Investigating wintertime North Pacific Jet variability and disturbance-driven rainfall over Hawai‘i using the Self-Organizing Map (SOM)

Interannual rainfall variability in Hawai‘i is strongly influenced by the largescale circulation over the North Pacific, natural modes of climate variability (e.g., PNA and ENSO), and wintertime rainfall disturbances (e.g., Kona lows and cold fronts). In this study, the Self-Organizing Map (SOM) was applied to investigate the relationship between the large-scale circulation over the North Pacific, and its relationship to natural climate modes and wintertime rainfall disturbances in Hawai‘i. The SOM was trained with daily 250-hPa zonal wind anomalies from European Centre for Medium-Range Weather Forecasts reanalysis (ERA5). Results indicate that a zonally retracted jet is associated with the negative PNA/ENSO phase and above-normal rainfall across Hawai‘i, while a zonally extended jet is associated with the positive PNA/ENSO phase and below-normal rainfall across Hawai‘i. Further, to investigate future changes in the variability of the large-scale circulation, 250-hPa wind data from 21 CMIP6 models were projected onto the ERA5-trained SOM. The ensemble-mean wind changes (future–historical) suggest that overall, the jet will experience general poleward shift in a future climate. The SOM node frequency changes provide a more complex description of how the NPJ is projected to change in a future climate. Our results suggest that a zonally retracted jet will become more frequent in a future climate (2071–2100), which could lead to increased frequencies in Kona lows and increases in wet season rainfall. In contrast, our results also suggest that a zonally extended jet will become more frequent in a future climate, which could lead to decreased Kona low activity and decreases in wet season rainfall. Overall, the results suggest increased variability in a future climate and highlight the complexity and uncertainty of the future NPJ changes, making it difficult to explain the ensemble mean NPJ changes with a single physical mechanism.