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Prof Mark P. Baldwin

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• Marie Curie Career Integration Grant outcomes

Prof Mark P. Baldwin

Marie Curie Career Integration Grant outcomes

Summary

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.

Talks at meetings

SPARC General Assembly talk, January 2014, NZ

SPARC meeting talk Granada, Spain, January 2015

Brewer-Dobson workshop talk, Grindelwald, Switzerland, August 2015

AMS New Orleans talk, January 2016

SHARP Belin talk, February 2016

SPARC DynVar Helsinki, talk, June 2016

Tokyo Middle Atmosphere meeting talk, September 2016

Oxford QBO meeting talk, September 2016

Researcher training was very successful. The PhD student, Simon Clark, had a successful viva.

Papers published: