Interannual and interdecadal rainfall variations in the Hawaiian Islands

Hawaii rainfall has exhibited both interannual and interdecadal variations. On the interannual time scale, Hawaii tends to be dry during most El Nino events, but low rainfall also occurred in the absence of El Nino. On the interdecadal time scale, Hawaii rainfall is negatively and significantly correlated with the Pacific decadal oscillation ( PDO) signal; an epoch of low rainfall persists from the mid-1970s to 2001, which is preceded by an epoch of high rainfall lasting for nearly 28 yr. Difference patterns in winter [November-December-January-February-March (NDJFM)] rainfall are investigated for composites of extremely dry and wet winters during the dry and wet epochs, respectively. These patterns (i.e., DRY minus WET) are then compared to the difference in constructive match conditions of El Nino and PDO (i.e., El Nino/+PDO minus La Nina/-PDO). Relative to the El Nino/PDO stage, the magnitude of dryness during the rainfall-based stage is enhanced. The corresponding large-scale atmospheric circulation composites are studied. Similar patterns are revealed between these two stages. However, anomalously stronger and deeper sinking motions over Hawaii are revealed in the height-latitude section of the rainfall-based analysis compared to the El Nino/PDO stage. Moreover, an anomalous zonal circulation cell is well established over the subtropical North Pacific with a pronounced descending branch over Hawaii in the rainfall-based stage. The band of anomalous surface westerlies to the north of Hawaii, and the deep sinking motion as well as the anomalously vertically integrated moisture flux divergence over Hawaii are all unfavorable for rainfall in Hawaii.