Item

Abstract

Stratospheric

climate

has

changed

significantly

during

the last decades.

The causes

of these

changes

are discussed

on the

basis

of two different

general

circulation

model

experi-

ments

forced

by observed

greenhouse

gas

and ozone

concentration.

There is a clear and signifi-

cant

response

of the lower stratosphere

temperature

and

geopotential

in the model

simulations

forced

by observed

ozone

changes

that is in accord

with observed

trends

in summer

in middle

and

high latitudes

of the northern

hemisphere.

Little effect

is seen

in the tropics.

In spring

there

occur

the strongest

anomalies/trends

in both

hemispheres

at polar latitudes;

however,

the model

responseis late

by 1 to 2 months

and

is much

weaker

than

the observed

effects.

The ozone-forced

model

in winter of both hemispheres

produces

slight

warming or no change

instead

of the slight

cooling

observed.

The effects

of enhanced

greenhouse

gases

as taken

from a transient

IPCC sce-

nario

AGCM run

do enhance

the

cooling

in high

latitudesin spring,

but the

effect

is much

smaller

than

observed.

Hence

neither

of the two forcings

(reduced

ozone

and increased

greenhouse

gas-

es) in the cold seasonsis able to produce

the recent

stratospheric

and tropospheric

trend

patterns

alone.

These

trends

clearly

resemblea natural

mode

of variability

both

in the model

and in the

real worldø

This mode

associates

a strengthened

polar night vortex

with an enhanced

North At-

lantic

oscillation.

The excitation

of this

mode

cannot

yet be attributed

to anthropogenic

forcing.