Climate and Ocean Indicators

August 1, 2016 (updated 12/22/2017)

Variations in large-scale climate patterns are influential in shaping the physical environment of marine organisms, and affect many aspects of their physiology such as feeding, migration, and reproductive success. With significant climatological changes predicted to occur in coming decades, it is increasingly important to understand the major physical forces impacting West Hawaiʻi and the effects these forces may have on the biology and management of the ecosystem. Here we present a number of climate and oceanographic indicators which are useful for tracking and predicting changes in the natural environment of West Hawaiʻi's marine ecosystem.

Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is often described as a long-lived El Niño-like pattern of Pacific climate variability. As seen with the better-known El Niño Southern Oscillation (ENSO), extremes in the PDO pattern are marked by widespread variations in temperature, wind patterns, ocean mixing, and biological productivity (Polovina et al., 1994). The extreme phases of the PDO have been classified as being either warm or cool, as defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean. When SSTs are anomalously warm in the northeastern and tropical Pacific and when sea level pressures are below average over the north Pacific, the PDO has a positive value (i.e., warm phase) (Mantua & Hare, 2002).

Warm and cool phases of the PDO tend to prevail for multiple decades with punctuated and short-lived, intermittent reversals (Figure 17). For example, a warm phase dominated over the late 1970s to late 1990s; however, a short-lived cool phase was observed from 1989 to 1992. The PDO was in a cool phase much of the past two decades. More recently, the PDO has switched to a warm phase, which may lead to an increase in the storminess of the North Pacific (Bond & Harrison, 2000) and when coincident with a positive phase of ENSO (i.e., El Niño), can result in an increase in hurricane activity in Hawaiʻi during the summer months (Rooney et al., 2008).

Figure 17. Pacific Decadal Oscillation Index from 1950 to present. Positive (red) values represent Warm Phase conditions and negative (blue) represent Cool Phase conditions. Black line is a 3-year moving average. Data source: NOAA's National Centers for Environmental Information.
Figure 17. Pacific Decadal Oscillation Index from 1950 to present. Positive (red) values represent Warm Phase conditions and negative (blue) represent Cool Phase conditions. Black line is a 3-year moving average. Data source: NOAA's National Centers for Environmental Information.

El Niño Southern Oscillation

The El Niño Southern Oscillation (ENSO) is an irregular, large-scale ocean-atmosphere climate phenomenon. El Niño represents the warm phase of the ENSO cycle, characterized by weakening of the trade winds across much of the Pacific and warming of ocean temperatures in the Equatorial Pacific. El Niño events typically last 9-15 months, with peak forcing occurring in the northern hemisphere winter. La Niña represents the cool phase and is associated with stronger than normal trade winds and the anomalously cool ocean temperatures (Philander, 1990). On average, La Niña is a less extreme anomaly than El Niño but tends to last longer, approximately 1-3 years.

The Multivariate ENSO Index (MEI) is an indicator of ENSO strength: positive values represent El Niño conditions while negative values represent La Niña conditions (Figure 18). Throughout the past half-century, ENSO has oscillated between El Niño and La Niña numerous times. However, since 1976, there has been a shift towards increased frequency and strength in El Niño conditions. Most notable were the strong El Niños observed in 1982-1983 and 1997- 1998, and recently in 2015. Although no two El Niños are the same, changes in local climate and oceanographic conditions, such as lower than average precipitation (see Rainfall) and larger-than-average wave events (see Wave Forcing), are often observed in the in Hawaiʻi during a strong El Niño.

Figure 18. Multivariate ENSO Index from January 1950 to December 2015. Positive (red) values represent El Nino conditions and negative (blue) represent La Nina conditions. Black line represents an 18 month moving average. Data source: NOAA's Earth System Research Laboratory.
Figure 18. Multivariate ENSO Index from January 1950 to December 2015. Positive (red) values represent El Nino conditions and negative (blue) represent La Nina conditions. Black line represents an 18 month moving average. Data source: NOAA's Earth System Research Laboratory.

Rainfall

Tracking the status and trends in rainfall patterns is important for a variety of resource management issues in West Hawaiʻi. Changes in rainfall dictate the amount and intensity of ground water and surface water transport to the marine environment, which can influence nearshore salinity and temperature, as well as suspended sediment and nutrient concentrations.

The Hawaiian Islands have one of the most diverse rainfall patterns on earth. The 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).

West Hawaiʻi's rainfall patterns are somewhat unique for the Hawaiian Islands. Rainfall is principally driven by well-developed and reliable land and sea breezes that predominate in the region 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.

Using data compiled for the Rainfall Atlas of Hawaiʻi (Giambelluca et al., 2012), we combined monthly data from 15 rainfall data sets obtained from locations spread along West Hawaiʻi from 1950 to 2012 (Figure 19). Over this historical record, rainfall in the region exhibited somewhat consistent seasonal and inter annual patterns. However, since the mid-90s, rainfall patterns have been at or below the long-term average while the intensity of short-term events has increased over the same time period. Similar changes in rainfall patterns have been observed state wide (Fletcher, 2010).

Figure 19. Monthly rainfall (mm) from West Hawaiʻi. Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1950-2012; solid line) and +/- 1 standard deviation (dashed line). Rainfall data represent an average of 15 separate rain gauges located throughout West Hawaiʻi. Data source: University of Hawaiʻi Mānoa Rainfall Atlas of Hawaiʻi (Giambelluca et al. 2012).
Figure 19. Monthly rainfall (mm) from West Hawaiʻi. Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1950-2012; solid line) and +/- 1 standard deviation (dashed line). Rainfall data represent an average of 15 separate rain gauges located throughout West Hawaiʻi. Data source: University of Hawaiʻi Mānoa Rainfall Atlas of Hawaiʻi (Giambelluca et al. 2012).

Sea Level

Tracking the status and trends in sea level is important for coastal communities and nearshore marine ecosystems. Over long time periods, sea level rise can lead to chronic coastal erosion, coastal flooding, and drainage problems. Moreover, long-term increases in sea level exacerbate short-term fluctuations in coastal sea level driven by waves, storms, and extreme tides. Continued sea level rise will increase inundation of coastal roads and communities and result in salt intrusion into coastal wetlands, groundwater systems, taro fields, and anchialine pools (Fletcher, 2010).

Long-term sea level measurements (1990-2015) from Kawaihae indicate an increasing trend (Figure 20), with a sea level rise rate estimated at 3.79 mm per year (Vitousek et al., 2009). Based on sea level rise rates and taking into account the global acceleration in sea level rise reported in the literature, it's estimated that mean sea level will increase in West Hawaiʻi by 0.19 m and 0.48 m by 2050 and 2100, respectively (estimates are relative to 2008 mean sea level) (Vitousek et al., 2009).

Figure 20. Daily sea level data from Kawaihae Harbor (black line). Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1990-2015; solid line) and +/- 1 standard deviation (dashed line). Data source: University of Hawaiʻi Sea Level Center.
Figure 20. Daily sea level data from Kawaihae Harbor (black line). Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1990-2015; solid line) and +/- 1 standard deviation (dashed line). Data source: University of Hawaiʻi Sea Level Center.

Eddy Activity

The combination of prevailing northeasterly trade winds and island topography results in the formation of vigorous mesoscale (~ 100 km) eddies in the lee of Hawaiʻi Island (Lumpkin, 1998). Eddy formation is attributed to a funneling of the northeast trade winds through the Alenuihāhā Channel, between Hawaiʻi Island and Maui, as well as around the southern flank of Mauna Loa at South Point. As a result, cyclonic (counterclockwise) ocean eddies are commonly formed in the North region and anticyclonic (clockwise) eddies are commonly formed in the South region of West Hawaiʻi. The strength and location of these eddies depends heavily on the strength of the trade winds and the stage of eddy development (Bathen, 1975).

Eddies have important biological implications for West Hawaiʻi. Cyclonic eddies, for example, can drive upwelling of cooler, nutrient rich water that influences ocean temperatures and fuels a localized increase in phytoplankton production (Seki et al., 2002), an essential source of energy for higher trophic groups. The presence and strength of eddies may also have a negative effect on certain organisms. For example, Fox et al., (2012) found significant negative correlations between annual patterns of cold-core cyclonic mesoscale eddies and young-of-the-year totals of several fish species on the west coast of the island of Hawaiʻi.

Eddy Kinetic Energy (EKE) is a measure of eddy activity. Higher EKE values are an indicator of increased eddy activity and, therefore, a potentially greater influence on marine ecosystem processes. We split EKE into North and South in order to capture the strength of the cyclonic and anticyclonic eddies in the respective regions (Figure 21). EKE has a prominent North-South split, with the South region characterized by more active eddy activity. EKE in both regions indicates historical time periods of increased activity, such as 1999, 2005-2007, and recently in 2013.

Figure 21. Monthly Eddy Kinetic Energy from 1992-2015 for the North (green; upper panel) and South (blue; lower panel) sections of West Hawaiʻi. Horizontal lines represent the long-term mean (1992-2015; solid line) and +/- 1 standard deviation (dashed line). Data Source: Aviso Geostrophic Currents.
Figure 21. Monthly Eddy Kinetic Energy from 1992-2015 for the North (green; upper panel) and South (blue; lower panel) sections of West Hawaiʻi. Horizontal lines represent the long-term mean (1992-2015; solid line) and +/- 1 standard deviation (dashed line). Data Source: Aviso Geostrophic Currents.

Sea Surface Temperature

Sea surface temperature (SST) plays an important role in a number of ecological processes and varies over a broad range of temporal scales. SST can vary in response to diel, intra-seasonal (e.g., mesoscale eddies), seasonal, interannual (e.g., ENSO) and decadal (e.g., PDO) forcing. Ecosystem responses can include changes in primary productivity, species migration patterns, and if anomalous enough, coral mortality (Hoegh-Guldberg, 1999). In West Hawaiʻi, regional satellite-derived SST shows both strong seasonal and interannual variability (Figure 22). Seasonally, ocean temperatures are coolest in March (24.8°C) and warmest in September (26.9°C). However, this seasonal cycle can vary from year-to-year owing to large-scale ocean-atmosphere climate phenomena such as ENSO and PDO. Although the dynamics between local temperature changes and large-scale climate forcing is complex, generalizations can be made based on historical information. In general, ocean temperatures tend to be warmer than average during El Niño conditions and warm phases of the PDO and cooler than average during La Niña conditions and cold phases of the PDO. The recent increase in SST observed in West Hawaiʻi is likely indicative of these large-scale processes influencing regional scale conditions.

Figure 22. Weekly sea surface temperature for the entire region of West Hawaiʻi from January 1985 – October 2015 (black line). Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1985-2015; solid solid) and +/- 1 standard deviation (dashed line). Values Data source: NOAA's Coral Reef Watch.
Figure 22. Weekly sea surface temperature for the entire region of West Hawaiʻi from January 1985 – October 2015 (black line). Red line represents a 12 month moving average. Horizontal lines represent the long-term mean (1985-2015; solid solid) and +/- 1 standard deviation (dashed line). Values Data source: NOAA's Coral Reef Watch.

Thermal Stress

Thermal stress is an indicator of increased SST that exceeds typical maximum summertime temperatures, resulting in stress-inducing conditions for coral reef ecosystems. Coral bleaching—the loss of corals' photosynthetic symbionts—can result if thermal stress is > 1°C above the maximum summertime temperatures and sustained for extended periods of time.

Over the last three decades, West Hawaiʻi's coral reefs have experienced multiple years with thermal stress (Figure 23), although rarely have ocean temperatures exceeded the coral reef bleaching threshold (> 1°C thermal stress) until 2015. The 2015 thermal stress event resulted in satellite-derived temperatures reaching a maximum of ~ 2.5°C above typical summertime temperatures. Rising ocean temperatures and the associated increase in thermal stress are expected to increase the frequency and severity of coral bleaching events in the future (Pandolfi et al., 2011).

Figure 23. Annual maximum Thermal Stress HotSpot for West Hawaiʻi from January 1985 – October 2015. Values represent temperatures that exceeded the maximum climatological monthly mean. No data represents years that experienced no thermal stress anomalies. Data Source: NOAA's Coral Reef Watch.
Figure 23. Annual maximum Thermal Stress HotSpot for West Hawaiʻi from January 1985 – October 2015. Values represent temperatures that exceeded the maximum climatological monthly mean. No data represents years that experienced no thermal stress anomalies. Data Source: NOAA's Coral Reef Watch.

Coral Bleaching Event of 2015

The 2015 warming event resulted in widespread and severe coral bleaching across West Hawaiʻi. Reef-level temperatures were anomalously warm throughout the summer, reaching as high as 30.3°C (86.5°F) in September, far exceeding the coral reef bleaching threshold for the region (see figure below). The overall severity of bleaching, or the percentage of corals bleached at a given site, was estimated at 30-80%, with some geographic areas exhibiting upwards of 90% coral bleaching (DAR and The Nature Conservancy, unpublished data). Coral bleaching was widespread across coral species, although some (e.g., Pavona duerdina and Pocillopora damicornis; 100% bleached) were hit harder than others (e.g., Montipora incrassate and Leptastrea purpure; 40-47% bleached). The two most dominant reef-building coral species in West Hawaiʻi, Porites lobata and Porites compressa, exhibited > 50% coral bleaching.

Reef-level temperature (15 m) information obtained from a DAR long-term monitoring site located in the North region shows the 
        anomalously warm temperatures observed in 2015. Temperatures peaked at 30.3°C (86.5°F) in late September, surpassing the 
        coral bleaching threshold by 2.4 °C (Data source: DAR).
Reef-level temperature (15 m) information obtained from a DAR long-term monitoring site located in the North region shows the anomalously warm temperatures observed in 2015. Temperatures peaked at 30.3°C (86.5°F) in late September, surpassing the coral bleaching threshold by 2.4 °C (Data source: DAR).

Coral bleaching does not necessarily result in coral mortality; corals can survive temporarily in the absence of their photosynthetic symbionts and completely recover from a bleaching event. However, local human stressors such as sedimentation, excess nutrient input, and removal of herbivorous fishes can impede their ability to recover. Projected future increases in ocean temperatures are expected to increase the frequency and severity of coral bleaching in Hawaiʻi. Effective management strategies that mitigate local human stressors and thereby bolster coral reef resiliency to future bleaching events are critical in this era of rapid climate change.

Coral bleaching was widespread in West Hawaiʻi in 2015. Here, a photograph of a bleached pocillopora damicornis coral 
        is shown. An estimated 40-80% of corals bleached in the region.
Coral bleaching was widespread in West Hawaiʻi in 2015. Here, a photograph of a bleached pocillopora damicornis coral is shown. An estimated 40-80% of corals bleached in the region.
Photo courtesy of The Nature Conservancy.
Photo courtesy of The Nature Conservancy.

Wave Forcing

Wave forcing is a major environmental forcing mechanism in marine ecosystems. Variations in wave forcing can influence important ecological processes such as coral reef development (Dollar & Tribble, 1993), spatiotemporal patterning in benthic and reef fish communities (Friedlander et al., 2003), sediment transport and resuspension (Storlazzi et al., 2004), and shoreline and beach morphology (Rooney & Fletcher, 2005). Wave forcing can drive mixing of the upper water column that can reduce ocean temperatures during warming events (McClanahan et al., 2005) and potentially enhance surface nutrient availability (Wolanski & Delesalle, 1995).

Hawaiʻi receives large ocean swell from extra-tropical storms in the northwest Pacific. During winter months, the Aleutian low intensifies and the strong winds associated with these storms produce large swell events that travel for thousands of miles until reaching Hawaiʻi (Rooney et al., 2008). However, because seven of the main eight Hawaiian Islands lie to the northwest of Hawaiʻi Island, significant wave shadowing (i.e., blocking) occurs, dramatically reducing nearshore wave forcing along West Hawaiʻi (Vitousek et al., 2009). This blocking effect is particularly accentuated north of Keahole Point. As such, we have split waves into North and South West Hawaiʻi (Figure 24).

Figure 24. Monthly peak wave power (kW m -1) calculated for the North (green; upper panel) and South (blue; lower panel) 
        regions in West Hawaiʻi. Data represent maximum daily values in each month, from 1979-2013. Horizontal lines represent the 
        long-term mean (1979-2013; solid line) and + 1 standard deviation (dashed line). Data Source: Li et al., (2016); International 
        Pacific Research Center (IPRC) at the University of Hawaiʻi.
Figure 24. Monthly peak wave power (kW m-1) calculated for the North (green; upper panel) and South (blue; lower panel) regions in West Hawaiʻi. Data represent maximum daily values in each month, from 1979-2013. Horizontal lines represent the long-term mean (1979-2013; solid line) and + 1 standard deviation (dashed line). Data Source: Li et al., (2016); International Pacific Research Center (IPRC) at the University of Hawaiʻi.

Although wave height is frequently used in ecological research and is often easier to contextualize, it underrepresents wave conditions owing to the importance of wave period in determining the overall impact of wave forcing (Gove et al., 2013). Wave power (kW m-1), a calculation that includes both wave period and wave height, is a more realistic estimate of wave forcing, and therefore, a more ecologically relevant indicator with which to assess wave forcing on marine ecosystems (Gove et al., 2013).

Wave power in West Hawaiʻi is highly seasonal with wintertime months typically experiencing significantly greater wave forcing that summertime months (Figure 24). Wave forcing is, on average, greater in the South compared to the North. Peak swell events are also typically greater to the South. From 1979 to 2013, wave events in 1980, 1986, and 2004 stand out in both records as the largest events of the 35-year record.

Phytoplankton Biomass

Phytoplankton production is an essential source of energy in the marine environment. The extent and availability of phytoplankton biomass drives the trophic-structure of entire marine ecosystems (Iverson, 1990), dictating the distribution and production of the world's fisheries (Chassot et al., 2010). The ecological impacts of increased phytoplankton biomass are especially acute near coral reef ecosystems as they predominantly reside in nutrient impoverished waters that lack new production (Hamner & Hauri, 1981). Changes in phytoplankton biomass are predominantly driven by changes in nutrient concentrations. Nutrients can increase through a variety of natural processes that bring deeper waters to the upper surface of the ocean. For example, bathymetric influences on ocean currents can drive turbulent mixing, lee eddy, and wake affects that increase nutrients on the lee side of an island. Internal waves, generated from tidal currents interacting with underlying bathymetry, can also drive vertical changes in the background stratification that deliver cooler, nutrient-rich waters to the near surface (Leichter et al., 1998). However, human activities can also increase nearshore nutrient levels that artificially elevate planktonic production in coastal marine ecosystems (Vitousek et al., 1997). Sources of nutrients include urban development and agricultural land use as well as wastewater effluent (e.g., on-site waste disposal systems) and storm outfalls (Smith et al., 1999; Anderson et al., 2002). Precipitation events and outflow from rivers can mobilize and carry land-based pollutants and other terrigenous input that can also stimulate phytoplankton production (Anderson et al., 2002).

We use chlorophyll-a, a widely used proxy for phytoplankton biomass, as an indicator for changes in phytoplankton production. Owing to existence of cyclonic and anticyclonic eddies off West Hawaiʻi and other geographically variable factors that can influence phytoplankton production (e.g., wave forcing), we split the region into North and South in order to more effectively track changes in phytoplankton biomass in the region (Figure 25). Phytoplankton biomass is generally greater in the North compared with the South, and shows a seasonal pattern, with peaks occurring in October to December. However, there is clear intra-seasonal and interannual variability observed in both regions. More recently, phytoplankton biomass was considerably and anomalously low in 2014, but has since increased above long-term averages in both regions.

Figure 25. Eight-day chlorophyll-a (proxy for phytoplankton biomass) information from 2002 - 2015 for the North (green, 
        upper panel) and South (blue; lower panel) regions in West Hawaiʻi. Horizontal lines represent the long-term mean (solid) 
        and ± 1 standard deviation (dashed). Data Source: NASA's Moderate Resolution Imaging Spectroradiometer.
Figure 25. Eight-day chlorophyll-a (proxy for phytoplankton biomass) information from 2002 - 2015 for the North (green, upper panel) and South (blue; lower panel) regions in West Hawaiʻi. Horizontal lines represent the long-term mean (solid) and ± 1 standard deviation (dashed). Data Source: NASA's Moderate Resolution Imaging Spectroradiometer.