What’s going on? We have gone from relentless rain to unusually dry in just 6-8 weeks. From almost UK-wide flood warnings in February/early March to little or no rain from March 19 to April 16.
This was the 500mb height anomaly for February.
Credit: Simon Cardy
This was the result. A record wet February for the UK.
Credit: Met Office
As for the first 14 days of April. The deep trough has lifted into NE Europe while the strong high over Iberia pushes into the Low Countries and UK.
Credit: Simon Cardy
A remarkable flip in rainfall anomaly over the UK between February and April.
Credit: Met Office
The cause of the relentless rain during February is likely down to firstly a record strong Indian Ocean Dipole last autumn and warm phase of the MJO throughout the winter.
The record +IOD last autumn may have enhanced a very strong SSWE over Antarctica which produced record warmth over the Southern Hemisphere. We had a literal ‘polar opposite’ with very strong SSWE (weak polar vortex) over Antarctica while the Arctic received a record strong polar vortex. The combination of both +IOD and persistent warm phases of the MJO, likely led to these polar vortex contrasts.
The strong PV in the Arctic drove a continuously strong positive Arctic Oscillation (Arctic low) and unusually strong subtropical high underneath. This clash super charged a highly zonal polar jet stream driving mild, moist ocean air into Europe. Warm waters south and west of the UK helped enhance rainfall.
Why the 360 flip?
From soggy to dry? What’s changed? One key factor is the seasonal shift from low winter sun strength to high spring sun strength which naturally weakens and eventually destroys the polar vortex.
As the sun gains strength over the higher latitudes, the subtropical ridge begins it’s journey away from equator towards the pole at the same time the polar vortex weakens. This process of subtropical high lifting north with the sun and weakening of the PV slows down the westerlies. The weakening of the westerlies has allowed ridge building to return and with the MJO finally progressing, combined with current Atlantic SST profile, ridging has not only returned but held it’s ground. The first negative North Atlantic Oscillation since November is helping maintain ridging.
As stated in Thursday’s video, this ridging looks set to persist in it’s current northerly position and so expect the dry conditions to continue at a latitude from Iceland to Scandinavia but this leads to lower heights beneath so expect wetter weather bubbling over Iberia and France to periodically lift northward into southern Ireland and UK.
Credit: Tropical Tidbits
Interestingly the CFSv2 sees the mean ridge position shifting towards Iceland allowing a late season cold intrusion to slide westward.
Credit: Tropical Tidbits
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