The first week of August saw the Jet Stream dipping below the UK allowing depressions to form along it and positioning us in the cooler air on the north side although a brief change in wind direction on the 4th, coming from the south, allowed the thermometer to reach 23.7C. This period also produced five wet days, with 10.1mm on the 7th.
On the 10th the jet stream looped to the west allowing a ridge of high pressure to form bringing drier and warmer air. A similar situation in the middle of the month saw the thermometer rise to 23.9C being 2.8C above the 37-year average and the warmest day of the month.
All the while a tropical storm formed in Mid Atlantic named Fred, number six of the 2021 season, noted as it was earlier than normal for August. This was shortly followed by another two tropical storms, number six and seven of the season, named Grace and Henri. Both developed into hurricanes but were downgraded after they reached the Mexican and American coastlines although they still created havoc with intense rainfall and caused much damage.
The third week was noted for average temperatures and mainly cloudy days with the wind veering into the northwest on the 16th and 17th, a cooler direction. The long sea run around the high pressure meant the air mass had time to absorb moisture thus overcast days.
Shortly afterwards, as the another depression arrived, the wind swung into the southwest bringing many hours of rain on the 21st totalling 17.7mm, which was the wettest day since 26th December.
On the 23rd the Jet Stream had repositioned and was forming a loop to the north of the UK. For the rest of the month the wind came from the northeast or north-northeast. This run of air over the cool North Sea allowed it to absorb moisture that gave us many cloudy days in the last week. This ‘blocking high’ kept weather fronts from advancing on the UK
On the 29th the thermometer dropped to 5.3C under a short lived clear sky making it the coldest night since 23rd June and 6.4C below the 37-year average.
It will not be surprising to find that the August 2021 data confirms it was not a good summer month with the mean temperature being 0.3C below the 37-year average. An analysis shows that although the mean daytime temperatures was 1.C below average it was contrasted by an above average temperature for the minimum of +0.3C, cooler by day but warmer by night.
There were extreme temperatures with 23.9C on the 10th and 14th but a very cool day on the 30th under the chilly north-easterly breeze and thick cloud when the thermometer refused to rise above 16.6C, which was 4.5C below average.
The total rainfall of 64.2mm was just 2.7mm short of the 37-year average. The extremes were recorded in 1992 with 139.5mm and 1995 with just 5.3mm. Evaporation from ground sources and plant life almost equalled the rainfall having lost 60.3mm to the atmosphere. Around 3mm was lost daily from the 24th to the 26th.
August will also be remembered for the reduced amount of sunshine especially at the end of the month, just 166 hours. The sunniest day occurred on the 26th with 9.4 hours but 4 days later under the thick cloud just 45 minutes was recorded.
Although we did not enjoy a good summer month when some forecasters predicted a heat wave, we did not suffer winds gusting to 150mph, 400mm of rainfall in one day, forest fires that burnt huge areas and hundreds of homes or peak temperatures in the high 40C that occurred around the world. So we must be thankful that our maritime place in the world moderates our weather.
The mean temperature for summer 2021 was 16.9C being 0.8C above average. It is interesting to note that in the late 1980s and early 1990s the mean temperature was 15.2C but since that time the graph as seen a slow but steady increase so that during the last few years the average has risen to 16.1C.
Rainfall for the three months amounted to 163mm being 18mm below the 37-year average. The very wet summer of 2012 produced 328mm of precipitation but the very dry summer of 1995 brought just 53mm.
Not surprisingly for the summer month, evaporation was greater than rainfall with 233mm lost to the atmosphere.
Much publicity was given recently to the international report on climate change with references to excessive rainfall and extreme temperatures. A contributor to climate change is the reported change to the Gulf Stream.
Although the UK is firmly in the northern hemisphere, it enjoys relatively warm, wet and windy weather, and milder winters than could be expected – and much of that is thanks to the Gulf Stream, the ocean current that bathes Britain’s shore with water drawn from the tropics.
But the Gulf Stream is just part of a major system of currents that circulate through the Atlantic Ocean, easing the climate in many parts of northern Europe. However, this vital system could be on the brink of collapse, scientists warned this month – an event that would dramatically and catastrophically reshape the world’s weather patterns. So what is going on?
The Atlantic Meridional Overturning Circulation – AMOC – works like a conveyor built, drawing cold waters from the north down to the ocean floor and carrying them southward. In parallel, warmer waters from the tropics are carried to the ocean surface and transported north.
The salty ocean water is crucial to the process. As the Met Office explains: “As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt. Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean.”
AMOC ensures that water in the world’s oceans is continually mixed, distributing heat and energy evenly around the world. Scientists therefore see it as a crucial stabilising force in the global weather system.
Scientists have already determined that the AMOC’s circulatory system is weaker than it has been for the past 1,000 years.
Now a new study by the Potsdam Institute for Climate Impact Research, published in the journal Nature Climate Change, suggests it is losing stability and is at risk of collapsing, falling from a fast, strong current system to a slow, weak and ineffectual one.
Climate change is likely a major culprit. As temperatures rise, ice sheets and glaciers are melting. That is causing huge volumes of freshwater to pour into the ocean, disrupting the salinity of northern seas.
Freshwater is lighter than saltwater and so is less inclined to sink to the depths and be pulled southwards, disrupting the conveyor-belt effect of the AMOC.
The AMOC is a major driver of the Gulf Stream. If it collapses or is substantially weakened, it could push the Gulf Stream further south from where it flows now, warns Professor Tim Palmer, Royal Society Research Professor at the University of Oxford. That would bring cooler weather to northern Europe.
This would mark what is known as a climate “tipping point” – an invisible threshold, which once passed could trigger a cascade of other climate impacts that will be irreversible.
In short, scientists don’t know. It could be 70 years away or 700 years away. But they did not expect to detect this level of instability in the AMOC this early on in climate change.
It’s important to note that this is just one study. Other scientists have suggested it is still too early to say conclusively whether the AMOC is nearing a tipping point. But almost all are nervous about rising greenhouse gas emissions and their potential impact on ocean currents.