The influence of the stratosphere on tropospheric circulation and implications for forecasting

1. The influence of the stratosphere on tropospheric circulation and implications for forecasting Nili Harnik Department of Geophysics and Planetary Sciences, Tel Aviv University

2. An anomalous winter over the Atlantic-Europe-Mediterranean region 300 mb zonal wind Dec 2009 – Feb 2010 300 mb zonal wind Climatology Dec-Feb

3. The atmospheric layers- vertical temperature structure O2 absorption 0.01mb 0.1mb Atmospheric “heat sources” Ozone layer 1mb 10mb surface 100mb

4. Dec daily ncoming radiation at top of atmosphere Observations C W W C Dec net daily solar radiation at surface C C W C W C

5. Differential heating Rotation – Coriolis force to balance it : Geostrophic balance - pressure gradient cold No rotation With rotation Jet warm Geostrophic and hydrostatic balances yield a thermal wind balance: vertical shear

6. Observations The stratospheric polar night jet W C C C W The zonal mean wind is in thermal wind balance with the zonal mean temperature

7. 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 10 mb total geopotential height 10mb polar cap temperature, 2001-2 Sudden warming Cold vortex 15 Dec 2001

8. 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

9. How does this perturbation happen? Stationary planetary Rossby waves, forced by mountains and air-sea assymetries propagate upwards on the vortex. troposphere stratosphere

10. 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

11. 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. Baldwin and Dunkerton, 2001

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

13. How this affects predictability: (Gerber et al, 2009 GRL) 12 % 88 % Ensemble forecasts of sudden warming show stratospheric predictability of around 20 days. 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)

14. Observed auto correlation (“memory”) time scales of the zonal mean flow Stratospheric time scales are longer. Tropospheric time scales are longer during the stratospheric active season: NH Dec-Feb (winter) SH Nov-Dec (spring) height season Baldwin et al, Science 2003

15. 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