Variability of Hawaiian Winter Rainfall During La Nina Events Since 1956

Rainfall in Hawaii during La Niņa years has undergone abnormal variability since the early 1980s. Traditionally, Hawaii receives greater than normal precipitation during the La Niņa wet seasons. Recently, La Niņa years have experienced less than normal rainfall. A drying trend in Hawaiian precipitation during La Niņa years is evident. A change-point analysis determined that the shift in precipitation occurred in 1983, forming the two epochs used for comparison in this study. The first epoch runs from 1956 to 1982 and the second epoch from 1983 to 2010. Location specific changes in rainfall anomalies from epoch 1 to 2 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 Niņa wet seasons, thus changing La Niņa 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 Niņa wet seasons. Moisture transport analysis shows that variations in circulation structures play a dominant role in the reduction of moisture flux convergence in the Hawaii 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.