Cabo Frio Tide Cycle

Tide cycle and upwelling at Cabo Frio (RJ)

Cabo Frio, on Rio de Janeiro's Costa do Sol, runs on a semi-diurnal tide. You get two highs and two lows per lunar day, with roughly 12 h 25 min between one high and the next. The local range stays modest, usually around ~1.2 m from spring high to spring low. A cosine model captures it well enough: h(t) = A · cos(2π·t / 12.42 h), where A is the amplitude and t counts the time since the last high tide.

The region is known for its coastal upwelling (ressurgência). Persistent NE winds push surface water offshore through Ekman transport, and cold, nutrient-rich water from the South Atlantic Central Water (SACW) wells up to take its place, pulling the sea temperature down to 15-18 °C even in midsummer. That cold water feeds heavy marine productivity, which is why the local fishing economy thrives and why the water at Recife do Farol (Arraial do Cabo) is so clear for diving. Official tide tables come from the Marinha do Brasil — DHN.

Applications

Tourism in Búzios, Arraial do Cabo and Cabo Frio. People use this to time scuba diving and snorkelling trips for the slack-tide window, to schedule surf and SUP sessions, to plan recreational fishing, and to make sense of the official DHN tables before heading out by boat.

FAQ

Why is the sea so cold at Cabo Frio? It comes down to wind-driven upwelling. NE winds shove the surface water away, and the cold SACW deep water rises to fill the gap.

When is the best time to dive? Aim for slack tide, close to high or low water, when the currents go quiet and visibility peaks. A few days of NE wind beforehand makes it even better.

Where can I get the official tide table? The Marinha do Brasil DHN publishes them at marinha.mil.br/chm for the Cabo Frio port (station 60230).