West Hawai‘i Climate and Ocean Indicators
Last updated: 2019
Large-scale climate patterns shape the physical environment of marine organisms, influencing feeding, migration, and reproductive success. Significant climatological changes, including increased sea surface temperature, sea level rise, decreased ocean pH, and shifting storm patterns are predicted to occur in coming decades.
It is increasingly important to understand the major physical forces impacting West Hawai‘i and the effects these forces may have on the biology and management of the ecosystem. Climate and oceanographic indicators are presented here to help track and predict changes in the natural environment of West Hawai‘i’s marine ecosystem.
The Hawaiian Islands have one of the most diverse rainfall patterns on Earth. Persistent trade winds, mountainous terrain, and diel heating and cooling of the land interact to produce areas of uplift in distinct spatial patterns associated with the islands’ topography. The resulting clouds and rainfall produced by this uplift lead to dramatic differences in mean rainfall over short distances (Giambelluca et al. 2012).
Rainfall patterns in West Hawai‘i are somewhat unique for the Hawaiian Islands. Rainfall is principally driven by well-developed and reliable land and sea breezes owing to a combination of diel land heating and a blocking of the trade winds by Mauna Loa and Mauna Kea. This diurnal pattern is particularly strong during the summer months.
Changes in rainfall will influence groundwater and surface water transport to the marine environment, which can impact nearshore salinity and temperature, as well as suspended sediment and nutrient concentrations.
Monthly rainfall and the monthly Standardized Precipitation Index (SPI) are shown from 1975 to 2018 from three locations in West Hawai‘i: Waikoloa, Opihihale (Captain Cook), and Hōnaunau. The monthly SPI was developed by the National Center for Atmospheric Research (NCAR) and represents a standardized approach to calculate monthly rainfall, facilitating the comparison of rainfall anomalies from separate regions with differing climates (Keyantash 2018). In short, SPI standardizes rainfall at a given location and can be interpreted as the number of standard deviations by which the observed anomaly deviates from the long-term mean. SPI values of 1, 1.5, and 2 above/below zero represent rainfall conditions categorized as moderately wet/moderately dry, very wet/very dry, and extremely wet/extremely dry. More information on SPI is provided by NOAA’s National Weather Service (http://www.prh.noaa.gov/hnl/hydro/pages/spi_web_page.php). Waikoloa, Opihihale, and Hōnaunau have experienced progressively increased rainfall, with climatological monthly maximum rainfall values of 14.7 in (374 mm), 27.4 in (696 mm), and 34.5 in (876 mm), respectively. The highest rainfall event over the 42-year time series was at Hōnaunau in September 2015, when 86 in (2,183 mm) of rain was recorded in a single month. SPI values from each of the three locations show periods of very wet (e.g., 1980, 1989, 1997) and very dry (e.g., 1995, 2003 2010) conditions. The overarching trend over the 42 years has been a shift towards dryer conditions. The total number of months that exceeded the very dry threshold (-1.5) at Waikoloa, Opihihale, and Hōnaunau was 2.6, 2.3, and 1.9 times higher during the most recent 20-year time period (1997–2016) compared to the previous 20 years (1976–1996). Conversely, the two time periods were nearly similar (between 80 and 100%) with respect to the number of months that exceeded the very wet threshold (1.5).
Long-term sea level rise can lead to chronic coastal erosion, coastal flooding, and drainage problems. Long-term sea level rise also exacerbates short-term fluctuations in coastal sea level driven by waves, storms, and extreme tides. Tracking the status and trends in sea level is critically important for coastal planning and management of nearshore marine ecosystems.
Long-term sea level measurements (1990–2018) from Kawaihae indicate a clear positive trend, increasing by 0.27 m (0.89 ft) in the past 28 years. Over the next 30 to 70 years, properties, infrastructure, and critical habitat located on or near the West Hawai‘i’s shorelines will increasingly be flooded, eroded, or completely lost to the sea. Example areas that will be exposed to chronic flooding include Ka‘ūpūlehu, Kawaihae, and South Point. Portions of the Hualālai Resort in Ka‘ūpūlehu, an economically important tourist destination in North Kona, would be permanently flooded with 1 m (3.3 ft) of sea level rise, which is expected by the end of this century (Hawai‘i Climate Change Mitigation and Adaptations Commission, 2017). As sea level continues to rise, low-lying, populated coastal communities, such as Puakō would experience increased frequency and extent of flooding. Beaches, such as those between Kailua-Kona and Kawaihae, will increasingly be eroded and permanently lost. Native Hawaiian cultural and historical resources, many of which are located near the shoreline, will also be severely threatened and potentially lost with continued sea level rise. For more detailed information on the potential impacts of sea level rise in Hawai‘i, please see the Hawai‘i Climate Change Mitigation and Adaptations Commission’s Report 2017 (https://climateadaptation.hawaii. gov/wp-content/uploads/2017/12/SLR-Report_Dec2017.pdf).
Surface ocean temperatures in Hawai‘i can vary over a broad range of temporal scales owing to the oceanic setting and geographic location in the central-northern Pacific. Diel, intra-seasonal (e.g., mesoscale eddies), seasonal, interannual (e.g., ENSO), and decadal (e.g., PDO) forcing, as well as fluctuations in the rotational speed of the subtropical gyre all influence ocean temperatures in the main Hawaiian Islands.
Seasonal and interannual variability are readily discernible in ocean temperatures dating back to 1900. Seasonally, ocean temperatures are coolest in March (24.8°C; 76.6°F) and warmest in September (27°C; 80.6°F). This seasonal cycle can be shifted, accentuated, or dampened over longer time scales owing to large-scale ocean-atmosphere climate phenomena. Ocean temperature in 2015 clearly stands out as the warmest on record in Hawai‘i due to the confluence of local conditions and large-scale processes. We provide a generalized description of this anomalously warm year in the ‘Why Was 2015 So Hot In Hawai‘i?