Who is Sam Rossby and why should I care?
IF you are a cruising yachtsman doing your thing from Perth, around the south coast of Australia including Tasmania, as well as up the east coast to the Queensland/New South Wales border; the answer to the headline question is: everything!
Weather is what is happening in the atmosphere a couple of days either side of when you are looking. Climate is what has already happened, averaged out, over the previous 30 years.
Understanding how climate drivers influence the range of possible variation and likelihood in weather over the next one to three months, is knowledge of great value to the cruising yachtsman. What we will try to do is bridge the gap between the weather of the next few days against longer term expectations three to six months into the future.
This discussion mainly addresses the way that the polar jetstream, (PJ), at an altitude of five to ten kilometres, influences surface weather in Australian waters between latitudes of 25 degrees to 30 degrees south and as far south as the bravery of the yachtsman leads him or her to venture. See Image 1 on page 25.
The southern annular mode (SAM) is a band of low pressure, strong westerly winds and often wet, unsettled weather between 40°S and 60°S. Looked at from a great height, such as space, the SAM is seen as flowlines of air movement from west to east with a wavy pattern of crests and troughs.
This band of westerlies contains the Roaring 40s with, along its southern border, the polar front (PF), which is the breeding ground for frontal depressions July issue, page 50.
