Swellnet Dispatch The Stationary, Flat-Topped High

In: Swellnet Dispatch by Stu Nettle 10 Comments Tue 23rd Nov '10

Tags: meteorology , surf forecasting , trade winds

click to enlarge

Why do Queensland and northern NSW get so much swell during summer? Why, even in the absence of any cyclones, does the stretch from Noosa to Yamba have extended periods of small but fun waves?

The answer to the above questions lie in the interaction between high pressure systems and the easterly trade wind belt.

As most of you will know, our weather systems move from west to east, yet from late Spring to early Autumn it is not uncommon for large high pressure systems to stall over the Tasman Sea. These 'stationary highs' can sit in the same place for up to a week and sometimes, when the pattern is very entrenched, one system can move on only to be quickly replaced by yet another stationary high.

Now, it isn't the whole weather sytem that interests us but only the northern half of it. Imagine, if you will, a high pressure system with its centre 500 kilometres off the coast of Sydney and the isobars rotating counter-clockwise around it. The typical summer pattern is for the high pressure system to take on a 'flat topped' look. That is, rather than rotating in a circle around the high pressure system, the pattern of isobars are flat along the top (or the northern-quadrant) of the system (see image 1). This is due to the high pressure system intersecting with the easterly trade wind belt.

Trade winds are semi-permanent winds that blow in the tropics between 15 and 25 degrees north and south of the equator. They are so-called because of the effect their discovery had on oceanic trading. The predictable nature of the trade winds (their location and their strength) created dependable trade routes. Sea merchants could reliably schedule trips if they utilised the trade winds.

Off the east coast of Australia there is a belt of trade winds that originate in the South Pacific and travel in an easterly direction toward the Australian continent. On their own the trades aren't able to kick up much of a swell - they are simply too light. However, when the trade winds combine with a stationary high pressure system the effect is exagerrated. The isobars bunch together creating a steeper pressure gradient that can remain in place for extended periods of time.

The result is stronger winds that blow in a straight line over a large stretch of water. And that creates swell...lots of it.

Fortunately for QLD and northern NSW surfers the place where high pressure systems intersect with the trade wind belt is directly offshore from their coast. Therefore the isobars are pointed directly toward them and, as long as the 'stationary, flat-topped high' remains in place, then they will get waves.

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