Influence of ENSO, ENSO Modoki, and the IPO on tropical cyclogenesis: a spatial analysis of the southwest Pacific region

Tropical cyclones (TCs) pose a significant risk to the nations and territories of the southwest Pacific (SWP). The spatio-temporal variability of TCs makes it challenging to forecast where and when a TC is likely to develop. Therefore, the aim of this study is to better understand the link between large-scale climatic variability, the environmental conditions required for tropical cyclogenesis (TC genesis), and the spatial variability of TC activity. Three modes of climate variability are investigated: (1) El Nino/Southern Oscillation (ENSO), (2) ENSO Modoki, and (3) the Interdecadal Pacific Oscillation (IPO); along with TC genesis parameters: sea surface temperature; 700 hPa relative humidity; 700 hPa vorticity; and vertical wind shear (difference between 200 hPa and 850 hPa winds). Our findings reaffirm the well-established northeast/southwest modulation of TC activity according to El Nino (EN)/La Nina (LN) using an extended TC dataset (1945-2011). In addition, new insights into how ENSO Modoki and the IPO modulate TC activity according to phase and season are identified. Importantly, we show that depending on phase, the IPO can enhance or alter the spatial modulation of TC genesis during ENSO/ENSO Modoki events, in favour of the northeast/southwest modulations typical of IPO positive/negative events. This is particularly the case during the latter half of the SWP TC season. For example, EN events that occur within IPO positive (negative) epochs result in a shift of TC activity up to 1087 km (1288 km) further east (west) during February to April, compared to the typical location of TC activity during EN events. Importantly, these statistical relationships are also associated with anomalously favourable genesis parameters, providing some insights into the physical mechanisms behind the modulations. The findings of this study provide baseline metrics with which to compare climate model simulations and may also facilitate improved seasonal outlooks and better quantification of TC-related risks for the vulnerable island nations and territories of the SWP.