The influence of the stratosphere on tropospheric circulation and implications

for forecasting

Nili HarnikDepartment of Geophysics and Planetary

Sciences, Tel Aviv University

300 mb zonal wind

Dec 2009 – Feb 2010

300 mb zonal wind

Climatology Dec-Feb

An anomalous winter over the Atlantic-Europe-Mediterranean region

10mb

1mb

100mb

0.01mb

0.1mbAtmospheric “heat sources”

surface

Ozone layer

O2 absorption

The atmospheric layers- vertical temperature structure

Dec net daily solar radiation at surface

Dec daily ncoming radiation at top of atmosphere

W

W

W

W

C C

C

C

C

C

Observations

Geostrophic and hydrostatic balances yield a thermal wind balance: vertical shear

- pressure gradient

cold

warm

JetNo rotation

With rotation

Rotation – Coriolis force to balance it : Geostrophic balance

Differential heating

Observations

WC C

W C

The zonal mean wind is in thermal wind balance with the zonal mean temperature

The stratospheric polar night jet

10mb polar cap temperature, 2001-2

Sudden warming

The stratospheric polar vortex, looking from above, is a cold polar cap.

Sometimes, however, this cap warms by 10s of degrees in a few days Sudden stratospheric warming

Cold vortex 15 Dec 2001

10 mb total geopotential height

Wave breaking polar vortex animation

Shown: Potential vorticity- a conserved dynamical quantity (angular momentum) which depicts the vortex structure.

Anomaly starts small from below and grows upwards

How does this perturbation happen?

Stationary planetary Rossby waves, forced by mountains and air-sea assymetries propagate upwards on the vortex.

troposphere stratosphere

The waves which propagate upwards sometimes break, like sea waves, mixing vortex air with its surroundings

But sometimes the waves are reflected back down to the troposphere.

This type of coupling, and what determines which behavior we see is less well understood.

Perlwitz and Harnik (2003,2004), Harnik, 2009

Baldwin and Dunkerton, 2001

While the breakup is caused by waves propagating upwards, it starts above and slowly progresses downward.

In the troposphere, it tends to push the jet equatorwards.

The tropospheric “storm tracks” are likewise shifted equatorwards.

European climate extremes and the North Atlantic Oscillation, Scaife et al, J Clim, 2008

Scaife et al (2008, J Clim) correctly simulated extreme surface conditions over Europe, only when imposing the observed positive wind anomaly in the lower stratosphere.

With strat. anomaly

control

Observations

Ensemble forecasts initiated just before a sudden warming showed 12 like-singend and 88 oppositely signed tropospheric responses (3.1x10-14 likelihood that is random)

Ensemble forecasts of sudden warming show stratospheric predictability of around 20 days.

12 %

88 %

How this affects predictability: (Gerber et al, 2009 GRL)

he

igh

t

season

Baldwin et al, Science 2003

Stratospheric time scales are longer.

Tropospheric time scales are longer during the stratospheric active season:

NH Dec-Feb (winter)

SH Nov-Dec (spring)

Observed auto correlation (“memory”) time scales of the zonal mean flow

Conclusions:

•Planetary Rossby waves propagate upwards on the stratospheric winter polar vortex, occasionally leading to a sudden warming.

•Sudden warmings propagate downwards to the lower stratosphere, where they nudge the tropospheric jet stream and weather systems to a more equatorwards postion.

•The longer predictability of the stratosphere during these times can add predictability to the troposphere.

We can gain predictaiblity by resolving the stratosphere in models

•The Atlantic-European sector is most sensitive to this.

So what about the current winter:

The jet stream anomaly started before the main sudden warming (Jan 23) but the warming can explain its persistence in this anomalous state