Two houses have been damaged by a landslide at Great Mackerel Beach to the north of Sydney. The incident happened slightly after midnight, local time, on Monday 26 October 2020. A further eight properties were evacuated following the incident, due to fears of further slippage, but the residents of two of these have now been allowed to return. The incident came amid heavy rains as a series of storm systems have swept across much of New South Wales over the past two days, causing flooding in many areas.
Storm systems form due to heating of air over the sea. As the air is heated the the air pressure drops and the air rises, causing new air to rush in from outside the forming storm zone. If this zone is sufficiently large, then it will be influenced by the Coriolis Effect, which loosely speaking means the winds closer to the equator will be faster than those further away, causing the storm to rotate, clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. Such storms can carry large amounts of water, evaporated from the surface of the sea by the warmth of the Sun, which can then be released abruptly as rain or hail when the moist air encounters cooler conditions over land (the amount of water the air can hold relates directly to its temperature).
The South Pacific is currently in an El Niño climate phase, which typically brings dryer conditions in Australia. However it also brings very high temperatures to the region, which can lead to the creation of more storm systems, with associated flash flooding events. Flash floods are a common problem in South Australia (and other areas with a dry climate) as protracted periods of dry weather can cause topsoil to dry out completely, making it vulnerable to being blown away by the wind. When rain does arrive it then falls on exposed bedrock, which is much less absorbent, triggering flash flooding as the water escapes over the surface of the ground rather than sinking into it. These floods wash away more topsoil, making the problem progressively worse over time.
The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.
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