Uitleg over het 'reflection event' van de huidige SSW

Door de zeer kundige poster Met4Cast van Netweather:

"Reflection events are relatively rare although evidence suggests they are becoming more frequent, likely as a result of climate change.

We had a reflection event back in January, you may remember how good things were looking and comparisons to 1947 were being made in terms of how sustained blocking was expected to become. Tropospheric drivers were very, very good and we had a major SSW unfolding, that SSW however reflected back down into the troposphere and completely shifted the pattern to something far more unfavourable.

Unfortunately, the same thing has happened again. The stratosphere is the most important driver during our winters and it can override all other tropospheric drivers, even if they are all favourable. Wave reflections happen on average 40% of the time with varying degrees of intensity / impact etc.

Based on the research I've done on wave reflection events, they typically strengthen the Atlantic trough and Pacific high, this pattern is unfavourable for further "attacks" on the stratosphere and we see a rapid re-bound in u-wind strength following a reflection event, as we are seeing here.

The issue now is whether the impacts from the reflection event (stronger tPV over Cananda / Atlantic) wane in time for favourable tropical forcing to be able to promote blocking? Because time is limited.

The MJO cycle will begin to weaken into phase 8. Phase 8 of the MJO will lead to a fall in AAM tendency. Here are the composites for phase 7-8 of the MJO

Generally a cold producing pattern for the UK/NW Europe, here's the current NWP output:

Instead of high pressure in the Atlantic as per the MJO composites we've got the exact opposite. I am very certain that had we seen an absorption event rather than a wave reflection event, we would probably be seeing significant amounts of Atlantic / Greenland blocking and the first proper cold spell of the winter. Alas, the wave reflection happened and we have a pattern that could not be more different to the MJO composites.

This really, really shows why you can't just take the MJO in isolation and assume phase X = what the composite shows, it's far more complicated than that. Tropical and sub-tropical forcing always needs to be assessed in relation to the stratosphere.

This is also why we're seeing some signs of height rises to the NE, the stronger tPV over Canada and Atlantic trough essentially "push" the area of amplification east, the pattern wants to amplify, tropospheric drivers are in phases where blocking is promoted but the strongest driver of all, the stratosphere (and the wave reflection) is working against that idea.. and winning.

With the MJO cycle continuing eastward into phase 8 (and likely weakening from there), AAM tendency will turn negative, MJO forcing will begin to wane and tropospheric drivers will begin to switch towards favouring more westerly regimes.

If the wave reflection event can wane (typically 2-4 weeks!) before tropical forcing changes, we may see amplification and have some potential for colder weather post mid month, but the window is shortening due to changes in tropical forcing as the current cycle moves towards finishing."