1 a Hyperactive End to the Atlantic Hurricane Season: October–November 2020
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1 A Hyperactive End to the Atlantic Hurricane Season: October–November 2020 2 3 Philip J. Klotzbach* 4 Department of Atmospheric Science 5 Colorado State University 6 Fort Collins CO 80523 7 8 Kimberly M. Wood# 9 Department of Geosciences 10 Mississippi State University 11 Mississippi State MS 39762 12 13 Michael M. Bell 14 Department of Atmospheric Science 15 Colorado State University 16 Fort Collins CO 80523 17 1 18 Eric S. Blake 19 National Hurricane Center 1 Early Online Release: This preliminary version has been accepted for publication in Bulletin of the American Meteorological Society, may be fully cited, and has been assigned DOI 10.1175/BAMS-D-20-0312.1. The final typeset copyedited article will replace the EOR at the above DOI when it is published. © 2021 American Meteorological Society Unauthenticated | Downloaded 09/26/21 05:03 AM UTC 20 National Oceanic and Atmospheric Administration 21 Miami FL 33165 22 23 Steven G. Bowen 24 Aon 25 Chicago IL 60601 26 27 Louis-Philippe Caron 28 Ouranos 29 Montreal Canada H3A 1B9 30 31 Barcelona Supercomputing Center 32 Barcelona Spain 08034 33 34 Jennifer M. Collins 35 School of Geosciences 36 University of South Florida 37 Tampa FL 33620 38 2 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 39 Ethan J. Gibney 40 UCAR/Cooperative Programs for the Advancement of Earth System Science 41 San Diego, CA 92127 42 43 Carl J. Schreck III 44 North Carolina Institute for Climate Studies, Cooperative Institute for Satellite Earth System 45 Studies (CISESS) 46 North Carolina State University 47 Asheville NC 28801 48 49 Ryan E. Truchelut 50 WeatherTiger, LLC 51 Tallahassee FL 32302 52 53 54 55 56 57 58 3 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 59 60 61 Submitted to the Bulletin of the American Meteorological Society 62 Date: 4 February 2021 63 Revised: 12 June 2021 64 65 *Corresponding author address: Philip J. Klotzbach, Department of Atmospheric Science, 66 Colorado State University, Fort Collins, CO 80523 email: [email protected] 67 68 #Klotzbach and Wood contributed equally to this work. 4 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 69 Abstract 70 The active 2020 Atlantic hurricane season produced 30 named storms, 14 hurricanes, and 7 71 major hurricanes (Category 3+ on the Saffir-Simpson Hurricane Wind Scale). Though the season 72 was active overall, the final two months (October–November) raised 2020 into the upper echelon 73 of Atlantic hurricane activity for integrated metrics such as Accumulated Cyclone Energy 74 (ACE). This study focuses on October–November 2020, when 7 named storms, 6 hurricanes, and 75 5 major hurricanes formed and produced ACE of 74 * 104 kt2. Since 1950, October–November 76 2020 ranks tied for 3rd for named storms, 1st for hurricanes and major hurricanes, and 2nd for 77 ACE. Six named storms also underwent rapid intensification (≥30 kt intensification in ≤24 hr) in 78 October–November 2020—the most on record. 79 80 This manuscript includes a climatological analysis of October–November tropical cyclones 81 (TCs) and their primary formation regions. In 2020, anomalously low wind shear in the western 82 Caribbean and Gulf of Mexico, likely driven by a moderate intensity La Niña event and 83 anomalously high sea surface temperatures (SSTs) in the Caribbean provided dynamic and 84 thermodynamic conditions that were much more conducive than normal for late-season TC 85 formation and rapid intensification. This study also highlights October–November 2020 86 landfalls, including Hurricanes Delta and Zeta in Louisiana and in Mexico and Hurricanes Eta 87 and Iota in Nicaragua. The active late season in the Caribbean would have been anticipated by a 88 statistical model using the July–September-averaged ENSO Longitude Index and Atlantic warm 89 pool SSTs as predictors. 90 Capsule Summary 5 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 91 October–November 2020 had record-breaking levels of Atlantic hurricane activity, likely 92 supported by anomalously high sea surface temperatures and anomalously weak vertical wind 93 shear in the Caribbean. 94 95 Keywords 96 hurricanes, tropical cyclones, La Niña, subseasonal forecasting 97 6 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 98 Introduction 99 The 2020 Atlantic hurricane season was very active by most tropical cyclone (TC) metrics, 100 producing a total of 30 named storms, 14 hurricanes, and 7 major hurricanes (maximum 101 sustained winds ≥96 kt; Category 3-4-5 on the Saffir-Simpson Hurricane Wind Scale). The 102 formation of 30 named storms broke the Atlantic single-season record of 28 named storms set in 103 2005, and only 2005 produced more hurricanes with a total of 15. Seven major hurricanes 104 formed in 2020, tying 2005 for the most major hurricanes on record dating back to 1851 105 (Landsea and Franklin 2013). Though the 2020 Atlantic hurricane season was on record pace for 106 named storm formations through September (Table 1), the extremely active October–November 107 (Oct–Nov) drove integrated metrics such as named storm days, major hurricane days, and 108 Accumulated Cyclone Energy1 (ACE; Bell et al. 2000) to well above-average levels by the end 109 of the season. Through 30 September, the 2020 Atlantic hurricane season produced 23 named 110 storms, six ahead of the prior record of 17 named storms set in 2005 and equaled in 2011. 111 However, during this time period the Atlantic only generated 19 hurricane days, just above the 112 1950–2019 average of 18.75 hurricane days. Also, total ACE reached 106 * 104 kt2 by 30 113 September (~130% of the 1950–2019 average). 114 115 October–November 2020 was extremely active with 7 named storms, 6 hurricanes, and 5 major 116 hurricanes forming during the two-month period (Table 1). In addition, these two months 117 generated 74 * 104 kt2 of ACE, producing the 2nd-most Oct–Nov ACE since 1950 (trailing only 1 Accumulated Cyclone Energy is an integrated metric accounting for frequency, intensity, and duration of storms and is calculated by squaring the maximum sustained wind speed at each six-hourly interval when a tropical or subtropical cyclone has maximum sustained winds ≥34 kt (e.g., a named storm) and then dividing by 10000. 7 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 118 2016 which produced 79 * 104 kt2 of ACE). The 1950–2019 Oct–Nov average is 2.7 named 119 storms, 1.5 hurricanes, 0.5 major hurricanes, and 23 * 104 kt2 of ACE. The five major hurricanes 120 that formed in Oct–Nov 2020 (Delta, Epsilon, Zeta, Eta, and Iota) were the most on record for 121 the Atlantic basin during this two-month period. No other season on record had more than two 122 major hurricane formations in Oct–Nov. These two months in 2020 also generated five hurricane 123 formations in the Caribbean Sea (Gamma, Delta, Zeta, Eta, and Iota), breaking the old record of 124 three set in 1870 and equaled in 2010. All six Oct–Nov hurricanes hit land, causing extensive 125 damage and loss of life (Table 2). Hurricane Gamma made landfall along the Yucatan Peninsula, 126 and Delta and Zeta both made landfall in the Yucatan Peninsula and subsequently in Louisiana. 127 Zeta’s Yucatan landfall occurred near Gamma’s landfall location a few weeks earlier. Hurricanes 128 Eta and Iota made landfall at nearly the same location in Nicaragua as Category 4 hurricanes less 129 than two weeks apart, causing catastrophic damage, especially due to flooding and associated 130 mudslides. Eta and Iota combined to cause $7 billion USD in Honduras alone, which is ~30% of 131 Honduras’s Gross Domestic Product of $24 billion USD (Aon 2021). These storms also 132 combined to cause over 230 direct fatalities in Central America and southern Mexico, which 133 thankfully was substantially fewer than the thousands of casualties resulting from Hurricane 134 Mitch in 1998. 135 136 This manuscript examines the climatology of Oct–Nov Atlantic TCs, including its interannual 137 variability as modulated by ENSO as well as its multi-decadal variability that is likely modulated 138 by the Atlantic multidecadal oscillation (AMO; Goldenberg et al. 2001; Klotzbach and Gray 139 2008; Klotzbach et al. 2018). We also discuss observed activity in Oct–Nov 2020 and then 140 examine the large-scale drivers of the extremely active latter portion of the 2020 Atlantic 8 Unauthenticated | Downloaded 09/26/21 05:03 AM UTC Accepted for publication in Bulletin of the American Meteorological Society. DOI 10.1175/BAMS-D-20-0312.1. 141 hurricane season to compare against both the long-term climatology and 2005 specifically – 142 another recent very active season in the Caribbean. Lastly, we examine the potential 143 predictability of Oct–Nov Caribbean TC activity using a two-predictor model composed of the 144 July–September-averaged ENSO Longitude Index (ELI; Williams and Patricola 2018) and July– 145 September-averaged Atlantic warm pool (AWP; Wang and Lee 2007) conditions.