Southern hemisphere tropical weather

Tropical weather in the southern hemisphere does not really follow a season similar to that seen in the North Atlantic and Eastern Pacific. Instead, tropical storms have developed throughout the year and the tropical season can be said to last from July all the way to June. Of course their seasons are opposite of the seasons in the northern hemisphere so the heaviest activity occurs from November through April.


The chart above shows the total number of tropical cyclones from 1969 - 2006 in both the southern Indian and the South Pacific Oceans by month. The average number of storms in a given year is 28.25 and every year had over 20 named storms.

Reference: Southern Hemisphere Monthly Storm Distribution (Australian Severe Weather)

While the southern hemisphere is clearly more active than any of the northern basins, the potential for landfall is much less since the land area in the south is less than in the north.

Anja was the first storm of the year forming on November 15th, the start of the peak period for the south. This storm is expected to not make landfall prior to breaking apart later in the week.

We have not covered southern storms in the past. As information is available, we will attempt to provide links and content for readers in the southern latitudes. The sources I utilize may not have detailed information for southern cyclones and much of that information may be in French (see previous post).

Additionally, those of us in the north need to keep in mind that the rotation of the Earth causes tropical cyclones to rotate clockwise in the southern hemisphere as can be seen in this picture of Anja.

Likewise the directional tendencies will be to the south and east rather than to the north and west as we see in the Atlantic and Eastern Pacific.

Photo source: Anja premier cyclone tropical (Grandbaie.mu)

Another difference between the northern and the southern hemisphere is the tropical cyclone classification system shown above. These classifications differ from the Saffir-Simpson scale used in the Atlantic and Eastern Pacific basins. Categories are based on the averaged 10 minute sustained winds in knots converted to km/hr.

Source: Tropical Cyclone Map Intensity Scale Information (Australian Severe Weather)

The definitions in the following table are from the Southern Hemisphere classification system (as used by the Australian BoM), and all maps on this website have been plotted using it. JTWC best track data has been converted from 1-minute average winds in knots, to the Southern Hemisphere standard of 10-minute average winds in knots, then converted to km/h. The conversion used is "JTWC 1-min" * 0.88 * 1.852. Operational data is also converted using this formula where 10-min average winds are not available.

The Hurricane and Typhoon classification systems are different.