All of the 17 substantially above average wet years (greater than 0.5 standardized anomalies) occurred prior to 2006 (Figure 5). In 10 of the 15 years since 2007, wet-season precipitation was below average, with 4 of the remaining 5 years being very near average. Larger total acres burned by wildfires are more likely to occur in the year following an El Niño event.ĭespite great spatial variability in precipitation amounts across the islands, annual rainfall has decreased throughout the island chain (Figures 5 and 6), particularly during recent years in the wet season. Moreover, El Niño events have occurred more frequently over the last two decades. In fact, a drying trend in Hawaiian rainfall during La Niña years is evident since 1956. Since the early 1980s, Hawai‘i has experienced drier conditions during the wet season of La Niña years. Hawai‘i historically experienced drier than normal conditions during the El Niño wet season and greater than normal rainfall during the La Niña wet season. Total annual rainfall sometimes exceeds 300 inches along the windward slopes of mountains, while it averages less than 20 inches in leeward coastal areas and on the highest mountain slopes. The interaction of mountainous terrain, persistent trade winds, heating and cooling of the land, and other factors results in dramatic differences in average rainfall over short distances. The wet season runs from November through April, with weaker and less frequent trade winds and a significant amount of rain from mid-latitude storms. Hawai‘i experiences a drier season from May through October, when warm, steady trade winds cause light to moderate showers. Precipitation varies greatly according to season and location. Historically, temperatures in Hawai‘i have been tightly coupled with the decadal variability of the atmospheric circulation and sea surface temperature anomalies in the Pacific Basin (known as the Pacific Decadal Oscillation) however, since the 1970s, increasing temperatures are more consistent with an increase in the frequency of the trade wind inversion (a layer above the surface where temperature increases with height) and a decrease in the frequency of trade winds (steady, persistent northeasterly winds). The annual number of days below freezing is decreasing over time, as is the daily temperature range, largely due to nighttime warming. The rate of temperature increase is greatest at high elevations, far exceeding the global average rate of change. Statewide, the number of hot days and the number of very warm nights were well above average during the 2015–2020 period, with values more than double the respective long-term averages (Figures 3 and 4). Temperatures in Honolulu have increased by 2.6☏ since 1950 and have consistently been above the 1951–1980 average since 1975 (Figure 2). Since 1950, temperatures across the Hawaiian Islands have risen by about 2☏, with a sharp increase in warming over the last decade (Figure 1). Data: GHCN-Monthly from 5 long-term stations (observations) and CMIP5 (projections). Less warming is expected under a lower emissions future (the coldest end-of-century projections being about 2☏ warmer than the historical average green shading) and more warming under a higher emissions future (the hottest end-of-century projections being about 10☏ warmer than the hottest year in the historical record red shading). Historically unprecedented warming is projected during this century. Observed temperatures are generally within the envelope of model simulations of the historical period (gray shading). Shading indicates the range of annual temperatures from the set of models. Temperatures in Hawai‘i (orange line) have risen by about 2☏ since 1950, with a sharp increase in warming over the last decade. Projected changes for 2006–2100 are from global climate models for two possible futures: one in which greenhouse gas emissions continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Figure 1: Observed and projected changes (compared to the 1951–1980 average) in near-surface air temperature for Hawai‘i.

Honolulu tide graph