Honolulu has seen 45 days with record highs so far this year, including 29 days between June and August. That’s the equivalent of more than two record highs every week. Beginning Aug. 10, Honolulu hit 90 degrees each of the next 37 days.
But even more impressive have been the nighttime lows. From 1950 to 2018, only 14 nights failed to drop below 80 degrees. This year has featured 19 such nights. The combination of toasty daytime highs and even steamier nighttime lows has helped 2019 claim the top spot for having the hottest calendar day on record in Honolulu — and snag second and third place in the process, while tying twice for fourth.
Honolulu also hit 95 degrees on the final day of August. That set a record for the hottest August temperature recorded in more than a century of bookkeeping, as well as tied the record for hottest year-round temperature.
All told, Honolulu recorded its hottest summer in the books, coming in 0.3 degrees warmer than 2005, the previous record holder.
It’s not just a phenomenon local to Honolulu. In fact, of the four long-running climate sites spread along the Hawaiian archipelago, three of them saw their warmest summer on record. Only Hilo, on the Big Island, did not.
In Lihue, a town nestled on the eastern fringe of Hawaii’s northwesternmost populated island, high-temperature records were set or surpassed every day between Aug. 24 and Sept. 12. Since the summer solstice, 48 days have featured record highs or ties, 44 nights had record high lows, and exactly zero days/nights had record lows.
Of the more than 300 temperature records to be tied or broken at the four Hawaiian climate stations this year, only five have been for record cold temperatures.
Lihue also saw 16 nights that stayed above 80 degrees, with only five nights having been recorded in the five-plus decades of numbers before that.
It’s no coincidence that Hawaii’s seeing a flurry of record-high temperatures. As climate change causes warming, the pendulum will continue to swing toward record highs outpacing record lows at an exponentially accelerating clip. All the while, the background temperature continues to sneak upward, Honolulu having warmed about 2.4 degrees Fahrenheit since 1950.
What makes this year so extreme? Ocean waters, in particular, have been abnormally warm. But they’re not the sole cause. They were warmer in 2015, when air temperatures weren’t as hot as this summer.
However, this year, compared with 2015, a much broader area of above-average sea surface temperatures exists for thousands of miles east and northeast of Hawaii, from which direction the trade winds blow. Weather Underground reported that August 2019 was the warmest month for global ocean water temperatures on record, based on records dating to 1854. That would have an enormous role in boosting temperatures and humidity.
Indeed, we do see that signature acutely; we can look at Green Island, an uninhabited sand mound 1,000 miles up the Hawaiian chain along the Midway Atoll, and we see a half-degree dew point (a measure of humidity) increase in 2019 over 2015, a likely reflection of the warmer ocean temperatures to the east.
The position and strength of dominant high pressure also play a role. Summer air pressure averages at the Hawaiian climate stations we checked were about 0.04 inches greater this summer than in 2015; keep in mind that average surface pressure is about 29.94 inches there. So while the 0.04 inches may not seem significant, it’s the equivalent of adding about 10 yards’ worth of air on top of the atmosphere above you. The more weight above, the more compressional heating, and the more warming. That’s why high-pressure “heat domes” are so effective at heating things up.
In any case, this summer fits like a puzzle piece into a continued warming trend across the Aloha State. As carbon dioxide emissions continue and climate change’s pace quickens, we may expect record-breaking temperatures like this to become even more routine. The previously abnormal is now the new normal.