Hawaiian drought and the Southern Oscillation

Abstract

AbstractLong‐term rainfall records at key locations on the Hawaiian Islands and the Southern Oscillation Index (SOI) are examined to reveal the impact of large‐scale atmospheric circulation on Hawaiian rainfall. By compositing six major drought winters, it is found that they were preceded by a persistently low phase of the Southern Oscillation starting from March of the preceding year. Correlation analysis indicates that the spring SOI is not significantly correlated to rainfall in the subsequent seasons. In contrast, changes of the SOI in summer lead corresponding changes of rainfall by two to three seasons. Similarly, changes of the SOI in autumn lead changes of rainfall by one to two seasons. These results are also confirmed by scatter diagrams. Although a large negative SOI anomaly in summer or autumn can be regarded as a precursor to a drought for the following winter or spring, the inverse relation, namely, a positive SOI anomaly leading abundant rainfall does not necessarily follow. As an empirical rule, a value for the summer SOI of −2 or below, or a value for the autumn SOI of −1 or below, or both, indicate the occurrence of a dry winter.During drought episodes, Hawaii was located under an area of strong subsidence, and synoptic systems that normally produce winter and spring rainfall in the Hawaiian Islands were retarded. An anomalously cold sea‐surface temperature in the north‐central Pacific may also further enhance and prolong the dryness by reducing the rate of evaporation from the sea surface and by stabilizing the overlying air.Individual forecasts carried out for recent years reveal that severe drought during an El Niño/Southern Oscillation (ENSO) winter could be predicted two seasons in advance using simple regression models. For the non‐ENSO drought winter, forecasting ability degrades somewhat. Overall, by means of a cross‐validation technique, antecedent conditions of the large‐scale pressure see‐saw are useful predictors for Hawaiian winter rainfall anomalies, particularly when an ENSO‐like phenomenon has started to develop.