The variable impact of ENSO events on regional dengue/DHF in Indonesia Paula Arcari 1 and Nigel Tapper 2 1 School of Global, Urban and Social Studies, RMIT University, Melbourne, Australia 2 School of Geography and Environmental Sciences, Monash University, Melbourne, Australia Correspondence: Paula Arcari (email: paula.arcari@rmit.edu.au) Although studies have demonstrated significant associations between ENSO events and dengue fe- ver, few have explored regional impacts on dengue fever of separate events. This study explores the impacts of two ENSO events on regional patterns of dengue/ dengue haemorrhagic fever (DHF) in- cidence in Indonesia. Data consist of monthly cases of dengue/DHF from 1992 to 2001 for each of Indonesia’s 27 provinces, and monthly figures for rainfall, rainfall anomalies, temperature, relative humidity and the Southern Oscillation Index (SOI). We conducted Pearson correlation analyses for each independent variable against dengue/DHF incidence, using a direct month-by-month correla- tion and applying a lag of between one and six months to each variable with respect to dengue/DHF incidence. Based on the SOI value, we identified two ENSO events between 1992 and 2001. To explore each event, we created two dummy variables and in regression analyses for eight provinces. The variance of between 12.9 per cent and 24.5 per cent in provincial dengue/DHF incidence is explained by two or three climate variables in each of the provinces (p < 0.01 to 0.1). During the 1997/98 event, the explained variance increased by between 7 per cent and 15 per cent in provinces whose climate regimes were most affected by this event. This study demonstrates that indicators of ENSO such as the SOI may assist in the forecast of potential dengue/DHF incidence and distribution in Indonesia. Keywords: dengue fever, ENSO, humidity, Indonesia, rainfall, SOI, temperature Introduction We are currently experiencing a series of the warmest years since records began in 1880, and 2016 is on track to be the warmest year yet, even after the effects of the recent El Nino, which emerged in early 2015, have subsided (Arndt, 2015; NASA 2016). The fre- quency, persistence and intensity of the El Nino Southern Oscillation (ENSO), affecting regional variations in precipitation and temperature in the tropics and subtropics, have increased since the 1970s compared with the previous 100 years (Intergovernmental Panel on Climate Change, 2001). As predicted by National Oceanic and Atmospheric Administration’s Climate Prediction Centre, the recent event persisted through early 2016 to become a so-called Super El Nino on par with that of 1997–98 and, including that in 1982–83, stands among the three strongest El Nino events on record since 1950 (Halpert, quoted in The Weather Network, 2015; NOAA 2016). In the last decade, increasing attention has focused on associations between ENSO events or El Nino and a variety of human health problems including vector-borne dis- eases such as dengue fever (Glantz, 1996; McMichael et al., 1996; Epstein, 1999; Kirono & Tapper, 1999a; Kovats et al., 2003; Hu et al., 2004; Marlier et al., 2013). The World Health Organization (WHO) describes dengue fever as a major public health problem (category A) and acknowledges it as one of the most significant arboviral infectious diseases that affects 50–100 million people each year. Because of misdiagnosis and underreporting, this figure could be up to four times higher (WHO, 1993; 2009; Murray et al., 2013). Temperature and rainfall together can play significant roles in the life cycle doi:10.1111/sjtg.12179 Singapore Journal of Tropical Geography 38 (2017) 5–24 © 2017 Department of Geography, National University of Singapore and John Wiley & Sons Australia, Ltd