Tag Archives: Summer

CYCLONIC ACTIVITY IN THE SOUTHERN HEMISPHERE

Last December 21 at 11:12 UTC our star, the Sun, reached its most southerly declination of 23.5 S marked by the Tropic of Capricorn, an imaginary line south of the equator running near Sao Paulo and Rio de Janeiro in Brazil, Pretoria in South Africa, Maputo in Mozambique, Alice Springs in Australia, Noku-alofa capital of the archipelago of Tonga in the South Pacific, and Antofagasta on the coast of Chile. This event marks what is known as the winter solstice in the northern hemisphere or summer solstice in the southern hemisphere.

A lot happened on that date of 21 December 2011 as the tilt of the Earth’s axis reached its maximum pulling the north pole farthest away from the Sun, and bringing the south pole to its closest point from our star. This day marked the onset of winter in the northern hemisphere, and of summer in the southern hemisphere. On that day all points on Earth north of latitude 66.5 N had total darkness for 24 hours, while those south of latitude 66.5 S – the Antarctic polar circle – had 24 hours of daylight.

All of these happenings were set in motion, programmed in a manner of speaking, billions of years ago when the energy of the Universe caused the birth of the star we know as the Sun and subsequently generated the solar system, including our Earth, orbiting it. The driving orbital mechanics, or the celestial dance of the spheres as some would call it, result in a planet Earth that every 24 hours completes a full revolution around an axis, which in turn oscillates back and forth with a swing of 47 degrees of amplitude taking one year to complete a full oscillation, at the same time that it takes the planet to complete a full orbit around the Sun.

The above is quite a simplified description of a process that causes what we know as seasons, meaning winter, spring, summer and autumn, on Earth and the variability in climate associated with the same. In reality the orbital mechanics of the Earth are a lot more complicated than what has been described, as they also involve the slow change in the direction the Earth’s axis points toward causing to slowly draw a circle in the sky and the whole planet to wobble. Then there is the gradual change in the geometry of the Earth’s orbit around the Sun from nearly circular to a more elongated elliptical shape, and other changes caused mainly by the gravitational tug of Sun and the Moon resulting in what is known as the precession of the equinoxes, a process that over a cycle of 26,000 years causes the dates if the seasons to gradually change as the time of the year when the Earth comes closest to the Sun or perihelion also changes. This complex combinations of movements and interaction between celestial bodies drives the Earth through a series of cycles ranging from daily, to annual, and others lasting respectively  26,000, 40,000 and 100,000 years.

Of interest to us today, for this article, is how the combination of changes on the tilt of the Earth’s axis as it completes its annual orbit around the Sun drives the process of cyclogenesis, the generation of tropical cyclones, around several oceanic basins.

In the case of Earth’s southern hemisphere and tropical cyclone generation in 2012, it all started on 23 September 2011, date of the autumnal equinox, when the Sun was directly above the equator on its way south and the oceans of the southern hemisphere started receiving more hours of sunlight and energy. By the time the tilt of the Earth’s axis reached its maximum bringing the Sun over the Tropic of Capricorn, marking the onset of summer in the southern hemisphere, the reach of warmer sea surface waters had extended considerably over the southern tropics, and with this the formation of tropical waves, cells of disturbed weather, regions of low pressure and other potential contributors to cyclonic activity have all become more frequent and prevalent over certain regions.

Sea surface temperatures for the autumnal equinox on 23 September 2011
Sea surface temperature over eastern Pacific on 23 September 2011
Sea surface temperatures over eastern Pacific at onset of winter on 12/23/2011
Sea surface temperatures over Atlantic for the vernal solstice on 12/23/2011
Sea surface temperatures over eastern Pacific on 02/12/2012
Sea surface temperatures over Atlantic on 02/12/2012

It is clear that the southern Indian Ocean, both at its western end near Africa and its eastern sub-basin off Western Australia, as well as the South Pacific, are already primed for their respective 2012 tropical cyclone seasons.

View of tropical cyclone activity over the southern Indian Ocean on 13 February 2012 showing category 4 cyclone Giovanna approaching landfall over eastern Madagascar

In fact, on 13 February 2012, we already have cyclonic activity represented by a dangerous category 4 cyclone GIOVANNA about to make landfall near Tomasina on the eastern coast of Madagascar, a cell of low pressure showing signs of potential further development to the southwest of Indonesia and west of Australia, and far on the other side of Australia over the South Pacific a long lived tropical cyclone JASMINE now downgraded to tropical storm strength impacting Nuku-alofa the capital of Tonga. In addition satellite images show plenty of moisture in the atmosphere, areas of disturbed stormy weather, and atmospheric patterns that appear favorable for cyclogenesis over various regions in the southern hemisphere. All interests in those regions must pay attention, be alert and remain prepared, and above all continue to practice mitigation!

A weakened tropical cyclone JASMINE approaches Nuku-alofa, in the Tonga archipelago, as a tropical storm on 13 February 2012
Composite full Earth disk satellite image showing a good portion of the Pacific and Indian Oceans as well as Australia, Australasia and parts of Asia on 13 February 2012

Elsewhere satellites also show plenty of storm activity, but of the winter kind, over portions of Canada, the USA, Europe, and the northern regions of the Pacific and Atlantic oceans. Relative to this it is sobering to learn that many of these winter storm events have resulted in death, injury and damage to property in several places, mainly in Europe.

Full Earth disk composite satellite image of western hemisphere on 13 February 2012 showing storms and rain over much of South America and winter storms over Canada, portions of the USA, and the North Atlantic

REFERENCES:

1. J. D. Hays, J. Imbrie, N. J. Shackleton: “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages.” Science 194 (1976): 1121-1132.

2. D. Brouwer, G. M. Clemence. Methods of Celestial Mechanics. New York, 1961