SPCZ and cyclone future evolutions

This study assesses the future changes in the South Pacific Convergence Zone (SPCZ) and the tropical cyclone (TC) activity over the South Pacific for the late twenty-first century (2070-2100). The Weather Research and Forecast model (WRF) at 1° resolution with a nest at 1/5° resolution is used. The model is forced in the present-day by the NCEP2 reanalysis to which seasonal anomalies (RCP8.5 minus present) of 30 IPCC-AR5 models are added to simulate the future (2070-2100) climate. Analyses for the current climate are first performed to evaluate the model ability to reproduce the SPCZ, and the tropical cyclone activity. In the historical simulation (1980-2016), the SPCZ slope is -0.21 and the mean latitude position is 12.9°S, whereas in Global Precipitation Climatology Project (GPCP) data the slope is -0.28, and the mean latitude position is 12.7°S. For the inter-annual variability, we show that the modeled SPCZ compares well to observations. The mean seasonal and spatial distribution of TC genesis and occurrence are in good agreement with the Joint Typhoon Warning Center (JTWC) data. The 20-km-nested model is able to simulate some extremely intense cat 4-5 TCs. Over the whole genesis area, 5.1 ± 3.0 cyclones are produced seasonally in the model, compared with 6.6 ± 3.0 in the JTWC data. In the model, the spatial pattern of TC activity is shifted to the southwest during La Niña and to northeast during El Niño and strongly east during "strong" El Nino even such as the 1997-1998 and 1982-1983 events, as observed in JTWC data. In the future climate, the SPCZ is wetter and has a tendency to become more slanted to the south in the eastern part. Ninos are seen to become stronger with zonal SPCZs and we detail the processes at play. Cyclonic activity is slightly decreasing with cyclone intensities and precipitation increasing.