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Stratospheric variability and its impact on surface climate
Andreas Marc Fischer(1,2), Isla Simpson(3), Stefan Brönnimann(2), Eugene Rozanov(2,4), Martin
Schraner(2)
(1) Federal Office of Meteorology and Climatology, MeteoSwiss, Kraehbuehlstr. 58, 8044 Zurich, Switzerland
(2) Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland. (andreas.fischer@env.ethz.ch)
(3) Department of Physics, Imperial College, London, UK
(2) PMOD/WRC, Dorfstrasse 33, 7260 Davos, Switzerland.
1. Introduction
The stratospheric layer plays a key role in communicating
climate variability over vast regions of the Earth’s
atmosphere. Some of its variability is attributable to natural
drivers such as variations in El Niño Southern Oscillation
(ENSO), solar irradiance, or volcanic eruptions and is
manifest on different timescales from days to seasons and
even longer timescales. Through downward wave
propagation, variability in the stratosphere can impact on
tropospheric climate variability modes and hence surface
climate on different spatial scales. One of the most
prominent stratospheric influence are sudden stratospheric
warmings (SSWs) leaving an imprint on weather at the
ground even a few weeks later (Baldwin and Dunkerton,
2001).
Figure 1 shows the observed surface air temperature (SAT)
and sea level pressure (SLP) response with respect to five
major tropical volcanic eruptions and to ENSO since 1880.
The volcanic signal in the first boreal winter months after
the eruption is characterized by a cooling over the oceans
and a warming over the northern extra-tropical land masses.
Several dynamical feedback mechanisms including the
propagation of planetary waves have been proposed to
explain the pathway of climate anomalies originating from
the stratosphere (see e.g., Stenchikov et al., 2004).
Over recent years much attention has been drawn to the
climatic effect of El Niño on the northern extra-tropical
stratosphere and its manifestation on surface. While the
surface climate response over the North Pacific and North
American region is well known, the signal over the North
Atlantic European sector (negative North Atlantic
Oscillation accompanied by cold (mild) temperatures over
Northern (Southern) Europe) is subject to a large variability
among individual El Niño events which complicates its
interpretation. Ineson and Scaife (2009) provided evidence
for a global teleconnection pathway from the Pacific region
to Europe via the stratosphere. They showed that, in
presence of SSWs, the stratosphere plays an active role in
the manifestation of the European regional climate pattern.
A better knowledge of the impact of stratospheric variability
on regional surface climate is therefore highly relevant with
respect to detection and attribution studies as well as
improvements of seasonal prediction schemes.
volcanic
ENSO
-1.8
-0.6
-0.6
-0.2
0.6
0.2
1.8
0.6
(°C)
(°C/°C)
Figure 1. Effect of tropical volcanic eruptions (top)
and ENSO (bottom) on boreal winter (Jan-Mar) SAT
and SLP since 1880. Temperature and SLP data
were detrended. The top panel shows a composite of
the first winters after five tropical eruptions
(Krakatoa, Santa Maria, Mt. Agung, El Chichón,
Pinatubo). The bottom panel shows regression
coefficients using a NINO3.4 index (Sep-Feb
average) after removing two winters after each
major volcanic eruption
2. Model Simulations
To study stratosphere-troposphere exchange processes
global chemistry-climate models (CCMs) have proven to
be indispensable tools as they incorporate all relevant
dynamical, radiative, and chemical processes in the
atmosphere (Eyring et al., 2006).
Here we present results of two different kinds of
simulations with the CCM SOCOL (Schraner et al.,
2008): (a) ensemble simulations (9 members) in transient
mode across the 20 th century (Fischer et al., 2008a); (b)
time-slice simulations (20 ensemble members) of an
anomalously strong El Niño (1940-42) and a weak La
Niña (1975-76) (Fischer et al., 2008b). SOCOL is a
combination of the middle atmosphere version of
ECHAM4 (MPI, Hamburg) and the chemistry-transport
model MEZON (PMOD/WRC, Davos). The simulations