Hawaiian summer rainfall: Two distinguishing variability regimes

Contributed by Xiao Luo: luoxiao.rf@gmail.com

Surrounded by the subtropical Pacific Ocean and immersed in persistent trade winds, the Hawaiian Islands experience distinct seasonality in rainfall: a wet winter from November to April and a dry summer from May to October. Summer precipitation in Hawai‘i accounts for 40% of the annual total and provides important water sources. However, our knowledge about its variability remains limited. In this study we show that statewide Hawai’i summer rainfall (HSR) variability exhibits two distinct regimes: quasi-biennial (QB, ~2 years) and interdecadal (~30-40 years). 

Figure. 1 The time series and spectrum of statewide Hawai‘i summer rainfall (HSR) anomalies from 1920 to 2012. (a) The time series of HSR (color bar) and its interdecadal component (>7 years period component, black solid line). The black dashed line indicates the linear trend in HSR during 1920-2012. (b) The power spectrum of HSR, the blue (red) dashed line indicates the 95% (90%) confidence bounds. (c) The quasi-biennial component of the normalized HSR and the quasi-biennial component of Oceanic Niño Index (ONI) from December to the next February.

The QB variation is linked to alternating occurrences of the Western North Pacific (WNP) cyclone and anticyclone in successive years. The cyclone-induced southwest anomalies generate moisture convergence and ascending motion that favors abundant rainfall. The turn-about from the cyclone to anticyclone is associated with the intrinsic biennial component of El Nino-Southern Oscillation and involves a positive feedback between atmospheric Rossby waves and the underlying dipolar sea surface temperature anomalies. 

Figure 2 Seasonal evolution of regressions on the (a-d) QB HSR index and (e-h) QB ONI on the quasi-biennial time scale. (a-d) Regressions on QB HSR index during 1920-2012 in (a) MJJA(0), (b) SOND(0), (c) JFMA(0), and (d) MJJA(1). Regressions in (a-d) correspond to rainfall anomaly of 20 mm/month on QB time scale. (e-h) are the same as in (a-d), except that the regressions are based on QB ONI during 1960-2012. Regressed fields are precipitation anomalies over land (in units of mm/month), SST anomalies over ocean (in units of °C), and 850 hPa wind anomalies (arrows) in units of m/s. 

The interdecadal variation of HSR is largely modulated by the Pacific Decadal Oscillation through affecting upstream low-level humidity that affects topographic rainfall. With the updated data to 2019 from the 10 representative stations, this study shows the long-term summer rainfall trend is quite weak during 1920-2019. This first description of the major physical drivers of summer rainfall variability provides key information for seasonal rainfall prediction in Hawai‘i. A deeper understanding of summer rainfall variability and the major drivers can help develop appropriate variability-based climate divisions that characterize the State’s spatial and temporal variability.

This work has been published in Geophysical Research Letters, 


Authors: Xiao Luo, Bin Wang, Abby G. Frazier, and Thomas W. Giambelluca

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