Meridional flow

Meridional flow is characterized by significant, dramatic curves and meanders, contrasting with the relatively straight, west-to-east path of zonal flow.

Meridional flow is associated with the vigorous mixing of air masses. The deep, wavy troughs (southward dips) and ridges (northward bulges) in the jet stream allow cold, polar air to penetrate deep into mid-latitude regions and, conversely, allow warm, tropical air to surge poleward. This leads to unpredictable and often extreme weather events, including significant temperature swings, intense storms, and heavy precipitation.

The weather associated with meridional flow is highly variable and can change rapidly. For example, in winter, a strong meridional flow pattern can bring a deep trough of frigid arctic air to the southern United States, causing an unseasonal cold snap. Conversely, a strong ridge can bring an intense heat wave to regions like western Europe. Since weather systems move more slowly and can intensify within these deep meanders, major storms, such as nor'easters or blizzards, are more common and can become more severe under meridional flow.

The Role of Rossby Waves

Meridional flow is directly related to the behavior of Rossby waves (also known as planetary waves), which are large-scale waves in the atmosphere.

In periods of strong meridional flow, Rossby waves have large amplitudes. This means their meanders are pronounced and deep, creating the significant troughs and ridges that characterize this flow pattern. This allows for the extensive mixing of warm and cold air masses, leading to variable weather conditions. The large amplitude of these waves also causes weather systems to move more slowly and often allows them to intensify.

Conversely, during periods of zonal flow, Rossby waves have a small amplitude, resulting in a straighter, less wavy jet stream and more stable weather.

What causes meridional flow?

Several factors contribute to the development of meridional flow patterns:

• Weakening of the pole-to-equator temperature gradient: When the temperature difference between the poles and the equator is less pronounced, the jet stream tends to weaken and become more unstable. This can cause the jet stream to slow down and develop larger, more pronounced meanders, leading to meridional flow.
• Blocking patterns: These are high-pressure systems that become nearly stationary in the atmosphere, effectively "blocking" the normal west-to-east flow of weather systems. These blocks force the jet stream to take a large, circuitous path around them, creating a strong meridional pattern.
• Geographic features: Large mountain ranges, such as the Rocky Mountains or the Himalayas, can disrupt the zonal flow and induce larger waves in the jet stream.
• Atmospheric teleconnections: These are large-scale, long-distance climate patterns, like the El Niño-Southern Oscillation (ENSO) or the Arctic Oscillation (AO). Shifts in these teleconnections can alter the global circulation patterns, influencing the frequency and strength of meridional flow.

The transition between zonal and meridional flow is a key part of weather forecasting, as it signals a change in the likelihood of extreme weather events.