Guide to intraseasonal variation

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AlphaToOmega
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Guide to intraseasonal variation

#1 Postby AlphaToOmega » Sat Sep 04, 2021 4:14 pm

This is a simple guide for people wondering about intraseasonal variation. This is not meant to be a safety guide for Saharan Dust storms or any other topic mentioned in this point.



Overview

Inactive hurricane seasons can have bursts of activity, and active hurricane seasons can have lulls of activity. There are three major forces governing intraseasonal variation: the Madden-Julian Oscillation, Convectively Coupled Kelvin waves, and Saharan Dust outbreaks.

The Madden-Julian Oscillation is a wave that moves eastward across the world that creates rising air in some parts of the world and sinking air in other parts of the world. The Madden-Julian Oscillation is typically strongest in the spring and early summer; during the autumn, it is typically weakest in the autumn. It impacts rainfall patterns and tropical cyclone activity in certain basins.

Convectively Coupled Kelvin waves are smaller waves of rising air. Like the Madden-Julian Oscillation, they travel eastward. Also like the Madden-Julian Oscillation, they impact tropical cyclone activity.

Between June and August, dust from the Saharan Desert travels through the Atlantic, creating dust storms for people on the East Coast of the United States. Saharan Dust storms lower air quality, provide iron for ecosystems, and inhibit tropical cyclogenesis in the North Atlantic Ocean.

Madden-Julian Oscillation

The Madden-Julian Oscillation measures the velocity potential anomalies (amount of rising air) in certain parts of the world. Negative velocity potential anomalies indicate rising air, and positive velocity potential anomalies indicate sinking air. Rising air promotes moist air with low shear, and it also promotes positive sea surface temperature anomalies; sinking air promotes dry air with high wind shear, and it also promotes negative sea surface temperature anomalies. Rising air is favorable for tropical cyclogenesis; sinking air is unfavorable for tropical cyclogenesis.

The Madden-Julian Oscillation is split into nine phases: Phase I, Phase II, Phase III, Phase IV, Phase V, Phase VI, Phase VII, Phase VIII, and the null phase. During Phase I, there is rising air over the Americas and Africa; there is sinking air over Southeast Asia and the West Pacific Ocean. During Phase II, there is rising air over Africa and the Indian Ocean; there is sinking air over the Pacific Ocean. During Phase III, there is rising air over the Indian Ocean and Southeast Asia; there is sinking air over the East Pacific Ocean, the Americas, and the Atlantic Ocean. During Phase IV, there is rising air over the Americas, the Atlantic, and West Africa; there is sinking air over the Indian Ocean and Southeast Asia. During Phase V, there is rising air over Southeast Asia and the West Pacific Ocean; there is sinking air over the Americas and Africa. During Phase VI, there is rising air over the Pacific Ocean; there is sinking air over the Atlantic Ocean, Africa, and Indian Ocean. During Phase VII, there is rising air over the Pacific Ocean and the Americas; there is sinking air over the Atlantic Ocean, Africa, and the Indian Ocean. During Phase VIII, there is rising air over the East Pacific Ocean, the Americas, the Atlantic Ocean, and Africa; there is sinking air over the Indian Ocean, Southeast Asia, and the West Pacific Ocean. During the null phase, the state Madden-Julian Oscillation cannot be determined. A diagram of the following is shown below (orange indicates sinking air; blue indicates rising air):

Image

Where there is rising air, precipitation in the tropics is going to be above-average. Where there is sinking air, precipitation in the tropics is going to be below-average. For tropical cyclogenesis in the North Atlantic Ocean, Phases I, II, III, and IV are favorable for tropical cyclogenesis; Phases V, VI, VII, and VIII are unfavorable for tropical cyclogenesis in the North Atlantic Ocean. Phases I and II typically favor development in the Northwest Atlantic Ocean; Phases III and IV typically favor development in the Northeast Atlantic Ocean. During the peak of the North Atlantic hurricane season (August to October), the Madden-Julian Oscillation matters little, for it is weakest during the autumn. A lull of activity does not imply an inactive season, but a burst of activity does not imply an active season.

Convectively Coupled Kelvin waves

Convectively Coupled Kelvin waves are waves of rising air that move across eastward. Like the Madden-Julian Oscillation, they influence tropical cyclogenesis. However, they are much smaller and more fast-moving than the Madden-Julian Oscillation.

Convectively Coupled Kelvin waves can interact with tropical waves, giving them favorable conditions for tropical cyclogenesis, and allow them to intensify into strong tropical cyclones. They can allow tropical cyclones to form even during unfavorable Madden-Julian Oscillation phases.

Saharan Dust storms

Saharan Dust storms are waves of dust from the Saharan Desert that travel westward from Africa to the Americas. Occasionally the amount of Saharan dust in the Atlantic can be large enough to create a Saharan Dust storm. Saharan Dust storms worsen air quality and provide iron to ecosystems. They also hinder North Atlantic tropical cyclogenesis because dry air disrupts convection. Saharan Dust storms impact development in the Main Development Region more than they impact development in the Gulf of Mexico or in the Caribbean Sea.

Strong Saharan Dust storms do not indicate inactive North Atlantic hurricane seasons. For example, June 2020 featured the Godzilla Dust Storm, one of the strongest dust storms on record. The 2020 North Atlantic Hurricane Season was the most active North Atlantic hurricane season on record despite the Godzilla Dust Storm.

Forecasting

Almost all climate models (such as the CFSv2 (American), CanSIPS (Canadian), and JMA (Japanese)) have some way to forecast the Madden-Julian Oscillation. They all have velocity potential anomaly maps. Global models (such as the GFS (American) and ECMWF (European)) have Madden-Julian Oscillation phase diagrams. Global models also have velocity potential maps. Because Convectively Coupled Kelvin waves are smaller than, faster than, and shorter-lived than Madden-Julian Oscillation phases, global model velocity potential maps are the best way to forecast Convectively Coupled Kelvin waves. To forecast Saharan Dust storms, dust extinction maps are used, which are provided by models such as the NASA GEOS-5.

Summary

The Madden-Julian Oscillation is a wave of rising and sinking air that always moves eastward. Phases I through IV are favorable for tropical cyclogenesis in the North Atlantic Ocean, and Phases V through VIII are unfavorable for tropical cyclogenesis in the North Atlantic Ocean. Where there is rising air, precipitation in the tropics is going to be above-average; where there is sinking air, precipitation in the tropics is going to be below-average. The Madden-Julian Oscillation influences when storms might form, but it does not influence how active a hurricane season might be. Convectively Coupled Kelvin Waves are masses of rising air that move eastward, and they can interact with tropical waves to produce powerful tropical cyclones. Saharan Dust storms are waves of Saharan dust that move across the North Atlantic Ocean that can hinder tropical cyclogenesis; however, strong Saharan dust storms do not indicate inactive North Atlantic hurricane seasons. People either use climate models or global models to forecast the Madden-Julian Oscillation, and they also use global models to forecast Convectively Coupled Kelvin waves. Dust extraction forecasts from models such as the NASA GEOS-5 are used to forecast Saharan Dust storms.
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