Tropical Indian Ocean Influence on Northwest Pacific Tropical Cyclones in Summer following Strong El Nino

In the summer following a strong El Nino, tropical cyclone (TC) number decreases over the Northwest (NW) Pacific despite little change in local sea surface temperature. The authors' analysis suggests El Nino-induced tropical Indian Ocean (TIO) warming as the cause. The TIO warming forces a warm tropospheric Kelvin wave that propagates into the western Pacific. Inducing surface divergence off the equator, the tropospheric Kelvin wave suppresses convection and induces an anomalous anticyclone over the NW Pacific, both anomalies unfavorable for TCs. The westerly vertical shear associated with the warm Kelvin wave reduces the magnitude of vertical shear in the South China Sea and strengthens it in the NW Pacific, an east-west variation that causes TC activity to increase and decrease in respective regions. These results help improve seasonal TC prediction.