RESEARCH LETTER Extreme Rainfall Associated With Hurricane Maria Over 10.1029/2019GL082077 Puerto Rico and Its Connections to Climate Special Section: Variability and Change The Three Major Hurricanes of 2017: Harvey, Irma and Maria David Keellings1 and José J. Hernández Ayala2 1Department of Geography, University of Alabama, Tuscaloosa, AL, USA, 2Department of Geography, Environment, and Key Points: Planning, Sonoma State University, Rohnert Park, CA, USA • Hurricane Maria was the most extreme rainfall event when compared to 129 tropical cyclones • Return periods for Hurricane Abstract Hurricane Maria was associated with record‐breaking rainfall over Puerto Rico and caused Maria's precipitation decreased by at unprecedented flooding and landslides. Here we analyze the extreme rainfall produced by Hurricane least half across Puerto Rico, Maria using 35 stations with daily precipitation data from 1956–2016. A covariate‐based extreme value indicating increased likelihood in recent years analysis point process approach that accounts for natural climate variability and long‐term climate change • The probability of Maria's heaviest influences on extreme rainfall is applied. Hurricane Maria produced the single largest maximum rainfall precipitation has likely increased as event since 1956 and had the highest total averaged precipitation of 129 storms that have impacted the a result of long‐term climate trends island since 1956. Return periods for an event of Hurricane Maria's precipitation magnitude decreased in Supporting Information: 48.6% of stations across Puerto Rico and at least halved when averaged across the island. Within the most • Supporting Information S1 affected areas it is likely that the probability of precipitation of Maria's magnitude has increased by a factor • Figure S1 greater than 1 (best estimate 4.85) as a result of long‐term climate trends. Plain Language Summary Hurricane Maria was associated with record‐breaking rainfall over Correspondence to: Puerto Rico, which caused unprecedented flooding and landslides across the island and led to widespread D. Keellings, devastation. Here we analyze the extreme rainfall produced by Hurricane Maria using 35 historical weather [email protected] stations with daily precipitation data from 1956–2016. We use a statistical analysis technique to determine how unusual Maria's rainfall was and if Maria's rainfall can be attributed to climate variability and/or Citation: climate change. We find that Hurricane Maria produced the single largest maximum rainfall event since Keellings, D., & Hernández Ayala, J. J. (2019). Extreme rainfall associated with 1956 and had the highest precipitation of 129 storms that have impacted the island since 1956. Our study Hurricane Maria over Puerto Rico and concludes that extreme precipitation, like that of Hurricane Maria, has become much more likely in recent its connections to climate variability years and long‐term trends in atmospheric and sea surface temperature are both linked to increased and change. Geophysical Research Letters, 46, 2964–2973. https://doi.org/ precipitation in Puerto Rico. These results place Maria prominently in the context of extreme storms that 10.1029/2019GL082077 have impacted Puerto Rico and indicate that such events are becoming increasingly likely. Received 17 JAN 2019 Accepted 25 FEB 2019 1. Introduction Accepted article online 4 MAR 2019 Published online 12 MAR 2019 On 20 September 2017, Hurricane Maria made landfall on the southeast coast of Puerto Rico as a strong Category 4 hurricane. Tropical cyclones (TCs) are not uncommon to the island, the long‐term average for TC landfalls in the northern Caribbean where Puerto Rico is located is one per year (Pielke et al., 2003), the National Hurricane Center Report (2018) on Maria highlights it as the strongest hurricane to make land- fall on the island since 1928 (Pasch et al., 2018). Maria broke rainfall records that resulted in unprecedented flooding and mudslides and combined with sustained winds of 249 km/hr at landfall that contributed to a near complete loss of the electrical grid and municipal water supplies for 3.4 million residents (Pasch et al., 2018). All infrastructure was affected with 80% destruction of communication systems, including uti- lity poles and cellular towers, and ultimately the storm resulted in a cost of 90 billion dollars in damage between Puerto Rico and the U.S. Virgin Islands, which exceeds the previously most costly storm to affect Puerto Rico directly, Hurricane Georges in 1998, by 85 billion dollars (Pasch et al., 2018). The official death toll of Hurricane Maria is 64 deaths, though a mortality study by Kishore et al. (2018) found the number of excess deaths associated with Maria is more than 70 times the official estimate bringing the total count closer to 5,000. Disaster‐related deaths are difficult to determine. While direct causes of death, such as flying debris or drowning, are relatively easy to assign, indirect deaths resulting from delayed medical treatment or wor- sening of preexisting conditions are much more difficult to capture (Kishore et al., 2018). Further hindering ‐ fi ©2019. American Geophysical Union. this effort in Puerto Rico is the requirement that every hurricane related death be con rmed by the Institute All Rights Reserved. of Forensic Sciences, which requires bodies to be brought to San Juan and delays the issuance of a death KEELLINGS AND HERNÁNDEZ AYALA 2964 Geophysical Research Letters 10.1029/2019GL082077 certificate (Kishore et al., 2018). A recent report concluded that the total excess mortality associated with Hurricane Maria in Puerto Rico was 2,975, which is 46 times the official government estimate (Santos‐ Burgoa et al., 2018). Additionally, estimates of between 114,000 and 212,000 residents are expected to migrate away from the island in the first year, and up to 14% of the total population by the end of the second year following the event (Meléndez & Hinojosa, 2017). Several recent studies have focused on the relationship between climate change and the intensity of hurri- canes. Mann and Emanuel (2006) found that the underlying factors in the increasing trends of Atlantic hur- ricane intensity appear to be the influence of primarily anthropogenic forced large‐scale warming, while Elsner (2006) found causal evidence that increasing near‐surface air temperatures lead to an increase in sea surface temperature (SST) as evidence to support a hypothesis of human‐induced climate change influ- encing the intensity of TCs in the Atlantic. No trend in hurricane frequency has been detected. Other studies have looked at the influences that teleconnections, such as the El Nino–Southern Oscillation (ENSO), and anthropogenic influences, such as carbon dioxide (CO2), have on hurricane rainfall variability. A recent study by Risser and Wehner (2017) used a covariate‐based extreme value analysis (EVA) approach where they found that human‐induced climate change likely increased Hurricane Harvey's total rainfall by at least 19% and increased the chance of the observed precipitation by a factor of at least 3.5. Emanuel (2017) exam- ined the annual probability of Hurricane Harvey's observed rainfall finding that it had become 6 times more likely since the end of the twentieth century and that a similar magnitude event will be roughly 18 times more likely by 2081–2100. Another study found that Hurricane Harvey was 3 times more likely due to anthropogenic climate change (Van Oldenborgh et al., 2017). Patricola and Wehner (2018) examined the anthropogenic influence on major TCs finding that relative to preindustrial conditions, climate change has intensified extreme rainfall in Hurricanes Katrina, Irma, and Maria. Here we focus on two main questions regarding the rainfall associated with Hurricane Maria. How does the extreme rainfall associated with Hurricane Maria compare to the precipitation climatology of TCs in Puerto Rico and how much of the rain attributed to the storm can be explained by natural climate variability and anthropogenic climate change? To address these questions, data from 47 sites are used to estimate Hurricane Maria's rainfall over the 3‐day period the storm was within a 500‐km radius of the island. Historical data from 35 stations are used in an EVA point process model to examine the relationship between modes of natural variability (North Atlantic Oscillation [NAO], Atlantic Multidecadal Oscillation [AMO], and ENSO), long‐term trends associated with anthropogenic climate change (atmospheric CO2, global tem- perature, SST, and Cloud cover), and the extreme rainfall associated with Hurricane Maria. Changes in return periods associated with Maria's peak precipitation are estimated for each of 35 historical stations. 2. Data Six‐hourly TC positions were extracted from the International Best Track Archive for Climate Stewardship for the years 1970–2017 (Knapp et al., 2010). A Geographic Information System was utilized to calculate a 500‐km buffer around the island, allowing us to define the portion of Maria's track that was within the radius. Maria spent 3 days within the radius from 19 to 21 September 2017. This method has been used by researchers examining TC rainfall in Puerto Rico and the extreme floods associated with those events (Hernández Ayala et al., 2017; Hernández Ayala & Matyas, 2016, 2018). Those studies identified 86 TCs within the 500‐km radius around Puerto Rico from 1970–2010. In this study we expanded their data set to include an extra 43 TCs that were within the 500‐km radius during 1956–1970 and 2011–2017 for a total of 129 storms. Daily rainfall totals were obtained from the National Centers for Environmental Information (NCEI) for 19 stations and from the U.S. Geological Survey for 28 rain gauges with daily data for Hurricane Maria, for a total of 47 sites. Daily rainfall totals were obtained from the NCEI for 35 historical stations with a minimum of 70% of observations during the hurricane season that spans from 1 June to 30 November for a period that begins 1 June 1956 and ends 30 November 2016.