ATLANTIC CLIMATE · Variability and Trends in Atlantic Climate

Variability and Trends in Atlantic Climate

At mid-to-high latitudes, the stratosphere contains >25% of the column of atmospheric mass. In Northern Hemisphere winter large-scale vertically-propagating waves drive a synchronised meridional circulation that moves mass into and out of the Arctic (>65ºN), modulating adiabatic warming of the stratospheric polar air column, and altering the strength of the stratospheric polar vortex. These stratospheric changes are associated with substantial effects on surface weather and climate, especially on Northern Annular Mode (NAM) with associated long-lasting shifts in the jet streams, storm tracks, precipitation, and likelihood of blocking events. Despite unambiguous observations of this phenomenon, as well as numerical simulations, a quantitative physical explanation of this downward coupling remains elusive. In this project we demonstrate that amplification of the polar stratospheric pressure signal involves a positive dynamical feedback process within the troposphere. An initial Arctic tropospheric sea level pressure (SLP) signal (e.g., from a sudden stratospheric warming) reduces latitudinal pressure gradients and poleward heat transport within the troposphere, leading to increased cooling of the Arctic lower troposphere. This cooling induces higher pressure over the Arctic region (Hoskins et al. 1985), thus amplifying the original stratospheric signal. This positive dynamical feedback process appears to operate in both hemispheres, and is consistent with the observed lag in the tropospheric response to stratospheric signals. The feedback process itself is not unique to stratospheric forcing, and may explain why the annular modes are easily forced, tend to self-amplify, and have relatively long time scales. Thus, for the first time, we have been able to quantify stratosphere-troposphere coupling, and we have developed a diagnostic that can be used to assess the fidelity of stratosphere-troposphere coupling in nearly any model. In the future, climate models (such as used in IPCC reports) can be evaluated to see if their representation of the stratosphere is realistic. Similarly, weather forecast models can be evaluated. Career Prospect: I have a permanent job as Professor of Climate Science at the University of Exeter. My career prospects are to remain in this position, or possibly to move to a different UK institution (although I plan to stay at Exeter). I head a research group with 3 PhD students, and I run a NERC project that funds 5 post-docs; one of these post-docs worked directly in my group. I did secure additional funding that helps to advance this CIG project. I also participate in an FP7 project (StratoClim) that partially supported one post-doc. Transfer of Knowledge: I have supervised 3 PhD students. Two of these have had successful vivas, including Simon Clark, who was supported by this CIG. I have supervised directly one post-doc who secured her own independent fellowship (2 years) and recently moved to Barcelona to take this new position. I assisted in organising internal seminars and hosting visitors, although I was not the primary organiser. I have regular teaching duties in Mathematics that include climate science, how to conduct research, how to publish, and data graphics & typography. I have developed collaborations with Thomas Birner, who was at Colorado State, and has now moved to the EU, to the University of Munich. I also developed a collaboration with Blanca Ayarzeguena, who was my post-doc, and who is now in Barcelona on an independent fellowship. I have many other collaborations, but mainly they existed prior to the CIG. Dissemination: Below in the report I mention 7 specific conferences/meetings at which I disseminated results. My outreach activities have been mainly aimed at per-university prospective students. I gave presentations designed to convince them to pursue research in my area. These students are a "non-scientific audience", although it is my intent to convince them to become scientists. The project has not received any direct press coverage; however, results from the project have affected the Met Office. Here is an example. https://www.theguardian.com/uk-news/2018/feb/19/sudden-stratospheric-warming-set-to-bring-lengthy-cold-snap-to-uk The project results and findings can be seen by following the link to my University of Exeter home page: http://emps.exeter.ac.uk/mathematics/staff/mb482

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