Extreme weather

Drought

Drought is generally defined as an extended period of low or absent rainfall relative to the expected average for a region. Meteorological drought describes a prolonged period with less than average precipitation while a hydrologic drought occurs when available water reserves (in lakes, reservoirs and groundwater stores known as aquifers) fall below acceptable levels. The latter condition can arise even when there have been recent rains.

In many tropical countries, extremely arid conditions are an everyday state of affairs. Global belts of hot, dry sinking air girdle the tropics of Cancer and Capricorn. This is a result of a circulatory movement of air that initially rises close to the Equator along the Inter-Tropical Convergence Zone (ITCZ). Shedding its moisture there as convectional rain, the air moves pole-wards at high altitude before being forced to descend at the tropics as a result of jet stream activity. Now hot and dry, this descending air is responsible for high pressure and arid desert conditions throughout much of the tropics. The entire circulatory system, comprising of low-pressure along the ITCZ and high pressure at the tropics, is known as the Hadley Cell.

Some of the tropical air that subsides over the tropics in the northern hemisphere will often travel further northwards into Europe instead of returning to the ITCZ. If it passes mainly over land then it is termed a tropical continental air mass and is the usual cause of extended drought in the UK. During our very hottest summers, such as 1976, 2004 and 2006, clear blue skies – and lack of rain - are experienced as a constant. This indicates that hot dry Saharan air has advanced over mainland Europe into southern parts of the UK to form a ridge of high pressure. Meteorologists call this a blocking anticyclone. It can be recognised on weather charts as a circular or pear-shaped pattern of widely-spaced isobars with a 1026-1040 mb high pressure centre and light winds which diverge clockwise from the centre in the Northern hemisphere.

Is climate change increasing the frequency and severity of drought in the UK?

Warmer temperatures resulting from climate change will not simply be experienced around the world as a uniform and linear rise. With greater inputs of energy, complex systems such as the Hadley Cell will begin to function in modified ways. As a result, periods of drought in the tropics might last longer or become less predictable. For instance, scientists know that cycles of periodic drought currently operate in tropical regions such as sub-Saharan Africa and the Middle East. However, past observed trends will become a less reliable guide for future weather prediction as global temperatures rise.

The UK may also be facing a less predictable future where drought is concerned. The apparently increased frequency of extreme drought may be a result of changes in the behaviour of tropical air masses. For instance, back in 1976 an unusually high surface pressure persisted over southern England for much of the year (part of a ridge that extended northwards all the way from the Azores High). At the time, this was regarded as a very unusual situation. It was the driest period that had been recorded since 1727 and scientists calculated that there was only a 1 in 200 chance of its re-occurrence in any subsequent year.

However, some scientists now believe that the conditions experienced during the 1976 drought may return more frequently in future, perhaps once every fifty (or even thirty) years. The drought conditions experienced in south-east England during 2006 – a mere 30 years on from 1976 – may be used as evidence to support this hypothesis (especially when one recalls that record summer temperatures were also reached in the UK in 2004, while 20,000 deaths were attributed to a heat-wave in mainland Europe during 2003).

Looking towards the future, there is a strong likelihood that the UK is set to experience such dry conditions with ever-increasing frequency. The recent findings of the Intergovernmental Panel on Climate Change (IPCC) suggest that unless action is taken to keep the global temperature rise below 2C then summers such as those experienced in 1976 and 2006 could become the norm. London summer temperatures could reach 40C and above by 2050 according to climate models.

Student Practice Question:

Examine the causes and consequences of anticyclonic weather conditions in the UK

As well as referencing key ideas such as the origins of tropical air masses (and the importance of convergent flow in the upper troposphere, perhaps referring to Rossby Waves), a good response to this essay might ask whether climate change could be a cause of anticyclonic conditions dominating for longer periods of the summer, especially in southern England.