Extreme flows and unusual water levels near a Caribbean coral reef: was this a case of a “perfect storm”? Tal Ezer & William D. Heyman & Chris Houser & Björn Kjerfve Received: 11 July 2011 / Accepted: 10 April 2012 / Published online: 9 May 2012 # Springer-Verlag 2012 Abstract Observations of currents aimed to study the flow near a spawning aggregation reef, Gladden Spit off the coast of Belize, reveal unusually strong currents on 19–20 October 2009 (the current speed was over 1 ms -1 , when the mean and standard deviation are 0.2±0.12 ms -1 ). Dur- ing this short time, the water level was raised by 60–70 cm above normal in one place, but lowered by 10–20 cm in another location just 2 km away. The temperature dropped by over 2°C within a few hours. Analyses of local and remote sensing data suggest that a rare combination of an offshore Caribbean cyclonic eddy, a short-lived local tropi- cal storm, and a Spring tide, all occurred at the same time and creating a “perfect storm” condition that resulted in the unusual event. High-resolution simulations and momentum balance analysis demonstrate how the unique shape of the coral reef amplified the coastal current through nonlinear flow–topography interactions. The suggested mechanism for the water level change is different than the classical wind- driven storm surge process. The study has implications for the influence of external forcing on mixing processes and physi- cal–biological interactions near coral reefs. Keywords Numerical model . Caribbean coral reef . Storm surge . Flow–topography interaction 1 Introduction The Mesoamerican Barrier Reef System in the western Caribbean Sea stretches along the coasts of Mexico, Belize, Guatemala, and Honduras (Fig. 1a) and serves as an essen- tial source of marine biodiversity and productivity for the region (Miloslavich et al. 2010). Studies of the Belize Barrier Reef identified spawning aggregation sites for many species of Caribbean fishes (Heyman et al. 2005, 2007, 2008; Heyman and Kjerfve 2008). Those spawning aggregation sites have a unique geomorphology with large horizontal curvatures and convex steep slopes (Heyman and Requena 2002; Kobara and Heyman 2008; Wright and Heyman 2008), suggesting that flow–topography interactions and intense mixing there help disperse larvae and eggs from these sites and then transport them into more protected areas (Ezer et al. 2011). The exact forcing mechanisms of small-scale flow variability (scales of a few meters to a few kilometers) in the vicinity of the reefs are not yet fully understood, though both remote forcing by Caribbean eddies (Ezer et al. 2005) and local variability by tides, winds, and internal waves may play a role (Ezer et al. 2011). Responsible Editor: Leo Oey This article is part of the Topical Collection on the 3rd International Workshop on Modelling the Ocean 2011 T. Ezer (*) Old Dominion University, Norfolk, VA, USA e-mail: tezer@odu.edu W. D. Heyman : C. Houser Texas A&M University, College Station, TX, USA W. D. Heyman e-mail: wheyman@geog.tamu.edu C. Houser e-mail: chouser@geog.tamu.edu B. Kjerfve World Maritime University, Malmö, Sweden B. Kjerfve e-mail: kjerfve@wmu.se Ocean Dynamics (2012) 62:1043–1057 DOI 10.1007/s10236-012-0545-5