'Watsonian Dip’

23rd Jul 2010

With the boat work nearly complete we originally planned to head off tomorrow very early and sail up to Cape Upstart. However, the weather has turned and is blowing 30 kts with heavy rain predicted.

While this is annoying, it is very much the nature of cruising, as is the predisposition of cruising folk to sit out bad weather when they must. The underlying theme that applies here is to NOT be in a rush and not get into bad weather whenever it can be avoided. A message I have received from a number of ‘old salts’ in recent times and thus one I am happy to adhere to.

Of interest in all this is the fact that such weather is not typical of this region at this time of year. This is the dry season up north yet as the chart shows, its raining all the way to the tip of Cape York. Now please forgive my indulgence but as a minor climatologist, this meteorological scenario compels me to rabbit on a little with a 'know it all' explanation of this phenomenon:

As the chart shows, the massive High in the Bight, with it’s central pressure of 1032hP, is generating an unusually steep gradient in Qld. (note the isobar spacing as compared to NSW area). We all know from schooldays that the resultant wind will be East / Sth-East and will bring moist air to the coast. Even so, the predicted weather is well beyond the nature of what this High, in isolation, ought to produce.

The answer to this apparent oddity, is to be found in the trough ( the southward dipping isobars) extending below the gulf. A seemingly innocuous aberration, this feature usually results from a large scale upper air oscillations generated around the equator. They tend therefore to be very influential and enduring as their vertical and lateral extents are quite huge. So significant, yet familiar, are these things to our East Australian weather that long ago, a fellow teacher of meteorology coined the unofficial name of ‘Watsonian Dip’, referring to the southward dip in the isobars and claiming all this in his own name - Watson! For some resason , this has always stayed with me so a Watsonian dip it is!

So; what all this result in is a situation where the incoming moist air from the high, is accelerated into the trough (like going downhill, hence the wind speed) then 'dragged' upward, on a massive scale, towards the high altitude source of this synoptic feature. This uplifting of course results in cooling and rain, usually lots of it for a few days.

Therefore, its safe to say that we will almost certainly spend tomorrow here, predominantly because of the weather but (speaking honestly) in part because it’s my birthday today and I might just have a red or two tonight!

Cheers to the Watsonian!