Sustained Increases in Lower Tropospheric Subsidence over the Central Tropical North Pacific Drives a Decline in High Elevation Precipitation in Hawai‘i.

Abstract:

The trade wind inversion (TWI) is a prominent feature of the climate of Hawai‘i. When the TWI is present, the vertical development of clouds is suppressed and the transport of moisture to high elevation ecosystems above the inversion on Hawai‘i’s highest mountains is inhibited. In this study, 34 years (1979-2013) of TWI data were analyzed to determine the long-term characteristics of TWI occurrence frequency and its effects on moisture regimens at high elevations for the 6-month dry and wet seasons in Hawai‘i. An assessment of linear trends in each season shows that on average, TWI occurrence frequency has increased by 6% and 5% per decade for the dry and wet seasons, respectively. Application of change point analysis to the time series reveals that a prominent upward shift in TWI occurrence frequency occurred after 1990, resulting in a 15% increase in mean TWI persistence across the island chain during the most recent ~2 decades (1991 – 2013) when compared to a period preceding this shift (1973 – 1990). This change has generally coincided with decreases in precipitation at high elevations (> 1900 m), especially during the wet season. An analysis of mean precipitation from 21 high elevation stations in Hawai‘i for 1991-2013 in relation to 1973-1990 shows changes of -178 to 159 mm/season for the dry-season and -969 to -29 mm/season for the wet season. The increased persistence of the TWI and coincident decreases in precipitation can be explained by amplified subsidence within the regional Hadley cell circulation over this time period. An analysis of the vertical velocity variable, Omega, at the 500 hPa pressure level shows statistically significant average increases in subsidence of 35% (dry-season) and 28% (wet-season) for the 10˚ longitudinal band centered over the highest Hawaiian Islands (20˚ N Latitude) when these ~2-decade pre- and post-shift periods are compared.