Why do explosive cyclones frequently occur over warm ocean currents?: An answer to this mystery

It is well known that extratropical cyclones frequently rapidly intensify over the Kuroshio/Kuroshio Extension and the Gulf Steam in boreal winter. The disturbances called “bomb” or explosive cyclones cause extreme events including violent winds, heavy precipitation, and high waves. We explored why the rapid intensification of the cyclones occurs over the warm ocean currents using high-resolution regional cloud-resolving numerical experiments. Results showed that moisture and sensible heat supply from the warm currents enhance the rapid development of cyclones through a positive feedback process between a cold conveyor belt (CCB) and latent heating (LH) as below. When a cyclone grew over the warm currents, moisture and sensible heat supply from the ocean increased especially along the CCB. This is because that the overlap between the warmer sea water of the warm currents and cold and dry air of the CCB is favorable for the increase in the moisture and heat supply from ocean. This moisture and heat supply moistened and destabilized the near-surface air along the CCB. The CCB transported the modified air into a frontal zone characterized by the zonal thermal gradient just north of the cyclone center. As a result, the static instability was released and the moisture evaporated from the warm currents condensed around the front. This led to the generation of LH. The enhanced LH further intensified the cyclone and its associated CCB. We confirmed that such a CCB-LH feedback process played a key process in the rapid development of cyclones over both the Kuroshio/Kuroshio Extension and the Gulf Steam. Thus, we propose that this feedback process is one of the answers to the mystery of why extratropical cyclones rapidly develop over the warm currents.