<P> The westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles . The westerlies are particularly strong, especially in the Southern Hemisphere, in areas where land is absent, because land amplifies the flow pattern, making the current more north - south oriented, slowing the westerlies . The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40 and 50 degrees latitude . The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse . </P> <P> If the Earth were a non-rotating planet, solar heating would cause winds across the mid-latitudes to blow in a poleward direction, away from the subtropical ridge . However, the Coriolis effect caused by the rotation of Earth causes winds to steer to the right of what would otherwise be expected across the Northern Hemisphere, and left of what would be expected in the Southern Hemisphere . This is why winds across the Northern Hemisphere tend to blow from the southwest, but they tend to be from the northwest in the Southern Hemisphere . When pressures are lower over the poles, the strength of the westerlies increases, which has the effect of warming the mid-latitudes . This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer . When it is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the equator, which brings cold air into the mid-latitudes . </P> <P> Throughout the year, the westerlies vary in strength with the polar cyclone . As the cyclone reaches its maximum intensity in winter, the westerlies increase in strength . As the cyclone reaches its weakest intensity in summer, the Westerlies weaken . An example of the impact of the westerlies is when dust plumes, originating in the Gobi desert combine with pollutants and spread large distances downwind, or eastward, into North America . The westerlies can be particularly strong, especially in the Southern Hemisphere, where there is less land in the middle latitudes to cause the progression of west to east winds to slow down . In the Southern hemisphere, because of the stormy and cloudy conditions, it is usual to refer to the westerlies as the roaring forties, furious fifties, or shrieking sixties according to the varying degrees of latitude . </P> <P> Due to persistent winds from west to east on the poleward sides of the subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres . The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the westerlies of each hemisphere . The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary of an ocean . These western ocean currents transport warm, tropical water polewards toward the polar regions . Ships crossing both oceans have taken advantage of the ocean currents for centuries . </P>

The effect of the westerlies is felt to a greater extent in europe than in north america