Possible Causes for the Unprecedented Low/High Tropical Cyclone Activities in the Northern Pacific/Atlantic in 2023 El Niño

This study reported the unprecedented tropical cyclone (TC) activity in the western North Pacific (WNP) and North Atlantic (NA) during the developing year of the 2023/2024 El Niño. The possible causes behind these unusual features were addressed. In contrast to previous El Niño events, an unusual low/high TC genesis number in the WNP/NA was identified during the typhoon season (June–November) in 2023. Meanwhile, the mean TC genesis location in the WNP exhibited a La Niña‐like northwestward shift, rarely observed in an El Niño developing year. An observational diagnosis on TC‐genesis‐related large‐scale dynamics and thermodynamics revealed that the lower/higher TC numbers in the WNP/NA were primarily attributed to an anticyclonic/cyclonic anomaly linked to trans‐basin sea surface temperature anomalies in the tropics and extratropics. Additionally, weaker intraseasonal oscillation activity compared to previous El Niños also partially contributed to fewer TCs in the WNP. Plain Language Summary During the 2023 El Niño, tropical cyclone (TC) activity in the western North Pacific (WNP) and North Atlantic (NA) was unusual. Extremely few TCs formed in the WNP during the typhoon season (June–November), while more TCs were observed in the NA compared to the climatology, opposite to the feature of TC genesis in previous El Niño events. Additionally, the mean genesis location of WNP TCs shifted northwestward, contrary to the typical southeastward shift seen in past El Niño events. These anomalies were linked to an anticyclonic anomaly in the WNP and a cyclonic anomaly in the NA, which respectively hindered and facilitated favorable large‐scale environments related to TC genesis. Diagnostic analysis revealed that these circulation anomalies were associated with trans‐basin sea surface temperature anomalies (SSTAs). Extremely warm SSTAs in the northern Indian Ocean, tropical NA (TNA), and extratropical regions, along with a negative Pacific meridional mode, favored the anticyclonic anomaly in the WNP. In contrast, the extremely warm SSTA in the TNA facilitated the cyclonic anomaly in the NA. Additionally, weak intraseasonal oscillation activity also contributed to the low number of WNP TCs. Key Points Unlike previous El Niño events, unusually fewer TCs, with a northwestward shift in genesis location, were observed in the WNP in 2023 By contrast, the number of TCs in the North Atlantic (NA) was approximately twice that of the composite mean of previous El Niños In additi