Ocean and Coastal Research 2022, v70:e22023 1 Wave climate and extremes on a mesotidal atoll lagoon Mirella B. Costa 1,2 , Eduardo Siegle 3, * 1 Universidade de São Paulo - Instituto Oceanográfico - Programa de Pós-Graduação em Oceanografia (Praça do Oceanográfico,191 - São Paulo - 05508-120 - SP - Brazil) 2 Universidade Federal de Pernambuco - Departamento de Oceanografia (Rua da Arquitetura, sn - Recife - 50740-550 - PE - Brazil) 3 Universidade de São Paulo - Instituto Oceanográfico (Praça do Oceanográfico,191 - São Paulo - 05508-120 - SP - Brazil) * Corresponding author: mirella.costa@ufpe.br Submitted: 12-April-2022 Approved: 24-May-2022 Associate Editor: Piero Mazzini INTRODUCTION Geomorphic and ecological processes on atolls and coral reef lagoons are closely linked to local wave and tide regimes (Woodroffe and Biribo 2011). As waves approach a reef and break, they generate a rise in the dynamic sea level relative to the offshore mean sea level, initiating water trans- port across the rim towards the lagoon (Atkinson et al. 1981; Tartinville et al. 1997; Kraines et al. 1999; Lowe and Falter 2015). The directions of these wave-induced currents will depend on the swell and wind wave direction, as well as on the wave refraction controlled by the reef morphology (Costa et al. 2019). The side facing the largest waves will have large amounts of water pushed over the reef rim, while water will drain to the ocean on the leeward side (Callaghan et al. 2006). This process may be modulated by tides (Costa et al. 2017a). The combination of tide and wave-driven flows through the rim are critical for ecological functions of reef systems, such as renewal of water and oxygen and their uptake by corals (Jokiel 1978; Nakamori et al. 1992; Hearn et al. 2001), removal of metabolic waste and flushing of reef lagoons (Frith and Mason 1986; Kench 1998a; Dumas et al. 2012), and the dispersal and recruitment of larvae (Hamner and Wolanski 1988; Black 1993; Abelson and Denny 1997). However, depending © 2022 The authors. This is an open access article distributed under the terms of the Creative Commons license. Ocean and Coastal Research http://doi.org/10.1590/2675-2824070.22033mbc Original Article ISSN 2675-2824 Processes on atolls and coral reef lagoons are defned by local wave and tide climates. Based on the reconstruction of lagoon wave climate by downscaling ofshore historical wave information, this study aims to assess the wave climate and wave extremes on a mesotidal atoll lagoon (Rocas Atoll, Brazil). Results show that the transformation of ofshore waves, when propagating into the lagoon, results in a diferent and far more homogeneous wave climate. Atoll morphology and tides play a key role in controlling the directional and energy characteristics of the lagoon wave climate. Refraction processes and the reef passage make the side protected from the dominant SE waves the most energetic zone for lagoon wave propagation. Lagoon waves tend to propagate in opposite directions to ofshore wind waves, showing the refraction efect of approaching waves when crossing the atoll rim and the channel. The tidal efect on the dissipation process is also a flter for extreme wave conditions. We highlight the dependence of the resulting lagoon wave climate on atoll morphology, thereby addressing its implications to geomorphological process inside the atoll. Abstract Descriptors: Wave propagation, Reef wave transformation, Rocas Atoll.