Westerlies

The westerlies, also called prevailing westerlies or anti-trades, are a major system of global wind patterns. These winds blow from the west toward the east across the middle latitudes of Earth. They are located between 30 and 60 degrees latitude on both sides of the planet.

These winds are vital because they drive weather systems and move heat around the globe. They act as a massive conveyor belt for both air and ocean water. Without them, the distribution of temperature on Earth would look very different.

The movement of the westerlies is caused by a combination of solar heating and Earth's rotation. If the Earth did not spin, solar heating would cause winds to blow straight toward the poles. This would happen as air moves away from the subtropical ridge toward the colder poles. However, the rotation of the Earth creates the Coriolis effect. This effect deflects the path of the moving air. In the Northern Hemisphere, the Coriolis effect pulls the winds to the right. This results in winds that blow from the southwest. In the Southern Hemisphere, the effect pulls winds to the left. This creates winds that blow from the northwest.

The strength of these winds changes based on atmospheric pressure and the seasons. The westerlies are linked to the intensity of the polar cyclone. During the winter, the polar cyclone reaches its maximum intensity. This causes the westerlies to increase in strength. In the summer, the polar cyclone is at its weakest. This causes the westerlies to weaken as well. Pressure levels near the poles also play a major role. When the Arctic oscillation is positive, pressure is lower over the poles. This strengthens the westerlies and warms the middle latitudes. If pressure is higher over the poles, the flow becomes more meridional. This means the wind blows from the pole toward the equator, bringing cold air to the middle latitudes.

Geography has a massive impact on how strong these winds feel. The westerlies are much more powerful in the Southern Hemisphere than in the Northern Hemisphere. This is because there is much less land in the south to slow them down. Land masses can amplify flow patterns and make them more north-south oriented. This slows the west-to-east movement. In the vast oceans of the south, the winds remain unobstructed. Because of the stormy and cloudy conditions, people have given these winds descriptive names. Between 40 and 50 degrees south, they are called the roaring forties. Further south, they are known as the furious fifties or the shrieking sixties. In places like Chile, Argentina, Tasmania, and New Zealand, they are called the Brave West winds.

These winds are responsible for steering extratropical cyclones across the planet. An extratropical cyclone is a large-scale, low-pressure weather system. These systems form in the middle latitudes rather than the tropics. They are often called mid-latitude cyclones or simply depressions. The westerlies steer these systems in a general west-to-east direction.

These cyclones are important because they drive much of the everyday weather on Earth. Sometimes, a tropical cyclone can interact with these systems. If a tropical cyclone crosses the subtropical ridge, it can undergo recurvature. This means the westerly flow catches the storm and turns it toward the pole. An example of this occurred with Typhoon Ioke in 2006.

The westerlies also drive the movement of the world's great ocean currents. Persistent winds from the west push surface waters toward the east. This process is influenced by western intensification, which makes currents on the western boundaries of ocean basins stronger. In the Northern Hemisphere, the Gulf Stream is a famous example. It is a strong current that helps transport warm, tropical water toward the poles. In the North Pacific, the Kuroshio current acts in a similar way. In the Southern Hemisphere, the impact is even larger due to the open ocean. The Antarctic Circumpolar Current, or West Wind Drift, flows from west to east around Antarctica. At approximately 125 Sverdrups, it is the largest ocean current in the world.

Beyond weather and oceans, the westerlies can transport materials across entire continents. For example, dust plumes from the Gobi Desert can be picked up by these winds. The westerlies carry this dust and various pollutants eastward across long distances. This can eventually lead to these materials reaching North America. This shows how connected different parts of the Earth are through the atmosphere. The westerlies serve as a primary link between the tropics, the middle latitudes, and the polar regions. They move energy, water, and even dust in a continuous global cycle.