Atmospheric River

An atmospheric river is a long, narrow band of highly concentrated water vapor in the atmosphere. These rivers in the sky are responsible for transporting large amounts of moisture over long distances. When atmospheric rivers make landfall, they can bring heavy rainfall and contribute to significant weather events, including storms, floods, and snowstorms. Read here to learn more.

California is presently dealing with an atmospheric river (AR), a remarkable meteorological phenomenon that might dump up to 8 trillion gallons of rain on the state.

This surge is the result of a second Pineapple Express weather system that passed over California in less than a week, bringing hurricane-force winds and a lot of rain.

The term “Pineapple Express” is sometimes used to refer to atmospheric rivers that originate near Hawaii and travel eastwards towards the west coast of North America.

Atmospheric River (AR)

Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.

• While ARs can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.
• Exceptionally strong atmospheric rivers can transport up to 15 times that amount.
• When the ARs make landfall, they often release this water vapor in the form of rain or snow.

Characteristics of Atmospheric Rivers

Atmospheric rivers are typically formed in the tropics or subtropics where warm ocean surfaces evaporate water into the air.

• They often develop within the low-level jet stream, a fast-flowing, narrow air current.
• The structure of an AR is like a narrow ribbon or corridor of moisture, often extending from the equator to higher latitudes.
• These rivers in the sky can transport vast amounts of water vapor, equivalent to the flow of the Amazon River, the world’s largest river, but in the form of water vapor.

Impact of ARs

ARs play a vital role in the water cycle and contribute to water resources in the regions they impact.

• Consecutive atmospheric rivers, known as AR families, can cause significant flooding.
• When atmospheric rivers make landfall, they release the stored moisture in the form of precipitation, which can lead to heavy rainfall, snowfall, or a combination of both.
• The heavy precipitation associated with ARs can contribute to flooding, landslides, and snowstorms, depending on the geographic location and season.
• In some regions, ARs are crucial for maintaining water supply as they contribute a significant portion of annual precipitation.
• In some areas, ARs contribute significantly to snowpack, affecting water availability during drier seasons. Conversely, their absence can contribute to drought conditions.
• Monitoring and forecasting ARs are crucial for predicting and preparing for extreme weather events, helping mitigate potential damages.
• The variability in precipitation brought by ARs can impact hydropower generation in regions that rely on rivers for energy production.

Mechanism of ARs

Not all ARs cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply.

ARs are a key feature in the global water cycle and are closely tied to both water supply and flood risks, particularly in the western United States.

• ARs occur all over the world, most commonly in the mid-latitudes.
• They form when large-scale weather patterns align to create narrow channels, or filaments, of intense moisture transport.
• These start over warm water, typically tropical oceans, and are guided toward the coast by low-level jet streams ahead of cold fronts of extratropical cyclones.

The cause of AR families is an active area of research.

• Compared with single atmospheric river events, AR families tend to be associated with lower atmospheric pressure heights across the North Pacific, higher pressure heights over the subtropics, a stronger and more zonally elongated jet stream, and warmer tropical air temperatures.
• Large-scale weather patterns and climate phenomena such as the Madden-Julian Oscillation, also play an important role in the generation of AR families.

How is global warming affecting atmospheric rivers?

Global warming is influencing ARs in several ways, contributing to changes in their frequency, intensity, and behavior.

The impact of global warming on atmospheric rivers is a complex interaction that involves changes in temperature, atmospheric circulation patterns, and the hydrological cycle.

• Increased Water Vapor Content: Global warming leads to higher sea surface temperatures, increasing evaporation rates. This, in turn, results in a greater amount of water vapor in the atmosphere, providing more moisture for ARs to transport.
• Changes in Atmospheric Circulation: Warming of the Earth’s surface alters atmospheric circulation patterns. Changes in temperature gradients and atmospheric pressure can influence the positioning and intensity of the jet stream, which plays a crucial role in the formation and movement of atmospheric rivers.
• Amplification of Extreme Events: Global warming contributes to the intensification of extreme weather events, including heavy precipitation associated with atmospheric rivers. Warmer air can hold more moisture, leading to more intense rainfall or snowfall when atmospheric rivers make landfall.
• Shifts in Timing and Location: There is evidence suggesting shifts in the timing, location, and frequency of atmospheric rivers due to global warming. Changes in the atmospheric circulation patterns may alter the typical pathways of these rivers.
• Impact on Snowpack: Atmospheric rivers play a crucial role in maintaining snowpack in certain regions. Changes in their behavior, including the shift from snow to rain, can affect snow accumulation and lead to alterations in water availability during the melt season.
• Sea Level Rise: The warming of the Earth contributes to the melting of glaciers and ice caps, resulting in sea level rise. Higher sea levels can influence the interactions between atmospheric rivers and coastal areas, potentially exacerbating flooding.
• Hydrological Cycle Changes: Global warming influences the hydrological cycle, affecting precipitation patterns and the distribution of water vapor in the atmosphere. These changes can influence the characteristics of atmospheric rivers.
• Compound Extreme Events: Global warming increases the likelihood of compound extreme events, where multiple climate-related factors interact. For example, warming temperatures, altered precipitation patterns, and changes in atmospheric circulation can combine to influence the behavior of atmospheric rivers.

Also read: Hydrologic Cycle

Conclusion

Understanding and monitoring atmospheric rivers are essential for weather forecasting, disaster preparedness, and water resource management. Advances in technology and atmospheric science have improved the ability to predict and track these phenomena, providing valuable information for mitigating the impacts of extreme weather events.

-Article by Swathi Satish