Very warm conditions have dominated this December, especially in SE England due to a persistent warm SW sub-tropical airmass. Here in Reigate the mean monthly December temperature so far is over 10C, nearly 5C above the longer term average for the month (5.3C). Records for daily maximum and highest minimum temperatures have been tumbling as 850hPa airmass temperatures have risen 5-10C warmer than normal, making it feel more like May or even June than December, especially overnight! In addition, the air arriving tonight is loaded with Saharan dust.
Reigate daily temperatures have been consistently hitting double figures and recent nights have barely dropped below 10C.
It is the warmest start to December for years but will it break the all-time absolute Tmax record of 18.3C?
Oddly enough probably not, because the December absolute Tmax record was set in the lee of mountains in Scotland where, in the right conditions, a special foehn effect can lift temperatures beyond the reach of even the warmest airmass that we are likely to get in the next few days. This airmass is cloudy too, so the sun is less likely to break through.
Nevertheless, daily and local records here in the SE are tumbling and the duration of this warm spell is unusual. The cause is a stubborn high pressure over Europe and a trough in the Atlantic feeding a persistent SW airflow from the warm sub-tropics, places like the Azores.
At this time of year the Saharan HIGH pressure is pretty strong and the southern edge of this creates winds that pour across the Sahara Desert from the east, heading across the continent to the Atlantic.
The Bodélé Depression in Chad is very likely to be the source of any dust arriving in the UK over the next few days. Weather stations nearby have recorded windy conditions almost entirely from the east.
The Bodélé Depression produces more than half of all Saharan dust, partly due to the way super-geostrophic wind circulate around the Saharan high pressure and are funneled through a mountain barrier into the depression, accelerating wind which then lofts prodigious amounts of dust into the air.
Some 700,000 tonnes of dust can be lofted into the air every day in this location. The result is some extraordinary rock formations due to the eroding effect of the sand blasting the rocks.
Desert sand tends to be lofted into the lower atmosphere, up to the boundary layer at most some 1km-2km or so above the surface but it can reach higher altitudes in some conditions and be transported further afield. From Chad it is blown by trade winds into the Atlantic. A lot of this dust continues across to America but, depending on conditions, some of it can be gathered into the mid-latitude circulation and make its way to Europe. Spot the journey of the dust on these sat pics. taken since late November.
By Thursday the desert dust is forecast to join more local particulate pollution in a warm sector to bump up pollution levels in the UK. It also coincides with exceptionally mild airmass. A breeze tomorrow should reduce the threat of a “toxic cloud” developing as stated in some media. Nevertheless, it is worth considering that desert dust is an entirely natural and vital part of the atmospheric circulation.
The outlook is for continued warm/mild weather to continue into Christmas, potentially more unsettled at times as the jetstream perks up and takes on a more direct zonal attack across the Atlantic. The bigger reason for the mild mid-latitude weather is an exceptionally powerful polar vortex that is keeping pressure and temperature very low in the polar stratosphere.
The lower tropospheric jetstream is subsequently strong and “locks in” the cold to the polar regions. This situation is summarized by a strongly positive North Atlantic Oscillation: indicated by high pressure over the Azores and low over Iceland. Until this situation changes the chances of sustained cold for us are slim. The only hope for sustained cold this winter in a mega-El Nino year is said by experts in long range forecasting, to be a sudden stratospheric warming that will break down the polar vortex. That can occurs most commonly Jan-Feb. Here’s hoping!
More on the importance of desert dust in the atmosphere can be found in the post here https://rgsweather.com/2015/04/14/dust/