The Effects of Trade Wind Inversion Variability on High Elevation Climates in Hawaiʻi

Abstract

The primary objective of this dissertation is to characterize the spatial and temporal variability of climate at high elevation in Hawai‘i and to identify the proximal causes of observed change. First, up to 25 years (1988-2013) of climate data obtained from 11 weather stations located within the HaleNet climate network on Maui, Hawai‘i, were used to characterize the climate along a 2030-m leeward and 810-m windward elevation transect. Ten climate variables were assessed for trends occurring over the period of record at each station for the 6-month dry (May-October) and wet (November-April) seasons. In general a drying trend was identified along both leeward and windward transects during the dry season. At the five highest elevation sites (2120 to 2990 m), significant increases in solar radiation (2 to 4% decade-1), vapor pressure deficit (9 to 10% decade-1), potential evapotranspiration (3 to 7% decade-1), and the number of zero rainfall days (4 to 5% decade-1), and significant decreases in relative humidity (-3 to 5% decade-1), and rainfall (-3 to -8% decade-1) were identified at two or more of these sites over the period of record. Increases in Trade Wind Inversion (TWI) frequency of occurrence and or decreases in TWI base height were identified as potential agents driving these changes. A second part of this research is to test this hypothesis. To accomplish this, forty years (1973-2013) of radiosonde data obtained from the atmospheric sounding stations located at Hilo and Līhu‘e were used to calculate TWI frequency, base height and strength variability over time. Results indicate a 16% average increase in TWI frequency of occurrence beginning in the early 1990’s. TWI frequency was significantly correlated with the vertical wind velocity variable “omega” obtained from 4 reanalysis data sets, suggesting that observed increases in TWI frequency are a result of increased atmospheric subsidence over the Hawai‘i region. Increased TWI frequency has resulted in 16 and 40% average decreases in high elevation rainfall for the dry and wet seasons, respectively. The upward shift in TWI frequency is not explained by phase changes in the El Niño Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO). Over shorter time periods, however, such as the post-TWI shift period (1991-2013), both ENSO and PDO indices were found to be significantly correlated with TWI frequency, but the sign of the relationship is opposite for the two season. The final component of this research is to identify the role that ENSO and PDO have on TWI frequency of occurrence on ~20 year time scales and to quantify the subsequent effects that TWI variability has on climate variables at mid and high elevations in Hawai‘i. Results show that, during the dry season, mean TWI frequency is higher during the cool phases of ENSO and PDO relative to the warm phases and an opposite relationship is observed in the wet season. Cool phase ENSO and PDO conditions that have dominated over the most recent period of record may help to explain a 4% decade-1 increase and a 2% decade-1 decrease in TWI frequency for the dry and wet seasons, respectively. The opposite response of TWI frequency during the dry and wet seasons helps to explain why high elevation energy and moisture regimes have opposite patterns of change within each season over the same period. The increase in TWI frequency explain some of the observed increases in solar radiation and decreases in rainfall and relative humidity observed at high elevations but much of the variability is still unaccounted for.