Stratospheric sudden warming and elevated stratopause events

The stratospheric sudden warming (SSW) phenomenon is occasionally observed in the Arctic, in which the stratospheric temperature increases in a few days by several tens of degrees. An example is shown in Figure 1 in a time–altitude cross-section of the Arctic temperature from November 2008 to June 2009, plotted using data from the Microwave Limb Sounder (MLS) on the Aura satellite. From the figure, we can see that around January 20th, the temperature at an altitude of about 30 km suddenly increased by 50 K from 210 K to 260 K.
This phenomenon can also be seen as a drop in the position of the polar region’s stratopause, the region of maximum temperature, from its normal winter altitude of around 55 km to an altitude of about 30 km.
Interestingly, the stratopause was then seen to weaken, and from around February 5, 2009, a new stratopause started to form at an altitude of 80 km; this then gradually descended to around 50 km, which is the normal altitude for the stratopause in summer. Such a significant change in the stratopause around the 5th of February was first observed in 2006, and similar phenomena were observed in a few years when major SSW events occurred.

**Figure 1** Time-height section of temperature from Aura MLS averaged over 70N-80N in November 2008 to June 2009.

Gravity-wave-permitting general circulation model (GCM) simulations have previously simulated the occurrence of both SSW and elevated stratopause phenomena. One example is shown in the first panel of Figure 2, showing that a maximum temperature region was observed at an altitude of 75 km roughly 15 days after an SSW event, much like the stratopause jump that was actually observed in the atmosphere (as presented in Figure 1).
We had examined this event in the model simulations in detail. The time-altitude section of the zonal-mean zonal wind for 50-70N is shown in the second panel of Figure 2: during the winter, the zonal wind normally blows eastward (positive values), but the simulation results show that it blows in a westward direction (negative values) when a SSW occurred. This westward wind was persistent over 20 days at altitudes below 40 km, until a strong eastward wind appeared at an altitude of 60 km and gradually descended into the region.

**Figure 2** An event of the stratospheric sudden warming and elevated stratopause simulated by a gravity-wave permitting general circulation model. (Tomikawa et al., 2012)

The third and fourth panels of Figure 2 show the forcings by planetary-scale Rossby waves (planetary waves for short) and by gravity waves, respectively. Negative planetary wave forcing, centered at an altitude of around 50 km, was observed just before the SSW event, suggesting that it was a direct cause of the SSW. As for gravity wave forcing, large negative values were seen at altitudes of around 70 km just before the SSW (left end of the figure), suggesting that this maintains the stratopause (more details here). The disappearance of the stratopause after the SSW can be attributed to the weakening of gravity wave forcing, indicated by its positive values. As both the SSW and the westward wind weaken, the negative gravity wave forcing then recovers; however, it is still weaker than normal, resulting in the stratopause appearing at a higher altitude than usual.

An interesting point to notice is that after the SSW event ended, positive gravity wave forcing (i.e., an eastward acceleration of the zonal wind) was observed at altitudes below 40 km, and was gradually found to decrease in altitude with time. This gradual descent corresponds well with the gradual recovery of the eastward wind. The results of our study indicate that planetary waves are responsible for triggering a SSW, but that gravity waves play a crucial role in the recovery process after the event.

From Tomikawa et al. 2012

The list of SSW and SFW from JRA-55: XLSX