Q Climate change
Left & above: The flood waters at the Augrabies falls. Image: kruger.com
patterns have fuelled the climate change debate, polarising opinion and beliefs. Climate change sceptics point to frozen Europe as evidence that global warming is a myth, while images of scorched and drought-ravaged Africa in 2009/10 reinforce the global warming lobby. So what on earth is happening to climate and weather patterns? Climate and weather Before answering this, we need to distinguish between climate and weather. Climate represents a broadly regional condition that is typical, such as ‘temperate’ (for example the UK), ‘equatorial’ (e.g. central Africa), ‘Mediterranean’ (e.g. Cape Town) or ‘polar’ (e.g. the Arctic and Antarctica). Such climate regimes often experience seasonal variability (e.g. winter/summer) and long-term climate change over centuries or more. However, weather is what we experience daily. It is local and changeable over periods of hours, days, weeks or longer; for example seasonal weather patterns. However, weather patterns can also be subject to global modification over inter-annual or decadal time-scales. One such modification we experience is due to El Niño and La Niña events.
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El Niño and La Niña The term El Niño, meaning ‘the boy’ (Christ-Child) in Spanish, was coined by Peruvian fishermen in 1975 to describe the appearance of unusually warm water that occurs every three to six years along the western South American coast at about Christmas time. Now, El Niño events more broadly describe warming of the central and eastern equatorial Pacific, which reduces the atmospheric pressure gradient between the eastern and western Pacific. The associated fluctuation in tropical sea level pressure in the eastern and western Pacific is known as the Southern Oscillation, so
that El Niño events form part of the broader coupled ocean-atmosphere phenomenon known as the El NiñoSouthern Oscillation, or ENSO. In normal (La Niña) years, easterly trade winds blow right across the surface of the equatorial Pacific from Peru/Chile to Indonesia in the western Pacific basin. Apart from helping to set up coastal upwelling off Peru, these winds also create general divergent open-ocean upwelling right across the eastern half of the equatorial Pacific. This sets up a surface temperature gradient of less than 20 °C in the east to more than 30 °C in the west, resulting in a shallow thermocline in the east, but a much deeper one in the west. The prevailing trade-winds also ‘pileup’ water in the west where ocean levels are several centimetres higher than in the east. Off Peru and Chile, local along-shore winds drive coastal upwelling that brings nutrient-rich water to the surface, resulting in a diatomdominated phytoplankton community that supports the well-known and profitable anchoveta fishery. By contrast, El Niño Southern Oscillation (ENSO) events are associated with anomalous warming of the central and eastern equatorial Pacific on average every three to five years, although fossil records indicate that it used to follow a decadal pattern. El Niño, the opposite of La Niña, occurs when easterly trade winds lose intensity, allowing warm water from Indonesia and eastern Australia to flood eastwards across the Pacific, ‘capping’ upwelling waters of the general equatorial and coastal Peru/ Chile upwelling regions (see El Niño conditions). This raises surface temperatures in the eastern equatorial Pacific to as much as the 29 °C. Surface ocean warming deepens the ocean’s
A satellite image of the snow blanketing Ireland. Image: Wikimedia commons
Power lines brought down by an ice storm in extreme weather conditions. Image: Wikimedia commons
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