Zonal flow

Zonal flow occurs when the jet stream—fast-moving currents of air in the upper troposphere—follows a relatively straight path from west to east. This is in contrast to meridional flow, where the jet stream curves dramatically north and south.

This straight, latitudinal pattern tends to suppress the mixing of air masses. It limits the intrusion of cold polar air or warm tropical air into mid-latitude regions. As a result, zonal flow often leads to mild, consistent, and persistent weather, with fewer temperature extremes or significant precipitation events.

The weather associated with zonal flow varies by season and region, but it's generally predictable and moderate. For example, in winter, zonal flow can bring prolonged periods of cloudy, wet, but relatively mild weather to places like northern Europe or the Pacific Northwest. In summer, it might lead to cooler, breezy conditions with a reduced risk of heatwaves or severe thunderstorms. Since the atmosphere remains relatively stable under zonal flow, major storms are less common, and weather systems tend to move quickly without intensifying.

The role of Rossby waves

Zonal flow is closely connected to the behavior of Rossby waves (also known as planetary waves). These are large-scale waves that naturally occur in rotating fluids, like Earth's atmosphere and oceans, due to the variation in the Coriolis effect with latitude.

In periods of strong zonal flow, Rossby waves tend to have small amplitudes. This means their meanders are relatively weak, allowing the jet stream to maintain its largely straight, west-to-east path. This minimizes the northward and southward movement of air masses, contributing to stable weather.

Conversely, during periods of meridional flow, Rossby waves become much larger in amplitude. They create significant troughs (southward dips) and ridges (northward bulges) in the jet stream. These deep meanders facilitate the vigorous mixing of cold polar air with warm tropical air, leading to more extreme and variable weather.

What causes zonal flow?

Several fundamental factors drive Earth's atmospheric circulation and influence the prevalence of zonal flow:

• Differential heating: The most crucial factor is the uneven distribution of solar radiation across the Earth's surface. The equator receives significantly more direct sunlight and heat than the poles, creating a large temperature difference.
• Earth's rotation (Coriolis effect): As warmed air at the equator rises and moves poleward, Earth's rotation deflects it. This deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect is responsible for the formation of large-scale wind patterns, including the westerly winds in the mid-latitudes where the jet streams are located.
• Conservation of angular momentum: As air moves poleward from the equator, it conserves its angular momentum. Since the Earth's radius of rotation is smaller at higher latitudes, the air parcel must increase its zonal (east-west) speed to maintain its angular momentum. This contributes to the strong westerly flow in the upper atmosphere.
• Atmospheric circulation cells: The Earth's atmosphere has three major circulation cells in each hemisphere: the Hadley, Ferrel, and Polar cells. These cells work to redistribute heat from the equator to the poles. A strong and stable interaction between these cells, driven by the temperature gradient, tends to favor a more zonal pattern in the upper-level flow.

When the temperature difference between the equator and the poles is particularly strong and relatively uniform, it generally produces a more robust and straight jet stream, thereby promoting zonal flow. Disruptions to this temperature gradient or strong geographical features (like mountain ranges) can induce larger Rossby waves, leading to more meridional flow patterns.

Understanding these underlying dynamics, including the influence of Rossby waves and the driving forces of global circulation, is essential for predicting and interpreting atmospheric behavior. Shifts between zonal and meridional patterns often signal changes in the likelihood of extreme weather events.