Variability of Hawaiʻian winter rainfall during La Niña events since 1956

Abstract

Rainfall trends in Hawaiʻi during La Niña years have undergone abnormal variability since the early 1980s, making seasonal forecasts more challenging to perform. Traditionally, Hawaiʻi receives greater than normal precipitation during La Niña wet seasons. Recently, the wet seasons of La Niña years have experienced less than normal rainfall. A change-point analysis has been performed to determine that the shift in precipitation occurs in 1983, forming two epochs in the study used for comparison. The first epoch runs from 1956 to 1982 and the second epoch spreads from 1983 to 2010. A normalized rainfall anomaly assessment is performed, illustrating the drying trend in Hawaiʻian precipitation during La Niña years. Furthermore, location specific changes in rainfall anomalies throughout the Hawaiʻian Islands are examined. Influenced by variations in tropical sea surface temperatures, circulation features in the northern Pacific Ocean have changed during La Niña wet seasons. Strengthening, broadening, and westward shifting of the eastern North Pacific subtropical high, coupled with an eastward elongation and intensification of the subtropical jet stream, are two main influencers when considering the lack of precipitation during the recent La Niña wet seasons. Moisture transport analysis shows that variations in circulation structures are the greatest cause for a reduction of moisture in the Hawaiʻian region during the second epoch. Additionally, a storm track analysis reveals that the changes found in the aforementioned circulation features are creating a less favorable environment for the development of Kona lows and midlatitude fronts in the vicinity of Hawaiʻi. A case study concerning North Pacific precipitation analysis of two contrasting years is also performed.