From the arid 14,000-foot peak of Mauna Kea on the Big Island to the lush valleys fed by feet of annual rainfall on Kauai, Hawaii has no shortage of extreme weather. But it wasn’t until earlier this year that climate divisions were finally defined for the 50th state. This new data set, which establishes official maps of climatically similar regions in Hawaii, will help ensure the state is included in climate analyses. nationals that were previously available only to the continental United States and Alaska.
Waiting in the 50th state
From historic county boundaries to Zoning Improvement Plan (ZIP) codes developed by the US Postal Service in the 1960s, individual US states have been subdivided and categorized continually.
Initial efforts to divide the 48 contiguous United States states into climate regions began in the early 20th century. But some of those divisions seemed to have been based more on geography, agricultural land use, or even the ease of communication by mail. It wasn’t until the 1950s that state climatologists began incorporating climate data to create so-called climate divisions for each of the lower 48 states.
These regions, which today vary in number from 1 to 10 per state, encompass areas that are climatically similar in key indicators such as precipitation and surface temperature. Alaska received its own climate divisions (a record 13) in 2015. But one state was, until earlier this year, conspicuously untabulated.
“We are excluded from a huge percentage of so-called national weather and climate-related products.”
“Hawaii had no official climate divisions,” said Xiao Luo, an atmospheric scientist at the University of Hawai’i at Mānoa in Honolulu. Luo and his colleagues have used precipitation data from 1990 to 2019 to define climate divisions for the 50th state.
Hawaii is the latest state to receive official climate divisions, said Thomas Giambelluca, a climate scientist at the University of Hawai’i at Mānoa and a member of the research team. “We are the last ones”. Hawaii’s lack of climate divisions means the state has been omitted from analyzes such as the U.S. Gridded Standardized Precipitation Index and the National Temperature Index.
“We are excluded from a huge percentage of so-called national products related to weather and climate,” Giambelluca said.
From stations to the network
To define Hawaii’s climate divisions, Luo and his colleagues extracted monthly precipitation data from more than 600 rain gauges spread across seven of Hawaii’s eight main islands: the Big Island, Kauai, Oahu, Maui, Molokai, Lanai and Kahoolawe. (Niihau, Hawaii’s westernmost island, was not represented due to lack of data.) The researchers then interpolated between those measurements to define a gridded data set of precipitation measurements with a resolution of 250 meters.
That was a critical step, said Chris Daly, a geospatial climatologist at Oregon State University in Corvallis who was not involved in the research. “Complete spatial information is needed, not just station data.” And it’s a challenging task, said Daly, who helped define climate divisions for Alaska. “It takes a lot of data and a lot of expertise to be able to create these data sets.”
Looking for patterns
Luo and his colleagues then used an algorithm to group the gridded data into groups. The goal was to group together regions that exhibit similar precipitation patterns, said Abby Frazier, a climatologist at Clark University in Worcester, Massachusetts, and a member of the research team. “We’re trying to figure out which regions of Hawaii have similar climates.”
The team reran the analyzes considering between 8 and 16 groups before settling on a dozen groups. “The 12 divisions capture the variability of rainfall across the state,” Luo said. As a sanity test of their results, the researchers verified that the 12 regions also reflected differences in surface temperature.
“We have these incredibly humid places, but we also have these extremely dry desert places.”
Twelve climate divisions might, at first glance, seem excessive for such a small state. After all, Idaho, Michigan, Mississippi, New York, Ohio, Pennsylvania, Texas, Washington State and Wyoming each have only 10 climate divisions, and each of those states is substantially larger than Hawaii. The only state with more climate divisions is Alaska, but it is also more than 60 times larger than Hawaii.
However, it’s important to remember that Hawaii is extremely diverse, climatically speaking, said Frazier, who completed his graduate studies at the University of Hawai’i at Mānoa. “We have these incredibly humid places, but we also have these extremely dry desert places.”
This research is long overdue, Daly said, and the findings make sense. “The people who work on it know a lot about Hawaii’s climate.” These results were published in the Bulletin of the American Meteorological Society.
There is still more to do, Frazier said. The climate division’s data sets that are considered national aren’t there yet, he said. “There is still nothing for Puerto Rico, Guam and American Samoa.”
—Katherine Kornei (@KatherineKornei), scientific writer
Citation: Kornei, K. (2024), Hawaii Finally Gets Its Own Climate Divisions, Eos, 105, https://doi.org/10.1029/2024EO240250. Published June 6, 2024.
Text 2024. The authors. CC BY-NC-ND 3.0
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