In fact, there is one major additional factor that modifies the resulting wind so that it does not flow perfectly straight from the high surface pressure to the low surface pressure. This factor is called the Coriolis Force - we experience something similar when we try to walk across a spinning merry-go-round - the outer part of the merry-go-round is moving faster than the inner part - so we are thrown by an apparent force as we try to walk toward the center. The same happens to the atmosphere as it tries to move across our rotating planet. The effect can be calculated - it turns out to depend only on latitude - it is stronger at higher latitudes and is in the opposite direction in the Northern Hemisphere compared to the Southern Hemisphere. The final force we consider is that of friction - as air tries to flow close to the surface, so friction with the surface tries to slow the air motion. This is referred to as the friction force. Thus, as a first approximation, the seasonal mean wind in the tropics comes about through initiation from the pressure gradient force, but with the resulting wind modified by the Coriolis force and friction effect.

We consider a situation where SST rises from west to east by 2C over 2,000km (see Figure 1.5). We can infer the surface pressure gradient from the SST, and then infer the near-surface wind from the surface pressure (knowing the Coriolis force and the effect of friction)(See PDF for more details). The resulting wind at 5N is

U=3.8 m/s (west-to-east component of the wind - usually referred to as the zonal component of the wind),

V=-3.2 m/s (south-to-north component of the wind - usually referred to as the meridional component of the wind)

That is the wind blows westerly, toward the warmer SST, but partly turned Northerly by the Coriolis effect.

The resulting wind at 5S is

U=3.8 m/s (west-to-east component of the wind),

V=3.2 m/s (south-to-north component of the wind)

That is, the wind blows westerly, toward the warmer SST, but this time partly turned Southerly by the Coriolis effect. The contrast is due to us now being in the Southern Hemisphere - that is, the direction of the kick given to the wind by the Coriolis force changes depending on which hemisphere we are in.