Rio de Janeiro

Rio de Janeiro fronts the South Atlantic on Brazil's south-eastern coast, on the long curve of Guanabara Bay between the granite domes of Sugarloaf and the Corcovado, with Copacabana, Ipanema, and Leblon on the open Atlantic-facing south side and the working port and downtown on the bay side facing Niterói across the water. The tide here is a small mixed semidiurnal signal — two highs and two lows of unequal size each day, the asymmetry varying through the lunar month — with a mean range at the Ilha Fiscal harbour gauge inside Guanabara Bay of about 0.6 metres, climbing past 1.0 metre on the largest spring tides and dropping close to flat on neaps. The astronomical forcing is small because the South Atlantic is broad and the continental shelf relatively narrow; the propagating tide reaches the coast as a near-progressive wave rather than building through resonance. What matters more on a day-to-day basis is wind setup from the south-quadrant frentes that sweep up from the South Atlantic — sustained southerly winds can lift apparent water level 30 to 50 cm above predicted, and the same wind builds the famous Copacabana surf and the closeout shorebreak on the open Atlantic-facing beaches. Ipanema and Leblon read the swell more than the tide; Vermelha Beach at the foot of Sugarloaf, the protected swimming at Botafogo, and the long Niterói Beach across the bay all swing through the smaller astronomical signal. The Marinha do Brasil's Centro de Hidrografia da Marinha (CHM) runs the authoritative gauge network and publishes the official tide tables; Open-Meteo Marine drives the gridded predictions on this site.