Ocean circulation patterns around Curaçao: From regional currents to coastal connectivity

Abstract

Ocean currents flowing around islands generate distinctive local circulation patterns in the surrounding waters. As the flow encounters the landmass and is forced to divert around it, eddies form in the island's wake, flow separates along the coastline and zones of mixing and retention develop. These features depend on a combination of factors, including the strength and direction of the incoming current, which varies over daily, seasonal and interannual timescales. The circulation around an island is therefore dynamic and ecologically significant: it determines where and how nutrients, larvae and land-derived substances are transported through coastal waters, directly linking terrestrial activities to the health of nearby marine ecosystems. Reducing non-climate stressors such as anthropogenic pollution can help coral reefs build resilience against climate change, but requires understanding how ocean circulation transports these substances from land to reef sites around islands. This thesis examines current-island interactions in a case study of Curaçao, a small island in the southern Caribbean Sea where the northwest-directed Caribbean Current drives the dominant circulation. The island's southern coastline hosts both its population and extensive fringing coral reefs that have lost over 50% of their coral cover between 1982 and 2015. The central question is: How do ocean currents shape circulation patterns and coastal connectivity around Curaçao, and what are the implications for coral reef ecosystems? The main finding of this thesis is that two distinct flow regimes govern circulation around Curaçao. A strong northwest-directed flow, driven by the Caribbean Current, transports water predominantly away from the island. A weaker regime, characterised by cyclonic eddies or low-energy conditions, enhances connectivity in all directions, allowing land-derived substances to reach otherwise isolated reef areas. Using a high-resolution hydrodynamic model and Lagrangian particle tracking (a method that traces the trajectories of virtual particles through simulated flow fields), this thesis quantifies how these regimes control coastal connectivity around Curaçao and between the island and neighbouring coastlines. Beyond surface circulation, offshore-to-nearshore transport operates independently of the two flow regimes. The transport of offshore waters to nearshore reef areas increases from west to east along the southern coastline, while coastal upwelling is found predominantly in the west, with implications for water renewal, nutrient delivery and thermal regulation of reef environments. The two flow regimes also vary over time. Using 32 years of sea surface height data, this thesis links local flow conditions around Curaçao to the broader Caribbean Current system. The northwest-directed regime dominates during December to February and June to August, while the eddy regime prevails during September to November when the Caribbean Current and winds weaken. This seasonality has direct ecological consequences: wet season rain events flush large amounts of terrestrial material into coastal waters precisely when the eddy regime is most likely to spread that material to reefs across the island. Together, these findings reveal when and where land-derived stressors are most likely to reach reef sites, providing a physical foundation for targeted management strategies supporting the resilience of Curaçao's coral reefs in a changing climate.