Present Climate of Hawai'i

The eight main Hawaiian Islands stretch between 19° and 22° north latitude. This places them within the tropics, and also within the belt of persistent northeast trade winds (Figure 9.13). This geography, combined with the high topography of many Hawaiian peaks, gives rise to large variations in climate across the islands—Hawai’i Island alone has some of the most extreme climate gradients of any place on earth.. Additionally, as half of the land area of Hawai’i lies within eight kilometers (five miles) of the ocean, the ocean is an important control on climate.

Figure 9.13: The effect of the northeast trade winds is seen in the shiny, highly reflective, calm water southwest of Hawai’i Island, where the ocean lies in the lee of the island’s big volcanoes.

Effect of the Ocean

Hawai’i is a small archipelago in the center of the world’s largest ocean. Water has a very high heat capacity (i.e., a lot of energy is required to raise the temperature of water). This means that the annual temperature variation of the ocean is small. Around the Hawaiian Islands the ocean surface temperature falls between 24°C (75°F) in winter and 27°C (81°F) in summer. The seasonal variation in land surface temperature for coastal Hawai’i is similar, about 5°C (9°F) from winter to summer. In a continental setting the seasonal land temperature variation would be much larger; for example, in Chicago the seasonal variation is 25°C (45°F). Thus the ocean dominates climate in Hawai’i’s coastal areas.

Effect of Latitude

As mentioned earlier, Hawai’i lies between 19° and 22° north latitude, just south of the Tropic of Cancer. At this latitude, the global circulation of the atmosphere plays a significant role in climate. Incoming sunlight warms the Earth’s surface, and it does so year-round at equatorial latitudes. The air directly above the surface is warmed and rises. The rising air expands and then cools, allowing water vapor to condense and fall as rain; worldwide equatorial latitudes are therefore characterized by meteorological low pressures and high rainfall (Figure 9.14). The rising air also flows poleward when it reaches neutral buoyancy with its surroundings, where it continues to cool and eventually becomes denser and sinks. This occurs at a latitude of ~30° in both hemispheres. The sinking air is compressed and consequently warmed, and so it becomes strongly undersaturated with water vapor and has a low relative humidity. Near the latitudes of 30°N and 30°S are zones of high pressure and exceptionally dry climate, Earth’s “desert” latitudes. At the surface, the air completes its circuit by flowing back toward the equator. The surface airflow is deflected westward by the Earth’s rotation, creating the trade winds. This circulation pattern is known as a Hadley cell—named after 17th century meteorologist George Hadley— and is an important part of atmospheric circulation.

Figure 9.14: Aspects of general atmospheric circulation important for Hawai’i’s climate. The islands lie within the tropics, in a belt of persistent northeast trade winds, and beneath the descending limb of the Hadley circulation cell. A stable high-pressure system—the North Pacific Anticyclone (H)—remains north-northeast of the islands throughout the year.

Hawai’i, in the northern tropics, is located beneath the descending limb of the Hadley cell. Thus, the air above the islands warms as it approaches the surface, with a temperature gradient running from warm to cool air with increasing altitude. At the same time, the air heated by contact with the warm earth surface also cools with increasing altitude. Most of the time (80 - 90% of days) these two cooling trends are not continuous, and there is instead a temperature discontinuity located at an altitude of about 2000 meters (6000 feet). This arises from the more rapid cooling rate of the moist lower air relative to the cooling rate of the dry upper air. This discontinuity is called the trade wind inversion (Figure 9.15). The inversion is easily seen from any vantage point in the islands, as it creates a ceiling for cloud formation (Figure 9.16). Because of this control on cloud position, the inversion also functions as a control on the distribution of rainfall across the islands.

Figure 9.15: The rate of cooling of moist boundary layer air is faster than the rate of warming of dry descending Hadley cell air. The temperature (density) discontinuity prevents boundary layer air from mixing with upper layer air, and so it creates an upper ceiling for cloud formation.

Figure 9.16: Trade wind inversion. The summit of Hualālai, at sunset, rises above the inversion layer while hazy and humid boundary layer air remains below. The Hualālai summit is 2521 meters (8271 feet), and here the inversion is ~1980 meters (~6500 feet).

Effect of Topography

The most interesting control on the climate of Hawai’i is the high topographic relief of the islands. The islands of Hawai’i, Maui, Kaua’i, Moloka’i, and O’ahu all have summits that are above 1200 meters (4000 feet) in elevation. On Hawai’i Island the peaks of Mauna Kea and Mauna Loa are each above 4180 meters (13,700 feet). Without these summits, Hawai’i would be a warm and humid place with relatively low rainfall. However, the presence of these huge mountains changes the local climate dramatically, which, in turn, leads to the great diversity of climate zones found in Hawai’i (Figure 9.17).

The air above the ocean—the boundary layer—has a high relative humidity because it is in contact with the warm tropical ocean. Northeast trade winds carry this moisture-laden air to the Hawaiian Islands. The mountainous islands divert the airflow both around and over the topographic obstructions. Air that rises over the mountains expands and cools, and the moisture acquired from the ocean condenses and rains out. The windward sides of each island are therefore places with frequent and abundant precipitation (Figure 9.18). As the air continues down the leeward slopes it is at first compressed, and subsequently warms, and no additional moisture condenses; the leeward island shores are therefore very dry. This topography-induced upward airflow creates orographic precipitation on windward slopes and a rain shadow on the leeward side. On most of the Hawaiian Islands, the maximum rainfall occurs at 610 - 910 meters (2000 - 3000 feet) above sea level, although the two wettest spots in the islands are slightly higher in elevation. Wai’ale’ale on Kaua’i (1570 meters [5150 feet]) and Big Bog on Maui (1650 meters [5400 feet]) vie with each other for the title of wettest spot in the US, and indeed at ~1000 centimeters (~400 inches) of annual rainfall they are two of the wettest spots on Earth.

Areas of high topography are dry, as the trade wind inversion prevents clouds from rising high enough for orographic precipitation to occur there. The high summits and leeward slopes receive most of their annual precipitation during winter storms, when high-altitude, low pressure systems develop in the subtropics. These kona (Hawaiian for leeward) storms bring extended periods of rain and even snow (the latter on the summits of Mauna Kea, Mauna Loa, and Haleakalā).

Climate Gradients

Ocean, atmosphere, and topography interact in Hawai’i to make the islands a land of diverse climate with extreme climate gradients. The maps of rainfall and temperature distribution in Hawai’i (see Figures 9.17 and 9.18) clearly show the range and proximity of these variations. On Maui, the distance is only 32 kilometers (20 miles) from Big Bog (the wettest location, with 1029 centimeters [405 inches] per year) to Kihei (the driest, at 28 centimeters [11 inches] per year). On Kaua’i the distance from the wettest spot, Wai’ale’ale (1143 centimeters [450 inches] per year) to the driest spot, Polihale Beach (46 centimeters [18 inches] per year), is only 26 kilometers (16 miles). This gives Kaua’i a rainfall gradient of 38 centimeters per year per kilometer (24 inches per year per mile). Superimpose this gradient onto New York City, with its rainfall level of 127 centimeters (50 inches) per year: could we imagine a desert just three kilometers (two miles) west of Central Park?

Figure 9.17: Mean annual air temperature in Hawai’i. Air temperature is inversely related to elevation. Thus the temperature variations map large-scale topographic features as well.

Figure 9.18: Mean annual rainfall in the Hawaiian Islands. Northeast trade winds combine with topography to create a strongly asymmetrical rainfall distribution. Leeward (west) areas receive little rainfall while windward (east) slopes are some of the wettest places on Earth.

Large changes over short distances characterize the Hawaiian Islands and drive natural processes as well as human activity. Rates of weathering and erosion are much higher in areas of high rainfall. Therefore the islands’ windward slopes are more deeply incised by stream erosion. In dry areas erosion rates are lower; however, episodic winter storms can lead to large sediment loads discharged to the ocean from arid areas with little vegetative cover. Sedimentation events have a negative impact on coral reefs, as the corals require clear, sediment-free water for optimal growth. The proximity of different climate regimes gives Hawai’i a highly diverse set of ecosystems. As colonizing organisms move into the numerous different ecological niches, they undergo an adaptive radiation of species, resulting in one of the most highly endemic and unique groups of organisms on the planet. Rates of soil formation are also dependent on climate. Sufficient weathering is required to break down parent material, yet too much weathering will remove nutrients, ultimately rendering soils infertile. Native ecosystems, as well as agricultural systems, function best in conditions of optimal rainfall and soil fertility, yet these parameters can and do change over very short distances.

Climatic diversity and steep climate gradients make Hawai’i a unique natural laboratory for basic scientific research and applied agricultural research. The high-altitude, cloud-free summits of Mauna Kea and Mauna Loa are ideal sites for astronomical and atmospheric research, respectively. These same climatic features draw tourists from around the world and influence the development of human communities in the islands. Not surprisingly, most development occurs on the sunny leeward sides of the islands, but, unfortunately, most water resources are found on the windward sides. This paradox is both a problem and an opportunity for sustainable resource management, both now and in the future.

Climate Change

Hawai’i’s steep climate gradients also provide a unique opportunity to study the effects of climate change. When global temperatures rise or fall, Hawai’i’s ecosystems migrate up or down the mountainsides. This phenomenon can be observed for past climates through the analyses of fossil pollen grains. During glacial epochs, the summits of Mauna Kea, Mauna Loa, and Haleakalā were covered by ice caps. Clear evidence of glaciation is seen on the slopes of Mauna Kea today, where terminal moraines mark the maximum extent of ice, 18,000 years ago. Additionally, ancient sand dunes—now lithified to calcareous sandstone—mark the position of sea level highstands during interglacial periods.

Climate scientists have long identified the summits of Hawaiian volcanoes as ideal sites for atmospheric study. In 1956, NOAA established the Mauna Loa Observatory (MLO) at an elevation of 3500 meters (11,500 feet) on the north flank of Mauna Loa. MLO is well above the trade wind inversion, and it is located more than 3200 kilometers (2000 miles) from any continental landmass. Instrumentation at MLO therefore samples very clean air in the upper atmosphere, and MLO is the oldest and most important baseline station for analysis of atmospheric composition.

Atmospheric carbon dioxide is among the many parameters measured at MLO. The CO₂ record extends from 1958 to present, and it shows the influence of both natural and anthropogenic processes (Figure 9.19). The zigzag pattern is the result of seasonal photosynthesis in the northern hemisphere. In spring and summer, the growth and increased photosynthetic activity of plants draws CO₂ out of the atmosphere, while CO₂ accumulates in the atmosphere during fall and winter when plants are dormant. The overall upward trend is caused by human activity. Industrialization, fossil fuel combustion, and deforestation all contribute CO₂ to the atmosphere, adding it at a rate much faster than natural processes can remove it. Analyses of ancient atmosphere samples preserved in glacial ice cores show CO₂ levels to be 180 parts per million (ppm) at the height of the last ice age and 280 ppm at its end. The amount of CO₂ in the atmosphere has been increasing at a rapid rate since the start of the industrial revolution, and it has accelerated since the end of World War II. In May 2013, measurements at MLO reached 400 ppm CO₂ for the first time.

While some atmospheric CO₂ is necessary to keep Earth warm enough to be a habitable planet, the unprecedentedly rapid input of CO₂ to the atmosphere by human beings is cause for concern. Everything we know about atmospheric physics and chemistry tells us that increased CO₂ leads to a warmer planet. Multiple paleoclimate data sets verify this conclusion, and modern measurements confirm that we are living in an increasingly warmer world. The MLO data from Hawai’i are our oldest and most reliable direct measurements of anthropogenic atmospheric change.

Rainfall Extremes

Two locations in Hawai’i—Mt. Wai’ale’ale on Kaua’i and the more recently monitored Big Bog on Haleakalā, Maui— average 1029 cm (405 inches) of annual rainfall. Many areas on the islands’ leeward coasts receive less than 50 cm (20 inches) of annual rainfall. The village of Puakō on Hawai’i’s kona coast averages 23 cm (9 inches) per year and is the driest inhabited spot in the islands, while the summit of Mauna Kea receives only 20 centimeters (8 inches) per year—the driest place in the state, with the same rainfall as Phoenix, Arizona.

The truly remarkable aspect of this large variation in rainfall is the short distance that separates rainy and dry areas in Hawai’i. The wettest (Big Bog) and driest (Mauna Kea) places in Hawai’i are only 121 kilometers (75 miles) apart. On the continental US, Olympic National Park in Washington is the rainiest spot (380 centimeters [150 inches] per year) while Death Valley, California, is the driest (6.4 centimeters [2.5 inches] per year). This variation in rainfall is not nearly as large as that measured in Hawai’i, and the two locations are 1450 kilometers (900 miles) apart.

Globally, the rainest places on Earth are on the windward slopes of the world’s largest mountain ranges. Puerto Lopez de Micay, Colombia, is in the Andean foothills, while Cherrapunji, India, receives orographic rainfall from moist air flowing during the monsoon season from the Indian Ocean to the Himalayas. Cherrapunji has only recently had its title as the “Rainiest Spot on Earth” washed away by Puerto Lopez, but it still holds the record for most rainfall in a single year: 2647 centimeters (1042 inches) in 1961.

Figure 9.19: Measured concentration of atmospheric carbon dioxide (1958 to present) at MLO.