![]() ![]() This is due to coastal topography and the shape of the ocean bottom which plays a major role in determining the tidal patterns around the country. It has a full spectrum of tidal regimes, and a tidal range varying from 1m to over 10m. Australia is however very complex from a tidal point of view. South Africa has a relatively simple tidal system with semi-diurnal tides and a relatively small tidal range. Details of the tides at any given place are governed by the responses of the ocean to the gravitational forces. Thus the gravitational theory became established as the basis for all tidal science. He also showed why there are two tides for each lunar transit, the reason why spring and neap tides occurred, why diurnal tides are largest when the moon was furthest from the plane of the equator and why the equinoxial tides are larger in general than those at the solstices. He described in his “ Principia Mathematica ” how the tides arose from the gravitational attraction of the moon and the sun on the earth. ![]() However a scientific explanation for the tidal phenomenon had to wait for Sir Isaac Newton and his universal theory of gravitation which was published in 1687. In the late 10 th century, the Arabs had already begun to relate the timing of the tides to the cycles of the moon. One ascribed the tides to the breathing cycle of a giant whale. ![]() In the beginning tides were explained by all sorts of legends. It was this ignorance of tides that led to the loss of Caesar’s war galleys on the English shores, he failed to pull them up high enough to avoid the returning tide. Tides are very often neglected or taken for granted, “they are just the sea advancing and retreating once or twice a day.” The Ancient Greeks and Romans weren’t particularly concerned with the tides at all, since in the Mediterranean they are almost imperceptible. Despite substantial variations among individual events, on an average these persistent events exhibit a strong non-linear relationship between the wave activity and wave activity flux - a characteristic traffic jam behavior in the atmosphere.Tides: The periodic vertical movement of water on the Earth’s Surface Almost all of these persistent events are found to occur in clusters collocated with the quasi-stationary ridge over the Euro-Atlantic sector and the Pacific sector as predicted by local wave-activity theory. Using the wave activity budget and a feature tracking algorithm, we perform a careful analysis of all the major persistent anomalies of the jet stream during the Northern Hemisphere winter. In my talk I will show that atmospheric blocks are persistent events with anomalously large wave activity whose dynamics can be explained by the finite-amplitude local wave activity framework. Despite decades of research there has been no consensus on the mechanisms that lead to atmospheric blocks. Such phenomena are known as atmospheric blockings. However, under certain conditions these jet meanderings become stationary and disrupt the eastward flow of eddies causing extreme temperature conditions that can persist from a few days to more than a week. The atmospheric jet stream is an eastward river of wind in the midlatitudes that steers and meanders across the globe due to planetary waves and eddies. Pragallva Barpanda, CIRES/ NOAA PSL, University of Colorado, Boulder What causes atmospheric blocks? - A new perspective using the finite amplitude wave activity theory Seminars Atmosphere Ocean Science Colloquium
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