Tropical cyclone genesis over the western North Pacific impacted by SST anomalies from other basins while El Niño decays

Tropical cyclone (TC) genesis over the western North Pacific (WNP) in the TC active season July–October after El Niño peaks in the preceding December is revealed to be either deficient or excessive, depending on how El Niño is able to remotely create and maintain sea-surface temperature (SST) anomalies in the North Atlantic and Indian Ocean during spring while El Niño decays. A reduction of TC genesis (Group 1) is coincident with a warming in the Indian Ocean and a positive tripolar SST pattern in the North Atlantic, while enhanced TC genesis (Group 2) corresponds to the configuration without significant SST anomalies in the North Atlantic and in the Indian Ocean. Relevant physical mechanisms are investigated through observational data analysis and confirmed with model simulations. In Group 1, El Niño SST anomalies enhance convection over the central-eastern equatorial Pacific. The related latent heat release excites Rossby wave trains which can efficiently propagate energy from the eastern tropical Pacific into Central America and further into the subtropical North Atlantic. The warm SST anomalies found in the Indian Ocean and in the North Atlantic can force robust anomalous easterlies in the tropical WNP, producing unfavourable dynamic and thermodynamic conditions for TC genesis. Group 2 shows a quite distinct situation. Actually, the late winter anomalous westerlies are much weaker over Central America in Group 2, providing a configuration of inhibition for an efficient wave energy propagation from the tropics into the subtropical North Atlantic. The warm SST over the Maritime Continent is then responsible for the increase of TC genesis over the South China Sea and around the Philippines by exciting a cyclonic circulation in the TC active season.