Impact of MJO Propagation Speed on Active Atlantic Tropical Cyclone Activity Periods.

The Madden‐Julian Oscillation (MJO) is often used for subseasonal forecasting of tropical cyclone (TC) activity. However, TC activity still has considerable variability even given the state of the MJO. This study evaluates the connection between MJO propagation speed with Atlantic TC activity and possible physical mechanisms guiding this relation. We find the Atlantic sees the highest accumulated cyclone energy (ACE) during MJO phase 2. However, the odds of above average ACE in the Atlantic is greatest during slow MJO propagation. We find that slow propagation of the MJO results in lower vertical wind shear anomalies over the Caribbean and main development region compared with typical MJO propagation. Typical MJO propagation produces an amplified height pattern and lower height anomalies along the region of the tropical upper tropospheric trough which is known to impede Atlantic TC activity. Slow MJO propagation sees weaker height anomalies over the Atlantic. Plain Language Summary: The Madden‐Julian Oscillation (MJO) is a large region of storminess and winds that moves slowly eastward from the Indian ocean eastward into the Pacific over the course of 40–90 days. When the MJO is over the Indian Ocean it produces more hurricanes in the Atlantic because it reduces wind shear which is the difference in winds at different heights of the atmosphere. When the MJO moves slowly or is nearly stationary over the Indian Ocean there is even more hurricane activity in the Atlantic. When the MJO moves at a normal pace, it influences the jet stream which can then dip into the Atlantic creating high wind shear. When the MJO moves slowly there is less shear over the Atlantic. Key Points: The Atlantic basin sees the most subseasonal tropical cyclone activity when the Madden‐Julian Oscillation (MJO) is moving slowly prior to entering phase 2Under normal MJO propagation higher shear occurs in the main development region than during slow propagating MJOsUnder slow propagating MJO regimes the 500 mb geopotential height pattern has smaller wavelengths and weaker anomalies over the Atlantic [ABSTRACT FROM AUTHOR]