Westerlies wind and coriolis force?

Question

I have understood that coriolis force causes an apparant deflection of motion due to the rotation of the earth. For the example of trade winds, wind blows from Egypt will be deflected and ends up somewhere near the west shore of africa instead of somewhere right below egypt. Using the same concept, westerlies wind blowing from the southern of usa should end up somewhere near the west shore of usa. But in fact, westerlies winds blow from south-west to north-east. Can anyone please please explain this? Thank you very much.

I have understood that coriolis force causes an apparant deflection of motion due to the rotation of the earth. For the example of trade winds, wind blows from Egypt will be deflected and ends up somewhere near the west shore of africa instead of somewhere right below egypt. Using the same concept, westerlies wind blowing from the southern of usa should end up somewhere near the west shore of usa. But in fact, westerlies winds blow from south-west to north-east. Can anyone please please explain this? Thank you very much.

Answer 1

This is a very good question. What you need to understand is that a balance is established as the wind moves. The Coriolis acceleration is not the only force you see. There is also a pressure gradient force and a force caused by surface friction. When there is a perfect balance the wind is said to be in geostrophic balance. But since this never occurs perfectly the winds tend to cross the isobars rather than run parallel to them. Around a low pressure center the pressure gradient acts inward toward the center of the low. The Coriolis acceleration acts to the right of the flow or in the opposite direction of the pressure gradient. The friction acts along the direction of motion. The result is wind that tends to spiral into the low. An opposite situation is established with an anticyclone. Hope this helps a little. I could do it much better on a blackboard.

Answer 2

There is a belt of high pressure around the globe in each hemisphere called the sub-tropical ridge. It is centred between 30° and 40° depending on the season. Winds blow out of the high towards the equator but are deflected by coriolis to produce the northeast trade winds in the northern hemisphere and the southeast trades in the southern hemisphere. Polewards of the highs, the winds are blowing towards the poles but are deflected by coriolis to the west. This produces the westerly wind belt. Polewards of the sup-tropical ridge, the polar front lows reinforce the westerlies which can blow anywhere from southwest to northwest.