Variability of Hawaiian winter rainfall during La Niña events since 1956

Abstract

Rainfall in Hawaii during La Niña years has undergone abnormal variability since the early 1980s. Traditionally, Hawaii receives greater-than-normal precipitation during the La Niña wet seasons. Recently, La Niña years have experienced less-than-normal rainfall. A drying trend in Hawaiian precipitation during La Niña years is evident. A changepoint analysis determined that the shift in precipitation occurred in 1983, forming the two epochs used for comparison in this study. The first epoch (E1) runs from 1956 to 1982 and the second epoch (E2) from 1983 to 2010. Location-specific changes in rainfall anomalies from E1 to E2 throughout the Hawaiian Islands are examined, illustrating that the greatest difference in rainfall between epochs is found on the climatologically drier sides (i.e., south and west) of the islands. Variations in tropical sea surface temperatures and circulation features in the northern Pacific Ocean have changed during La Niña wet seasons, thus changing La Niña-year rainfall. The 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 influences when considering the lack of precipitation during the recent La Niña wet seasons. Moisture transport analysis shows that variations in circulation structures play a dominant role in the reduction of moisture flux convergence in the Hawaiian 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 Hawaii.