Revisiting oceanic elves and lightning occurrence rate during El Niño and la Nina episodes in a 10-year time frame

The Imager of Sprite and Upper Atmospheric Lightning (ISUAL) on board the Taiwanese satellite Formosat-2 has accomplished the first decade operating in the orbit. A comprehensive method defining the standardized anomaly of elve occurrence rate by limb-viewed observation is demonstrated. We revisit the occurrence of elves during El Niño and La Nina and extend the time series analysis to November 2015. The variation of the enhanced anomaly of elve occurrence density (AEOD) follows the change of sea surface temperature anomaly (SSTA) in SSTA-horseshoe, SSTA-Pacific and SSTA-Indian Ocean during the warm and cold phases, whereas the variation of the anomaly of lightning flash density (ALFD) is rather ambiguous. The correlation of the Southern Oscillation Index (SOI) with the Elve Anomaly Index (EAI) remains significant in Tahiti, while the best correlation between SOI and elves is found in the western/central Pacific Ocean with a correlation coefficient as high as 0.7. In the Indian Ocean, the correlation in the 10-year time frame is moderate, but the relation is obvious during the transition phase between a strong El Niño and the following La Nina, in the region where the synoptic circulation is affected. The weaker convection available potential energy (CAPE) in the upwelling region of the synoptic circulation provides an environment conducive to the development of elve-producing thunderstorms. This work substantiates the relation between elves/lightning and ENSO in a longer10-year time frame that elves are more sensitive than lightning to ENSO and the variation of the synoptic circulation.