Meridional flow is a pattern of large-scale atmospheric movement that significantly influences global weather. This term describes the north-to-south or south-to-north movement of air, which acts as a major mechanism for transferring energy and moisture. When this flow pattern dominates, it causes substantial mixing of air masses, bringing unseasonable temperatures and moisture to regions far from their source. A strong meridional flow often correlates with extended periods of extreme or unusual weather events.
Defining Meridional Flow
Meridional flow is the component of atmospheric circulation that moves along lines of longitude, running perpendicular to the equator. This flow pattern is a fundamental part of the Earth’s effort to balance the immense difference in solar heating between the equator and the poles. The sun heats the tropics much more intensely than the polar regions, creating an energy imbalance. Meridional flow resolves this by acting as a heat exchange system, transporting warm air poleward and cold air equatorward. This persistent north-south exchange of air masses helps regulate global temperatures and distributes heat energy across the planet.
The Contrast: Meridional vs. Zonal Flow
The atmosphere is characterized by two primary circulation patterns: meridional flow and zonal flow. Zonal flow describes air movement that is predominantly west-to-east, running parallel to the lines of latitude. When zonal flow dominates, weather systems move quickly across regions, leading to milder and more predictable conditions. This straight path limits the exchange of air masses, keeping cold polar air separate from warm tropical air.
In contrast, meridional flow features a pronounced, wavy path that meanders significantly north and south. This pattern is much slower and more unstable, actively promoting the mixing of cold and warm air masses. The meandering nature of meridional flow allows weather systems to linger and intensify, often resulting in periods of weather instability and extremes.
What Drives Meridional Movement?
The primary driver of significant meridional movement in the mid-latitudes is the jet stream, a fast-moving current of air in the upper troposphere. Meridional flow occurs when the jet stream path becomes highly curved, or “amplified,” developing deep troughs and high ridges. These large, wave-like disturbances are known as Rossby waves, which naturally occur due to Earth’s rotation and the difference in the Coriolis effect with latitude.
When the temperature contrast between the Arctic and the tropics is large, the jet stream tends to be strong and fast, maintaining a more direct, zonal path. When this temperature gradient weakens, the jet stream slows down and becomes unstable, developing larger-amplitude Rossby waves that force the flow into a north-south track. The troughs represent southward dips, allowing cold air to surge toward the equator, while the ridges permit warm air to move toward the poles.
Impact on Weather Systems
A strong meridional flow pattern has significant effects on regional weather, often leading to prolonged and intense conditions. The deep troughs and ridges in the jet stream allow for the vigorous mixing of air masses, causing dramatic temperature swings. For instance, a persistent southward trough can bring frigid Arctic air far into the southern United States, leading to severe cold waves. Conversely, a sustained northward ridge can deliver tropical warmth deep into northern latitudes, resulting in intense heat waves.
A significant consequence of meridional flow is atmospheric blocking, where weather systems essentially become stuck over a region. These blocking high-pressure systems force the jet stream to take a long, circuitous path around them, which slows the movement of storms and fronts. This stagnation can lead to persistent weather patterns, such as prolonged periods of heavy rain and potential flooding under a slow-moving low-pressure system, or extended drought conditions beneath a stationary high-pressure ridge.