drag. These changes have resulted in a significant improvement to the IGCM’s representation of the mean climate as well as its representation of stratospheric processes such as sudden stratospheric warmings. The IGCM4’s physical changes and climatology are described in this paper

The Effects of Different Sudden Stratospheric Warming Type on the Ocean:Ocean Impacts of SSWs

There is a confirmed link between sudden stratospheric warmings (SSWs) and surface weather. Here we find significant differences in the strength of surface and ocean responses for splitting and displacement SSWs, classified using a new straightforward moment analysis technique. In an intermediate general circulation model splitting SSWs possess an enhanced ability to affect the surface climate demonstrating the need to treat the two types individually. Following SSWs the North Atlantic surface wind stress curl weakens, compared to its climatological winter state, for over 30 days: this is also evident in NCEP/NCAR reanalysis. The effect of anomalies associated with SSWs on the ocean is analysed in the Intermediate General Circulation Model 4. The splitting SSW composite displays strong anomalies in the implied Ekman heat flux and net atmosphere-surface flux, modifying the mixed layer heat budget. Our results highlight that different SSW types need to be simulated in coupled stratospheric/tropospheric/ocean models

Improving together: better science writing through peer learning

Science, in our case the climate and geosciences, is increasingly interdisciplinary. Scientists must therefore communicate across disciplinary boundaries. For this communication to be successful, scientists must write clearly and concisely, yet the historically poor standard of scientific writing does not seem to be improving. Scientific writing must improve, and the key to long-term improvement lies with the early-career scientist (ECS). Many interventions exist for an ECS to improve their writing, like style guides and courses. However, momentum is often difficult to maintain after these interventions are completed. Continuity is key to improving writing. This paper introduces the ClimateSnack project, which aims to motivate ECSs to develop and continue to improve their writing and communication skills. The project adopts a peer-learning framework where ECSs voluntarily form writing groups at different institutes around the world. The group members learn, discuss, and improve their writing skills together. Several ClimateSnack writing groups have formed. This paper examines why some of the groups have flourished and others have dissolved. We identify the challenges involved in making a writing group successful and effective, notably the leadership of self-organized groups, and both individual and institutional time management. Within some of the groups, peer learning clearly offers a powerful tool to improve writing as well as bringing other benefits, including improved general communication skills and increased confidence